Prograde and retrograde diagenetic andmetamorphic evolution in metapelitic rocks

of Sierra Espuna (Spain)

I ABAD 1 F N IE TO 2 D R PEACOR 3AND N VEL ILLA 4

1Departamento de Geologga Universidad de Jaen 23071 Jaen Spain 2Instituto Andaluz de Ciencias de la Tierra yDepartamento de Mineralogga y Petrologga Universidad de Granada 18002 Granada Spain 3Department of

Geological Sciences University of Michigan Ann Arbor MI 48109-1063 USA and 4Departamento de Mineralogga yPetrologga Universidad de Granada 18002 Granada Spain

(Received 2 April 2002 revised 22 July 2002)

ABSTRACT An unusually complete sequence of pelitic rocks ranging from diagenetic togreenschist-facies metamorphic grades occurs in southern Sierra Espuna Spain Prograde andretrograde reactions have been studied by X-ray diffraction and electron microscopy (SEM TEM andAEM) The prograde reaction series with reactions facilitated by tectonic stress includes (1) R4interstratified illite-smectite in the diagenetic Malaguide Complex that preserves the variableorientation of original smectite packets and has 1Md polytypism (2) chemically heterogeneous illiteand Na-K dioctahedral white micas that progressively evolve toward chemical and texturalequilibrium in the anchizonal Intermediate Units and (3) thick defect-free packets of phengiteparagonite and clinochlore which have a typical metamorphic texture in the Alpujarride ComplexTwo superimposed retrograde episodes produced (a) sudoite at near-peak metamorphic conditionsand (b) dioctahedral smectite during low-temperature retrograde diagenesis

KEYWORDS XRD illite crystallinity electron microscopy very low-grade metamorphism sudoitemicas

The full sequence from earliest diagenesis togreenschist-facies metamorphism has not yet beenobserved in a single continuous sequence ofmetapelitic zones It is nevertheless possible todistinguish two kinds of diagenetic to low-grademetamorphic sequences (a) those affected bypassive sedimentation as in Gulf Coast sediments(eg Ahn amp Peacor 1986) or the Basque-Cantabrian Basin (Nieto et al 1996) and (b)those subject to tectonic stress eg the WelshSedimentary Basin (Merriman amp Roberts 1985)and the metasediments of the Gaspe PeninsulaQuebec (Jiang et al 1994) Studies of a variety ofsequences are essential in order to understand the

significance of factors such as initial sedimentlithology and subsequent tectonic history In rocksof the Sierra Espuna area Spain low-grademetamorphism has been triggered by tectonicactivity The rocks of this area include an unusuallylarge range of diagenesis in the MalaguideComplex from a very low-temperature environmentto greenschist-facies metamorphism in theAlpujarride Complex with a clockwise PTtevolution described by Azanon amp Crespo-Blanc(2000) Only the very lowest-grade diagenesis forwhich smectite-rich clay minerals are diagnostic isnot represented As such nearly complete low-grade metamorphic sequences are rare making theEspuna area a very interesting region for studying acomplete range of evolution from diagenesisthrough to epizonal metamorphism Thus it ispossible to try to establish the prograde and

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Clay Minerals (2003)38 1ndash23

2003 The Mineralogical Society

retrograde processes that determine typical low-grade mineral assemblages and their texturalcharacteristics

As mineral reactions may not be complete inlow-grade metasediments the assemblages are indisequilibrium and the recognition of geneticrelations between the different phases and theidentification of paragenetic associations is amajor problem The very fine-grained texture ofthese rocks makes it difficult to examine them evenby scanning electron microscopy (SEM) usingbackscattered electron (BSE) imaging and so atransmission electron microscope (TEM) study wasused to understand the textural relationships andchemistry of phyllosilicates in them at lattice scaleThe aim of this research is to advance ourknowledge of low-grade metamorphic processes inorder to characterize prograde events and todifferentiate them from retrograde overprints Todo so a detailed study of the rocks must be carriedout The sequence of the Espuna area has been thesubject of a TEM study of the regional retrogradealteration of chlorite to smectite (Nieto et al 1994)and therefore is especially suitable for studyingp ro g ra de - r e t r o g r ad e t r a ns fo rm a t i o n s i nphyllosilicates

G E O L O G I C A L S E T T I N G A N DM A T E R I A L S

Sierra Espuna is located in the Internal Zones of theBetic Cordillera (southern Spain) the westernmostEuropean Alpine chain It has been the subject ofdetailed studies of cartographic and stratigraphicrelations (Paquet 1969) Metamorphic relationswere studied by Makel amp Rondeel (1979) andstructural aspects were investigated by Makel(1981 1985) Lonergan (1991) and Lonergan etal (1994) The most recent studies of Sierra Espunafocused on stratigraphy (Martotildeacuten-Martotildeacuten amp Martotildeacuten-Algarra 1997) and redefinition of the tectonic units(Sanz de Galdeano et al 2001) The nomenclatureused in this work is based on that used in the latterreference

Three large tectonic complexes have beendistinguished in the Internal Zones of the BeticCordillera tectonically superimposed in the order(from bottom to top) Nevado-Filabride Alpujarrideand Malaguide Complexes Each complexcomprises multiple superimposed tectonic unitsSanz de Galdeano et al (2001) concluded that thereare five imbricated tectonic units in the southern

part of Sierra Espuna (Fig 1) The upper onebelongs to the Malaguide Complex (Morron deTotana) below which there are three IntermediateUnits (La Santa Yechar and Jaboneros) withtransitional stratigraphic and metamorphic featuresand an intermediate tectonic position between theMalaguide and Alpujarride Complexes The lowesttectonic unit is Los Molinos an Alpujarride UnitAccording to Makel amp Rondeel (1979) theMalaguide and Intermediate Units were affectedonly by very low-grade metamorphism or bydiagenesis whereas the Alpujarride Unit wasaffected by greenschist-facies metamorphism

Thirty nine samples were collected fromoutcrops taking care that they were free ofsurface alteration effects The Los MolinosYechar La Santa and Morron de Totana unitseach include two formations (Makel amp Rondeel1979) (a) a Permian-Triassic formation composedof conglomerates sandstones and red pelitic rocksand (b) an Upper Triassic carbonate formation inwhich dolomitic rocks predominate Only detritalnon-carbonate rocks were sampled throughout thedifferent units except Jaboneros the outcrops ofwhich are small and beyond the limits of thestudied section In general the rocks are red andgreen metapelites intercalated with thick induratedsandstone beds The Los Molinos Unit is a greyphyllite and rocks at the Yechar Unit are browngreywackes with variable proportions of matrixconsisting in part of phyllosilicates cemented bycarbonates Quartz grains are abundant Theproportions of clay minerals are relatively smalland these samples are therefore considered to besemipelitic rocks The La Santa Unit rockstectonically superimposed on the Yechar Unit arered and green metapelites The stratigraphicsuccession is very similar to that of the YecharUnit In the upper unit Morron de Totana thestratigraphic sequence is complicated by lateralfacies changes (carbonate indentations) Thesampled horizons are red-orange pelites withintercalated conglomerates and carbonates

A N A L Y T I C A L ME T H O D S

Samples were studied by optical microscopy andX-ray diffraction (XRD) in order to determine theoverall trends in mineral assemblages and gradeThe matrix materials are too fine grained to beresolved even by BSE imaging except for sampleswith the highest metamorphic grade Therefore

2 I Abad et al

HRTEM was required to characterize the texturalrelationships between the minerals especially thephyllosilicates

X-ray diffraction

Unaltered samples were obtained from wellbelow the outcrop surface and away from jointsto minimize alteration by weathering Samples werewashed and after coarse crushing homogeneousrock chips were used for preparation of samples for

XRD Whole-rock samples and clay fractions(lt2 mm) were studied using a Philips PW 1710powder diffractometer with Cu-Ka radiationgraphite monochromator and automatic divergenceslit at the Departamento de Mineralog otildeacutea yPetrologotildeacutea of the Universidad de Granada Thelt2 mm fractions were separated by repeatedextraction of supernatant liquid subsequent tosettling Oriented aggregates were prepared bysedimentation on glass slides Ethylene-glycol(EG) treatment was carried out on some samples

FIG 1 Geological map of Sierra Espuna (based on Sanz de Galdeano et al 2001) (1) Tertiary and Quaternaryterrains (2) (a) pelites and sandstones (b) carbonate rocks (3) pelites sandstones and conglomerates (4) pelitesand greywackes (5) (7) (9) and (11) carbonate rocks (6) and (8) pelites and sandstones (10) and (12) phyllitesand quartzites (13) sample locations (14) thrust-nappe (15) anticline The tectonostratigraphic column givingthe highest and lowest units in a thrust stack is shown on the right All these strata are Triassic except for (4)

which consists of Upper Palaeozoic sedimentary rocks

Prograde and retrograde metamorphism 3

to corroborate the identification of smectite andorillite-smectite mixed-layers Preparation of samplesand experimental conditions for illite ` crystallinityrsquorsquo(IC) measurements were carried out according toIGCP 294 IC Working Group recommendations(Kisch 1991) Our IC measurements (y) weretransformed into CIS values (x) according to theequation y = 0674x + 0052 (r = 0999) obtained inour laboratory using the international standards ofWarr amp Rice (1994) The IC values were measuredfor the lt2 mm fractions lt2 mm EG-treated fractionsand for the bulk-rock samples The b cellparameters of micas and chlorites were obtainedfrom the (060) peaks measured on slices of rock cutnormal to the sample foliation For all spacingmeasurements quartz from the sample itself wasused as internal standard

Electron microscopy

Following the XRD and optical studies sevenrepresentative samples of the different tectonostrati-graphic levels were selected for electron micro-scopy study on the basis of the IC values and themineral assemblages Carbon-coated slices wereexamined by SEM using back-scattered electron(BSE) imaging and energy-dispersive X-ray (EDX)analysis in order to obtain textural and chemicaldata These observations were carried out using aZeiss DSM 950 SEM equipped with an X-ray LinkAnalytical QX-20 energy-dispersive X-ray system(EDX) at the Centro de Instrumentacion Cientotildeacutefica(CIC) Granada An accelerating voltage of 20 kVwith a beam current of 1 ndash2 nA and counting timeof 100 s were used to analyse the phyllosilicates bySEM using both natural and synthetic standardsalbite (Na) periclase (Mg) wollastonite (Si andCa) and orthoclase (K) and synthetic Al2O3 (Al)Fe2O3 (Fe) and MnTiO3 (Ti and Mn) Ion-milledand carbon-coated specimens were studied by TEMwith two microscopes (1) a Philips CM12 scanningtransmission electron microscope (STEM) operatedat an accelerating voltage of 120 kV and beamcurrent of ~10 mA equipped with a KevexQuantum EDX system (University of Michigan)and (2) a Philips CM20 (STEM) equipped with anEDAX solid-state EDX detector operated at200 kV with a LaB6 filament (CIC) Lattice-fringe images of phyllosilicates were obtained using(00l) reflections Records of TEM data included144 lattice-fringe images and 94 selected areaelectron diffraction (SAED) patterns

The EDX spectra of ion-milled samples wereobtained using a raster of 10006200 AEcirc in scanningmode to minimize alkali diffusion and volatilizationwith the long axis oriented parallel to the length ofphyllosilicate packets Analytical electron microscopy(AEM) quantitative chemical analyses were obtainedfrom spectra using ion-milled standards of muscovitealbite clinochlore fayalite and sphene to derivek-values following the procedure of Jiang et al(1990) The AEM analyses were also obtained frompowders dispersed over holey C-coated formvar Cugrids after first verifying the monomineraliccharacter of each grain with a SAED pattern Suchanalyses permit larger areas to be used in scanningtransmission mode (STEM) (area of 1 mm61 mm)with greater accuracy because elements especiallyalkalis cannot diffuse away from the area analysedShorter counting times (30 s) were used for K tofurther minimize loss (Nieto et al 1996) Albitebiotite spessartine muscovite olivine titanite MnSand CaS were used as standards to derive k-factorsfor the transformation of intensity ratios to concen-tration ratios following the procedures of Cliff ampLorimer (1975)

The structural formulae of micas were calculatedon the basis of 22 negative charges O10(OH)2Although these analytical techniques cannot distin-guish between Fe3+ and Fe2+ Guidotti et al (1994)showed that the opaque-mineral assemblage (hema-tite in these samples) can be used to estimate theFe3+Fe2+ ratio in muscovite Even in low redox-potential parageneses ~50 of the Fe in muscoviteis Fe3+ and values close to 85 can be reached inless reducing environments Therefore in thecalculations of formulae it has been assumed that75 of the Fe in the micas is Fe3+

Due to the very fine-grained nature of thesamples the results obtained by AEM weresometimes rejected due to overlap of the beam onmaterial other than the mineral analysed Whenmica contamination was slight the chlorite formulawas recalculated after subtracting the proportions ofelements determined by normalizing to the K andNa contents resulting in a formula based on 28negative charges (Nieto 1997)

Electron microprobe

The EMPA analyses of detrital micas werecarried out using a Cameca SX50 electron micro-probe operated at 20 kV with a beam current of30 nA and a beam diameter of lt5 mm The

4 I Abad et al

standards were albite orthoclase periclase wollas-tonite and synthetic oxides (Al2O3 Fe2O3MnTiO3) Data were reduced using the proceduresof Pouchou amp Pichoir (1985)

R E S U L T S

Mineralogy and crystal-chemical parameters

The XRD patterns of bulk samples show thatquartz and white mica primarily phengite are theprincipal phases in all samples trioctahedralchlorite is very common smectite mixed-layeredillite-smectite and intermediate Na-K mica para-gonite feldspars carbonates and hematite arepresent only in some samples (Table 1) A di-trioctahedral chlorite sudoite according to Fransoletand Schreyerrsquos (1984) criteria has been detected insamples from the La Santa and Morron de Totanaunits The XRD patterns of sudoite usually show adistinctly more intense (003) reflection character-istic of Al-rich chlorites In general the mineralassemblages are similar and typical of very low-grade metamorphism

The basal spacing of mica is ~998 AEcirc (s = 001)varying with no clear trend throughout the

sequence An overall average value of 9019 AEcirc (s= 0010) obtained for the b cell parameter(Table 1) is consistent with medium-pressureregional metamorphism according to Guidotti ampSassi (1986)

Figure 2 shows the IC values for the lt2 mm vslt2 mm EG-treated fractions corresponding to all unitsexcept for the Los Molinos Unit for whichparagonite is a major phase whose peaks interferewith measurement of IC values The IC values of thelt2 mm fraction for the Morron de Totana Unitcorrespond to the zone of diagenesis with significantvariation in values in different samples as typical ofthat grade For the La Santa Unit the data indicatelate diagenetic conditions The IC values vary theleast for samples from the Yechar Unit andcorrespond to the low anchizone (according to thenomenclature of Merriman amp Peacor 1999) Inaddition Fig 2 shows that for the Morron de Totanasamples the IC values of the lt2 mm fraction arelarger than those of the EG-treated samples implyingthe presence of smectite layers interstratified withillite The IC values for the bulk samples aregenerally smaller than those of the lt2 mm separatesas consistent with a significant contribution ofdetrital mica in the untreated samples (Table 1)

FIG 2 Plot of IC values for the lt2 mm untreated vs lt2 mm EG-treated fractions

Prograde and retrograde metamorphism 5

Whole-rock analyses

Chemical analyses of the major elements ofselected samples were carried out using X-ray

fluorescence (XRF) in a Philips PW 104010spectrometer at the CIC The data (Table 2) werecompared with the average of Post-Archaean

TABLE 1 Crystal-chemical parameters and bulk mineralogy determined by XRD

White mica Chlorite Mineral compositiond001 Illite crystallinity (CIS) d001 Qtz K-white mica

Samples lt2 mm bulk b lt2 mm lt2 mm EG bulk lt2 mm bulk (all the samples)

Los Molinos UnitEs-101 9972 9966 9012 ndash ndash ndash 142 142 Chl Hem Pg Cc Dol SmEs-102 9978 9980 9002 ndash ndash ndash 142 142 Chl Hem Pg Cc SmEs-103 9970 9975 ndash ndash ndash ndash 142 142 Chl Hem Kfs Pg Cc SmEs-104 9969 ndash ndash ndash ndash ndash 142 ndash Chl Hem Pg Cc Sm

Yechar UnitEs-51 9977 9972 9019 038 035 020 1417 1417 Chl Hem Dol SmEs-52 9974 9970 9016 041 046 022 1417 1416 Chl Kfs SmEs-53 9978 9971 9016 037 040 022 1417 1417 Chl Hem Kfs SmEs-54 9979 9978 9019 041 ndash 025 1416 1416 Chl Hem SmEs-55 9975 9987 ndash 038 041 020 1416 1418 Chl Hem Cc SmEs-56 9957 9967 9010 043 044 022 ndash ndash Hem Cc DolEs-57 9958 ndash 9019 050 049 ndash 1418 ndash Chl Cc Dol

La Santa UnitEs-1 9976 ndash 9019 050 044 071 1417 ndash Chl Hem Kfs SmEs-2 9974 9977 9003 062 049 029 1417 1417 Chl Kfs SmEs-3 9963 ndash 9018 058 050 031 1420 ndash Chl Hem Cc SmEs-4 9987 9987 9007 075 077 049 1416 1417 Chl Hem DolEs-5 9993 9983 ndash 093 ndash 084 1417 1417 Su Chl Hem Ill-Pg PgEs-6 9988 9982 ndash 089 092 078 1417 1416 Chl Hem Cc Dol Ill-PgEs-7 9990 9981 ndash 098 095 069 1418 1419 Chl Hem Cc DolEs-8 9985 9975 9007 066 ndash 046 1417 1416 Chl Hem Cc Dol Ill-Pg

Morron de Totana UnitEs-71 9977 ndash 8994 ndash ndash ndash ndash ndash Chl Kfs HemEs-72 ndash ndash ndash ndash ndash ndash ndash ndash Chl Hem GtEs-73 9970 ndash ndash ndash ndash ndash ndash ndash Kfs Cc DolEs-74 9991 ndash ndash 069 071 ndash ndash ndash Kfs Hem GtEs-75 9972 9991 8989 068 064 ndash ndash ndash Chl Kfs Hem DolEs-76 10005 ndash 064 052 ndash ndash ndash ndash Chl Hem Gt DolEs-77 9979 ndash 9011 077 059 083 ndash ndash Chl Hem Dol I-SEs-78 9972 ndash 084 068 071 ndash ndash ndash Chl Cc Sm GtEs-79 9984 ndash 9028 098 090 061 ndash ndash Chl Kfs Hem DolEs-70 9984 ndash 096 089 035 1416 1418 ndash Dol Chl SmEs-81 9980 ndash ndash 099 061 ndash 142 ndash Chl Hm Cc DolEs-82 9972 ndash ndash 090 083 ndash ndash ndash Chl Hm Cc Dol SmEs-85 9972 ndash 9008 058 050 050 142 ndash Chl Su Hem Sm KfsEs-86 9951 9977 9013 092 084 047 142 ndash Chl Su Hem Sm KfsEs-87 9974 9008 083 ndash 050 142 ndash ndash Chl Su Hem Sm KfsEs-91 9974 ndash ndash 104 108 093 ndash ndash Chl Hem KfsEs-92 9972 ndash 9028 083 074 086 ndash ndash Chl Dol KfsEs-93 9980 9983 ndash 109 096 061 ndash ndash Chl Hem DolEs-94 9964 9985 9024 098 093 080 ndash ndash Chl Hem Dol KfsEs-95 9978 ndash 9027 104 093 ndash ndash ndash Dol

Mineral abbreviations according to Kretz (1983) Ill-Pg Intermediate Na-K mica I-S Illite-smectiteinterstratified layers Su Sudoite In bold samples studied by TEM

6 I Abad et al

Australian Shales (PAAS) (Taylor amp McLennan1985) The values for samples of this study aresimilar to them The loss on ignition (LOI) is verylarge in some samples because of the presence ofcarbonates

Electron microscopy observations (SEM andTEM)

Textural aspects Due to the fine-grainedcharacter of these rocks characterization of texturesis possible only by TEM except for the highestmetamorphic-grade samples (Es-102 and Es-53) forwhich grain sizes are large enough to be resolvedby BSE imaging The HRTEM characterizationconfirms the general mineralogical data determinedby XRD for the different units of the studied areaand have allowed us to clarify some specificrelations eg the mixed-layered illite-smectite inthe Morron de Totana Unit

Los Molinos Unit samples are phyllosilicate-richphyllites with well-developed slaty cleavage(Fig 3a) composed of preferentially oriented micaand chlorite alternating with deformed elongatedquartz grains carbonate aggregates and hematitegrains Paragonite and muscovite coexist as separatecrystals Lattice-fringe images show that both micasand chlorite occur as large defect-free crystalsmore than several thousand angstroms thick

(Fig 4) Lattice-fringe images (not shown) withboth 00l and hkl fringes show that crystals arecomposed virtually entirely of coherently relatedlayers but with some stacking disorder in chloriteThese features are in accord with the typicalgreenschist facies of the Alpujarride Complex(Azanon amp Crespo-Blanc 2000) On the otherhand dioctahedral smectite commonly occursreplacing layers of trioctahedral chlorite describedby Nieto et al (1994) as the result of retrogradealteration by hydrothermal fluids

The sample studied from the basal portion of theYechar Unit (Es-53) is a quartz-rich slate with well-developed cleavage (Fig 3b) High-resolutionimages show that the main phyllosilicate is K-richdioctahedral mica with lesser amounts of chloriteand dioctahedral smectite the smectite occurring asaltered layers of primary metamorphic chloriteAlthough phyllosilicates generally form parallel orlow-angle intergrowths preferentially orientedparallel to cleavage Fig 5 shows a low-magnifica-tion image with well-defined high-angle grainboundaries of interlocking grains Such texturesare typical of a metamorphic origin The packetsare 500 AEcirc thick (Fig 6) much smaller than typicaldetrital grains (gt5000 AEcirc ) They also have composi-tions which differentiate them from detrital micasas described below (see chemical characterizationof phyllosilicates) In general the phyllosilicate

TABLE 2 Whole-rock analyses of major elements (wt oxide)

Samples SiO2 TiO2 Al2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O P2O5 LOI

Es-102 5340 077 1790 557 100 013 158 722 152 254 013 847Es-51 5270 083 1752 441 190 014 514 359 028 503 016 845Es-52 7620 093 1210 070 120 ndash 185 037 143 251 017 239Es-53 5990 089 1890 583 140 ndash 292 021 043 489 014 408Es-56 6640 061 1200 350 020 ndash 190 384 011 427 012 616Es-57 6147 065 1154 369 086 005 354 522 ndash 403 012 909Es-1 6008 086 1820 524 170 ndash 299 029 031 484 014 416Es-2 6510 091 1770 139 190 ndash 322 033 038 427 016 470Es-3 5152 089 2030 590 185 006 429 241 ndash 619 018 642Es-5 6260 090 1750 688 020 ndash 263 030 065 237 012 493Es-6 6891 080 1321 434 086 009 169 293 050 234 010 536Es-7 5158 087 2109 671 142 006 275 379 086 347 018 766Es-75 7238 071 1251 238 118 003 154 143 ndash 337 009 438Es-77 3442 055 1252 516 087 005 956 1337 ndash 361 009 2160Es-79 4996 064 1062 335 129 024 676 935 ndash 301 008 1590Es-86 7600 083 1208 122 144 002 205 048 061 324 014 282Es-87 6285 106 1705 516 175 004 299 034 033 493 018 396Es-91 5881 083 1707 409 151 003 338 217 ndash 569 017 645Es-95 5913 075 1802 368 104 002 410 095 009 717 016 520

Prograde and retrograde metamorphism 7

packets are almost defect-free with only rare layerterminations (Fig 6) The polytypes are 2M formicas and semi-random for chlorites

Two samples have been studied from the LaSanta Unit Sample Es-2 a chlorite-rich shale isfrom the basal part of the unit and sample Es-5 aquartz-rich shale is from the upper part In Es-2elongated quartz-rich and phyllosilicate-richdomains alternate at the scale of millimetresparallel to slaty cleavage The principal phyllosili-cate is muscovite with well-defined 2M polytypismThe (hk0) reflections tend to be spread normal to cmuch more than in sample Es-53 due to the

presence of thinner packets in subparallel orienta-tions The trioctahedral chlorite (clinochlore) isoften altered to dioctahedral smectite as determinedin part by AEM data as a result of retrogradealteration processes (eg Fig 7 in Nieto et al1994) The most common defects are layerterminations voids low-angle grain boundariesand dislocations

Sample Es-5 shows incipient slaty cleavagedefined by subparallel preferentially orientedpackets of illite sudoite trioctahedral chloriteparagonite and intermediate Na-K mica as illu-strated in Fig 7 Hematite is commonly included

FIG 4 Lattice-fringe image showing a portion of a defect-free well-crystallized chlorite packet of metamorphicorigin in phyllite Es-102 (Los Molinos Unit)

FIG 3 BSE images showing the well-developed slaty cleavage of two samples (a) Es-102 (Los Molinos Unit) aphyllite in which the grains with the brightest contrast are hematite and the darkest ones quartz Chlorite andmuscovite grains are characterized by their elongated habit and intermediate brightness (stronger in chlorite thanin muscovite) (b) Es-53 (Yechar Unit) in which stretched quartz grains are more abundant and responsible for itssemipelitic character grains with high contrast are hematite or TiO2 Muscovite crystals are indicated by arrows

8 I Abad et al

within illite packets as elongated individual grainsup to ~3006500 AEcirc in size or as aggregates(Fig 7) they show up clearly due to their darkcontrast Sudoite and chlorite were identified on thebasis of SAED and AEM data Separate grains ofeach were observed to be intergrown with texturestypical of metamorphic origins for both Grain sizesof sudoite (gt500 AEcirc ) and clinochlore are approxi-mately equal implying formation of both at gradeswhich were not substantially different Howeverwhere grains of each intersect at high angles thesudoite ubiquitously transects clinochlore packetsAs shown in Fig 8 clinochlore but not sudoitemay exhibit strain contrast These relations implybut do not prove that sudoite post-dated clino-chlore Obvious alteration of clinochlore todioctahedral smectite was ubiquitous but possiblealteration effects were rare in sudoite occurring asdefects such as voids and wavy layers Because thecomposition of sudoite is similar to that ofdioctahedral smectite it was not possible to verifythe presence of small numbers of smectite layerswith AEM data Nevertheless the observationsclearly show that alteration of chlorite to smectitewas relegated entirely or almost entirely toclinochlore

Muscovite occurs in crystals which average~500 AEcirc in thickness and are characterized by 2Mpolytypism features which are typical of meta-morphic mica of anchizonal grade The Na-K micaoccurs in packets of similar thickness The

FIG 6 Lattice-fringe image showing the relation between mica and chlorite packets The arrows indicate theposition of layer terminations The inset SAED pattern illustrates the low-angle relation between the two phases

(Yechar Unit)

FIG 5 Low-magnification image with well-definedhigh-angle grain boundaries and interlocking relations

among muscovite crystals (Yechar Unit)

Prograde and retrograde metamorphism 9

composition (see below) is quite variable corre-sponding to Na-rich muscovite commonly but somegrains have compositions within the limits of thesolvus between muscovite and paragonite and thusare metastable Na-K micas give SAED patternswhich have relatively sharp reflections over a largerange as consistent with large defect-free crystals

Sudoite crystals comprise units with 14 AEcirc

periodicities which in some areas are 28 AEcirc Lattice-fringe images show the presence of inter-

stratified individual 7 AEcirc layers which in some areasform small packets containing no more than eight7 AEcirc layers The AEM analyses (see below) of suchareas are typical of those of homogeneous sudoitecontaining only 14 AEcirc layers implying a composi-tion of the 7 AEcirc layers similar to that of sudoite andnot to an exotic septechlorite such as kaolinite orberthierine The contrast of both 14 AEcirc and 7 AEcirc

fringes appear to be identical further consistentwith a close chemical relation

FIG 7 Textural relations between phyllosilicates (sudoite illite Na-K mica) and hematite (dark contrast) (LaSanta Unit)

FIG 8 Lattice-fringe image showing the coexistence of two types of chlorite trioctahedral chlorite (chl) andsudoite (su) Sudoite transects chlorite which shows strain contrast associated with curved and kinked layersIdentifications were based in part on AEM analyses (see Table 6 Es-511 and 12 analyses) (La Santa Unit)

10 I Abad et al

Three samples corresponding to different butequivalent levels were selected from the Morron deTotana Unit for TEM study (see Fig 1 peliticrocks intercalated with carbonate rocks) Es-77 atthe bottom of the unit Es-86 which is located in anintermediate position and Es-95 from the top of thecross-section There is no incipient slaty cleavage inthese samples Sample Es-77 is a very fine-grainedrock consisting of quartz white mica chlorite andaggregates of carbonate grains Minor hematiteproduces a deep red colour Lattice-fringe images

show that dioctahedral clay minerals have 10 and20 AEcirc periodicities occur as packets 100 ndash200 AEcirc

thick and have subparallel orientations Accordingto Guthrie amp Veblen (1989) smectite and illiteinterlayers can produce different contrast and canonly be differentiated in lattice fringe-imagesobtained with well-defined electron beam (001)orientations and defocus conditions Such contrastdifferences were observed in several lattice-fringeimages The light contrast for fringes in Fig 9represents the smectite interlayer as shown in part

FIG 9 TEM image showing 10 AEcirc periodicity in R4 illite-rich I-S Contrast differences imply the presence ofillite-like (the dark fringes) and smectite-like (the light fringes) layers Packets usually have four or more illite

layers and the SAED pattern shows predominantly 50 AEcirc periodicity (Morron de Totana Unit)

FIG 10 Trioctahedral chlorite with intercalations of 10 AEcirc layers (upper arrow) which damage quickly under theelectron beam The chlorite packets show strain contrast microfolds and 7 AEcirc periodicities corresponding toberthierine (left arrow) or greater values eg 28 AEcirc (see analysis in Table 6 Es-867) (Morron de Totana Unit)

Prograde and retrograde metamorphism 11

by the continuity of such fringes with openinterlayer spaces produced by contraction due to

dehydration The sequence on average is that ofR4 mixed-layering ie a single smectite-likeinterlayer between four or more illite-like inter-layers Although some packets with more than fourlayers of illite are present repeats of four illite-likelayers predominate as shown by the 50 AEcirc

periodicity of SAED patterns (Fig 9 inset) Theillite-smectite (I-S) has typical 1Md polytypism Theobservation of smectite interstratified with illite isin accord with the changes observed in XRDdiagrams after EG treatment (Fig 2) In additionAEM analyses of areas similar to that of Fig 9have relatively high Si and minor Ca contentsfurther confirming the mixed layering

Some packets which give rise to fringes with10 AEcirc spacing produce SAED patterns typical oftwo-layer polytypism The contrast of all fringes isrelatively constant Such packets have thicknessestypical of diagenetic clays and two orders ofmagnitude smaller than typical detrital grains Thecompositions of such grains (see below) haveinterlayer-cation contents typical of illite They aretherefore inferred to be illite which formed duringlate diagenesis and coexisting with the R4 I-S

Sample Es-86 is composed mainly of detritalgrains of quartz randomly-oriented grains ofchlorite and white mica all immersed in aphyllosilicate-rich fine-grained matrix At theTEM scale chlorite is the most common phyllosi-licate in the matrix As in sample Es-5 trioctahe-dral chlorite and sudoite coexist There areinterstratified packets of 10 AEcirc layers only a fewlayers thick (lt150 AEcirc ) which are quickly damaged

FIG 11 Di-trioctahedral chlorite (sudoite) very similarin appearance to the trioctahedral chlorite shown inFig 10 7 AEcirc layers are marked by an arrow and 10 AEcirc

layers are present in the poorly resolved area in thecentre of the image (see analysis in Table 6 Es-862)

(Morron de Totana Unit)

FIG 12 Low-magnification image illustrating illite-like packets The inset SAED pattern shows only a smallnumber of diffuse poorly defined reflections The packets are 100 ndash200 AEcirc thick (Morron de Totana Unit)

12 I Abad et al

under the electron beam (Figs 10 and 11) TheAEM analyses of such material show an enhancedSi content and the presence of small concentrationsof alkali cations These data are consistent withretrograde alteration of chlorite as observed byNieto et al (1994) They differ however from theresults of Nieto et al (1994) and those describedabove for sample Es-5 in that both tri-trioctahedraland di-trioctahedral chlorite are affected In TEMimages strain features in chlorites such as contrastnormal to the layers or microfolds were observed(Fig 10) Mica is less abundant than sudoiteoccurring in packets 100 ndash400 AEcirc thick The SAEDpatterns exhibit 2M polytypism but with diffuse-ness parallel to c in 0kl rows At low magnifica-tion subparallel and slightly curved illite- or illite-smectite-like packets are present they coalescehave abundant layer terminations and show intensecontrasts (Fig 12)

Sample Es-95 is a very fine-grained rock madeup of large detrital grains of quartz in a fine-grained matrix of phyllosilicates mainly whitemicas and opaque minerals Some biotite grainswere observed as verified by AEM data Biotitewas observed only in separated grains on holey

C-coated Cu grids so it is not possible to describethe textural relations for it Authigenic biotite isinconsistent with the low grade of this samplehowever and it is therefore inferred to be of detritalorigin The AEM analyses show deviations frompure biotite however as consistent with alterationto chlorite the usual process in detrital biotite inrocks undergoing diagenesis Lattice-fringe imagesof ion-milled samples show that individual micapackets are very thin with no more than 10 layersisolated and dispersed in a matrix consisting ofmaterial which gives no SAED pattern Thesepackets are wavy discontinuous and with anasto-mosing layers (Fig 13) Despite these texturalfeatures SAED data indicate that there are 2Mand 1Md polytypes

For all samples prismatic crystals of Ti-oxidedispersed Fe-oxyhydroxides zircon and tourmalineare common accessory minerals

Chemical characterization of phyllosilicates

Dioctahedral mica Table 3 shows the composi-tion of K-rich dioctahedral micas of authigenic ormetamorphic origin corresponding to the samples

FIG 13 General aspect of sample Es-95 The image shows arrays of wavy discontinuous and anastomosingpackets of illite and smectite (Morron de Totana Unit)

Prograde and retrograde metamorphism 13

selected from each unit The Yechar and LosMolinos units are characterized by compositionsapproaching those of evolved micas with a sum ofinterlayer cations slightly less than 1 apfu typicalof phengite or muscovite rather than authigenic

illite In contrast the La Santa and Morron deTotana units show heterogeneous mica composi-tions and display deficiencies in interlayer cations(Fig 14a) This last feature is more evident in theMorron de Totana analyses where the illitic

TABLE 3 Representative chemical compositions for K-rich dioctahedral micas normalized to O10(OH)2

Si IVAl VIAl Fe Mg Ti VIS K Na S inter

Los Molinos Unit (1)Es-10213 337 063 178 010 008 002 198 075 008 085Es-10221 308 092 195 005 001 001 202 082 009 091Es-10231 319 081 191 005 003 001 200 077 012 090Es-10232 314 086 188 006 007 001 202 072 021 093

Yechar Unit (1)Es-5311 332 068 163 015 022 002 204 090 004 093Es-5312 310 090 180 003 013 004 201 089 010 099Es-5322 338 062 170 012 015 003 200 086 001 086Es-5323 319 081 176 016 012 003 206 087 002 089

Yechar Unit (2)Es-53 1 317 083 163 021 026 000 210 084 000 084Es-53 2 320 080 165 011 027 003 207 087 000 087Es-53 3 317 083 155 025 024 002 206 093 000 093Es-53 4 328 072 160 020 024 000 203 092 000 092Es-53 5 326 074 164 019 016 000 199 096 000 096Es-53 6 326 074 158 020 028 000 206 089 000 089Es-53 7 319 081 154 025 026 000 205 097 000 097Es-53 8 325 075 166 016 025 000 206 085 000 085Es-53 9 324 076 167 019 019 000 205 085 000 085

La Santa Unit (2)Es-2 1 311 089 157 019 032 002 210 088 005 094Es-2 3 336 064 148 020 043 001 212 072 003 075Es-2 6 313 087 156 024 029 001 210 075 017 092Es-2 7 313 087 155 024 029 001 209 084 011 095Es-2 8 331 069 157 015 032 000 205 087 004 092Es-2 10 328 072 160 018 027 003 208 068 008 075Es-5 4 303 097 174 015 023 001 213 073 009 082Es-5 10 301 099 189 006 015 000 210 065 021 086Es-5 7 307 093 159 033 016 000 208 075 018 093Es-5 8rsquo 301 099 160 031 025 000 216 067 016 083

Morron de Totana Unit (2) (analyses corresponding to R4 Ill-Sm may also be present see text)Es-77 13 327 073 172 013 027 000 212 067 000 067Es-77 15 316 084 195 003 013 000 211 063 000 063Es-77 16 328 072 182 010 015 000 207 068 000 068Es-77 17 338 062 147 032 019 000 199 093 000 093Es-86 9 334 066 160 017 034 000 210 072 000 072Es-86 10 357 043 151 012 046 000 209 065 000 065Es-86 11 353 047 156 012 036 000 203 077 000 077Es-86 13 324 076 152 025 035 000 211 084 000 084Es-86 14 324 076 173 013 019 000 205 084 000 084Es- 95 2 323 077 158 016 019 004 198 103 000 103Es- 95 5 336 064 169 013 025 000 207 070 000 070Es- 95 6 336 064 148 027 033 000 209 078 000 078

(1) SEMEDX analyses (2) TEMAEM analyses

14 I Abad et al

character is consistent with the presence of the R4mixed-layer illite-smectite described above Most ofthe micas are phengitic in composition (Fig 14ab)with an Fe(Fe+Mg) ratio that varies over the range02 ndash085 However the detrital-like grains aresimilar to those of end-member muscovite (seebelow detrital muscovite vs metamorphic musco-vite or Table 5) with much lower Fe and Mgcontents Data plotted in Fig 14bc confirm theassumption of a significant ferrimuscovitic substitu-tion (Guidotti et al 1994) Some of the K-micaanalysed in the Los Molinos and La Santa unitswere found to have significant Na contents (up to02 apfu) Although Ti is unusual in dioctahedralmicas it has been measured in several analyses

(lt005 apfu) The most evident feature in theplots is the scattering of data in the La Santa andMorron de Totana units ie where diageneticconditions are not exceeded In contrast theYechar Unit analyses define a narrow field in thediagrams and the Los Molinos data are even closerto the theoretical muscovite composition (Table 3)

Typical compositions of micas intermediate tomuscovite and paragonite called Na-K micas in thispaper are shown in Table 4 The compositions arequite variable even in the same sample Whencompared with analyses of K-rich dioctahedralmicas (Table 3) and paragonite analyses are seento span the entire range from muscovite toparagonite Some analyses (eg Es-5 9) fall within

FIG 14 Plots of values of chemical compositions of K-rich dioctahedral phyllosilicates solid circles and linesindicate values for theoretical muscovite and corresponding exchange vectors respectively

Prograde and retrograde metamorphism 15

the limits of the solvus between muscovite andparagonite the limbs of that solvus having theapproximate compositions ms60pg40 and ms20pg80

(Li et al 1994a)Detrital muscovite vs metamorphic muscovite

Detrital-like micas of sample Es-95 the lowest-grade one are large enough to be analysed(Table 5) by electron microprobe (EMPA) Thesedata have been compared with the AEM data ofother samples Compositions of detrital-like grainsof the lowest-grade sample and the detrital micas ofthe anchizonal-grade sample Es-53 (Fig 15) arevery similar and close to that of end-membermuscovite These data suggest that the detrital micahas a common source for the different units thatcomprise Sierra Espuna Figure 15 also comparesthe data from authigenic micas of the samesamples which are clearly more phengitic (Fe+Mg= 015 ndash075 apfu) than the detrital ones (Fe+Mg= 01 ndash02 apfu)

Chlorite Trioctahedral and di-trioctahedral(sudoite) chlorites have been analysed (Table 6)The former corresponding to the species clino-chlore is common in most of the samples unlikesudoite which is absent from most (Table 1)Sudoite was identified because the sum ofoctahedral cations is nearly equal to 5 apfu andthe Si and Al contents are higher than intrioctahedral chlorite (Fig 16a) A slight contam-ination by interlayer cations (K and Na) has beendetected therefore the formulae were recalculatedin order to eliminate those impurities (seeAnalytical Methods) Although total Fe has beenconsidered as Fe2+ according to Fransolet ampBourguignon (1978) the Fe3+ content in sudoitemay be greater than the Fe2+ content As severalanalyses have an octahedral sum slightly higherthan the ideal value of 5 apfu this may indicate atrioctahedral substitution andor the presence ofsignificant Fe3+ No Ti or Mn were detected The

Fe(Fe+Mg) ratios do not define a clear trendthroughout the sequence (Fig 16b) nevertheless itis much greater in chlorite in the Morron de Totanaanalyses falling in the characteristic range ofdiagenetic chlorites (Li et al 1994b) There is anincrease in Mg content (from 15 to 4 apfu)concomitant with increase in grade as compatiblewith some other sequences (eg Bevins et al1991)

D I S C U S S I O N

Dioctahedral phyllosilicates chemicalcompositions IC values and polytypes

On the basis of their large size homogeneity ofindividual grains and well-crystallized aspectdetrital dioctahedral mica grains were identified inseveral samples with the exception of those ofgreenschist facies This confirms the idea of the

TABLE 4 Chemical compositions for Na-K micas normalized to O10(OH)2

Si IVAl VIAl Fe Mg Ti VIS Ca K Na S inter

Es-5 14 313 087 190 004 009 002 204 000 034 048 083Es-5 15 321 079 176 009 037 000 222 000 015 038 053Es-5 4 318 082 187 004 019 000 209 000 013 061 074Es-5 12 329 071 172 008 029 000 208 000 039 036 076Es-5 13 323 077 179 005 028 000 212 000 029 041 069Es-5 9 296 104 163 019 029 000 211 000 048 056 104Es-5 8 292 108 156 043 008 001 209 003 014 080 096

FIG 15 Plot of Si vs Al contents of detrital andauthigenic micas of samples Es-95 and Es-53 Solidsymbols = detrital micas open symbols = authigenic

micas

16 I Abad et al

relative stability of muscovite in most diageneticenvironments (Crowley 1991) The compositions ofthe detrital grains of samples from different unitsare similar The lack of heterogeneity which isexpected for mica from a complex source implies acommon source area for all samples

The IC values define a trend from diagenetic toanchizonal conditions but with smaller IC values inbulk-rock fractions consistent with the contributionof detrital mica The coexistence of mature micawith lower-grade I-S or illite in the sub-greenschistfacies rocks shows that those rocks have not

TABLE 5 EMPA data for detrital-like crystals of mica normalized to O10(OH)2

Si IVAl VIAl Fe Mg Ti VIS Ca K Na K+Na+Ca

Es-95z11 313 087 193 005 004 001 203 000 065 022 087Es-95z11 314 086 190 006 007 001 204 000 068 019 087Es-95z1 2 314 086 191 005 009 001 206 000 070 006 077Es-95z2 1 314 086 176 012 010 003 201 002 095 004 101Es-95z2 2 317 083 177 012 011 003 202 000 095 004 099Es-95z4 1 305 095 188 005 007 004 204 000 081 009 089Es-95z4 2 311 089 187 006 007 004 204 000 078 009 086

TABLE 6 Representative chemical compositions for chlorite normalized to O10(OH)8

Si IVAl VIAl Fe Mg VIS FeFe+Mg

Yechar UnitEs53-1 273 127 112 121 376 609 024Es53-4 287 113 125 070 400 596 015Es53-12 303 097 130 146 307 583 032Es53-13 277 123 125 122 354 601 026Es53-15 307 093 146 094 323 564 023Es53-42 271 129 152 160 294 606 035

La Santa UnitEs2-4 319 081 147 148 273 569 035Es2-5 365 035 183 118 225 526 034Es5-1 354 046 293 023 172 488 012Es5-5 361 039 292 021 178 490 010Es5-10 309 091 262 028 221 511 011Es5-16 310 090 197 112 234 543 032Es5-17 350 050 252 076 170 499 031Es-5 10rsquo 364 036 213 097 202 512 032Es-5 11 291 109 143 136 314 593 030Es-5 12 382 018 229 087 179 495 033Es-5 13 316 084 302 021 172 494 011Es-5 16rsquo 346 054 341 013 101 455 011Es-5 17rsquo 300 100 282 033 194 509 014Es-5 18 285 115 285 026 204 515 011

Morron de Totana UnitEs-86 2 377 023 253 088 149 490 037Es-86 4 284 116 166 230 181 578 056Es-86 5 350 050 191 192 151 534 056Es-86 6 366 034 189 191 149 529 056Es-86 7 315 085 129 273 176 578 061

indicates sudoite analyses

Prograde and retrograde metamorphism 17

reached stable chemical or textural equilibrium atleast at the mm scale In addition the lack ofchemical equilibrium among diagenetic I-S andillite at the nm scale is demonstrated by their highlyvariable Fe+Mg contents (006ndash075 apfu) Theaverages of the sum of interlayer cations are closeto 1 apfu in the Los Molinos and Yechar units asconsistent with mica but values of 06 apfucorresponding to I-S or illitic compositions are alsopresent in the Morron de Totana Unit Except forsamples Es-102 and Es-53 the variable composi-tions represent a metastable state with composi-tions of micas determined by a combination ofphengitic ferrimuscovitic and illitic vectors asshown by Fig 14ab

Paragonite is generally considered an indexmineral for the anchizone (Frey 1987) but Li etal (1994a) have shown that paragonite coexistswith K-rich mica in grades as low as that of the latediagenetic zone for a prograde sequence Samplesin which muscovite paragonite and intermediateNa-K mica coexist as in sample Es-5 areconsistent with metastability both because morethan two such micas coexist and because theircompositions are incompatible with the solvus inthe muscovite-paragonite system (see below) Whilethe 2M polytype typical of metamorphic micas is

the most common in these samples the 1Md

polytype also occurs in some diagenetic samplesparticularly those from the Morron de Totana UnitThe 1Md polytype is metastable relative to the 2Mpolytype and the coexistence of both polytypesfurther demonstrates a lack of equilibrium at the nmscale Dong amp Peacor (1996) showed that thedisorder noted in the diagenetic phyllosilicates bySAED is a consequence of the general high entropyof phases formed at very low temperatures thatproduces for example stacking defects separatingpackets with no more than 10 layers These packetsprogressively thicken with increasing metamorphicgrade and the 2M polytype then becomes the mostcommon polytype in authigenic micas Thenm-scale size of the lower-grade clays and theregular increase in size with increasing gradereflect the lack of textural equilibrium in the low-grade samples

A wide range of sample characteristics thusimply that the lower-grade samples are in statesof metastable equilibrium those factors includingcoexistence of mature mica with I-S and illitevariable chemical compositions incompatiblecompositions of white micas small crystal sizesdefect-rich states and polytypism Such dataemphasize the notion of lsquoreaction progressrsquo relative

FIG 16 (a) Plot of Al vs Fe+Mg contents of chlorites illustrating the contrast in composition between tri-trioctahedral clinochlore or chamosite and di-trioctahedral sudoite (b) plot of Si vs Fe(Fe+Mg) ratio of

chlorites showing an absence of compositional trends throughout the sampled sequence

18 I Abad et al

to Ostwald-step rule processes metastable dioctahe-dral clay systems undergoing reactions toward thestate of stable chemical and textural equilibriumapproached in greenschist-facies samples (Essene ampPeacor 1995)

Prograde evolution based on TEM datacorrelation with the Basque-Cantabriansequence (Northern Spain)

With increasing grade TEM data confirm thatcrystal thickness increases crystal defect densitydecreases compositions become less variable andmicrotextural relations are simpler (eg Peacor1992) This trend is consistent with three well-characterized low-grade settings the Gulf Coastsequence (Ahn amp Peacor 1986) and the WelshBasin (Merriman et al 1990) both extensionalbasins and the Helvetic Alps (Livi et al 1997)The latter is a fold-and-thrust belt with similarseries of prograde transitions to those observed inSierra Espuna in which pelites have passed througha sequence of metastable states in the lithologicalsequence mudstone and shale slate and phyllite

The mudstones of the Morron de Totana Unit arecharacterized by the presence of illite-smectite R4mixed-layers mica packets of no more than 10 ndash40layers and the occurrence of 1Md polytypism TheLa Santa Unit samples represent a step in theprograde evolution with thicker packets than in theprevious samples (200 ndash700 AEcirc ) illitic or phengiticcompositions the absence of illite-smectite mixedlayers as well as the presence of discreteintermediate Na-K micas and paragonite Althoughthere is a tendency towards non-preferred orienta-tion of crystals it is not so evident as in the Morronde Totana samples In the Yechar Unit (anchizone)and more clearly in the Los Molinos phyllites(greenschist facies) the microtextural relations areless complex the compositions are less variablewith phengite and trioctahedral chlorite crystalthickness are greater (gt400 AEcirc in the Yechar Unitand gt1000 AEcirc in the Los Molinos Unit) with thepackets being defect-free and with a very crystallineaspect This description has features in commonwith the three well-characterized low-grade settingspreviously noted Only the mineral assemblagesvary somewhat presumably as a function of thedifferences in bulk-rock composition

A detailed comparison with the Basque-Cantabrian Basin (N Spain) shows that the lowest-grade conditions of Sierra Espuna overlap most of

those of that sequence which is characterized bypassive burial diagenesis (8000 m thick) (Nieto etal 1996) Only in the deepest sample of theBasque-Cantabrian sequence is illite-smectite mixedlayering absent Figure 12 of this study is verysimilar to Fig 7 in Nieto et al (1996) and bothimages correspond to diagenetic samples fromdifferent settings nevertheless they illustratesimilar mica packets 100 ndash200 AEcirc thick withparallel-to-subparallel boundaries the orientationof which is inferred to represent beddingTherefore there is a parallelism between thesesequences down to the scale of individual clay-mineral grains even though they developed underdifferent conditions with stress presumably playinga significant role in the Sierra Espuna units Thetexture of the least metamorphosed sample (Fig 13)is reminiscent of the typical textural relations ofsmectite described by Nieto et al (1996) (seeFig 4) Although it corresponds to illite-richinterstratified illite-smectite the clay mineralgrains conserved the grain size and anastomosingdiscontinuous array of original smectite layers

Despite the similarities between the twosequences the chemical behaviour of the micasare clearly different since the grade correspondingto the phengitic compositions of the Yechar Unit isnot reached in the Basque-Cantabrian Basin wherethe deepest and highest-grade sample has illiticcompositions equivalent to those of the La SantaUnit That is in a passive burial metamorphiccontext as in the Basque-Cantabrian Basin thefactors responsible for increasing the rate ofchemical reactions are less significant than in thesequences that were affected by tectonic stressTectonic stress introduces defects which in turnincreases the rate of dissolution of strained grainsthus increasing the rate of dissolution-crystallizationreactions typical of clay minerals Perhaps moreimportantly regional tectonic stress is known tocause mobilization of fluids (eg Oliver 1986) Thepresence and activity of such fluids is a prime causeof increased rates of dissolution ion diffusion andcrystallization Tectonic activity thus may have atwo-fold effect in increasing rate of clay-mineralreactions The phengite component may in part be ameasure of the reaction progress depending on thereaction kinetics in very low-grade rocksAccording to Merriman amp Peacor (1999) whereheat flow is low as in accretionary settingsphengite contents are typically high perhapsbecause there is insufficient thermal energy to

Prograde and retrograde metamorphism 19

cause equilibration of the phengite component ofneoformed illite by contrast with extensional basinswhere this component is typically small

Coexistence of trioctahedral chlorite andsudoite Implications for multiple metamorphicevents

In metamorphic rocks the coexistence of mineralsthat formed at different times and conditions on aPTt path ie not in equilibrium is a commonlyobserved feature where the metamorphic history iscomplex Gross differences in the grain size ofcoexisting phases has long been recognized bymetamorphic petrologists as an indicator of non-equilibrium mineral assemblages Historically muchless attention has been paid to such relations in sub-greenschist-facies rocks because prior to the intro-duction of SEM+TEM techniques their small grainsize prevented observations of textural relationsStudy of such rocks eg by XRD provides littleinformation on textural relations beyond determina-tion of degree of preferred orientation

Study by XRD of the samples showed thatsudoite and trioctahedral chlorite coexist If theyhad formed at the same time and under the sameP-T conditions their compositions would define thelimbs of a solvus The TEM images show howeverthat wherever they are in contact the sudoitetransects the packets of trioctahedral chlorite (egFig 8) Furthermore as illustrated in Fig 8 thetrioctahedral chlorite is commonly deformed anddisplays strain contrast in TEM images Thesetextural relations suggest that sudoite was intro-duced subsequent to the formation of trioctahedralchlorite Similar observations were made byGiorgetti et al (1998) who concluded that texturesin Verrucano metasediments showed that sudoitepost-dated chloritoid pyrophyllite and trioctahedralchlorite ie sudoite is in disequilibrium with thelatter minerals and formed as a retrograde phase Inrocks of the Sierra Espuna area differences in bulk-rock composition are small (Table 2) and do notcorrespond to the presence or absence of sudoite(Table 1) ie the random occurrence of sudoiteindependent of bulk-rock composition is compa-tible with a lack of equilibrium relations Theserelations collectively imply that sudoite wasintroduced at some time subsequent to theformation of mineral assemblages at peak meta-morphic conditions On the other hand the size ofsudoite packets is similar to that of trioctahedral

chlorite as consistent with formation at conditionsnot much different from those for chlorite

In samples from the Morron de Totana bothsudoite and trioctahedral chlorite were affected byretrograde alteration to smectite The TEM imagesshow sudoite (Fig 11) and trioctahedral chlorite(Fig 10) with interstratified layers of smectite withspacings of 10 AEcirc due to collapse and dehydration inthe TEM environment and even packets of smectite150 AEcirc thick within chlorite crystals These relationsimply that both sudoite and trioctahedral chloritehave been affected by retrograde alteration at thevery low temperatures consistent with smectiteformation although the presence of more subtlealteration features in sudoite of some samplessuggests that it was less susceptible to alteration

We therefore conclude that at least three episodesof phyllosilicate formation can be recognized inrocks from the Sierra Espuna-area Trioctahedralchlorite was presumably produced at the highestmetamorphicdiagenetic grade in equilibrium withparagonite and phengite Subsequently sudoite wasintroduced but at P-T conditions which could nothave been much different than those of peakmetamorphic grade Finally at very low-T condi-tions hydrothermal fluids caused partial alterationof trioctahedral chlorite and sudoite to smectite

Retrograde products formed by the reaction offluids with mineral phases generated during anearlier prograde event were described previouslyby Nieto et al (1994) and Zhao et al (1999) Thosereactions occurred on a regional scale but similarreactions have been observed on a more localizedscale The fluid activity associated with a majorfault zone is a common source for these reactions(eg Jiang et al 1990) The structure of SierraEspuna (Fig 1) is based on imbrication processesthat superimpose units that have subsequently beenaffected by faults and folds and that have under-gone significant uplift at least 800 m from the lateMiocene to the present (Sanz de Galdeano et al2001) Given this evolution it is easy to justify theretrograde processes that usually seem to be relatedwith tectonic events during which fluid flow andfluidrock ratios are enhanced (Merriman amp Peacor1999) Moreover according to Nieto et al (1994)the uplift of the Betic Cordillera would furnish theintroduction of phreatic water into the systemproducing a reactive environment adequate for thegenesis of smectite Geological conditions aretherefore consistent with post-peak metamorphicconditions as implied by the TEM observations

20 I Abad et al

The introduction of sudoite as a major phaseduring metamorphism implies the possibility ofadditional overprints on a simple prograde assem-blage the evidence for which might be subtle TheNa-rich micas at all grades have compositionswhich either fall within the solvus or are too Na-rich to correspond to the limb of the solvus In thehighest-grade rocks such micas coexist withmuscovite Those relations are incompatible withchemical equilibrium despite the fact that relationsfor muscovite and other minerals imply an approachto chemical equilibrium Furthermore there is aclose association of Na-rich micas with sudoite (egFig 7) Those relations imply the possibility thatNa-micas were introduced concomitant withsudoite P Mata (pers comm) has shown forexample that overprinting of an epizonal assem-blage by hydrothermal solutions resulted in theintroduction of both an Al-rich variety of chlorite(cookeite) and paragonite in pelites of the CamerosBasin Spain The textural relations such as cross-cutting textures which are so striking for sudoitewere not observed for Na-rich micas Neverthelessthe possibility of such overprinting must beconsidered

In spite of all these prograde-retrogradeprocesses in the Los Molinos Unit a smallnumber of homogeneous phases coexist in chemicalequilibrium and the fine-grained metamorphicphyllosilicates are clearly parallel to cleavage witha typical metamorphic aspect Finally all thetextural and compositional changes that haveoperated in the Espuna area during the metamorphicevolution proceed toward a simpler system repre-senting both textural and chemical equilibriumwhich is characteristic of greenschist-facies condi-tions

Roles of detrital and authigenic phyllosilicatesduring slaty cleavage development

Van der Pluijm et al (1998) focused on therespective roles of the phyllosilicates of detritalorigin and those formed during diagenesis andmetamorphism and observed that detrital phyllosi-licates remain preferentially oriented parallel tobedding in pelites with slaty cleavage whereas fine-grained metamorphic phyllosilicates are orientedparallel to cleavage

Although it is usually difficult to determine thepreferred orientations of phyllosilicates in TEMimages as the grade increases better-aligned

packets occur in the cleavage orientation Thetexture of the least metamorphosed sample inwhich even incipient slaty cleavage is absent isreminiscent of the typical textural relation ofsmectites described in the literature The illite-smectite mixed-layers conserve the textural featuresof possible previous smectite such as anastomosingfringes curved and discontinuous with variations ininterplanar spacing d001 These features indicatethat the minimum energy conditions to initiatemechanical transformations have not been reachedin the lower-grade samples as postulated by Ho etal (1996) since slaty cleavage develops as a resultof interactive mechanical deformation and ther-mally driven dissolution and crystallizationprocesses (Knipe 1981) In contrast in the LosMolinos Unit large crystals of chlorite and whitemica (gt1000 AEcirc ) occur which are of metamorphicorigin but with no detrital grains (Fig 3a)Phyllosilicates are preferentially oriented parallelto slaty cleavage in response to increasedtemperature and tectonic strain (Merriman ampPeacor 1999) The oriented microfabrics and slatycleavage well developed in the rocks of this unitmust be a product of a higher thermal and strainenergy environment reached in the deepest part ofthe sequence As pointed out by Van der Pluijm etal (1998) a continuous range of orientations isevidence for mechanical rotation of grains whereasthe lack of such transitional orientations impliesthat cleavage-parallel phyllosilicates originatedthrough dissolution of bedding-parallel and crystal-lization in the slaty cleavage orientation Asobserved in this study for the Intermediate Unitsgrains with orientations which are transitional frombedding-parallel to slaty cleavage orientation areonly rarely observed Cleavage formation is there-fore inferred to have occurred entirely or nearlyentirely through dissolution and crystallization

ACKNOWLEDGMENTS

We thank MM Abad Ortega from the Centro deInstrumentacio n Cient otilde fica of the Universidad deGranada for her help with HRTEM and I Nieto forher essential help with sample preparation Financialsupport was supplied by Research Project nordm BT 2000-0582 FPI research grant to IA both of the SpanishMinistry of Science and Technology Research GroupRNM-0179 of the Junta de Andaluc otildeacutea and to DRP byNSF grant EAR-9814391

Prograde and retrograde metamorphism 21

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Ahn JH amp Peacor DR (1986) Transmission andanalytical electron microscopy of the smectite-to-illite transition Clays and Clay Minerals 34165 ndash179

Azanon JM amp Crespo-Blanc A (2000) Exhumationduring a continental collision inferred from thetectonometamorphic evolution of the AlpujarrideComplex in the central Betics (Alboran Domain SESpain) Tectonics 19 549 ndash565

Bevins RE Robinson D amp Rowbotham G (1991)Compositional variations in mafic phyllosilicatesfrom regional low-grade metabasites and applicationof the chlorite geother mometer Journal ofMetamorphic Geology 9 711 ndash721

Cliff G amp Lorimer GW (1975) The quantitativeanalysis of thin specimens Journal of Microscopy 103 203 ndash207

Crowley SF (1991) Diagenetic modification of detritalmuscovite an example from the Great LimestoneCyclothem (Carboniferous) of Co Durham UKClay Minerals 26 91 ndash103

Dong H amp Peacor DR (1996) TEM observations ofcoherent stacking relations in smectite and illite ofshales evidence for MacEwan crystallites anddominance of 2M1 polytypes Clays and ClayMinerals 44 257 ndash275

Essene E amp Peacor DR (1995) Clay mineral thermo-metry a critical perspective Clays and ClayMinerals 43 540 ndash553

Fransolet AM amp Bourguignon P (1978) Dioctahedralchlorite in quartz veins from Ardennes BelgiumThe Canadian Mineralogist 16 365 ndash373

Fransolet AM amp Schreyer W (1984) Sudoite ditrioctahedral chlorite a stable low-temperature phasein the system MgO-Al2O3-SiO2-H2O Contributionsto Mineralogy and Petrology 86 409 ndash417

Frey M (1987) Very low-grade metamorphism of clasticsedimentary rocks Pp 9 ndash58 in Low TemperatureMetamorphism (M Frey editor) Blackie GlasgowUK

Giorgetti G Goffe B Memmi I amp Nieto F (1998)Metamorphic evolution of Verrucano metasedimentsin northern Apennines new petrological constraintsEuropean Journal of Mineralogy 10 1295 ndash1308

Guidotti CV amp Sassi FP (1986) Classification andcorrelation of metamorphic facies series by means ofmuscovite b0 data from low grade metapelites NeuesJahrbuch fur Mineralogie Abhandlungen 153363 ndash380

Guidotti CV Yates MG Dyar MD amp Taylor ME(1994) Petrogenetic implications of the Fe3+ contentof muscovi te in pel it ic sch is ts AmericanMineralogist 79 793 ndash795

Guthrie GD amp Veblen DR (1989) High resolutionelectron microscopy of mixed-layer illitesmectite

Computer simulations Clays and Clay Minerals 371 ndash11

Ho N Peacor DR amp Van der Pluijm BA (1996)Contrasting roles of detrital and authigenic phyllo-silicates during slaty cleavage development Journalof Structural Geology 18 615 ndash623

Jiang WT Peacor DR Merriman RJ amp Roberts B(1990) Transmission and analytical electron micro-scopic study of mixed layer illitesmectite formed asan apparent replacement product of diagenetic illiteClays and Clay Minerals 38 449 ndash468

Jiang WT Peacor DR amp Buseck PR (1994) Chloritegeothermometry Contamination and apparent octa-hedral vacancies Clays and Clay Minerals 42593 ndash605

Kisch HJ (1991) Development of slaty cleavage anddegree of very-low-grade metamorphism a reviewJournal of Metamorphic Geology 9 735 ndash750

Knipe JR (1981) The interaction of deformation andmetamorphi sm in slates Tectonophysics 78249 ndash272

Kretz R (1983) Symbols for rock-forming mineralsAmerican Mineralogist 68 277 ndash279

Li G Peacor DR Merriman RJ amp Roberts B (1994a)The diagenetic to low grade metamorphism evolu-tion of matrix white mica in the system muscovite-paragonite in a mudrock from Central Wales UKClays and Clay Minerals 42 369 ndash381

Li G Peacor DR Merriman RJ Roberts B amp Vander Pluijm BA (1994b) TEM and AEM constraintson the origin and significance of chlorite-mica stacksin slates an example from Central Wales UKJournal of Structural Geology 16 1139 ndash1157

Livi KJT Veblen DR Ferry JM amp Frey M (1997)Evolution of 21 layered silicates in low-grademetamorphosed Liass ic sha les of CentralSwitzerland Journal of Metamorphic Geology 15323 ndash344

Lonergan L (1991) Structural evolution of the SierraEspuna Betic Cordillera SE Spain PhD thesisOxford University UK

Lonergan L Platt JP amp Gallagher L (1994) TheInternal-External Zone Boundary in the eastern BeticCordillera SE Spain Journal of Structural Geology16 175 ndash188

Makel GH (1981) Differences in tectonic evolution ofsuperimposed Malaguide and Alpujarride tectonicunits in the Espuna area (Betic Cordilleras Spain)Geology in Mijnbouw 60 203 ndash208

Makel GH (1985) The geology of the MalaguideComplex and its bearing on the geodynamicevolution of the Betic-Rif orogen (southern Spainand northern Morocco) GUA papers of Geology Ser1 22 263 pp

Makel GH amp Rondeel HE (1979) Differences instratigraphy and metamorphism between superposedMalaguide and Alpujarride units in the Espuna area

22 I Abad et al

retrograde processes that determine typical low-grade mineral assemblages and their texturalcharacteristics

As mineral reactions may not be complete inlow-grade metasediments the assemblages are indisequilibrium and the recognition of geneticrelations between the different phases and theidentification of paragenetic associations is amajor problem The very fine-grained texture ofthese rocks makes it difficult to examine them evenby scanning electron microscopy (SEM) usingbackscattered electron (BSE) imaging and so atransmission electron microscope (TEM) study wasused to understand the textural relationships andchemistry of phyllosilicates in them at lattice scaleThe aim of this research is to advance ourknowledge of low-grade metamorphic processes inorder to characterize prograde events and todifferentiate them from retrograde overprints Todo so a detailed study of the rocks must be carriedout The sequence of the Espuna area has been thesubject of a TEM study of the regional retrogradealteration of chlorite to smectite (Nieto et al 1994)and therefore is especially suitable for studyingp ro g ra de - r e t r o g r ad e t r a ns fo rm a t i o n s i nphyllosilicates

G E O L O G I C A L S E T T I N G A N DM A T E R I A L S

Sierra Espuna is located in the Internal Zones of theBetic Cordillera (southern Spain) the westernmostEuropean Alpine chain It has been the subject ofdetailed studies of cartographic and stratigraphicrelations (Paquet 1969) Metamorphic relationswere studied by Makel amp Rondeel (1979) andstructural aspects were investigated by Makel(1981 1985) Lonergan (1991) and Lonergan etal (1994) The most recent studies of Sierra Espunafocused on stratigraphy (Martotildeacuten-Martotildeacuten amp Martotildeacuten-Algarra 1997) and redefinition of the tectonic units(Sanz de Galdeano et al 2001) The nomenclatureused in this work is based on that used in the latterreference

Three large tectonic complexes have beendistinguished in the Internal Zones of the BeticCordillera tectonically superimposed in the order(from bottom to top) Nevado-Filabride Alpujarrideand Malaguide Complexes Each complexcomprises multiple superimposed tectonic unitsSanz de Galdeano et al (2001) concluded that thereare five imbricated tectonic units in the southern

part of Sierra Espuna (Fig 1) The upper onebelongs to the Malaguide Complex (Morron deTotana) below which there are three IntermediateUnits (La Santa Yechar and Jaboneros) withtransitional stratigraphic and metamorphic featuresand an intermediate tectonic position between theMalaguide and Alpujarride Complexes The lowesttectonic unit is Los Molinos an Alpujarride UnitAccording to Makel amp Rondeel (1979) theMalaguide and Intermediate Units were affectedonly by very low-grade metamorphism or bydiagenesis whereas the Alpujarride Unit wasaffected by greenschist-facies metamorphism

Thirty nine samples were collected fromoutcrops taking care that they were free ofsurface alteration effects The Los MolinosYechar La Santa and Morron de Totana unitseach include two formations (Makel amp Rondeel1979) (a) a Permian-Triassic formation composedof conglomerates sandstones and red pelitic rocksand (b) an Upper Triassic carbonate formation inwhich dolomitic rocks predominate Only detritalnon-carbonate rocks were sampled throughout thedifferent units except Jaboneros the outcrops ofwhich are small and beyond the limits of thestudied section In general the rocks are red andgreen metapelites intercalated with thick induratedsandstone beds The Los Molinos Unit is a greyphyllite and rocks at the Yechar Unit are browngreywackes with variable proportions of matrixconsisting in part of phyllosilicates cemented bycarbonates Quartz grains are abundant Theproportions of clay minerals are relatively smalland these samples are therefore considered to besemipelitic rocks The La Santa Unit rockstectonically superimposed on the Yechar Unit arered and green metapelites The stratigraphicsuccession is very similar to that of the YecharUnit In the upper unit Morron de Totana thestratigraphic sequence is complicated by lateralfacies changes (carbonate indentations) Thesampled horizons are red-orange pelites withintercalated conglomerates and carbonates

A N A L Y T I C A L ME T H O D S

Samples were studied by optical microscopy andX-ray diffraction (XRD) in order to determine theoverall trends in mineral assemblages and gradeThe matrix materials are too fine grained to beresolved even by BSE imaging except for sampleswith the highest metamorphic grade Therefore

2 I Abad et al

HRTEM was required to characterize the texturalrelationships between the minerals especially thephyllosilicates

X-ray diffraction

Unaltered samples were obtained from wellbelow the outcrop surface and away from jointsto minimize alteration by weathering Samples werewashed and after coarse crushing homogeneousrock chips were used for preparation of samples for

XRD Whole-rock samples and clay fractions(lt2 mm) were studied using a Philips PW 1710powder diffractometer with Cu-Ka radiationgraphite monochromator and automatic divergenceslit at the Departamento de Mineralog otildeacutea yPetrologotildeacutea of the Universidad de Granada Thelt2 mm fractions were separated by repeatedextraction of supernatant liquid subsequent tosettling Oriented aggregates were prepared bysedimentation on glass slides Ethylene-glycol(EG) treatment was carried out on some samples

FIG 1 Geological map of Sierra Espuna (based on Sanz de Galdeano et al 2001) (1) Tertiary and Quaternaryterrains (2) (a) pelites and sandstones (b) carbonate rocks (3) pelites sandstones and conglomerates (4) pelitesand greywackes (5) (7) (9) and (11) carbonate rocks (6) and (8) pelites and sandstones (10) and (12) phyllitesand quartzites (13) sample locations (14) thrust-nappe (15) anticline The tectonostratigraphic column givingthe highest and lowest units in a thrust stack is shown on the right All these strata are Triassic except for (4)

which consists of Upper Palaeozoic sedimentary rocks

Prograde and retrograde metamorphism 3

to corroborate the identification of smectite andorillite-smectite mixed-layers Preparation of samplesand experimental conditions for illite ` crystallinityrsquorsquo(IC) measurements were carried out according toIGCP 294 IC Working Group recommendations(Kisch 1991) Our IC measurements (y) weretransformed into CIS values (x) according to theequation y = 0674x + 0052 (r = 0999) obtained inour laboratory using the international standards ofWarr amp Rice (1994) The IC values were measuredfor the lt2 mm fractions lt2 mm EG-treated fractionsand for the bulk-rock samples The b cellparameters of micas and chlorites were obtainedfrom the (060) peaks measured on slices of rock cutnormal to the sample foliation For all spacingmeasurements quartz from the sample itself wasused as internal standard

Electron microscopy

Following the XRD and optical studies sevenrepresentative samples of the different tectonostrati-graphic levels were selected for electron micro-scopy study on the basis of the IC values and themineral assemblages Carbon-coated slices wereexamined by SEM using back-scattered electron(BSE) imaging and energy-dispersive X-ray (EDX)analysis in order to obtain textural and chemicaldata These observations were carried out using aZeiss DSM 950 SEM equipped with an X-ray LinkAnalytical QX-20 energy-dispersive X-ray system(EDX) at the Centro de Instrumentacion Cientotildeacutefica(CIC) Granada An accelerating voltage of 20 kVwith a beam current of 1 ndash2 nA and counting timeof 100 s were used to analyse the phyllosilicates bySEM using both natural and synthetic standardsalbite (Na) periclase (Mg) wollastonite (Si andCa) and orthoclase (K) and synthetic Al2O3 (Al)Fe2O3 (Fe) and MnTiO3 (Ti and Mn) Ion-milledand carbon-coated specimens were studied by TEMwith two microscopes (1) a Philips CM12 scanningtransmission electron microscope (STEM) operatedat an accelerating voltage of 120 kV and beamcurrent of ~10 mA equipped with a KevexQuantum EDX system (University of Michigan)and (2) a Philips CM20 (STEM) equipped with anEDAX solid-state EDX detector operated at200 kV with a LaB6 filament (CIC) Lattice-fringe images of phyllosilicates were obtained using(00l) reflections Records of TEM data included144 lattice-fringe images and 94 selected areaelectron diffraction (SAED) patterns

The EDX spectra of ion-milled samples wereobtained using a raster of 10006200 AEcirc in scanningmode to minimize alkali diffusion and volatilizationwith the long axis oriented parallel to the length ofphyllosilicate packets Analytical electron microscopy(AEM) quantitative chemical analyses were obtainedfrom spectra using ion-milled standards of muscovitealbite clinochlore fayalite and sphene to derivek-values following the procedure of Jiang et al(1990) The AEM analyses were also obtained frompowders dispersed over holey C-coated formvar Cugrids after first verifying the monomineraliccharacter of each grain with a SAED pattern Suchanalyses permit larger areas to be used in scanningtransmission mode (STEM) (area of 1 mm61 mm)with greater accuracy because elements especiallyalkalis cannot diffuse away from the area analysedShorter counting times (30 s) were used for K tofurther minimize loss (Nieto et al 1996) Albitebiotite spessartine muscovite olivine titanite MnSand CaS were used as standards to derive k-factorsfor the transformation of intensity ratios to concen-tration ratios following the procedures of Cliff ampLorimer (1975)

The structural formulae of micas were calculatedon the basis of 22 negative charges O10(OH)2Although these analytical techniques cannot distin-guish between Fe3+ and Fe2+ Guidotti et al (1994)showed that the opaque-mineral assemblage (hema-tite in these samples) can be used to estimate theFe3+Fe2+ ratio in muscovite Even in low redox-potential parageneses ~50 of the Fe in muscoviteis Fe3+ and values close to 85 can be reached inless reducing environments Therefore in thecalculations of formulae it has been assumed that75 of the Fe in the micas is Fe3+

Due to the very fine-grained nature of thesamples the results obtained by AEM weresometimes rejected due to overlap of the beam onmaterial other than the mineral analysed Whenmica contamination was slight the chlorite formulawas recalculated after subtracting the proportions ofelements determined by normalizing to the K andNa contents resulting in a formula based on 28negative charges (Nieto 1997)

Electron microprobe

The EMPA analyses of detrital micas werecarried out using a Cameca SX50 electron micro-probe operated at 20 kV with a beam current of30 nA and a beam diameter of lt5 mm The

4 I Abad et al

standards were albite orthoclase periclase wollas-tonite and synthetic oxides (Al2O3 Fe2O3MnTiO3) Data were reduced using the proceduresof Pouchou amp Pichoir (1985)

R E S U L T S

Mineralogy and crystal-chemical parameters

The XRD patterns of bulk samples show thatquartz and white mica primarily phengite are theprincipal phases in all samples trioctahedralchlorite is very common smectite mixed-layeredillite-smectite and intermediate Na-K mica para-gonite feldspars carbonates and hematite arepresent only in some samples (Table 1) A di-trioctahedral chlorite sudoite according to Fransoletand Schreyerrsquos (1984) criteria has been detected insamples from the La Santa and Morron de Totanaunits The XRD patterns of sudoite usually show adistinctly more intense (003) reflection character-istic of Al-rich chlorites In general the mineralassemblages are similar and typical of very low-grade metamorphism

The basal spacing of mica is ~998 AEcirc (s = 001)varying with no clear trend throughout the

sequence An overall average value of 9019 AEcirc (s= 0010) obtained for the b cell parameter(Table 1) is consistent with medium-pressureregional metamorphism according to Guidotti ampSassi (1986)

Figure 2 shows the IC values for the lt2 mm vslt2 mm EG-treated fractions corresponding to all unitsexcept for the Los Molinos Unit for whichparagonite is a major phase whose peaks interferewith measurement of IC values The IC values of thelt2 mm fraction for the Morron de Totana Unitcorrespond to the zone of diagenesis with significantvariation in values in different samples as typical ofthat grade For the La Santa Unit the data indicatelate diagenetic conditions The IC values vary theleast for samples from the Yechar Unit andcorrespond to the low anchizone (according to thenomenclature of Merriman amp Peacor 1999) Inaddition Fig 2 shows that for the Morron de Totanasamples the IC values of the lt2 mm fraction arelarger than those of the EG-treated samples implyingthe presence of smectite layers interstratified withillite The IC values for the bulk samples aregenerally smaller than those of the lt2 mm separatesas consistent with a significant contribution ofdetrital mica in the untreated samples (Table 1)

FIG 2 Plot of IC values for the lt2 mm untreated vs lt2 mm EG-treated fractions

Prograde and retrograde metamorphism 5

Whole-rock analyses

Chemical analyses of the major elements ofselected samples were carried out using X-ray

fluorescence (XRF) in a Philips PW 104010spectrometer at the CIC The data (Table 2) werecompared with the average of Post-Archaean

TABLE 1 Crystal-chemical parameters and bulk mineralogy determined by XRD

White mica Chlorite Mineral compositiond001 Illite crystallinity (CIS) d001 Qtz K-white mica

Samples lt2 mm bulk b lt2 mm lt2 mm EG bulk lt2 mm bulk (all the samples)

Los Molinos UnitEs-101 9972 9966 9012 ndash ndash ndash 142 142 Chl Hem Pg Cc Dol SmEs-102 9978 9980 9002 ndash ndash ndash 142 142 Chl Hem Pg Cc SmEs-103 9970 9975 ndash ndash ndash ndash 142 142 Chl Hem Kfs Pg Cc SmEs-104 9969 ndash ndash ndash ndash ndash 142 ndash Chl Hem Pg Cc Sm

Yechar UnitEs-51 9977 9972 9019 038 035 020 1417 1417 Chl Hem Dol SmEs-52 9974 9970 9016 041 046 022 1417 1416 Chl Kfs SmEs-53 9978 9971 9016 037 040 022 1417 1417 Chl Hem Kfs SmEs-54 9979 9978 9019 041 ndash 025 1416 1416 Chl Hem SmEs-55 9975 9987 ndash 038 041 020 1416 1418 Chl Hem Cc SmEs-56 9957 9967 9010 043 044 022 ndash ndash Hem Cc DolEs-57 9958 ndash 9019 050 049 ndash 1418 ndash Chl Cc Dol

La Santa UnitEs-1 9976 ndash 9019 050 044 071 1417 ndash Chl Hem Kfs SmEs-2 9974 9977 9003 062 049 029 1417 1417 Chl Kfs SmEs-3 9963 ndash 9018 058 050 031 1420 ndash Chl Hem Cc SmEs-4 9987 9987 9007 075 077 049 1416 1417 Chl Hem DolEs-5 9993 9983 ndash 093 ndash 084 1417 1417 Su Chl Hem Ill-Pg PgEs-6 9988 9982 ndash 089 092 078 1417 1416 Chl Hem Cc Dol Ill-PgEs-7 9990 9981 ndash 098 095 069 1418 1419 Chl Hem Cc DolEs-8 9985 9975 9007 066 ndash 046 1417 1416 Chl Hem Cc Dol Ill-Pg

Morron de Totana UnitEs-71 9977 ndash 8994 ndash ndash ndash ndash ndash Chl Kfs HemEs-72 ndash ndash ndash ndash ndash ndash ndash ndash Chl Hem GtEs-73 9970 ndash ndash ndash ndash ndash ndash ndash Kfs Cc DolEs-74 9991 ndash ndash 069 071 ndash ndash ndash Kfs Hem GtEs-75 9972 9991 8989 068 064 ndash ndash ndash Chl Kfs Hem DolEs-76 10005 ndash 064 052 ndash ndash ndash ndash Chl Hem Gt DolEs-77 9979 ndash 9011 077 059 083 ndash ndash Chl Hem Dol I-SEs-78 9972 ndash 084 068 071 ndash ndash ndash Chl Cc Sm GtEs-79 9984 ndash 9028 098 090 061 ndash ndash Chl Kfs Hem DolEs-70 9984 ndash 096 089 035 1416 1418 ndash Dol Chl SmEs-81 9980 ndash ndash 099 061 ndash 142 ndash Chl Hm Cc DolEs-82 9972 ndash ndash 090 083 ndash ndash ndash Chl Hm Cc Dol SmEs-85 9972 ndash 9008 058 050 050 142 ndash Chl Su Hem Sm KfsEs-86 9951 9977 9013 092 084 047 142 ndash Chl Su Hem Sm KfsEs-87 9974 9008 083 ndash 050 142 ndash ndash Chl Su Hem Sm KfsEs-91 9974 ndash ndash 104 108 093 ndash ndash Chl Hem KfsEs-92 9972 ndash 9028 083 074 086 ndash ndash Chl Dol KfsEs-93 9980 9983 ndash 109 096 061 ndash ndash Chl Hem DolEs-94 9964 9985 9024 098 093 080 ndash ndash Chl Hem Dol KfsEs-95 9978 ndash 9027 104 093 ndash ndash ndash Dol

Mineral abbreviations according to Kretz (1983) Ill-Pg Intermediate Na-K mica I-S Illite-smectiteinterstratified layers Su Sudoite In bold samples studied by TEM

6 I Abad et al

Australian Shales (PAAS) (Taylor amp McLennan1985) The values for samples of this study aresimilar to them The loss on ignition (LOI) is verylarge in some samples because of the presence ofcarbonates

Electron microscopy observations (SEM andTEM)

Textural aspects Due to the fine-grainedcharacter of these rocks characterization of texturesis possible only by TEM except for the highestmetamorphic-grade samples (Es-102 and Es-53) forwhich grain sizes are large enough to be resolvedby BSE imaging The HRTEM characterizationconfirms the general mineralogical data determinedby XRD for the different units of the studied areaand have allowed us to clarify some specificrelations eg the mixed-layered illite-smectite inthe Morron de Totana Unit

Los Molinos Unit samples are phyllosilicate-richphyllites with well-developed slaty cleavage(Fig 3a) composed of preferentially oriented micaand chlorite alternating with deformed elongatedquartz grains carbonate aggregates and hematitegrains Paragonite and muscovite coexist as separatecrystals Lattice-fringe images show that both micasand chlorite occur as large defect-free crystalsmore than several thousand angstroms thick

(Fig 4) Lattice-fringe images (not shown) withboth 00l and hkl fringes show that crystals arecomposed virtually entirely of coherently relatedlayers but with some stacking disorder in chloriteThese features are in accord with the typicalgreenschist facies of the Alpujarride Complex(Azanon amp Crespo-Blanc 2000) On the otherhand dioctahedral smectite commonly occursreplacing layers of trioctahedral chlorite describedby Nieto et al (1994) as the result of retrogradealteration by hydrothermal fluids

The sample studied from the basal portion of theYechar Unit (Es-53) is a quartz-rich slate with well-developed cleavage (Fig 3b) High-resolutionimages show that the main phyllosilicate is K-richdioctahedral mica with lesser amounts of chloriteand dioctahedral smectite the smectite occurring asaltered layers of primary metamorphic chloriteAlthough phyllosilicates generally form parallel orlow-angle intergrowths preferentially orientedparallel to cleavage Fig 5 shows a low-magnifica-tion image with well-defined high-angle grainboundaries of interlocking grains Such texturesare typical of a metamorphic origin The packetsare 500 AEcirc thick (Fig 6) much smaller than typicaldetrital grains (gt5000 AEcirc ) They also have composi-tions which differentiate them from detrital micasas described below (see chemical characterizationof phyllosilicates) In general the phyllosilicate

TABLE 2 Whole-rock analyses of major elements (wt oxide)

Samples SiO2 TiO2 Al2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O P2O5 LOI

Es-102 5340 077 1790 557 100 013 158 722 152 254 013 847Es-51 5270 083 1752 441 190 014 514 359 028 503 016 845Es-52 7620 093 1210 070 120 ndash 185 037 143 251 017 239Es-53 5990 089 1890 583 140 ndash 292 021 043 489 014 408Es-56 6640 061 1200 350 020 ndash 190 384 011 427 012 616Es-57 6147 065 1154 369 086 005 354 522 ndash 403 012 909Es-1 6008 086 1820 524 170 ndash 299 029 031 484 014 416Es-2 6510 091 1770 139 190 ndash 322 033 038 427 016 470Es-3 5152 089 2030 590 185 006 429 241 ndash 619 018 642Es-5 6260 090 1750 688 020 ndash 263 030 065 237 012 493Es-6 6891 080 1321 434 086 009 169 293 050 234 010 536Es-7 5158 087 2109 671 142 006 275 379 086 347 018 766Es-75 7238 071 1251 238 118 003 154 143 ndash 337 009 438Es-77 3442 055 1252 516 087 005 956 1337 ndash 361 009 2160Es-79 4996 064 1062 335 129 024 676 935 ndash 301 008 1590Es-86 7600 083 1208 122 144 002 205 048 061 324 014 282Es-87 6285 106 1705 516 175 004 299 034 033 493 018 396Es-91 5881 083 1707 409 151 003 338 217 ndash 569 017 645Es-95 5913 075 1802 368 104 002 410 095 009 717 016 520

Prograde and retrograde metamorphism 7

packets are almost defect-free with only rare layerterminations (Fig 6) The polytypes are 2M formicas and semi-random for chlorites

Two samples have been studied from the LaSanta Unit Sample Es-2 a chlorite-rich shale isfrom the basal part of the unit and sample Es-5 aquartz-rich shale is from the upper part In Es-2elongated quartz-rich and phyllosilicate-richdomains alternate at the scale of millimetresparallel to slaty cleavage The principal phyllosili-cate is muscovite with well-defined 2M polytypismThe (hk0) reflections tend to be spread normal to cmuch more than in sample Es-53 due to the

presence of thinner packets in subparallel orienta-tions The trioctahedral chlorite (clinochlore) isoften altered to dioctahedral smectite as determinedin part by AEM data as a result of retrogradealteration processes (eg Fig 7 in Nieto et al1994) The most common defects are layerterminations voids low-angle grain boundariesand dislocations

Sample Es-5 shows incipient slaty cleavagedefined by subparallel preferentially orientedpackets of illite sudoite trioctahedral chloriteparagonite and intermediate Na-K mica as illu-strated in Fig 7 Hematite is commonly included

FIG 4 Lattice-fringe image showing a portion of a defect-free well-crystallized chlorite packet of metamorphicorigin in phyllite Es-102 (Los Molinos Unit)

FIG 3 BSE images showing the well-developed slaty cleavage of two samples (a) Es-102 (Los Molinos Unit) aphyllite in which the grains with the brightest contrast are hematite and the darkest ones quartz Chlorite andmuscovite grains are characterized by their elongated habit and intermediate brightness (stronger in chlorite thanin muscovite) (b) Es-53 (Yechar Unit) in which stretched quartz grains are more abundant and responsible for itssemipelitic character grains with high contrast are hematite or TiO2 Muscovite crystals are indicated by arrows

8 I Abad et al

within illite packets as elongated individual grainsup to ~3006500 AEcirc in size or as aggregates(Fig 7) they show up clearly due to their darkcontrast Sudoite and chlorite were identified on thebasis of SAED and AEM data Separate grains ofeach were observed to be intergrown with texturestypical of metamorphic origins for both Grain sizesof sudoite (gt500 AEcirc ) and clinochlore are approxi-mately equal implying formation of both at gradeswhich were not substantially different Howeverwhere grains of each intersect at high angles thesudoite ubiquitously transects clinochlore packetsAs shown in Fig 8 clinochlore but not sudoitemay exhibit strain contrast These relations implybut do not prove that sudoite post-dated clino-chlore Obvious alteration of clinochlore todioctahedral smectite was ubiquitous but possiblealteration effects were rare in sudoite occurring asdefects such as voids and wavy layers Because thecomposition of sudoite is similar to that ofdioctahedral smectite it was not possible to verifythe presence of small numbers of smectite layerswith AEM data Nevertheless the observationsclearly show that alteration of chlorite to smectitewas relegated entirely or almost entirely toclinochlore

Muscovite occurs in crystals which average~500 AEcirc in thickness and are characterized by 2Mpolytypism features which are typical of meta-morphic mica of anchizonal grade The Na-K micaoccurs in packets of similar thickness The

FIG 6 Lattice-fringe image showing the relation between mica and chlorite packets The arrows indicate theposition of layer terminations The inset SAED pattern illustrates the low-angle relation between the two phases

(Yechar Unit)

FIG 5 Low-magnification image with well-definedhigh-angle grain boundaries and interlocking relations

among muscovite crystals (Yechar Unit)

Prograde and retrograde metamorphism 9

composition (see below) is quite variable corre-sponding to Na-rich muscovite commonly but somegrains have compositions within the limits of thesolvus between muscovite and paragonite and thusare metastable Na-K micas give SAED patternswhich have relatively sharp reflections over a largerange as consistent with large defect-free crystals

Sudoite crystals comprise units with 14 AEcirc

periodicities which in some areas are 28 AEcirc Lattice-fringe images show the presence of inter-

stratified individual 7 AEcirc layers which in some areasform small packets containing no more than eight7 AEcirc layers The AEM analyses (see below) of suchareas are typical of those of homogeneous sudoitecontaining only 14 AEcirc layers implying a composi-tion of the 7 AEcirc layers similar to that of sudoite andnot to an exotic septechlorite such as kaolinite orberthierine The contrast of both 14 AEcirc and 7 AEcirc

fringes appear to be identical further consistentwith a close chemical relation

FIG 7 Textural relations between phyllosilicates (sudoite illite Na-K mica) and hematite (dark contrast) (LaSanta Unit)

FIG 8 Lattice-fringe image showing the coexistence of two types of chlorite trioctahedral chlorite (chl) andsudoite (su) Sudoite transects chlorite which shows strain contrast associated with curved and kinked layersIdentifications were based in part on AEM analyses (see Table 6 Es-511 and 12 analyses) (La Santa Unit)

10 I Abad et al

Three samples corresponding to different butequivalent levels were selected from the Morron deTotana Unit for TEM study (see Fig 1 peliticrocks intercalated with carbonate rocks) Es-77 atthe bottom of the unit Es-86 which is located in anintermediate position and Es-95 from the top of thecross-section There is no incipient slaty cleavage inthese samples Sample Es-77 is a very fine-grainedrock consisting of quartz white mica chlorite andaggregates of carbonate grains Minor hematiteproduces a deep red colour Lattice-fringe images

show that dioctahedral clay minerals have 10 and20 AEcirc periodicities occur as packets 100 ndash200 AEcirc

thick and have subparallel orientations Accordingto Guthrie amp Veblen (1989) smectite and illiteinterlayers can produce different contrast and canonly be differentiated in lattice fringe-imagesobtained with well-defined electron beam (001)orientations and defocus conditions Such contrastdifferences were observed in several lattice-fringeimages The light contrast for fringes in Fig 9represents the smectite interlayer as shown in part

FIG 9 TEM image showing 10 AEcirc periodicity in R4 illite-rich I-S Contrast differences imply the presence ofillite-like (the dark fringes) and smectite-like (the light fringes) layers Packets usually have four or more illite

layers and the SAED pattern shows predominantly 50 AEcirc periodicity (Morron de Totana Unit)

FIG 10 Trioctahedral chlorite with intercalations of 10 AEcirc layers (upper arrow) which damage quickly under theelectron beam The chlorite packets show strain contrast microfolds and 7 AEcirc periodicities corresponding toberthierine (left arrow) or greater values eg 28 AEcirc (see analysis in Table 6 Es-867) (Morron de Totana Unit)

Prograde and retrograde metamorphism 11

by the continuity of such fringes with openinterlayer spaces produced by contraction due to

dehydration The sequence on average is that ofR4 mixed-layering ie a single smectite-likeinterlayer between four or more illite-like inter-layers Although some packets with more than fourlayers of illite are present repeats of four illite-likelayers predominate as shown by the 50 AEcirc

periodicity of SAED patterns (Fig 9 inset) Theillite-smectite (I-S) has typical 1Md polytypism Theobservation of smectite interstratified with illite isin accord with the changes observed in XRDdiagrams after EG treatment (Fig 2) In additionAEM analyses of areas similar to that of Fig 9have relatively high Si and minor Ca contentsfurther confirming the mixed layering

Some packets which give rise to fringes with10 AEcirc spacing produce SAED patterns typical oftwo-layer polytypism The contrast of all fringes isrelatively constant Such packets have thicknessestypical of diagenetic clays and two orders ofmagnitude smaller than typical detrital grains Thecompositions of such grains (see below) haveinterlayer-cation contents typical of illite They aretherefore inferred to be illite which formed duringlate diagenesis and coexisting with the R4 I-S

Sample Es-86 is composed mainly of detritalgrains of quartz randomly-oriented grains ofchlorite and white mica all immersed in aphyllosilicate-rich fine-grained matrix At theTEM scale chlorite is the most common phyllosi-licate in the matrix As in sample Es-5 trioctahe-dral chlorite and sudoite coexist There areinterstratified packets of 10 AEcirc layers only a fewlayers thick (lt150 AEcirc ) which are quickly damaged

FIG 11 Di-trioctahedral chlorite (sudoite) very similarin appearance to the trioctahedral chlorite shown inFig 10 7 AEcirc layers are marked by an arrow and 10 AEcirc

layers are present in the poorly resolved area in thecentre of the image (see analysis in Table 6 Es-862)

(Morron de Totana Unit)

FIG 12 Low-magnification image illustrating illite-like packets The inset SAED pattern shows only a smallnumber of diffuse poorly defined reflections The packets are 100 ndash200 AEcirc thick (Morron de Totana Unit)

12 I Abad et al

under the electron beam (Figs 10 and 11) TheAEM analyses of such material show an enhancedSi content and the presence of small concentrationsof alkali cations These data are consistent withretrograde alteration of chlorite as observed byNieto et al (1994) They differ however from theresults of Nieto et al (1994) and those describedabove for sample Es-5 in that both tri-trioctahedraland di-trioctahedral chlorite are affected In TEMimages strain features in chlorites such as contrastnormal to the layers or microfolds were observed(Fig 10) Mica is less abundant than sudoiteoccurring in packets 100 ndash400 AEcirc thick The SAEDpatterns exhibit 2M polytypism but with diffuse-ness parallel to c in 0kl rows At low magnifica-tion subparallel and slightly curved illite- or illite-smectite-like packets are present they coalescehave abundant layer terminations and show intensecontrasts (Fig 12)

Sample Es-95 is a very fine-grained rock madeup of large detrital grains of quartz in a fine-grained matrix of phyllosilicates mainly whitemicas and opaque minerals Some biotite grainswere observed as verified by AEM data Biotitewas observed only in separated grains on holey

C-coated Cu grids so it is not possible to describethe textural relations for it Authigenic biotite isinconsistent with the low grade of this samplehowever and it is therefore inferred to be of detritalorigin The AEM analyses show deviations frompure biotite however as consistent with alterationto chlorite the usual process in detrital biotite inrocks undergoing diagenesis Lattice-fringe imagesof ion-milled samples show that individual micapackets are very thin with no more than 10 layersisolated and dispersed in a matrix consisting ofmaterial which gives no SAED pattern Thesepackets are wavy discontinuous and with anasto-mosing layers (Fig 13) Despite these texturalfeatures SAED data indicate that there are 2Mand 1Md polytypes

For all samples prismatic crystals of Ti-oxidedispersed Fe-oxyhydroxides zircon and tourmalineare common accessory minerals

Chemical characterization of phyllosilicates

Dioctahedral mica Table 3 shows the composi-tion of K-rich dioctahedral micas of authigenic ormetamorphic origin corresponding to the samples

FIG 13 General aspect of sample Es-95 The image shows arrays of wavy discontinuous and anastomosingpackets of illite and smectite (Morron de Totana Unit)

Prograde and retrograde metamorphism 13

selected from each unit The Yechar and LosMolinos units are characterized by compositionsapproaching those of evolved micas with a sum ofinterlayer cations slightly less than 1 apfu typicalof phengite or muscovite rather than authigenic

illite In contrast the La Santa and Morron deTotana units show heterogeneous mica composi-tions and display deficiencies in interlayer cations(Fig 14a) This last feature is more evident in theMorron de Totana analyses where the illitic

TABLE 3 Representative chemical compositions for K-rich dioctahedral micas normalized to O10(OH)2

Si IVAl VIAl Fe Mg Ti VIS K Na S inter

Los Molinos Unit (1)Es-10213 337 063 178 010 008 002 198 075 008 085Es-10221 308 092 195 005 001 001 202 082 009 091Es-10231 319 081 191 005 003 001 200 077 012 090Es-10232 314 086 188 006 007 001 202 072 021 093

Yechar Unit (1)Es-5311 332 068 163 015 022 002 204 090 004 093Es-5312 310 090 180 003 013 004 201 089 010 099Es-5322 338 062 170 012 015 003 200 086 001 086Es-5323 319 081 176 016 012 003 206 087 002 089

Yechar Unit (2)Es-53 1 317 083 163 021 026 000 210 084 000 084Es-53 2 320 080 165 011 027 003 207 087 000 087Es-53 3 317 083 155 025 024 002 206 093 000 093Es-53 4 328 072 160 020 024 000 203 092 000 092Es-53 5 326 074 164 019 016 000 199 096 000 096Es-53 6 326 074 158 020 028 000 206 089 000 089Es-53 7 319 081 154 025 026 000 205 097 000 097Es-53 8 325 075 166 016 025 000 206 085 000 085Es-53 9 324 076 167 019 019 000 205 085 000 085

La Santa Unit (2)Es-2 1 311 089 157 019 032 002 210 088 005 094Es-2 3 336 064 148 020 043 001 212 072 003 075Es-2 6 313 087 156 024 029 001 210 075 017 092Es-2 7 313 087 155 024 029 001 209 084 011 095Es-2 8 331 069 157 015 032 000 205 087 004 092Es-2 10 328 072 160 018 027 003 208 068 008 075Es-5 4 303 097 174 015 023 001 213 073 009 082Es-5 10 301 099 189 006 015 000 210 065 021 086Es-5 7 307 093 159 033 016 000 208 075 018 093Es-5 8rsquo 301 099 160 031 025 000 216 067 016 083

Morron de Totana Unit (2) (analyses corresponding to R4 Ill-Sm may also be present see text)Es-77 13 327 073 172 013 027 000 212 067 000 067Es-77 15 316 084 195 003 013 000 211 063 000 063Es-77 16 328 072 182 010 015 000 207 068 000 068Es-77 17 338 062 147 032 019 000 199 093 000 093Es-86 9 334 066 160 017 034 000 210 072 000 072Es-86 10 357 043 151 012 046 000 209 065 000 065Es-86 11 353 047 156 012 036 000 203 077 000 077Es-86 13 324 076 152 025 035 000 211 084 000 084Es-86 14 324 076 173 013 019 000 205 084 000 084Es- 95 2 323 077 158 016 019 004 198 103 000 103Es- 95 5 336 064 169 013 025 000 207 070 000 070Es- 95 6 336 064 148 027 033 000 209 078 000 078

(1) SEMEDX analyses (2) TEMAEM analyses

14 I Abad et al

character is consistent with the presence of the R4mixed-layer illite-smectite described above Most ofthe micas are phengitic in composition (Fig 14ab)with an Fe(Fe+Mg) ratio that varies over the range02 ndash085 However the detrital-like grains aresimilar to those of end-member muscovite (seebelow detrital muscovite vs metamorphic musco-vite or Table 5) with much lower Fe and Mgcontents Data plotted in Fig 14bc confirm theassumption of a significant ferrimuscovitic substitu-tion (Guidotti et al 1994) Some of the K-micaanalysed in the Los Molinos and La Santa unitswere found to have significant Na contents (up to02 apfu) Although Ti is unusual in dioctahedralmicas it has been measured in several analyses

(lt005 apfu) The most evident feature in theplots is the scattering of data in the La Santa andMorron de Totana units ie where diageneticconditions are not exceeded In contrast theYechar Unit analyses define a narrow field in thediagrams and the Los Molinos data are even closerto the theoretical muscovite composition (Table 3)

Typical compositions of micas intermediate tomuscovite and paragonite called Na-K micas in thispaper are shown in Table 4 The compositions arequite variable even in the same sample Whencompared with analyses of K-rich dioctahedralmicas (Table 3) and paragonite analyses are seento span the entire range from muscovite toparagonite Some analyses (eg Es-5 9) fall within

FIG 14 Plots of values of chemical compositions of K-rich dioctahedral phyllosilicates solid circles and linesindicate values for theoretical muscovite and corresponding exchange vectors respectively

Prograde and retrograde metamorphism 15

the limits of the solvus between muscovite andparagonite the limbs of that solvus having theapproximate compositions ms60pg40 and ms20pg80

(Li et al 1994a)Detrital muscovite vs metamorphic muscovite

Detrital-like micas of sample Es-95 the lowest-grade one are large enough to be analysed(Table 5) by electron microprobe (EMPA) Thesedata have been compared with the AEM data ofother samples Compositions of detrital-like grainsof the lowest-grade sample and the detrital micas ofthe anchizonal-grade sample Es-53 (Fig 15) arevery similar and close to that of end-membermuscovite These data suggest that the detrital micahas a common source for the different units thatcomprise Sierra Espuna Figure 15 also comparesthe data from authigenic micas of the samesamples which are clearly more phengitic (Fe+Mg= 015 ndash075 apfu) than the detrital ones (Fe+Mg= 01 ndash02 apfu)

Chlorite Trioctahedral and di-trioctahedral(sudoite) chlorites have been analysed (Table 6)The former corresponding to the species clino-chlore is common in most of the samples unlikesudoite which is absent from most (Table 1)Sudoite was identified because the sum ofoctahedral cations is nearly equal to 5 apfu andthe Si and Al contents are higher than intrioctahedral chlorite (Fig 16a) A slight contam-ination by interlayer cations (K and Na) has beendetected therefore the formulae were recalculatedin order to eliminate those impurities (seeAnalytical Methods) Although total Fe has beenconsidered as Fe2+ according to Fransolet ampBourguignon (1978) the Fe3+ content in sudoitemay be greater than the Fe2+ content As severalanalyses have an octahedral sum slightly higherthan the ideal value of 5 apfu this may indicate atrioctahedral substitution andor the presence ofsignificant Fe3+ No Ti or Mn were detected The

Fe(Fe+Mg) ratios do not define a clear trendthroughout the sequence (Fig 16b) nevertheless itis much greater in chlorite in the Morron de Totanaanalyses falling in the characteristic range ofdiagenetic chlorites (Li et al 1994b) There is anincrease in Mg content (from 15 to 4 apfu)concomitant with increase in grade as compatiblewith some other sequences (eg Bevins et al1991)

D I S C U S S I O N

Dioctahedral phyllosilicates chemicalcompositions IC values and polytypes

On the basis of their large size homogeneity ofindividual grains and well-crystallized aspectdetrital dioctahedral mica grains were identified inseveral samples with the exception of those ofgreenschist facies This confirms the idea of the

TABLE 4 Chemical compositions for Na-K micas normalized to O10(OH)2

Si IVAl VIAl Fe Mg Ti VIS Ca K Na S inter

Es-5 14 313 087 190 004 009 002 204 000 034 048 083Es-5 15 321 079 176 009 037 000 222 000 015 038 053Es-5 4 318 082 187 004 019 000 209 000 013 061 074Es-5 12 329 071 172 008 029 000 208 000 039 036 076Es-5 13 323 077 179 005 028 000 212 000 029 041 069Es-5 9 296 104 163 019 029 000 211 000 048 056 104Es-5 8 292 108 156 043 008 001 209 003 014 080 096

FIG 15 Plot of Si vs Al contents of detrital andauthigenic micas of samples Es-95 and Es-53 Solidsymbols = detrital micas open symbols = authigenic

micas

16 I Abad et al

relative stability of muscovite in most diageneticenvironments (Crowley 1991) The compositions ofthe detrital grains of samples from different unitsare similar The lack of heterogeneity which isexpected for mica from a complex source implies acommon source area for all samples

The IC values define a trend from diagenetic toanchizonal conditions but with smaller IC values inbulk-rock fractions consistent with the contributionof detrital mica The coexistence of mature micawith lower-grade I-S or illite in the sub-greenschistfacies rocks shows that those rocks have not

TABLE 5 EMPA data for detrital-like crystals of mica normalized to O10(OH)2

Si IVAl VIAl Fe Mg Ti VIS Ca K Na K+Na+Ca

Es-95z11 313 087 193 005 004 001 203 000 065 022 087Es-95z11 314 086 190 006 007 001 204 000 068 019 087Es-95z1 2 314 086 191 005 009 001 206 000 070 006 077Es-95z2 1 314 086 176 012 010 003 201 002 095 004 101Es-95z2 2 317 083 177 012 011 003 202 000 095 004 099Es-95z4 1 305 095 188 005 007 004 204 000 081 009 089Es-95z4 2 311 089 187 006 007 004 204 000 078 009 086

TABLE 6 Representative chemical compositions for chlorite normalized to O10(OH)8

Si IVAl VIAl Fe Mg VIS FeFe+Mg

Yechar UnitEs53-1 273 127 112 121 376 609 024Es53-4 287 113 125 070 400 596 015Es53-12 303 097 130 146 307 583 032Es53-13 277 123 125 122 354 601 026Es53-15 307 093 146 094 323 564 023Es53-42 271 129 152 160 294 606 035

La Santa UnitEs2-4 319 081 147 148 273 569 035Es2-5 365 035 183 118 225 526 034Es5-1 354 046 293 023 172 488 012Es5-5 361 039 292 021 178 490 010Es5-10 309 091 262 028 221 511 011Es5-16 310 090 197 112 234 543 032Es5-17 350 050 252 076 170 499 031Es-5 10rsquo 364 036 213 097 202 512 032Es-5 11 291 109 143 136 314 593 030Es-5 12 382 018 229 087 179 495 033Es-5 13 316 084 302 021 172 494 011Es-5 16rsquo 346 054 341 013 101 455 011Es-5 17rsquo 300 100 282 033 194 509 014Es-5 18 285 115 285 026 204 515 011

Morron de Totana UnitEs-86 2 377 023 253 088 149 490 037Es-86 4 284 116 166 230 181 578 056Es-86 5 350 050 191 192 151 534 056Es-86 6 366 034 189 191 149 529 056Es-86 7 315 085 129 273 176 578 061

indicates sudoite analyses

Prograde and retrograde metamorphism 17

reached stable chemical or textural equilibrium atleast at the mm scale In addition the lack ofchemical equilibrium among diagenetic I-S andillite at the nm scale is demonstrated by their highlyvariable Fe+Mg contents (006ndash075 apfu) Theaverages of the sum of interlayer cations are closeto 1 apfu in the Los Molinos and Yechar units asconsistent with mica but values of 06 apfucorresponding to I-S or illitic compositions are alsopresent in the Morron de Totana Unit Except forsamples Es-102 and Es-53 the variable composi-tions represent a metastable state with composi-tions of micas determined by a combination ofphengitic ferrimuscovitic and illitic vectors asshown by Fig 14ab

Paragonite is generally considered an indexmineral for the anchizone (Frey 1987) but Li etal (1994a) have shown that paragonite coexistswith K-rich mica in grades as low as that of the latediagenetic zone for a prograde sequence Samplesin which muscovite paragonite and intermediateNa-K mica coexist as in sample Es-5 areconsistent with metastability both because morethan two such micas coexist and because theircompositions are incompatible with the solvus inthe muscovite-paragonite system (see below) Whilethe 2M polytype typical of metamorphic micas is

the most common in these samples the 1Md

polytype also occurs in some diagenetic samplesparticularly those from the Morron de Totana UnitThe 1Md polytype is metastable relative to the 2Mpolytype and the coexistence of both polytypesfurther demonstrates a lack of equilibrium at the nmscale Dong amp Peacor (1996) showed that thedisorder noted in the diagenetic phyllosilicates bySAED is a consequence of the general high entropyof phases formed at very low temperatures thatproduces for example stacking defects separatingpackets with no more than 10 layers These packetsprogressively thicken with increasing metamorphicgrade and the 2M polytype then becomes the mostcommon polytype in authigenic micas Thenm-scale size of the lower-grade clays and theregular increase in size with increasing gradereflect the lack of textural equilibrium in the low-grade samples

A wide range of sample characteristics thusimply that the lower-grade samples are in statesof metastable equilibrium those factors includingcoexistence of mature mica with I-S and illitevariable chemical compositions incompatiblecompositions of white micas small crystal sizesdefect-rich states and polytypism Such dataemphasize the notion of lsquoreaction progressrsquo relative

FIG 16 (a) Plot of Al vs Fe+Mg contents of chlorites illustrating the contrast in composition between tri-trioctahedral clinochlore or chamosite and di-trioctahedral sudoite (b) plot of Si vs Fe(Fe+Mg) ratio of

chlorites showing an absence of compositional trends throughout the sampled sequence

18 I Abad et al

to Ostwald-step rule processes metastable dioctahe-dral clay systems undergoing reactions toward thestate of stable chemical and textural equilibriumapproached in greenschist-facies samples (Essene ampPeacor 1995)

Prograde evolution based on TEM datacorrelation with the Basque-Cantabriansequence (Northern Spain)

With increasing grade TEM data confirm thatcrystal thickness increases crystal defect densitydecreases compositions become less variable andmicrotextural relations are simpler (eg Peacor1992) This trend is consistent with three well-characterized low-grade settings the Gulf Coastsequence (Ahn amp Peacor 1986) and the WelshBasin (Merriman et al 1990) both extensionalbasins and the Helvetic Alps (Livi et al 1997)The latter is a fold-and-thrust belt with similarseries of prograde transitions to those observed inSierra Espuna in which pelites have passed througha sequence of metastable states in the lithologicalsequence mudstone and shale slate and phyllite

The mudstones of the Morron de Totana Unit arecharacterized by the presence of illite-smectite R4mixed-layers mica packets of no more than 10 ndash40layers and the occurrence of 1Md polytypism TheLa Santa Unit samples represent a step in theprograde evolution with thicker packets than in theprevious samples (200 ndash700 AEcirc ) illitic or phengiticcompositions the absence of illite-smectite mixedlayers as well as the presence of discreteintermediate Na-K micas and paragonite Althoughthere is a tendency towards non-preferred orienta-tion of crystals it is not so evident as in the Morronde Totana samples In the Yechar Unit (anchizone)and more clearly in the Los Molinos phyllites(greenschist facies) the microtextural relations areless complex the compositions are less variablewith phengite and trioctahedral chlorite crystalthickness are greater (gt400 AEcirc in the Yechar Unitand gt1000 AEcirc in the Los Molinos Unit) with thepackets being defect-free and with a very crystallineaspect This description has features in commonwith the three well-characterized low-grade settingspreviously noted Only the mineral assemblagesvary somewhat presumably as a function of thedifferences in bulk-rock composition

A detailed comparison with the Basque-Cantabrian Basin (N Spain) shows that the lowest-grade conditions of Sierra Espuna overlap most of

those of that sequence which is characterized bypassive burial diagenesis (8000 m thick) (Nieto etal 1996) Only in the deepest sample of theBasque-Cantabrian sequence is illite-smectite mixedlayering absent Figure 12 of this study is verysimilar to Fig 7 in Nieto et al (1996) and bothimages correspond to diagenetic samples fromdifferent settings nevertheless they illustratesimilar mica packets 100 ndash200 AEcirc thick withparallel-to-subparallel boundaries the orientationof which is inferred to represent beddingTherefore there is a parallelism between thesesequences down to the scale of individual clay-mineral grains even though they developed underdifferent conditions with stress presumably playinga significant role in the Sierra Espuna units Thetexture of the least metamorphosed sample (Fig 13)is reminiscent of the typical textural relations ofsmectite described by Nieto et al (1996) (seeFig 4) Although it corresponds to illite-richinterstratified illite-smectite the clay mineralgrains conserved the grain size and anastomosingdiscontinuous array of original smectite layers

Despite the similarities between the twosequences the chemical behaviour of the micasare clearly different since the grade correspondingto the phengitic compositions of the Yechar Unit isnot reached in the Basque-Cantabrian Basin wherethe deepest and highest-grade sample has illiticcompositions equivalent to those of the La SantaUnit That is in a passive burial metamorphiccontext as in the Basque-Cantabrian Basin thefactors responsible for increasing the rate ofchemical reactions are less significant than in thesequences that were affected by tectonic stressTectonic stress introduces defects which in turnincreases the rate of dissolution of strained grainsthus increasing the rate of dissolution-crystallizationreactions typical of clay minerals Perhaps moreimportantly regional tectonic stress is known tocause mobilization of fluids (eg Oliver 1986) Thepresence and activity of such fluids is a prime causeof increased rates of dissolution ion diffusion andcrystallization Tectonic activity thus may have atwo-fold effect in increasing rate of clay-mineralreactions The phengite component may in part be ameasure of the reaction progress depending on thereaction kinetics in very low-grade rocksAccording to Merriman amp Peacor (1999) whereheat flow is low as in accretionary settingsphengite contents are typically high perhapsbecause there is insufficient thermal energy to

Prograde and retrograde metamorphism 19

cause equilibration of the phengite component ofneoformed illite by contrast with extensional basinswhere this component is typically small

Coexistence of trioctahedral chlorite andsudoite Implications for multiple metamorphicevents

In metamorphic rocks the coexistence of mineralsthat formed at different times and conditions on aPTt path ie not in equilibrium is a commonlyobserved feature where the metamorphic history iscomplex Gross differences in the grain size ofcoexisting phases has long been recognized bymetamorphic petrologists as an indicator of non-equilibrium mineral assemblages Historically muchless attention has been paid to such relations in sub-greenschist-facies rocks because prior to the intro-duction of SEM+TEM techniques their small grainsize prevented observations of textural relationsStudy of such rocks eg by XRD provides littleinformation on textural relations beyond determina-tion of degree of preferred orientation

Study by XRD of the samples showed thatsudoite and trioctahedral chlorite coexist If theyhad formed at the same time and under the sameP-T conditions their compositions would define thelimbs of a solvus The TEM images show howeverthat wherever they are in contact the sudoitetransects the packets of trioctahedral chlorite (egFig 8) Furthermore as illustrated in Fig 8 thetrioctahedral chlorite is commonly deformed anddisplays strain contrast in TEM images Thesetextural relations suggest that sudoite was intro-duced subsequent to the formation of trioctahedralchlorite Similar observations were made byGiorgetti et al (1998) who concluded that texturesin Verrucano metasediments showed that sudoitepost-dated chloritoid pyrophyllite and trioctahedralchlorite ie sudoite is in disequilibrium with thelatter minerals and formed as a retrograde phase Inrocks of the Sierra Espuna area differences in bulk-rock composition are small (Table 2) and do notcorrespond to the presence or absence of sudoite(Table 1) ie the random occurrence of sudoiteindependent of bulk-rock composition is compa-tible with a lack of equilibrium relations Theserelations collectively imply that sudoite wasintroduced at some time subsequent to theformation of mineral assemblages at peak meta-morphic conditions On the other hand the size ofsudoite packets is similar to that of trioctahedral

chlorite as consistent with formation at conditionsnot much different from those for chlorite

In samples from the Morron de Totana bothsudoite and trioctahedral chlorite were affected byretrograde alteration to smectite The TEM imagesshow sudoite (Fig 11) and trioctahedral chlorite(Fig 10) with interstratified layers of smectite withspacings of 10 AEcirc due to collapse and dehydration inthe TEM environment and even packets of smectite150 AEcirc thick within chlorite crystals These relationsimply that both sudoite and trioctahedral chloritehave been affected by retrograde alteration at thevery low temperatures consistent with smectiteformation although the presence of more subtlealteration features in sudoite of some samplessuggests that it was less susceptible to alteration

We therefore conclude that at least three episodesof phyllosilicate formation can be recognized inrocks from the Sierra Espuna-area Trioctahedralchlorite was presumably produced at the highestmetamorphicdiagenetic grade in equilibrium withparagonite and phengite Subsequently sudoite wasintroduced but at P-T conditions which could nothave been much different than those of peakmetamorphic grade Finally at very low-T condi-tions hydrothermal fluids caused partial alterationof trioctahedral chlorite and sudoite to smectite

Retrograde products formed by the reaction offluids with mineral phases generated during anearlier prograde event were described previouslyby Nieto et al (1994) and Zhao et al (1999) Thosereactions occurred on a regional scale but similarreactions have been observed on a more localizedscale The fluid activity associated with a majorfault zone is a common source for these reactions(eg Jiang et al 1990) The structure of SierraEspuna (Fig 1) is based on imbrication processesthat superimpose units that have subsequently beenaffected by faults and folds and that have under-gone significant uplift at least 800 m from the lateMiocene to the present (Sanz de Galdeano et al2001) Given this evolution it is easy to justify theretrograde processes that usually seem to be relatedwith tectonic events during which fluid flow andfluidrock ratios are enhanced (Merriman amp Peacor1999) Moreover according to Nieto et al (1994)the uplift of the Betic Cordillera would furnish theintroduction of phreatic water into the systemproducing a reactive environment adequate for thegenesis of smectite Geological conditions aretherefore consistent with post-peak metamorphicconditions as implied by the TEM observations

20 I Abad et al

The introduction of sudoite as a major phaseduring metamorphism implies the possibility ofadditional overprints on a simple prograde assem-blage the evidence for which might be subtle TheNa-rich micas at all grades have compositionswhich either fall within the solvus or are too Na-rich to correspond to the limb of the solvus In thehighest-grade rocks such micas coexist withmuscovite Those relations are incompatible withchemical equilibrium despite the fact that relationsfor muscovite and other minerals imply an approachto chemical equilibrium Furthermore there is aclose association of Na-rich micas with sudoite (egFig 7) Those relations imply the possibility thatNa-micas were introduced concomitant withsudoite P Mata (pers comm) has shown forexample that overprinting of an epizonal assem-blage by hydrothermal solutions resulted in theintroduction of both an Al-rich variety of chlorite(cookeite) and paragonite in pelites of the CamerosBasin Spain The textural relations such as cross-cutting textures which are so striking for sudoitewere not observed for Na-rich micas Neverthelessthe possibility of such overprinting must beconsidered

In spite of all these prograde-retrogradeprocesses in the Los Molinos Unit a smallnumber of homogeneous phases coexist in chemicalequilibrium and the fine-grained metamorphicphyllosilicates are clearly parallel to cleavage witha typical metamorphic aspect Finally all thetextural and compositional changes that haveoperated in the Espuna area during the metamorphicevolution proceed toward a simpler system repre-senting both textural and chemical equilibriumwhich is characteristic of greenschist-facies condi-tions

Roles of detrital and authigenic phyllosilicatesduring slaty cleavage development

Van der Pluijm et al (1998) focused on therespective roles of the phyllosilicates of detritalorigin and those formed during diagenesis andmetamorphism and observed that detrital phyllosi-licates remain preferentially oriented parallel tobedding in pelites with slaty cleavage whereas fine-grained metamorphic phyllosilicates are orientedparallel to cleavage

Although it is usually difficult to determine thepreferred orientations of phyllosilicates in TEMimages as the grade increases better-aligned

packets occur in the cleavage orientation Thetexture of the least metamorphosed sample inwhich even incipient slaty cleavage is absent isreminiscent of the typical textural relation ofsmectites described in the literature The illite-smectite mixed-layers conserve the textural featuresof possible previous smectite such as anastomosingfringes curved and discontinuous with variations ininterplanar spacing d001 These features indicatethat the minimum energy conditions to initiatemechanical transformations have not been reachedin the lower-grade samples as postulated by Ho etal (1996) since slaty cleavage develops as a resultof interactive mechanical deformation and ther-mally driven dissolution and crystallizationprocesses (Knipe 1981) In contrast in the LosMolinos Unit large crystals of chlorite and whitemica (gt1000 AEcirc ) occur which are of metamorphicorigin but with no detrital grains (Fig 3a)Phyllosilicates are preferentially oriented parallelto slaty cleavage in response to increasedtemperature and tectonic strain (Merriman ampPeacor 1999) The oriented microfabrics and slatycleavage well developed in the rocks of this unitmust be a product of a higher thermal and strainenergy environment reached in the deepest part ofthe sequence As pointed out by Van der Pluijm etal (1998) a continuous range of orientations isevidence for mechanical rotation of grains whereasthe lack of such transitional orientations impliesthat cleavage-parallel phyllosilicates originatedthrough dissolution of bedding-parallel and crystal-lization in the slaty cleavage orientation Asobserved in this study for the Intermediate Unitsgrains with orientations which are transitional frombedding-parallel to slaty cleavage orientation areonly rarely observed Cleavage formation is there-fore inferred to have occurred entirely or nearlyentirely through dissolution and crystallization

ACKNOWLEDGMENTS

We thank MM Abad Ortega from the Centro deInstrumentacio n Cient otilde fica of the Universidad deGranada for her help with HRTEM and I Nieto forher essential help with sample preparation Financialsupport was supplied by Research Project nordm BT 2000-0582 FPI research grant to IA both of the SpanishMinistry of Science and Technology Research GroupRNM-0179 of the Junta de Andaluc otildeacutea and to DRP byNSF grant EAR-9814391

Prograde and retrograde metamorphism 21

REFERENCES

Ahn JH amp Peacor DR (1986) Transmission andanalytical electron microscopy of the smectite-to-illite transition Clays and Clay Minerals 34165 ndash179

Azanon JM amp Crespo-Blanc A (2000) Exhumationduring a continental collision inferred from thetectonometamorphic evolution of the AlpujarrideComplex in the central Betics (Alboran Domain SESpain) Tectonics 19 549 ndash565

Bevins RE Robinson D amp Rowbotham G (1991)Compositional variations in mafic phyllosilicatesfrom regional low-grade metabasites and applicationof the chlorite geother mometer Journal ofMetamorphic Geology 9 711 ndash721

Cliff G amp Lorimer GW (1975) The quantitativeanalysis of thin specimens Journal of Microscopy 103 203 ndash207

Crowley SF (1991) Diagenetic modification of detritalmuscovite an example from the Great LimestoneCyclothem (Carboniferous) of Co Durham UKClay Minerals 26 91 ndash103

Dong H amp Peacor DR (1996) TEM observations ofcoherent stacking relations in smectite and illite ofshales evidence for MacEwan crystallites anddominance of 2M1 polytypes Clays and ClayMinerals 44 257 ndash275

Essene E amp Peacor DR (1995) Clay mineral thermo-metry a critical perspective Clays and ClayMinerals 43 540 ndash553

Fransolet AM amp Bourguignon P (1978) Dioctahedralchlorite in quartz veins from Ardennes BelgiumThe Canadian Mineralogist 16 365 ndash373

Fransolet AM amp Schreyer W (1984) Sudoite ditrioctahedral chlorite a stable low-temperature phasein the system MgO-Al2O3-SiO2-H2O Contributionsto Mineralogy and Petrology 86 409 ndash417

Frey M (1987) Very low-grade metamorphism of clasticsedimentary rocks Pp 9 ndash58 in Low TemperatureMetamorphism (M Frey editor) Blackie GlasgowUK

Giorgetti G Goffe B Memmi I amp Nieto F (1998)Metamorphic evolution of Verrucano metasedimentsin northern Apennines new petrological constraintsEuropean Journal of Mineralogy 10 1295 ndash1308

Guidotti CV amp Sassi FP (1986) Classification andcorrelation of metamorphic facies series by means ofmuscovite b0 data from low grade metapelites NeuesJahrbuch fur Mineralogie Abhandlungen 153363 ndash380

Guidotti CV Yates MG Dyar MD amp Taylor ME(1994) Petrogenetic implications of the Fe3+ contentof muscovi te in pel it ic sch is ts AmericanMineralogist 79 793 ndash795

Guthrie GD amp Veblen DR (1989) High resolutionelectron microscopy of mixed-layer illitesmectite

Computer simulations Clays and Clay Minerals 371 ndash11

Ho N Peacor DR amp Van der Pluijm BA (1996)Contrasting roles of detrital and authigenic phyllo-silicates during slaty cleavage development Journalof Structural Geology 18 615 ndash623

Jiang WT Peacor DR Merriman RJ amp Roberts B(1990) Transmission and analytical electron micro-scopic study of mixed layer illitesmectite formed asan apparent replacement product of diagenetic illiteClays and Clay Minerals 38 449 ndash468

Jiang WT Peacor DR amp Buseck PR (1994) Chloritegeothermometry Contamination and apparent octa-hedral vacancies Clays and Clay Minerals 42593 ndash605

Kisch HJ (1991) Development of slaty cleavage anddegree of very-low-grade metamorphism a reviewJournal of Metamorphic Geology 9 735 ndash750

Knipe JR (1981) The interaction of deformation andmetamorphi sm in slates Tectonophysics 78249 ndash272

Kretz R (1983) Symbols for rock-forming mineralsAmerican Mineralogist 68 277 ndash279

Li G Peacor DR Merriman RJ amp Roberts B (1994a)The diagenetic to low grade metamorphism evolu-tion of matrix white mica in the system muscovite-paragonite in a mudrock from Central Wales UKClays and Clay Minerals 42 369 ndash381

Li G Peacor DR Merriman RJ Roberts B amp Vander Pluijm BA (1994b) TEM and AEM constraintson the origin and significance of chlorite-mica stacksin slates an example from Central Wales UKJournal of Structural Geology 16 1139 ndash1157

Livi KJT Veblen DR Ferry JM amp Frey M (1997)Evolution of 21 layered silicates in low-grademetamorphosed Liass ic sha les of CentralSwitzerland Journal of Metamorphic Geology 15323 ndash344

Lonergan L (1991) Structural evolution of the SierraEspuna Betic Cordillera SE Spain PhD thesisOxford University UK

Lonergan L Platt JP amp Gallagher L (1994) TheInternal-External Zone Boundary in the eastern BeticCordillera SE Spain Journal of Structural Geology16 175 ndash188

Makel GH (1981) Differences in tectonic evolution ofsuperimposed Malaguide and Alpujarride tectonicunits in the Espuna area (Betic Cordilleras Spain)Geology in Mijnbouw 60 203 ndash208

Makel GH (1985) The geology of the MalaguideComplex and its bearing on the geodynamicevolution of the Betic-Rif orogen (southern Spainand northern Morocco) GUA papers of Geology Ser1 22 263 pp

Makel GH amp Rondeel HE (1979) Differences instratigraphy and metamorphism between superposedMalaguide and Alpujarride units in the Espuna area

22 I Abad et al

HRTEM was required to characterize the texturalrelationships between the minerals especially thephyllosilicates

X-ray diffraction

Unaltered samples were obtained from wellbelow the outcrop surface and away from jointsto minimize alteration by weathering Samples werewashed and after coarse crushing homogeneousrock chips were used for preparation of samples for

XRD Whole-rock samples and clay fractions(lt2 mm) were studied using a Philips PW 1710powder diffractometer with Cu-Ka radiationgraphite monochromator and automatic divergenceslit at the Departamento de Mineralog otildeacutea yPetrologotildeacutea of the Universidad de Granada Thelt2 mm fractions were separated by repeatedextraction of supernatant liquid subsequent tosettling Oriented aggregates were prepared bysedimentation on glass slides Ethylene-glycol(EG) treatment was carried out on some samples

FIG 1 Geological map of Sierra Espuna (based on Sanz de Galdeano et al 2001) (1) Tertiary and Quaternaryterrains (2) (a) pelites and sandstones (b) carbonate rocks (3) pelites sandstones and conglomerates (4) pelitesand greywackes (5) (7) (9) and (11) carbonate rocks (6) and (8) pelites and sandstones (10) and (12) phyllitesand quartzites (13) sample locations (14) thrust-nappe (15) anticline The tectonostratigraphic column givingthe highest and lowest units in a thrust stack is shown on the right All these strata are Triassic except for (4)

which consists of Upper Palaeozoic sedimentary rocks

Prograde and retrograde metamorphism 3

to corroborate the identification of smectite andorillite-smectite mixed-layers Preparation of samplesand experimental conditions for illite ` crystallinityrsquorsquo(IC) measurements were carried out according toIGCP 294 IC Working Group recommendations(Kisch 1991) Our IC measurements (y) weretransformed into CIS values (x) according to theequation y = 0674x + 0052 (r = 0999) obtained inour laboratory using the international standards ofWarr amp Rice (1994) The IC values were measuredfor the lt2 mm fractions lt2 mm EG-treated fractionsand for the bulk-rock samples The b cellparameters of micas and chlorites were obtainedfrom the (060) peaks measured on slices of rock cutnormal to the sample foliation For all spacingmeasurements quartz from the sample itself wasused as internal standard

Electron microscopy

Following the XRD and optical studies sevenrepresentative samples of the different tectonostrati-graphic levels were selected for electron micro-scopy study on the basis of the IC values and themineral assemblages Carbon-coated slices wereexamined by SEM using back-scattered electron(BSE) imaging and energy-dispersive X-ray (EDX)analysis in order to obtain textural and chemicaldata These observations were carried out using aZeiss DSM 950 SEM equipped with an X-ray LinkAnalytical QX-20 energy-dispersive X-ray system(EDX) at the Centro de Instrumentacion Cientotildeacutefica(CIC) Granada An accelerating voltage of 20 kVwith a beam current of 1 ndash2 nA and counting timeof 100 s were used to analyse the phyllosilicates bySEM using both natural and synthetic standardsalbite (Na) periclase (Mg) wollastonite (Si andCa) and orthoclase (K) and synthetic Al2O3 (Al)Fe2O3 (Fe) and MnTiO3 (Ti and Mn) Ion-milledand carbon-coated specimens were studied by TEMwith two microscopes (1) a Philips CM12 scanningtransmission electron microscope (STEM) operatedat an accelerating voltage of 120 kV and beamcurrent of ~10 mA equipped with a KevexQuantum EDX system (University of Michigan)and (2) a Philips CM20 (STEM) equipped with anEDAX solid-state EDX detector operated at200 kV with a LaB6 filament (CIC) Lattice-fringe images of phyllosilicates were obtained using(00l) reflections Records of TEM data included144 lattice-fringe images and 94 selected areaelectron diffraction (SAED) patterns

The EDX spectra of ion-milled samples wereobtained using a raster of 10006200 AEcirc in scanningmode to minimize alkali diffusion and volatilizationwith the long axis oriented parallel to the length ofphyllosilicate packets Analytical electron microscopy(AEM) quantitative chemical analyses were obtainedfrom spectra using ion-milled standards of muscovitealbite clinochlore fayalite and sphene to derivek-values following the procedure of Jiang et al(1990) The AEM analyses were also obtained frompowders dispersed over holey C-coated formvar Cugrids after first verifying the monomineraliccharacter of each grain with a SAED pattern Suchanalyses permit larger areas to be used in scanningtransmission mode (STEM) (area of 1 mm61 mm)with greater accuracy because elements especiallyalkalis cannot diffuse away from the area analysedShorter counting times (30 s) were used for K tofurther minimize loss (Nieto et al 1996) Albitebiotite spessartine muscovite olivine titanite MnSand CaS were used as standards to derive k-factorsfor the transformation of intensity ratios to concen-tration ratios following the procedures of Cliff ampLorimer (1975)

The structural formulae of micas were calculatedon the basis of 22 negative charges O10(OH)2Although these analytical techniques cannot distin-guish between Fe3+ and Fe2+ Guidotti et al (1994)showed that the opaque-mineral assemblage (hema-tite in these samples) can be used to estimate theFe3+Fe2+ ratio in muscovite Even in low redox-potential parageneses ~50 of the Fe in muscoviteis Fe3+ and values close to 85 can be reached inless reducing environments Therefore in thecalculations of formulae it has been assumed that75 of the Fe in the micas is Fe3+

Due to the very fine-grained nature of thesamples the results obtained by AEM weresometimes rejected due to overlap of the beam onmaterial other than the mineral analysed Whenmica contamination was slight the chlorite formulawas recalculated after subtracting the proportions ofelements determined by normalizing to the K andNa contents resulting in a formula based on 28negative charges (Nieto 1997)

Electron microprobe

The EMPA analyses of detrital micas werecarried out using a Cameca SX50 electron micro-probe operated at 20 kV with a beam current of30 nA and a beam diameter of lt5 mm The

4 I Abad et al

standards were albite orthoclase periclase wollas-tonite and synthetic oxides (Al2O3 Fe2O3MnTiO3) Data were reduced using the proceduresof Pouchou amp Pichoir (1985)

R E S U L T S

Mineralogy and crystal-chemical parameters

The XRD patterns of bulk samples show thatquartz and white mica primarily phengite are theprincipal phases in all samples trioctahedralchlorite is very common smectite mixed-layeredillite-smectite and intermediate Na-K mica para-gonite feldspars carbonates and hematite arepresent only in some samples (Table 1) A di-trioctahedral chlorite sudoite according to Fransoletand Schreyerrsquos (1984) criteria has been detected insamples from the La Santa and Morron de Totanaunits The XRD patterns of sudoite usually show adistinctly more intense (003) reflection character-istic of Al-rich chlorites In general the mineralassemblages are similar and typical of very low-grade metamorphism

The basal spacing of mica is ~998 AEcirc (s = 001)varying with no clear trend throughout the

sequence An overall average value of 9019 AEcirc (s= 0010) obtained for the b cell parameter(Table 1) is consistent with medium-pressureregional metamorphism according to Guidotti ampSassi (1986)

Figure 2 shows the IC values for the lt2 mm vslt2 mm EG-treated fractions corresponding to all unitsexcept for the Los Molinos Unit for whichparagonite is a major phase whose peaks interferewith measurement of IC values The IC values of thelt2 mm fraction for the Morron de Totana Unitcorrespond to the zone of diagenesis with significantvariation in values in different samples as typical ofthat grade For the La Santa Unit the data indicatelate diagenetic conditions The IC values vary theleast for samples from the Yechar Unit andcorrespond to the low anchizone (according to thenomenclature of Merriman amp Peacor 1999) Inaddition Fig 2 shows that for the Morron de Totanasamples the IC values of the lt2 mm fraction arelarger than those of the EG-treated samples implyingthe presence of smectite layers interstratified withillite The IC values for the bulk samples aregenerally smaller than those of the lt2 mm separatesas consistent with a significant contribution ofdetrital mica in the untreated samples (Table 1)

FIG 2 Plot of IC values for the lt2 mm untreated vs lt2 mm EG-treated fractions

Prograde and retrograde metamorphism 5

Whole-rock analyses

Chemical analyses of the major elements ofselected samples were carried out using X-ray

fluorescence (XRF) in a Philips PW 104010spectrometer at the CIC The data (Table 2) werecompared with the average of Post-Archaean

TABLE 1 Crystal-chemical parameters and bulk mineralogy determined by XRD

White mica Chlorite Mineral compositiond001 Illite crystallinity (CIS) d001 Qtz K-white mica

Samples lt2 mm bulk b lt2 mm lt2 mm EG bulk lt2 mm bulk (all the samples)

Los Molinos UnitEs-101 9972 9966 9012 ndash ndash ndash 142 142 Chl Hem Pg Cc Dol SmEs-102 9978 9980 9002 ndash ndash ndash 142 142 Chl Hem Pg Cc SmEs-103 9970 9975 ndash ndash ndash ndash 142 142 Chl Hem Kfs Pg Cc SmEs-104 9969 ndash ndash ndash ndash ndash 142 ndash Chl Hem Pg Cc Sm

Yechar UnitEs-51 9977 9972 9019 038 035 020 1417 1417 Chl Hem Dol SmEs-52 9974 9970 9016 041 046 022 1417 1416 Chl Kfs SmEs-53 9978 9971 9016 037 040 022 1417 1417 Chl Hem Kfs SmEs-54 9979 9978 9019 041 ndash 025 1416 1416 Chl Hem SmEs-55 9975 9987 ndash 038 041 020 1416 1418 Chl Hem Cc SmEs-56 9957 9967 9010 043 044 022 ndash ndash Hem Cc DolEs-57 9958 ndash 9019 050 049 ndash 1418 ndash Chl Cc Dol

La Santa UnitEs-1 9976 ndash 9019 050 044 071 1417 ndash Chl Hem Kfs SmEs-2 9974 9977 9003 062 049 029 1417 1417 Chl Kfs SmEs-3 9963 ndash 9018 058 050 031 1420 ndash Chl Hem Cc SmEs-4 9987 9987 9007 075 077 049 1416 1417 Chl Hem DolEs-5 9993 9983 ndash 093 ndash 084 1417 1417 Su Chl Hem Ill-Pg PgEs-6 9988 9982 ndash 089 092 078 1417 1416 Chl Hem Cc Dol Ill-PgEs-7 9990 9981 ndash 098 095 069 1418 1419 Chl Hem Cc DolEs-8 9985 9975 9007 066 ndash 046 1417 1416 Chl Hem Cc Dol Ill-Pg

Morron de Totana UnitEs-71 9977 ndash 8994 ndash ndash ndash ndash ndash Chl Kfs HemEs-72 ndash ndash ndash ndash ndash ndash ndash ndash Chl Hem GtEs-73 9970 ndash ndash ndash ndash ndash ndash ndash Kfs Cc DolEs-74 9991 ndash ndash 069 071 ndash ndash ndash Kfs Hem GtEs-75 9972 9991 8989 068 064 ndash ndash ndash Chl Kfs Hem DolEs-76 10005 ndash 064 052 ndash ndash ndash ndash Chl Hem Gt DolEs-77 9979 ndash 9011 077 059 083 ndash ndash Chl Hem Dol I-SEs-78 9972 ndash 084 068 071 ndash ndash ndash Chl Cc Sm GtEs-79 9984 ndash 9028 098 090 061 ndash ndash Chl Kfs Hem DolEs-70 9984 ndash 096 089 035 1416 1418 ndash Dol Chl SmEs-81 9980 ndash ndash 099 061 ndash 142 ndash Chl Hm Cc DolEs-82 9972 ndash ndash 090 083 ndash ndash ndash Chl Hm Cc Dol SmEs-85 9972 ndash 9008 058 050 050 142 ndash Chl Su Hem Sm KfsEs-86 9951 9977 9013 092 084 047 142 ndash Chl Su Hem Sm KfsEs-87 9974 9008 083 ndash 050 142 ndash ndash Chl Su Hem Sm KfsEs-91 9974 ndash ndash 104 108 093 ndash ndash Chl Hem KfsEs-92 9972 ndash 9028 083 074 086 ndash ndash Chl Dol KfsEs-93 9980 9983 ndash 109 096 061 ndash ndash Chl Hem DolEs-94 9964 9985 9024 098 093 080 ndash ndash Chl Hem Dol KfsEs-95 9978 ndash 9027 104 093 ndash ndash ndash Dol

Mineral abbreviations according to Kretz (1983) Ill-Pg Intermediate Na-K mica I-S Illite-smectiteinterstratified layers Su Sudoite In bold samples studied by TEM

6 I Abad et al

Australian Shales (PAAS) (Taylor amp McLennan1985) The values for samples of this study aresimilar to them The loss on ignition (LOI) is verylarge in some samples because of the presence ofcarbonates

Electron microscopy observations (SEM andTEM)

Textural aspects Due to the fine-grainedcharacter of these rocks characterization of texturesis possible only by TEM except for the highestmetamorphic-grade samples (Es-102 and Es-53) forwhich grain sizes are large enough to be resolvedby BSE imaging The HRTEM characterizationconfirms the general mineralogical data determinedby XRD for the different units of the studied areaand have allowed us to clarify some specificrelations eg the mixed-layered illite-smectite inthe Morron de Totana Unit

Los Molinos Unit samples are phyllosilicate-richphyllites with well-developed slaty cleavage(Fig 3a) composed of preferentially oriented micaand chlorite alternating with deformed elongatedquartz grains carbonate aggregates and hematitegrains Paragonite and muscovite coexist as separatecrystals Lattice-fringe images show that both micasand chlorite occur as large defect-free crystalsmore than several thousand angstroms thick

(Fig 4) Lattice-fringe images (not shown) withboth 00l and hkl fringes show that crystals arecomposed virtually entirely of coherently relatedlayers but with some stacking disorder in chloriteThese features are in accord with the typicalgreenschist facies of the Alpujarride Complex(Azanon amp Crespo-Blanc 2000) On the otherhand dioctahedral smectite commonly occursreplacing layers of trioctahedral chlorite describedby Nieto et al (1994) as the result of retrogradealteration by hydrothermal fluids

The sample studied from the basal portion of theYechar Unit (Es-53) is a quartz-rich slate with well-developed cleavage (Fig 3b) High-resolutionimages show that the main phyllosilicate is K-richdioctahedral mica with lesser amounts of chloriteand dioctahedral smectite the smectite occurring asaltered layers of primary metamorphic chloriteAlthough phyllosilicates generally form parallel orlow-angle intergrowths preferentially orientedparallel to cleavage Fig 5 shows a low-magnifica-tion image with well-defined high-angle grainboundaries of interlocking grains Such texturesare typical of a metamorphic origin The packetsare 500 AEcirc thick (Fig 6) much smaller than typicaldetrital grains (gt5000 AEcirc ) They also have composi-tions which differentiate them from detrital micasas described below (see chemical characterizationof phyllosilicates) In general the phyllosilicate

TABLE 2 Whole-rock analyses of major elements (wt oxide)

Samples SiO2 TiO2 Al2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O P2O5 LOI

Es-102 5340 077 1790 557 100 013 158 722 152 254 013 847Es-51 5270 083 1752 441 190 014 514 359 028 503 016 845Es-52 7620 093 1210 070 120 ndash 185 037 143 251 017 239Es-53 5990 089 1890 583 140 ndash 292 021 043 489 014 408Es-56 6640 061 1200 350 020 ndash 190 384 011 427 012 616Es-57 6147 065 1154 369 086 005 354 522 ndash 403 012 909Es-1 6008 086 1820 524 170 ndash 299 029 031 484 014 416Es-2 6510 091 1770 139 190 ndash 322 033 038 427 016 470Es-3 5152 089 2030 590 185 006 429 241 ndash 619 018 642Es-5 6260 090 1750 688 020 ndash 263 030 065 237 012 493Es-6 6891 080 1321 434 086 009 169 293 050 234 010 536Es-7 5158 087 2109 671 142 006 275 379 086 347 018 766Es-75 7238 071 1251 238 118 003 154 143 ndash 337 009 438Es-77 3442 055 1252 516 087 005 956 1337 ndash 361 009 2160Es-79 4996 064 1062 335 129 024 676 935 ndash 301 008 1590Es-86 7600 083 1208 122 144 002 205 048 061 324 014 282Es-87 6285 106 1705 516 175 004 299 034 033 493 018 396Es-91 5881 083 1707 409 151 003 338 217 ndash 569 017 645Es-95 5913 075 1802 368 104 002 410 095 009 717 016 520

Prograde and retrograde metamorphism 7

packets are almost defect-free with only rare layerterminations (Fig 6) The polytypes are 2M formicas and semi-random for chlorites

Two samples have been studied from the LaSanta Unit Sample Es-2 a chlorite-rich shale isfrom the basal part of the unit and sample Es-5 aquartz-rich shale is from the upper part In Es-2elongated quartz-rich and phyllosilicate-richdomains alternate at the scale of millimetresparallel to slaty cleavage The principal phyllosili-cate is muscovite with well-defined 2M polytypismThe (hk0) reflections tend to be spread normal to cmuch more than in sample Es-53 due to the

presence of thinner packets in subparallel orienta-tions The trioctahedral chlorite (clinochlore) isoften altered to dioctahedral smectite as determinedin part by AEM data as a result of retrogradealteration processes (eg Fig 7 in Nieto et al1994) The most common defects are layerterminations voids low-angle grain boundariesand dislocations

Sample Es-5 shows incipient slaty cleavagedefined by subparallel preferentially orientedpackets of illite sudoite trioctahedral chloriteparagonite and intermediate Na-K mica as illu-strated in Fig 7 Hematite is commonly included

FIG 4 Lattice-fringe image showing a portion of a defect-free well-crystallized chlorite packet of metamorphicorigin in phyllite Es-102 (Los Molinos Unit)

FIG 3 BSE images showing the well-developed slaty cleavage of two samples (a) Es-102 (Los Molinos Unit) aphyllite in which the grains with the brightest contrast are hematite and the darkest ones quartz Chlorite andmuscovite grains are characterized by their elongated habit and intermediate brightness (stronger in chlorite thanin muscovite) (b) Es-53 (Yechar Unit) in which stretched quartz grains are more abundant and responsible for itssemipelitic character grains with high contrast are hematite or TiO2 Muscovite crystals are indicated by arrows

8 I Abad et al

within illite packets as elongated individual grainsup to ~3006500 AEcirc in size or as aggregates(Fig 7) they show up clearly due to their darkcontrast Sudoite and chlorite were identified on thebasis of SAED and AEM data Separate grains ofeach were observed to be intergrown with texturestypical of metamorphic origins for both Grain sizesof sudoite (gt500 AEcirc ) and clinochlore are approxi-mately equal implying formation of both at gradeswhich were not substantially different Howeverwhere grains of each intersect at high angles thesudoite ubiquitously transects clinochlore packetsAs shown in Fig 8 clinochlore but not sudoitemay exhibit strain contrast These relations implybut do not prove that sudoite post-dated clino-chlore Obvious alteration of clinochlore todioctahedral smectite was ubiquitous but possiblealteration effects were rare in sudoite occurring asdefects such as voids and wavy layers Because thecomposition of sudoite is similar to that ofdioctahedral smectite it was not possible to verifythe presence of small numbers of smectite layerswith AEM data Nevertheless the observationsclearly show that alteration of chlorite to smectitewas relegated entirely or almost entirely toclinochlore

Muscovite occurs in crystals which average~500 AEcirc in thickness and are characterized by 2Mpolytypism features which are typical of meta-morphic mica of anchizonal grade The Na-K micaoccurs in packets of similar thickness The

FIG 6 Lattice-fringe image showing the relation between mica and chlorite packets The arrows indicate theposition of layer terminations The inset SAED pattern illustrates the low-angle relation between the two phases

(Yechar Unit)

FIG 5 Low-magnification image with well-definedhigh-angle grain boundaries and interlocking relations

among muscovite crystals (Yechar Unit)

Prograde and retrograde metamorphism 9

composition (see below) is quite variable corre-sponding to Na-rich muscovite commonly but somegrains have compositions within the limits of thesolvus between muscovite and paragonite and thusare metastable Na-K micas give SAED patternswhich have relatively sharp reflections over a largerange as consistent with large defect-free crystals

Sudoite crystals comprise units with 14 AEcirc

periodicities which in some areas are 28 AEcirc Lattice-fringe images show the presence of inter-

stratified individual 7 AEcirc layers which in some areasform small packets containing no more than eight7 AEcirc layers The AEM analyses (see below) of suchareas are typical of those of homogeneous sudoitecontaining only 14 AEcirc layers implying a composi-tion of the 7 AEcirc layers similar to that of sudoite andnot to an exotic septechlorite such as kaolinite orberthierine The contrast of both 14 AEcirc and 7 AEcirc

fringes appear to be identical further consistentwith a close chemical relation

FIG 7 Textural relations between phyllosilicates (sudoite illite Na-K mica) and hematite (dark contrast) (LaSanta Unit)

FIG 8 Lattice-fringe image showing the coexistence of two types of chlorite trioctahedral chlorite (chl) andsudoite (su) Sudoite transects chlorite which shows strain contrast associated with curved and kinked layersIdentifications were based in part on AEM analyses (see Table 6 Es-511 and 12 analyses) (La Santa Unit)

10 I Abad et al

Three samples corresponding to different butequivalent levels were selected from the Morron deTotana Unit for TEM study (see Fig 1 peliticrocks intercalated with carbonate rocks) Es-77 atthe bottom of the unit Es-86 which is located in anintermediate position and Es-95 from the top of thecross-section There is no incipient slaty cleavage inthese samples Sample Es-77 is a very fine-grainedrock consisting of quartz white mica chlorite andaggregates of carbonate grains Minor hematiteproduces a deep red colour Lattice-fringe images

show that dioctahedral clay minerals have 10 and20 AEcirc periodicities occur as packets 100 ndash200 AEcirc

thick and have subparallel orientations Accordingto Guthrie amp Veblen (1989) smectite and illiteinterlayers can produce different contrast and canonly be differentiated in lattice fringe-imagesobtained with well-defined electron beam (001)orientations and defocus conditions Such contrastdifferences were observed in several lattice-fringeimages The light contrast for fringes in Fig 9represents the smectite interlayer as shown in part

FIG 9 TEM image showing 10 AEcirc periodicity in R4 illite-rich I-S Contrast differences imply the presence ofillite-like (the dark fringes) and smectite-like (the light fringes) layers Packets usually have four or more illite

layers and the SAED pattern shows predominantly 50 AEcirc periodicity (Morron de Totana Unit)

FIG 10 Trioctahedral chlorite with intercalations of 10 AEcirc layers (upper arrow) which damage quickly under theelectron beam The chlorite packets show strain contrast microfolds and 7 AEcirc periodicities corresponding toberthierine (left arrow) or greater values eg 28 AEcirc (see analysis in Table 6 Es-867) (Morron de Totana Unit)

Prograde and retrograde metamorphism 11

by the continuity of such fringes with openinterlayer spaces produced by contraction due to

dehydration The sequence on average is that ofR4 mixed-layering ie a single smectite-likeinterlayer between four or more illite-like inter-layers Although some packets with more than fourlayers of illite are present repeats of four illite-likelayers predominate as shown by the 50 AEcirc

periodicity of SAED patterns (Fig 9 inset) Theillite-smectite (I-S) has typical 1Md polytypism Theobservation of smectite interstratified with illite isin accord with the changes observed in XRDdiagrams after EG treatment (Fig 2) In additionAEM analyses of areas similar to that of Fig 9have relatively high Si and minor Ca contentsfurther confirming the mixed layering

Some packets which give rise to fringes with10 AEcirc spacing produce SAED patterns typical oftwo-layer polytypism The contrast of all fringes isrelatively constant Such packets have thicknessestypical of diagenetic clays and two orders ofmagnitude smaller than typical detrital grains Thecompositions of such grains (see below) haveinterlayer-cation contents typical of illite They aretherefore inferred to be illite which formed duringlate diagenesis and coexisting with the R4 I-S

Sample Es-86 is composed mainly of detritalgrains of quartz randomly-oriented grains ofchlorite and white mica all immersed in aphyllosilicate-rich fine-grained matrix At theTEM scale chlorite is the most common phyllosi-licate in the matrix As in sample Es-5 trioctahe-dral chlorite and sudoite coexist There areinterstratified packets of 10 AEcirc layers only a fewlayers thick (lt150 AEcirc ) which are quickly damaged

FIG 11 Di-trioctahedral chlorite (sudoite) very similarin appearance to the trioctahedral chlorite shown inFig 10 7 AEcirc layers are marked by an arrow and 10 AEcirc

layers are present in the poorly resolved area in thecentre of the image (see analysis in Table 6 Es-862)

(Morron de Totana Unit)

FIG 12 Low-magnification image illustrating illite-like packets The inset SAED pattern shows only a smallnumber of diffuse poorly defined reflections The packets are 100 ndash200 AEcirc thick (Morron de Totana Unit)

12 I Abad et al

under the electron beam (Figs 10 and 11) TheAEM analyses of such material show an enhancedSi content and the presence of small concentrationsof alkali cations These data are consistent withretrograde alteration of chlorite as observed byNieto et al (1994) They differ however from theresults of Nieto et al (1994) and those describedabove for sample Es-5 in that both tri-trioctahedraland di-trioctahedral chlorite are affected In TEMimages strain features in chlorites such as contrastnormal to the layers or microfolds were observed(Fig 10) Mica is less abundant than sudoiteoccurring in packets 100 ndash400 AEcirc thick The SAEDpatterns exhibit 2M polytypism but with diffuse-ness parallel to c in 0kl rows At low magnifica-tion subparallel and slightly curved illite- or illite-smectite-like packets are present they coalescehave abundant layer terminations and show intensecontrasts (Fig 12)

Sample Es-95 is a very fine-grained rock madeup of large detrital grains of quartz in a fine-grained matrix of phyllosilicates mainly whitemicas and opaque minerals Some biotite grainswere observed as verified by AEM data Biotitewas observed only in separated grains on holey

C-coated Cu grids so it is not possible to describethe textural relations for it Authigenic biotite isinconsistent with the low grade of this samplehowever and it is therefore inferred to be of detritalorigin The AEM analyses show deviations frompure biotite however as consistent with alterationto chlorite the usual process in detrital biotite inrocks undergoing diagenesis Lattice-fringe imagesof ion-milled samples show that individual micapackets are very thin with no more than 10 layersisolated and dispersed in a matrix consisting ofmaterial which gives no SAED pattern Thesepackets are wavy discontinuous and with anasto-mosing layers (Fig 13) Despite these texturalfeatures SAED data indicate that there are 2Mand 1Md polytypes

For all samples prismatic crystals of Ti-oxidedispersed Fe-oxyhydroxides zircon and tourmalineare common accessory minerals

Chemical characterization of phyllosilicates

Dioctahedral mica Table 3 shows the composi-tion of K-rich dioctahedral micas of authigenic ormetamorphic origin corresponding to the samples

FIG 13 General aspect of sample Es-95 The image shows arrays of wavy discontinuous and anastomosingpackets of illite and smectite (Morron de Totana Unit)

Prograde and retrograde metamorphism 13

selected from each unit The Yechar and LosMolinos units are characterized by compositionsapproaching those of evolved micas with a sum ofinterlayer cations slightly less than 1 apfu typicalof phengite or muscovite rather than authigenic

illite In contrast the La Santa and Morron deTotana units show heterogeneous mica composi-tions and display deficiencies in interlayer cations(Fig 14a) This last feature is more evident in theMorron de Totana analyses where the illitic

TABLE 3 Representative chemical compositions for K-rich dioctahedral micas normalized to O10(OH)2

Si IVAl VIAl Fe Mg Ti VIS K Na S inter

Los Molinos Unit (1)Es-10213 337 063 178 010 008 002 198 075 008 085Es-10221 308 092 195 005 001 001 202 082 009 091Es-10231 319 081 191 005 003 001 200 077 012 090Es-10232 314 086 188 006 007 001 202 072 021 093

Yechar Unit (1)Es-5311 332 068 163 015 022 002 204 090 004 093Es-5312 310 090 180 003 013 004 201 089 010 099Es-5322 338 062 170 012 015 003 200 086 001 086Es-5323 319 081 176 016 012 003 206 087 002 089

Yechar Unit (2)Es-53 1 317 083 163 021 026 000 210 084 000 084Es-53 2 320 080 165 011 027 003 207 087 000 087Es-53 3 317 083 155 025 024 002 206 093 000 093Es-53 4 328 072 160 020 024 000 203 092 000 092Es-53 5 326 074 164 019 016 000 199 096 000 096Es-53 6 326 074 158 020 028 000 206 089 000 089Es-53 7 319 081 154 025 026 000 205 097 000 097Es-53 8 325 075 166 016 025 000 206 085 000 085Es-53 9 324 076 167 019 019 000 205 085 000 085

La Santa Unit (2)Es-2 1 311 089 157 019 032 002 210 088 005 094Es-2 3 336 064 148 020 043 001 212 072 003 075Es-2 6 313 087 156 024 029 001 210 075 017 092Es-2 7 313 087 155 024 029 001 209 084 011 095Es-2 8 331 069 157 015 032 000 205 087 004 092Es-2 10 328 072 160 018 027 003 208 068 008 075Es-5 4 303 097 174 015 023 001 213 073 009 082Es-5 10 301 099 189 006 015 000 210 065 021 086Es-5 7 307 093 159 033 016 000 208 075 018 093Es-5 8rsquo 301 099 160 031 025 000 216 067 016 083

Morron de Totana Unit (2) (analyses corresponding to R4 Ill-Sm may also be present see text)Es-77 13 327 073 172 013 027 000 212 067 000 067Es-77 15 316 084 195 003 013 000 211 063 000 063Es-77 16 328 072 182 010 015 000 207 068 000 068Es-77 17 338 062 147 032 019 000 199 093 000 093Es-86 9 334 066 160 017 034 000 210 072 000 072Es-86 10 357 043 151 012 046 000 209 065 000 065Es-86 11 353 047 156 012 036 000 203 077 000 077Es-86 13 324 076 152 025 035 000 211 084 000 084Es-86 14 324 076 173 013 019 000 205 084 000 084Es- 95 2 323 077 158 016 019 004 198 103 000 103Es- 95 5 336 064 169 013 025 000 207 070 000 070Es- 95 6 336 064 148 027 033 000 209 078 000 078

(1) SEMEDX analyses (2) TEMAEM analyses

14 I Abad et al

character is consistent with the presence of the R4mixed-layer illite-smectite described above Most ofthe micas are phengitic in composition (Fig 14ab)with an Fe(Fe+Mg) ratio that varies over the range02 ndash085 However the detrital-like grains aresimilar to those of end-member muscovite (seebelow detrital muscovite vs metamorphic musco-vite or Table 5) with much lower Fe and Mgcontents Data plotted in Fig 14bc confirm theassumption of a significant ferrimuscovitic substitu-tion (Guidotti et al 1994) Some of the K-micaanalysed in the Los Molinos and La Santa unitswere found to have significant Na contents (up to02 apfu) Although Ti is unusual in dioctahedralmicas it has been measured in several analyses

(lt005 apfu) The most evident feature in theplots is the scattering of data in the La Santa andMorron de Totana units ie where diageneticconditions are not exceeded In contrast theYechar Unit analyses define a narrow field in thediagrams and the Los Molinos data are even closerto the theoretical muscovite composition (Table 3)

Typical compositions of micas intermediate tomuscovite and paragonite called Na-K micas in thispaper are shown in Table 4 The compositions arequite variable even in the same sample Whencompared with analyses of K-rich dioctahedralmicas (Table 3) and paragonite analyses are seento span the entire range from muscovite toparagonite Some analyses (eg Es-5 9) fall within

FIG 14 Plots of values of chemical compositions of K-rich dioctahedral phyllosilicates solid circles and linesindicate values for theoretical muscovite and corresponding exchange vectors respectively

Prograde and retrograde metamorphism 15

the limits of the solvus between muscovite andparagonite the limbs of that solvus having theapproximate compositions ms60pg40 and ms20pg80

(Li et al 1994a)Detrital muscovite vs metamorphic muscovite

Detrital-like micas of sample Es-95 the lowest-grade one are large enough to be analysed(Table 5) by electron microprobe (EMPA) Thesedata have been compared with the AEM data ofother samples Compositions of detrital-like grainsof the lowest-grade sample and the detrital micas ofthe anchizonal-grade sample Es-53 (Fig 15) arevery similar and close to that of end-membermuscovite These data suggest that the detrital micahas a common source for the different units thatcomprise Sierra Espuna Figure 15 also comparesthe data from authigenic micas of the samesamples which are clearly more phengitic (Fe+Mg= 015 ndash075 apfu) than the detrital ones (Fe+Mg= 01 ndash02 apfu)

Chlorite Trioctahedral and di-trioctahedral(sudoite) chlorites have been analysed (Table 6)The former corresponding to the species clino-chlore is common in most of the samples unlikesudoite which is absent from most (Table 1)Sudoite was identified because the sum ofoctahedral cations is nearly equal to 5 apfu andthe Si and Al contents are higher than intrioctahedral chlorite (Fig 16a) A slight contam-ination by interlayer cations (K and Na) has beendetected therefore the formulae were recalculatedin order to eliminate those impurities (seeAnalytical Methods) Although total Fe has beenconsidered as Fe2+ according to Fransolet ampBourguignon (1978) the Fe3+ content in sudoitemay be greater than the Fe2+ content As severalanalyses have an octahedral sum slightly higherthan the ideal value of 5 apfu this may indicate atrioctahedral substitution andor the presence ofsignificant Fe3+ No Ti or Mn were detected The

Fe(Fe+Mg) ratios do not define a clear trendthroughout the sequence (Fig 16b) nevertheless itis much greater in chlorite in the Morron de Totanaanalyses falling in the characteristic range ofdiagenetic chlorites (Li et al 1994b) There is anincrease in Mg content (from 15 to 4 apfu)concomitant with increase in grade as compatiblewith some other sequences (eg Bevins et al1991)

D I S C U S S I O N

Dioctahedral phyllosilicates chemicalcompositions IC values and polytypes

On the basis of their large size homogeneity ofindividual grains and well-crystallized aspectdetrital dioctahedral mica grains were identified inseveral samples with the exception of those ofgreenschist facies This confirms the idea of the

TABLE 4 Chemical compositions for Na-K micas normalized to O10(OH)2

Si IVAl VIAl Fe Mg Ti VIS Ca K Na S inter

Es-5 14 313 087 190 004 009 002 204 000 034 048 083Es-5 15 321 079 176 009 037 000 222 000 015 038 053Es-5 4 318 082 187 004 019 000 209 000 013 061 074Es-5 12 329 071 172 008 029 000 208 000 039 036 076Es-5 13 323 077 179 005 028 000 212 000 029 041 069Es-5 9 296 104 163 019 029 000 211 000 048 056 104Es-5 8 292 108 156 043 008 001 209 003 014 080 096

FIG 15 Plot of Si vs Al contents of detrital andauthigenic micas of samples Es-95 and Es-53 Solidsymbols = detrital micas open symbols = authigenic

micas

16 I Abad et al

relative stability of muscovite in most diageneticenvironments (Crowley 1991) The compositions ofthe detrital grains of samples from different unitsare similar The lack of heterogeneity which isexpected for mica from a complex source implies acommon source area for all samples

The IC values define a trend from diagenetic toanchizonal conditions but with smaller IC values inbulk-rock fractions consistent with the contributionof detrital mica The coexistence of mature micawith lower-grade I-S or illite in the sub-greenschistfacies rocks shows that those rocks have not

TABLE 5 EMPA data for detrital-like crystals of mica normalized to O10(OH)2

Si IVAl VIAl Fe Mg Ti VIS Ca K Na K+Na+Ca

Es-95z11 313 087 193 005 004 001 203 000 065 022 087Es-95z11 314 086 190 006 007 001 204 000 068 019 087Es-95z1 2 314 086 191 005 009 001 206 000 070 006 077Es-95z2 1 314 086 176 012 010 003 201 002 095 004 101Es-95z2 2 317 083 177 012 011 003 202 000 095 004 099Es-95z4 1 305 095 188 005 007 004 204 000 081 009 089Es-95z4 2 311 089 187 006 007 004 204 000 078 009 086

TABLE 6 Representative chemical compositions for chlorite normalized to O10(OH)8

Si IVAl VIAl Fe Mg VIS FeFe+Mg

Yechar UnitEs53-1 273 127 112 121 376 609 024Es53-4 287 113 125 070 400 596 015Es53-12 303 097 130 146 307 583 032Es53-13 277 123 125 122 354 601 026Es53-15 307 093 146 094 323 564 023Es53-42 271 129 152 160 294 606 035

La Santa UnitEs2-4 319 081 147 148 273 569 035Es2-5 365 035 183 118 225 526 034Es5-1 354 046 293 023 172 488 012Es5-5 361 039 292 021 178 490 010Es5-10 309 091 262 028 221 511 011Es5-16 310 090 197 112 234 543 032Es5-17 350 050 252 076 170 499 031Es-5 10rsquo 364 036 213 097 202 512 032Es-5 11 291 109 143 136 314 593 030Es-5 12 382 018 229 087 179 495 033Es-5 13 316 084 302 021 172 494 011Es-5 16rsquo 346 054 341 013 101 455 011Es-5 17rsquo 300 100 282 033 194 509 014Es-5 18 285 115 285 026 204 515 011

Morron de Totana UnitEs-86 2 377 023 253 088 149 490 037Es-86 4 284 116 166 230 181 578 056Es-86 5 350 050 191 192 151 534 056Es-86 6 366 034 189 191 149 529 056Es-86 7 315 085 129 273 176 578 061

indicates sudoite analyses

Prograde and retrograde metamorphism 17

reached stable chemical or textural equilibrium atleast at the mm scale In addition the lack ofchemical equilibrium among diagenetic I-S andillite at the nm scale is demonstrated by their highlyvariable Fe+Mg contents (006ndash075 apfu) Theaverages of the sum of interlayer cations are closeto 1 apfu in the Los Molinos and Yechar units asconsistent with mica but values of 06 apfucorresponding to I-S or illitic compositions are alsopresent in the Morron de Totana Unit Except forsamples Es-102 and Es-53 the variable composi-tions represent a metastable state with composi-tions of micas determined by a combination ofphengitic ferrimuscovitic and illitic vectors asshown by Fig 14ab

Paragonite is generally considered an indexmineral for the anchizone (Frey 1987) but Li etal (1994a) have shown that paragonite coexistswith K-rich mica in grades as low as that of the latediagenetic zone for a prograde sequence Samplesin which muscovite paragonite and intermediateNa-K mica coexist as in sample Es-5 areconsistent with metastability both because morethan two such micas coexist and because theircompositions are incompatible with the solvus inthe muscovite-paragonite system (see below) Whilethe 2M polytype typical of metamorphic micas is

the most common in these samples the 1Md

polytype also occurs in some diagenetic samplesparticularly those from the Morron de Totana UnitThe 1Md polytype is metastable relative to the 2Mpolytype and the coexistence of both polytypesfurther demonstrates a lack of equilibrium at the nmscale Dong amp Peacor (1996) showed that thedisorder noted in the diagenetic phyllosilicates bySAED is a consequence of the general high entropyof phases formed at very low temperatures thatproduces for example stacking defects separatingpackets with no more than 10 layers These packetsprogressively thicken with increasing metamorphicgrade and the 2M polytype then becomes the mostcommon polytype in authigenic micas Thenm-scale size of the lower-grade clays and theregular increase in size with increasing gradereflect the lack of textural equilibrium in the low-grade samples

A wide range of sample characteristics thusimply that the lower-grade samples are in statesof metastable equilibrium those factors includingcoexistence of mature mica with I-S and illitevariable chemical compositions incompatiblecompositions of white micas small crystal sizesdefect-rich states and polytypism Such dataemphasize the notion of lsquoreaction progressrsquo relative

FIG 16 (a) Plot of Al vs Fe+Mg contents of chlorites illustrating the contrast in composition between tri-trioctahedral clinochlore or chamosite and di-trioctahedral sudoite (b) plot of Si vs Fe(Fe+Mg) ratio of

chlorites showing an absence of compositional trends throughout the sampled sequence

18 I Abad et al

to Ostwald-step rule processes metastable dioctahe-dral clay systems undergoing reactions toward thestate of stable chemical and textural equilibriumapproached in greenschist-facies samples (Essene ampPeacor 1995)

Prograde evolution based on TEM datacorrelation with the Basque-Cantabriansequence (Northern Spain)

With increasing grade TEM data confirm thatcrystal thickness increases crystal defect densitydecreases compositions become less variable andmicrotextural relations are simpler (eg Peacor1992) This trend is consistent with three well-characterized low-grade settings the Gulf Coastsequence (Ahn amp Peacor 1986) and the WelshBasin (Merriman et al 1990) both extensionalbasins and the Helvetic Alps (Livi et al 1997)The latter is a fold-and-thrust belt with similarseries of prograde transitions to those observed inSierra Espuna in which pelites have passed througha sequence of metastable states in the lithologicalsequence mudstone and shale slate and phyllite

The mudstones of the Morron de Totana Unit arecharacterized by the presence of illite-smectite R4mixed-layers mica packets of no more than 10 ndash40layers and the occurrence of 1Md polytypism TheLa Santa Unit samples represent a step in theprograde evolution with thicker packets than in theprevious samples (200 ndash700 AEcirc ) illitic or phengiticcompositions the absence of illite-smectite mixedlayers as well as the presence of discreteintermediate Na-K micas and paragonite Althoughthere is a tendency towards non-preferred orienta-tion of crystals it is not so evident as in the Morronde Totana samples In the Yechar Unit (anchizone)and more clearly in the Los Molinos phyllites(greenschist facies) the microtextural relations areless complex the compositions are less variablewith phengite and trioctahedral chlorite crystalthickness are greater (gt400 AEcirc in the Yechar Unitand gt1000 AEcirc in the Los Molinos Unit) with thepackets being defect-free and with a very crystallineaspect This description has features in commonwith the three well-characterized low-grade settingspreviously noted Only the mineral assemblagesvary somewhat presumably as a function of thedifferences in bulk-rock composition

A detailed comparison with the Basque-Cantabrian Basin (N Spain) shows that the lowest-grade conditions of Sierra Espuna overlap most of

those of that sequence which is characterized bypassive burial diagenesis (8000 m thick) (Nieto etal 1996) Only in the deepest sample of theBasque-Cantabrian sequence is illite-smectite mixedlayering absent Figure 12 of this study is verysimilar to Fig 7 in Nieto et al (1996) and bothimages correspond to diagenetic samples fromdifferent settings nevertheless they illustratesimilar mica packets 100 ndash200 AEcirc thick withparallel-to-subparallel boundaries the orientationof which is inferred to represent beddingTherefore there is a parallelism between thesesequences down to the scale of individual clay-mineral grains even though they developed underdifferent conditions with stress presumably playinga significant role in the Sierra Espuna units Thetexture of the least metamorphosed sample (Fig 13)is reminiscent of the typical textural relations ofsmectite described by Nieto et al (1996) (seeFig 4) Although it corresponds to illite-richinterstratified illite-smectite the clay mineralgrains conserved the grain size and anastomosingdiscontinuous array of original smectite layers

Despite the similarities between the twosequences the chemical behaviour of the micasare clearly different since the grade correspondingto the phengitic compositions of the Yechar Unit isnot reached in the Basque-Cantabrian Basin wherethe deepest and highest-grade sample has illiticcompositions equivalent to those of the La SantaUnit That is in a passive burial metamorphiccontext as in the Basque-Cantabrian Basin thefactors responsible for increasing the rate ofchemical reactions are less significant than in thesequences that were affected by tectonic stressTectonic stress introduces defects which in turnincreases the rate of dissolution of strained grainsthus increasing the rate of dissolution-crystallizationreactions typical of clay minerals Perhaps moreimportantly regional tectonic stress is known tocause mobilization of fluids (eg Oliver 1986) Thepresence and activity of such fluids is a prime causeof increased rates of dissolution ion diffusion andcrystallization Tectonic activity thus may have atwo-fold effect in increasing rate of clay-mineralreactions The phengite component may in part be ameasure of the reaction progress depending on thereaction kinetics in very low-grade rocksAccording to Merriman amp Peacor (1999) whereheat flow is low as in accretionary settingsphengite contents are typically high perhapsbecause there is insufficient thermal energy to

Prograde and retrograde metamorphism 19

cause equilibration of the phengite component ofneoformed illite by contrast with extensional basinswhere this component is typically small

Coexistence of trioctahedral chlorite andsudoite Implications for multiple metamorphicevents

In metamorphic rocks the coexistence of mineralsthat formed at different times and conditions on aPTt path ie not in equilibrium is a commonlyobserved feature where the metamorphic history iscomplex Gross differences in the grain size ofcoexisting phases has long been recognized bymetamorphic petrologists as an indicator of non-equilibrium mineral assemblages Historically muchless attention has been paid to such relations in sub-greenschist-facies rocks because prior to the intro-duction of SEM+TEM techniques their small grainsize prevented observations of textural relationsStudy of such rocks eg by XRD provides littleinformation on textural relations beyond determina-tion of degree of preferred orientation

Study by XRD of the samples showed thatsudoite and trioctahedral chlorite coexist If theyhad formed at the same time and under the sameP-T conditions their compositions would define thelimbs of a solvus The TEM images show howeverthat wherever they are in contact the sudoitetransects the packets of trioctahedral chlorite (egFig 8) Furthermore as illustrated in Fig 8 thetrioctahedral chlorite is commonly deformed anddisplays strain contrast in TEM images Thesetextural relations suggest that sudoite was intro-duced subsequent to the formation of trioctahedralchlorite Similar observations were made byGiorgetti et al (1998) who concluded that texturesin Verrucano metasediments showed that sudoitepost-dated chloritoid pyrophyllite and trioctahedralchlorite ie sudoite is in disequilibrium with thelatter minerals and formed as a retrograde phase Inrocks of the Sierra Espuna area differences in bulk-rock composition are small (Table 2) and do notcorrespond to the presence or absence of sudoite(Table 1) ie the random occurrence of sudoiteindependent of bulk-rock composition is compa-tible with a lack of equilibrium relations Theserelations collectively imply that sudoite wasintroduced at some time subsequent to theformation of mineral assemblages at peak meta-morphic conditions On the other hand the size ofsudoite packets is similar to that of trioctahedral

chlorite as consistent with formation at conditionsnot much different from those for chlorite

In samples from the Morron de Totana bothsudoite and trioctahedral chlorite were affected byretrograde alteration to smectite The TEM imagesshow sudoite (Fig 11) and trioctahedral chlorite(Fig 10) with interstratified layers of smectite withspacings of 10 AEcirc due to collapse and dehydration inthe TEM environment and even packets of smectite150 AEcirc thick within chlorite crystals These relationsimply that both sudoite and trioctahedral chloritehave been affected by retrograde alteration at thevery low temperatures consistent with smectiteformation although the presence of more subtlealteration features in sudoite of some samplessuggests that it was less susceptible to alteration

We therefore conclude that at least three episodesof phyllosilicate formation can be recognized inrocks from the Sierra Espuna-area Trioctahedralchlorite was presumably produced at the highestmetamorphicdiagenetic grade in equilibrium withparagonite and phengite Subsequently sudoite wasintroduced but at P-T conditions which could nothave been much different than those of peakmetamorphic grade Finally at very low-T condi-tions hydrothermal fluids caused partial alterationof trioctahedral chlorite and sudoite to smectite

Retrograde products formed by the reaction offluids with mineral phases generated during anearlier prograde event were described previouslyby Nieto et al (1994) and Zhao et al (1999) Thosereactions occurred on a regional scale but similarreactions have been observed on a more localizedscale The fluid activity associated with a majorfault zone is a common source for these reactions(eg Jiang et al 1990) The structure of SierraEspuna (Fig 1) is based on imbrication processesthat superimpose units that have subsequently beenaffected by faults and folds and that have under-gone significant uplift at least 800 m from the lateMiocene to the present (Sanz de Galdeano et al2001) Given this evolution it is easy to justify theretrograde processes that usually seem to be relatedwith tectonic events during which fluid flow andfluidrock ratios are enhanced (Merriman amp Peacor1999) Moreover according to Nieto et al (1994)the uplift of the Betic Cordillera would furnish theintroduction of phreatic water into the systemproducing a reactive environment adequate for thegenesis of smectite Geological conditions aretherefore consistent with post-peak metamorphicconditions as implied by the TEM observations

20 I Abad et al

The introduction of sudoite as a major phaseduring metamorphism implies the possibility ofadditional overprints on a simple prograde assem-blage the evidence for which might be subtle TheNa-rich micas at all grades have compositionswhich either fall within the solvus or are too Na-rich to correspond to the limb of the solvus In thehighest-grade rocks such micas coexist withmuscovite Those relations are incompatible withchemical equilibrium despite the fact that relationsfor muscovite and other minerals imply an approachto chemical equilibrium Furthermore there is aclose association of Na-rich micas with sudoite (egFig 7) Those relations imply the possibility thatNa-micas were introduced concomitant withsudoite P Mata (pers comm) has shown forexample that overprinting of an epizonal assem-blage by hydrothermal solutions resulted in theintroduction of both an Al-rich variety of chlorite(cookeite) and paragonite in pelites of the CamerosBasin Spain The textural relations such as cross-cutting textures which are so striking for sudoitewere not observed for Na-rich micas Neverthelessthe possibility of such overprinting must beconsidered

In spite of all these prograde-retrogradeprocesses in the Los Molinos Unit a smallnumber of homogeneous phases coexist in chemicalequilibrium and the fine-grained metamorphicphyllosilicates are clearly parallel to cleavage witha typical metamorphic aspect Finally all thetextural and compositional changes that haveoperated in the Espuna area during the metamorphicevolution proceed toward a simpler system repre-senting both textural and chemical equilibriumwhich is characteristic of greenschist-facies condi-tions

Roles of detrital and authigenic phyllosilicatesduring slaty cleavage development

Van der Pluijm et al (1998) focused on therespective roles of the phyllosilicates of detritalorigin and those formed during diagenesis andmetamorphism and observed that detrital phyllosi-licates remain preferentially oriented parallel tobedding in pelites with slaty cleavage whereas fine-grained metamorphic phyllosilicates are orientedparallel to cleavage

Although it is usually difficult to determine thepreferred orientations of phyllosilicates in TEMimages as the grade increases better-aligned

packets occur in the cleavage orientation Thetexture of the least metamorphosed sample inwhich even incipient slaty cleavage is absent isreminiscent of the typical textural relation ofsmectites described in the literature The illite-smectite mixed-layers conserve the textural featuresof possible previous smectite such as anastomosingfringes curved and discontinuous with variations ininterplanar spacing d001 These features indicatethat the minimum energy conditions to initiatemechanical transformations have not been reachedin the lower-grade samples as postulated by Ho etal (1996) since slaty cleavage develops as a resultof interactive mechanical deformation and ther-mally driven dissolution and crystallizationprocesses (Knipe 1981) In contrast in the LosMolinos Unit large crystals of chlorite and whitemica (gt1000 AEcirc ) occur which are of metamorphicorigin but with no detrital grains (Fig 3a)Phyllosilicates are preferentially oriented parallelto slaty cleavage in response to increasedtemperature and tectonic strain (Merriman ampPeacor 1999) The oriented microfabrics and slatycleavage well developed in the rocks of this unitmust be a product of a higher thermal and strainenergy environment reached in the deepest part ofthe sequence As pointed out by Van der Pluijm etal (1998) a continuous range of orientations isevidence for mechanical rotation of grains whereasthe lack of such transitional orientations impliesthat cleavage-parallel phyllosilicates originatedthrough dissolution of bedding-parallel and crystal-lization in the slaty cleavage orientation Asobserved in this study for the Intermediate Unitsgrains with orientations which are transitional frombedding-parallel to slaty cleavage orientation areonly rarely observed Cleavage formation is there-fore inferred to have occurred entirely or nearlyentirely through dissolution and crystallization

ACKNOWLEDGMENTS

We thank MM Abad Ortega from the Centro deInstrumentacio n Cient otilde fica of the Universidad deGranada for her help with HRTEM and I Nieto forher essential help with sample preparation Financialsupport was supplied by Research Project nordm BT 2000-0582 FPI research grant to IA both of the SpanishMinistry of Science and Technology Research GroupRNM-0179 of the Junta de Andaluc otildeacutea and to DRP byNSF grant EAR-9814391

Prograde and retrograde metamorphism 21

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Ahn JH amp Peacor DR (1986) Transmission andanalytical electron microscopy of the smectite-to-illite transition Clays and Clay Minerals 34165 ndash179

Azanon JM amp Crespo-Blanc A (2000) Exhumationduring a continental collision inferred from thetectonometamorphic evolution of the AlpujarrideComplex in the central Betics (Alboran Domain SESpain) Tectonics 19 549 ndash565

Bevins RE Robinson D amp Rowbotham G (1991)Compositional variations in mafic phyllosilicatesfrom regional low-grade metabasites and applicationof the chlorite geother mometer Journal ofMetamorphic Geology 9 711 ndash721

Cliff G amp Lorimer GW (1975) The quantitativeanalysis of thin specimens Journal of Microscopy 103 203 ndash207

Crowley SF (1991) Diagenetic modification of detritalmuscovite an example from the Great LimestoneCyclothem (Carboniferous) of Co Durham UKClay Minerals 26 91 ndash103

Dong H amp Peacor DR (1996) TEM observations ofcoherent stacking relations in smectite and illite ofshales evidence for MacEwan crystallites anddominance of 2M1 polytypes Clays and ClayMinerals 44 257 ndash275

Essene E amp Peacor DR (1995) Clay mineral thermo-metry a critical perspective Clays and ClayMinerals 43 540 ndash553

Fransolet AM amp Bourguignon P (1978) Dioctahedralchlorite in quartz veins from Ardennes BelgiumThe Canadian Mineralogist 16 365 ndash373

Fransolet AM amp Schreyer W (1984) Sudoite ditrioctahedral chlorite a stable low-temperature phasein the system MgO-Al2O3-SiO2-H2O Contributionsto Mineralogy and Petrology 86 409 ndash417

Frey M (1987) Very low-grade metamorphism of clasticsedimentary rocks Pp 9 ndash58 in Low TemperatureMetamorphism (M Frey editor) Blackie GlasgowUK

Giorgetti G Goffe B Memmi I amp Nieto F (1998)Metamorphic evolution of Verrucano metasedimentsin northern Apennines new petrological constraintsEuropean Journal of Mineralogy 10 1295 ndash1308

Guidotti CV amp Sassi FP (1986) Classification andcorrelation of metamorphic facies series by means ofmuscovite b0 data from low grade metapelites NeuesJahrbuch fur Mineralogie Abhandlungen 153363 ndash380

Guidotti CV Yates MG Dyar MD amp Taylor ME(1994) Petrogenetic implications of the Fe3+ contentof muscovi te in pel it ic sch is ts AmericanMineralogist 79 793 ndash795

Guthrie GD amp Veblen DR (1989) High resolutionelectron microscopy of mixed-layer illitesmectite

Computer simulations Clays and Clay Minerals 371 ndash11

Ho N Peacor DR amp Van der Pluijm BA (1996)Contrasting roles of detrital and authigenic phyllo-silicates during slaty cleavage development Journalof Structural Geology 18 615 ndash623

Jiang WT Peacor DR Merriman RJ amp Roberts B(1990) Transmission and analytical electron micro-scopic study of mixed layer illitesmectite formed asan apparent replacement product of diagenetic illiteClays and Clay Minerals 38 449 ndash468

Jiang WT Peacor DR amp Buseck PR (1994) Chloritegeothermometry Contamination and apparent octa-hedral vacancies Clays and Clay Minerals 42593 ndash605

Kisch HJ (1991) Development of slaty cleavage anddegree of very-low-grade metamorphism a reviewJournal of Metamorphic Geology 9 735 ndash750

Knipe JR (1981) The interaction of deformation andmetamorphi sm in slates Tectonophysics 78249 ndash272

Kretz R (1983) Symbols for rock-forming mineralsAmerican Mineralogist 68 277 ndash279

Li G Peacor DR Merriman RJ amp Roberts B (1994a)The diagenetic to low grade metamorphism evolu-tion of matrix white mica in the system muscovite-paragonite in a mudrock from Central Wales UKClays and Clay Minerals 42 369 ndash381

Li G Peacor DR Merriman RJ Roberts B amp Vander Pluijm BA (1994b) TEM and AEM constraintson the origin and significance of chlorite-mica stacksin slates an example from Central Wales UKJournal of Structural Geology 16 1139 ndash1157

Livi KJT Veblen DR Ferry JM amp Frey M (1997)Evolution of 21 layered silicates in low-grademetamorphosed Liass ic sha les of CentralSwitzerland Journal of Metamorphic Geology 15323 ndash344

Lonergan L (1991) Structural evolution of the SierraEspuna Betic Cordillera SE Spain PhD thesisOxford University UK

Lonergan L Platt JP amp Gallagher L (1994) TheInternal-External Zone Boundary in the eastern BeticCordillera SE Spain Journal of Structural Geology16 175 ndash188

Makel GH (1981) Differences in tectonic evolution ofsuperimposed Malaguide and Alpujarride tectonicunits in the Espuna area (Betic Cordilleras Spain)Geology in Mijnbouw 60 203 ndash208

Makel GH (1985) The geology of the MalaguideComplex and its bearing on the geodynamicevolution of the Betic-Rif orogen (southern Spainand northern Morocco) GUA papers of Geology Ser1 22 263 pp

Makel GH amp Rondeel HE (1979) Differences instratigraphy and metamorphism between superposedMalaguide and Alpujarride units in the Espuna area

22 I Abad et al

to corroborate the identification of smectite andorillite-smectite mixed-layers Preparation of samplesand experimental conditions for illite ` crystallinityrsquorsquo(IC) measurements were carried out according toIGCP 294 IC Working Group recommendations(Kisch 1991) Our IC measurements (y) weretransformed into CIS values (x) according to theequation y = 0674x + 0052 (r = 0999) obtained inour laboratory using the international standards ofWarr amp Rice (1994) The IC values were measuredfor the lt2 mm fractions lt2 mm EG-treated fractionsand for the bulk-rock samples The b cellparameters of micas and chlorites were obtainedfrom the (060) peaks measured on slices of rock cutnormal to the sample foliation For all spacingmeasurements quartz from the sample itself wasused as internal standard

Electron microscopy

Following the XRD and optical studies sevenrepresentative samples of the different tectonostrati-graphic levels were selected for electron micro-scopy study on the basis of the IC values and themineral assemblages Carbon-coated slices wereexamined by SEM using back-scattered electron(BSE) imaging and energy-dispersive X-ray (EDX)analysis in order to obtain textural and chemicaldata These observations were carried out using aZeiss DSM 950 SEM equipped with an X-ray LinkAnalytical QX-20 energy-dispersive X-ray system(EDX) at the Centro de Instrumentacion Cientotildeacutefica(CIC) Granada An accelerating voltage of 20 kVwith a beam current of 1 ndash2 nA and counting timeof 100 s were used to analyse the phyllosilicates bySEM using both natural and synthetic standardsalbite (Na) periclase (Mg) wollastonite (Si andCa) and orthoclase (K) and synthetic Al2O3 (Al)Fe2O3 (Fe) and MnTiO3 (Ti and Mn) Ion-milledand carbon-coated specimens were studied by TEMwith two microscopes (1) a Philips CM12 scanningtransmission electron microscope (STEM) operatedat an accelerating voltage of 120 kV and beamcurrent of ~10 mA equipped with a KevexQuantum EDX system (University of Michigan)and (2) a Philips CM20 (STEM) equipped with anEDAX solid-state EDX detector operated at200 kV with a LaB6 filament (CIC) Lattice-fringe images of phyllosilicates were obtained using(00l) reflections Records of TEM data included144 lattice-fringe images and 94 selected areaelectron diffraction (SAED) patterns

The EDX spectra of ion-milled samples wereobtained using a raster of 10006200 AEcirc in scanningmode to minimize alkali diffusion and volatilizationwith the long axis oriented parallel to the length ofphyllosilicate packets Analytical electron microscopy(AEM) quantitative chemical analyses were obtainedfrom spectra using ion-milled standards of muscovitealbite clinochlore fayalite and sphene to derivek-values following the procedure of Jiang et al(1990) The AEM analyses were also obtained frompowders dispersed over holey C-coated formvar Cugrids after first verifying the monomineraliccharacter of each grain with a SAED pattern Suchanalyses permit larger areas to be used in scanningtransmission mode (STEM) (area of 1 mm61 mm)with greater accuracy because elements especiallyalkalis cannot diffuse away from the area analysedShorter counting times (30 s) were used for K tofurther minimize loss (Nieto et al 1996) Albitebiotite spessartine muscovite olivine titanite MnSand CaS were used as standards to derive k-factorsfor the transformation of intensity ratios to concen-tration ratios following the procedures of Cliff ampLorimer (1975)

The structural formulae of micas were calculatedon the basis of 22 negative charges O10(OH)2Although these analytical techniques cannot distin-guish between Fe3+ and Fe2+ Guidotti et al (1994)showed that the opaque-mineral assemblage (hema-tite in these samples) can be used to estimate theFe3+Fe2+ ratio in muscovite Even in low redox-potential parageneses ~50 of the Fe in muscoviteis Fe3+ and values close to 85 can be reached inless reducing environments Therefore in thecalculations of formulae it has been assumed that75 of the Fe in the micas is Fe3+

Due to the very fine-grained nature of thesamples the results obtained by AEM weresometimes rejected due to overlap of the beam onmaterial other than the mineral analysed Whenmica contamination was slight the chlorite formulawas recalculated after subtracting the proportions ofelements determined by normalizing to the K andNa contents resulting in a formula based on 28negative charges (Nieto 1997)

Electron microprobe

The EMPA analyses of detrital micas werecarried out using a Cameca SX50 electron micro-probe operated at 20 kV with a beam current of30 nA and a beam diameter of lt5 mm The

4 I Abad et al

standards were albite orthoclase periclase wollas-tonite and synthetic oxides (Al2O3 Fe2O3MnTiO3) Data were reduced using the proceduresof Pouchou amp Pichoir (1985)

R E S U L T S

Mineralogy and crystal-chemical parameters

The XRD patterns of bulk samples show thatquartz and white mica primarily phengite are theprincipal phases in all samples trioctahedralchlorite is very common smectite mixed-layeredillite-smectite and intermediate Na-K mica para-gonite feldspars carbonates and hematite arepresent only in some samples (Table 1) A di-trioctahedral chlorite sudoite according to Fransoletand Schreyerrsquos (1984) criteria has been detected insamples from the La Santa and Morron de Totanaunits The XRD patterns of sudoite usually show adistinctly more intense (003) reflection character-istic of Al-rich chlorites In general the mineralassemblages are similar and typical of very low-grade metamorphism

The basal spacing of mica is ~998 AEcirc (s = 001)varying with no clear trend throughout the

sequence An overall average value of 9019 AEcirc (s= 0010) obtained for the b cell parameter(Table 1) is consistent with medium-pressureregional metamorphism according to Guidotti ampSassi (1986)

Figure 2 shows the IC values for the lt2 mm vslt2 mm EG-treated fractions corresponding to all unitsexcept for the Los Molinos Unit for whichparagonite is a major phase whose peaks interferewith measurement of IC values The IC values of thelt2 mm fraction for the Morron de Totana Unitcorrespond to the zone of diagenesis with significantvariation in values in different samples as typical ofthat grade For the La Santa Unit the data indicatelate diagenetic conditions The IC values vary theleast for samples from the Yechar Unit andcorrespond to the low anchizone (according to thenomenclature of Merriman amp Peacor 1999) Inaddition Fig 2 shows that for the Morron de Totanasamples the IC values of the lt2 mm fraction arelarger than those of the EG-treated samples implyingthe presence of smectite layers interstratified withillite The IC values for the bulk samples aregenerally smaller than those of the lt2 mm separatesas consistent with a significant contribution ofdetrital mica in the untreated samples (Table 1)

FIG 2 Plot of IC values for the lt2 mm untreated vs lt2 mm EG-treated fractions

Prograde and retrograde metamorphism 5

Whole-rock analyses

Chemical analyses of the major elements ofselected samples were carried out using X-ray

fluorescence (XRF) in a Philips PW 104010spectrometer at the CIC The data (Table 2) werecompared with the average of Post-Archaean

TABLE 1 Crystal-chemical parameters and bulk mineralogy determined by XRD

White mica Chlorite Mineral compositiond001 Illite crystallinity (CIS) d001 Qtz K-white mica

Samples lt2 mm bulk b lt2 mm lt2 mm EG bulk lt2 mm bulk (all the samples)

Los Molinos UnitEs-101 9972 9966 9012 ndash ndash ndash 142 142 Chl Hem Pg Cc Dol SmEs-102 9978 9980 9002 ndash ndash ndash 142 142 Chl Hem Pg Cc SmEs-103 9970 9975 ndash ndash ndash ndash 142 142 Chl Hem Kfs Pg Cc SmEs-104 9969 ndash ndash ndash ndash ndash 142 ndash Chl Hem Pg Cc Sm

Yechar UnitEs-51 9977 9972 9019 038 035 020 1417 1417 Chl Hem Dol SmEs-52 9974 9970 9016 041 046 022 1417 1416 Chl Kfs SmEs-53 9978 9971 9016 037 040 022 1417 1417 Chl Hem Kfs SmEs-54 9979 9978 9019 041 ndash 025 1416 1416 Chl Hem SmEs-55 9975 9987 ndash 038 041 020 1416 1418 Chl Hem Cc SmEs-56 9957 9967 9010 043 044 022 ndash ndash Hem Cc DolEs-57 9958 ndash 9019 050 049 ndash 1418 ndash Chl Cc Dol

La Santa UnitEs-1 9976 ndash 9019 050 044 071 1417 ndash Chl Hem Kfs SmEs-2 9974 9977 9003 062 049 029 1417 1417 Chl Kfs SmEs-3 9963 ndash 9018 058 050 031 1420 ndash Chl Hem Cc SmEs-4 9987 9987 9007 075 077 049 1416 1417 Chl Hem DolEs-5 9993 9983 ndash 093 ndash 084 1417 1417 Su Chl Hem Ill-Pg PgEs-6 9988 9982 ndash 089 092 078 1417 1416 Chl Hem Cc Dol Ill-PgEs-7 9990 9981 ndash 098 095 069 1418 1419 Chl Hem Cc DolEs-8 9985 9975 9007 066 ndash 046 1417 1416 Chl Hem Cc Dol Ill-Pg

Morron de Totana UnitEs-71 9977 ndash 8994 ndash ndash ndash ndash ndash Chl Kfs HemEs-72 ndash ndash ndash ndash ndash ndash ndash ndash Chl Hem GtEs-73 9970 ndash ndash ndash ndash ndash ndash ndash Kfs Cc DolEs-74 9991 ndash ndash 069 071 ndash ndash ndash Kfs Hem GtEs-75 9972 9991 8989 068 064 ndash ndash ndash Chl Kfs Hem DolEs-76 10005 ndash 064 052 ndash ndash ndash ndash Chl Hem Gt DolEs-77 9979 ndash 9011 077 059 083 ndash ndash Chl Hem Dol I-SEs-78 9972 ndash 084 068 071 ndash ndash ndash Chl Cc Sm GtEs-79 9984 ndash 9028 098 090 061 ndash ndash Chl Kfs Hem DolEs-70 9984 ndash 096 089 035 1416 1418 ndash Dol Chl SmEs-81 9980 ndash ndash 099 061 ndash 142 ndash Chl Hm Cc DolEs-82 9972 ndash ndash 090 083 ndash ndash ndash Chl Hm Cc Dol SmEs-85 9972 ndash 9008 058 050 050 142 ndash Chl Su Hem Sm KfsEs-86 9951 9977 9013 092 084 047 142 ndash Chl Su Hem Sm KfsEs-87 9974 9008 083 ndash 050 142 ndash ndash Chl Su Hem Sm KfsEs-91 9974 ndash ndash 104 108 093 ndash ndash Chl Hem KfsEs-92 9972 ndash 9028 083 074 086 ndash ndash Chl Dol KfsEs-93 9980 9983 ndash 109 096 061 ndash ndash Chl Hem DolEs-94 9964 9985 9024 098 093 080 ndash ndash Chl Hem Dol KfsEs-95 9978 ndash 9027 104 093 ndash ndash ndash Dol

Mineral abbreviations according to Kretz (1983) Ill-Pg Intermediate Na-K mica I-S Illite-smectiteinterstratified layers Su Sudoite In bold samples studied by TEM

6 I Abad et al

Australian Shales (PAAS) (Taylor amp McLennan1985) The values for samples of this study aresimilar to them The loss on ignition (LOI) is verylarge in some samples because of the presence ofcarbonates

Electron microscopy observations (SEM andTEM)

Textural aspects Due to the fine-grainedcharacter of these rocks characterization of texturesis possible only by TEM except for the highestmetamorphic-grade samples (Es-102 and Es-53) forwhich grain sizes are large enough to be resolvedby BSE imaging The HRTEM characterizationconfirms the general mineralogical data determinedby XRD for the different units of the studied areaand have allowed us to clarify some specificrelations eg the mixed-layered illite-smectite inthe Morron de Totana Unit

Los Molinos Unit samples are phyllosilicate-richphyllites with well-developed slaty cleavage(Fig 3a) composed of preferentially oriented micaand chlorite alternating with deformed elongatedquartz grains carbonate aggregates and hematitegrains Paragonite and muscovite coexist as separatecrystals Lattice-fringe images show that both micasand chlorite occur as large defect-free crystalsmore than several thousand angstroms thick

(Fig 4) Lattice-fringe images (not shown) withboth 00l and hkl fringes show that crystals arecomposed virtually entirely of coherently relatedlayers but with some stacking disorder in chloriteThese features are in accord with the typicalgreenschist facies of the Alpujarride Complex(Azanon amp Crespo-Blanc 2000) On the otherhand dioctahedral smectite commonly occursreplacing layers of trioctahedral chlorite describedby Nieto et al (1994) as the result of retrogradealteration by hydrothermal fluids

The sample studied from the basal portion of theYechar Unit (Es-53) is a quartz-rich slate with well-developed cleavage (Fig 3b) High-resolutionimages show that the main phyllosilicate is K-richdioctahedral mica with lesser amounts of chloriteand dioctahedral smectite the smectite occurring asaltered layers of primary metamorphic chloriteAlthough phyllosilicates generally form parallel orlow-angle intergrowths preferentially orientedparallel to cleavage Fig 5 shows a low-magnifica-tion image with well-defined high-angle grainboundaries of interlocking grains Such texturesare typical of a metamorphic origin The packetsare 500 AEcirc thick (Fig 6) much smaller than typicaldetrital grains (gt5000 AEcirc ) They also have composi-tions which differentiate them from detrital micasas described below (see chemical characterizationof phyllosilicates) In general the phyllosilicate

TABLE 2 Whole-rock analyses of major elements (wt oxide)

Samples SiO2 TiO2 Al2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O P2O5 LOI

Es-102 5340 077 1790 557 100 013 158 722 152 254 013 847Es-51 5270 083 1752 441 190 014 514 359 028 503 016 845Es-52 7620 093 1210 070 120 ndash 185 037 143 251 017 239Es-53 5990 089 1890 583 140 ndash 292 021 043 489 014 408Es-56 6640 061 1200 350 020 ndash 190 384 011 427 012 616Es-57 6147 065 1154 369 086 005 354 522 ndash 403 012 909Es-1 6008 086 1820 524 170 ndash 299 029 031 484 014 416Es-2 6510 091 1770 139 190 ndash 322 033 038 427 016 470Es-3 5152 089 2030 590 185 006 429 241 ndash 619 018 642Es-5 6260 090 1750 688 020 ndash 263 030 065 237 012 493Es-6 6891 080 1321 434 086 009 169 293 050 234 010 536Es-7 5158 087 2109 671 142 006 275 379 086 347 018 766Es-75 7238 071 1251 238 118 003 154 143 ndash 337 009 438Es-77 3442 055 1252 516 087 005 956 1337 ndash 361 009 2160Es-79 4996 064 1062 335 129 024 676 935 ndash 301 008 1590Es-86 7600 083 1208 122 144 002 205 048 061 324 014 282Es-87 6285 106 1705 516 175 004 299 034 033 493 018 396Es-91 5881 083 1707 409 151 003 338 217 ndash 569 017 645Es-95 5913 075 1802 368 104 002 410 095 009 717 016 520

Prograde and retrograde metamorphism 7

packets are almost defect-free with only rare layerterminations (Fig 6) The polytypes are 2M formicas and semi-random for chlorites

Two samples have been studied from the LaSanta Unit Sample Es-2 a chlorite-rich shale isfrom the basal part of the unit and sample Es-5 aquartz-rich shale is from the upper part In Es-2elongated quartz-rich and phyllosilicate-richdomains alternate at the scale of millimetresparallel to slaty cleavage The principal phyllosili-cate is muscovite with well-defined 2M polytypismThe (hk0) reflections tend to be spread normal to cmuch more than in sample Es-53 due to the

presence of thinner packets in subparallel orienta-tions The trioctahedral chlorite (clinochlore) isoften altered to dioctahedral smectite as determinedin part by AEM data as a result of retrogradealteration processes (eg Fig 7 in Nieto et al1994) The most common defects are layerterminations voids low-angle grain boundariesand dislocations

Sample Es-5 shows incipient slaty cleavagedefined by subparallel preferentially orientedpackets of illite sudoite trioctahedral chloriteparagonite and intermediate Na-K mica as illu-strated in Fig 7 Hematite is commonly included

FIG 4 Lattice-fringe image showing a portion of a defect-free well-crystallized chlorite packet of metamorphicorigin in phyllite Es-102 (Los Molinos Unit)

FIG 3 BSE images showing the well-developed slaty cleavage of two samples (a) Es-102 (Los Molinos Unit) aphyllite in which the grains with the brightest contrast are hematite and the darkest ones quartz Chlorite andmuscovite grains are characterized by their elongated habit and intermediate brightness (stronger in chlorite thanin muscovite) (b) Es-53 (Yechar Unit) in which stretched quartz grains are more abundant and responsible for itssemipelitic character grains with high contrast are hematite or TiO2 Muscovite crystals are indicated by arrows

8 I Abad et al

within illite packets as elongated individual grainsup to ~3006500 AEcirc in size or as aggregates(Fig 7) they show up clearly due to their darkcontrast Sudoite and chlorite were identified on thebasis of SAED and AEM data Separate grains ofeach were observed to be intergrown with texturestypical of metamorphic origins for both Grain sizesof sudoite (gt500 AEcirc ) and clinochlore are approxi-mately equal implying formation of both at gradeswhich were not substantially different Howeverwhere grains of each intersect at high angles thesudoite ubiquitously transects clinochlore packetsAs shown in Fig 8 clinochlore but not sudoitemay exhibit strain contrast These relations implybut do not prove that sudoite post-dated clino-chlore Obvious alteration of clinochlore todioctahedral smectite was ubiquitous but possiblealteration effects were rare in sudoite occurring asdefects such as voids and wavy layers Because thecomposition of sudoite is similar to that ofdioctahedral smectite it was not possible to verifythe presence of small numbers of smectite layerswith AEM data Nevertheless the observationsclearly show that alteration of chlorite to smectitewas relegated entirely or almost entirely toclinochlore

Muscovite occurs in crystals which average~500 AEcirc in thickness and are characterized by 2Mpolytypism features which are typical of meta-morphic mica of anchizonal grade The Na-K micaoccurs in packets of similar thickness The

FIG 6 Lattice-fringe image showing the relation between mica and chlorite packets The arrows indicate theposition of layer terminations The inset SAED pattern illustrates the low-angle relation between the two phases

(Yechar Unit)

FIG 5 Low-magnification image with well-definedhigh-angle grain boundaries and interlocking relations

among muscovite crystals (Yechar Unit)

Prograde and retrograde metamorphism 9

composition (see below) is quite variable corre-sponding to Na-rich muscovite commonly but somegrains have compositions within the limits of thesolvus between muscovite and paragonite and thusare metastable Na-K micas give SAED patternswhich have relatively sharp reflections over a largerange as consistent with large defect-free crystals

Sudoite crystals comprise units with 14 AEcirc

periodicities which in some areas are 28 AEcirc Lattice-fringe images show the presence of inter-

stratified individual 7 AEcirc layers which in some areasform small packets containing no more than eight7 AEcirc layers The AEM analyses (see below) of suchareas are typical of those of homogeneous sudoitecontaining only 14 AEcirc layers implying a composi-tion of the 7 AEcirc layers similar to that of sudoite andnot to an exotic septechlorite such as kaolinite orberthierine The contrast of both 14 AEcirc and 7 AEcirc

fringes appear to be identical further consistentwith a close chemical relation

FIG 7 Textural relations between phyllosilicates (sudoite illite Na-K mica) and hematite (dark contrast) (LaSanta Unit)

FIG 8 Lattice-fringe image showing the coexistence of two types of chlorite trioctahedral chlorite (chl) andsudoite (su) Sudoite transects chlorite which shows strain contrast associated with curved and kinked layersIdentifications were based in part on AEM analyses (see Table 6 Es-511 and 12 analyses) (La Santa Unit)

10 I Abad et al

Three samples corresponding to different butequivalent levels were selected from the Morron deTotana Unit for TEM study (see Fig 1 peliticrocks intercalated with carbonate rocks) Es-77 atthe bottom of the unit Es-86 which is located in anintermediate position and Es-95 from the top of thecross-section There is no incipient slaty cleavage inthese samples Sample Es-77 is a very fine-grainedrock consisting of quartz white mica chlorite andaggregates of carbonate grains Minor hematiteproduces a deep red colour Lattice-fringe images

show that dioctahedral clay minerals have 10 and20 AEcirc periodicities occur as packets 100 ndash200 AEcirc

thick and have subparallel orientations Accordingto Guthrie amp Veblen (1989) smectite and illiteinterlayers can produce different contrast and canonly be differentiated in lattice fringe-imagesobtained with well-defined electron beam (001)orientations and defocus conditions Such contrastdifferences were observed in several lattice-fringeimages The light contrast for fringes in Fig 9represents the smectite interlayer as shown in part

FIG 9 TEM image showing 10 AEcirc periodicity in R4 illite-rich I-S Contrast differences imply the presence ofillite-like (the dark fringes) and smectite-like (the light fringes) layers Packets usually have four or more illite

layers and the SAED pattern shows predominantly 50 AEcirc periodicity (Morron de Totana Unit)

FIG 10 Trioctahedral chlorite with intercalations of 10 AEcirc layers (upper arrow) which damage quickly under theelectron beam The chlorite packets show strain contrast microfolds and 7 AEcirc periodicities corresponding toberthierine (left arrow) or greater values eg 28 AEcirc (see analysis in Table 6 Es-867) (Morron de Totana Unit)

Prograde and retrograde metamorphism 11

by the continuity of such fringes with openinterlayer spaces produced by contraction due to

dehydration The sequence on average is that ofR4 mixed-layering ie a single smectite-likeinterlayer between four or more illite-like inter-layers Although some packets with more than fourlayers of illite are present repeats of four illite-likelayers predominate as shown by the 50 AEcirc

periodicity of SAED patterns (Fig 9 inset) Theillite-smectite (I-S) has typical 1Md polytypism Theobservation of smectite interstratified with illite isin accord with the changes observed in XRDdiagrams after EG treatment (Fig 2) In additionAEM analyses of areas similar to that of Fig 9have relatively high Si and minor Ca contentsfurther confirming the mixed layering

Some packets which give rise to fringes with10 AEcirc spacing produce SAED patterns typical oftwo-layer polytypism The contrast of all fringes isrelatively constant Such packets have thicknessestypical of diagenetic clays and two orders ofmagnitude smaller than typical detrital grains Thecompositions of such grains (see below) haveinterlayer-cation contents typical of illite They aretherefore inferred to be illite which formed duringlate diagenesis and coexisting with the R4 I-S

Sample Es-86 is composed mainly of detritalgrains of quartz randomly-oriented grains ofchlorite and white mica all immersed in aphyllosilicate-rich fine-grained matrix At theTEM scale chlorite is the most common phyllosi-licate in the matrix As in sample Es-5 trioctahe-dral chlorite and sudoite coexist There areinterstratified packets of 10 AEcirc layers only a fewlayers thick (lt150 AEcirc ) which are quickly damaged

FIG 11 Di-trioctahedral chlorite (sudoite) very similarin appearance to the trioctahedral chlorite shown inFig 10 7 AEcirc layers are marked by an arrow and 10 AEcirc

layers are present in the poorly resolved area in thecentre of the image (see analysis in Table 6 Es-862)

(Morron de Totana Unit)

FIG 12 Low-magnification image illustrating illite-like packets The inset SAED pattern shows only a smallnumber of diffuse poorly defined reflections The packets are 100 ndash200 AEcirc thick (Morron de Totana Unit)

12 I Abad et al

under the electron beam (Figs 10 and 11) TheAEM analyses of such material show an enhancedSi content and the presence of small concentrationsof alkali cations These data are consistent withretrograde alteration of chlorite as observed byNieto et al (1994) They differ however from theresults of Nieto et al (1994) and those describedabove for sample Es-5 in that both tri-trioctahedraland di-trioctahedral chlorite are affected In TEMimages strain features in chlorites such as contrastnormal to the layers or microfolds were observed(Fig 10) Mica is less abundant than sudoiteoccurring in packets 100 ndash400 AEcirc thick The SAEDpatterns exhibit 2M polytypism but with diffuse-ness parallel to c in 0kl rows At low magnifica-tion subparallel and slightly curved illite- or illite-smectite-like packets are present they coalescehave abundant layer terminations and show intensecontrasts (Fig 12)

Sample Es-95 is a very fine-grained rock madeup of large detrital grains of quartz in a fine-grained matrix of phyllosilicates mainly whitemicas and opaque minerals Some biotite grainswere observed as verified by AEM data Biotitewas observed only in separated grains on holey

C-coated Cu grids so it is not possible to describethe textural relations for it Authigenic biotite isinconsistent with the low grade of this samplehowever and it is therefore inferred to be of detritalorigin The AEM analyses show deviations frompure biotite however as consistent with alterationto chlorite the usual process in detrital biotite inrocks undergoing diagenesis Lattice-fringe imagesof ion-milled samples show that individual micapackets are very thin with no more than 10 layersisolated and dispersed in a matrix consisting ofmaterial which gives no SAED pattern Thesepackets are wavy discontinuous and with anasto-mosing layers (Fig 13) Despite these texturalfeatures SAED data indicate that there are 2Mand 1Md polytypes

For all samples prismatic crystals of Ti-oxidedispersed Fe-oxyhydroxides zircon and tourmalineare common accessory minerals

Chemical characterization of phyllosilicates

Dioctahedral mica Table 3 shows the composi-tion of K-rich dioctahedral micas of authigenic ormetamorphic origin corresponding to the samples

FIG 13 General aspect of sample Es-95 The image shows arrays of wavy discontinuous and anastomosingpackets of illite and smectite (Morron de Totana Unit)

Prograde and retrograde metamorphism 13

selected from each unit The Yechar and LosMolinos units are characterized by compositionsapproaching those of evolved micas with a sum ofinterlayer cations slightly less than 1 apfu typicalof phengite or muscovite rather than authigenic

illite In contrast the La Santa and Morron deTotana units show heterogeneous mica composi-tions and display deficiencies in interlayer cations(Fig 14a) This last feature is more evident in theMorron de Totana analyses where the illitic

TABLE 3 Representative chemical compositions for K-rich dioctahedral micas normalized to O10(OH)2

Si IVAl VIAl Fe Mg Ti VIS K Na S inter

Los Molinos Unit (1)Es-10213 337 063 178 010 008 002 198 075 008 085Es-10221 308 092 195 005 001 001 202 082 009 091Es-10231 319 081 191 005 003 001 200 077 012 090Es-10232 314 086 188 006 007 001 202 072 021 093

Yechar Unit (1)Es-5311 332 068 163 015 022 002 204 090 004 093Es-5312 310 090 180 003 013 004 201 089 010 099Es-5322 338 062 170 012 015 003 200 086 001 086Es-5323 319 081 176 016 012 003 206 087 002 089

Yechar Unit (2)Es-53 1 317 083 163 021 026 000 210 084 000 084Es-53 2 320 080 165 011 027 003 207 087 000 087Es-53 3 317 083 155 025 024 002 206 093 000 093Es-53 4 328 072 160 020 024 000 203 092 000 092Es-53 5 326 074 164 019 016 000 199 096 000 096Es-53 6 326 074 158 020 028 000 206 089 000 089Es-53 7 319 081 154 025 026 000 205 097 000 097Es-53 8 325 075 166 016 025 000 206 085 000 085Es-53 9 324 076 167 019 019 000 205 085 000 085

La Santa Unit (2)Es-2 1 311 089 157 019 032 002 210 088 005 094Es-2 3 336 064 148 020 043 001 212 072 003 075Es-2 6 313 087 156 024 029 001 210 075 017 092Es-2 7 313 087 155 024 029 001 209 084 011 095Es-2 8 331 069 157 015 032 000 205 087 004 092Es-2 10 328 072 160 018 027 003 208 068 008 075Es-5 4 303 097 174 015 023 001 213 073 009 082Es-5 10 301 099 189 006 015 000 210 065 021 086Es-5 7 307 093 159 033 016 000 208 075 018 093Es-5 8rsquo 301 099 160 031 025 000 216 067 016 083

Morron de Totana Unit (2) (analyses corresponding to R4 Ill-Sm may also be present see text)Es-77 13 327 073 172 013 027 000 212 067 000 067Es-77 15 316 084 195 003 013 000 211 063 000 063Es-77 16 328 072 182 010 015 000 207 068 000 068Es-77 17 338 062 147 032 019 000 199 093 000 093Es-86 9 334 066 160 017 034 000 210 072 000 072Es-86 10 357 043 151 012 046 000 209 065 000 065Es-86 11 353 047 156 012 036 000 203 077 000 077Es-86 13 324 076 152 025 035 000 211 084 000 084Es-86 14 324 076 173 013 019 000 205 084 000 084Es- 95 2 323 077 158 016 019 004 198 103 000 103Es- 95 5 336 064 169 013 025 000 207 070 000 070Es- 95 6 336 064 148 027 033 000 209 078 000 078

(1) SEMEDX analyses (2) TEMAEM analyses

14 I Abad et al

character is consistent with the presence of the R4mixed-layer illite-smectite described above Most ofthe micas are phengitic in composition (Fig 14ab)with an Fe(Fe+Mg) ratio that varies over the range02 ndash085 However the detrital-like grains aresimilar to those of end-member muscovite (seebelow detrital muscovite vs metamorphic musco-vite or Table 5) with much lower Fe and Mgcontents Data plotted in Fig 14bc confirm theassumption of a significant ferrimuscovitic substitu-tion (Guidotti et al 1994) Some of the K-micaanalysed in the Los Molinos and La Santa unitswere found to have significant Na contents (up to02 apfu) Although Ti is unusual in dioctahedralmicas it has been measured in several analyses

(lt005 apfu) The most evident feature in theplots is the scattering of data in the La Santa andMorron de Totana units ie where diageneticconditions are not exceeded In contrast theYechar Unit analyses define a narrow field in thediagrams and the Los Molinos data are even closerto the theoretical muscovite composition (Table 3)

Typical compositions of micas intermediate tomuscovite and paragonite called Na-K micas in thispaper are shown in Table 4 The compositions arequite variable even in the same sample Whencompared with analyses of K-rich dioctahedralmicas (Table 3) and paragonite analyses are seento span the entire range from muscovite toparagonite Some analyses (eg Es-5 9) fall within

FIG 14 Plots of values of chemical compositions of K-rich dioctahedral phyllosilicates solid circles and linesindicate values for theoretical muscovite and corresponding exchange vectors respectively

Prograde and retrograde metamorphism 15

the limits of the solvus between muscovite andparagonite the limbs of that solvus having theapproximate compositions ms60pg40 and ms20pg80

(Li et al 1994a)Detrital muscovite vs metamorphic muscovite

Detrital-like micas of sample Es-95 the lowest-grade one are large enough to be analysed(Table 5) by electron microprobe (EMPA) Thesedata have been compared with the AEM data ofother samples Compositions of detrital-like grainsof the lowest-grade sample and the detrital micas ofthe anchizonal-grade sample Es-53 (Fig 15) arevery similar and close to that of end-membermuscovite These data suggest that the detrital micahas a common source for the different units thatcomprise Sierra Espuna Figure 15 also comparesthe data from authigenic micas of the samesamples which are clearly more phengitic (Fe+Mg= 015 ndash075 apfu) than the detrital ones (Fe+Mg= 01 ndash02 apfu)

Chlorite Trioctahedral and di-trioctahedral(sudoite) chlorites have been analysed (Table 6)The former corresponding to the species clino-chlore is common in most of the samples unlikesudoite which is absent from most (Table 1)Sudoite was identified because the sum ofoctahedral cations is nearly equal to 5 apfu andthe Si and Al contents are higher than intrioctahedral chlorite (Fig 16a) A slight contam-ination by interlayer cations (K and Na) has beendetected therefore the formulae were recalculatedin order to eliminate those impurities (seeAnalytical Methods) Although total Fe has beenconsidered as Fe2+ according to Fransolet ampBourguignon (1978) the Fe3+ content in sudoitemay be greater than the Fe2+ content As severalanalyses have an octahedral sum slightly higherthan the ideal value of 5 apfu this may indicate atrioctahedral substitution andor the presence ofsignificant Fe3+ No Ti or Mn were detected The

Fe(Fe+Mg) ratios do not define a clear trendthroughout the sequence (Fig 16b) nevertheless itis much greater in chlorite in the Morron de Totanaanalyses falling in the characteristic range ofdiagenetic chlorites (Li et al 1994b) There is anincrease in Mg content (from 15 to 4 apfu)concomitant with increase in grade as compatiblewith some other sequences (eg Bevins et al1991)

D I S C U S S I O N

Dioctahedral phyllosilicates chemicalcompositions IC values and polytypes

On the basis of their large size homogeneity ofindividual grains and well-crystallized aspectdetrital dioctahedral mica grains were identified inseveral samples with the exception of those ofgreenschist facies This confirms the idea of the

TABLE 4 Chemical compositions for Na-K micas normalized to O10(OH)2

Si IVAl VIAl Fe Mg Ti VIS Ca K Na S inter

Es-5 14 313 087 190 004 009 002 204 000 034 048 083Es-5 15 321 079 176 009 037 000 222 000 015 038 053Es-5 4 318 082 187 004 019 000 209 000 013 061 074Es-5 12 329 071 172 008 029 000 208 000 039 036 076Es-5 13 323 077 179 005 028 000 212 000 029 041 069Es-5 9 296 104 163 019 029 000 211 000 048 056 104Es-5 8 292 108 156 043 008 001 209 003 014 080 096

FIG 15 Plot of Si vs Al contents of detrital andauthigenic micas of samples Es-95 and Es-53 Solidsymbols = detrital micas open symbols = authigenic

micas

16 I Abad et al

relative stability of muscovite in most diageneticenvironments (Crowley 1991) The compositions ofthe detrital grains of samples from different unitsare similar The lack of heterogeneity which isexpected for mica from a complex source implies acommon source area for all samples

The IC values define a trend from diagenetic toanchizonal conditions but with smaller IC values inbulk-rock fractions consistent with the contributionof detrital mica The coexistence of mature micawith lower-grade I-S or illite in the sub-greenschistfacies rocks shows that those rocks have not

TABLE 5 EMPA data for detrital-like crystals of mica normalized to O10(OH)2

Si IVAl VIAl Fe Mg Ti VIS Ca K Na K+Na+Ca

Es-95z11 313 087 193 005 004 001 203 000 065 022 087Es-95z11 314 086 190 006 007 001 204 000 068 019 087Es-95z1 2 314 086 191 005 009 001 206 000 070 006 077Es-95z2 1 314 086 176 012 010 003 201 002 095 004 101Es-95z2 2 317 083 177 012 011 003 202 000 095 004 099Es-95z4 1 305 095 188 005 007 004 204 000 081 009 089Es-95z4 2 311 089 187 006 007 004 204 000 078 009 086

TABLE 6 Representative chemical compositions for chlorite normalized to O10(OH)8

Si IVAl VIAl Fe Mg VIS FeFe+Mg

Yechar UnitEs53-1 273 127 112 121 376 609 024Es53-4 287 113 125 070 400 596 015Es53-12 303 097 130 146 307 583 032Es53-13 277 123 125 122 354 601 026Es53-15 307 093 146 094 323 564 023Es53-42 271 129 152 160 294 606 035

La Santa UnitEs2-4 319 081 147 148 273 569 035Es2-5 365 035 183 118 225 526 034Es5-1 354 046 293 023 172 488 012Es5-5 361 039 292 021 178 490 010Es5-10 309 091 262 028 221 511 011Es5-16 310 090 197 112 234 543 032Es5-17 350 050 252 076 170 499 031Es-5 10rsquo 364 036 213 097 202 512 032Es-5 11 291 109 143 136 314 593 030Es-5 12 382 018 229 087 179 495 033Es-5 13 316 084 302 021 172 494 011Es-5 16rsquo 346 054 341 013 101 455 011Es-5 17rsquo 300 100 282 033 194 509 014Es-5 18 285 115 285 026 204 515 011

Morron de Totana UnitEs-86 2 377 023 253 088 149 490 037Es-86 4 284 116 166 230 181 578 056Es-86 5 350 050 191 192 151 534 056Es-86 6 366 034 189 191 149 529 056Es-86 7 315 085 129 273 176 578 061

indicates sudoite analyses

Prograde and retrograde metamorphism 17

reached stable chemical or textural equilibrium atleast at the mm scale In addition the lack ofchemical equilibrium among diagenetic I-S andillite at the nm scale is demonstrated by their highlyvariable Fe+Mg contents (006ndash075 apfu) Theaverages of the sum of interlayer cations are closeto 1 apfu in the Los Molinos and Yechar units asconsistent with mica but values of 06 apfucorresponding to I-S or illitic compositions are alsopresent in the Morron de Totana Unit Except forsamples Es-102 and Es-53 the variable composi-tions represent a metastable state with composi-tions of micas determined by a combination ofphengitic ferrimuscovitic and illitic vectors asshown by Fig 14ab

Paragonite is generally considered an indexmineral for the anchizone (Frey 1987) but Li etal (1994a) have shown that paragonite coexistswith K-rich mica in grades as low as that of the latediagenetic zone for a prograde sequence Samplesin which muscovite paragonite and intermediateNa-K mica coexist as in sample Es-5 areconsistent with metastability both because morethan two such micas coexist and because theircompositions are incompatible with the solvus inthe muscovite-paragonite system (see below) Whilethe 2M polytype typical of metamorphic micas is

the most common in these samples the 1Md

polytype also occurs in some diagenetic samplesparticularly those from the Morron de Totana UnitThe 1Md polytype is metastable relative to the 2Mpolytype and the coexistence of both polytypesfurther demonstrates a lack of equilibrium at the nmscale Dong amp Peacor (1996) showed that thedisorder noted in the diagenetic phyllosilicates bySAED is a consequence of the general high entropyof phases formed at very low temperatures thatproduces for example stacking defects separatingpackets with no more than 10 layers These packetsprogressively thicken with increasing metamorphicgrade and the 2M polytype then becomes the mostcommon polytype in authigenic micas Thenm-scale size of the lower-grade clays and theregular increase in size with increasing gradereflect the lack of textural equilibrium in the low-grade samples

A wide range of sample characteristics thusimply that the lower-grade samples are in statesof metastable equilibrium those factors includingcoexistence of mature mica with I-S and illitevariable chemical compositions incompatiblecompositions of white micas small crystal sizesdefect-rich states and polytypism Such dataemphasize the notion of lsquoreaction progressrsquo relative

FIG 16 (a) Plot of Al vs Fe+Mg contents of chlorites illustrating the contrast in composition between tri-trioctahedral clinochlore or chamosite and di-trioctahedral sudoite (b) plot of Si vs Fe(Fe+Mg) ratio of

chlorites showing an absence of compositional trends throughout the sampled sequence

18 I Abad et al

to Ostwald-step rule processes metastable dioctahe-dral clay systems undergoing reactions toward thestate of stable chemical and textural equilibriumapproached in greenschist-facies samples (Essene ampPeacor 1995)

Prograde evolution based on TEM datacorrelation with the Basque-Cantabriansequence (Northern Spain)

With increasing grade TEM data confirm thatcrystal thickness increases crystal defect densitydecreases compositions become less variable andmicrotextural relations are simpler (eg Peacor1992) This trend is consistent with three well-characterized low-grade settings the Gulf Coastsequence (Ahn amp Peacor 1986) and the WelshBasin (Merriman et al 1990) both extensionalbasins and the Helvetic Alps (Livi et al 1997)The latter is a fold-and-thrust belt with similarseries of prograde transitions to those observed inSierra Espuna in which pelites have passed througha sequence of metastable states in the lithologicalsequence mudstone and shale slate and phyllite

The mudstones of the Morron de Totana Unit arecharacterized by the presence of illite-smectite R4mixed-layers mica packets of no more than 10 ndash40layers and the occurrence of 1Md polytypism TheLa Santa Unit samples represent a step in theprograde evolution with thicker packets than in theprevious samples (200 ndash700 AEcirc ) illitic or phengiticcompositions the absence of illite-smectite mixedlayers as well as the presence of discreteintermediate Na-K micas and paragonite Althoughthere is a tendency towards non-preferred orienta-tion of crystals it is not so evident as in the Morronde Totana samples In the Yechar Unit (anchizone)and more clearly in the Los Molinos phyllites(greenschist facies) the microtextural relations areless complex the compositions are less variablewith phengite and trioctahedral chlorite crystalthickness are greater (gt400 AEcirc in the Yechar Unitand gt1000 AEcirc in the Los Molinos Unit) with thepackets being defect-free and with a very crystallineaspect This description has features in commonwith the three well-characterized low-grade settingspreviously noted Only the mineral assemblagesvary somewhat presumably as a function of thedifferences in bulk-rock composition

A detailed comparison with the Basque-Cantabrian Basin (N Spain) shows that the lowest-grade conditions of Sierra Espuna overlap most of

those of that sequence which is characterized bypassive burial diagenesis (8000 m thick) (Nieto etal 1996) Only in the deepest sample of theBasque-Cantabrian sequence is illite-smectite mixedlayering absent Figure 12 of this study is verysimilar to Fig 7 in Nieto et al (1996) and bothimages correspond to diagenetic samples fromdifferent settings nevertheless they illustratesimilar mica packets 100 ndash200 AEcirc thick withparallel-to-subparallel boundaries the orientationof which is inferred to represent beddingTherefore there is a parallelism between thesesequences down to the scale of individual clay-mineral grains even though they developed underdifferent conditions with stress presumably playinga significant role in the Sierra Espuna units Thetexture of the least metamorphosed sample (Fig 13)is reminiscent of the typical textural relations ofsmectite described by Nieto et al (1996) (seeFig 4) Although it corresponds to illite-richinterstratified illite-smectite the clay mineralgrains conserved the grain size and anastomosingdiscontinuous array of original smectite layers

Despite the similarities between the twosequences the chemical behaviour of the micasare clearly different since the grade correspondingto the phengitic compositions of the Yechar Unit isnot reached in the Basque-Cantabrian Basin wherethe deepest and highest-grade sample has illiticcompositions equivalent to those of the La SantaUnit That is in a passive burial metamorphiccontext as in the Basque-Cantabrian Basin thefactors responsible for increasing the rate ofchemical reactions are less significant than in thesequences that were affected by tectonic stressTectonic stress introduces defects which in turnincreases the rate of dissolution of strained grainsthus increasing the rate of dissolution-crystallizationreactions typical of clay minerals Perhaps moreimportantly regional tectonic stress is known tocause mobilization of fluids (eg Oliver 1986) Thepresence and activity of such fluids is a prime causeof increased rates of dissolution ion diffusion andcrystallization Tectonic activity thus may have atwo-fold effect in increasing rate of clay-mineralreactions The phengite component may in part be ameasure of the reaction progress depending on thereaction kinetics in very low-grade rocksAccording to Merriman amp Peacor (1999) whereheat flow is low as in accretionary settingsphengite contents are typically high perhapsbecause there is insufficient thermal energy to

Prograde and retrograde metamorphism 19

cause equilibration of the phengite component ofneoformed illite by contrast with extensional basinswhere this component is typically small

Coexistence of trioctahedral chlorite andsudoite Implications for multiple metamorphicevents

In metamorphic rocks the coexistence of mineralsthat formed at different times and conditions on aPTt path ie not in equilibrium is a commonlyobserved feature where the metamorphic history iscomplex Gross differences in the grain size ofcoexisting phases has long been recognized bymetamorphic petrologists as an indicator of non-equilibrium mineral assemblages Historically muchless attention has been paid to such relations in sub-greenschist-facies rocks because prior to the intro-duction of SEM+TEM techniques their small grainsize prevented observations of textural relationsStudy of such rocks eg by XRD provides littleinformation on textural relations beyond determina-tion of degree of preferred orientation

Study by XRD of the samples showed thatsudoite and trioctahedral chlorite coexist If theyhad formed at the same time and under the sameP-T conditions their compositions would define thelimbs of a solvus The TEM images show howeverthat wherever they are in contact the sudoitetransects the packets of trioctahedral chlorite (egFig 8) Furthermore as illustrated in Fig 8 thetrioctahedral chlorite is commonly deformed anddisplays strain contrast in TEM images Thesetextural relations suggest that sudoite was intro-duced subsequent to the formation of trioctahedralchlorite Similar observations were made byGiorgetti et al (1998) who concluded that texturesin Verrucano metasediments showed that sudoitepost-dated chloritoid pyrophyllite and trioctahedralchlorite ie sudoite is in disequilibrium with thelatter minerals and formed as a retrograde phase Inrocks of the Sierra Espuna area differences in bulk-rock composition are small (Table 2) and do notcorrespond to the presence or absence of sudoite(Table 1) ie the random occurrence of sudoiteindependent of bulk-rock composition is compa-tible with a lack of equilibrium relations Theserelations collectively imply that sudoite wasintroduced at some time subsequent to theformation of mineral assemblages at peak meta-morphic conditions On the other hand the size ofsudoite packets is similar to that of trioctahedral

chlorite as consistent with formation at conditionsnot much different from those for chlorite

In samples from the Morron de Totana bothsudoite and trioctahedral chlorite were affected byretrograde alteration to smectite The TEM imagesshow sudoite (Fig 11) and trioctahedral chlorite(Fig 10) with interstratified layers of smectite withspacings of 10 AEcirc due to collapse and dehydration inthe TEM environment and even packets of smectite150 AEcirc thick within chlorite crystals These relationsimply that both sudoite and trioctahedral chloritehave been affected by retrograde alteration at thevery low temperatures consistent with smectiteformation although the presence of more subtlealteration features in sudoite of some samplessuggests that it was less susceptible to alteration

We therefore conclude that at least three episodesof phyllosilicate formation can be recognized inrocks from the Sierra Espuna-area Trioctahedralchlorite was presumably produced at the highestmetamorphicdiagenetic grade in equilibrium withparagonite and phengite Subsequently sudoite wasintroduced but at P-T conditions which could nothave been much different than those of peakmetamorphic grade Finally at very low-T condi-tions hydrothermal fluids caused partial alterationof trioctahedral chlorite and sudoite to smectite

Retrograde products formed by the reaction offluids with mineral phases generated during anearlier prograde event were described previouslyby Nieto et al (1994) and Zhao et al (1999) Thosereactions occurred on a regional scale but similarreactions have been observed on a more localizedscale The fluid activity associated with a majorfault zone is a common source for these reactions(eg Jiang et al 1990) The structure of SierraEspuna (Fig 1) is based on imbrication processesthat superimpose units that have subsequently beenaffected by faults and folds and that have under-gone significant uplift at least 800 m from the lateMiocene to the present (Sanz de Galdeano et al2001) Given this evolution it is easy to justify theretrograde processes that usually seem to be relatedwith tectonic events during which fluid flow andfluidrock ratios are enhanced (Merriman amp Peacor1999) Moreover according to Nieto et al (1994)the uplift of the Betic Cordillera would furnish theintroduction of phreatic water into the systemproducing a reactive environment adequate for thegenesis of smectite Geological conditions aretherefore consistent with post-peak metamorphicconditions as implied by the TEM observations

20 I Abad et al

The introduction of sudoite as a major phaseduring metamorphism implies the possibility ofadditional overprints on a simple prograde assem-blage the evidence for which might be subtle TheNa-rich micas at all grades have compositionswhich either fall within the solvus or are too Na-rich to correspond to the limb of the solvus In thehighest-grade rocks such micas coexist withmuscovite Those relations are incompatible withchemical equilibrium despite the fact that relationsfor muscovite and other minerals imply an approachto chemical equilibrium Furthermore there is aclose association of Na-rich micas with sudoite (egFig 7) Those relations imply the possibility thatNa-micas were introduced concomitant withsudoite P Mata (pers comm) has shown forexample that overprinting of an epizonal assem-blage by hydrothermal solutions resulted in theintroduction of both an Al-rich variety of chlorite(cookeite) and paragonite in pelites of the CamerosBasin Spain The textural relations such as cross-cutting textures which are so striking for sudoitewere not observed for Na-rich micas Neverthelessthe possibility of such overprinting must beconsidered

In spite of all these prograde-retrogradeprocesses in the Los Molinos Unit a smallnumber of homogeneous phases coexist in chemicalequilibrium and the fine-grained metamorphicphyllosilicates are clearly parallel to cleavage witha typical metamorphic aspect Finally all thetextural and compositional changes that haveoperated in the Espuna area during the metamorphicevolution proceed toward a simpler system repre-senting both textural and chemical equilibriumwhich is characteristic of greenschist-facies condi-tions

Roles of detrital and authigenic phyllosilicatesduring slaty cleavage development

Van der Pluijm et al (1998) focused on therespective roles of the phyllosilicates of detritalorigin and those formed during diagenesis andmetamorphism and observed that detrital phyllosi-licates remain preferentially oriented parallel tobedding in pelites with slaty cleavage whereas fine-grained metamorphic phyllosilicates are orientedparallel to cleavage

Although it is usually difficult to determine thepreferred orientations of phyllosilicates in TEMimages as the grade increases better-aligned

packets occur in the cleavage orientation Thetexture of the least metamorphosed sample inwhich even incipient slaty cleavage is absent isreminiscent of the typical textural relation ofsmectites described in the literature The illite-smectite mixed-layers conserve the textural featuresof possible previous smectite such as anastomosingfringes curved and discontinuous with variations ininterplanar spacing d001 These features indicatethat the minimum energy conditions to initiatemechanical transformations have not been reachedin the lower-grade samples as postulated by Ho etal (1996) since slaty cleavage develops as a resultof interactive mechanical deformation and ther-mally driven dissolution and crystallizationprocesses (Knipe 1981) In contrast in the LosMolinos Unit large crystals of chlorite and whitemica (gt1000 AEcirc ) occur which are of metamorphicorigin but with no detrital grains (Fig 3a)Phyllosilicates are preferentially oriented parallelto slaty cleavage in response to increasedtemperature and tectonic strain (Merriman ampPeacor 1999) The oriented microfabrics and slatycleavage well developed in the rocks of this unitmust be a product of a higher thermal and strainenergy environment reached in the deepest part ofthe sequence As pointed out by Van der Pluijm etal (1998) a continuous range of orientations isevidence for mechanical rotation of grains whereasthe lack of such transitional orientations impliesthat cleavage-parallel phyllosilicates originatedthrough dissolution of bedding-parallel and crystal-lization in the slaty cleavage orientation Asobserved in this study for the Intermediate Unitsgrains with orientations which are transitional frombedding-parallel to slaty cleavage orientation areonly rarely observed Cleavage formation is there-fore inferred to have occurred entirely or nearlyentirely through dissolution and crystallization

ACKNOWLEDGMENTS

We thank MM Abad Ortega from the Centro deInstrumentacio n Cient otilde fica of the Universidad deGranada for her help with HRTEM and I Nieto forher essential help with sample preparation Financialsupport was supplied by Research Project nordm BT 2000-0582 FPI research grant to IA both of the SpanishMinistry of Science and Technology Research GroupRNM-0179 of the Junta de Andaluc otildeacutea and to DRP byNSF grant EAR-9814391

Prograde and retrograde metamorphism 21

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Azanon JM amp Crespo-Blanc A (2000) Exhumationduring a continental collision inferred from thetectonometamorphic evolution of the AlpujarrideComplex in the central Betics (Alboran Domain SESpain) Tectonics 19 549 ndash565

Bevins RE Robinson D amp Rowbotham G (1991)Compositional variations in mafic phyllosilicatesfrom regional low-grade metabasites and applicationof the chlorite geother mometer Journal ofMetamorphic Geology 9 711 ndash721

Cliff G amp Lorimer GW (1975) The quantitativeanalysis of thin specimens Journal of Microscopy 103 203 ndash207

Crowley SF (1991) Diagenetic modification of detritalmuscovite an example from the Great LimestoneCyclothem (Carboniferous) of Co Durham UKClay Minerals 26 91 ndash103

Dong H amp Peacor DR (1996) TEM observations ofcoherent stacking relations in smectite and illite ofshales evidence for MacEwan crystallites anddominance of 2M1 polytypes Clays and ClayMinerals 44 257 ndash275

Essene E amp Peacor DR (1995) Clay mineral thermo-metry a critical perspective Clays and ClayMinerals 43 540 ndash553

Fransolet AM amp Bourguignon P (1978) Dioctahedralchlorite in quartz veins from Ardennes BelgiumThe Canadian Mineralogist 16 365 ndash373

Fransolet AM amp Schreyer W (1984) Sudoite ditrioctahedral chlorite a stable low-temperature phasein the system MgO-Al2O3-SiO2-H2O Contributionsto Mineralogy and Petrology 86 409 ndash417

Frey M (1987) Very low-grade metamorphism of clasticsedimentary rocks Pp 9 ndash58 in Low TemperatureMetamorphism (M Frey editor) Blackie GlasgowUK

Giorgetti G Goffe B Memmi I amp Nieto F (1998)Metamorphic evolution of Verrucano metasedimentsin northern Apennines new petrological constraintsEuropean Journal of Mineralogy 10 1295 ndash1308

Guidotti CV amp Sassi FP (1986) Classification andcorrelation of metamorphic facies series by means ofmuscovite b0 data from low grade metapelites NeuesJahrbuch fur Mineralogie Abhandlungen 153363 ndash380

Guidotti CV Yates MG Dyar MD amp Taylor ME(1994) Petrogenetic implications of the Fe3+ contentof muscovi te in pel it ic sch is ts AmericanMineralogist 79 793 ndash795

Guthrie GD amp Veblen DR (1989) High resolutionelectron microscopy of mixed-layer illitesmectite

Computer simulations Clays and Clay Minerals 371 ndash11

Ho N Peacor DR amp Van der Pluijm BA (1996)Contrasting roles of detrital and authigenic phyllo-silicates during slaty cleavage development Journalof Structural Geology 18 615 ndash623

Jiang WT Peacor DR Merriman RJ amp Roberts B(1990) Transmission and analytical electron micro-scopic study of mixed layer illitesmectite formed asan apparent replacement product of diagenetic illiteClays and Clay Minerals 38 449 ndash468

Jiang WT Peacor DR amp Buseck PR (1994) Chloritegeothermometry Contamination and apparent octa-hedral vacancies Clays and Clay Minerals 42593 ndash605

Kisch HJ (1991) Development of slaty cleavage anddegree of very-low-grade metamorphism a reviewJournal of Metamorphic Geology 9 735 ndash750

Knipe JR (1981) The interaction of deformation andmetamorphi sm in slates Tectonophysics 78249 ndash272

Kretz R (1983) Symbols for rock-forming mineralsAmerican Mineralogist 68 277 ndash279

Li G Peacor DR Merriman RJ amp Roberts B (1994a)The diagenetic to low grade metamorphism evolu-tion of matrix white mica in the system muscovite-paragonite in a mudrock from Central Wales UKClays and Clay Minerals 42 369 ndash381

Li G Peacor DR Merriman RJ Roberts B amp Vander Pluijm BA (1994b) TEM and AEM constraintson the origin and significance of chlorite-mica stacksin slates an example from Central Wales UKJournal of Structural Geology 16 1139 ndash1157

Livi KJT Veblen DR Ferry JM amp Frey M (1997)Evolution of 21 layered silicates in low-grademetamorphosed Liass ic sha les of CentralSwitzerland Journal of Metamorphic Geology 15323 ndash344

Lonergan L (1991) Structural evolution of the SierraEspuna Betic Cordillera SE Spain PhD thesisOxford University UK

Lonergan L Platt JP amp Gallagher L (1994) TheInternal-External Zone Boundary in the eastern BeticCordillera SE Spain Journal of Structural Geology16 175 ndash188

Makel GH (1981) Differences in tectonic evolution ofsuperimposed Malaguide and Alpujarride tectonicunits in the Espuna area (Betic Cordilleras Spain)Geology in Mijnbouw 60 203 ndash208

Makel GH (1985) The geology of the MalaguideComplex and its bearing on the geodynamicevolution of the Betic-Rif orogen (southern Spainand northern Morocco) GUA papers of Geology Ser1 22 263 pp

Makel GH amp Rondeel HE (1979) Differences instratigraphy and metamorphism between superposedMalaguide and Alpujarride units in the Espuna area

22 I Abad et al

standards were albite orthoclase periclase wollas-tonite and synthetic oxides (Al2O3 Fe2O3MnTiO3) Data were reduced using the proceduresof Pouchou amp Pichoir (1985)

R E S U L T S

Mineralogy and crystal-chemical parameters

The XRD patterns of bulk samples show thatquartz and white mica primarily phengite are theprincipal phases in all samples trioctahedralchlorite is very common smectite mixed-layeredillite-smectite and intermediate Na-K mica para-gonite feldspars carbonates and hematite arepresent only in some samples (Table 1) A di-trioctahedral chlorite sudoite according to Fransoletand Schreyerrsquos (1984) criteria has been detected insamples from the La Santa and Morron de Totanaunits The XRD patterns of sudoite usually show adistinctly more intense (003) reflection character-istic of Al-rich chlorites In general the mineralassemblages are similar and typical of very low-grade metamorphism

The basal spacing of mica is ~998 AEcirc (s = 001)varying with no clear trend throughout the

sequence An overall average value of 9019 AEcirc (s= 0010) obtained for the b cell parameter(Table 1) is consistent with medium-pressureregional metamorphism according to Guidotti ampSassi (1986)

Figure 2 shows the IC values for the lt2 mm vslt2 mm EG-treated fractions corresponding to all unitsexcept for the Los Molinos Unit for whichparagonite is a major phase whose peaks interferewith measurement of IC values The IC values of thelt2 mm fraction for the Morron de Totana Unitcorrespond to the zone of diagenesis with significantvariation in values in different samples as typical ofthat grade For the La Santa Unit the data indicatelate diagenetic conditions The IC values vary theleast for samples from the Yechar Unit andcorrespond to the low anchizone (according to thenomenclature of Merriman amp Peacor 1999) Inaddition Fig 2 shows that for the Morron de Totanasamples the IC values of the lt2 mm fraction arelarger than those of the EG-treated samples implyingthe presence of smectite layers interstratified withillite The IC values for the bulk samples aregenerally smaller than those of the lt2 mm separatesas consistent with a significant contribution ofdetrital mica in the untreated samples (Table 1)

FIG 2 Plot of IC values for the lt2 mm untreated vs lt2 mm EG-treated fractions

Prograde and retrograde metamorphism 5

Whole-rock analyses

Chemical analyses of the major elements ofselected samples were carried out using X-ray

fluorescence (XRF) in a Philips PW 104010spectrometer at the CIC The data (Table 2) werecompared with the average of Post-Archaean

TABLE 1 Crystal-chemical parameters and bulk mineralogy determined by XRD

White mica Chlorite Mineral compositiond001 Illite crystallinity (CIS) d001 Qtz K-white mica

Samples lt2 mm bulk b lt2 mm lt2 mm EG bulk lt2 mm bulk (all the samples)

Los Molinos UnitEs-101 9972 9966 9012 ndash ndash ndash 142 142 Chl Hem Pg Cc Dol SmEs-102 9978 9980 9002 ndash ndash ndash 142 142 Chl Hem Pg Cc SmEs-103 9970 9975 ndash ndash ndash ndash 142 142 Chl Hem Kfs Pg Cc SmEs-104 9969 ndash ndash ndash ndash ndash 142 ndash Chl Hem Pg Cc Sm

Yechar UnitEs-51 9977 9972 9019 038 035 020 1417 1417 Chl Hem Dol SmEs-52 9974 9970 9016 041 046 022 1417 1416 Chl Kfs SmEs-53 9978 9971 9016 037 040 022 1417 1417 Chl Hem Kfs SmEs-54 9979 9978 9019 041 ndash 025 1416 1416 Chl Hem SmEs-55 9975 9987 ndash 038 041 020 1416 1418 Chl Hem Cc SmEs-56 9957 9967 9010 043 044 022 ndash ndash Hem Cc DolEs-57 9958 ndash 9019 050 049 ndash 1418 ndash Chl Cc Dol

La Santa UnitEs-1 9976 ndash 9019 050 044 071 1417 ndash Chl Hem Kfs SmEs-2 9974 9977 9003 062 049 029 1417 1417 Chl Kfs SmEs-3 9963 ndash 9018 058 050 031 1420 ndash Chl Hem Cc SmEs-4 9987 9987 9007 075 077 049 1416 1417 Chl Hem DolEs-5 9993 9983 ndash 093 ndash 084 1417 1417 Su Chl Hem Ill-Pg PgEs-6 9988 9982 ndash 089 092 078 1417 1416 Chl Hem Cc Dol Ill-PgEs-7 9990 9981 ndash 098 095 069 1418 1419 Chl Hem Cc DolEs-8 9985 9975 9007 066 ndash 046 1417 1416 Chl Hem Cc Dol Ill-Pg

Morron de Totana UnitEs-71 9977 ndash 8994 ndash ndash ndash ndash ndash Chl Kfs HemEs-72 ndash ndash ndash ndash ndash ndash ndash ndash Chl Hem GtEs-73 9970 ndash ndash ndash ndash ndash ndash ndash Kfs Cc DolEs-74 9991 ndash ndash 069 071 ndash ndash ndash Kfs Hem GtEs-75 9972 9991 8989 068 064 ndash ndash ndash Chl Kfs Hem DolEs-76 10005 ndash 064 052 ndash ndash ndash ndash Chl Hem Gt DolEs-77 9979 ndash 9011 077 059 083 ndash ndash Chl Hem Dol I-SEs-78 9972 ndash 084 068 071 ndash ndash ndash Chl Cc Sm GtEs-79 9984 ndash 9028 098 090 061 ndash ndash Chl Kfs Hem DolEs-70 9984 ndash 096 089 035 1416 1418 ndash Dol Chl SmEs-81 9980 ndash ndash 099 061 ndash 142 ndash Chl Hm Cc DolEs-82 9972 ndash ndash 090 083 ndash ndash ndash Chl Hm Cc Dol SmEs-85 9972 ndash 9008 058 050 050 142 ndash Chl Su Hem Sm KfsEs-86 9951 9977 9013 092 084 047 142 ndash Chl Su Hem Sm KfsEs-87 9974 9008 083 ndash 050 142 ndash ndash Chl Su Hem Sm KfsEs-91 9974 ndash ndash 104 108 093 ndash ndash Chl Hem KfsEs-92 9972 ndash 9028 083 074 086 ndash ndash Chl Dol KfsEs-93 9980 9983 ndash 109 096 061 ndash ndash Chl Hem DolEs-94 9964 9985 9024 098 093 080 ndash ndash Chl Hem Dol KfsEs-95 9978 ndash 9027 104 093 ndash ndash ndash Dol

Mineral abbreviations according to Kretz (1983) Ill-Pg Intermediate Na-K mica I-S Illite-smectiteinterstratified layers Su Sudoite In bold samples studied by TEM

6 I Abad et al

Australian Shales (PAAS) (Taylor amp McLennan1985) The values for samples of this study aresimilar to them The loss on ignition (LOI) is verylarge in some samples because of the presence ofcarbonates

Electron microscopy observations (SEM andTEM)

Textural aspects Due to the fine-grainedcharacter of these rocks characterization of texturesis possible only by TEM except for the highestmetamorphic-grade samples (Es-102 and Es-53) forwhich grain sizes are large enough to be resolvedby BSE imaging The HRTEM characterizationconfirms the general mineralogical data determinedby XRD for the different units of the studied areaand have allowed us to clarify some specificrelations eg the mixed-layered illite-smectite inthe Morron de Totana Unit

Los Molinos Unit samples are phyllosilicate-richphyllites with well-developed slaty cleavage(Fig 3a) composed of preferentially oriented micaand chlorite alternating with deformed elongatedquartz grains carbonate aggregates and hematitegrains Paragonite and muscovite coexist as separatecrystals Lattice-fringe images show that both micasand chlorite occur as large defect-free crystalsmore than several thousand angstroms thick

(Fig 4) Lattice-fringe images (not shown) withboth 00l and hkl fringes show that crystals arecomposed virtually entirely of coherently relatedlayers but with some stacking disorder in chloriteThese features are in accord with the typicalgreenschist facies of the Alpujarride Complex(Azanon amp Crespo-Blanc 2000) On the otherhand dioctahedral smectite commonly occursreplacing layers of trioctahedral chlorite describedby Nieto et al (1994) as the result of retrogradealteration by hydrothermal fluids

The sample studied from the basal portion of theYechar Unit (Es-53) is a quartz-rich slate with well-developed cleavage (Fig 3b) High-resolutionimages show that the main phyllosilicate is K-richdioctahedral mica with lesser amounts of chloriteand dioctahedral smectite the smectite occurring asaltered layers of primary metamorphic chloriteAlthough phyllosilicates generally form parallel orlow-angle intergrowths preferentially orientedparallel to cleavage Fig 5 shows a low-magnifica-tion image with well-defined high-angle grainboundaries of interlocking grains Such texturesare typical of a metamorphic origin The packetsare 500 AEcirc thick (Fig 6) much smaller than typicaldetrital grains (gt5000 AEcirc ) They also have composi-tions which differentiate them from detrital micasas described below (see chemical characterizationof phyllosilicates) In general the phyllosilicate

TABLE 2 Whole-rock analyses of major elements (wt oxide)

Samples SiO2 TiO2 Al2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O P2O5 LOI

Es-102 5340 077 1790 557 100 013 158 722 152 254 013 847Es-51 5270 083 1752 441 190 014 514 359 028 503 016 845Es-52 7620 093 1210 070 120 ndash 185 037 143 251 017 239Es-53 5990 089 1890 583 140 ndash 292 021 043 489 014 408Es-56 6640 061 1200 350 020 ndash 190 384 011 427 012 616Es-57 6147 065 1154 369 086 005 354 522 ndash 403 012 909Es-1 6008 086 1820 524 170 ndash 299 029 031 484 014 416Es-2 6510 091 1770 139 190 ndash 322 033 038 427 016 470Es-3 5152 089 2030 590 185 006 429 241 ndash 619 018 642Es-5 6260 090 1750 688 020 ndash 263 030 065 237 012 493Es-6 6891 080 1321 434 086 009 169 293 050 234 010 536Es-7 5158 087 2109 671 142 006 275 379 086 347 018 766Es-75 7238 071 1251 238 118 003 154 143 ndash 337 009 438Es-77 3442 055 1252 516 087 005 956 1337 ndash 361 009 2160Es-79 4996 064 1062 335 129 024 676 935 ndash 301 008 1590Es-86 7600 083 1208 122 144 002 205 048 061 324 014 282Es-87 6285 106 1705 516 175 004 299 034 033 493 018 396Es-91 5881 083 1707 409 151 003 338 217 ndash 569 017 645Es-95 5913 075 1802 368 104 002 410 095 009 717 016 520

Prograde and retrograde metamorphism 7

packets are almost defect-free with only rare layerterminations (Fig 6) The polytypes are 2M formicas and semi-random for chlorites

Two samples have been studied from the LaSanta Unit Sample Es-2 a chlorite-rich shale isfrom the basal part of the unit and sample Es-5 aquartz-rich shale is from the upper part In Es-2elongated quartz-rich and phyllosilicate-richdomains alternate at the scale of millimetresparallel to slaty cleavage The principal phyllosili-cate is muscovite with well-defined 2M polytypismThe (hk0) reflections tend to be spread normal to cmuch more than in sample Es-53 due to the

presence of thinner packets in subparallel orienta-tions The trioctahedral chlorite (clinochlore) isoften altered to dioctahedral smectite as determinedin part by AEM data as a result of retrogradealteration processes (eg Fig 7 in Nieto et al1994) The most common defects are layerterminations voids low-angle grain boundariesand dislocations

Sample Es-5 shows incipient slaty cleavagedefined by subparallel preferentially orientedpackets of illite sudoite trioctahedral chloriteparagonite and intermediate Na-K mica as illu-strated in Fig 7 Hematite is commonly included

FIG 4 Lattice-fringe image showing a portion of a defect-free well-crystallized chlorite packet of metamorphicorigin in phyllite Es-102 (Los Molinos Unit)

FIG 3 BSE images showing the well-developed slaty cleavage of two samples (a) Es-102 (Los Molinos Unit) aphyllite in which the grains with the brightest contrast are hematite and the darkest ones quartz Chlorite andmuscovite grains are characterized by their elongated habit and intermediate brightness (stronger in chlorite thanin muscovite) (b) Es-53 (Yechar Unit) in which stretched quartz grains are more abundant and responsible for itssemipelitic character grains with high contrast are hematite or TiO2 Muscovite crystals are indicated by arrows

8 I Abad et al

within illite packets as elongated individual grainsup to ~3006500 AEcirc in size or as aggregates(Fig 7) they show up clearly due to their darkcontrast Sudoite and chlorite were identified on thebasis of SAED and AEM data Separate grains ofeach were observed to be intergrown with texturestypical of metamorphic origins for both Grain sizesof sudoite (gt500 AEcirc ) and clinochlore are approxi-mately equal implying formation of both at gradeswhich were not substantially different Howeverwhere grains of each intersect at high angles thesudoite ubiquitously transects clinochlore packetsAs shown in Fig 8 clinochlore but not sudoitemay exhibit strain contrast These relations implybut do not prove that sudoite post-dated clino-chlore Obvious alteration of clinochlore todioctahedral smectite was ubiquitous but possiblealteration effects were rare in sudoite occurring asdefects such as voids and wavy layers Because thecomposition of sudoite is similar to that ofdioctahedral smectite it was not possible to verifythe presence of small numbers of smectite layerswith AEM data Nevertheless the observationsclearly show that alteration of chlorite to smectitewas relegated entirely or almost entirely toclinochlore

Muscovite occurs in crystals which average~500 AEcirc in thickness and are characterized by 2Mpolytypism features which are typical of meta-morphic mica of anchizonal grade The Na-K micaoccurs in packets of similar thickness The

FIG 6 Lattice-fringe image showing the relation between mica and chlorite packets The arrows indicate theposition of layer terminations The inset SAED pattern illustrates the low-angle relation between the two phases

(Yechar Unit)

FIG 5 Low-magnification image with well-definedhigh-angle grain boundaries and interlocking relations

among muscovite crystals (Yechar Unit)

Prograde and retrograde metamorphism 9

composition (see below) is quite variable corre-sponding to Na-rich muscovite commonly but somegrains have compositions within the limits of thesolvus between muscovite and paragonite and thusare metastable Na-K micas give SAED patternswhich have relatively sharp reflections over a largerange as consistent with large defect-free crystals

Sudoite crystals comprise units with 14 AEcirc

periodicities which in some areas are 28 AEcirc Lattice-fringe images show the presence of inter-

stratified individual 7 AEcirc layers which in some areasform small packets containing no more than eight7 AEcirc layers The AEM analyses (see below) of suchareas are typical of those of homogeneous sudoitecontaining only 14 AEcirc layers implying a composi-tion of the 7 AEcirc layers similar to that of sudoite andnot to an exotic septechlorite such as kaolinite orberthierine The contrast of both 14 AEcirc and 7 AEcirc

fringes appear to be identical further consistentwith a close chemical relation

FIG 7 Textural relations between phyllosilicates (sudoite illite Na-K mica) and hematite (dark contrast) (LaSanta Unit)

FIG 8 Lattice-fringe image showing the coexistence of two types of chlorite trioctahedral chlorite (chl) andsudoite (su) Sudoite transects chlorite which shows strain contrast associated with curved and kinked layersIdentifications were based in part on AEM analyses (see Table 6 Es-511 and 12 analyses) (La Santa Unit)

10 I Abad et al

Three samples corresponding to different butequivalent levels were selected from the Morron deTotana Unit for TEM study (see Fig 1 peliticrocks intercalated with carbonate rocks) Es-77 atthe bottom of the unit Es-86 which is located in anintermediate position and Es-95 from the top of thecross-section There is no incipient slaty cleavage inthese samples Sample Es-77 is a very fine-grainedrock consisting of quartz white mica chlorite andaggregates of carbonate grains Minor hematiteproduces a deep red colour Lattice-fringe images

show that dioctahedral clay minerals have 10 and20 AEcirc periodicities occur as packets 100 ndash200 AEcirc

thick and have subparallel orientations Accordingto Guthrie amp Veblen (1989) smectite and illiteinterlayers can produce different contrast and canonly be differentiated in lattice fringe-imagesobtained with well-defined electron beam (001)orientations and defocus conditions Such contrastdifferences were observed in several lattice-fringeimages The light contrast for fringes in Fig 9represents the smectite interlayer as shown in part

FIG 9 TEM image showing 10 AEcirc periodicity in R4 illite-rich I-S Contrast differences imply the presence ofillite-like (the dark fringes) and smectite-like (the light fringes) layers Packets usually have four or more illite

layers and the SAED pattern shows predominantly 50 AEcirc periodicity (Morron de Totana Unit)

FIG 10 Trioctahedral chlorite with intercalations of 10 AEcirc layers (upper arrow) which damage quickly under theelectron beam The chlorite packets show strain contrast microfolds and 7 AEcirc periodicities corresponding toberthierine (left arrow) or greater values eg 28 AEcirc (see analysis in Table 6 Es-867) (Morron de Totana Unit)

Prograde and retrograde metamorphism 11

by the continuity of such fringes with openinterlayer spaces produced by contraction due to

dehydration The sequence on average is that ofR4 mixed-layering ie a single smectite-likeinterlayer between four or more illite-like inter-layers Although some packets with more than fourlayers of illite are present repeats of four illite-likelayers predominate as shown by the 50 AEcirc

periodicity of SAED patterns (Fig 9 inset) Theillite-smectite (I-S) has typical 1Md polytypism Theobservation of smectite interstratified with illite isin accord with the changes observed in XRDdiagrams after EG treatment (Fig 2) In additionAEM analyses of areas similar to that of Fig 9have relatively high Si and minor Ca contentsfurther confirming the mixed layering

Some packets which give rise to fringes with10 AEcirc spacing produce SAED patterns typical oftwo-layer polytypism The contrast of all fringes isrelatively constant Such packets have thicknessestypical of diagenetic clays and two orders ofmagnitude smaller than typical detrital grains Thecompositions of such grains (see below) haveinterlayer-cation contents typical of illite They aretherefore inferred to be illite which formed duringlate diagenesis and coexisting with the R4 I-S

Sample Es-86 is composed mainly of detritalgrains of quartz randomly-oriented grains ofchlorite and white mica all immersed in aphyllosilicate-rich fine-grained matrix At theTEM scale chlorite is the most common phyllosi-licate in the matrix As in sample Es-5 trioctahe-dral chlorite and sudoite coexist There areinterstratified packets of 10 AEcirc layers only a fewlayers thick (lt150 AEcirc ) which are quickly damaged

FIG 11 Di-trioctahedral chlorite (sudoite) very similarin appearance to the trioctahedral chlorite shown inFig 10 7 AEcirc layers are marked by an arrow and 10 AEcirc

layers are present in the poorly resolved area in thecentre of the image (see analysis in Table 6 Es-862)

(Morron de Totana Unit)

FIG 12 Low-magnification image illustrating illite-like packets The inset SAED pattern shows only a smallnumber of diffuse poorly defined reflections The packets are 100 ndash200 AEcirc thick (Morron de Totana Unit)

12 I Abad et al

under the electron beam (Figs 10 and 11) TheAEM analyses of such material show an enhancedSi content and the presence of small concentrationsof alkali cations These data are consistent withretrograde alteration of chlorite as observed byNieto et al (1994) They differ however from theresults of Nieto et al (1994) and those describedabove for sample Es-5 in that both tri-trioctahedraland di-trioctahedral chlorite are affected In TEMimages strain features in chlorites such as contrastnormal to the layers or microfolds were observed(Fig 10) Mica is less abundant than sudoiteoccurring in packets 100 ndash400 AEcirc thick The SAEDpatterns exhibit 2M polytypism but with diffuse-ness parallel to c in 0kl rows At low magnifica-tion subparallel and slightly curved illite- or illite-smectite-like packets are present they coalescehave abundant layer terminations and show intensecontrasts (Fig 12)

Sample Es-95 is a very fine-grained rock madeup of large detrital grains of quartz in a fine-grained matrix of phyllosilicates mainly whitemicas and opaque minerals Some biotite grainswere observed as verified by AEM data Biotitewas observed only in separated grains on holey

C-coated Cu grids so it is not possible to describethe textural relations for it Authigenic biotite isinconsistent with the low grade of this samplehowever and it is therefore inferred to be of detritalorigin The AEM analyses show deviations frompure biotite however as consistent with alterationto chlorite the usual process in detrital biotite inrocks undergoing diagenesis Lattice-fringe imagesof ion-milled samples show that individual micapackets are very thin with no more than 10 layersisolated and dispersed in a matrix consisting ofmaterial which gives no SAED pattern Thesepackets are wavy discontinuous and with anasto-mosing layers (Fig 13) Despite these texturalfeatures SAED data indicate that there are 2Mand 1Md polytypes

For all samples prismatic crystals of Ti-oxidedispersed Fe-oxyhydroxides zircon and tourmalineare common accessory minerals

Chemical characterization of phyllosilicates

Dioctahedral mica Table 3 shows the composi-tion of K-rich dioctahedral micas of authigenic ormetamorphic origin corresponding to the samples

FIG 13 General aspect of sample Es-95 The image shows arrays of wavy discontinuous and anastomosingpackets of illite and smectite (Morron de Totana Unit)

Prograde and retrograde metamorphism 13

selected from each unit The Yechar and LosMolinos units are characterized by compositionsapproaching those of evolved micas with a sum ofinterlayer cations slightly less than 1 apfu typicalof phengite or muscovite rather than authigenic

illite In contrast the La Santa and Morron deTotana units show heterogeneous mica composi-tions and display deficiencies in interlayer cations(Fig 14a) This last feature is more evident in theMorron de Totana analyses where the illitic

TABLE 3 Representative chemical compositions for K-rich dioctahedral micas normalized to O10(OH)2

Si IVAl VIAl Fe Mg Ti VIS K Na S inter

Los Molinos Unit (1)Es-10213 337 063 178 010 008 002 198 075 008 085Es-10221 308 092 195 005 001 001 202 082 009 091Es-10231 319 081 191 005 003 001 200 077 012 090Es-10232 314 086 188 006 007 001 202 072 021 093

Yechar Unit (1)Es-5311 332 068 163 015 022 002 204 090 004 093Es-5312 310 090 180 003 013 004 201 089 010 099Es-5322 338 062 170 012 015 003 200 086 001 086Es-5323 319 081 176 016 012 003 206 087 002 089

Yechar Unit (2)Es-53 1 317 083 163 021 026 000 210 084 000 084Es-53 2 320 080 165 011 027 003 207 087 000 087Es-53 3 317 083 155 025 024 002 206 093 000 093Es-53 4 328 072 160 020 024 000 203 092 000 092Es-53 5 326 074 164 019 016 000 199 096 000 096Es-53 6 326 074 158 020 028 000 206 089 000 089Es-53 7 319 081 154 025 026 000 205 097 000 097Es-53 8 325 075 166 016 025 000 206 085 000 085Es-53 9 324 076 167 019 019 000 205 085 000 085

La Santa Unit (2)Es-2 1 311 089 157 019 032 002 210 088 005 094Es-2 3 336 064 148 020 043 001 212 072 003 075Es-2 6 313 087 156 024 029 001 210 075 017 092Es-2 7 313 087 155 024 029 001 209 084 011 095Es-2 8 331 069 157 015 032 000 205 087 004 092Es-2 10 328 072 160 018 027 003 208 068 008 075Es-5 4 303 097 174 015 023 001 213 073 009 082Es-5 10 301 099 189 006 015 000 210 065 021 086Es-5 7 307 093 159 033 016 000 208 075 018 093Es-5 8rsquo 301 099 160 031 025 000 216 067 016 083

Morron de Totana Unit (2) (analyses corresponding to R4 Ill-Sm may also be present see text)Es-77 13 327 073 172 013 027 000 212 067 000 067Es-77 15 316 084 195 003 013 000 211 063 000 063Es-77 16 328 072 182 010 015 000 207 068 000 068Es-77 17 338 062 147 032 019 000 199 093 000 093Es-86 9 334 066 160 017 034 000 210 072 000 072Es-86 10 357 043 151 012 046 000 209 065 000 065Es-86 11 353 047 156 012 036 000 203 077 000 077Es-86 13 324 076 152 025 035 000 211 084 000 084Es-86 14 324 076 173 013 019 000 205 084 000 084Es- 95 2 323 077 158 016 019 004 198 103 000 103Es- 95 5 336 064 169 013 025 000 207 070 000 070Es- 95 6 336 064 148 027 033 000 209 078 000 078

(1) SEMEDX analyses (2) TEMAEM analyses

14 I Abad et al

character is consistent with the presence of the R4mixed-layer illite-smectite described above Most ofthe micas are phengitic in composition (Fig 14ab)with an Fe(Fe+Mg) ratio that varies over the range02 ndash085 However the detrital-like grains aresimilar to those of end-member muscovite (seebelow detrital muscovite vs metamorphic musco-vite or Table 5) with much lower Fe and Mgcontents Data plotted in Fig 14bc confirm theassumption of a significant ferrimuscovitic substitu-tion (Guidotti et al 1994) Some of the K-micaanalysed in the Los Molinos and La Santa unitswere found to have significant Na contents (up to02 apfu) Although Ti is unusual in dioctahedralmicas it has been measured in several analyses

(lt005 apfu) The most evident feature in theplots is the scattering of data in the La Santa andMorron de Totana units ie where diageneticconditions are not exceeded In contrast theYechar Unit analyses define a narrow field in thediagrams and the Los Molinos data are even closerto the theoretical muscovite composition (Table 3)

Typical compositions of micas intermediate tomuscovite and paragonite called Na-K micas in thispaper are shown in Table 4 The compositions arequite variable even in the same sample Whencompared with analyses of K-rich dioctahedralmicas (Table 3) and paragonite analyses are seento span the entire range from muscovite toparagonite Some analyses (eg Es-5 9) fall within

FIG 14 Plots of values of chemical compositions of K-rich dioctahedral phyllosilicates solid circles and linesindicate values for theoretical muscovite and corresponding exchange vectors respectively

Prograde and retrograde metamorphism 15

the limits of the solvus between muscovite andparagonite the limbs of that solvus having theapproximate compositions ms60pg40 and ms20pg80

(Li et al 1994a)Detrital muscovite vs metamorphic muscovite

Detrital-like micas of sample Es-95 the lowest-grade one are large enough to be analysed(Table 5) by electron microprobe (EMPA) Thesedata have been compared with the AEM data ofother samples Compositions of detrital-like grainsof the lowest-grade sample and the detrital micas ofthe anchizonal-grade sample Es-53 (Fig 15) arevery similar and close to that of end-membermuscovite These data suggest that the detrital micahas a common source for the different units thatcomprise Sierra Espuna Figure 15 also comparesthe data from authigenic micas of the samesamples which are clearly more phengitic (Fe+Mg= 015 ndash075 apfu) than the detrital ones (Fe+Mg= 01 ndash02 apfu)

Chlorite Trioctahedral and di-trioctahedral(sudoite) chlorites have been analysed (Table 6)The former corresponding to the species clino-chlore is common in most of the samples unlikesudoite which is absent from most (Table 1)Sudoite was identified because the sum ofoctahedral cations is nearly equal to 5 apfu andthe Si and Al contents are higher than intrioctahedral chlorite (Fig 16a) A slight contam-ination by interlayer cations (K and Na) has beendetected therefore the formulae were recalculatedin order to eliminate those impurities (seeAnalytical Methods) Although total Fe has beenconsidered as Fe2+ according to Fransolet ampBourguignon (1978) the Fe3+ content in sudoitemay be greater than the Fe2+ content As severalanalyses have an octahedral sum slightly higherthan the ideal value of 5 apfu this may indicate atrioctahedral substitution andor the presence ofsignificant Fe3+ No Ti or Mn were detected The

Fe(Fe+Mg) ratios do not define a clear trendthroughout the sequence (Fig 16b) nevertheless itis much greater in chlorite in the Morron de Totanaanalyses falling in the characteristic range ofdiagenetic chlorites (Li et al 1994b) There is anincrease in Mg content (from 15 to 4 apfu)concomitant with increase in grade as compatiblewith some other sequences (eg Bevins et al1991)

D I S C U S S I O N

Dioctahedral phyllosilicates chemicalcompositions IC values and polytypes

On the basis of their large size homogeneity ofindividual grains and well-crystallized aspectdetrital dioctahedral mica grains were identified inseveral samples with the exception of those ofgreenschist facies This confirms the idea of the

TABLE 4 Chemical compositions for Na-K micas normalized to O10(OH)2

Si IVAl VIAl Fe Mg Ti VIS Ca K Na S inter

Es-5 14 313 087 190 004 009 002 204 000 034 048 083Es-5 15 321 079 176 009 037 000 222 000 015 038 053Es-5 4 318 082 187 004 019 000 209 000 013 061 074Es-5 12 329 071 172 008 029 000 208 000 039 036 076Es-5 13 323 077 179 005 028 000 212 000 029 041 069Es-5 9 296 104 163 019 029 000 211 000 048 056 104Es-5 8 292 108 156 043 008 001 209 003 014 080 096

FIG 15 Plot of Si vs Al contents of detrital andauthigenic micas of samples Es-95 and Es-53 Solidsymbols = detrital micas open symbols = authigenic

micas

16 I Abad et al

relative stability of muscovite in most diageneticenvironments (Crowley 1991) The compositions ofthe detrital grains of samples from different unitsare similar The lack of heterogeneity which isexpected for mica from a complex source implies acommon source area for all samples

The IC values define a trend from diagenetic toanchizonal conditions but with smaller IC values inbulk-rock fractions consistent with the contributionof detrital mica The coexistence of mature micawith lower-grade I-S or illite in the sub-greenschistfacies rocks shows that those rocks have not

TABLE 5 EMPA data for detrital-like crystals of mica normalized to O10(OH)2

Si IVAl VIAl Fe Mg Ti VIS Ca K Na K+Na+Ca

Es-95z11 313 087 193 005 004 001 203 000 065 022 087Es-95z11 314 086 190 006 007 001 204 000 068 019 087Es-95z1 2 314 086 191 005 009 001 206 000 070 006 077Es-95z2 1 314 086 176 012 010 003 201 002 095 004 101Es-95z2 2 317 083 177 012 011 003 202 000 095 004 099Es-95z4 1 305 095 188 005 007 004 204 000 081 009 089Es-95z4 2 311 089 187 006 007 004 204 000 078 009 086

TABLE 6 Representative chemical compositions for chlorite normalized to O10(OH)8

Si IVAl VIAl Fe Mg VIS FeFe+Mg

Yechar UnitEs53-1 273 127 112 121 376 609 024Es53-4 287 113 125 070 400 596 015Es53-12 303 097 130 146 307 583 032Es53-13 277 123 125 122 354 601 026Es53-15 307 093 146 094 323 564 023Es53-42 271 129 152 160 294 606 035

La Santa UnitEs2-4 319 081 147 148 273 569 035Es2-5 365 035 183 118 225 526 034Es5-1 354 046 293 023 172 488 012Es5-5 361 039 292 021 178 490 010Es5-10 309 091 262 028 221 511 011Es5-16 310 090 197 112 234 543 032Es5-17 350 050 252 076 170 499 031Es-5 10rsquo 364 036 213 097 202 512 032Es-5 11 291 109 143 136 314 593 030Es-5 12 382 018 229 087 179 495 033Es-5 13 316 084 302 021 172 494 011Es-5 16rsquo 346 054 341 013 101 455 011Es-5 17rsquo 300 100 282 033 194 509 014Es-5 18 285 115 285 026 204 515 011

Morron de Totana UnitEs-86 2 377 023 253 088 149 490 037Es-86 4 284 116 166 230 181 578 056Es-86 5 350 050 191 192 151 534 056Es-86 6 366 034 189 191 149 529 056Es-86 7 315 085 129 273 176 578 061

indicates sudoite analyses

Prograde and retrograde metamorphism 17

reached stable chemical or textural equilibrium atleast at the mm scale In addition the lack ofchemical equilibrium among diagenetic I-S andillite at the nm scale is demonstrated by their highlyvariable Fe+Mg contents (006ndash075 apfu) Theaverages of the sum of interlayer cations are closeto 1 apfu in the Los Molinos and Yechar units asconsistent with mica but values of 06 apfucorresponding to I-S or illitic compositions are alsopresent in the Morron de Totana Unit Except forsamples Es-102 and Es-53 the variable composi-tions represent a metastable state with composi-tions of micas determined by a combination ofphengitic ferrimuscovitic and illitic vectors asshown by Fig 14ab

Paragonite is generally considered an indexmineral for the anchizone (Frey 1987) but Li etal (1994a) have shown that paragonite coexistswith K-rich mica in grades as low as that of the latediagenetic zone for a prograde sequence Samplesin which muscovite paragonite and intermediateNa-K mica coexist as in sample Es-5 areconsistent with metastability both because morethan two such micas coexist and because theircompositions are incompatible with the solvus inthe muscovite-paragonite system (see below) Whilethe 2M polytype typical of metamorphic micas is

the most common in these samples the 1Md

polytype also occurs in some diagenetic samplesparticularly those from the Morron de Totana UnitThe 1Md polytype is metastable relative to the 2Mpolytype and the coexistence of both polytypesfurther demonstrates a lack of equilibrium at the nmscale Dong amp Peacor (1996) showed that thedisorder noted in the diagenetic phyllosilicates bySAED is a consequence of the general high entropyof phases formed at very low temperatures thatproduces for example stacking defects separatingpackets with no more than 10 layers These packetsprogressively thicken with increasing metamorphicgrade and the 2M polytype then becomes the mostcommon polytype in authigenic micas Thenm-scale size of the lower-grade clays and theregular increase in size with increasing gradereflect the lack of textural equilibrium in the low-grade samples

A wide range of sample characteristics thusimply that the lower-grade samples are in statesof metastable equilibrium those factors includingcoexistence of mature mica with I-S and illitevariable chemical compositions incompatiblecompositions of white micas small crystal sizesdefect-rich states and polytypism Such dataemphasize the notion of lsquoreaction progressrsquo relative

FIG 16 (a) Plot of Al vs Fe+Mg contents of chlorites illustrating the contrast in composition between tri-trioctahedral clinochlore or chamosite and di-trioctahedral sudoite (b) plot of Si vs Fe(Fe+Mg) ratio of

chlorites showing an absence of compositional trends throughout the sampled sequence

18 I Abad et al

to Ostwald-step rule processes metastable dioctahe-dral clay systems undergoing reactions toward thestate of stable chemical and textural equilibriumapproached in greenschist-facies samples (Essene ampPeacor 1995)

Prograde evolution based on TEM datacorrelation with the Basque-Cantabriansequence (Northern Spain)

With increasing grade TEM data confirm thatcrystal thickness increases crystal defect densitydecreases compositions become less variable andmicrotextural relations are simpler (eg Peacor1992) This trend is consistent with three well-characterized low-grade settings the Gulf Coastsequence (Ahn amp Peacor 1986) and the WelshBasin (Merriman et al 1990) both extensionalbasins and the Helvetic Alps (Livi et al 1997)The latter is a fold-and-thrust belt with similarseries of prograde transitions to those observed inSierra Espuna in which pelites have passed througha sequence of metastable states in the lithologicalsequence mudstone and shale slate and phyllite

The mudstones of the Morron de Totana Unit arecharacterized by the presence of illite-smectite R4mixed-layers mica packets of no more than 10 ndash40layers and the occurrence of 1Md polytypism TheLa Santa Unit samples represent a step in theprograde evolution with thicker packets than in theprevious samples (200 ndash700 AEcirc ) illitic or phengiticcompositions the absence of illite-smectite mixedlayers as well as the presence of discreteintermediate Na-K micas and paragonite Althoughthere is a tendency towards non-preferred orienta-tion of crystals it is not so evident as in the Morronde Totana samples In the Yechar Unit (anchizone)and more clearly in the Los Molinos phyllites(greenschist facies) the microtextural relations areless complex the compositions are less variablewith phengite and trioctahedral chlorite crystalthickness are greater (gt400 AEcirc in the Yechar Unitand gt1000 AEcirc in the Los Molinos Unit) with thepackets being defect-free and with a very crystallineaspect This description has features in commonwith the three well-characterized low-grade settingspreviously noted Only the mineral assemblagesvary somewhat presumably as a function of thedifferences in bulk-rock composition

A detailed comparison with the Basque-Cantabrian Basin (N Spain) shows that the lowest-grade conditions of Sierra Espuna overlap most of

those of that sequence which is characterized bypassive burial diagenesis (8000 m thick) (Nieto etal 1996) Only in the deepest sample of theBasque-Cantabrian sequence is illite-smectite mixedlayering absent Figure 12 of this study is verysimilar to Fig 7 in Nieto et al (1996) and bothimages correspond to diagenetic samples fromdifferent settings nevertheless they illustratesimilar mica packets 100 ndash200 AEcirc thick withparallel-to-subparallel boundaries the orientationof which is inferred to represent beddingTherefore there is a parallelism between thesesequences down to the scale of individual clay-mineral grains even though they developed underdifferent conditions with stress presumably playinga significant role in the Sierra Espuna units Thetexture of the least metamorphosed sample (Fig 13)is reminiscent of the typical textural relations ofsmectite described by Nieto et al (1996) (seeFig 4) Although it corresponds to illite-richinterstratified illite-smectite the clay mineralgrains conserved the grain size and anastomosingdiscontinuous array of original smectite layers

Despite the similarities between the twosequences the chemical behaviour of the micasare clearly different since the grade correspondingto the phengitic compositions of the Yechar Unit isnot reached in the Basque-Cantabrian Basin wherethe deepest and highest-grade sample has illiticcompositions equivalent to those of the La SantaUnit That is in a passive burial metamorphiccontext as in the Basque-Cantabrian Basin thefactors responsible for increasing the rate ofchemical reactions are less significant than in thesequences that were affected by tectonic stressTectonic stress introduces defects which in turnincreases the rate of dissolution of strained grainsthus increasing the rate of dissolution-crystallizationreactions typical of clay minerals Perhaps moreimportantly regional tectonic stress is known tocause mobilization of fluids (eg Oliver 1986) Thepresence and activity of such fluids is a prime causeof increased rates of dissolution ion diffusion andcrystallization Tectonic activity thus may have atwo-fold effect in increasing rate of clay-mineralreactions The phengite component may in part be ameasure of the reaction progress depending on thereaction kinetics in very low-grade rocksAccording to Merriman amp Peacor (1999) whereheat flow is low as in accretionary settingsphengite contents are typically high perhapsbecause there is insufficient thermal energy to

Prograde and retrograde metamorphism 19

cause equilibration of the phengite component ofneoformed illite by contrast with extensional basinswhere this component is typically small

Coexistence of trioctahedral chlorite andsudoite Implications for multiple metamorphicevents

In metamorphic rocks the coexistence of mineralsthat formed at different times and conditions on aPTt path ie not in equilibrium is a commonlyobserved feature where the metamorphic history iscomplex Gross differences in the grain size ofcoexisting phases has long been recognized bymetamorphic petrologists as an indicator of non-equilibrium mineral assemblages Historically muchless attention has been paid to such relations in sub-greenschist-facies rocks because prior to the intro-duction of SEM+TEM techniques their small grainsize prevented observations of textural relationsStudy of such rocks eg by XRD provides littleinformation on textural relations beyond determina-tion of degree of preferred orientation

Study by XRD of the samples showed thatsudoite and trioctahedral chlorite coexist If theyhad formed at the same time and under the sameP-T conditions their compositions would define thelimbs of a solvus The TEM images show howeverthat wherever they are in contact the sudoitetransects the packets of trioctahedral chlorite (egFig 8) Furthermore as illustrated in Fig 8 thetrioctahedral chlorite is commonly deformed anddisplays strain contrast in TEM images Thesetextural relations suggest that sudoite was intro-duced subsequent to the formation of trioctahedralchlorite Similar observations were made byGiorgetti et al (1998) who concluded that texturesin Verrucano metasediments showed that sudoitepost-dated chloritoid pyrophyllite and trioctahedralchlorite ie sudoite is in disequilibrium with thelatter minerals and formed as a retrograde phase Inrocks of the Sierra Espuna area differences in bulk-rock composition are small (Table 2) and do notcorrespond to the presence or absence of sudoite(Table 1) ie the random occurrence of sudoiteindependent of bulk-rock composition is compa-tible with a lack of equilibrium relations Theserelations collectively imply that sudoite wasintroduced at some time subsequent to theformation of mineral assemblages at peak meta-morphic conditions On the other hand the size ofsudoite packets is similar to that of trioctahedral

chlorite as consistent with formation at conditionsnot much different from those for chlorite

In samples from the Morron de Totana bothsudoite and trioctahedral chlorite were affected byretrograde alteration to smectite The TEM imagesshow sudoite (Fig 11) and trioctahedral chlorite(Fig 10) with interstratified layers of smectite withspacings of 10 AEcirc due to collapse and dehydration inthe TEM environment and even packets of smectite150 AEcirc thick within chlorite crystals These relationsimply that both sudoite and trioctahedral chloritehave been affected by retrograde alteration at thevery low temperatures consistent with smectiteformation although the presence of more subtlealteration features in sudoite of some samplessuggests that it was less susceptible to alteration

We therefore conclude that at least three episodesof phyllosilicate formation can be recognized inrocks from the Sierra Espuna-area Trioctahedralchlorite was presumably produced at the highestmetamorphicdiagenetic grade in equilibrium withparagonite and phengite Subsequently sudoite wasintroduced but at P-T conditions which could nothave been much different than those of peakmetamorphic grade Finally at very low-T condi-tions hydrothermal fluids caused partial alterationof trioctahedral chlorite and sudoite to smectite

Retrograde products formed by the reaction offluids with mineral phases generated during anearlier prograde event were described previouslyby Nieto et al (1994) and Zhao et al (1999) Thosereactions occurred on a regional scale but similarreactions have been observed on a more localizedscale The fluid activity associated with a majorfault zone is a common source for these reactions(eg Jiang et al 1990) The structure of SierraEspuna (Fig 1) is based on imbrication processesthat superimpose units that have subsequently beenaffected by faults and folds and that have under-gone significant uplift at least 800 m from the lateMiocene to the present (Sanz de Galdeano et al2001) Given this evolution it is easy to justify theretrograde processes that usually seem to be relatedwith tectonic events during which fluid flow andfluidrock ratios are enhanced (Merriman amp Peacor1999) Moreover according to Nieto et al (1994)the uplift of the Betic Cordillera would furnish theintroduction of phreatic water into the systemproducing a reactive environment adequate for thegenesis of smectite Geological conditions aretherefore consistent with post-peak metamorphicconditions as implied by the TEM observations

20 I Abad et al

The introduction of sudoite as a major phaseduring metamorphism implies the possibility ofadditional overprints on a simple prograde assem-blage the evidence for which might be subtle TheNa-rich micas at all grades have compositionswhich either fall within the solvus or are too Na-rich to correspond to the limb of the solvus In thehighest-grade rocks such micas coexist withmuscovite Those relations are incompatible withchemical equilibrium despite the fact that relationsfor muscovite and other minerals imply an approachto chemical equilibrium Furthermore there is aclose association of Na-rich micas with sudoite (egFig 7) Those relations imply the possibility thatNa-micas were introduced concomitant withsudoite P Mata (pers comm) has shown forexample that overprinting of an epizonal assem-blage by hydrothermal solutions resulted in theintroduction of both an Al-rich variety of chlorite(cookeite) and paragonite in pelites of the CamerosBasin Spain The textural relations such as cross-cutting textures which are so striking for sudoitewere not observed for Na-rich micas Neverthelessthe possibility of such overprinting must beconsidered

In spite of all these prograde-retrogradeprocesses in the Los Molinos Unit a smallnumber of homogeneous phases coexist in chemicalequilibrium and the fine-grained metamorphicphyllosilicates are clearly parallel to cleavage witha typical metamorphic aspect Finally all thetextural and compositional changes that haveoperated in the Espuna area during the metamorphicevolution proceed toward a simpler system repre-senting both textural and chemical equilibriumwhich is characteristic of greenschist-facies condi-tions

Roles of detrital and authigenic phyllosilicatesduring slaty cleavage development

Van der Pluijm et al (1998) focused on therespective roles of the phyllosilicates of detritalorigin and those formed during diagenesis andmetamorphism and observed that detrital phyllosi-licates remain preferentially oriented parallel tobedding in pelites with slaty cleavage whereas fine-grained metamorphic phyllosilicates are orientedparallel to cleavage

Although it is usually difficult to determine thepreferred orientations of phyllosilicates in TEMimages as the grade increases better-aligned

packets occur in the cleavage orientation Thetexture of the least metamorphosed sample inwhich even incipient slaty cleavage is absent isreminiscent of the typical textural relation ofsmectites described in the literature The illite-smectite mixed-layers conserve the textural featuresof possible previous smectite such as anastomosingfringes curved and discontinuous with variations ininterplanar spacing d001 These features indicatethat the minimum energy conditions to initiatemechanical transformations have not been reachedin the lower-grade samples as postulated by Ho etal (1996) since slaty cleavage develops as a resultof interactive mechanical deformation and ther-mally driven dissolution and crystallizationprocesses (Knipe 1981) In contrast in the LosMolinos Unit large crystals of chlorite and whitemica (gt1000 AEcirc ) occur which are of metamorphicorigin but with no detrital grains (Fig 3a)Phyllosilicates are preferentially oriented parallelto slaty cleavage in response to increasedtemperature and tectonic strain (Merriman ampPeacor 1999) The oriented microfabrics and slatycleavage well developed in the rocks of this unitmust be a product of a higher thermal and strainenergy environment reached in the deepest part ofthe sequence As pointed out by Van der Pluijm etal (1998) a continuous range of orientations isevidence for mechanical rotation of grains whereasthe lack of such transitional orientations impliesthat cleavage-parallel phyllosilicates originatedthrough dissolution of bedding-parallel and crystal-lization in the slaty cleavage orientation Asobserved in this study for the Intermediate Unitsgrains with orientations which are transitional frombedding-parallel to slaty cleavage orientation areonly rarely observed Cleavage formation is there-fore inferred to have occurred entirely or nearlyentirely through dissolution and crystallization

ACKNOWLEDGMENTS

We thank MM Abad Ortega from the Centro deInstrumentacio n Cient otilde fica of the Universidad deGranada for her help with HRTEM and I Nieto forher essential help with sample preparation Financialsupport was supplied by Research Project nordm BT 2000-0582 FPI research grant to IA both of the SpanishMinistry of Science and Technology Research GroupRNM-0179 of the Junta de Andaluc otildeacutea and to DRP byNSF grant EAR-9814391

Prograde and retrograde metamorphism 21

REFERENCES

Ahn JH amp Peacor DR (1986) Transmission andanalytical electron microscopy of the smectite-to-illite transition Clays and Clay Minerals 34165 ndash179

Azanon JM amp Crespo-Blanc A (2000) Exhumationduring a continental collision inferred from thetectonometamorphic evolution of the AlpujarrideComplex in the central Betics (Alboran Domain SESpain) Tectonics 19 549 ndash565

Bevins RE Robinson D amp Rowbotham G (1991)Compositional variations in mafic phyllosilicatesfrom regional low-grade metabasites and applicationof the chlorite geother mometer Journal ofMetamorphic Geology 9 711 ndash721

Cliff G amp Lorimer GW (1975) The quantitativeanalysis of thin specimens Journal of Microscopy 103 203 ndash207

Crowley SF (1991) Diagenetic modification of detritalmuscovite an example from the Great LimestoneCyclothem (Carboniferous) of Co Durham UKClay Minerals 26 91 ndash103

Dong H amp Peacor DR (1996) TEM observations ofcoherent stacking relations in smectite and illite ofshales evidence for MacEwan crystallites anddominance of 2M1 polytypes Clays and ClayMinerals 44 257 ndash275

Essene E amp Peacor DR (1995) Clay mineral thermo-metry a critical perspective Clays and ClayMinerals 43 540 ndash553

Fransolet AM amp Bourguignon P (1978) Dioctahedralchlorite in quartz veins from Ardennes BelgiumThe Canadian Mineralogist 16 365 ndash373

Fransolet AM amp Schreyer W (1984) Sudoite ditrioctahedral chlorite a stable low-temperature phasein the system MgO-Al2O3-SiO2-H2O Contributionsto Mineralogy and Petrology 86 409 ndash417

Frey M (1987) Very low-grade metamorphism of clasticsedimentary rocks Pp 9 ndash58 in Low TemperatureMetamorphism (M Frey editor) Blackie GlasgowUK

Giorgetti G Goffe B Memmi I amp Nieto F (1998)Metamorphic evolution of Verrucano metasedimentsin northern Apennines new petrological constraintsEuropean Journal of Mineralogy 10 1295 ndash1308

Guidotti CV amp Sassi FP (1986) Classification andcorrelation of metamorphic facies series by means ofmuscovite b0 data from low grade metapelites NeuesJahrbuch fur Mineralogie Abhandlungen 153363 ndash380

Guidotti CV Yates MG Dyar MD amp Taylor ME(1994) Petrogenetic implications of the Fe3+ contentof muscovi te in pel it ic sch is ts AmericanMineralogist 79 793 ndash795

Guthrie GD amp Veblen DR (1989) High resolutionelectron microscopy of mixed-layer illitesmectite

Computer simulations Clays and Clay Minerals 371 ndash11

Ho N Peacor DR amp Van der Pluijm BA (1996)Contrasting roles of detrital and authigenic phyllo-silicates during slaty cleavage development Journalof Structural Geology 18 615 ndash623

Jiang WT Peacor DR Merriman RJ amp Roberts B(1990) Transmission and analytical electron micro-scopic study of mixed layer illitesmectite formed asan apparent replacement product of diagenetic illiteClays and Clay Minerals 38 449 ndash468

Jiang WT Peacor DR amp Buseck PR (1994) Chloritegeothermometry Contamination and apparent octa-hedral vacancies Clays and Clay Minerals 42593 ndash605

Kisch HJ (1991) Development of slaty cleavage anddegree of very-low-grade metamorphism a reviewJournal of Metamorphic Geology 9 735 ndash750

Knipe JR (1981) The interaction of deformation andmetamorphi sm in slates Tectonophysics 78249 ndash272

Kretz R (1983) Symbols for rock-forming mineralsAmerican Mineralogist 68 277 ndash279

Li G Peacor DR Merriman RJ amp Roberts B (1994a)The diagenetic to low grade metamorphism evolu-tion of matrix white mica in the system muscovite-paragonite in a mudrock from Central Wales UKClays and Clay Minerals 42 369 ndash381

Li G Peacor DR Merriman RJ Roberts B amp Vander Pluijm BA (1994b) TEM and AEM constraintson the origin and significance of chlorite-mica stacksin slates an example from Central Wales UKJournal of Structural Geology 16 1139 ndash1157

Livi KJT Veblen DR Ferry JM amp Frey M (1997)Evolution of 21 layered silicates in low-grademetamorphosed Liass ic sha les of CentralSwitzerland Journal of Metamorphic Geology 15323 ndash344

Lonergan L (1991) Structural evolution of the SierraEspuna Betic Cordillera SE Spain PhD thesisOxford University UK

Lonergan L Platt JP amp Gallagher L (1994) TheInternal-External Zone Boundary in the eastern BeticCordillera SE Spain Journal of Structural Geology16 175 ndash188

Makel GH (1981) Differences in tectonic evolution ofsuperimposed Malaguide and Alpujarride tectonicunits in the Espuna area (Betic Cordilleras Spain)Geology in Mijnbouw 60 203 ndash208

Makel GH (1985) The geology of the MalaguideComplex and its bearing on the geodynamicevolution of the Betic-Rif orogen (southern Spainand northern Morocco) GUA papers of Geology Ser1 22 263 pp

Makel GH amp Rondeel HE (1979) Differences instratigraphy and metamorphism between superposedMalaguide and Alpujarride units in the Espuna area

22 I Abad et al

Whole-rock analyses

Chemical analyses of the major elements ofselected samples were carried out using X-ray

fluorescence (XRF) in a Philips PW 104010spectrometer at the CIC The data (Table 2) werecompared with the average of Post-Archaean

TABLE 1 Crystal-chemical parameters and bulk mineralogy determined by XRD

White mica Chlorite Mineral compositiond001 Illite crystallinity (CIS) d001 Qtz K-white mica

Samples lt2 mm bulk b lt2 mm lt2 mm EG bulk lt2 mm bulk (all the samples)

Los Molinos UnitEs-101 9972 9966 9012 ndash ndash ndash 142 142 Chl Hem Pg Cc Dol SmEs-102 9978 9980 9002 ndash ndash ndash 142 142 Chl Hem Pg Cc SmEs-103 9970 9975 ndash ndash ndash ndash 142 142 Chl Hem Kfs Pg Cc SmEs-104 9969 ndash ndash ndash ndash ndash 142 ndash Chl Hem Pg Cc Sm

Yechar UnitEs-51 9977 9972 9019 038 035 020 1417 1417 Chl Hem Dol SmEs-52 9974 9970 9016 041 046 022 1417 1416 Chl Kfs SmEs-53 9978 9971 9016 037 040 022 1417 1417 Chl Hem Kfs SmEs-54 9979 9978 9019 041 ndash 025 1416 1416 Chl Hem SmEs-55 9975 9987 ndash 038 041 020 1416 1418 Chl Hem Cc SmEs-56 9957 9967 9010 043 044 022 ndash ndash Hem Cc DolEs-57 9958 ndash 9019 050 049 ndash 1418 ndash Chl Cc Dol

La Santa UnitEs-1 9976 ndash 9019 050 044 071 1417 ndash Chl Hem Kfs SmEs-2 9974 9977 9003 062 049 029 1417 1417 Chl Kfs SmEs-3 9963 ndash 9018 058 050 031 1420 ndash Chl Hem Cc SmEs-4 9987 9987 9007 075 077 049 1416 1417 Chl Hem DolEs-5 9993 9983 ndash 093 ndash 084 1417 1417 Su Chl Hem Ill-Pg PgEs-6 9988 9982 ndash 089 092 078 1417 1416 Chl Hem Cc Dol Ill-PgEs-7 9990 9981 ndash 098 095 069 1418 1419 Chl Hem Cc DolEs-8 9985 9975 9007 066 ndash 046 1417 1416 Chl Hem Cc Dol Ill-Pg

Morron de Totana UnitEs-71 9977 ndash 8994 ndash ndash ndash ndash ndash Chl Kfs HemEs-72 ndash ndash ndash ndash ndash ndash ndash ndash Chl Hem GtEs-73 9970 ndash ndash ndash ndash ndash ndash ndash Kfs Cc DolEs-74 9991 ndash ndash 069 071 ndash ndash ndash Kfs Hem GtEs-75 9972 9991 8989 068 064 ndash ndash ndash Chl Kfs Hem DolEs-76 10005 ndash 064 052 ndash ndash ndash ndash Chl Hem Gt DolEs-77 9979 ndash 9011 077 059 083 ndash ndash Chl Hem Dol I-SEs-78 9972 ndash 084 068 071 ndash ndash ndash Chl Cc Sm GtEs-79 9984 ndash 9028 098 090 061 ndash ndash Chl Kfs Hem DolEs-70 9984 ndash 096 089 035 1416 1418 ndash Dol Chl SmEs-81 9980 ndash ndash 099 061 ndash 142 ndash Chl Hm Cc DolEs-82 9972 ndash ndash 090 083 ndash ndash ndash Chl Hm Cc Dol SmEs-85 9972 ndash 9008 058 050 050 142 ndash Chl Su Hem Sm KfsEs-86 9951 9977 9013 092 084 047 142 ndash Chl Su Hem Sm KfsEs-87 9974 9008 083 ndash 050 142 ndash ndash Chl Su Hem Sm KfsEs-91 9974 ndash ndash 104 108 093 ndash ndash Chl Hem KfsEs-92 9972 ndash 9028 083 074 086 ndash ndash Chl Dol KfsEs-93 9980 9983 ndash 109 096 061 ndash ndash Chl Hem DolEs-94 9964 9985 9024 098 093 080 ndash ndash Chl Hem Dol KfsEs-95 9978 ndash 9027 104 093 ndash ndash ndash Dol

Mineral abbreviations according to Kretz (1983) Ill-Pg Intermediate Na-K mica I-S Illite-smectiteinterstratified layers Su Sudoite In bold samples studied by TEM

6 I Abad et al

Australian Shales (PAAS) (Taylor amp McLennan1985) The values for samples of this study aresimilar to them The loss on ignition (LOI) is verylarge in some samples because of the presence ofcarbonates

Electron microscopy observations (SEM andTEM)

Textural aspects Due to the fine-grainedcharacter of these rocks characterization of texturesis possible only by TEM except for the highestmetamorphic-grade samples (Es-102 and Es-53) forwhich grain sizes are large enough to be resolvedby BSE imaging The HRTEM characterizationconfirms the general mineralogical data determinedby XRD for the different units of the studied areaand have allowed us to clarify some specificrelations eg the mixed-layered illite-smectite inthe Morron de Totana Unit

Los Molinos Unit samples are phyllosilicate-richphyllites with well-developed slaty cleavage(Fig 3a) composed of preferentially oriented micaand chlorite alternating with deformed elongatedquartz grains carbonate aggregates and hematitegrains Paragonite and muscovite coexist as separatecrystals Lattice-fringe images show that both micasand chlorite occur as large defect-free crystalsmore than several thousand angstroms thick

(Fig 4) Lattice-fringe images (not shown) withboth 00l and hkl fringes show that crystals arecomposed virtually entirely of coherently relatedlayers but with some stacking disorder in chloriteThese features are in accord with the typicalgreenschist facies of the Alpujarride Complex(Azanon amp Crespo-Blanc 2000) On the otherhand dioctahedral smectite commonly occursreplacing layers of trioctahedral chlorite describedby Nieto et al (1994) as the result of retrogradealteration by hydrothermal fluids

The sample studied from the basal portion of theYechar Unit (Es-53) is a quartz-rich slate with well-developed cleavage (Fig 3b) High-resolutionimages show that the main phyllosilicate is K-richdioctahedral mica with lesser amounts of chloriteand dioctahedral smectite the smectite occurring asaltered layers of primary metamorphic chloriteAlthough phyllosilicates generally form parallel orlow-angle intergrowths preferentially orientedparallel to cleavage Fig 5 shows a low-magnifica-tion image with well-defined high-angle grainboundaries of interlocking grains Such texturesare typical of a metamorphic origin The packetsare 500 AEcirc thick (Fig 6) much smaller than typicaldetrital grains (gt5000 AEcirc ) They also have composi-tions which differentiate them from detrital micasas described below (see chemical characterizationof phyllosilicates) In general the phyllosilicate

TABLE 2 Whole-rock analyses of major elements (wt oxide)

Samples SiO2 TiO2 Al2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O P2O5 LOI

Es-102 5340 077 1790 557 100 013 158 722 152 254 013 847Es-51 5270 083 1752 441 190 014 514 359 028 503 016 845Es-52 7620 093 1210 070 120 ndash 185 037 143 251 017 239Es-53 5990 089 1890 583 140 ndash 292 021 043 489 014 408Es-56 6640 061 1200 350 020 ndash 190 384 011 427 012 616Es-57 6147 065 1154 369 086 005 354 522 ndash 403 012 909Es-1 6008 086 1820 524 170 ndash 299 029 031 484 014 416Es-2 6510 091 1770 139 190 ndash 322 033 038 427 016 470Es-3 5152 089 2030 590 185 006 429 241 ndash 619 018 642Es-5 6260 090 1750 688 020 ndash 263 030 065 237 012 493Es-6 6891 080 1321 434 086 009 169 293 050 234 010 536Es-7 5158 087 2109 671 142 006 275 379 086 347 018 766Es-75 7238 071 1251 238 118 003 154 143 ndash 337 009 438Es-77 3442 055 1252 516 087 005 956 1337 ndash 361 009 2160Es-79 4996 064 1062 335 129 024 676 935 ndash 301 008 1590Es-86 7600 083 1208 122 144 002 205 048 061 324 014 282Es-87 6285 106 1705 516 175 004 299 034 033 493 018 396Es-91 5881 083 1707 409 151 003 338 217 ndash 569 017 645Es-95 5913 075 1802 368 104 002 410 095 009 717 016 520

Prograde and retrograde metamorphism 7

packets are almost defect-free with only rare layerterminations (Fig 6) The polytypes are 2M formicas and semi-random for chlorites

Two samples have been studied from the LaSanta Unit Sample Es-2 a chlorite-rich shale isfrom the basal part of the unit and sample Es-5 aquartz-rich shale is from the upper part In Es-2elongated quartz-rich and phyllosilicate-richdomains alternate at the scale of millimetresparallel to slaty cleavage The principal phyllosili-cate is muscovite with well-defined 2M polytypismThe (hk0) reflections tend to be spread normal to cmuch more than in sample Es-53 due to the

presence of thinner packets in subparallel orienta-tions The trioctahedral chlorite (clinochlore) isoften altered to dioctahedral smectite as determinedin part by AEM data as a result of retrogradealteration processes (eg Fig 7 in Nieto et al1994) The most common defects are layerterminations voids low-angle grain boundariesand dislocations

Sample Es-5 shows incipient slaty cleavagedefined by subparallel preferentially orientedpackets of illite sudoite trioctahedral chloriteparagonite and intermediate Na-K mica as illu-strated in Fig 7 Hematite is commonly included

FIG 4 Lattice-fringe image showing a portion of a defect-free well-crystallized chlorite packet of metamorphicorigin in phyllite Es-102 (Los Molinos Unit)

FIG 3 BSE images showing the well-developed slaty cleavage of two samples (a) Es-102 (Los Molinos Unit) aphyllite in which the grains with the brightest contrast are hematite and the darkest ones quartz Chlorite andmuscovite grains are characterized by their elongated habit and intermediate brightness (stronger in chlorite thanin muscovite) (b) Es-53 (Yechar Unit) in which stretched quartz grains are more abundant and responsible for itssemipelitic character grains with high contrast are hematite or TiO2 Muscovite crystals are indicated by arrows

8 I Abad et al

within illite packets as elongated individual grainsup to ~3006500 AEcirc in size or as aggregates(Fig 7) they show up clearly due to their darkcontrast Sudoite and chlorite were identified on thebasis of SAED and AEM data Separate grains ofeach were observed to be intergrown with texturestypical of metamorphic origins for both Grain sizesof sudoite (gt500 AEcirc ) and clinochlore are approxi-mately equal implying formation of both at gradeswhich were not substantially different Howeverwhere grains of each intersect at high angles thesudoite ubiquitously transects clinochlore packetsAs shown in Fig 8 clinochlore but not sudoitemay exhibit strain contrast These relations implybut do not prove that sudoite post-dated clino-chlore Obvious alteration of clinochlore todioctahedral smectite was ubiquitous but possiblealteration effects were rare in sudoite occurring asdefects such as voids and wavy layers Because thecomposition of sudoite is similar to that ofdioctahedral smectite it was not possible to verifythe presence of small numbers of smectite layerswith AEM data Nevertheless the observationsclearly show that alteration of chlorite to smectitewas relegated entirely or almost entirely toclinochlore

Muscovite occurs in crystals which average~500 AEcirc in thickness and are characterized by 2Mpolytypism features which are typical of meta-morphic mica of anchizonal grade The Na-K micaoccurs in packets of similar thickness The

FIG 6 Lattice-fringe image showing the relation between mica and chlorite packets The arrows indicate theposition of layer terminations The inset SAED pattern illustrates the low-angle relation between the two phases

(Yechar Unit)

FIG 5 Low-magnification image with well-definedhigh-angle grain boundaries and interlocking relations

among muscovite crystals (Yechar Unit)

Prograde and retrograde metamorphism 9

composition (see below) is quite variable corre-sponding to Na-rich muscovite commonly but somegrains have compositions within the limits of thesolvus between muscovite and paragonite and thusare metastable Na-K micas give SAED patternswhich have relatively sharp reflections over a largerange as consistent with large defect-free crystals

Sudoite crystals comprise units with 14 AEcirc

periodicities which in some areas are 28 AEcirc Lattice-fringe images show the presence of inter-

stratified individual 7 AEcirc layers which in some areasform small packets containing no more than eight7 AEcirc layers The AEM analyses (see below) of suchareas are typical of those of homogeneous sudoitecontaining only 14 AEcirc layers implying a composi-tion of the 7 AEcirc layers similar to that of sudoite andnot to an exotic septechlorite such as kaolinite orberthierine The contrast of both 14 AEcirc and 7 AEcirc

fringes appear to be identical further consistentwith a close chemical relation

FIG 7 Textural relations between phyllosilicates (sudoite illite Na-K mica) and hematite (dark contrast) (LaSanta Unit)

FIG 8 Lattice-fringe image showing the coexistence of two types of chlorite trioctahedral chlorite (chl) andsudoite (su) Sudoite transects chlorite which shows strain contrast associated with curved and kinked layersIdentifications were based in part on AEM analyses (see Table 6 Es-511 and 12 analyses) (La Santa Unit)

10 I Abad et al

Three samples corresponding to different butequivalent levels were selected from the Morron deTotana Unit for TEM study (see Fig 1 peliticrocks intercalated with carbonate rocks) Es-77 atthe bottom of the unit Es-86 which is located in anintermediate position and Es-95 from the top of thecross-section There is no incipient slaty cleavage inthese samples Sample Es-77 is a very fine-grainedrock consisting of quartz white mica chlorite andaggregates of carbonate grains Minor hematiteproduces a deep red colour Lattice-fringe images

show that dioctahedral clay minerals have 10 and20 AEcirc periodicities occur as packets 100 ndash200 AEcirc

thick and have subparallel orientations Accordingto Guthrie amp Veblen (1989) smectite and illiteinterlayers can produce different contrast and canonly be differentiated in lattice fringe-imagesobtained with well-defined electron beam (001)orientations and defocus conditions Such contrastdifferences were observed in several lattice-fringeimages The light contrast for fringes in Fig 9represents the smectite interlayer as shown in part

FIG 9 TEM image showing 10 AEcirc periodicity in R4 illite-rich I-S Contrast differences imply the presence ofillite-like (the dark fringes) and smectite-like (the light fringes) layers Packets usually have four or more illite

layers and the SAED pattern shows predominantly 50 AEcirc periodicity (Morron de Totana Unit)

FIG 10 Trioctahedral chlorite with intercalations of 10 AEcirc layers (upper arrow) which damage quickly under theelectron beam The chlorite packets show strain contrast microfolds and 7 AEcirc periodicities corresponding toberthierine (left arrow) or greater values eg 28 AEcirc (see analysis in Table 6 Es-867) (Morron de Totana Unit)

Prograde and retrograde metamorphism 11

by the continuity of such fringes with openinterlayer spaces produced by contraction due to

dehydration The sequence on average is that ofR4 mixed-layering ie a single smectite-likeinterlayer between four or more illite-like inter-layers Although some packets with more than fourlayers of illite are present repeats of four illite-likelayers predominate as shown by the 50 AEcirc

periodicity of SAED patterns (Fig 9 inset) Theillite-smectite (I-S) has typical 1Md polytypism Theobservation of smectite interstratified with illite isin accord with the changes observed in XRDdiagrams after EG treatment (Fig 2) In additionAEM analyses of areas similar to that of Fig 9have relatively high Si and minor Ca contentsfurther confirming the mixed layering

Some packets which give rise to fringes with10 AEcirc spacing produce SAED patterns typical oftwo-layer polytypism The contrast of all fringes isrelatively constant Such packets have thicknessestypical of diagenetic clays and two orders ofmagnitude smaller than typical detrital grains Thecompositions of such grains (see below) haveinterlayer-cation contents typical of illite They aretherefore inferred to be illite which formed duringlate diagenesis and coexisting with the R4 I-S

Sample Es-86 is composed mainly of detritalgrains of quartz randomly-oriented grains ofchlorite and white mica all immersed in aphyllosilicate-rich fine-grained matrix At theTEM scale chlorite is the most common phyllosi-licate in the matrix As in sample Es-5 trioctahe-dral chlorite and sudoite coexist There areinterstratified packets of 10 AEcirc layers only a fewlayers thick (lt150 AEcirc ) which are quickly damaged

FIG 11 Di-trioctahedral chlorite (sudoite) very similarin appearance to the trioctahedral chlorite shown inFig 10 7 AEcirc layers are marked by an arrow and 10 AEcirc

layers are present in the poorly resolved area in thecentre of the image (see analysis in Table 6 Es-862)

(Morron de Totana Unit)

FIG 12 Low-magnification image illustrating illite-like packets The inset SAED pattern shows only a smallnumber of diffuse poorly defined reflections The packets are 100 ndash200 AEcirc thick (Morron de Totana Unit)

12 I Abad et al

under the electron beam (Figs 10 and 11) TheAEM analyses of such material show an enhancedSi content and the presence of small concentrationsof alkali cations These data are consistent withretrograde alteration of chlorite as observed byNieto et al (1994) They differ however from theresults of Nieto et al (1994) and those describedabove for sample Es-5 in that both tri-trioctahedraland di-trioctahedral chlorite are affected In TEMimages strain features in chlorites such as contrastnormal to the layers or microfolds were observed(Fig 10) Mica is less abundant than sudoiteoccurring in packets 100 ndash400 AEcirc thick The SAEDpatterns exhibit 2M polytypism but with diffuse-ness parallel to c in 0kl rows At low magnifica-tion subparallel and slightly curved illite- or illite-smectite-like packets are present they coalescehave abundant layer terminations and show intensecontrasts (Fig 12)

Sample Es-95 is a very fine-grained rock madeup of large detrital grains of quartz in a fine-grained matrix of phyllosilicates mainly whitemicas and opaque minerals Some biotite grainswere observed as verified by AEM data Biotitewas observed only in separated grains on holey

C-coated Cu grids so it is not possible to describethe textural relations for it Authigenic biotite isinconsistent with the low grade of this samplehowever and it is therefore inferred to be of detritalorigin The AEM analyses show deviations frompure biotite however as consistent with alterationto chlorite the usual process in detrital biotite inrocks undergoing diagenesis Lattice-fringe imagesof ion-milled samples show that individual micapackets are very thin with no more than 10 layersisolated and dispersed in a matrix consisting ofmaterial which gives no SAED pattern Thesepackets are wavy discontinuous and with anasto-mosing layers (Fig 13) Despite these texturalfeatures SAED data indicate that there are 2Mand 1Md polytypes

For all samples prismatic crystals of Ti-oxidedispersed Fe-oxyhydroxides zircon and tourmalineare common accessory minerals

Chemical characterization of phyllosilicates

Dioctahedral mica Table 3 shows the composi-tion of K-rich dioctahedral micas of authigenic ormetamorphic origin corresponding to the samples

FIG 13 General aspect of sample Es-95 The image shows arrays of wavy discontinuous and anastomosingpackets of illite and smectite (Morron de Totana Unit)

Prograde and retrograde metamorphism 13

selected from each unit The Yechar and LosMolinos units are characterized by compositionsapproaching those of evolved micas with a sum ofinterlayer cations slightly less than 1 apfu typicalof phengite or muscovite rather than authigenic

illite In contrast the La Santa and Morron deTotana units show heterogeneous mica composi-tions and display deficiencies in interlayer cations(Fig 14a) This last feature is more evident in theMorron de Totana analyses where the illitic

TABLE 3 Representative chemical compositions for K-rich dioctahedral micas normalized to O10(OH)2

Si IVAl VIAl Fe Mg Ti VIS K Na S inter

Los Molinos Unit (1)Es-10213 337 063 178 010 008 002 198 075 008 085Es-10221 308 092 195 005 001 001 202 082 009 091Es-10231 319 081 191 005 003 001 200 077 012 090Es-10232 314 086 188 006 007 001 202 072 021 093

Yechar Unit (1)Es-5311 332 068 163 015 022 002 204 090 004 093Es-5312 310 090 180 003 013 004 201 089 010 099Es-5322 338 062 170 012 015 003 200 086 001 086Es-5323 319 081 176 016 012 003 206 087 002 089

Yechar Unit (2)Es-53 1 317 083 163 021 026 000 210 084 000 084Es-53 2 320 080 165 011 027 003 207 087 000 087Es-53 3 317 083 155 025 024 002 206 093 000 093Es-53 4 328 072 160 020 024 000 203 092 000 092Es-53 5 326 074 164 019 016 000 199 096 000 096Es-53 6 326 074 158 020 028 000 206 089 000 089Es-53 7 319 081 154 025 026 000 205 097 000 097Es-53 8 325 075 166 016 025 000 206 085 000 085Es-53 9 324 076 167 019 019 000 205 085 000 085

La Santa Unit (2)Es-2 1 311 089 157 019 032 002 210 088 005 094Es-2 3 336 064 148 020 043 001 212 072 003 075Es-2 6 313 087 156 024 029 001 210 075 017 092Es-2 7 313 087 155 024 029 001 209 084 011 095Es-2 8 331 069 157 015 032 000 205 087 004 092Es-2 10 328 072 160 018 027 003 208 068 008 075Es-5 4 303 097 174 015 023 001 213 073 009 082Es-5 10 301 099 189 006 015 000 210 065 021 086Es-5 7 307 093 159 033 016 000 208 075 018 093Es-5 8rsquo 301 099 160 031 025 000 216 067 016 083

Morron de Totana Unit (2) (analyses corresponding to R4 Ill-Sm may also be present see text)Es-77 13 327 073 172 013 027 000 212 067 000 067Es-77 15 316 084 195 003 013 000 211 063 000 063Es-77 16 328 072 182 010 015 000 207 068 000 068Es-77 17 338 062 147 032 019 000 199 093 000 093Es-86 9 334 066 160 017 034 000 210 072 000 072Es-86 10 357 043 151 012 046 000 209 065 000 065Es-86 11 353 047 156 012 036 000 203 077 000 077Es-86 13 324 076 152 025 035 000 211 084 000 084Es-86 14 324 076 173 013 019 000 205 084 000 084Es- 95 2 323 077 158 016 019 004 198 103 000 103Es- 95 5 336 064 169 013 025 000 207 070 000 070Es- 95 6 336 064 148 027 033 000 209 078 000 078

(1) SEMEDX analyses (2) TEMAEM analyses

14 I Abad et al

character is consistent with the presence of the R4mixed-layer illite-smectite described above Most ofthe micas are phengitic in composition (Fig 14ab)with an Fe(Fe+Mg) ratio that varies over the range02 ndash085 However the detrital-like grains aresimilar to those of end-member muscovite (seebelow detrital muscovite vs metamorphic musco-vite or Table 5) with much lower Fe and Mgcontents Data plotted in Fig 14bc confirm theassumption of a significant ferrimuscovitic substitu-tion (Guidotti et al 1994) Some of the K-micaanalysed in the Los Molinos and La Santa unitswere found to have significant Na contents (up to02 apfu) Although Ti is unusual in dioctahedralmicas it has been measured in several analyses

(lt005 apfu) The most evident feature in theplots is the scattering of data in the La Santa andMorron de Totana units ie where diageneticconditions are not exceeded In contrast theYechar Unit analyses define a narrow field in thediagrams and the Los Molinos data are even closerto the theoretical muscovite composition (Table 3)

Typical compositions of micas intermediate tomuscovite and paragonite called Na-K micas in thispaper are shown in Table 4 The compositions arequite variable even in the same sample Whencompared with analyses of K-rich dioctahedralmicas (Table 3) and paragonite analyses are seento span the entire range from muscovite toparagonite Some analyses (eg Es-5 9) fall within

FIG 14 Plots of values of chemical compositions of K-rich dioctahedral phyllosilicates solid circles and linesindicate values for theoretical muscovite and corresponding exchange vectors respectively

Prograde and retrograde metamorphism 15

the limits of the solvus between muscovite andparagonite the limbs of that solvus having theapproximate compositions ms60pg40 and ms20pg80

(Li et al 1994a)Detrital muscovite vs metamorphic muscovite

Detrital-like micas of sample Es-95 the lowest-grade one are large enough to be analysed(Table 5) by electron microprobe (EMPA) Thesedata have been compared with the AEM data ofother samples Compositions of detrital-like grainsof the lowest-grade sample and the detrital micas ofthe anchizonal-grade sample Es-53 (Fig 15) arevery similar and close to that of end-membermuscovite These data suggest that the detrital micahas a common source for the different units thatcomprise Sierra Espuna Figure 15 also comparesthe data from authigenic micas of the samesamples which are clearly more phengitic (Fe+Mg= 015 ndash075 apfu) than the detrital ones (Fe+Mg= 01 ndash02 apfu)

Chlorite Trioctahedral and di-trioctahedral(sudoite) chlorites have been analysed (Table 6)The former corresponding to the species clino-chlore is common in most of the samples unlikesudoite which is absent from most (Table 1)Sudoite was identified because the sum ofoctahedral cations is nearly equal to 5 apfu andthe Si and Al contents are higher than intrioctahedral chlorite (Fig 16a) A slight contam-ination by interlayer cations (K and Na) has beendetected therefore the formulae were recalculatedin order to eliminate those impurities (seeAnalytical Methods) Although total Fe has beenconsidered as Fe2+ according to Fransolet ampBourguignon (1978) the Fe3+ content in sudoitemay be greater than the Fe2+ content As severalanalyses have an octahedral sum slightly higherthan the ideal value of 5 apfu this may indicate atrioctahedral substitution andor the presence ofsignificant Fe3+ No Ti or Mn were detected The

Fe(Fe+Mg) ratios do not define a clear trendthroughout the sequence (Fig 16b) nevertheless itis much greater in chlorite in the Morron de Totanaanalyses falling in the characteristic range ofdiagenetic chlorites (Li et al 1994b) There is anincrease in Mg content (from 15 to 4 apfu)concomitant with increase in grade as compatiblewith some other sequences (eg Bevins et al1991)

D I S C U S S I O N

Dioctahedral phyllosilicates chemicalcompositions IC values and polytypes

On the basis of their large size homogeneity ofindividual grains and well-crystallized aspectdetrital dioctahedral mica grains were identified inseveral samples with the exception of those ofgreenschist facies This confirms the idea of the

TABLE 4 Chemical compositions for Na-K micas normalized to O10(OH)2

Si IVAl VIAl Fe Mg Ti VIS Ca K Na S inter

Es-5 14 313 087 190 004 009 002 204 000 034 048 083Es-5 15 321 079 176 009 037 000 222 000 015 038 053Es-5 4 318 082 187 004 019 000 209 000 013 061 074Es-5 12 329 071 172 008 029 000 208 000 039 036 076Es-5 13 323 077 179 005 028 000 212 000 029 041 069Es-5 9 296 104 163 019 029 000 211 000 048 056 104Es-5 8 292 108 156 043 008 001 209 003 014 080 096

FIG 15 Plot of Si vs Al contents of detrital andauthigenic micas of samples Es-95 and Es-53 Solidsymbols = detrital micas open symbols = authigenic

micas

16 I Abad et al

relative stability of muscovite in most diageneticenvironments (Crowley 1991) The compositions ofthe detrital grains of samples from different unitsare similar The lack of heterogeneity which isexpected for mica from a complex source implies acommon source area for all samples

The IC values define a trend from diagenetic toanchizonal conditions but with smaller IC values inbulk-rock fractions consistent with the contributionof detrital mica The coexistence of mature micawith lower-grade I-S or illite in the sub-greenschistfacies rocks shows that those rocks have not

TABLE 5 EMPA data for detrital-like crystals of mica normalized to O10(OH)2

Si IVAl VIAl Fe Mg Ti VIS Ca K Na K+Na+Ca

Es-95z11 313 087 193 005 004 001 203 000 065 022 087Es-95z11 314 086 190 006 007 001 204 000 068 019 087Es-95z1 2 314 086 191 005 009 001 206 000 070 006 077Es-95z2 1 314 086 176 012 010 003 201 002 095 004 101Es-95z2 2 317 083 177 012 011 003 202 000 095 004 099Es-95z4 1 305 095 188 005 007 004 204 000 081 009 089Es-95z4 2 311 089 187 006 007 004 204 000 078 009 086

TABLE 6 Representative chemical compositions for chlorite normalized to O10(OH)8

Si IVAl VIAl Fe Mg VIS FeFe+Mg

Yechar UnitEs53-1 273 127 112 121 376 609 024Es53-4 287 113 125 070 400 596 015Es53-12 303 097 130 146 307 583 032Es53-13 277 123 125 122 354 601 026Es53-15 307 093 146 094 323 564 023Es53-42 271 129 152 160 294 606 035

La Santa UnitEs2-4 319 081 147 148 273 569 035Es2-5 365 035 183 118 225 526 034Es5-1 354 046 293 023 172 488 012Es5-5 361 039 292 021 178 490 010Es5-10 309 091 262 028 221 511 011Es5-16 310 090 197 112 234 543 032Es5-17 350 050 252 076 170 499 031Es-5 10rsquo 364 036 213 097 202 512 032Es-5 11 291 109 143 136 314 593 030Es-5 12 382 018 229 087 179 495 033Es-5 13 316 084 302 021 172 494 011Es-5 16rsquo 346 054 341 013 101 455 011Es-5 17rsquo 300 100 282 033 194 509 014Es-5 18 285 115 285 026 204 515 011

Morron de Totana UnitEs-86 2 377 023 253 088 149 490 037Es-86 4 284 116 166 230 181 578 056Es-86 5 350 050 191 192 151 534 056Es-86 6 366 034 189 191 149 529 056Es-86 7 315 085 129 273 176 578 061

indicates sudoite analyses

Prograde and retrograde metamorphism 17

reached stable chemical or textural equilibrium atleast at the mm scale In addition the lack ofchemical equilibrium among diagenetic I-S andillite at the nm scale is demonstrated by their highlyvariable Fe+Mg contents (006ndash075 apfu) Theaverages of the sum of interlayer cations are closeto 1 apfu in the Los Molinos and Yechar units asconsistent with mica but values of 06 apfucorresponding to I-S or illitic compositions are alsopresent in the Morron de Totana Unit Except forsamples Es-102 and Es-53 the variable composi-tions represent a metastable state with composi-tions of micas determined by a combination ofphengitic ferrimuscovitic and illitic vectors asshown by Fig 14ab

Paragonite is generally considered an indexmineral for the anchizone (Frey 1987) but Li etal (1994a) have shown that paragonite coexistswith K-rich mica in grades as low as that of the latediagenetic zone for a prograde sequence Samplesin which muscovite paragonite and intermediateNa-K mica coexist as in sample Es-5 areconsistent with metastability both because morethan two such micas coexist and because theircompositions are incompatible with the solvus inthe muscovite-paragonite system (see below) Whilethe 2M polytype typical of metamorphic micas is

the most common in these samples the 1Md

polytype also occurs in some diagenetic samplesparticularly those from the Morron de Totana UnitThe 1Md polytype is metastable relative to the 2Mpolytype and the coexistence of both polytypesfurther demonstrates a lack of equilibrium at the nmscale Dong amp Peacor (1996) showed that thedisorder noted in the diagenetic phyllosilicates bySAED is a consequence of the general high entropyof phases formed at very low temperatures thatproduces for example stacking defects separatingpackets with no more than 10 layers These packetsprogressively thicken with increasing metamorphicgrade and the 2M polytype then becomes the mostcommon polytype in authigenic micas Thenm-scale size of the lower-grade clays and theregular increase in size with increasing gradereflect the lack of textural equilibrium in the low-grade samples

A wide range of sample characteristics thusimply that the lower-grade samples are in statesof metastable equilibrium those factors includingcoexistence of mature mica with I-S and illitevariable chemical compositions incompatiblecompositions of white micas small crystal sizesdefect-rich states and polytypism Such dataemphasize the notion of lsquoreaction progressrsquo relative

FIG 16 (a) Plot of Al vs Fe+Mg contents of chlorites illustrating the contrast in composition between tri-trioctahedral clinochlore or chamosite and di-trioctahedral sudoite (b) plot of Si vs Fe(Fe+Mg) ratio of

chlorites showing an absence of compositional trends throughout the sampled sequence

18 I Abad et al

to Ostwald-step rule processes metastable dioctahe-dral clay systems undergoing reactions toward thestate of stable chemical and textural equilibriumapproached in greenschist-facies samples (Essene ampPeacor 1995)

Prograde evolution based on TEM datacorrelation with the Basque-Cantabriansequence (Northern Spain)

With increasing grade TEM data confirm thatcrystal thickness increases crystal defect densitydecreases compositions become less variable andmicrotextural relations are simpler (eg Peacor1992) This trend is consistent with three well-characterized low-grade settings the Gulf Coastsequence (Ahn amp Peacor 1986) and the WelshBasin (Merriman et al 1990) both extensionalbasins and the Helvetic Alps (Livi et al 1997)The latter is a fold-and-thrust belt with similarseries of prograde transitions to those observed inSierra Espuna in which pelites have passed througha sequence of metastable states in the lithologicalsequence mudstone and shale slate and phyllite

The mudstones of the Morron de Totana Unit arecharacterized by the presence of illite-smectite R4mixed-layers mica packets of no more than 10 ndash40layers and the occurrence of 1Md polytypism TheLa Santa Unit samples represent a step in theprograde evolution with thicker packets than in theprevious samples (200 ndash700 AEcirc ) illitic or phengiticcompositions the absence of illite-smectite mixedlayers as well as the presence of discreteintermediate Na-K micas and paragonite Althoughthere is a tendency towards non-preferred orienta-tion of crystals it is not so evident as in the Morronde Totana samples In the Yechar Unit (anchizone)and more clearly in the Los Molinos phyllites(greenschist facies) the microtextural relations areless complex the compositions are less variablewith phengite and trioctahedral chlorite crystalthickness are greater (gt400 AEcirc in the Yechar Unitand gt1000 AEcirc in the Los Molinos Unit) with thepackets being defect-free and with a very crystallineaspect This description has features in commonwith the three well-characterized low-grade settingspreviously noted Only the mineral assemblagesvary somewhat presumably as a function of thedifferences in bulk-rock composition

A detailed comparison with the Basque-Cantabrian Basin (N Spain) shows that the lowest-grade conditions of Sierra Espuna overlap most of

those of that sequence which is characterized bypassive burial diagenesis (8000 m thick) (Nieto etal 1996) Only in the deepest sample of theBasque-Cantabrian sequence is illite-smectite mixedlayering absent Figure 12 of this study is verysimilar to Fig 7 in Nieto et al (1996) and bothimages correspond to diagenetic samples fromdifferent settings nevertheless they illustratesimilar mica packets 100 ndash200 AEcirc thick withparallel-to-subparallel boundaries the orientationof which is inferred to represent beddingTherefore there is a parallelism between thesesequences down to the scale of individual clay-mineral grains even though they developed underdifferent conditions with stress presumably playinga significant role in the Sierra Espuna units Thetexture of the least metamorphosed sample (Fig 13)is reminiscent of the typical textural relations ofsmectite described by Nieto et al (1996) (seeFig 4) Although it corresponds to illite-richinterstratified illite-smectite the clay mineralgrains conserved the grain size and anastomosingdiscontinuous array of original smectite layers

Despite the similarities between the twosequences the chemical behaviour of the micasare clearly different since the grade correspondingto the phengitic compositions of the Yechar Unit isnot reached in the Basque-Cantabrian Basin wherethe deepest and highest-grade sample has illiticcompositions equivalent to those of the La SantaUnit That is in a passive burial metamorphiccontext as in the Basque-Cantabrian Basin thefactors responsible for increasing the rate ofchemical reactions are less significant than in thesequences that were affected by tectonic stressTectonic stress introduces defects which in turnincreases the rate of dissolution of strained grainsthus increasing the rate of dissolution-crystallizationreactions typical of clay minerals Perhaps moreimportantly regional tectonic stress is known tocause mobilization of fluids (eg Oliver 1986) Thepresence and activity of such fluids is a prime causeof increased rates of dissolution ion diffusion andcrystallization Tectonic activity thus may have atwo-fold effect in increasing rate of clay-mineralreactions The phengite component may in part be ameasure of the reaction progress depending on thereaction kinetics in very low-grade rocksAccording to Merriman amp Peacor (1999) whereheat flow is low as in accretionary settingsphengite contents are typically high perhapsbecause there is insufficient thermal energy to

Prograde and retrograde metamorphism 19

cause equilibration of the phengite component ofneoformed illite by contrast with extensional basinswhere this component is typically small

Coexistence of trioctahedral chlorite andsudoite Implications for multiple metamorphicevents

In metamorphic rocks the coexistence of mineralsthat formed at different times and conditions on aPTt path ie not in equilibrium is a commonlyobserved feature where the metamorphic history iscomplex Gross differences in the grain size ofcoexisting phases has long been recognized bymetamorphic petrologists as an indicator of non-equilibrium mineral assemblages Historically muchless attention has been paid to such relations in sub-greenschist-facies rocks because prior to the intro-duction of SEM+TEM techniques their small grainsize prevented observations of textural relationsStudy of such rocks eg by XRD provides littleinformation on textural relations beyond determina-tion of degree of preferred orientation

Study by XRD of the samples showed thatsudoite and trioctahedral chlorite coexist If theyhad formed at the same time and under the sameP-T conditions their compositions would define thelimbs of a solvus The TEM images show howeverthat wherever they are in contact the sudoitetransects the packets of trioctahedral chlorite (egFig 8) Furthermore as illustrated in Fig 8 thetrioctahedral chlorite is commonly deformed anddisplays strain contrast in TEM images Thesetextural relations suggest that sudoite was intro-duced subsequent to the formation of trioctahedralchlorite Similar observations were made byGiorgetti et al (1998) who concluded that texturesin Verrucano metasediments showed that sudoitepost-dated chloritoid pyrophyllite and trioctahedralchlorite ie sudoite is in disequilibrium with thelatter minerals and formed as a retrograde phase Inrocks of the Sierra Espuna area differences in bulk-rock composition are small (Table 2) and do notcorrespond to the presence or absence of sudoite(Table 1) ie the random occurrence of sudoiteindependent of bulk-rock composition is compa-tible with a lack of equilibrium relations Theserelations collectively imply that sudoite wasintroduced at some time subsequent to theformation of mineral assemblages at peak meta-morphic conditions On the other hand the size ofsudoite packets is similar to that of trioctahedral

chlorite as consistent with formation at conditionsnot much different from those for chlorite

In samples from the Morron de Totana bothsudoite and trioctahedral chlorite were affected byretrograde alteration to smectite The TEM imagesshow sudoite (Fig 11) and trioctahedral chlorite(Fig 10) with interstratified layers of smectite withspacings of 10 AEcirc due to collapse and dehydration inthe TEM environment and even packets of smectite150 AEcirc thick within chlorite crystals These relationsimply that both sudoite and trioctahedral chloritehave been affected by retrograde alteration at thevery low temperatures consistent with smectiteformation although the presence of more subtlealteration features in sudoite of some samplessuggests that it was less susceptible to alteration

We therefore conclude that at least three episodesof phyllosilicate formation can be recognized inrocks from the Sierra Espuna-area Trioctahedralchlorite was presumably produced at the highestmetamorphicdiagenetic grade in equilibrium withparagonite and phengite Subsequently sudoite wasintroduced but at P-T conditions which could nothave been much different than those of peakmetamorphic grade Finally at very low-T condi-tions hydrothermal fluids caused partial alterationof trioctahedral chlorite and sudoite to smectite

Retrograde products formed by the reaction offluids with mineral phases generated during anearlier prograde event were described previouslyby Nieto et al (1994) and Zhao et al (1999) Thosereactions occurred on a regional scale but similarreactions have been observed on a more localizedscale The fluid activity associated with a majorfault zone is a common source for these reactions(eg Jiang et al 1990) The structure of SierraEspuna (Fig 1) is based on imbrication processesthat superimpose units that have subsequently beenaffected by faults and folds and that have under-gone significant uplift at least 800 m from the lateMiocene to the present (Sanz de Galdeano et al2001) Given this evolution it is easy to justify theretrograde processes that usually seem to be relatedwith tectonic events during which fluid flow andfluidrock ratios are enhanced (Merriman amp Peacor1999) Moreover according to Nieto et al (1994)the uplift of the Betic Cordillera would furnish theintroduction of phreatic water into the systemproducing a reactive environment adequate for thegenesis of smectite Geological conditions aretherefore consistent with post-peak metamorphicconditions as implied by the TEM observations

20 I Abad et al

The introduction of sudoite as a major phaseduring metamorphism implies the possibility ofadditional overprints on a simple prograde assem-blage the evidence for which might be subtle TheNa-rich micas at all grades have compositionswhich either fall within the solvus or are too Na-rich to correspond to the limb of the solvus In thehighest-grade rocks such micas coexist withmuscovite Those relations are incompatible withchemical equilibrium despite the fact that relationsfor muscovite and other minerals imply an approachto chemical equilibrium Furthermore there is aclose association of Na-rich micas with sudoite (egFig 7) Those relations imply the possibility thatNa-micas were introduced concomitant withsudoite P Mata (pers comm) has shown forexample that overprinting of an epizonal assem-blage by hydrothermal solutions resulted in theintroduction of both an Al-rich variety of chlorite(cookeite) and paragonite in pelites of the CamerosBasin Spain The textural relations such as cross-cutting textures which are so striking for sudoitewere not observed for Na-rich micas Neverthelessthe possibility of such overprinting must beconsidered

In spite of all these prograde-retrogradeprocesses in the Los Molinos Unit a smallnumber of homogeneous phases coexist in chemicalequilibrium and the fine-grained metamorphicphyllosilicates are clearly parallel to cleavage witha typical metamorphic aspect Finally all thetextural and compositional changes that haveoperated in the Espuna area during the metamorphicevolution proceed toward a simpler system repre-senting both textural and chemical equilibriumwhich is characteristic of greenschist-facies condi-tions

Roles of detrital and authigenic phyllosilicatesduring slaty cleavage development

Van der Pluijm et al (1998) focused on therespective roles of the phyllosilicates of detritalorigin and those formed during diagenesis andmetamorphism and observed that detrital phyllosi-licates remain preferentially oriented parallel tobedding in pelites with slaty cleavage whereas fine-grained metamorphic phyllosilicates are orientedparallel to cleavage

Although it is usually difficult to determine thepreferred orientations of phyllosilicates in TEMimages as the grade increases better-aligned

packets occur in the cleavage orientation Thetexture of the least metamorphosed sample inwhich even incipient slaty cleavage is absent isreminiscent of the typical textural relation ofsmectites described in the literature The illite-smectite mixed-layers conserve the textural featuresof possible previous smectite such as anastomosingfringes curved and discontinuous with variations ininterplanar spacing d001 These features indicatethat the minimum energy conditions to initiatemechanical transformations have not been reachedin the lower-grade samples as postulated by Ho etal (1996) since slaty cleavage develops as a resultof interactive mechanical deformation and ther-mally driven dissolution and crystallizationprocesses (Knipe 1981) In contrast in the LosMolinos Unit large crystals of chlorite and whitemica (gt1000 AEcirc ) occur which are of metamorphicorigin but with no detrital grains (Fig 3a)Phyllosilicates are preferentially oriented parallelto slaty cleavage in response to increasedtemperature and tectonic strain (Merriman ampPeacor 1999) The oriented microfabrics and slatycleavage well developed in the rocks of this unitmust be a product of a higher thermal and strainenergy environment reached in the deepest part ofthe sequence As pointed out by Van der Pluijm etal (1998) a continuous range of orientations isevidence for mechanical rotation of grains whereasthe lack of such transitional orientations impliesthat cleavage-parallel phyllosilicates originatedthrough dissolution of bedding-parallel and crystal-lization in the slaty cleavage orientation Asobserved in this study for the Intermediate Unitsgrains with orientations which are transitional frombedding-parallel to slaty cleavage orientation areonly rarely observed Cleavage formation is there-fore inferred to have occurred entirely or nearlyentirely through dissolution and crystallization

ACKNOWLEDGMENTS

We thank MM Abad Ortega from the Centro deInstrumentacio n Cient otilde fica of the Universidad deGranada for her help with HRTEM and I Nieto forher essential help with sample preparation Financialsupport was supplied by Research Project nordm BT 2000-0582 FPI research grant to IA both of the SpanishMinistry of Science and Technology Research GroupRNM-0179 of the Junta de Andaluc otildeacutea and to DRP byNSF grant EAR-9814391

Prograde and retrograde metamorphism 21

REFERENCES

Ahn JH amp Peacor DR (1986) Transmission andanalytical electron microscopy of the smectite-to-illite transition Clays and Clay Minerals 34165 ndash179

Azanon JM amp Crespo-Blanc A (2000) Exhumationduring a continental collision inferred from thetectonometamorphic evolution of the AlpujarrideComplex in the central Betics (Alboran Domain SESpain) Tectonics 19 549 ndash565

Bevins RE Robinson D amp Rowbotham G (1991)Compositional variations in mafic phyllosilicatesfrom regional low-grade metabasites and applicationof the chlorite geother mometer Journal ofMetamorphic Geology 9 711 ndash721

Cliff G amp Lorimer GW (1975) The quantitativeanalysis of thin specimens Journal of Microscopy 103 203 ndash207

Crowley SF (1991) Diagenetic modification of detritalmuscovite an example from the Great LimestoneCyclothem (Carboniferous) of Co Durham UKClay Minerals 26 91 ndash103

Dong H amp Peacor DR (1996) TEM observations ofcoherent stacking relations in smectite and illite ofshales evidence for MacEwan crystallites anddominance of 2M1 polytypes Clays and ClayMinerals 44 257 ndash275

Essene E amp Peacor DR (1995) Clay mineral thermo-metry a critical perspective Clays and ClayMinerals 43 540 ndash553

Fransolet AM amp Bourguignon P (1978) Dioctahedralchlorite in quartz veins from Ardennes BelgiumThe Canadian Mineralogist 16 365 ndash373

Fransolet AM amp Schreyer W (1984) Sudoite ditrioctahedral chlorite a stable low-temperature phasein the system MgO-Al2O3-SiO2-H2O Contributionsto Mineralogy and Petrology 86 409 ndash417

Frey M (1987) Very low-grade metamorphism of clasticsedimentary rocks Pp 9 ndash58 in Low TemperatureMetamorphism (M Frey editor) Blackie GlasgowUK

Giorgetti G Goffe B Memmi I amp Nieto F (1998)Metamorphic evolution of Verrucano metasedimentsin northern Apennines new petrological constraintsEuropean Journal of Mineralogy 10 1295 ndash1308

Guidotti CV amp Sassi FP (1986) Classification andcorrelation of metamorphic facies series by means ofmuscovite b0 data from low grade metapelites NeuesJahrbuch fur Mineralogie Abhandlungen 153363 ndash380

Guidotti CV Yates MG Dyar MD amp Taylor ME(1994) Petrogenetic implications of the Fe3+ contentof muscovi te in pel it ic sch is ts AmericanMineralogist 79 793 ndash795

Guthrie GD amp Veblen DR (1989) High resolutionelectron microscopy of mixed-layer illitesmectite

Computer simulations Clays and Clay Minerals 371 ndash11

Ho N Peacor DR amp Van der Pluijm BA (1996)Contrasting roles of detrital and authigenic phyllo-silicates during slaty cleavage development Journalof Structural Geology 18 615 ndash623

Jiang WT Peacor DR Merriman RJ amp Roberts B(1990) Transmission and analytical electron micro-scopic study of mixed layer illitesmectite formed asan apparent replacement product of diagenetic illiteClays and Clay Minerals 38 449 ndash468

Jiang WT Peacor DR amp Buseck PR (1994) Chloritegeothermometry Contamination and apparent octa-hedral vacancies Clays and Clay Minerals 42593 ndash605

Kisch HJ (1991) Development of slaty cleavage anddegree of very-low-grade metamorphism a reviewJournal of Metamorphic Geology 9 735 ndash750

Knipe JR (1981) The interaction of deformation andmetamorphi sm in slates Tectonophysics 78249 ndash272

Kretz R (1983) Symbols for rock-forming mineralsAmerican Mineralogist 68 277 ndash279

Li G Peacor DR Merriman RJ amp Roberts B (1994a)The diagenetic to low grade metamorphism evolu-tion of matrix white mica in the system muscovite-paragonite in a mudrock from Central Wales UKClays and Clay Minerals 42 369 ndash381

Li G Peacor DR Merriman RJ Roberts B amp Vander Pluijm BA (1994b) TEM and AEM constraintson the origin and significance of chlorite-mica stacksin slates an example from Central Wales UKJournal of Structural Geology 16 1139 ndash1157

Livi KJT Veblen DR Ferry JM amp Frey M (1997)Evolution of 21 layered silicates in low-grademetamorphosed Liass ic sha les of CentralSwitzerland Journal of Metamorphic Geology 15323 ndash344

Lonergan L (1991) Structural evolution of the SierraEspuna Betic Cordillera SE Spain PhD thesisOxford University UK

Lonergan L Platt JP amp Gallagher L (1994) TheInternal-External Zone Boundary in the eastern BeticCordillera SE Spain Journal of Structural Geology16 175 ndash188

Makel GH (1981) Differences in tectonic evolution ofsuperimposed Malaguide and Alpujarride tectonicunits in the Espuna area (Betic Cordilleras Spain)Geology in Mijnbouw 60 203 ndash208

Makel GH (1985) The geology of the MalaguideComplex and its bearing on the geodynamicevolution of the Betic-Rif orogen (southern Spainand northern Morocco) GUA papers of Geology Ser1 22 263 pp

Makel GH amp Rondeel HE (1979) Differences instratigraphy and metamorphism between superposedMalaguide and Alpujarride units in the Espuna area

22 I Abad et al

Australian Shales (PAAS) (Taylor amp McLennan1985) The values for samples of this study aresimilar to them The loss on ignition (LOI) is verylarge in some samples because of the presence ofcarbonates

Electron microscopy observations (SEM andTEM)

Textural aspects Due to the fine-grainedcharacter of these rocks characterization of texturesis possible only by TEM except for the highestmetamorphic-grade samples (Es-102 and Es-53) forwhich grain sizes are large enough to be resolvedby BSE imaging The HRTEM characterizationconfirms the general mineralogical data determinedby XRD for the different units of the studied areaand have allowed us to clarify some specificrelations eg the mixed-layered illite-smectite inthe Morron de Totana Unit

Los Molinos Unit samples are phyllosilicate-richphyllites with well-developed slaty cleavage(Fig 3a) composed of preferentially oriented micaand chlorite alternating with deformed elongatedquartz grains carbonate aggregates and hematitegrains Paragonite and muscovite coexist as separatecrystals Lattice-fringe images show that both micasand chlorite occur as large defect-free crystalsmore than several thousand angstroms thick

(Fig 4) Lattice-fringe images (not shown) withboth 00l and hkl fringes show that crystals arecomposed virtually entirely of coherently relatedlayers but with some stacking disorder in chloriteThese features are in accord with the typicalgreenschist facies of the Alpujarride Complex(Azanon amp Crespo-Blanc 2000) On the otherhand dioctahedral smectite commonly occursreplacing layers of trioctahedral chlorite describedby Nieto et al (1994) as the result of retrogradealteration by hydrothermal fluids

The sample studied from the basal portion of theYechar Unit (Es-53) is a quartz-rich slate with well-developed cleavage (Fig 3b) High-resolutionimages show that the main phyllosilicate is K-richdioctahedral mica with lesser amounts of chloriteand dioctahedral smectite the smectite occurring asaltered layers of primary metamorphic chloriteAlthough phyllosilicates generally form parallel orlow-angle intergrowths preferentially orientedparallel to cleavage Fig 5 shows a low-magnifica-tion image with well-defined high-angle grainboundaries of interlocking grains Such texturesare typical of a metamorphic origin The packetsare 500 AEcirc thick (Fig 6) much smaller than typicaldetrital grains (gt5000 AEcirc ) They also have composi-tions which differentiate them from detrital micasas described below (see chemical characterizationof phyllosilicates) In general the phyllosilicate

TABLE 2 Whole-rock analyses of major elements (wt oxide)

Samples SiO2 TiO2 Al2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O P2O5 LOI

Es-102 5340 077 1790 557 100 013 158 722 152 254 013 847Es-51 5270 083 1752 441 190 014 514 359 028 503 016 845Es-52 7620 093 1210 070 120 ndash 185 037 143 251 017 239Es-53 5990 089 1890 583 140 ndash 292 021 043 489 014 408Es-56 6640 061 1200 350 020 ndash 190 384 011 427 012 616Es-57 6147 065 1154 369 086 005 354 522 ndash 403 012 909Es-1 6008 086 1820 524 170 ndash 299 029 031 484 014 416Es-2 6510 091 1770 139 190 ndash 322 033 038 427 016 470Es-3 5152 089 2030 590 185 006 429 241 ndash 619 018 642Es-5 6260 090 1750 688 020 ndash 263 030 065 237 012 493Es-6 6891 080 1321 434 086 009 169 293 050 234 010 536Es-7 5158 087 2109 671 142 006 275 379 086 347 018 766Es-75 7238 071 1251 238 118 003 154 143 ndash 337 009 438Es-77 3442 055 1252 516 087 005 956 1337 ndash 361 009 2160Es-79 4996 064 1062 335 129 024 676 935 ndash 301 008 1590Es-86 7600 083 1208 122 144 002 205 048 061 324 014 282Es-87 6285 106 1705 516 175 004 299 034 033 493 018 396Es-91 5881 083 1707 409 151 003 338 217 ndash 569 017 645Es-95 5913 075 1802 368 104 002 410 095 009 717 016 520

Prograde and retrograde metamorphism 7

packets are almost defect-free with only rare layerterminations (Fig 6) The polytypes are 2M formicas and semi-random for chlorites

Two samples have been studied from the LaSanta Unit Sample Es-2 a chlorite-rich shale isfrom the basal part of the unit and sample Es-5 aquartz-rich shale is from the upper part In Es-2elongated quartz-rich and phyllosilicate-richdomains alternate at the scale of millimetresparallel to slaty cleavage The principal phyllosili-cate is muscovite with well-defined 2M polytypismThe (hk0) reflections tend to be spread normal to cmuch more than in sample Es-53 due to the

presence of thinner packets in subparallel orienta-tions The trioctahedral chlorite (clinochlore) isoften altered to dioctahedral smectite as determinedin part by AEM data as a result of retrogradealteration processes (eg Fig 7 in Nieto et al1994) The most common defects are layerterminations voids low-angle grain boundariesand dislocations

Sample Es-5 shows incipient slaty cleavagedefined by subparallel preferentially orientedpackets of illite sudoite trioctahedral chloriteparagonite and intermediate Na-K mica as illu-strated in Fig 7 Hematite is commonly included

FIG 4 Lattice-fringe image showing a portion of a defect-free well-crystallized chlorite packet of metamorphicorigin in phyllite Es-102 (Los Molinos Unit)

FIG 3 BSE images showing the well-developed slaty cleavage of two samples (a) Es-102 (Los Molinos Unit) aphyllite in which the grains with the brightest contrast are hematite and the darkest ones quartz Chlorite andmuscovite grains are characterized by their elongated habit and intermediate brightness (stronger in chlorite thanin muscovite) (b) Es-53 (Yechar Unit) in which stretched quartz grains are more abundant and responsible for itssemipelitic character grains with high contrast are hematite or TiO2 Muscovite crystals are indicated by arrows

8 I Abad et al

within illite packets as elongated individual grainsup to ~3006500 AEcirc in size or as aggregates(Fig 7) they show up clearly due to their darkcontrast Sudoite and chlorite were identified on thebasis of SAED and AEM data Separate grains ofeach were observed to be intergrown with texturestypical of metamorphic origins for both Grain sizesof sudoite (gt500 AEcirc ) and clinochlore are approxi-mately equal implying formation of both at gradeswhich were not substantially different Howeverwhere grains of each intersect at high angles thesudoite ubiquitously transects clinochlore packetsAs shown in Fig 8 clinochlore but not sudoitemay exhibit strain contrast These relations implybut do not prove that sudoite post-dated clino-chlore Obvious alteration of clinochlore todioctahedral smectite was ubiquitous but possiblealteration effects were rare in sudoite occurring asdefects such as voids and wavy layers Because thecomposition of sudoite is similar to that ofdioctahedral smectite it was not possible to verifythe presence of small numbers of smectite layerswith AEM data Nevertheless the observationsclearly show that alteration of chlorite to smectitewas relegated entirely or almost entirely toclinochlore

Muscovite occurs in crystals which average~500 AEcirc in thickness and are characterized by 2Mpolytypism features which are typical of meta-morphic mica of anchizonal grade The Na-K micaoccurs in packets of similar thickness The

FIG 6 Lattice-fringe image showing the relation between mica and chlorite packets The arrows indicate theposition of layer terminations The inset SAED pattern illustrates the low-angle relation between the two phases

(Yechar Unit)

FIG 5 Low-magnification image with well-definedhigh-angle grain boundaries and interlocking relations

among muscovite crystals (Yechar Unit)

Prograde and retrograde metamorphism 9

composition (see below) is quite variable corre-sponding to Na-rich muscovite commonly but somegrains have compositions within the limits of thesolvus between muscovite and paragonite and thusare metastable Na-K micas give SAED patternswhich have relatively sharp reflections over a largerange as consistent with large defect-free crystals

Sudoite crystals comprise units with 14 AEcirc

periodicities which in some areas are 28 AEcirc Lattice-fringe images show the presence of inter-

stratified individual 7 AEcirc layers which in some areasform small packets containing no more than eight7 AEcirc layers The AEM analyses (see below) of suchareas are typical of those of homogeneous sudoitecontaining only 14 AEcirc layers implying a composi-tion of the 7 AEcirc layers similar to that of sudoite andnot to an exotic septechlorite such as kaolinite orberthierine The contrast of both 14 AEcirc and 7 AEcirc

fringes appear to be identical further consistentwith a close chemical relation

FIG 7 Textural relations between phyllosilicates (sudoite illite Na-K mica) and hematite (dark contrast) (LaSanta Unit)

FIG 8 Lattice-fringe image showing the coexistence of two types of chlorite trioctahedral chlorite (chl) andsudoite (su) Sudoite transects chlorite which shows strain contrast associated with curved and kinked layersIdentifications were based in part on AEM analyses (see Table 6 Es-511 and 12 analyses) (La Santa Unit)

10 I Abad et al

Three samples corresponding to different butequivalent levels were selected from the Morron deTotana Unit for TEM study (see Fig 1 peliticrocks intercalated with carbonate rocks) Es-77 atthe bottom of the unit Es-86 which is located in anintermediate position and Es-95 from the top of thecross-section There is no incipient slaty cleavage inthese samples Sample Es-77 is a very fine-grainedrock consisting of quartz white mica chlorite andaggregates of carbonate grains Minor hematiteproduces a deep red colour Lattice-fringe images

show that dioctahedral clay minerals have 10 and20 AEcirc periodicities occur as packets 100 ndash200 AEcirc

thick and have subparallel orientations Accordingto Guthrie amp Veblen (1989) smectite and illiteinterlayers can produce different contrast and canonly be differentiated in lattice fringe-imagesobtained with well-defined electron beam (001)orientations and defocus conditions Such contrastdifferences were observed in several lattice-fringeimages The light contrast for fringes in Fig 9represents the smectite interlayer as shown in part

FIG 9 TEM image showing 10 AEcirc periodicity in R4 illite-rich I-S Contrast differences imply the presence ofillite-like (the dark fringes) and smectite-like (the light fringes) layers Packets usually have four or more illite

layers and the SAED pattern shows predominantly 50 AEcirc periodicity (Morron de Totana Unit)

FIG 10 Trioctahedral chlorite with intercalations of 10 AEcirc layers (upper arrow) which damage quickly under theelectron beam The chlorite packets show strain contrast microfolds and 7 AEcirc periodicities corresponding toberthierine (left arrow) or greater values eg 28 AEcirc (see analysis in Table 6 Es-867) (Morron de Totana Unit)

Prograde and retrograde metamorphism 11

by the continuity of such fringes with openinterlayer spaces produced by contraction due to

dehydration The sequence on average is that ofR4 mixed-layering ie a single smectite-likeinterlayer between four or more illite-like inter-layers Although some packets with more than fourlayers of illite are present repeats of four illite-likelayers predominate as shown by the 50 AEcirc

periodicity of SAED patterns (Fig 9 inset) Theillite-smectite (I-S) has typical 1Md polytypism Theobservation of smectite interstratified with illite isin accord with the changes observed in XRDdiagrams after EG treatment (Fig 2) In additionAEM analyses of areas similar to that of Fig 9have relatively high Si and minor Ca contentsfurther confirming the mixed layering

Some packets which give rise to fringes with10 AEcirc spacing produce SAED patterns typical oftwo-layer polytypism The contrast of all fringes isrelatively constant Such packets have thicknessestypical of diagenetic clays and two orders ofmagnitude smaller than typical detrital grains Thecompositions of such grains (see below) haveinterlayer-cation contents typical of illite They aretherefore inferred to be illite which formed duringlate diagenesis and coexisting with the R4 I-S

Sample Es-86 is composed mainly of detritalgrains of quartz randomly-oriented grains ofchlorite and white mica all immersed in aphyllosilicate-rich fine-grained matrix At theTEM scale chlorite is the most common phyllosi-licate in the matrix As in sample Es-5 trioctahe-dral chlorite and sudoite coexist There areinterstratified packets of 10 AEcirc layers only a fewlayers thick (lt150 AEcirc ) which are quickly damaged

FIG 11 Di-trioctahedral chlorite (sudoite) very similarin appearance to the trioctahedral chlorite shown inFig 10 7 AEcirc layers are marked by an arrow and 10 AEcirc

layers are present in the poorly resolved area in thecentre of the image (see analysis in Table 6 Es-862)

(Morron de Totana Unit)

FIG 12 Low-magnification image illustrating illite-like packets The inset SAED pattern shows only a smallnumber of diffuse poorly defined reflections The packets are 100 ndash200 AEcirc thick (Morron de Totana Unit)

12 I Abad et al

under the electron beam (Figs 10 and 11) TheAEM analyses of such material show an enhancedSi content and the presence of small concentrationsof alkali cations These data are consistent withretrograde alteration of chlorite as observed byNieto et al (1994) They differ however from theresults of Nieto et al (1994) and those describedabove for sample Es-5 in that both tri-trioctahedraland di-trioctahedral chlorite are affected In TEMimages strain features in chlorites such as contrastnormal to the layers or microfolds were observed(Fig 10) Mica is less abundant than sudoiteoccurring in packets 100 ndash400 AEcirc thick The SAEDpatterns exhibit 2M polytypism but with diffuse-ness parallel to c in 0kl rows At low magnifica-tion subparallel and slightly curved illite- or illite-smectite-like packets are present they coalescehave abundant layer terminations and show intensecontrasts (Fig 12)

Sample Es-95 is a very fine-grained rock madeup of large detrital grains of quartz in a fine-grained matrix of phyllosilicates mainly whitemicas and opaque minerals Some biotite grainswere observed as verified by AEM data Biotitewas observed only in separated grains on holey

C-coated Cu grids so it is not possible to describethe textural relations for it Authigenic biotite isinconsistent with the low grade of this samplehowever and it is therefore inferred to be of detritalorigin The AEM analyses show deviations frompure biotite however as consistent with alterationto chlorite the usual process in detrital biotite inrocks undergoing diagenesis Lattice-fringe imagesof ion-milled samples show that individual micapackets are very thin with no more than 10 layersisolated and dispersed in a matrix consisting ofmaterial which gives no SAED pattern Thesepackets are wavy discontinuous and with anasto-mosing layers (Fig 13) Despite these texturalfeatures SAED data indicate that there are 2Mand 1Md polytypes

For all samples prismatic crystals of Ti-oxidedispersed Fe-oxyhydroxides zircon and tourmalineare common accessory minerals

Chemical characterization of phyllosilicates

Dioctahedral mica Table 3 shows the composi-tion of K-rich dioctahedral micas of authigenic ormetamorphic origin corresponding to the samples

FIG 13 General aspect of sample Es-95 The image shows arrays of wavy discontinuous and anastomosingpackets of illite and smectite (Morron de Totana Unit)

Prograde and retrograde metamorphism 13

selected from each unit The Yechar and LosMolinos units are characterized by compositionsapproaching those of evolved micas with a sum ofinterlayer cations slightly less than 1 apfu typicalof phengite or muscovite rather than authigenic

illite In contrast the La Santa and Morron deTotana units show heterogeneous mica composi-tions and display deficiencies in interlayer cations(Fig 14a) This last feature is more evident in theMorron de Totana analyses where the illitic

TABLE 3 Representative chemical compositions for K-rich dioctahedral micas normalized to O10(OH)2

Si IVAl VIAl Fe Mg Ti VIS K Na S inter

Los Molinos Unit (1)Es-10213 337 063 178 010 008 002 198 075 008 085Es-10221 308 092 195 005 001 001 202 082 009 091Es-10231 319 081 191 005 003 001 200 077 012 090Es-10232 314 086 188 006 007 001 202 072 021 093

Yechar Unit (1)Es-5311 332 068 163 015 022 002 204 090 004 093Es-5312 310 090 180 003 013 004 201 089 010 099Es-5322 338 062 170 012 015 003 200 086 001 086Es-5323 319 081 176 016 012 003 206 087 002 089

Yechar Unit (2)Es-53 1 317 083 163 021 026 000 210 084 000 084Es-53 2 320 080 165 011 027 003 207 087 000 087Es-53 3 317 083 155 025 024 002 206 093 000 093Es-53 4 328 072 160 020 024 000 203 092 000 092Es-53 5 326 074 164 019 016 000 199 096 000 096Es-53 6 326 074 158 020 028 000 206 089 000 089Es-53 7 319 081 154 025 026 000 205 097 000 097Es-53 8 325 075 166 016 025 000 206 085 000 085Es-53 9 324 076 167 019 019 000 205 085 000 085

La Santa Unit (2)Es-2 1 311 089 157 019 032 002 210 088 005 094Es-2 3 336 064 148 020 043 001 212 072 003 075Es-2 6 313 087 156 024 029 001 210 075 017 092Es-2 7 313 087 155 024 029 001 209 084 011 095Es-2 8 331 069 157 015 032 000 205 087 004 092Es-2 10 328 072 160 018 027 003 208 068 008 075Es-5 4 303 097 174 015 023 001 213 073 009 082Es-5 10 301 099 189 006 015 000 210 065 021 086Es-5 7 307 093 159 033 016 000 208 075 018 093Es-5 8rsquo 301 099 160 031 025 000 216 067 016 083

Morron de Totana Unit (2) (analyses corresponding to R4 Ill-Sm may also be present see text)Es-77 13 327 073 172 013 027 000 212 067 000 067Es-77 15 316 084 195 003 013 000 211 063 000 063Es-77 16 328 072 182 010 015 000 207 068 000 068Es-77 17 338 062 147 032 019 000 199 093 000 093Es-86 9 334 066 160 017 034 000 210 072 000 072Es-86 10 357 043 151 012 046 000 209 065 000 065Es-86 11 353 047 156 012 036 000 203 077 000 077Es-86 13 324 076 152 025 035 000 211 084 000 084Es-86 14 324 076 173 013 019 000 205 084 000 084Es- 95 2 323 077 158 016 019 004 198 103 000 103Es- 95 5 336 064 169 013 025 000 207 070 000 070Es- 95 6 336 064 148 027 033 000 209 078 000 078

(1) SEMEDX analyses (2) TEMAEM analyses

14 I Abad et al

character is consistent with the presence of the R4mixed-layer illite-smectite described above Most ofthe micas are phengitic in composition (Fig 14ab)with an Fe(Fe+Mg) ratio that varies over the range02 ndash085 However the detrital-like grains aresimilar to those of end-member muscovite (seebelow detrital muscovite vs metamorphic musco-vite or Table 5) with much lower Fe and Mgcontents Data plotted in Fig 14bc confirm theassumption of a significant ferrimuscovitic substitu-tion (Guidotti et al 1994) Some of the K-micaanalysed in the Los Molinos and La Santa unitswere found to have significant Na contents (up to02 apfu) Although Ti is unusual in dioctahedralmicas it has been measured in several analyses

(lt005 apfu) The most evident feature in theplots is the scattering of data in the La Santa andMorron de Totana units ie where diageneticconditions are not exceeded In contrast theYechar Unit analyses define a narrow field in thediagrams and the Los Molinos data are even closerto the theoretical muscovite composition (Table 3)

Typical compositions of micas intermediate tomuscovite and paragonite called Na-K micas in thispaper are shown in Table 4 The compositions arequite variable even in the same sample Whencompared with analyses of K-rich dioctahedralmicas (Table 3) and paragonite analyses are seento span the entire range from muscovite toparagonite Some analyses (eg Es-5 9) fall within

FIG 14 Plots of values of chemical compositions of K-rich dioctahedral phyllosilicates solid circles and linesindicate values for theoretical muscovite and corresponding exchange vectors respectively

Prograde and retrograde metamorphism 15

the limits of the solvus between muscovite andparagonite the limbs of that solvus having theapproximate compositions ms60pg40 and ms20pg80

(Li et al 1994a)Detrital muscovite vs metamorphic muscovite

Detrital-like micas of sample Es-95 the lowest-grade one are large enough to be analysed(Table 5) by electron microprobe (EMPA) Thesedata have been compared with the AEM data ofother samples Compositions of detrital-like grainsof the lowest-grade sample and the detrital micas ofthe anchizonal-grade sample Es-53 (Fig 15) arevery similar and close to that of end-membermuscovite These data suggest that the detrital micahas a common source for the different units thatcomprise Sierra Espuna Figure 15 also comparesthe data from authigenic micas of the samesamples which are clearly more phengitic (Fe+Mg= 015 ndash075 apfu) than the detrital ones (Fe+Mg= 01 ndash02 apfu)

Chlorite Trioctahedral and di-trioctahedral(sudoite) chlorites have been analysed (Table 6)The former corresponding to the species clino-chlore is common in most of the samples unlikesudoite which is absent from most (Table 1)Sudoite was identified because the sum ofoctahedral cations is nearly equal to 5 apfu andthe Si and Al contents are higher than intrioctahedral chlorite (Fig 16a) A slight contam-ination by interlayer cations (K and Na) has beendetected therefore the formulae were recalculatedin order to eliminate those impurities (seeAnalytical Methods) Although total Fe has beenconsidered as Fe2+ according to Fransolet ampBourguignon (1978) the Fe3+ content in sudoitemay be greater than the Fe2+ content As severalanalyses have an octahedral sum slightly higherthan the ideal value of 5 apfu this may indicate atrioctahedral substitution andor the presence ofsignificant Fe3+ No Ti or Mn were detected The

Fe(Fe+Mg) ratios do not define a clear trendthroughout the sequence (Fig 16b) nevertheless itis much greater in chlorite in the Morron de Totanaanalyses falling in the characteristic range ofdiagenetic chlorites (Li et al 1994b) There is anincrease in Mg content (from 15 to 4 apfu)concomitant with increase in grade as compatiblewith some other sequences (eg Bevins et al1991)

D I S C U S S I O N

Dioctahedral phyllosilicates chemicalcompositions IC values and polytypes

On the basis of their large size homogeneity ofindividual grains and well-crystallized aspectdetrital dioctahedral mica grains were identified inseveral samples with the exception of those ofgreenschist facies This confirms the idea of the

TABLE 4 Chemical compositions for Na-K micas normalized to O10(OH)2

Si IVAl VIAl Fe Mg Ti VIS Ca K Na S inter

Es-5 14 313 087 190 004 009 002 204 000 034 048 083Es-5 15 321 079 176 009 037 000 222 000 015 038 053Es-5 4 318 082 187 004 019 000 209 000 013 061 074Es-5 12 329 071 172 008 029 000 208 000 039 036 076Es-5 13 323 077 179 005 028 000 212 000 029 041 069Es-5 9 296 104 163 019 029 000 211 000 048 056 104Es-5 8 292 108 156 043 008 001 209 003 014 080 096

FIG 15 Plot of Si vs Al contents of detrital andauthigenic micas of samples Es-95 and Es-53 Solidsymbols = detrital micas open symbols = authigenic

micas

16 I Abad et al

relative stability of muscovite in most diageneticenvironments (Crowley 1991) The compositions ofthe detrital grains of samples from different unitsare similar The lack of heterogeneity which isexpected for mica from a complex source implies acommon source area for all samples

The IC values define a trend from diagenetic toanchizonal conditions but with smaller IC values inbulk-rock fractions consistent with the contributionof detrital mica The coexistence of mature micawith lower-grade I-S or illite in the sub-greenschistfacies rocks shows that those rocks have not

TABLE 5 EMPA data for detrital-like crystals of mica normalized to O10(OH)2

Si IVAl VIAl Fe Mg Ti VIS Ca K Na K+Na+Ca

Es-95z11 313 087 193 005 004 001 203 000 065 022 087Es-95z11 314 086 190 006 007 001 204 000 068 019 087Es-95z1 2 314 086 191 005 009 001 206 000 070 006 077Es-95z2 1 314 086 176 012 010 003 201 002 095 004 101Es-95z2 2 317 083 177 012 011 003 202 000 095 004 099Es-95z4 1 305 095 188 005 007 004 204 000 081 009 089Es-95z4 2 311 089 187 006 007 004 204 000 078 009 086

TABLE 6 Representative chemical compositions for chlorite normalized to O10(OH)8

Si IVAl VIAl Fe Mg VIS FeFe+Mg

Yechar UnitEs53-1 273 127 112 121 376 609 024Es53-4 287 113 125 070 400 596 015Es53-12 303 097 130 146 307 583 032Es53-13 277 123 125 122 354 601 026Es53-15 307 093 146 094 323 564 023Es53-42 271 129 152 160 294 606 035

La Santa UnitEs2-4 319 081 147 148 273 569 035Es2-5 365 035 183 118 225 526 034Es5-1 354 046 293 023 172 488 012Es5-5 361 039 292 021 178 490 010Es5-10 309 091 262 028 221 511 011Es5-16 310 090 197 112 234 543 032Es5-17 350 050 252 076 170 499 031Es-5 10rsquo 364 036 213 097 202 512 032Es-5 11 291 109 143 136 314 593 030Es-5 12 382 018 229 087 179 495 033Es-5 13 316 084 302 021 172 494 011Es-5 16rsquo 346 054 341 013 101 455 011Es-5 17rsquo 300 100 282 033 194 509 014Es-5 18 285 115 285 026 204 515 011

Morron de Totana UnitEs-86 2 377 023 253 088 149 490 037Es-86 4 284 116 166 230 181 578 056Es-86 5 350 050 191 192 151 534 056Es-86 6 366 034 189 191 149 529 056Es-86 7 315 085 129 273 176 578 061

indicates sudoite analyses

Prograde and retrograde metamorphism 17

reached stable chemical or textural equilibrium atleast at the mm scale In addition the lack ofchemical equilibrium among diagenetic I-S andillite at the nm scale is demonstrated by their highlyvariable Fe+Mg contents (006ndash075 apfu) Theaverages of the sum of interlayer cations are closeto 1 apfu in the Los Molinos and Yechar units asconsistent with mica but values of 06 apfucorresponding to I-S or illitic compositions are alsopresent in the Morron de Totana Unit Except forsamples Es-102 and Es-53 the variable composi-tions represent a metastable state with composi-tions of micas determined by a combination ofphengitic ferrimuscovitic and illitic vectors asshown by Fig 14ab

Paragonite is generally considered an indexmineral for the anchizone (Frey 1987) but Li etal (1994a) have shown that paragonite coexistswith K-rich mica in grades as low as that of the latediagenetic zone for a prograde sequence Samplesin which muscovite paragonite and intermediateNa-K mica coexist as in sample Es-5 areconsistent with metastability both because morethan two such micas coexist and because theircompositions are incompatible with the solvus inthe muscovite-paragonite system (see below) Whilethe 2M polytype typical of metamorphic micas is

the most common in these samples the 1Md

polytype also occurs in some diagenetic samplesparticularly those from the Morron de Totana UnitThe 1Md polytype is metastable relative to the 2Mpolytype and the coexistence of both polytypesfurther demonstrates a lack of equilibrium at the nmscale Dong amp Peacor (1996) showed that thedisorder noted in the diagenetic phyllosilicates bySAED is a consequence of the general high entropyof phases formed at very low temperatures thatproduces for example stacking defects separatingpackets with no more than 10 layers These packetsprogressively thicken with increasing metamorphicgrade and the 2M polytype then becomes the mostcommon polytype in authigenic micas Thenm-scale size of the lower-grade clays and theregular increase in size with increasing gradereflect the lack of textural equilibrium in the low-grade samples

A wide range of sample characteristics thusimply that the lower-grade samples are in statesof metastable equilibrium those factors includingcoexistence of mature mica with I-S and illitevariable chemical compositions incompatiblecompositions of white micas small crystal sizesdefect-rich states and polytypism Such dataemphasize the notion of lsquoreaction progressrsquo relative

FIG 16 (a) Plot of Al vs Fe+Mg contents of chlorites illustrating the contrast in composition between tri-trioctahedral clinochlore or chamosite and di-trioctahedral sudoite (b) plot of Si vs Fe(Fe+Mg) ratio of

chlorites showing an absence of compositional trends throughout the sampled sequence

18 I Abad et al

to Ostwald-step rule processes metastable dioctahe-dral clay systems undergoing reactions toward thestate of stable chemical and textural equilibriumapproached in greenschist-facies samples (Essene ampPeacor 1995)

Prograde evolution based on TEM datacorrelation with the Basque-Cantabriansequence (Northern Spain)

With increasing grade TEM data confirm thatcrystal thickness increases crystal defect densitydecreases compositions become less variable andmicrotextural relations are simpler (eg Peacor1992) This trend is consistent with three well-characterized low-grade settings the Gulf Coastsequence (Ahn amp Peacor 1986) and the WelshBasin (Merriman et al 1990) both extensionalbasins and the Helvetic Alps (Livi et al 1997)The latter is a fold-and-thrust belt with similarseries of prograde transitions to those observed inSierra Espuna in which pelites have passed througha sequence of metastable states in the lithologicalsequence mudstone and shale slate and phyllite

The mudstones of the Morron de Totana Unit arecharacterized by the presence of illite-smectite R4mixed-layers mica packets of no more than 10 ndash40layers and the occurrence of 1Md polytypism TheLa Santa Unit samples represent a step in theprograde evolution with thicker packets than in theprevious samples (200 ndash700 AEcirc ) illitic or phengiticcompositions the absence of illite-smectite mixedlayers as well as the presence of discreteintermediate Na-K micas and paragonite Althoughthere is a tendency towards non-preferred orienta-tion of crystals it is not so evident as in the Morronde Totana samples In the Yechar Unit (anchizone)and more clearly in the Los Molinos phyllites(greenschist facies) the microtextural relations areless complex the compositions are less variablewith phengite and trioctahedral chlorite crystalthickness are greater (gt400 AEcirc in the Yechar Unitand gt1000 AEcirc in the Los Molinos Unit) with thepackets being defect-free and with a very crystallineaspect This description has features in commonwith the three well-characterized low-grade settingspreviously noted Only the mineral assemblagesvary somewhat presumably as a function of thedifferences in bulk-rock composition

A detailed comparison with the Basque-Cantabrian Basin (N Spain) shows that the lowest-grade conditions of Sierra Espuna overlap most of

those of that sequence which is characterized bypassive burial diagenesis (8000 m thick) (Nieto etal 1996) Only in the deepest sample of theBasque-Cantabrian sequence is illite-smectite mixedlayering absent Figure 12 of this study is verysimilar to Fig 7 in Nieto et al (1996) and bothimages correspond to diagenetic samples fromdifferent settings nevertheless they illustratesimilar mica packets 100 ndash200 AEcirc thick withparallel-to-subparallel boundaries the orientationof which is inferred to represent beddingTherefore there is a parallelism between thesesequences down to the scale of individual clay-mineral grains even though they developed underdifferent conditions with stress presumably playinga significant role in the Sierra Espuna units Thetexture of the least metamorphosed sample (Fig 13)is reminiscent of the typical textural relations ofsmectite described by Nieto et al (1996) (seeFig 4) Although it corresponds to illite-richinterstratified illite-smectite the clay mineralgrains conserved the grain size and anastomosingdiscontinuous array of original smectite layers

Despite the similarities between the twosequences the chemical behaviour of the micasare clearly different since the grade correspondingto the phengitic compositions of the Yechar Unit isnot reached in the Basque-Cantabrian Basin wherethe deepest and highest-grade sample has illiticcompositions equivalent to those of the La SantaUnit That is in a passive burial metamorphiccontext as in the Basque-Cantabrian Basin thefactors responsible for increasing the rate ofchemical reactions are less significant than in thesequences that were affected by tectonic stressTectonic stress introduces defects which in turnincreases the rate of dissolution of strained grainsthus increasing the rate of dissolution-crystallizationreactions typical of clay minerals Perhaps moreimportantly regional tectonic stress is known tocause mobilization of fluids (eg Oliver 1986) Thepresence and activity of such fluids is a prime causeof increased rates of dissolution ion diffusion andcrystallization Tectonic activity thus may have atwo-fold effect in increasing rate of clay-mineralreactions The phengite component may in part be ameasure of the reaction progress depending on thereaction kinetics in very low-grade rocksAccording to Merriman amp Peacor (1999) whereheat flow is low as in accretionary settingsphengite contents are typically high perhapsbecause there is insufficient thermal energy to

Prograde and retrograde metamorphism 19

cause equilibration of the phengite component ofneoformed illite by contrast with extensional basinswhere this component is typically small

Coexistence of trioctahedral chlorite andsudoite Implications for multiple metamorphicevents

In metamorphic rocks the coexistence of mineralsthat formed at different times and conditions on aPTt path ie not in equilibrium is a commonlyobserved feature where the metamorphic history iscomplex Gross differences in the grain size ofcoexisting phases has long been recognized bymetamorphic petrologists as an indicator of non-equilibrium mineral assemblages Historically muchless attention has been paid to such relations in sub-greenschist-facies rocks because prior to the intro-duction of SEM+TEM techniques their small grainsize prevented observations of textural relationsStudy of such rocks eg by XRD provides littleinformation on textural relations beyond determina-tion of degree of preferred orientation

Study by XRD of the samples showed thatsudoite and trioctahedral chlorite coexist If theyhad formed at the same time and under the sameP-T conditions their compositions would define thelimbs of a solvus The TEM images show howeverthat wherever they are in contact the sudoitetransects the packets of trioctahedral chlorite (egFig 8) Furthermore as illustrated in Fig 8 thetrioctahedral chlorite is commonly deformed anddisplays strain contrast in TEM images Thesetextural relations suggest that sudoite was intro-duced subsequent to the formation of trioctahedralchlorite Similar observations were made byGiorgetti et al (1998) who concluded that texturesin Verrucano metasediments showed that sudoitepost-dated chloritoid pyrophyllite and trioctahedralchlorite ie sudoite is in disequilibrium with thelatter minerals and formed as a retrograde phase Inrocks of the Sierra Espuna area differences in bulk-rock composition are small (Table 2) and do notcorrespond to the presence or absence of sudoite(Table 1) ie the random occurrence of sudoiteindependent of bulk-rock composition is compa-tible with a lack of equilibrium relations Theserelations collectively imply that sudoite wasintroduced at some time subsequent to theformation of mineral assemblages at peak meta-morphic conditions On the other hand the size ofsudoite packets is similar to that of trioctahedral

chlorite as consistent with formation at conditionsnot much different from those for chlorite

In samples from the Morron de Totana bothsudoite and trioctahedral chlorite were affected byretrograde alteration to smectite The TEM imagesshow sudoite (Fig 11) and trioctahedral chlorite(Fig 10) with interstratified layers of smectite withspacings of 10 AEcirc due to collapse and dehydration inthe TEM environment and even packets of smectite150 AEcirc thick within chlorite crystals These relationsimply that both sudoite and trioctahedral chloritehave been affected by retrograde alteration at thevery low temperatures consistent with smectiteformation although the presence of more subtlealteration features in sudoite of some samplessuggests that it was less susceptible to alteration

We therefore conclude that at least three episodesof phyllosilicate formation can be recognized inrocks from the Sierra Espuna-area Trioctahedralchlorite was presumably produced at the highestmetamorphicdiagenetic grade in equilibrium withparagonite and phengite Subsequently sudoite wasintroduced but at P-T conditions which could nothave been much different than those of peakmetamorphic grade Finally at very low-T condi-tions hydrothermal fluids caused partial alterationof trioctahedral chlorite and sudoite to smectite

Retrograde products formed by the reaction offluids with mineral phases generated during anearlier prograde event were described previouslyby Nieto et al (1994) and Zhao et al (1999) Thosereactions occurred on a regional scale but similarreactions have been observed on a more localizedscale The fluid activity associated with a majorfault zone is a common source for these reactions(eg Jiang et al 1990) The structure of SierraEspuna (Fig 1) is based on imbrication processesthat superimpose units that have subsequently beenaffected by faults and folds and that have under-gone significant uplift at least 800 m from the lateMiocene to the present (Sanz de Galdeano et al2001) Given this evolution it is easy to justify theretrograde processes that usually seem to be relatedwith tectonic events during which fluid flow andfluidrock ratios are enhanced (Merriman amp Peacor1999) Moreover according to Nieto et al (1994)the uplift of the Betic Cordillera would furnish theintroduction of phreatic water into the systemproducing a reactive environment adequate for thegenesis of smectite Geological conditions aretherefore consistent with post-peak metamorphicconditions as implied by the TEM observations

20 I Abad et al

The introduction of sudoite as a major phaseduring metamorphism implies the possibility ofadditional overprints on a simple prograde assem-blage the evidence for which might be subtle TheNa-rich micas at all grades have compositionswhich either fall within the solvus or are too Na-rich to correspond to the limb of the solvus In thehighest-grade rocks such micas coexist withmuscovite Those relations are incompatible withchemical equilibrium despite the fact that relationsfor muscovite and other minerals imply an approachto chemical equilibrium Furthermore there is aclose association of Na-rich micas with sudoite (egFig 7) Those relations imply the possibility thatNa-micas were introduced concomitant withsudoite P Mata (pers comm) has shown forexample that overprinting of an epizonal assem-blage by hydrothermal solutions resulted in theintroduction of both an Al-rich variety of chlorite(cookeite) and paragonite in pelites of the CamerosBasin Spain The textural relations such as cross-cutting textures which are so striking for sudoitewere not observed for Na-rich micas Neverthelessthe possibility of such overprinting must beconsidered

In spite of all these prograde-retrogradeprocesses in the Los Molinos Unit a smallnumber of homogeneous phases coexist in chemicalequilibrium and the fine-grained metamorphicphyllosilicates are clearly parallel to cleavage witha typical metamorphic aspect Finally all thetextural and compositional changes that haveoperated in the Espuna area during the metamorphicevolution proceed toward a simpler system repre-senting both textural and chemical equilibriumwhich is characteristic of greenschist-facies condi-tions

Roles of detrital and authigenic phyllosilicatesduring slaty cleavage development

Van der Pluijm et al (1998) focused on therespective roles of the phyllosilicates of detritalorigin and those formed during diagenesis andmetamorphism and observed that detrital phyllosi-licates remain preferentially oriented parallel tobedding in pelites with slaty cleavage whereas fine-grained metamorphic phyllosilicates are orientedparallel to cleavage

Although it is usually difficult to determine thepreferred orientations of phyllosilicates in TEMimages as the grade increases better-aligned

packets occur in the cleavage orientation Thetexture of the least metamorphosed sample inwhich even incipient slaty cleavage is absent isreminiscent of the typical textural relation ofsmectites described in the literature The illite-smectite mixed-layers conserve the textural featuresof possible previous smectite such as anastomosingfringes curved and discontinuous with variations ininterplanar spacing d001 These features indicatethat the minimum energy conditions to initiatemechanical transformations have not been reachedin the lower-grade samples as postulated by Ho etal (1996) since slaty cleavage develops as a resultof interactive mechanical deformation and ther-mally driven dissolution and crystallizationprocesses (Knipe 1981) In contrast in the LosMolinos Unit large crystals of chlorite and whitemica (gt1000 AEcirc ) occur which are of metamorphicorigin but with no detrital grains (Fig 3a)Phyllosilicates are preferentially oriented parallelto slaty cleavage in response to increasedtemperature and tectonic strain (Merriman ampPeacor 1999) The oriented microfabrics and slatycleavage well developed in the rocks of this unitmust be a product of a higher thermal and strainenergy environment reached in the deepest part ofthe sequence As pointed out by Van der Pluijm etal (1998) a continuous range of orientations isevidence for mechanical rotation of grains whereasthe lack of such transitional orientations impliesthat cleavage-parallel phyllosilicates originatedthrough dissolution of bedding-parallel and crystal-lization in the slaty cleavage orientation Asobserved in this study for the Intermediate Unitsgrains with orientations which are transitional frombedding-parallel to slaty cleavage orientation areonly rarely observed Cleavage formation is there-fore inferred to have occurred entirely or nearlyentirely through dissolution and crystallization

ACKNOWLEDGMENTS

We thank MM Abad Ortega from the Centro deInstrumentacio n Cient otilde fica of the Universidad deGranada for her help with HRTEM and I Nieto forher essential help with sample preparation Financialsupport was supplied by Research Project nordm BT 2000-0582 FPI research grant to IA both of the SpanishMinistry of Science and Technology Research GroupRNM-0179 of the Junta de Andaluc otildeacutea and to DRP byNSF grant EAR-9814391

Prograde and retrograde metamorphism 21

REFERENCES

Ahn JH amp Peacor DR (1986) Transmission andanalytical electron microscopy of the smectite-to-illite transition Clays and Clay Minerals 34165 ndash179

Azanon JM amp Crespo-Blanc A (2000) Exhumationduring a continental collision inferred from thetectonometamorphic evolution of the AlpujarrideComplex in the central Betics (Alboran Domain SESpain) Tectonics 19 549 ndash565

Bevins RE Robinson D amp Rowbotham G (1991)Compositional variations in mafic phyllosilicatesfrom regional low-grade metabasites and applicationof the chlorite geother mometer Journal ofMetamorphic Geology 9 711 ndash721

Cliff G amp Lorimer GW (1975) The quantitativeanalysis of thin specimens Journal of Microscopy 103 203 ndash207

Crowley SF (1991) Diagenetic modification of detritalmuscovite an example from the Great LimestoneCyclothem (Carboniferous) of Co Durham UKClay Minerals 26 91 ndash103

Dong H amp Peacor DR (1996) TEM observations ofcoherent stacking relations in smectite and illite ofshales evidence for MacEwan crystallites anddominance of 2M1 polytypes Clays and ClayMinerals 44 257 ndash275

Essene E amp Peacor DR (1995) Clay mineral thermo-metry a critical perspective Clays and ClayMinerals 43 540 ndash553

Fransolet AM amp Bourguignon P (1978) Dioctahedralchlorite in quartz veins from Ardennes BelgiumThe Canadian Mineralogist 16 365 ndash373

Fransolet AM amp Schreyer W (1984) Sudoite ditrioctahedral chlorite a stable low-temperature phasein the system MgO-Al2O3-SiO2-H2O Contributionsto Mineralogy and Petrology 86 409 ndash417

Frey M (1987) Very low-grade metamorphism of clasticsedimentary rocks Pp 9 ndash58 in Low TemperatureMetamorphism (M Frey editor) Blackie GlasgowUK

Giorgetti G Goffe B Memmi I amp Nieto F (1998)Metamorphic evolution of Verrucano metasedimentsin northern Apennines new petrological constraintsEuropean Journal of Mineralogy 10 1295 ndash1308

Guidotti CV amp Sassi FP (1986) Classification andcorrelation of metamorphic facies series by means ofmuscovite b0 data from low grade metapelites NeuesJahrbuch fur Mineralogie Abhandlungen 153363 ndash380

Guidotti CV Yates MG Dyar MD amp Taylor ME(1994) Petrogenetic implications of the Fe3+ contentof muscovi te in pel it ic sch is ts AmericanMineralogist 79 793 ndash795

Guthrie GD amp Veblen DR (1989) High resolutionelectron microscopy of mixed-layer illitesmectite

Computer simulations Clays and Clay Minerals 371 ndash11

Ho N Peacor DR amp Van der Pluijm BA (1996)Contrasting roles of detrital and authigenic phyllo-silicates during slaty cleavage development Journalof Structural Geology 18 615 ndash623

Jiang WT Peacor DR Merriman RJ amp Roberts B(1990) Transmission and analytical electron micro-scopic study of mixed layer illitesmectite formed asan apparent replacement product of diagenetic illiteClays and Clay Minerals 38 449 ndash468

Jiang WT Peacor DR amp Buseck PR (1994) Chloritegeothermometry Contamination and apparent octa-hedral vacancies Clays and Clay Minerals 42593 ndash605

Kisch HJ (1991) Development of slaty cleavage anddegree of very-low-grade metamorphism a reviewJournal of Metamorphic Geology 9 735 ndash750

Knipe JR (1981) The interaction of deformation andmetamorphi sm in slates Tectonophysics 78249 ndash272

Kretz R (1983) Symbols for rock-forming mineralsAmerican Mineralogist 68 277 ndash279

Li G Peacor DR Merriman RJ amp Roberts B (1994a)The diagenetic to low grade metamorphism evolu-tion of matrix white mica in the system muscovite-paragonite in a mudrock from Central Wales UKClays and Clay Minerals 42 369 ndash381

Li G Peacor DR Merriman RJ Roberts B amp Vander Pluijm BA (1994b) TEM and AEM constraintson the origin and significance of chlorite-mica stacksin slates an example from Central Wales UKJournal of Structural Geology 16 1139 ndash1157

Livi KJT Veblen DR Ferry JM amp Frey M (1997)Evolution of 21 layered silicates in low-grademetamorphosed Liass ic sha les of CentralSwitzerland Journal of Metamorphic Geology 15323 ndash344

Lonergan L (1991) Structural evolution of the SierraEspuna Betic Cordillera SE Spain PhD thesisOxford University UK

Lonergan L Platt JP amp Gallagher L (1994) TheInternal-External Zone Boundary in the eastern BeticCordillera SE Spain Journal of Structural Geology16 175 ndash188

Makel GH (1981) Differences in tectonic evolution ofsuperimposed Malaguide and Alpujarride tectonicunits in the Espuna area (Betic Cordilleras Spain)Geology in Mijnbouw 60 203 ndash208

Makel GH (1985) The geology of the MalaguideComplex and its bearing on the geodynamicevolution of the Betic-Rif orogen (southern Spainand northern Morocco) GUA papers of Geology Ser1 22 263 pp

Makel GH amp Rondeel HE (1979) Differences instratigraphy and metamorphism between superposedMalaguide and Alpujarride units in the Espuna area

22 I Abad et al

packets are almost defect-free with only rare layerterminations (Fig 6) The polytypes are 2M formicas and semi-random for chlorites

Two samples have been studied from the LaSanta Unit Sample Es-2 a chlorite-rich shale isfrom the basal part of the unit and sample Es-5 aquartz-rich shale is from the upper part In Es-2elongated quartz-rich and phyllosilicate-richdomains alternate at the scale of millimetresparallel to slaty cleavage The principal phyllosili-cate is muscovite with well-defined 2M polytypismThe (hk0) reflections tend to be spread normal to cmuch more than in sample Es-53 due to the

presence of thinner packets in subparallel orienta-tions The trioctahedral chlorite (clinochlore) isoften altered to dioctahedral smectite as determinedin part by AEM data as a result of retrogradealteration processes (eg Fig 7 in Nieto et al1994) The most common defects are layerterminations voids low-angle grain boundariesand dislocations

Sample Es-5 shows incipient slaty cleavagedefined by subparallel preferentially orientedpackets of illite sudoite trioctahedral chloriteparagonite and intermediate Na-K mica as illu-strated in Fig 7 Hematite is commonly included

FIG 4 Lattice-fringe image showing a portion of a defect-free well-crystallized chlorite packet of metamorphicorigin in phyllite Es-102 (Los Molinos Unit)

FIG 3 BSE images showing the well-developed slaty cleavage of two samples (a) Es-102 (Los Molinos Unit) aphyllite in which the grains with the brightest contrast are hematite and the darkest ones quartz Chlorite andmuscovite grains are characterized by their elongated habit and intermediate brightness (stronger in chlorite thanin muscovite) (b) Es-53 (Yechar Unit) in which stretched quartz grains are more abundant and responsible for itssemipelitic character grains with high contrast are hematite or TiO2 Muscovite crystals are indicated by arrows

8 I Abad et al

within illite packets as elongated individual grainsup to ~3006500 AEcirc in size or as aggregates(Fig 7) they show up clearly due to their darkcontrast Sudoite and chlorite were identified on thebasis of SAED and AEM data Separate grains ofeach were observed to be intergrown with texturestypical of metamorphic origins for both Grain sizesof sudoite (gt500 AEcirc ) and clinochlore are approxi-mately equal implying formation of both at gradeswhich were not substantially different Howeverwhere grains of each intersect at high angles thesudoite ubiquitously transects clinochlore packetsAs shown in Fig 8 clinochlore but not sudoitemay exhibit strain contrast These relations implybut do not prove that sudoite post-dated clino-chlore Obvious alteration of clinochlore todioctahedral smectite was ubiquitous but possiblealteration effects were rare in sudoite occurring asdefects such as voids and wavy layers Because thecomposition of sudoite is similar to that ofdioctahedral smectite it was not possible to verifythe presence of small numbers of smectite layerswith AEM data Nevertheless the observationsclearly show that alteration of chlorite to smectitewas relegated entirely or almost entirely toclinochlore

Muscovite occurs in crystals which average~500 AEcirc in thickness and are characterized by 2Mpolytypism features which are typical of meta-morphic mica of anchizonal grade The Na-K micaoccurs in packets of similar thickness The

FIG 6 Lattice-fringe image showing the relation between mica and chlorite packets The arrows indicate theposition of layer terminations The inset SAED pattern illustrates the low-angle relation between the two phases

(Yechar Unit)

FIG 5 Low-magnification image with well-definedhigh-angle grain boundaries and interlocking relations

among muscovite crystals (Yechar Unit)

Prograde and retrograde metamorphism 9

composition (see below) is quite variable corre-sponding to Na-rich muscovite commonly but somegrains have compositions within the limits of thesolvus between muscovite and paragonite and thusare metastable Na-K micas give SAED patternswhich have relatively sharp reflections over a largerange as consistent with large defect-free crystals

Sudoite crystals comprise units with 14 AEcirc

periodicities which in some areas are 28 AEcirc Lattice-fringe images show the presence of inter-

stratified individual 7 AEcirc layers which in some areasform small packets containing no more than eight7 AEcirc layers The AEM analyses (see below) of suchareas are typical of those of homogeneous sudoitecontaining only 14 AEcirc layers implying a composi-tion of the 7 AEcirc layers similar to that of sudoite andnot to an exotic septechlorite such as kaolinite orberthierine The contrast of both 14 AEcirc and 7 AEcirc

fringes appear to be identical further consistentwith a close chemical relation

FIG 7 Textural relations between phyllosilicates (sudoite illite Na-K mica) and hematite (dark contrast) (LaSanta Unit)

FIG 8 Lattice-fringe image showing the coexistence of two types of chlorite trioctahedral chlorite (chl) andsudoite (su) Sudoite transects chlorite which shows strain contrast associated with curved and kinked layersIdentifications were based in part on AEM analyses (see Table 6 Es-511 and 12 analyses) (La Santa Unit)

10 I Abad et al

Three samples corresponding to different butequivalent levels were selected from the Morron deTotana Unit for TEM study (see Fig 1 peliticrocks intercalated with carbonate rocks) Es-77 atthe bottom of the unit Es-86 which is located in anintermediate position and Es-95 from the top of thecross-section There is no incipient slaty cleavage inthese samples Sample Es-77 is a very fine-grainedrock consisting of quartz white mica chlorite andaggregates of carbonate grains Minor hematiteproduces a deep red colour Lattice-fringe images

show that dioctahedral clay minerals have 10 and20 AEcirc periodicities occur as packets 100 ndash200 AEcirc

thick and have subparallel orientations Accordingto Guthrie amp Veblen (1989) smectite and illiteinterlayers can produce different contrast and canonly be differentiated in lattice fringe-imagesobtained with well-defined electron beam (001)orientations and defocus conditions Such contrastdifferences were observed in several lattice-fringeimages The light contrast for fringes in Fig 9represents the smectite interlayer as shown in part

FIG 9 TEM image showing 10 AEcirc periodicity in R4 illite-rich I-S Contrast differences imply the presence ofillite-like (the dark fringes) and smectite-like (the light fringes) layers Packets usually have four or more illite

layers and the SAED pattern shows predominantly 50 AEcirc periodicity (Morron de Totana Unit)

FIG 10 Trioctahedral chlorite with intercalations of 10 AEcirc layers (upper arrow) which damage quickly under theelectron beam The chlorite packets show strain contrast microfolds and 7 AEcirc periodicities corresponding toberthierine (left arrow) or greater values eg 28 AEcirc (see analysis in Table 6 Es-867) (Morron de Totana Unit)

Prograde and retrograde metamorphism 11

by the continuity of such fringes with openinterlayer spaces produced by contraction due to

dehydration The sequence on average is that ofR4 mixed-layering ie a single smectite-likeinterlayer between four or more illite-like inter-layers Although some packets with more than fourlayers of illite are present repeats of four illite-likelayers predominate as shown by the 50 AEcirc

periodicity of SAED patterns (Fig 9 inset) Theillite-smectite (I-S) has typical 1Md polytypism Theobservation of smectite interstratified with illite isin accord with the changes observed in XRDdiagrams after EG treatment (Fig 2) In additionAEM analyses of areas similar to that of Fig 9have relatively high Si and minor Ca contentsfurther confirming the mixed layering

Some packets which give rise to fringes with10 AEcirc spacing produce SAED patterns typical oftwo-layer polytypism The contrast of all fringes isrelatively constant Such packets have thicknessestypical of diagenetic clays and two orders ofmagnitude smaller than typical detrital grains Thecompositions of such grains (see below) haveinterlayer-cation contents typical of illite They aretherefore inferred to be illite which formed duringlate diagenesis and coexisting with the R4 I-S

Sample Es-86 is composed mainly of detritalgrains of quartz randomly-oriented grains ofchlorite and white mica all immersed in aphyllosilicate-rich fine-grained matrix At theTEM scale chlorite is the most common phyllosi-licate in the matrix As in sample Es-5 trioctahe-dral chlorite and sudoite coexist There areinterstratified packets of 10 AEcirc layers only a fewlayers thick (lt150 AEcirc ) which are quickly damaged

FIG 11 Di-trioctahedral chlorite (sudoite) very similarin appearance to the trioctahedral chlorite shown inFig 10 7 AEcirc layers are marked by an arrow and 10 AEcirc

layers are present in the poorly resolved area in thecentre of the image (see analysis in Table 6 Es-862)

(Morron de Totana Unit)

FIG 12 Low-magnification image illustrating illite-like packets The inset SAED pattern shows only a smallnumber of diffuse poorly defined reflections The packets are 100 ndash200 AEcirc thick (Morron de Totana Unit)

12 I Abad et al

under the electron beam (Figs 10 and 11) TheAEM analyses of such material show an enhancedSi content and the presence of small concentrationsof alkali cations These data are consistent withretrograde alteration of chlorite as observed byNieto et al (1994) They differ however from theresults of Nieto et al (1994) and those describedabove for sample Es-5 in that both tri-trioctahedraland di-trioctahedral chlorite are affected In TEMimages strain features in chlorites such as contrastnormal to the layers or microfolds were observed(Fig 10) Mica is less abundant than sudoiteoccurring in packets 100 ndash400 AEcirc thick The SAEDpatterns exhibit 2M polytypism but with diffuse-ness parallel to c in 0kl rows At low magnifica-tion subparallel and slightly curved illite- or illite-smectite-like packets are present they coalescehave abundant layer terminations and show intensecontrasts (Fig 12)

Sample Es-95 is a very fine-grained rock madeup of large detrital grains of quartz in a fine-grained matrix of phyllosilicates mainly whitemicas and opaque minerals Some biotite grainswere observed as verified by AEM data Biotitewas observed only in separated grains on holey

C-coated Cu grids so it is not possible to describethe textural relations for it Authigenic biotite isinconsistent with the low grade of this samplehowever and it is therefore inferred to be of detritalorigin The AEM analyses show deviations frompure biotite however as consistent with alterationto chlorite the usual process in detrital biotite inrocks undergoing diagenesis Lattice-fringe imagesof ion-milled samples show that individual micapackets are very thin with no more than 10 layersisolated and dispersed in a matrix consisting ofmaterial which gives no SAED pattern Thesepackets are wavy discontinuous and with anasto-mosing layers (Fig 13) Despite these texturalfeatures SAED data indicate that there are 2Mand 1Md polytypes

For all samples prismatic crystals of Ti-oxidedispersed Fe-oxyhydroxides zircon and tourmalineare common accessory minerals

Chemical characterization of phyllosilicates

Dioctahedral mica Table 3 shows the composi-tion of K-rich dioctahedral micas of authigenic ormetamorphic origin corresponding to the samples

FIG 13 General aspect of sample Es-95 The image shows arrays of wavy discontinuous and anastomosingpackets of illite and smectite (Morron de Totana Unit)

Prograde and retrograde metamorphism 13

selected from each unit The Yechar and LosMolinos units are characterized by compositionsapproaching those of evolved micas with a sum ofinterlayer cations slightly less than 1 apfu typicalof phengite or muscovite rather than authigenic

illite In contrast the La Santa and Morron deTotana units show heterogeneous mica composi-tions and display deficiencies in interlayer cations(Fig 14a) This last feature is more evident in theMorron de Totana analyses where the illitic

TABLE 3 Representative chemical compositions for K-rich dioctahedral micas normalized to O10(OH)2

Si IVAl VIAl Fe Mg Ti VIS K Na S inter

Los Molinos Unit (1)Es-10213 337 063 178 010 008 002 198 075 008 085Es-10221 308 092 195 005 001 001 202 082 009 091Es-10231 319 081 191 005 003 001 200 077 012 090Es-10232 314 086 188 006 007 001 202 072 021 093

Yechar Unit (1)Es-5311 332 068 163 015 022 002 204 090 004 093Es-5312 310 090 180 003 013 004 201 089 010 099Es-5322 338 062 170 012 015 003 200 086 001 086Es-5323 319 081 176 016 012 003 206 087 002 089

Yechar Unit (2)Es-53 1 317 083 163 021 026 000 210 084 000 084Es-53 2 320 080 165 011 027 003 207 087 000 087Es-53 3 317 083 155 025 024 002 206 093 000 093Es-53 4 328 072 160 020 024 000 203 092 000 092Es-53 5 326 074 164 019 016 000 199 096 000 096Es-53 6 326 074 158 020 028 000 206 089 000 089Es-53 7 319 081 154 025 026 000 205 097 000 097Es-53 8 325 075 166 016 025 000 206 085 000 085Es-53 9 324 076 167 019 019 000 205 085 000 085

La Santa Unit (2)Es-2 1 311 089 157 019 032 002 210 088 005 094Es-2 3 336 064 148 020 043 001 212 072 003 075Es-2 6 313 087 156 024 029 001 210 075 017 092Es-2 7 313 087 155 024 029 001 209 084 011 095Es-2 8 331 069 157 015 032 000 205 087 004 092Es-2 10 328 072 160 018 027 003 208 068 008 075Es-5 4 303 097 174 015 023 001 213 073 009 082Es-5 10 301 099 189 006 015 000 210 065 021 086Es-5 7 307 093 159 033 016 000 208 075 018 093Es-5 8rsquo 301 099 160 031 025 000 216 067 016 083

Morron de Totana Unit (2) (analyses corresponding to R4 Ill-Sm may also be present see text)Es-77 13 327 073 172 013 027 000 212 067 000 067Es-77 15 316 084 195 003 013 000 211 063 000 063Es-77 16 328 072 182 010 015 000 207 068 000 068Es-77 17 338 062 147 032 019 000 199 093 000 093Es-86 9 334 066 160 017 034 000 210 072 000 072Es-86 10 357 043 151 012 046 000 209 065 000 065Es-86 11 353 047 156 012 036 000 203 077 000 077Es-86 13 324 076 152 025 035 000 211 084 000 084Es-86 14 324 076 173 013 019 000 205 084 000 084Es- 95 2 323 077 158 016 019 004 198 103 000 103Es- 95 5 336 064 169 013 025 000 207 070 000 070Es- 95 6 336 064 148 027 033 000 209 078 000 078

(1) SEMEDX analyses (2) TEMAEM analyses

14 I Abad et al

character is consistent with the presence of the R4mixed-layer illite-smectite described above Most ofthe micas are phengitic in composition (Fig 14ab)with an Fe(Fe+Mg) ratio that varies over the range02 ndash085 However the detrital-like grains aresimilar to those of end-member muscovite (seebelow detrital muscovite vs metamorphic musco-vite or Table 5) with much lower Fe and Mgcontents Data plotted in Fig 14bc confirm theassumption of a significant ferrimuscovitic substitu-tion (Guidotti et al 1994) Some of the K-micaanalysed in the Los Molinos and La Santa unitswere found to have significant Na contents (up to02 apfu) Although Ti is unusual in dioctahedralmicas it has been measured in several analyses

(lt005 apfu) The most evident feature in theplots is the scattering of data in the La Santa andMorron de Totana units ie where diageneticconditions are not exceeded In contrast theYechar Unit analyses define a narrow field in thediagrams and the Los Molinos data are even closerto the theoretical muscovite composition (Table 3)

Typical compositions of micas intermediate tomuscovite and paragonite called Na-K micas in thispaper are shown in Table 4 The compositions arequite variable even in the same sample Whencompared with analyses of K-rich dioctahedralmicas (Table 3) and paragonite analyses are seento span the entire range from muscovite toparagonite Some analyses (eg Es-5 9) fall within

FIG 14 Plots of values of chemical compositions of K-rich dioctahedral phyllosilicates solid circles and linesindicate values for theoretical muscovite and corresponding exchange vectors respectively

Prograde and retrograde metamorphism 15

the limits of the solvus between muscovite andparagonite the limbs of that solvus having theapproximate compositions ms60pg40 and ms20pg80

(Li et al 1994a)Detrital muscovite vs metamorphic muscovite

Detrital-like micas of sample Es-95 the lowest-grade one are large enough to be analysed(Table 5) by electron microprobe (EMPA) Thesedata have been compared with the AEM data ofother samples Compositions of detrital-like grainsof the lowest-grade sample and the detrital micas ofthe anchizonal-grade sample Es-53 (Fig 15) arevery similar and close to that of end-membermuscovite These data suggest that the detrital micahas a common source for the different units thatcomprise Sierra Espuna Figure 15 also comparesthe data from authigenic micas of the samesamples which are clearly more phengitic (Fe+Mg= 015 ndash075 apfu) than the detrital ones (Fe+Mg= 01 ndash02 apfu)

Chlorite Trioctahedral and di-trioctahedral(sudoite) chlorites have been analysed (Table 6)The former corresponding to the species clino-chlore is common in most of the samples unlikesudoite which is absent from most (Table 1)Sudoite was identified because the sum ofoctahedral cations is nearly equal to 5 apfu andthe Si and Al contents are higher than intrioctahedral chlorite (Fig 16a) A slight contam-ination by interlayer cations (K and Na) has beendetected therefore the formulae were recalculatedin order to eliminate those impurities (seeAnalytical Methods) Although total Fe has beenconsidered as Fe2+ according to Fransolet ampBourguignon (1978) the Fe3+ content in sudoitemay be greater than the Fe2+ content As severalanalyses have an octahedral sum slightly higherthan the ideal value of 5 apfu this may indicate atrioctahedral substitution andor the presence ofsignificant Fe3+ No Ti or Mn were detected The

Fe(Fe+Mg) ratios do not define a clear trendthroughout the sequence (Fig 16b) nevertheless itis much greater in chlorite in the Morron de Totanaanalyses falling in the characteristic range ofdiagenetic chlorites (Li et al 1994b) There is anincrease in Mg content (from 15 to 4 apfu)concomitant with increase in grade as compatiblewith some other sequences (eg Bevins et al1991)

D I S C U S S I O N

Dioctahedral phyllosilicates chemicalcompositions IC values and polytypes

On the basis of their large size homogeneity ofindividual grains and well-crystallized aspectdetrital dioctahedral mica grains were identified inseveral samples with the exception of those ofgreenschist facies This confirms the idea of the

TABLE 4 Chemical compositions for Na-K micas normalized to O10(OH)2

Si IVAl VIAl Fe Mg Ti VIS Ca K Na S inter

Es-5 14 313 087 190 004 009 002 204 000 034 048 083Es-5 15 321 079 176 009 037 000 222 000 015 038 053Es-5 4 318 082 187 004 019 000 209 000 013 061 074Es-5 12 329 071 172 008 029 000 208 000 039 036 076Es-5 13 323 077 179 005 028 000 212 000 029 041 069Es-5 9 296 104 163 019 029 000 211 000 048 056 104Es-5 8 292 108 156 043 008 001 209 003 014 080 096

FIG 15 Plot of Si vs Al contents of detrital andauthigenic micas of samples Es-95 and Es-53 Solidsymbols = detrital micas open symbols = authigenic

micas

16 I Abad et al

relative stability of muscovite in most diageneticenvironments (Crowley 1991) The compositions ofthe detrital grains of samples from different unitsare similar The lack of heterogeneity which isexpected for mica from a complex source implies acommon source area for all samples

The IC values define a trend from diagenetic toanchizonal conditions but with smaller IC values inbulk-rock fractions consistent with the contributionof detrital mica The coexistence of mature micawith lower-grade I-S or illite in the sub-greenschistfacies rocks shows that those rocks have not

TABLE 5 EMPA data for detrital-like crystals of mica normalized to O10(OH)2

Si IVAl VIAl Fe Mg Ti VIS Ca K Na K+Na+Ca

Es-95z11 313 087 193 005 004 001 203 000 065 022 087Es-95z11 314 086 190 006 007 001 204 000 068 019 087Es-95z1 2 314 086 191 005 009 001 206 000 070 006 077Es-95z2 1 314 086 176 012 010 003 201 002 095 004 101Es-95z2 2 317 083 177 012 011 003 202 000 095 004 099Es-95z4 1 305 095 188 005 007 004 204 000 081 009 089Es-95z4 2 311 089 187 006 007 004 204 000 078 009 086

TABLE 6 Representative chemical compositions for chlorite normalized to O10(OH)8

Si IVAl VIAl Fe Mg VIS FeFe+Mg

Yechar UnitEs53-1 273 127 112 121 376 609 024Es53-4 287 113 125 070 400 596 015Es53-12 303 097 130 146 307 583 032Es53-13 277 123 125 122 354 601 026Es53-15 307 093 146 094 323 564 023Es53-42 271 129 152 160 294 606 035

La Santa UnitEs2-4 319 081 147 148 273 569 035Es2-5 365 035 183 118 225 526 034Es5-1 354 046 293 023 172 488 012Es5-5 361 039 292 021 178 490 010Es5-10 309 091 262 028 221 511 011Es5-16 310 090 197 112 234 543 032Es5-17 350 050 252 076 170 499 031Es-5 10rsquo 364 036 213 097 202 512 032Es-5 11 291 109 143 136 314 593 030Es-5 12 382 018 229 087 179 495 033Es-5 13 316 084 302 021 172 494 011Es-5 16rsquo 346 054 341 013 101 455 011Es-5 17rsquo 300 100 282 033 194 509 014Es-5 18 285 115 285 026 204 515 011

Morron de Totana UnitEs-86 2 377 023 253 088 149 490 037Es-86 4 284 116 166 230 181 578 056Es-86 5 350 050 191 192 151 534 056Es-86 6 366 034 189 191 149 529 056Es-86 7 315 085 129 273 176 578 061

indicates sudoite analyses

Prograde and retrograde metamorphism 17

reached stable chemical or textural equilibrium atleast at the mm scale In addition the lack ofchemical equilibrium among diagenetic I-S andillite at the nm scale is demonstrated by their highlyvariable Fe+Mg contents (006ndash075 apfu) Theaverages of the sum of interlayer cations are closeto 1 apfu in the Los Molinos and Yechar units asconsistent with mica but values of 06 apfucorresponding to I-S or illitic compositions are alsopresent in the Morron de Totana Unit Except forsamples Es-102 and Es-53 the variable composi-tions represent a metastable state with composi-tions of micas determined by a combination ofphengitic ferrimuscovitic and illitic vectors asshown by Fig 14ab

Paragonite is generally considered an indexmineral for the anchizone (Frey 1987) but Li etal (1994a) have shown that paragonite coexistswith K-rich mica in grades as low as that of the latediagenetic zone for a prograde sequence Samplesin which muscovite paragonite and intermediateNa-K mica coexist as in sample Es-5 areconsistent with metastability both because morethan two such micas coexist and because theircompositions are incompatible with the solvus inthe muscovite-paragonite system (see below) Whilethe 2M polytype typical of metamorphic micas is

the most common in these samples the 1Md

polytype also occurs in some diagenetic samplesparticularly those from the Morron de Totana UnitThe 1Md polytype is metastable relative to the 2Mpolytype and the coexistence of both polytypesfurther demonstrates a lack of equilibrium at the nmscale Dong amp Peacor (1996) showed that thedisorder noted in the diagenetic phyllosilicates bySAED is a consequence of the general high entropyof phases formed at very low temperatures thatproduces for example stacking defects separatingpackets with no more than 10 layers These packetsprogressively thicken with increasing metamorphicgrade and the 2M polytype then becomes the mostcommon polytype in authigenic micas Thenm-scale size of the lower-grade clays and theregular increase in size with increasing gradereflect the lack of textural equilibrium in the low-grade samples

A wide range of sample characteristics thusimply that the lower-grade samples are in statesof metastable equilibrium those factors includingcoexistence of mature mica with I-S and illitevariable chemical compositions incompatiblecompositions of white micas small crystal sizesdefect-rich states and polytypism Such dataemphasize the notion of lsquoreaction progressrsquo relative

FIG 16 (a) Plot of Al vs Fe+Mg contents of chlorites illustrating the contrast in composition between tri-trioctahedral clinochlore or chamosite and di-trioctahedral sudoite (b) plot of Si vs Fe(Fe+Mg) ratio of

chlorites showing an absence of compositional trends throughout the sampled sequence

18 I Abad et al

to Ostwald-step rule processes metastable dioctahe-dral clay systems undergoing reactions toward thestate of stable chemical and textural equilibriumapproached in greenschist-facies samples (Essene ampPeacor 1995)

Prograde evolution based on TEM datacorrelation with the Basque-Cantabriansequence (Northern Spain)

With increasing grade TEM data confirm thatcrystal thickness increases crystal defect densitydecreases compositions become less variable andmicrotextural relations are simpler (eg Peacor1992) This trend is consistent with three well-characterized low-grade settings the Gulf Coastsequence (Ahn amp Peacor 1986) and the WelshBasin (Merriman et al 1990) both extensionalbasins and the Helvetic Alps (Livi et al 1997)The latter is a fold-and-thrust belt with similarseries of prograde transitions to those observed inSierra Espuna in which pelites have passed througha sequence of metastable states in the lithologicalsequence mudstone and shale slate and phyllite

The mudstones of the Morron de Totana Unit arecharacterized by the presence of illite-smectite R4mixed-layers mica packets of no more than 10 ndash40layers and the occurrence of 1Md polytypism TheLa Santa Unit samples represent a step in theprograde evolution with thicker packets than in theprevious samples (200 ndash700 AEcirc ) illitic or phengiticcompositions the absence of illite-smectite mixedlayers as well as the presence of discreteintermediate Na-K micas and paragonite Althoughthere is a tendency towards non-preferred orienta-tion of crystals it is not so evident as in the Morronde Totana samples In the Yechar Unit (anchizone)and more clearly in the Los Molinos phyllites(greenschist facies) the microtextural relations areless complex the compositions are less variablewith phengite and trioctahedral chlorite crystalthickness are greater (gt400 AEcirc in the Yechar Unitand gt1000 AEcirc in the Los Molinos Unit) with thepackets being defect-free and with a very crystallineaspect This description has features in commonwith the three well-characterized low-grade settingspreviously noted Only the mineral assemblagesvary somewhat presumably as a function of thedifferences in bulk-rock composition

A detailed comparison with the Basque-Cantabrian Basin (N Spain) shows that the lowest-grade conditions of Sierra Espuna overlap most of

those of that sequence which is characterized bypassive burial diagenesis (8000 m thick) (Nieto etal 1996) Only in the deepest sample of theBasque-Cantabrian sequence is illite-smectite mixedlayering absent Figure 12 of this study is verysimilar to Fig 7 in Nieto et al (1996) and bothimages correspond to diagenetic samples fromdifferent settings nevertheless they illustratesimilar mica packets 100 ndash200 AEcirc thick withparallel-to-subparallel boundaries the orientationof which is inferred to represent beddingTherefore there is a parallelism between thesesequences down to the scale of individual clay-mineral grains even though they developed underdifferent conditions with stress presumably playinga significant role in the Sierra Espuna units Thetexture of the least metamorphosed sample (Fig 13)is reminiscent of the typical textural relations ofsmectite described by Nieto et al (1996) (seeFig 4) Although it corresponds to illite-richinterstratified illite-smectite the clay mineralgrains conserved the grain size and anastomosingdiscontinuous array of original smectite layers

Despite the similarities between the twosequences the chemical behaviour of the micasare clearly different since the grade correspondingto the phengitic compositions of the Yechar Unit isnot reached in the Basque-Cantabrian Basin wherethe deepest and highest-grade sample has illiticcompositions equivalent to those of the La SantaUnit That is in a passive burial metamorphiccontext as in the Basque-Cantabrian Basin thefactors responsible for increasing the rate ofchemical reactions are less significant than in thesequences that were affected by tectonic stressTectonic stress introduces defects which in turnincreases the rate of dissolution of strained grainsthus increasing the rate of dissolution-crystallizationreactions typical of clay minerals Perhaps moreimportantly regional tectonic stress is known tocause mobilization of fluids (eg Oliver 1986) Thepresence and activity of such fluids is a prime causeof increased rates of dissolution ion diffusion andcrystallization Tectonic activity thus may have atwo-fold effect in increasing rate of clay-mineralreactions The phengite component may in part be ameasure of the reaction progress depending on thereaction kinetics in very low-grade rocksAccording to Merriman amp Peacor (1999) whereheat flow is low as in accretionary settingsphengite contents are typically high perhapsbecause there is insufficient thermal energy to

Prograde and retrograde metamorphism 19

cause equilibration of the phengite component ofneoformed illite by contrast with extensional basinswhere this component is typically small

Coexistence of trioctahedral chlorite andsudoite Implications for multiple metamorphicevents

In metamorphic rocks the coexistence of mineralsthat formed at different times and conditions on aPTt path ie not in equilibrium is a commonlyobserved feature where the metamorphic history iscomplex Gross differences in the grain size ofcoexisting phases has long been recognized bymetamorphic petrologists as an indicator of non-equilibrium mineral assemblages Historically muchless attention has been paid to such relations in sub-greenschist-facies rocks because prior to the intro-duction of SEM+TEM techniques their small grainsize prevented observations of textural relationsStudy of such rocks eg by XRD provides littleinformation on textural relations beyond determina-tion of degree of preferred orientation

Study by XRD of the samples showed thatsudoite and trioctahedral chlorite coexist If theyhad formed at the same time and under the sameP-T conditions their compositions would define thelimbs of a solvus The TEM images show howeverthat wherever they are in contact the sudoitetransects the packets of trioctahedral chlorite (egFig 8) Furthermore as illustrated in Fig 8 thetrioctahedral chlorite is commonly deformed anddisplays strain contrast in TEM images Thesetextural relations suggest that sudoite was intro-duced subsequent to the formation of trioctahedralchlorite Similar observations were made byGiorgetti et al (1998) who concluded that texturesin Verrucano metasediments showed that sudoitepost-dated chloritoid pyrophyllite and trioctahedralchlorite ie sudoite is in disequilibrium with thelatter minerals and formed as a retrograde phase Inrocks of the Sierra Espuna area differences in bulk-rock composition are small (Table 2) and do notcorrespond to the presence or absence of sudoite(Table 1) ie the random occurrence of sudoiteindependent of bulk-rock composition is compa-tible with a lack of equilibrium relations Theserelations collectively imply that sudoite wasintroduced at some time subsequent to theformation of mineral assemblages at peak meta-morphic conditions On the other hand the size ofsudoite packets is similar to that of trioctahedral

chlorite as consistent with formation at conditionsnot much different from those for chlorite

In samples from the Morron de Totana bothsudoite and trioctahedral chlorite were affected byretrograde alteration to smectite The TEM imagesshow sudoite (Fig 11) and trioctahedral chlorite(Fig 10) with interstratified layers of smectite withspacings of 10 AEcirc due to collapse and dehydration inthe TEM environment and even packets of smectite150 AEcirc thick within chlorite crystals These relationsimply that both sudoite and trioctahedral chloritehave been affected by retrograde alteration at thevery low temperatures consistent with smectiteformation although the presence of more subtlealteration features in sudoite of some samplessuggests that it was less susceptible to alteration

We therefore conclude that at least three episodesof phyllosilicate formation can be recognized inrocks from the Sierra Espuna-area Trioctahedralchlorite was presumably produced at the highestmetamorphicdiagenetic grade in equilibrium withparagonite and phengite Subsequently sudoite wasintroduced but at P-T conditions which could nothave been much different than those of peakmetamorphic grade Finally at very low-T condi-tions hydrothermal fluids caused partial alterationof trioctahedral chlorite and sudoite to smectite

Retrograde products formed by the reaction offluids with mineral phases generated during anearlier prograde event were described previouslyby Nieto et al (1994) and Zhao et al (1999) Thosereactions occurred on a regional scale but similarreactions have been observed on a more localizedscale The fluid activity associated with a majorfault zone is a common source for these reactions(eg Jiang et al 1990) The structure of SierraEspuna (Fig 1) is based on imbrication processesthat superimpose units that have subsequently beenaffected by faults and folds and that have under-gone significant uplift at least 800 m from the lateMiocene to the present (Sanz de Galdeano et al2001) Given this evolution it is easy to justify theretrograde processes that usually seem to be relatedwith tectonic events during which fluid flow andfluidrock ratios are enhanced (Merriman amp Peacor1999) Moreover according to Nieto et al (1994)the uplift of the Betic Cordillera would furnish theintroduction of phreatic water into the systemproducing a reactive environment adequate for thegenesis of smectite Geological conditions aretherefore consistent with post-peak metamorphicconditions as implied by the TEM observations

20 I Abad et al

The introduction of sudoite as a major phaseduring metamorphism implies the possibility ofadditional overprints on a simple prograde assem-blage the evidence for which might be subtle TheNa-rich micas at all grades have compositionswhich either fall within the solvus or are too Na-rich to correspond to the limb of the solvus In thehighest-grade rocks such micas coexist withmuscovite Those relations are incompatible withchemical equilibrium despite the fact that relationsfor muscovite and other minerals imply an approachto chemical equilibrium Furthermore there is aclose association of Na-rich micas with sudoite (egFig 7) Those relations imply the possibility thatNa-micas were introduced concomitant withsudoite P Mata (pers comm) has shown forexample that overprinting of an epizonal assem-blage by hydrothermal solutions resulted in theintroduction of both an Al-rich variety of chlorite(cookeite) and paragonite in pelites of the CamerosBasin Spain The textural relations such as cross-cutting textures which are so striking for sudoitewere not observed for Na-rich micas Neverthelessthe possibility of such overprinting must beconsidered

In spite of all these prograde-retrogradeprocesses in the Los Molinos Unit a smallnumber of homogeneous phases coexist in chemicalequilibrium and the fine-grained metamorphicphyllosilicates are clearly parallel to cleavage witha typical metamorphic aspect Finally all thetextural and compositional changes that haveoperated in the Espuna area during the metamorphicevolution proceed toward a simpler system repre-senting both textural and chemical equilibriumwhich is characteristic of greenschist-facies condi-tions

Roles of detrital and authigenic phyllosilicatesduring slaty cleavage development

Van der Pluijm et al (1998) focused on therespective roles of the phyllosilicates of detritalorigin and those formed during diagenesis andmetamorphism and observed that detrital phyllosi-licates remain preferentially oriented parallel tobedding in pelites with slaty cleavage whereas fine-grained metamorphic phyllosilicates are orientedparallel to cleavage

Although it is usually difficult to determine thepreferred orientations of phyllosilicates in TEMimages as the grade increases better-aligned

packets occur in the cleavage orientation Thetexture of the least metamorphosed sample inwhich even incipient slaty cleavage is absent isreminiscent of the typical textural relation ofsmectites described in the literature The illite-smectite mixed-layers conserve the textural featuresof possible previous smectite such as anastomosingfringes curved and discontinuous with variations ininterplanar spacing d001 These features indicatethat the minimum energy conditions to initiatemechanical transformations have not been reachedin the lower-grade samples as postulated by Ho etal (1996) since slaty cleavage develops as a resultof interactive mechanical deformation and ther-mally driven dissolution and crystallizationprocesses (Knipe 1981) In contrast in the LosMolinos Unit large crystals of chlorite and whitemica (gt1000 AEcirc ) occur which are of metamorphicorigin but with no detrital grains (Fig 3a)Phyllosilicates are preferentially oriented parallelto slaty cleavage in response to increasedtemperature and tectonic strain (Merriman ampPeacor 1999) The oriented microfabrics and slatycleavage well developed in the rocks of this unitmust be a product of a higher thermal and strainenergy environment reached in the deepest part ofthe sequence As pointed out by Van der Pluijm etal (1998) a continuous range of orientations isevidence for mechanical rotation of grains whereasthe lack of such transitional orientations impliesthat cleavage-parallel phyllosilicates originatedthrough dissolution of bedding-parallel and crystal-lization in the slaty cleavage orientation Asobserved in this study for the Intermediate Unitsgrains with orientations which are transitional frombedding-parallel to slaty cleavage orientation areonly rarely observed Cleavage formation is there-fore inferred to have occurred entirely or nearlyentirely through dissolution and crystallization

ACKNOWLEDGMENTS

We thank MM Abad Ortega from the Centro deInstrumentacio n Cient otilde fica of the Universidad deGranada for her help with HRTEM and I Nieto forher essential help with sample preparation Financialsupport was supplied by Research Project nordm BT 2000-0582 FPI research grant to IA both of the SpanishMinistry of Science and Technology Research GroupRNM-0179 of the Junta de Andaluc otildeacutea and to DRP byNSF grant EAR-9814391

Prograde and retrograde metamorphism 21

REFERENCES

Ahn JH amp Peacor DR (1986) Transmission andanalytical electron microscopy of the smectite-to-illite transition Clays and Clay Minerals 34165 ndash179

Azanon JM amp Crespo-Blanc A (2000) Exhumationduring a continental collision inferred from thetectonometamorphic evolution of the AlpujarrideComplex in the central Betics (Alboran Domain SESpain) Tectonics 19 549 ndash565

Bevins RE Robinson D amp Rowbotham G (1991)Compositional variations in mafic phyllosilicatesfrom regional low-grade metabasites and applicationof the chlorite geother mometer Journal ofMetamorphic Geology 9 711 ndash721

Cliff G amp Lorimer GW (1975) The quantitativeanalysis of thin specimens Journal of Microscopy 103 203 ndash207

Crowley SF (1991) Diagenetic modification of detritalmuscovite an example from the Great LimestoneCyclothem (Carboniferous) of Co Durham UKClay Minerals 26 91 ndash103

Dong H amp Peacor DR (1996) TEM observations ofcoherent stacking relations in smectite and illite ofshales evidence for MacEwan crystallites anddominance of 2M1 polytypes Clays and ClayMinerals 44 257 ndash275

Essene E amp Peacor DR (1995) Clay mineral thermo-metry a critical perspective Clays and ClayMinerals 43 540 ndash553

Fransolet AM amp Bourguignon P (1978) Dioctahedralchlorite in quartz veins from Ardennes BelgiumThe Canadian Mineralogist 16 365 ndash373

Fransolet AM amp Schreyer W (1984) Sudoite ditrioctahedral chlorite a stable low-temperature phasein the system MgO-Al2O3-SiO2-H2O Contributionsto Mineralogy and Petrology 86 409 ndash417

Frey M (1987) Very low-grade metamorphism of clasticsedimentary rocks Pp 9 ndash58 in Low TemperatureMetamorphism (M Frey editor) Blackie GlasgowUK

Giorgetti G Goffe B Memmi I amp Nieto F (1998)Metamorphic evolution of Verrucano metasedimentsin northern Apennines new petrological constraintsEuropean Journal of Mineralogy 10 1295 ndash1308

Guidotti CV amp Sassi FP (1986) Classification andcorrelation of metamorphic facies series by means ofmuscovite b0 data from low grade metapelites NeuesJahrbuch fur Mineralogie Abhandlungen 153363 ndash380

Guidotti CV Yates MG Dyar MD amp Taylor ME(1994) Petrogenetic implications of the Fe3+ contentof muscovi te in pel it ic sch is ts AmericanMineralogist 79 793 ndash795

Guthrie GD amp Veblen DR (1989) High resolutionelectron microscopy of mixed-layer illitesmectite

Computer simulations Clays and Clay Minerals 371 ndash11

Ho N Peacor DR amp Van der Pluijm BA (1996)Contrasting roles of detrital and authigenic phyllo-silicates during slaty cleavage development Journalof Structural Geology 18 615 ndash623

Jiang WT Peacor DR Merriman RJ amp Roberts B(1990) Transmission and analytical electron micro-scopic study of mixed layer illitesmectite formed asan apparent replacement product of diagenetic illiteClays and Clay Minerals 38 449 ndash468

Jiang WT Peacor DR amp Buseck PR (1994) Chloritegeothermometry Contamination and apparent octa-hedral vacancies Clays and Clay Minerals 42593 ndash605

Kisch HJ (1991) Development of slaty cleavage anddegree of very-low-grade metamorphism a reviewJournal of Metamorphic Geology 9 735 ndash750

Knipe JR (1981) The interaction of deformation andmetamorphi sm in slates Tectonophysics 78249 ndash272

Kretz R (1983) Symbols for rock-forming mineralsAmerican Mineralogist 68 277 ndash279

Li G Peacor DR Merriman RJ amp Roberts B (1994a)The diagenetic to low grade metamorphism evolu-tion of matrix white mica in the system muscovite-paragonite in a mudrock from Central Wales UKClays and Clay Minerals 42 369 ndash381

Li G Peacor DR Merriman RJ Roberts B amp Vander Pluijm BA (1994b) TEM and AEM constraintson the origin and significance of chlorite-mica stacksin slates an example from Central Wales UKJournal of Structural Geology 16 1139 ndash1157

Livi KJT Veblen DR Ferry JM amp Frey M (1997)Evolution of 21 layered silicates in low-grademetamorphosed Liass ic sha les of CentralSwitzerland Journal of Metamorphic Geology 15323 ndash344

Lonergan L (1991) Structural evolution of the SierraEspuna Betic Cordillera SE Spain PhD thesisOxford University UK

Lonergan L Platt JP amp Gallagher L (1994) TheInternal-External Zone Boundary in the eastern BeticCordillera SE Spain Journal of Structural Geology16 175 ndash188

Makel GH (1981) Differences in tectonic evolution ofsuperimposed Malaguide and Alpujarride tectonicunits in the Espuna area (Betic Cordilleras Spain)Geology in Mijnbouw 60 203 ndash208

Makel GH (1985) The geology of the MalaguideComplex and its bearing on the geodynamicevolution of the Betic-Rif orogen (southern Spainand northern Morocco) GUA papers of Geology Ser1 22 263 pp

Makel GH amp Rondeel HE (1979) Differences instratigraphy and metamorphism between superposedMalaguide and Alpujarride units in the Espuna area

22 I Abad et al

within illite packets as elongated individual grainsup to ~3006500 AEcirc in size or as aggregates(Fig 7) they show up clearly due to their darkcontrast Sudoite and chlorite were identified on thebasis of SAED and AEM data Separate grains ofeach were observed to be intergrown with texturestypical of metamorphic origins for both Grain sizesof sudoite (gt500 AEcirc ) and clinochlore are approxi-mately equal implying formation of both at gradeswhich were not substantially different Howeverwhere grains of each intersect at high angles thesudoite ubiquitously transects clinochlore packetsAs shown in Fig 8 clinochlore but not sudoitemay exhibit strain contrast These relations implybut do not prove that sudoite post-dated clino-chlore Obvious alteration of clinochlore todioctahedral smectite was ubiquitous but possiblealteration effects were rare in sudoite occurring asdefects such as voids and wavy layers Because thecomposition of sudoite is similar to that ofdioctahedral smectite it was not possible to verifythe presence of small numbers of smectite layerswith AEM data Nevertheless the observationsclearly show that alteration of chlorite to smectitewas relegated entirely or almost entirely toclinochlore

Muscovite occurs in crystals which average~500 AEcirc in thickness and are characterized by 2Mpolytypism features which are typical of meta-morphic mica of anchizonal grade The Na-K micaoccurs in packets of similar thickness The

FIG 6 Lattice-fringe image showing the relation between mica and chlorite packets The arrows indicate theposition of layer terminations The inset SAED pattern illustrates the low-angle relation between the two phases

(Yechar Unit)

FIG 5 Low-magnification image with well-definedhigh-angle grain boundaries and interlocking relations

among muscovite crystals (Yechar Unit)

Prograde and retrograde metamorphism 9

composition (see below) is quite variable corre-sponding to Na-rich muscovite commonly but somegrains have compositions within the limits of thesolvus between muscovite and paragonite and thusare metastable Na-K micas give SAED patternswhich have relatively sharp reflections over a largerange as consistent with large defect-free crystals

Sudoite crystals comprise units with 14 AEcirc

periodicities which in some areas are 28 AEcirc Lattice-fringe images show the presence of inter-

stratified individual 7 AEcirc layers which in some areasform small packets containing no more than eight7 AEcirc layers The AEM analyses (see below) of suchareas are typical of those of homogeneous sudoitecontaining only 14 AEcirc layers implying a composi-tion of the 7 AEcirc layers similar to that of sudoite andnot to an exotic septechlorite such as kaolinite orberthierine The contrast of both 14 AEcirc and 7 AEcirc

fringes appear to be identical further consistentwith a close chemical relation

FIG 7 Textural relations between phyllosilicates (sudoite illite Na-K mica) and hematite (dark contrast) (LaSanta Unit)

FIG 8 Lattice-fringe image showing the coexistence of two types of chlorite trioctahedral chlorite (chl) andsudoite (su) Sudoite transects chlorite which shows strain contrast associated with curved and kinked layersIdentifications were based in part on AEM analyses (see Table 6 Es-511 and 12 analyses) (La Santa Unit)

10 I Abad et al

Three samples corresponding to different butequivalent levels were selected from the Morron deTotana Unit for TEM study (see Fig 1 peliticrocks intercalated with carbonate rocks) Es-77 atthe bottom of the unit Es-86 which is located in anintermediate position and Es-95 from the top of thecross-section There is no incipient slaty cleavage inthese samples Sample Es-77 is a very fine-grainedrock consisting of quartz white mica chlorite andaggregates of carbonate grains Minor hematiteproduces a deep red colour Lattice-fringe images

show that dioctahedral clay minerals have 10 and20 AEcirc periodicities occur as packets 100 ndash200 AEcirc

thick and have subparallel orientations Accordingto Guthrie amp Veblen (1989) smectite and illiteinterlayers can produce different contrast and canonly be differentiated in lattice fringe-imagesobtained with well-defined electron beam (001)orientations and defocus conditions Such contrastdifferences were observed in several lattice-fringeimages The light contrast for fringes in Fig 9represents the smectite interlayer as shown in part

FIG 9 TEM image showing 10 AEcirc periodicity in R4 illite-rich I-S Contrast differences imply the presence ofillite-like (the dark fringes) and smectite-like (the light fringes) layers Packets usually have four or more illite

layers and the SAED pattern shows predominantly 50 AEcirc periodicity (Morron de Totana Unit)

FIG 10 Trioctahedral chlorite with intercalations of 10 AEcirc layers (upper arrow) which damage quickly under theelectron beam The chlorite packets show strain contrast microfolds and 7 AEcirc periodicities corresponding toberthierine (left arrow) or greater values eg 28 AEcirc (see analysis in Table 6 Es-867) (Morron de Totana Unit)

Prograde and retrograde metamorphism 11

by the continuity of such fringes with openinterlayer spaces produced by contraction due to

dehydration The sequence on average is that ofR4 mixed-layering ie a single smectite-likeinterlayer between four or more illite-like inter-layers Although some packets with more than fourlayers of illite are present repeats of four illite-likelayers predominate as shown by the 50 AEcirc

periodicity of SAED patterns (Fig 9 inset) Theillite-smectite (I-S) has typical 1Md polytypism Theobservation of smectite interstratified with illite isin accord with the changes observed in XRDdiagrams after EG treatment (Fig 2) In additionAEM analyses of areas similar to that of Fig 9have relatively high Si and minor Ca contentsfurther confirming the mixed layering

Some packets which give rise to fringes with10 AEcirc spacing produce SAED patterns typical oftwo-layer polytypism The contrast of all fringes isrelatively constant Such packets have thicknessestypical of diagenetic clays and two orders ofmagnitude smaller than typical detrital grains Thecompositions of such grains (see below) haveinterlayer-cation contents typical of illite They aretherefore inferred to be illite which formed duringlate diagenesis and coexisting with the R4 I-S

Sample Es-86 is composed mainly of detritalgrains of quartz randomly-oriented grains ofchlorite and white mica all immersed in aphyllosilicate-rich fine-grained matrix At theTEM scale chlorite is the most common phyllosi-licate in the matrix As in sample Es-5 trioctahe-dral chlorite and sudoite coexist There areinterstratified packets of 10 AEcirc layers only a fewlayers thick (lt150 AEcirc ) which are quickly damaged

FIG 11 Di-trioctahedral chlorite (sudoite) very similarin appearance to the trioctahedral chlorite shown inFig 10 7 AEcirc layers are marked by an arrow and 10 AEcirc

layers are present in the poorly resolved area in thecentre of the image (see analysis in Table 6 Es-862)

(Morron de Totana Unit)

FIG 12 Low-magnification image illustrating illite-like packets The inset SAED pattern shows only a smallnumber of diffuse poorly defined reflections The packets are 100 ndash200 AEcirc thick (Morron de Totana Unit)

12 I Abad et al

under the electron beam (Figs 10 and 11) TheAEM analyses of such material show an enhancedSi content and the presence of small concentrationsof alkali cations These data are consistent withretrograde alteration of chlorite as observed byNieto et al (1994) They differ however from theresults of Nieto et al (1994) and those describedabove for sample Es-5 in that both tri-trioctahedraland di-trioctahedral chlorite are affected In TEMimages strain features in chlorites such as contrastnormal to the layers or microfolds were observed(Fig 10) Mica is less abundant than sudoiteoccurring in packets 100 ndash400 AEcirc thick The SAEDpatterns exhibit 2M polytypism but with diffuse-ness parallel to c in 0kl rows At low magnifica-tion subparallel and slightly curved illite- or illite-smectite-like packets are present they coalescehave abundant layer terminations and show intensecontrasts (Fig 12)

Sample Es-95 is a very fine-grained rock madeup of large detrital grains of quartz in a fine-grained matrix of phyllosilicates mainly whitemicas and opaque minerals Some biotite grainswere observed as verified by AEM data Biotitewas observed only in separated grains on holey

C-coated Cu grids so it is not possible to describethe textural relations for it Authigenic biotite isinconsistent with the low grade of this samplehowever and it is therefore inferred to be of detritalorigin The AEM analyses show deviations frompure biotite however as consistent with alterationto chlorite the usual process in detrital biotite inrocks undergoing diagenesis Lattice-fringe imagesof ion-milled samples show that individual micapackets are very thin with no more than 10 layersisolated and dispersed in a matrix consisting ofmaterial which gives no SAED pattern Thesepackets are wavy discontinuous and with anasto-mosing layers (Fig 13) Despite these texturalfeatures SAED data indicate that there are 2Mand 1Md polytypes

For all samples prismatic crystals of Ti-oxidedispersed Fe-oxyhydroxides zircon and tourmalineare common accessory minerals

Chemical characterization of phyllosilicates

Dioctahedral mica Table 3 shows the composi-tion of K-rich dioctahedral micas of authigenic ormetamorphic origin corresponding to the samples

FIG 13 General aspect of sample Es-95 The image shows arrays of wavy discontinuous and anastomosingpackets of illite and smectite (Morron de Totana Unit)

Prograde and retrograde metamorphism 13

selected from each unit The Yechar and LosMolinos units are characterized by compositionsapproaching those of evolved micas with a sum ofinterlayer cations slightly less than 1 apfu typicalof phengite or muscovite rather than authigenic

illite In contrast the La Santa and Morron deTotana units show heterogeneous mica composi-tions and display deficiencies in interlayer cations(Fig 14a) This last feature is more evident in theMorron de Totana analyses where the illitic

TABLE 3 Representative chemical compositions for K-rich dioctahedral micas normalized to O10(OH)2

Si IVAl VIAl Fe Mg Ti VIS K Na S inter

Los Molinos Unit (1)Es-10213 337 063 178 010 008 002 198 075 008 085Es-10221 308 092 195 005 001 001 202 082 009 091Es-10231 319 081 191 005 003 001 200 077 012 090Es-10232 314 086 188 006 007 001 202 072 021 093

Yechar Unit (1)Es-5311 332 068 163 015 022 002 204 090 004 093Es-5312 310 090 180 003 013 004 201 089 010 099Es-5322 338 062 170 012 015 003 200 086 001 086Es-5323 319 081 176 016 012 003 206 087 002 089

Yechar Unit (2)Es-53 1 317 083 163 021 026 000 210 084 000 084Es-53 2 320 080 165 011 027 003 207 087 000 087Es-53 3 317 083 155 025 024 002 206 093 000 093Es-53 4 328 072 160 020 024 000 203 092 000 092Es-53 5 326 074 164 019 016 000 199 096 000 096Es-53 6 326 074 158 020 028 000 206 089 000 089Es-53 7 319 081 154 025 026 000 205 097 000 097Es-53 8 325 075 166 016 025 000 206 085 000 085Es-53 9 324 076 167 019 019 000 205 085 000 085

La Santa Unit (2)Es-2 1 311 089 157 019 032 002 210 088 005 094Es-2 3 336 064 148 020 043 001 212 072 003 075Es-2 6 313 087 156 024 029 001 210 075 017 092Es-2 7 313 087 155 024 029 001 209 084 011 095Es-2 8 331 069 157 015 032 000 205 087 004 092Es-2 10 328 072 160 018 027 003 208 068 008 075Es-5 4 303 097 174 015 023 001 213 073 009 082Es-5 10 301 099 189 006 015 000 210 065 021 086Es-5 7 307 093 159 033 016 000 208 075 018 093Es-5 8rsquo 301 099 160 031 025 000 216 067 016 083

Morron de Totana Unit (2) (analyses corresponding to R4 Ill-Sm may also be present see text)Es-77 13 327 073 172 013 027 000 212 067 000 067Es-77 15 316 084 195 003 013 000 211 063 000 063Es-77 16 328 072 182 010 015 000 207 068 000 068Es-77 17 338 062 147 032 019 000 199 093 000 093Es-86 9 334 066 160 017 034 000 210 072 000 072Es-86 10 357 043 151 012 046 000 209 065 000 065Es-86 11 353 047 156 012 036 000 203 077 000 077Es-86 13 324 076 152 025 035 000 211 084 000 084Es-86 14 324 076 173 013 019 000 205 084 000 084Es- 95 2 323 077 158 016 019 004 198 103 000 103Es- 95 5 336 064 169 013 025 000 207 070 000 070Es- 95 6 336 064 148 027 033 000 209 078 000 078

(1) SEMEDX analyses (2) TEMAEM analyses

14 I Abad et al

character is consistent with the presence of the R4mixed-layer illite-smectite described above Most ofthe micas are phengitic in composition (Fig 14ab)with an Fe(Fe+Mg) ratio that varies over the range02 ndash085 However the detrital-like grains aresimilar to those of end-member muscovite (seebelow detrital muscovite vs metamorphic musco-vite or Table 5) with much lower Fe and Mgcontents Data plotted in Fig 14bc confirm theassumption of a significant ferrimuscovitic substitu-tion (Guidotti et al 1994) Some of the K-micaanalysed in the Los Molinos and La Santa unitswere found to have significant Na contents (up to02 apfu) Although Ti is unusual in dioctahedralmicas it has been measured in several analyses

(lt005 apfu) The most evident feature in theplots is the scattering of data in the La Santa andMorron de Totana units ie where diageneticconditions are not exceeded In contrast theYechar Unit analyses define a narrow field in thediagrams and the Los Molinos data are even closerto the theoretical muscovite composition (Table 3)

Typical compositions of micas intermediate tomuscovite and paragonite called Na-K micas in thispaper are shown in Table 4 The compositions arequite variable even in the same sample Whencompared with analyses of K-rich dioctahedralmicas (Table 3) and paragonite analyses are seento span the entire range from muscovite toparagonite Some analyses (eg Es-5 9) fall within

FIG 14 Plots of values of chemical compositions of K-rich dioctahedral phyllosilicates solid circles and linesindicate values for theoretical muscovite and corresponding exchange vectors respectively

Prograde and retrograde metamorphism 15

the limits of the solvus between muscovite andparagonite the limbs of that solvus having theapproximate compositions ms60pg40 and ms20pg80

(Li et al 1994a)Detrital muscovite vs metamorphic muscovite

Detrital-like micas of sample Es-95 the lowest-grade one are large enough to be analysed(Table 5) by electron microprobe (EMPA) Thesedata have been compared with the AEM data ofother samples Compositions of detrital-like grainsof the lowest-grade sample and the detrital micas ofthe anchizonal-grade sample Es-53 (Fig 15) arevery similar and close to that of end-membermuscovite These data suggest that the detrital micahas a common source for the different units thatcomprise Sierra Espuna Figure 15 also comparesthe data from authigenic micas of the samesamples which are clearly more phengitic (Fe+Mg= 015 ndash075 apfu) than the detrital ones (Fe+Mg= 01 ndash02 apfu)

Chlorite Trioctahedral and di-trioctahedral(sudoite) chlorites have been analysed (Table 6)The former corresponding to the species clino-chlore is common in most of the samples unlikesudoite which is absent from most (Table 1)Sudoite was identified because the sum ofoctahedral cations is nearly equal to 5 apfu andthe Si and Al contents are higher than intrioctahedral chlorite (Fig 16a) A slight contam-ination by interlayer cations (K and Na) has beendetected therefore the formulae were recalculatedin order to eliminate those impurities (seeAnalytical Methods) Although total Fe has beenconsidered as Fe2+ according to Fransolet ampBourguignon (1978) the Fe3+ content in sudoitemay be greater than the Fe2+ content As severalanalyses have an octahedral sum slightly higherthan the ideal value of 5 apfu this may indicate atrioctahedral substitution andor the presence ofsignificant Fe3+ No Ti or Mn were detected The

Fe(Fe+Mg) ratios do not define a clear trendthroughout the sequence (Fig 16b) nevertheless itis much greater in chlorite in the Morron de Totanaanalyses falling in the characteristic range ofdiagenetic chlorites (Li et al 1994b) There is anincrease in Mg content (from 15 to 4 apfu)concomitant with increase in grade as compatiblewith some other sequences (eg Bevins et al1991)

D I S C U S S I O N

Dioctahedral phyllosilicates chemicalcompositions IC values and polytypes

On the basis of their large size homogeneity ofindividual grains and well-crystallized aspectdetrital dioctahedral mica grains were identified inseveral samples with the exception of those ofgreenschist facies This confirms the idea of the

TABLE 4 Chemical compositions for Na-K micas normalized to O10(OH)2

Si IVAl VIAl Fe Mg Ti VIS Ca K Na S inter

Es-5 14 313 087 190 004 009 002 204 000 034 048 083Es-5 15 321 079 176 009 037 000 222 000 015 038 053Es-5 4 318 082 187 004 019 000 209 000 013 061 074Es-5 12 329 071 172 008 029 000 208 000 039 036 076Es-5 13 323 077 179 005 028 000 212 000 029 041 069Es-5 9 296 104 163 019 029 000 211 000 048 056 104Es-5 8 292 108 156 043 008 001 209 003 014 080 096

FIG 15 Plot of Si vs Al contents of detrital andauthigenic micas of samples Es-95 and Es-53 Solidsymbols = detrital micas open symbols = authigenic

micas

16 I Abad et al

relative stability of muscovite in most diageneticenvironments (Crowley 1991) The compositions ofthe detrital grains of samples from different unitsare similar The lack of heterogeneity which isexpected for mica from a complex source implies acommon source area for all samples

The IC values define a trend from diagenetic toanchizonal conditions but with smaller IC values inbulk-rock fractions consistent with the contributionof detrital mica The coexistence of mature micawith lower-grade I-S or illite in the sub-greenschistfacies rocks shows that those rocks have not

TABLE 5 EMPA data for detrital-like crystals of mica normalized to O10(OH)2

Si IVAl VIAl Fe Mg Ti VIS Ca K Na K+Na+Ca

Es-95z11 313 087 193 005 004 001 203 000 065 022 087Es-95z11 314 086 190 006 007 001 204 000 068 019 087Es-95z1 2 314 086 191 005 009 001 206 000 070 006 077Es-95z2 1 314 086 176 012 010 003 201 002 095 004 101Es-95z2 2 317 083 177 012 011 003 202 000 095 004 099Es-95z4 1 305 095 188 005 007 004 204 000 081 009 089Es-95z4 2 311 089 187 006 007 004 204 000 078 009 086

TABLE 6 Representative chemical compositions for chlorite normalized to O10(OH)8

Si IVAl VIAl Fe Mg VIS FeFe+Mg

Yechar UnitEs53-1 273 127 112 121 376 609 024Es53-4 287 113 125 070 400 596 015Es53-12 303 097 130 146 307 583 032Es53-13 277 123 125 122 354 601 026Es53-15 307 093 146 094 323 564 023Es53-42 271 129 152 160 294 606 035

La Santa UnitEs2-4 319 081 147 148 273 569 035Es2-5 365 035 183 118 225 526 034Es5-1 354 046 293 023 172 488 012Es5-5 361 039 292 021 178 490 010Es5-10 309 091 262 028 221 511 011Es5-16 310 090 197 112 234 543 032Es5-17 350 050 252 076 170 499 031Es-5 10rsquo 364 036 213 097 202 512 032Es-5 11 291 109 143 136 314 593 030Es-5 12 382 018 229 087 179 495 033Es-5 13 316 084 302 021 172 494 011Es-5 16rsquo 346 054 341 013 101 455 011Es-5 17rsquo 300 100 282 033 194 509 014Es-5 18 285 115 285 026 204 515 011

Morron de Totana UnitEs-86 2 377 023 253 088 149 490 037Es-86 4 284 116 166 230 181 578 056Es-86 5 350 050 191 192 151 534 056Es-86 6 366 034 189 191 149 529 056Es-86 7 315 085 129 273 176 578 061

indicates sudoite analyses

Prograde and retrograde metamorphism 17

reached stable chemical or textural equilibrium atleast at the mm scale In addition the lack ofchemical equilibrium among diagenetic I-S andillite at the nm scale is demonstrated by their highlyvariable Fe+Mg contents (006ndash075 apfu) Theaverages of the sum of interlayer cations are closeto 1 apfu in the Los Molinos and Yechar units asconsistent with mica but values of 06 apfucorresponding to I-S or illitic compositions are alsopresent in the Morron de Totana Unit Except forsamples Es-102 and Es-53 the variable composi-tions represent a metastable state with composi-tions of micas determined by a combination ofphengitic ferrimuscovitic and illitic vectors asshown by Fig 14ab

Paragonite is generally considered an indexmineral for the anchizone (Frey 1987) but Li etal (1994a) have shown that paragonite coexistswith K-rich mica in grades as low as that of the latediagenetic zone for a prograde sequence Samplesin which muscovite paragonite and intermediateNa-K mica coexist as in sample Es-5 areconsistent with metastability both because morethan two such micas coexist and because theircompositions are incompatible with the solvus inthe muscovite-paragonite system (see below) Whilethe 2M polytype typical of metamorphic micas is

the most common in these samples the 1Md

polytype also occurs in some diagenetic samplesparticularly those from the Morron de Totana UnitThe 1Md polytype is metastable relative to the 2Mpolytype and the coexistence of both polytypesfurther demonstrates a lack of equilibrium at the nmscale Dong amp Peacor (1996) showed that thedisorder noted in the diagenetic phyllosilicates bySAED is a consequence of the general high entropyof phases formed at very low temperatures thatproduces for example stacking defects separatingpackets with no more than 10 layers These packetsprogressively thicken with increasing metamorphicgrade and the 2M polytype then becomes the mostcommon polytype in authigenic micas Thenm-scale size of the lower-grade clays and theregular increase in size with increasing gradereflect the lack of textural equilibrium in the low-grade samples

A wide range of sample characteristics thusimply that the lower-grade samples are in statesof metastable equilibrium those factors includingcoexistence of mature mica with I-S and illitevariable chemical compositions incompatiblecompositions of white micas small crystal sizesdefect-rich states and polytypism Such dataemphasize the notion of lsquoreaction progressrsquo relative

FIG 16 (a) Plot of Al vs Fe+Mg contents of chlorites illustrating the contrast in composition between tri-trioctahedral clinochlore or chamosite and di-trioctahedral sudoite (b) plot of Si vs Fe(Fe+Mg) ratio of

chlorites showing an absence of compositional trends throughout the sampled sequence

18 I Abad et al

to Ostwald-step rule processes metastable dioctahe-dral clay systems undergoing reactions toward thestate of stable chemical and textural equilibriumapproached in greenschist-facies samples (Essene ampPeacor 1995)

Prograde evolution based on TEM datacorrelation with the Basque-Cantabriansequence (Northern Spain)

With increasing grade TEM data confirm thatcrystal thickness increases crystal defect densitydecreases compositions become less variable andmicrotextural relations are simpler (eg Peacor1992) This trend is consistent with three well-characterized low-grade settings the Gulf Coastsequence (Ahn amp Peacor 1986) and the WelshBasin (Merriman et al 1990) both extensionalbasins and the Helvetic Alps (Livi et al 1997)The latter is a fold-and-thrust belt with similarseries of prograde transitions to those observed inSierra Espuna in which pelites have passed througha sequence of metastable states in the lithologicalsequence mudstone and shale slate and phyllite

The mudstones of the Morron de Totana Unit arecharacterized by the presence of illite-smectite R4mixed-layers mica packets of no more than 10 ndash40layers and the occurrence of 1Md polytypism TheLa Santa Unit samples represent a step in theprograde evolution with thicker packets than in theprevious samples (200 ndash700 AEcirc ) illitic or phengiticcompositions the absence of illite-smectite mixedlayers as well as the presence of discreteintermediate Na-K micas and paragonite Althoughthere is a tendency towards non-preferred orienta-tion of crystals it is not so evident as in the Morronde Totana samples In the Yechar Unit (anchizone)and more clearly in the Los Molinos phyllites(greenschist facies) the microtextural relations areless complex the compositions are less variablewith phengite and trioctahedral chlorite crystalthickness are greater (gt400 AEcirc in the Yechar Unitand gt1000 AEcirc in the Los Molinos Unit) with thepackets being defect-free and with a very crystallineaspect This description has features in commonwith the three well-characterized low-grade settingspreviously noted Only the mineral assemblagesvary somewhat presumably as a function of thedifferences in bulk-rock composition

A detailed comparison with the Basque-Cantabrian Basin (N Spain) shows that the lowest-grade conditions of Sierra Espuna overlap most of

those of that sequence which is characterized bypassive burial diagenesis (8000 m thick) (Nieto etal 1996) Only in the deepest sample of theBasque-Cantabrian sequence is illite-smectite mixedlayering absent Figure 12 of this study is verysimilar to Fig 7 in Nieto et al (1996) and bothimages correspond to diagenetic samples fromdifferent settings nevertheless they illustratesimilar mica packets 100 ndash200 AEcirc thick withparallel-to-subparallel boundaries the orientationof which is inferred to represent beddingTherefore there is a parallelism between thesesequences down to the scale of individual clay-mineral grains even though they developed underdifferent conditions with stress presumably playinga significant role in the Sierra Espuna units Thetexture of the least metamorphosed sample (Fig 13)is reminiscent of the typical textural relations ofsmectite described by Nieto et al (1996) (seeFig 4) Although it corresponds to illite-richinterstratified illite-smectite the clay mineralgrains conserved the grain size and anastomosingdiscontinuous array of original smectite layers

Despite the similarities between the twosequences the chemical behaviour of the micasare clearly different since the grade correspondingto the phengitic compositions of the Yechar Unit isnot reached in the Basque-Cantabrian Basin wherethe deepest and highest-grade sample has illiticcompositions equivalent to those of the La SantaUnit That is in a passive burial metamorphiccontext as in the Basque-Cantabrian Basin thefactors responsible for increasing the rate ofchemical reactions are less significant than in thesequences that were affected by tectonic stressTectonic stress introduces defects which in turnincreases the rate of dissolution of strained grainsthus increasing the rate of dissolution-crystallizationreactions typical of clay minerals Perhaps moreimportantly regional tectonic stress is known tocause mobilization of fluids (eg Oliver 1986) Thepresence and activity of such fluids is a prime causeof increased rates of dissolution ion diffusion andcrystallization Tectonic activity thus may have atwo-fold effect in increasing rate of clay-mineralreactions The phengite component may in part be ameasure of the reaction progress depending on thereaction kinetics in very low-grade rocksAccording to Merriman amp Peacor (1999) whereheat flow is low as in accretionary settingsphengite contents are typically high perhapsbecause there is insufficient thermal energy to

Prograde and retrograde metamorphism 19

cause equilibration of the phengite component ofneoformed illite by contrast with extensional basinswhere this component is typically small

Coexistence of trioctahedral chlorite andsudoite Implications for multiple metamorphicevents

In metamorphic rocks the coexistence of mineralsthat formed at different times and conditions on aPTt path ie not in equilibrium is a commonlyobserved feature where the metamorphic history iscomplex Gross differences in the grain size ofcoexisting phases has long been recognized bymetamorphic petrologists as an indicator of non-equilibrium mineral assemblages Historically muchless attention has been paid to such relations in sub-greenschist-facies rocks because prior to the intro-duction of SEM+TEM techniques their small grainsize prevented observations of textural relationsStudy of such rocks eg by XRD provides littleinformation on textural relations beyond determina-tion of degree of preferred orientation

Study by XRD of the samples showed thatsudoite and trioctahedral chlorite coexist If theyhad formed at the same time and under the sameP-T conditions their compositions would define thelimbs of a solvus The TEM images show howeverthat wherever they are in contact the sudoitetransects the packets of trioctahedral chlorite (egFig 8) Furthermore as illustrated in Fig 8 thetrioctahedral chlorite is commonly deformed anddisplays strain contrast in TEM images Thesetextural relations suggest that sudoite was intro-duced subsequent to the formation of trioctahedralchlorite Similar observations were made byGiorgetti et al (1998) who concluded that texturesin Verrucano metasediments showed that sudoitepost-dated chloritoid pyrophyllite and trioctahedralchlorite ie sudoite is in disequilibrium with thelatter minerals and formed as a retrograde phase Inrocks of the Sierra Espuna area differences in bulk-rock composition are small (Table 2) and do notcorrespond to the presence or absence of sudoite(Table 1) ie the random occurrence of sudoiteindependent of bulk-rock composition is compa-tible with a lack of equilibrium relations Theserelations collectively imply that sudoite wasintroduced at some time subsequent to theformation of mineral assemblages at peak meta-morphic conditions On the other hand the size ofsudoite packets is similar to that of trioctahedral

chlorite as consistent with formation at conditionsnot much different from those for chlorite

In samples from the Morron de Totana bothsudoite and trioctahedral chlorite were affected byretrograde alteration to smectite The TEM imagesshow sudoite (Fig 11) and trioctahedral chlorite(Fig 10) with interstratified layers of smectite withspacings of 10 AEcirc due to collapse and dehydration inthe TEM environment and even packets of smectite150 AEcirc thick within chlorite crystals These relationsimply that both sudoite and trioctahedral chloritehave been affected by retrograde alteration at thevery low temperatures consistent with smectiteformation although the presence of more subtlealteration features in sudoite of some samplessuggests that it was less susceptible to alteration

We therefore conclude that at least three episodesof phyllosilicate formation can be recognized inrocks from the Sierra Espuna-area Trioctahedralchlorite was presumably produced at the highestmetamorphicdiagenetic grade in equilibrium withparagonite and phengite Subsequently sudoite wasintroduced but at P-T conditions which could nothave been much different than those of peakmetamorphic grade Finally at very low-T condi-tions hydrothermal fluids caused partial alterationof trioctahedral chlorite and sudoite to smectite

Retrograde products formed by the reaction offluids with mineral phases generated during anearlier prograde event were described previouslyby Nieto et al (1994) and Zhao et al (1999) Thosereactions occurred on a regional scale but similarreactions have been observed on a more localizedscale The fluid activity associated with a majorfault zone is a common source for these reactions(eg Jiang et al 1990) The structure of SierraEspuna (Fig 1) is based on imbrication processesthat superimpose units that have subsequently beenaffected by faults and folds and that have under-gone significant uplift at least 800 m from the lateMiocene to the present (Sanz de Galdeano et al2001) Given this evolution it is easy to justify theretrograde processes that usually seem to be relatedwith tectonic events during which fluid flow andfluidrock ratios are enhanced (Merriman amp Peacor1999) Moreover according to Nieto et al (1994)the uplift of the Betic Cordillera would furnish theintroduction of phreatic water into the systemproducing a reactive environment adequate for thegenesis of smectite Geological conditions aretherefore consistent with post-peak metamorphicconditions as implied by the TEM observations

20 I Abad et al

The introduction of sudoite as a major phaseduring metamorphism implies the possibility ofadditional overprints on a simple prograde assem-blage the evidence for which might be subtle TheNa-rich micas at all grades have compositionswhich either fall within the solvus or are too Na-rich to correspond to the limb of the solvus In thehighest-grade rocks such micas coexist withmuscovite Those relations are incompatible withchemical equilibrium despite the fact that relationsfor muscovite and other minerals imply an approachto chemical equilibrium Furthermore there is aclose association of Na-rich micas with sudoite (egFig 7) Those relations imply the possibility thatNa-micas were introduced concomitant withsudoite P Mata (pers comm) has shown forexample that overprinting of an epizonal assem-blage by hydrothermal solutions resulted in theintroduction of both an Al-rich variety of chlorite(cookeite) and paragonite in pelites of the CamerosBasin Spain The textural relations such as cross-cutting textures which are so striking for sudoitewere not observed for Na-rich micas Neverthelessthe possibility of such overprinting must beconsidered

In spite of all these prograde-retrogradeprocesses in the Los Molinos Unit a smallnumber of homogeneous phases coexist in chemicalequilibrium and the fine-grained metamorphicphyllosilicates are clearly parallel to cleavage witha typical metamorphic aspect Finally all thetextural and compositional changes that haveoperated in the Espuna area during the metamorphicevolution proceed toward a simpler system repre-senting both textural and chemical equilibriumwhich is characteristic of greenschist-facies condi-tions

Roles of detrital and authigenic phyllosilicatesduring slaty cleavage development

Van der Pluijm et al (1998) focused on therespective roles of the phyllosilicates of detritalorigin and those formed during diagenesis andmetamorphism and observed that detrital phyllosi-licates remain preferentially oriented parallel tobedding in pelites with slaty cleavage whereas fine-grained metamorphic phyllosilicates are orientedparallel to cleavage

Although it is usually difficult to determine thepreferred orientations of phyllosilicates in TEMimages as the grade increases better-aligned

packets occur in the cleavage orientation Thetexture of the least metamorphosed sample inwhich even incipient slaty cleavage is absent isreminiscent of the typical textural relation ofsmectites described in the literature The illite-smectite mixed-layers conserve the textural featuresof possible previous smectite such as anastomosingfringes curved and discontinuous with variations ininterplanar spacing d001 These features indicatethat the minimum energy conditions to initiatemechanical transformations have not been reachedin the lower-grade samples as postulated by Ho etal (1996) since slaty cleavage develops as a resultof interactive mechanical deformation and ther-mally driven dissolution and crystallizationprocesses (Knipe 1981) In contrast in the LosMolinos Unit large crystals of chlorite and whitemica (gt1000 AEcirc ) occur which are of metamorphicorigin but with no detrital grains (Fig 3a)Phyllosilicates are preferentially oriented parallelto slaty cleavage in response to increasedtemperature and tectonic strain (Merriman ampPeacor 1999) The oriented microfabrics and slatycleavage well developed in the rocks of this unitmust be a product of a higher thermal and strainenergy environment reached in the deepest part ofthe sequence As pointed out by Van der Pluijm etal (1998) a continuous range of orientations isevidence for mechanical rotation of grains whereasthe lack of such transitional orientations impliesthat cleavage-parallel phyllosilicates originatedthrough dissolution of bedding-parallel and crystal-lization in the slaty cleavage orientation Asobserved in this study for the Intermediate Unitsgrains with orientations which are transitional frombedding-parallel to slaty cleavage orientation areonly rarely observed Cleavage formation is there-fore inferred to have occurred entirely or nearlyentirely through dissolution and crystallization

ACKNOWLEDGMENTS

We thank MM Abad Ortega from the Centro deInstrumentacio n Cient otilde fica of the Universidad deGranada for her help with HRTEM and I Nieto forher essential help with sample preparation Financialsupport was supplied by Research Project nordm BT 2000-0582 FPI research grant to IA both of the SpanishMinistry of Science and Technology Research GroupRNM-0179 of the Junta de Andaluc otildeacutea and to DRP byNSF grant EAR-9814391

Prograde and retrograde metamorphism 21

REFERENCES

Ahn JH amp Peacor DR (1986) Transmission andanalytical electron microscopy of the smectite-to-illite transition Clays and Clay Minerals 34165 ndash179

Azanon JM amp Crespo-Blanc A (2000) Exhumationduring a continental collision inferred from thetectonometamorphic evolution of the AlpujarrideComplex in the central Betics (Alboran Domain SESpain) Tectonics 19 549 ndash565

Bevins RE Robinson D amp Rowbotham G (1991)Compositional variations in mafic phyllosilicatesfrom regional low-grade metabasites and applicationof the chlorite geother mometer Journal ofMetamorphic Geology 9 711 ndash721

Cliff G amp Lorimer GW (1975) The quantitativeanalysis of thin specimens Journal of Microscopy 103 203 ndash207

Crowley SF (1991) Diagenetic modification of detritalmuscovite an example from the Great LimestoneCyclothem (Carboniferous) of Co Durham UKClay Minerals 26 91 ndash103

Dong H amp Peacor DR (1996) TEM observations ofcoherent stacking relations in smectite and illite ofshales evidence for MacEwan crystallites anddominance of 2M1 polytypes Clays and ClayMinerals 44 257 ndash275

Essene E amp Peacor DR (1995) Clay mineral thermo-metry a critical perspective Clays and ClayMinerals 43 540 ndash553

Fransolet AM amp Bourguignon P (1978) Dioctahedralchlorite in quartz veins from Ardennes BelgiumThe Canadian Mineralogist 16 365 ndash373

Fransolet AM amp Schreyer W (1984) Sudoite ditrioctahedral chlorite a stable low-temperature phasein the system MgO-Al2O3-SiO2-H2O Contributionsto Mineralogy and Petrology 86 409 ndash417

Frey M (1987) Very low-grade metamorphism of clasticsedimentary rocks Pp 9 ndash58 in Low TemperatureMetamorphism (M Frey editor) Blackie GlasgowUK

Giorgetti G Goffe B Memmi I amp Nieto F (1998)Metamorphic evolution of Verrucano metasedimentsin northern Apennines new petrological constraintsEuropean Journal of Mineralogy 10 1295 ndash1308

Guidotti CV amp Sassi FP (1986) Classification andcorrelation of metamorphic facies series by means ofmuscovite b0 data from low grade metapelites NeuesJahrbuch fur Mineralogie Abhandlungen 153363 ndash380

Guidotti CV Yates MG Dyar MD amp Taylor ME(1994) Petrogenetic implications of the Fe3+ contentof muscovi te in pel it ic sch is ts AmericanMineralogist 79 793 ndash795

Guthrie GD amp Veblen DR (1989) High resolutionelectron microscopy of mixed-layer illitesmectite

Computer simulations Clays and Clay Minerals 371 ndash11

Ho N Peacor DR amp Van der Pluijm BA (1996)Contrasting roles of detrital and authigenic phyllo-silicates during slaty cleavage development Journalof Structural Geology 18 615 ndash623

Jiang WT Peacor DR Merriman RJ amp Roberts B(1990) Transmission and analytical electron micro-scopic study of mixed layer illitesmectite formed asan apparent replacement product of diagenetic illiteClays and Clay Minerals 38 449 ndash468

Jiang WT Peacor DR amp Buseck PR (1994) Chloritegeothermometry Contamination and apparent octa-hedral vacancies Clays and Clay Minerals 42593 ndash605

Kisch HJ (1991) Development of slaty cleavage anddegree of very-low-grade metamorphism a reviewJournal of Metamorphic Geology 9 735 ndash750

Knipe JR (1981) The interaction of deformation andmetamorphi sm in slates Tectonophysics 78249 ndash272

Kretz R (1983) Symbols for rock-forming mineralsAmerican Mineralogist 68 277 ndash279

Li G Peacor DR Merriman RJ amp Roberts B (1994a)The diagenetic to low grade metamorphism evolu-tion of matrix white mica in the system muscovite-paragonite in a mudrock from Central Wales UKClays and Clay Minerals 42 369 ndash381

Li G Peacor DR Merriman RJ Roberts B amp Vander Pluijm BA (1994b) TEM and AEM constraintson the origin and significance of chlorite-mica stacksin slates an example from Central Wales UKJournal of Structural Geology 16 1139 ndash1157

Livi KJT Veblen DR Ferry JM amp Frey M (1997)Evolution of 21 layered silicates in low-grademetamorphosed Liass ic sha les of CentralSwitzerland Journal of Metamorphic Geology 15323 ndash344

Lonergan L (1991) Structural evolution of the SierraEspuna Betic Cordillera SE Spain PhD thesisOxford University UK

Lonergan L Platt JP amp Gallagher L (1994) TheInternal-External Zone Boundary in the eastern BeticCordillera SE Spain Journal of Structural Geology16 175 ndash188

Makel GH (1981) Differences in tectonic evolution ofsuperimposed Malaguide and Alpujarride tectonicunits in the Espuna area (Betic Cordilleras Spain)Geology in Mijnbouw 60 203 ndash208

Makel GH (1985) The geology of the MalaguideComplex and its bearing on the geodynamicevolution of the Betic-Rif orogen (southern Spainand northern Morocco) GUA papers of Geology Ser1 22 263 pp

Makel GH amp Rondeel HE (1979) Differences instratigraphy and metamorphism between superposedMalaguide and Alpujarride units in the Espuna area

22 I Abad et al

composition (see below) is quite variable corre-sponding to Na-rich muscovite commonly but somegrains have compositions within the limits of thesolvus between muscovite and paragonite and thusare metastable Na-K micas give SAED patternswhich have relatively sharp reflections over a largerange as consistent with large defect-free crystals

Sudoite crystals comprise units with 14 AEcirc

periodicities which in some areas are 28 AEcirc Lattice-fringe images show the presence of inter-

stratified individual 7 AEcirc layers which in some areasform small packets containing no more than eight7 AEcirc layers The AEM analyses (see below) of suchareas are typical of those of homogeneous sudoitecontaining only 14 AEcirc layers implying a composi-tion of the 7 AEcirc layers similar to that of sudoite andnot to an exotic septechlorite such as kaolinite orberthierine The contrast of both 14 AEcirc and 7 AEcirc

fringes appear to be identical further consistentwith a close chemical relation

FIG 7 Textural relations between phyllosilicates (sudoite illite Na-K mica) and hematite (dark contrast) (LaSanta Unit)

FIG 8 Lattice-fringe image showing the coexistence of two types of chlorite trioctahedral chlorite (chl) andsudoite (su) Sudoite transects chlorite which shows strain contrast associated with curved and kinked layersIdentifications were based in part on AEM analyses (see Table 6 Es-511 and 12 analyses) (La Santa Unit)

10 I Abad et al

Three samples corresponding to different butequivalent levels were selected from the Morron deTotana Unit for TEM study (see Fig 1 peliticrocks intercalated with carbonate rocks) Es-77 atthe bottom of the unit Es-86 which is located in anintermediate position and Es-95 from the top of thecross-section There is no incipient slaty cleavage inthese samples Sample Es-77 is a very fine-grainedrock consisting of quartz white mica chlorite andaggregates of carbonate grains Minor hematiteproduces a deep red colour Lattice-fringe images

show that dioctahedral clay minerals have 10 and20 AEcirc periodicities occur as packets 100 ndash200 AEcirc

thick and have subparallel orientations Accordingto Guthrie amp Veblen (1989) smectite and illiteinterlayers can produce different contrast and canonly be differentiated in lattice fringe-imagesobtained with well-defined electron beam (001)orientations and defocus conditions Such contrastdifferences were observed in several lattice-fringeimages The light contrast for fringes in Fig 9represents the smectite interlayer as shown in part

FIG 9 TEM image showing 10 AEcirc periodicity in R4 illite-rich I-S Contrast differences imply the presence ofillite-like (the dark fringes) and smectite-like (the light fringes) layers Packets usually have four or more illite

layers and the SAED pattern shows predominantly 50 AEcirc periodicity (Morron de Totana Unit)

FIG 10 Trioctahedral chlorite with intercalations of 10 AEcirc layers (upper arrow) which damage quickly under theelectron beam The chlorite packets show strain contrast microfolds and 7 AEcirc periodicities corresponding toberthierine (left arrow) or greater values eg 28 AEcirc (see analysis in Table 6 Es-867) (Morron de Totana Unit)

Prograde and retrograde metamorphism 11

by the continuity of such fringes with openinterlayer spaces produced by contraction due to

dehydration The sequence on average is that ofR4 mixed-layering ie a single smectite-likeinterlayer between four or more illite-like inter-layers Although some packets with more than fourlayers of illite are present repeats of four illite-likelayers predominate as shown by the 50 AEcirc

periodicity of SAED patterns (Fig 9 inset) Theillite-smectite (I-S) has typical 1Md polytypism Theobservation of smectite interstratified with illite isin accord with the changes observed in XRDdiagrams after EG treatment (Fig 2) In additionAEM analyses of areas similar to that of Fig 9have relatively high Si and minor Ca contentsfurther confirming the mixed layering

Some packets which give rise to fringes with10 AEcirc spacing produce SAED patterns typical oftwo-layer polytypism The contrast of all fringes isrelatively constant Such packets have thicknessestypical of diagenetic clays and two orders ofmagnitude smaller than typical detrital grains Thecompositions of such grains (see below) haveinterlayer-cation contents typical of illite They aretherefore inferred to be illite which formed duringlate diagenesis and coexisting with the R4 I-S

Sample Es-86 is composed mainly of detritalgrains of quartz randomly-oriented grains ofchlorite and white mica all immersed in aphyllosilicate-rich fine-grained matrix At theTEM scale chlorite is the most common phyllosi-licate in the matrix As in sample Es-5 trioctahe-dral chlorite and sudoite coexist There areinterstratified packets of 10 AEcirc layers only a fewlayers thick (lt150 AEcirc ) which are quickly damaged

FIG 11 Di-trioctahedral chlorite (sudoite) very similarin appearance to the trioctahedral chlorite shown inFig 10 7 AEcirc layers are marked by an arrow and 10 AEcirc

layers are present in the poorly resolved area in thecentre of the image (see analysis in Table 6 Es-862)

(Morron de Totana Unit)

FIG 12 Low-magnification image illustrating illite-like packets The inset SAED pattern shows only a smallnumber of diffuse poorly defined reflections The packets are 100 ndash200 AEcirc thick (Morron de Totana Unit)

12 I Abad et al

under the electron beam (Figs 10 and 11) TheAEM analyses of such material show an enhancedSi content and the presence of small concentrationsof alkali cations These data are consistent withretrograde alteration of chlorite as observed byNieto et al (1994) They differ however from theresults of Nieto et al (1994) and those describedabove for sample Es-5 in that both tri-trioctahedraland di-trioctahedral chlorite are affected In TEMimages strain features in chlorites such as contrastnormal to the layers or microfolds were observed(Fig 10) Mica is less abundant than sudoiteoccurring in packets 100 ndash400 AEcirc thick The SAEDpatterns exhibit 2M polytypism but with diffuse-ness parallel to c in 0kl rows At low magnifica-tion subparallel and slightly curved illite- or illite-smectite-like packets are present they coalescehave abundant layer terminations and show intensecontrasts (Fig 12)

Sample Es-95 is a very fine-grained rock madeup of large detrital grains of quartz in a fine-grained matrix of phyllosilicates mainly whitemicas and opaque minerals Some biotite grainswere observed as verified by AEM data Biotitewas observed only in separated grains on holey

C-coated Cu grids so it is not possible to describethe textural relations for it Authigenic biotite isinconsistent with the low grade of this samplehowever and it is therefore inferred to be of detritalorigin The AEM analyses show deviations frompure biotite however as consistent with alterationto chlorite the usual process in detrital biotite inrocks undergoing diagenesis Lattice-fringe imagesof ion-milled samples show that individual micapackets are very thin with no more than 10 layersisolated and dispersed in a matrix consisting ofmaterial which gives no SAED pattern Thesepackets are wavy discontinuous and with anasto-mosing layers (Fig 13) Despite these texturalfeatures SAED data indicate that there are 2Mand 1Md polytypes

For all samples prismatic crystals of Ti-oxidedispersed Fe-oxyhydroxides zircon and tourmalineare common accessory minerals

Chemical characterization of phyllosilicates

Dioctahedral mica Table 3 shows the composi-tion of K-rich dioctahedral micas of authigenic ormetamorphic origin corresponding to the samples

FIG 13 General aspect of sample Es-95 The image shows arrays of wavy discontinuous and anastomosingpackets of illite and smectite (Morron de Totana Unit)

Prograde and retrograde metamorphism 13

selected from each unit The Yechar and LosMolinos units are characterized by compositionsapproaching those of evolved micas with a sum ofinterlayer cations slightly less than 1 apfu typicalof phengite or muscovite rather than authigenic

illite In contrast the La Santa and Morron deTotana units show heterogeneous mica composi-tions and display deficiencies in interlayer cations(Fig 14a) This last feature is more evident in theMorron de Totana analyses where the illitic

TABLE 3 Representative chemical compositions for K-rich dioctahedral micas normalized to O10(OH)2

Si IVAl VIAl Fe Mg Ti VIS K Na S inter

Los Molinos Unit (1)Es-10213 337 063 178 010 008 002 198 075 008 085Es-10221 308 092 195 005 001 001 202 082 009 091Es-10231 319 081 191 005 003 001 200 077 012 090Es-10232 314 086 188 006 007 001 202 072 021 093

Yechar Unit (1)Es-5311 332 068 163 015 022 002 204 090 004 093Es-5312 310 090 180 003 013 004 201 089 010 099Es-5322 338 062 170 012 015 003 200 086 001 086Es-5323 319 081 176 016 012 003 206 087 002 089

Yechar Unit (2)Es-53 1 317 083 163 021 026 000 210 084 000 084Es-53 2 320 080 165 011 027 003 207 087 000 087Es-53 3 317 083 155 025 024 002 206 093 000 093Es-53 4 328 072 160 020 024 000 203 092 000 092Es-53 5 326 074 164 019 016 000 199 096 000 096Es-53 6 326 074 158 020 028 000 206 089 000 089Es-53 7 319 081 154 025 026 000 205 097 000 097Es-53 8 325 075 166 016 025 000 206 085 000 085Es-53 9 324 076 167 019 019 000 205 085 000 085

La Santa Unit (2)Es-2 1 311 089 157 019 032 002 210 088 005 094Es-2 3 336 064 148 020 043 001 212 072 003 075Es-2 6 313 087 156 024 029 001 210 075 017 092Es-2 7 313 087 155 024 029 001 209 084 011 095Es-2 8 331 069 157 015 032 000 205 087 004 092Es-2 10 328 072 160 018 027 003 208 068 008 075Es-5 4 303 097 174 015 023 001 213 073 009 082Es-5 10 301 099 189 006 015 000 210 065 021 086Es-5 7 307 093 159 033 016 000 208 075 018 093Es-5 8rsquo 301 099 160 031 025 000 216 067 016 083

Morron de Totana Unit (2) (analyses corresponding to R4 Ill-Sm may also be present see text)Es-77 13 327 073 172 013 027 000 212 067 000 067Es-77 15 316 084 195 003 013 000 211 063 000 063Es-77 16 328 072 182 010 015 000 207 068 000 068Es-77 17 338 062 147 032 019 000 199 093 000 093Es-86 9 334 066 160 017 034 000 210 072 000 072Es-86 10 357 043 151 012 046 000 209 065 000 065Es-86 11 353 047 156 012 036 000 203 077 000 077Es-86 13 324 076 152 025 035 000 211 084 000 084Es-86 14 324 076 173 013 019 000 205 084 000 084Es- 95 2 323 077 158 016 019 004 198 103 000 103Es- 95 5 336 064 169 013 025 000 207 070 000 070Es- 95 6 336 064 148 027 033 000 209 078 000 078

(1) SEMEDX analyses (2) TEMAEM analyses

14 I Abad et al

character is consistent with the presence of the R4mixed-layer illite-smectite described above Most ofthe micas are phengitic in composition (Fig 14ab)with an Fe(Fe+Mg) ratio that varies over the range02 ndash085 However the detrital-like grains aresimilar to those of end-member muscovite (seebelow detrital muscovite vs metamorphic musco-vite or Table 5) with much lower Fe and Mgcontents Data plotted in Fig 14bc confirm theassumption of a significant ferrimuscovitic substitu-tion (Guidotti et al 1994) Some of the K-micaanalysed in the Los Molinos and La Santa unitswere found to have significant Na contents (up to02 apfu) Although Ti is unusual in dioctahedralmicas it has been measured in several analyses

(lt005 apfu) The most evident feature in theplots is the scattering of data in the La Santa andMorron de Totana units ie where diageneticconditions are not exceeded In contrast theYechar Unit analyses define a narrow field in thediagrams and the Los Molinos data are even closerto the theoretical muscovite composition (Table 3)

Typical compositions of micas intermediate tomuscovite and paragonite called Na-K micas in thispaper are shown in Table 4 The compositions arequite variable even in the same sample Whencompared with analyses of K-rich dioctahedralmicas (Table 3) and paragonite analyses are seento span the entire range from muscovite toparagonite Some analyses (eg Es-5 9) fall within

FIG 14 Plots of values of chemical compositions of K-rich dioctahedral phyllosilicates solid circles and linesindicate values for theoretical muscovite and corresponding exchange vectors respectively

Prograde and retrograde metamorphism 15

the limits of the solvus between muscovite andparagonite the limbs of that solvus having theapproximate compositions ms60pg40 and ms20pg80

(Li et al 1994a)Detrital muscovite vs metamorphic muscovite

Detrital-like micas of sample Es-95 the lowest-grade one are large enough to be analysed(Table 5) by electron microprobe (EMPA) Thesedata have been compared with the AEM data ofother samples Compositions of detrital-like grainsof the lowest-grade sample and the detrital micas ofthe anchizonal-grade sample Es-53 (Fig 15) arevery similar and close to that of end-membermuscovite These data suggest that the detrital micahas a common source for the different units thatcomprise Sierra Espuna Figure 15 also comparesthe data from authigenic micas of the samesamples which are clearly more phengitic (Fe+Mg= 015 ndash075 apfu) than the detrital ones (Fe+Mg= 01 ndash02 apfu)

Chlorite Trioctahedral and di-trioctahedral(sudoite) chlorites have been analysed (Table 6)The former corresponding to the species clino-chlore is common in most of the samples unlikesudoite which is absent from most (Table 1)Sudoite was identified because the sum ofoctahedral cations is nearly equal to 5 apfu andthe Si and Al contents are higher than intrioctahedral chlorite (Fig 16a) A slight contam-ination by interlayer cations (K and Na) has beendetected therefore the formulae were recalculatedin order to eliminate those impurities (seeAnalytical Methods) Although total Fe has beenconsidered as Fe2+ according to Fransolet ampBourguignon (1978) the Fe3+ content in sudoitemay be greater than the Fe2+ content As severalanalyses have an octahedral sum slightly higherthan the ideal value of 5 apfu this may indicate atrioctahedral substitution andor the presence ofsignificant Fe3+ No Ti or Mn were detected The

Fe(Fe+Mg) ratios do not define a clear trendthroughout the sequence (Fig 16b) nevertheless itis much greater in chlorite in the Morron de Totanaanalyses falling in the characteristic range ofdiagenetic chlorites (Li et al 1994b) There is anincrease in Mg content (from 15 to 4 apfu)concomitant with increase in grade as compatiblewith some other sequences (eg Bevins et al1991)

D I S C U S S I O N

Dioctahedral phyllosilicates chemicalcompositions IC values and polytypes

On the basis of their large size homogeneity ofindividual grains and well-crystallized aspectdetrital dioctahedral mica grains were identified inseveral samples with the exception of those ofgreenschist facies This confirms the idea of the

TABLE 4 Chemical compositions for Na-K micas normalized to O10(OH)2

Si IVAl VIAl Fe Mg Ti VIS Ca K Na S inter

Es-5 14 313 087 190 004 009 002 204 000 034 048 083Es-5 15 321 079 176 009 037 000 222 000 015 038 053Es-5 4 318 082 187 004 019 000 209 000 013 061 074Es-5 12 329 071 172 008 029 000 208 000 039 036 076Es-5 13 323 077 179 005 028 000 212 000 029 041 069Es-5 9 296 104 163 019 029 000 211 000 048 056 104Es-5 8 292 108 156 043 008 001 209 003 014 080 096

FIG 15 Plot of Si vs Al contents of detrital andauthigenic micas of samples Es-95 and Es-53 Solidsymbols = detrital micas open symbols = authigenic

micas

16 I Abad et al

relative stability of muscovite in most diageneticenvironments (Crowley 1991) The compositions ofthe detrital grains of samples from different unitsare similar The lack of heterogeneity which isexpected for mica from a complex source implies acommon source area for all samples

The IC values define a trend from diagenetic toanchizonal conditions but with smaller IC values inbulk-rock fractions consistent with the contributionof detrital mica The coexistence of mature micawith lower-grade I-S or illite in the sub-greenschistfacies rocks shows that those rocks have not

TABLE 5 EMPA data for detrital-like crystals of mica normalized to O10(OH)2

Si IVAl VIAl Fe Mg Ti VIS Ca K Na K+Na+Ca

Es-95z11 313 087 193 005 004 001 203 000 065 022 087Es-95z11 314 086 190 006 007 001 204 000 068 019 087Es-95z1 2 314 086 191 005 009 001 206 000 070 006 077Es-95z2 1 314 086 176 012 010 003 201 002 095 004 101Es-95z2 2 317 083 177 012 011 003 202 000 095 004 099Es-95z4 1 305 095 188 005 007 004 204 000 081 009 089Es-95z4 2 311 089 187 006 007 004 204 000 078 009 086

TABLE 6 Representative chemical compositions for chlorite normalized to O10(OH)8

Si IVAl VIAl Fe Mg VIS FeFe+Mg

Yechar UnitEs53-1 273 127 112 121 376 609 024Es53-4 287 113 125 070 400 596 015Es53-12 303 097 130 146 307 583 032Es53-13 277 123 125 122 354 601 026Es53-15 307 093 146 094 323 564 023Es53-42 271 129 152 160 294 606 035

La Santa UnitEs2-4 319 081 147 148 273 569 035Es2-5 365 035 183 118 225 526 034Es5-1 354 046 293 023 172 488 012Es5-5 361 039 292 021 178 490 010Es5-10 309 091 262 028 221 511 011Es5-16 310 090 197 112 234 543 032Es5-17 350 050 252 076 170 499 031Es-5 10rsquo 364 036 213 097 202 512 032Es-5 11 291 109 143 136 314 593 030Es-5 12 382 018 229 087 179 495 033Es-5 13 316 084 302 021 172 494 011Es-5 16rsquo 346 054 341 013 101 455 011Es-5 17rsquo 300 100 282 033 194 509 014Es-5 18 285 115 285 026 204 515 011

Morron de Totana UnitEs-86 2 377 023 253 088 149 490 037Es-86 4 284 116 166 230 181 578 056Es-86 5 350 050 191 192 151 534 056Es-86 6 366 034 189 191 149 529 056Es-86 7 315 085 129 273 176 578 061

indicates sudoite analyses

Prograde and retrograde metamorphism 17

reached stable chemical or textural equilibrium atleast at the mm scale In addition the lack ofchemical equilibrium among diagenetic I-S andillite at the nm scale is demonstrated by their highlyvariable Fe+Mg contents (006ndash075 apfu) Theaverages of the sum of interlayer cations are closeto 1 apfu in the Los Molinos and Yechar units asconsistent with mica but values of 06 apfucorresponding to I-S or illitic compositions are alsopresent in the Morron de Totana Unit Except forsamples Es-102 and Es-53 the variable composi-tions represent a metastable state with composi-tions of micas determined by a combination ofphengitic ferrimuscovitic and illitic vectors asshown by Fig 14ab

Paragonite is generally considered an indexmineral for the anchizone (Frey 1987) but Li etal (1994a) have shown that paragonite coexistswith K-rich mica in grades as low as that of the latediagenetic zone for a prograde sequence Samplesin which muscovite paragonite and intermediateNa-K mica coexist as in sample Es-5 areconsistent with metastability both because morethan two such micas coexist and because theircompositions are incompatible with the solvus inthe muscovite-paragonite system (see below) Whilethe 2M polytype typical of metamorphic micas is

the most common in these samples the 1Md

polytype also occurs in some diagenetic samplesparticularly those from the Morron de Totana UnitThe 1Md polytype is metastable relative to the 2Mpolytype and the coexistence of both polytypesfurther demonstrates a lack of equilibrium at the nmscale Dong amp Peacor (1996) showed that thedisorder noted in the diagenetic phyllosilicates bySAED is a consequence of the general high entropyof phases formed at very low temperatures thatproduces for example stacking defects separatingpackets with no more than 10 layers These packetsprogressively thicken with increasing metamorphicgrade and the 2M polytype then becomes the mostcommon polytype in authigenic micas Thenm-scale size of the lower-grade clays and theregular increase in size with increasing gradereflect the lack of textural equilibrium in the low-grade samples

A wide range of sample characteristics thusimply that the lower-grade samples are in statesof metastable equilibrium those factors includingcoexistence of mature mica with I-S and illitevariable chemical compositions incompatiblecompositions of white micas small crystal sizesdefect-rich states and polytypism Such dataemphasize the notion of lsquoreaction progressrsquo relative

FIG 16 (a) Plot of Al vs Fe+Mg contents of chlorites illustrating the contrast in composition between tri-trioctahedral clinochlore or chamosite and di-trioctahedral sudoite (b) plot of Si vs Fe(Fe+Mg) ratio of

chlorites showing an absence of compositional trends throughout the sampled sequence

18 I Abad et al

to Ostwald-step rule processes metastable dioctahe-dral clay systems undergoing reactions toward thestate of stable chemical and textural equilibriumapproached in greenschist-facies samples (Essene ampPeacor 1995)

Prograde evolution based on TEM datacorrelation with the Basque-Cantabriansequence (Northern Spain)

With increasing grade TEM data confirm thatcrystal thickness increases crystal defect densitydecreases compositions become less variable andmicrotextural relations are simpler (eg Peacor1992) This trend is consistent with three well-characterized low-grade settings the Gulf Coastsequence (Ahn amp Peacor 1986) and the WelshBasin (Merriman et al 1990) both extensionalbasins and the Helvetic Alps (Livi et al 1997)The latter is a fold-and-thrust belt with similarseries of prograde transitions to those observed inSierra Espuna in which pelites have passed througha sequence of metastable states in the lithologicalsequence mudstone and shale slate and phyllite

The mudstones of the Morron de Totana Unit arecharacterized by the presence of illite-smectite R4mixed-layers mica packets of no more than 10 ndash40layers and the occurrence of 1Md polytypism TheLa Santa Unit samples represent a step in theprograde evolution with thicker packets than in theprevious samples (200 ndash700 AEcirc ) illitic or phengiticcompositions the absence of illite-smectite mixedlayers as well as the presence of discreteintermediate Na-K micas and paragonite Althoughthere is a tendency towards non-preferred orienta-tion of crystals it is not so evident as in the Morronde Totana samples In the Yechar Unit (anchizone)and more clearly in the Los Molinos phyllites(greenschist facies) the microtextural relations areless complex the compositions are less variablewith phengite and trioctahedral chlorite crystalthickness are greater (gt400 AEcirc in the Yechar Unitand gt1000 AEcirc in the Los Molinos Unit) with thepackets being defect-free and with a very crystallineaspect This description has features in commonwith the three well-characterized low-grade settingspreviously noted Only the mineral assemblagesvary somewhat presumably as a function of thedifferences in bulk-rock composition

A detailed comparison with the Basque-Cantabrian Basin (N Spain) shows that the lowest-grade conditions of Sierra Espuna overlap most of

those of that sequence which is characterized bypassive burial diagenesis (8000 m thick) (Nieto etal 1996) Only in the deepest sample of theBasque-Cantabrian sequence is illite-smectite mixedlayering absent Figure 12 of this study is verysimilar to Fig 7 in Nieto et al (1996) and bothimages correspond to diagenetic samples fromdifferent settings nevertheless they illustratesimilar mica packets 100 ndash200 AEcirc thick withparallel-to-subparallel boundaries the orientationof which is inferred to represent beddingTherefore there is a parallelism between thesesequences down to the scale of individual clay-mineral grains even though they developed underdifferent conditions with stress presumably playinga significant role in the Sierra Espuna units Thetexture of the least metamorphosed sample (Fig 13)is reminiscent of the typical textural relations ofsmectite described by Nieto et al (1996) (seeFig 4) Although it corresponds to illite-richinterstratified illite-smectite the clay mineralgrains conserved the grain size and anastomosingdiscontinuous array of original smectite layers

Despite the similarities between the twosequences the chemical behaviour of the micasare clearly different since the grade correspondingto the phengitic compositions of the Yechar Unit isnot reached in the Basque-Cantabrian Basin wherethe deepest and highest-grade sample has illiticcompositions equivalent to those of the La SantaUnit That is in a passive burial metamorphiccontext as in the Basque-Cantabrian Basin thefactors responsible for increasing the rate ofchemical reactions are less significant than in thesequences that were affected by tectonic stressTectonic stress introduces defects which in turnincreases the rate of dissolution of strained grainsthus increasing the rate of dissolution-crystallizationreactions typical of clay minerals Perhaps moreimportantly regional tectonic stress is known tocause mobilization of fluids (eg Oliver 1986) Thepresence and activity of such fluids is a prime causeof increased rates of dissolution ion diffusion andcrystallization Tectonic activity thus may have atwo-fold effect in increasing rate of clay-mineralreactions The phengite component may in part be ameasure of the reaction progress depending on thereaction kinetics in very low-grade rocksAccording to Merriman amp Peacor (1999) whereheat flow is low as in accretionary settingsphengite contents are typically high perhapsbecause there is insufficient thermal energy to

Prograde and retrograde metamorphism 19

cause equilibration of the phengite component ofneoformed illite by contrast with extensional basinswhere this component is typically small

Coexistence of trioctahedral chlorite andsudoite Implications for multiple metamorphicevents

In metamorphic rocks the coexistence of mineralsthat formed at different times and conditions on aPTt path ie not in equilibrium is a commonlyobserved feature where the metamorphic history iscomplex Gross differences in the grain size ofcoexisting phases has long been recognized bymetamorphic petrologists as an indicator of non-equilibrium mineral assemblages Historically muchless attention has been paid to such relations in sub-greenschist-facies rocks because prior to the intro-duction of SEM+TEM techniques their small grainsize prevented observations of textural relationsStudy of such rocks eg by XRD provides littleinformation on textural relations beyond determina-tion of degree of preferred orientation

Study by XRD of the samples showed thatsudoite and trioctahedral chlorite coexist If theyhad formed at the same time and under the sameP-T conditions their compositions would define thelimbs of a solvus The TEM images show howeverthat wherever they are in contact the sudoitetransects the packets of trioctahedral chlorite (egFig 8) Furthermore as illustrated in Fig 8 thetrioctahedral chlorite is commonly deformed anddisplays strain contrast in TEM images Thesetextural relations suggest that sudoite was intro-duced subsequent to the formation of trioctahedralchlorite Similar observations were made byGiorgetti et al (1998) who concluded that texturesin Verrucano metasediments showed that sudoitepost-dated chloritoid pyrophyllite and trioctahedralchlorite ie sudoite is in disequilibrium with thelatter minerals and formed as a retrograde phase Inrocks of the Sierra Espuna area differences in bulk-rock composition are small (Table 2) and do notcorrespond to the presence or absence of sudoite(Table 1) ie the random occurrence of sudoiteindependent of bulk-rock composition is compa-tible with a lack of equilibrium relations Theserelations collectively imply that sudoite wasintroduced at some time subsequent to theformation of mineral assemblages at peak meta-morphic conditions On the other hand the size ofsudoite packets is similar to that of trioctahedral

chlorite as consistent with formation at conditionsnot much different from those for chlorite

In samples from the Morron de Totana bothsudoite and trioctahedral chlorite were affected byretrograde alteration to smectite The TEM imagesshow sudoite (Fig 11) and trioctahedral chlorite(Fig 10) with interstratified layers of smectite withspacings of 10 AEcirc due to collapse and dehydration inthe TEM environment and even packets of smectite150 AEcirc thick within chlorite crystals These relationsimply that both sudoite and trioctahedral chloritehave been affected by retrograde alteration at thevery low temperatures consistent with smectiteformation although the presence of more subtlealteration features in sudoite of some samplessuggests that it was less susceptible to alteration

We therefore conclude that at least three episodesof phyllosilicate formation can be recognized inrocks from the Sierra Espuna-area Trioctahedralchlorite was presumably produced at the highestmetamorphicdiagenetic grade in equilibrium withparagonite and phengite Subsequently sudoite wasintroduced but at P-T conditions which could nothave been much different than those of peakmetamorphic grade Finally at very low-T condi-tions hydrothermal fluids caused partial alterationof trioctahedral chlorite and sudoite to smectite

Retrograde products formed by the reaction offluids with mineral phases generated during anearlier prograde event were described previouslyby Nieto et al (1994) and Zhao et al (1999) Thosereactions occurred on a regional scale but similarreactions have been observed on a more localizedscale The fluid activity associated with a majorfault zone is a common source for these reactions(eg Jiang et al 1990) The structure of SierraEspuna (Fig 1) is based on imbrication processesthat superimpose units that have subsequently beenaffected by faults and folds and that have under-gone significant uplift at least 800 m from the lateMiocene to the present (Sanz de Galdeano et al2001) Given this evolution it is easy to justify theretrograde processes that usually seem to be relatedwith tectonic events during which fluid flow andfluidrock ratios are enhanced (Merriman amp Peacor1999) Moreover according to Nieto et al (1994)the uplift of the Betic Cordillera would furnish theintroduction of phreatic water into the systemproducing a reactive environment adequate for thegenesis of smectite Geological conditions aretherefore consistent with post-peak metamorphicconditions as implied by the TEM observations

20 I Abad et al

The introduction of sudoite as a major phaseduring metamorphism implies the possibility ofadditional overprints on a simple prograde assem-blage the evidence for which might be subtle TheNa-rich micas at all grades have compositionswhich either fall within the solvus or are too Na-rich to correspond to the limb of the solvus In thehighest-grade rocks such micas coexist withmuscovite Those relations are incompatible withchemical equilibrium despite the fact that relationsfor muscovite and other minerals imply an approachto chemical equilibrium Furthermore there is aclose association of Na-rich micas with sudoite (egFig 7) Those relations imply the possibility thatNa-micas were introduced concomitant withsudoite P Mata (pers comm) has shown forexample that overprinting of an epizonal assem-blage by hydrothermal solutions resulted in theintroduction of both an Al-rich variety of chlorite(cookeite) and paragonite in pelites of the CamerosBasin Spain The textural relations such as cross-cutting textures which are so striking for sudoitewere not observed for Na-rich micas Neverthelessthe possibility of such overprinting must beconsidered

In spite of all these prograde-retrogradeprocesses in the Los Molinos Unit a smallnumber of homogeneous phases coexist in chemicalequilibrium and the fine-grained metamorphicphyllosilicates are clearly parallel to cleavage witha typical metamorphic aspect Finally all thetextural and compositional changes that haveoperated in the Espuna area during the metamorphicevolution proceed toward a simpler system repre-senting both textural and chemical equilibriumwhich is characteristic of greenschist-facies condi-tions

Roles of detrital and authigenic phyllosilicatesduring slaty cleavage development

Van der Pluijm et al (1998) focused on therespective roles of the phyllosilicates of detritalorigin and those formed during diagenesis andmetamorphism and observed that detrital phyllosi-licates remain preferentially oriented parallel tobedding in pelites with slaty cleavage whereas fine-grained metamorphic phyllosilicates are orientedparallel to cleavage

Although it is usually difficult to determine thepreferred orientations of phyllosilicates in TEMimages as the grade increases better-aligned

packets occur in the cleavage orientation Thetexture of the least metamorphosed sample inwhich even incipient slaty cleavage is absent isreminiscent of the typical textural relation ofsmectites described in the literature The illite-smectite mixed-layers conserve the textural featuresof possible previous smectite such as anastomosingfringes curved and discontinuous with variations ininterplanar spacing d001 These features indicatethat the minimum energy conditions to initiatemechanical transformations have not been reachedin the lower-grade samples as postulated by Ho etal (1996) since slaty cleavage develops as a resultof interactive mechanical deformation and ther-mally driven dissolution and crystallizationprocesses (Knipe 1981) In contrast in the LosMolinos Unit large crystals of chlorite and whitemica (gt1000 AEcirc ) occur which are of metamorphicorigin but with no detrital grains (Fig 3a)Phyllosilicates are preferentially oriented parallelto slaty cleavage in response to increasedtemperature and tectonic strain (Merriman ampPeacor 1999) The oriented microfabrics and slatycleavage well developed in the rocks of this unitmust be a product of a higher thermal and strainenergy environment reached in the deepest part ofthe sequence As pointed out by Van der Pluijm etal (1998) a continuous range of orientations isevidence for mechanical rotation of grains whereasthe lack of such transitional orientations impliesthat cleavage-parallel phyllosilicates originatedthrough dissolution of bedding-parallel and crystal-lization in the slaty cleavage orientation Asobserved in this study for the Intermediate Unitsgrains with orientations which are transitional frombedding-parallel to slaty cleavage orientation areonly rarely observed Cleavage formation is there-fore inferred to have occurred entirely or nearlyentirely through dissolution and crystallization

ACKNOWLEDGMENTS

We thank MM Abad Ortega from the Centro deInstrumentacio n Cient otilde fica of the Universidad deGranada for her help with HRTEM and I Nieto forher essential help with sample preparation Financialsupport was supplied by Research Project nordm BT 2000-0582 FPI research grant to IA both of the SpanishMinistry of Science and Technology Research GroupRNM-0179 of the Junta de Andaluc otildeacutea and to DRP byNSF grant EAR-9814391

Prograde and retrograde metamorphism 21

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Azanon JM amp Crespo-Blanc A (2000) Exhumationduring a continental collision inferred from thetectonometamorphic evolution of the AlpujarrideComplex in the central Betics (Alboran Domain SESpain) Tectonics 19 549 ndash565

Bevins RE Robinson D amp Rowbotham G (1991)Compositional variations in mafic phyllosilicatesfrom regional low-grade metabasites and applicationof the chlorite geother mometer Journal ofMetamorphic Geology 9 711 ndash721

Cliff G amp Lorimer GW (1975) The quantitativeanalysis of thin specimens Journal of Microscopy 103 203 ndash207

Crowley SF (1991) Diagenetic modification of detritalmuscovite an example from the Great LimestoneCyclothem (Carboniferous) of Co Durham UKClay Minerals 26 91 ndash103

Dong H amp Peacor DR (1996) TEM observations ofcoherent stacking relations in smectite and illite ofshales evidence for MacEwan crystallites anddominance of 2M1 polytypes Clays and ClayMinerals 44 257 ndash275

Essene E amp Peacor DR (1995) Clay mineral thermo-metry a critical perspective Clays and ClayMinerals 43 540 ndash553

Fransolet AM amp Bourguignon P (1978) Dioctahedralchlorite in quartz veins from Ardennes BelgiumThe Canadian Mineralogist 16 365 ndash373

Fransolet AM amp Schreyer W (1984) Sudoite ditrioctahedral chlorite a stable low-temperature phasein the system MgO-Al2O3-SiO2-H2O Contributionsto Mineralogy and Petrology 86 409 ndash417

Frey M (1987) Very low-grade metamorphism of clasticsedimentary rocks Pp 9 ndash58 in Low TemperatureMetamorphism (M Frey editor) Blackie GlasgowUK

Giorgetti G Goffe B Memmi I amp Nieto F (1998)Metamorphic evolution of Verrucano metasedimentsin northern Apennines new petrological constraintsEuropean Journal of Mineralogy 10 1295 ndash1308

Guidotti CV amp Sassi FP (1986) Classification andcorrelation of metamorphic facies series by means ofmuscovite b0 data from low grade metapelites NeuesJahrbuch fur Mineralogie Abhandlungen 153363 ndash380

Guidotti CV Yates MG Dyar MD amp Taylor ME(1994) Petrogenetic implications of the Fe3+ contentof muscovi te in pel it ic sch is ts AmericanMineralogist 79 793 ndash795

Guthrie GD amp Veblen DR (1989) High resolutionelectron microscopy of mixed-layer illitesmectite

Computer simulations Clays and Clay Minerals 371 ndash11

Ho N Peacor DR amp Van der Pluijm BA (1996)Contrasting roles of detrital and authigenic phyllo-silicates during slaty cleavage development Journalof Structural Geology 18 615 ndash623

Jiang WT Peacor DR Merriman RJ amp Roberts B(1990) Transmission and analytical electron micro-scopic study of mixed layer illitesmectite formed asan apparent replacement product of diagenetic illiteClays and Clay Minerals 38 449 ndash468

Jiang WT Peacor DR amp Buseck PR (1994) Chloritegeothermometry Contamination and apparent octa-hedral vacancies Clays and Clay Minerals 42593 ndash605

Kisch HJ (1991) Development of slaty cleavage anddegree of very-low-grade metamorphism a reviewJournal of Metamorphic Geology 9 735 ndash750

Knipe JR (1981) The interaction of deformation andmetamorphi sm in slates Tectonophysics 78249 ndash272

Kretz R (1983) Symbols for rock-forming mineralsAmerican Mineralogist 68 277 ndash279

Li G Peacor DR Merriman RJ amp Roberts B (1994a)The diagenetic to low grade metamorphism evolu-tion of matrix white mica in the system muscovite-paragonite in a mudrock from Central Wales UKClays and Clay Minerals 42 369 ndash381

Li G Peacor DR Merriman RJ Roberts B amp Vander Pluijm BA (1994b) TEM and AEM constraintson the origin and significance of chlorite-mica stacksin slates an example from Central Wales UKJournal of Structural Geology 16 1139 ndash1157

Livi KJT Veblen DR Ferry JM amp Frey M (1997)Evolution of 21 layered silicates in low-grademetamorphosed Liass ic sha les of CentralSwitzerland Journal of Metamorphic Geology 15323 ndash344

Lonergan L (1991) Structural evolution of the SierraEspuna Betic Cordillera SE Spain PhD thesisOxford University UK

Lonergan L Platt JP amp Gallagher L (1994) TheInternal-External Zone Boundary in the eastern BeticCordillera SE Spain Journal of Structural Geology16 175 ndash188

Makel GH (1981) Differences in tectonic evolution ofsuperimposed Malaguide and Alpujarride tectonicunits in the Espuna area (Betic Cordilleras Spain)Geology in Mijnbouw 60 203 ndash208

Makel GH (1985) The geology of the MalaguideComplex and its bearing on the geodynamicevolution of the Betic-Rif orogen (southern Spainand northern Morocco) GUA papers of Geology Ser1 22 263 pp

Makel GH amp Rondeel HE (1979) Differences instratigraphy and metamorphism between superposedMalaguide and Alpujarride units in the Espuna area

22 I Abad et al

Three samples corresponding to different butequivalent levels were selected from the Morron deTotana Unit for TEM study (see Fig 1 peliticrocks intercalated with carbonate rocks) Es-77 atthe bottom of the unit Es-86 which is located in anintermediate position and Es-95 from the top of thecross-section There is no incipient slaty cleavage inthese samples Sample Es-77 is a very fine-grainedrock consisting of quartz white mica chlorite andaggregates of carbonate grains Minor hematiteproduces a deep red colour Lattice-fringe images

show that dioctahedral clay minerals have 10 and20 AEcirc periodicities occur as packets 100 ndash200 AEcirc

thick and have subparallel orientations Accordingto Guthrie amp Veblen (1989) smectite and illiteinterlayers can produce different contrast and canonly be differentiated in lattice fringe-imagesobtained with well-defined electron beam (001)orientations and defocus conditions Such contrastdifferences were observed in several lattice-fringeimages The light contrast for fringes in Fig 9represents the smectite interlayer as shown in part

FIG 9 TEM image showing 10 AEcirc periodicity in R4 illite-rich I-S Contrast differences imply the presence ofillite-like (the dark fringes) and smectite-like (the light fringes) layers Packets usually have four or more illite

layers and the SAED pattern shows predominantly 50 AEcirc periodicity (Morron de Totana Unit)

FIG 10 Trioctahedral chlorite with intercalations of 10 AEcirc layers (upper arrow) which damage quickly under theelectron beam The chlorite packets show strain contrast microfolds and 7 AEcirc periodicities corresponding toberthierine (left arrow) or greater values eg 28 AEcirc (see analysis in Table 6 Es-867) (Morron de Totana Unit)

Prograde and retrograde metamorphism 11

by the continuity of such fringes with openinterlayer spaces produced by contraction due to

dehydration The sequence on average is that ofR4 mixed-layering ie a single smectite-likeinterlayer between four or more illite-like inter-layers Although some packets with more than fourlayers of illite are present repeats of four illite-likelayers predominate as shown by the 50 AEcirc

periodicity of SAED patterns (Fig 9 inset) Theillite-smectite (I-S) has typical 1Md polytypism Theobservation of smectite interstratified with illite isin accord with the changes observed in XRDdiagrams after EG treatment (Fig 2) In additionAEM analyses of areas similar to that of Fig 9have relatively high Si and minor Ca contentsfurther confirming the mixed layering

Some packets which give rise to fringes with10 AEcirc spacing produce SAED patterns typical oftwo-layer polytypism The contrast of all fringes isrelatively constant Such packets have thicknessestypical of diagenetic clays and two orders ofmagnitude smaller than typical detrital grains Thecompositions of such grains (see below) haveinterlayer-cation contents typical of illite They aretherefore inferred to be illite which formed duringlate diagenesis and coexisting with the R4 I-S

Sample Es-86 is composed mainly of detritalgrains of quartz randomly-oriented grains ofchlorite and white mica all immersed in aphyllosilicate-rich fine-grained matrix At theTEM scale chlorite is the most common phyllosi-licate in the matrix As in sample Es-5 trioctahe-dral chlorite and sudoite coexist There areinterstratified packets of 10 AEcirc layers only a fewlayers thick (lt150 AEcirc ) which are quickly damaged

FIG 11 Di-trioctahedral chlorite (sudoite) very similarin appearance to the trioctahedral chlorite shown inFig 10 7 AEcirc layers are marked by an arrow and 10 AEcirc

layers are present in the poorly resolved area in thecentre of the image (see analysis in Table 6 Es-862)

(Morron de Totana Unit)

FIG 12 Low-magnification image illustrating illite-like packets The inset SAED pattern shows only a smallnumber of diffuse poorly defined reflections The packets are 100 ndash200 AEcirc thick (Morron de Totana Unit)

12 I Abad et al

under the electron beam (Figs 10 and 11) TheAEM analyses of such material show an enhancedSi content and the presence of small concentrationsof alkali cations These data are consistent withretrograde alteration of chlorite as observed byNieto et al (1994) They differ however from theresults of Nieto et al (1994) and those describedabove for sample Es-5 in that both tri-trioctahedraland di-trioctahedral chlorite are affected In TEMimages strain features in chlorites such as contrastnormal to the layers or microfolds were observed(Fig 10) Mica is less abundant than sudoiteoccurring in packets 100 ndash400 AEcirc thick The SAEDpatterns exhibit 2M polytypism but with diffuse-ness parallel to c in 0kl rows At low magnifica-tion subparallel and slightly curved illite- or illite-smectite-like packets are present they coalescehave abundant layer terminations and show intensecontrasts (Fig 12)

Sample Es-95 is a very fine-grained rock madeup of large detrital grains of quartz in a fine-grained matrix of phyllosilicates mainly whitemicas and opaque minerals Some biotite grainswere observed as verified by AEM data Biotitewas observed only in separated grains on holey

C-coated Cu grids so it is not possible to describethe textural relations for it Authigenic biotite isinconsistent with the low grade of this samplehowever and it is therefore inferred to be of detritalorigin The AEM analyses show deviations frompure biotite however as consistent with alterationto chlorite the usual process in detrital biotite inrocks undergoing diagenesis Lattice-fringe imagesof ion-milled samples show that individual micapackets are very thin with no more than 10 layersisolated and dispersed in a matrix consisting ofmaterial which gives no SAED pattern Thesepackets are wavy discontinuous and with anasto-mosing layers (Fig 13) Despite these texturalfeatures SAED data indicate that there are 2Mand 1Md polytypes

For all samples prismatic crystals of Ti-oxidedispersed Fe-oxyhydroxides zircon and tourmalineare common accessory minerals

Chemical characterization of phyllosilicates

Dioctahedral mica Table 3 shows the composi-tion of K-rich dioctahedral micas of authigenic ormetamorphic origin corresponding to the samples

FIG 13 General aspect of sample Es-95 The image shows arrays of wavy discontinuous and anastomosingpackets of illite and smectite (Morron de Totana Unit)

Prograde and retrograde metamorphism 13

selected from each unit The Yechar and LosMolinos units are characterized by compositionsapproaching those of evolved micas with a sum ofinterlayer cations slightly less than 1 apfu typicalof phengite or muscovite rather than authigenic

illite In contrast the La Santa and Morron deTotana units show heterogeneous mica composi-tions and display deficiencies in interlayer cations(Fig 14a) This last feature is more evident in theMorron de Totana analyses where the illitic

TABLE 3 Representative chemical compositions for K-rich dioctahedral micas normalized to O10(OH)2

Si IVAl VIAl Fe Mg Ti VIS K Na S inter

Los Molinos Unit (1)Es-10213 337 063 178 010 008 002 198 075 008 085Es-10221 308 092 195 005 001 001 202 082 009 091Es-10231 319 081 191 005 003 001 200 077 012 090Es-10232 314 086 188 006 007 001 202 072 021 093

Yechar Unit (1)Es-5311 332 068 163 015 022 002 204 090 004 093Es-5312 310 090 180 003 013 004 201 089 010 099Es-5322 338 062 170 012 015 003 200 086 001 086Es-5323 319 081 176 016 012 003 206 087 002 089

Yechar Unit (2)Es-53 1 317 083 163 021 026 000 210 084 000 084Es-53 2 320 080 165 011 027 003 207 087 000 087Es-53 3 317 083 155 025 024 002 206 093 000 093Es-53 4 328 072 160 020 024 000 203 092 000 092Es-53 5 326 074 164 019 016 000 199 096 000 096Es-53 6 326 074 158 020 028 000 206 089 000 089Es-53 7 319 081 154 025 026 000 205 097 000 097Es-53 8 325 075 166 016 025 000 206 085 000 085Es-53 9 324 076 167 019 019 000 205 085 000 085

La Santa Unit (2)Es-2 1 311 089 157 019 032 002 210 088 005 094Es-2 3 336 064 148 020 043 001 212 072 003 075Es-2 6 313 087 156 024 029 001 210 075 017 092Es-2 7 313 087 155 024 029 001 209 084 011 095Es-2 8 331 069 157 015 032 000 205 087 004 092Es-2 10 328 072 160 018 027 003 208 068 008 075Es-5 4 303 097 174 015 023 001 213 073 009 082Es-5 10 301 099 189 006 015 000 210 065 021 086Es-5 7 307 093 159 033 016 000 208 075 018 093Es-5 8rsquo 301 099 160 031 025 000 216 067 016 083

Morron de Totana Unit (2) (analyses corresponding to R4 Ill-Sm may also be present see text)Es-77 13 327 073 172 013 027 000 212 067 000 067Es-77 15 316 084 195 003 013 000 211 063 000 063Es-77 16 328 072 182 010 015 000 207 068 000 068Es-77 17 338 062 147 032 019 000 199 093 000 093Es-86 9 334 066 160 017 034 000 210 072 000 072Es-86 10 357 043 151 012 046 000 209 065 000 065Es-86 11 353 047 156 012 036 000 203 077 000 077Es-86 13 324 076 152 025 035 000 211 084 000 084Es-86 14 324 076 173 013 019 000 205 084 000 084Es- 95 2 323 077 158 016 019 004 198 103 000 103Es- 95 5 336 064 169 013 025 000 207 070 000 070Es- 95 6 336 064 148 027 033 000 209 078 000 078

(1) SEMEDX analyses (2) TEMAEM analyses

14 I Abad et al

character is consistent with the presence of the R4mixed-layer illite-smectite described above Most ofthe micas are phengitic in composition (Fig 14ab)with an Fe(Fe+Mg) ratio that varies over the range02 ndash085 However the detrital-like grains aresimilar to those of end-member muscovite (seebelow detrital muscovite vs metamorphic musco-vite or Table 5) with much lower Fe and Mgcontents Data plotted in Fig 14bc confirm theassumption of a significant ferrimuscovitic substitu-tion (Guidotti et al 1994) Some of the K-micaanalysed in the Los Molinos and La Santa unitswere found to have significant Na contents (up to02 apfu) Although Ti is unusual in dioctahedralmicas it has been measured in several analyses

(lt005 apfu) The most evident feature in theplots is the scattering of data in the La Santa andMorron de Totana units ie where diageneticconditions are not exceeded In contrast theYechar Unit analyses define a narrow field in thediagrams and the Los Molinos data are even closerto the theoretical muscovite composition (Table 3)

Typical compositions of micas intermediate tomuscovite and paragonite called Na-K micas in thispaper are shown in Table 4 The compositions arequite variable even in the same sample Whencompared with analyses of K-rich dioctahedralmicas (Table 3) and paragonite analyses are seento span the entire range from muscovite toparagonite Some analyses (eg Es-5 9) fall within

FIG 14 Plots of values of chemical compositions of K-rich dioctahedral phyllosilicates solid circles and linesindicate values for theoretical muscovite and corresponding exchange vectors respectively

Prograde and retrograde metamorphism 15

the limits of the solvus between muscovite andparagonite the limbs of that solvus having theapproximate compositions ms60pg40 and ms20pg80

(Li et al 1994a)Detrital muscovite vs metamorphic muscovite

Detrital-like micas of sample Es-95 the lowest-grade one are large enough to be analysed(Table 5) by electron microprobe (EMPA) Thesedata have been compared with the AEM data ofother samples Compositions of detrital-like grainsof the lowest-grade sample and the detrital micas ofthe anchizonal-grade sample Es-53 (Fig 15) arevery similar and close to that of end-membermuscovite These data suggest that the detrital micahas a common source for the different units thatcomprise Sierra Espuna Figure 15 also comparesthe data from authigenic micas of the samesamples which are clearly more phengitic (Fe+Mg= 015 ndash075 apfu) than the detrital ones (Fe+Mg= 01 ndash02 apfu)

Chlorite Trioctahedral and di-trioctahedral(sudoite) chlorites have been analysed (Table 6)The former corresponding to the species clino-chlore is common in most of the samples unlikesudoite which is absent from most (Table 1)Sudoite was identified because the sum ofoctahedral cations is nearly equal to 5 apfu andthe Si and Al contents are higher than intrioctahedral chlorite (Fig 16a) A slight contam-ination by interlayer cations (K and Na) has beendetected therefore the formulae were recalculatedin order to eliminate those impurities (seeAnalytical Methods) Although total Fe has beenconsidered as Fe2+ according to Fransolet ampBourguignon (1978) the Fe3+ content in sudoitemay be greater than the Fe2+ content As severalanalyses have an octahedral sum slightly higherthan the ideal value of 5 apfu this may indicate atrioctahedral substitution andor the presence ofsignificant Fe3+ No Ti or Mn were detected The

Fe(Fe+Mg) ratios do not define a clear trendthroughout the sequence (Fig 16b) nevertheless itis much greater in chlorite in the Morron de Totanaanalyses falling in the characteristic range ofdiagenetic chlorites (Li et al 1994b) There is anincrease in Mg content (from 15 to 4 apfu)concomitant with increase in grade as compatiblewith some other sequences (eg Bevins et al1991)

D I S C U S S I O N

Dioctahedral phyllosilicates chemicalcompositions IC values and polytypes

On the basis of their large size homogeneity ofindividual grains and well-crystallized aspectdetrital dioctahedral mica grains were identified inseveral samples with the exception of those ofgreenschist facies This confirms the idea of the

TABLE 4 Chemical compositions for Na-K micas normalized to O10(OH)2

Si IVAl VIAl Fe Mg Ti VIS Ca K Na S inter

Es-5 14 313 087 190 004 009 002 204 000 034 048 083Es-5 15 321 079 176 009 037 000 222 000 015 038 053Es-5 4 318 082 187 004 019 000 209 000 013 061 074Es-5 12 329 071 172 008 029 000 208 000 039 036 076Es-5 13 323 077 179 005 028 000 212 000 029 041 069Es-5 9 296 104 163 019 029 000 211 000 048 056 104Es-5 8 292 108 156 043 008 001 209 003 014 080 096

FIG 15 Plot of Si vs Al contents of detrital andauthigenic micas of samples Es-95 and Es-53 Solidsymbols = detrital micas open symbols = authigenic

micas

16 I Abad et al

relative stability of muscovite in most diageneticenvironments (Crowley 1991) The compositions ofthe detrital grains of samples from different unitsare similar The lack of heterogeneity which isexpected for mica from a complex source implies acommon source area for all samples

The IC values define a trend from diagenetic toanchizonal conditions but with smaller IC values inbulk-rock fractions consistent with the contributionof detrital mica The coexistence of mature micawith lower-grade I-S or illite in the sub-greenschistfacies rocks shows that those rocks have not

TABLE 5 EMPA data for detrital-like crystals of mica normalized to O10(OH)2

Si IVAl VIAl Fe Mg Ti VIS Ca K Na K+Na+Ca

Es-95z11 313 087 193 005 004 001 203 000 065 022 087Es-95z11 314 086 190 006 007 001 204 000 068 019 087Es-95z1 2 314 086 191 005 009 001 206 000 070 006 077Es-95z2 1 314 086 176 012 010 003 201 002 095 004 101Es-95z2 2 317 083 177 012 011 003 202 000 095 004 099Es-95z4 1 305 095 188 005 007 004 204 000 081 009 089Es-95z4 2 311 089 187 006 007 004 204 000 078 009 086

TABLE 6 Representative chemical compositions for chlorite normalized to O10(OH)8

Si IVAl VIAl Fe Mg VIS FeFe+Mg

Yechar UnitEs53-1 273 127 112 121 376 609 024Es53-4 287 113 125 070 400 596 015Es53-12 303 097 130 146 307 583 032Es53-13 277 123 125 122 354 601 026Es53-15 307 093 146 094 323 564 023Es53-42 271 129 152 160 294 606 035

La Santa UnitEs2-4 319 081 147 148 273 569 035Es2-5 365 035 183 118 225 526 034Es5-1 354 046 293 023 172 488 012Es5-5 361 039 292 021 178 490 010Es5-10 309 091 262 028 221 511 011Es5-16 310 090 197 112 234 543 032Es5-17 350 050 252 076 170 499 031Es-5 10rsquo 364 036 213 097 202 512 032Es-5 11 291 109 143 136 314 593 030Es-5 12 382 018 229 087 179 495 033Es-5 13 316 084 302 021 172 494 011Es-5 16rsquo 346 054 341 013 101 455 011Es-5 17rsquo 300 100 282 033 194 509 014Es-5 18 285 115 285 026 204 515 011

Morron de Totana UnitEs-86 2 377 023 253 088 149 490 037Es-86 4 284 116 166 230 181 578 056Es-86 5 350 050 191 192 151 534 056Es-86 6 366 034 189 191 149 529 056Es-86 7 315 085 129 273 176 578 061

indicates sudoite analyses

Prograde and retrograde metamorphism 17

reached stable chemical or textural equilibrium atleast at the mm scale In addition the lack ofchemical equilibrium among diagenetic I-S andillite at the nm scale is demonstrated by their highlyvariable Fe+Mg contents (006ndash075 apfu) Theaverages of the sum of interlayer cations are closeto 1 apfu in the Los Molinos and Yechar units asconsistent with mica but values of 06 apfucorresponding to I-S or illitic compositions are alsopresent in the Morron de Totana Unit Except forsamples Es-102 and Es-53 the variable composi-tions represent a metastable state with composi-tions of micas determined by a combination ofphengitic ferrimuscovitic and illitic vectors asshown by Fig 14ab

Paragonite is generally considered an indexmineral for the anchizone (Frey 1987) but Li etal (1994a) have shown that paragonite coexistswith K-rich mica in grades as low as that of the latediagenetic zone for a prograde sequence Samplesin which muscovite paragonite and intermediateNa-K mica coexist as in sample Es-5 areconsistent with metastability both because morethan two such micas coexist and because theircompositions are incompatible with the solvus inthe muscovite-paragonite system (see below) Whilethe 2M polytype typical of metamorphic micas is

the most common in these samples the 1Md

polytype also occurs in some diagenetic samplesparticularly those from the Morron de Totana UnitThe 1Md polytype is metastable relative to the 2Mpolytype and the coexistence of both polytypesfurther demonstrates a lack of equilibrium at the nmscale Dong amp Peacor (1996) showed that thedisorder noted in the diagenetic phyllosilicates bySAED is a consequence of the general high entropyof phases formed at very low temperatures thatproduces for example stacking defects separatingpackets with no more than 10 layers These packetsprogressively thicken with increasing metamorphicgrade and the 2M polytype then becomes the mostcommon polytype in authigenic micas Thenm-scale size of the lower-grade clays and theregular increase in size with increasing gradereflect the lack of textural equilibrium in the low-grade samples

A wide range of sample characteristics thusimply that the lower-grade samples are in statesof metastable equilibrium those factors includingcoexistence of mature mica with I-S and illitevariable chemical compositions incompatiblecompositions of white micas small crystal sizesdefect-rich states and polytypism Such dataemphasize the notion of lsquoreaction progressrsquo relative

FIG 16 (a) Plot of Al vs Fe+Mg contents of chlorites illustrating the contrast in composition between tri-trioctahedral clinochlore or chamosite and di-trioctahedral sudoite (b) plot of Si vs Fe(Fe+Mg) ratio of

chlorites showing an absence of compositional trends throughout the sampled sequence

18 I Abad et al

to Ostwald-step rule processes metastable dioctahe-dral clay systems undergoing reactions toward thestate of stable chemical and textural equilibriumapproached in greenschist-facies samples (Essene ampPeacor 1995)

Prograde evolution based on TEM datacorrelation with the Basque-Cantabriansequence (Northern Spain)

With increasing grade TEM data confirm thatcrystal thickness increases crystal defect densitydecreases compositions become less variable andmicrotextural relations are simpler (eg Peacor1992) This trend is consistent with three well-characterized low-grade settings the Gulf Coastsequence (Ahn amp Peacor 1986) and the WelshBasin (Merriman et al 1990) both extensionalbasins and the Helvetic Alps (Livi et al 1997)The latter is a fold-and-thrust belt with similarseries of prograde transitions to those observed inSierra Espuna in which pelites have passed througha sequence of metastable states in the lithologicalsequence mudstone and shale slate and phyllite

The mudstones of the Morron de Totana Unit arecharacterized by the presence of illite-smectite R4mixed-layers mica packets of no more than 10 ndash40layers and the occurrence of 1Md polytypism TheLa Santa Unit samples represent a step in theprograde evolution with thicker packets than in theprevious samples (200 ndash700 AEcirc ) illitic or phengiticcompositions the absence of illite-smectite mixedlayers as well as the presence of discreteintermediate Na-K micas and paragonite Althoughthere is a tendency towards non-preferred orienta-tion of crystals it is not so evident as in the Morronde Totana samples In the Yechar Unit (anchizone)and more clearly in the Los Molinos phyllites(greenschist facies) the microtextural relations areless complex the compositions are less variablewith phengite and trioctahedral chlorite crystalthickness are greater (gt400 AEcirc in the Yechar Unitand gt1000 AEcirc in the Los Molinos Unit) with thepackets being defect-free and with a very crystallineaspect This description has features in commonwith the three well-characterized low-grade settingspreviously noted Only the mineral assemblagesvary somewhat presumably as a function of thedifferences in bulk-rock composition

A detailed comparison with the Basque-Cantabrian Basin (N Spain) shows that the lowest-grade conditions of Sierra Espuna overlap most of

those of that sequence which is characterized bypassive burial diagenesis (8000 m thick) (Nieto etal 1996) Only in the deepest sample of theBasque-Cantabrian sequence is illite-smectite mixedlayering absent Figure 12 of this study is verysimilar to Fig 7 in Nieto et al (1996) and bothimages correspond to diagenetic samples fromdifferent settings nevertheless they illustratesimilar mica packets 100 ndash200 AEcirc thick withparallel-to-subparallel boundaries the orientationof which is inferred to represent beddingTherefore there is a parallelism between thesesequences down to the scale of individual clay-mineral grains even though they developed underdifferent conditions with stress presumably playinga significant role in the Sierra Espuna units Thetexture of the least metamorphosed sample (Fig 13)is reminiscent of the typical textural relations ofsmectite described by Nieto et al (1996) (seeFig 4) Although it corresponds to illite-richinterstratified illite-smectite the clay mineralgrains conserved the grain size and anastomosingdiscontinuous array of original smectite layers

Despite the similarities between the twosequences the chemical behaviour of the micasare clearly different since the grade correspondingto the phengitic compositions of the Yechar Unit isnot reached in the Basque-Cantabrian Basin wherethe deepest and highest-grade sample has illiticcompositions equivalent to those of the La SantaUnit That is in a passive burial metamorphiccontext as in the Basque-Cantabrian Basin thefactors responsible for increasing the rate ofchemical reactions are less significant than in thesequences that were affected by tectonic stressTectonic stress introduces defects which in turnincreases the rate of dissolution of strained grainsthus increasing the rate of dissolution-crystallizationreactions typical of clay minerals Perhaps moreimportantly regional tectonic stress is known tocause mobilization of fluids (eg Oliver 1986) Thepresence and activity of such fluids is a prime causeof increased rates of dissolution ion diffusion andcrystallization Tectonic activity thus may have atwo-fold effect in increasing rate of clay-mineralreactions The phengite component may in part be ameasure of the reaction progress depending on thereaction kinetics in very low-grade rocksAccording to Merriman amp Peacor (1999) whereheat flow is low as in accretionary settingsphengite contents are typically high perhapsbecause there is insufficient thermal energy to

Prograde and retrograde metamorphism 19

cause equilibration of the phengite component ofneoformed illite by contrast with extensional basinswhere this component is typically small

Coexistence of trioctahedral chlorite andsudoite Implications for multiple metamorphicevents

In metamorphic rocks the coexistence of mineralsthat formed at different times and conditions on aPTt path ie not in equilibrium is a commonlyobserved feature where the metamorphic history iscomplex Gross differences in the grain size ofcoexisting phases has long been recognized bymetamorphic petrologists as an indicator of non-equilibrium mineral assemblages Historically muchless attention has been paid to such relations in sub-greenschist-facies rocks because prior to the intro-duction of SEM+TEM techniques their small grainsize prevented observations of textural relationsStudy of such rocks eg by XRD provides littleinformation on textural relations beyond determina-tion of degree of preferred orientation

Study by XRD of the samples showed thatsudoite and trioctahedral chlorite coexist If theyhad formed at the same time and under the sameP-T conditions their compositions would define thelimbs of a solvus The TEM images show howeverthat wherever they are in contact the sudoitetransects the packets of trioctahedral chlorite (egFig 8) Furthermore as illustrated in Fig 8 thetrioctahedral chlorite is commonly deformed anddisplays strain contrast in TEM images Thesetextural relations suggest that sudoite was intro-duced subsequent to the formation of trioctahedralchlorite Similar observations were made byGiorgetti et al (1998) who concluded that texturesin Verrucano metasediments showed that sudoitepost-dated chloritoid pyrophyllite and trioctahedralchlorite ie sudoite is in disequilibrium with thelatter minerals and formed as a retrograde phase Inrocks of the Sierra Espuna area differences in bulk-rock composition are small (Table 2) and do notcorrespond to the presence or absence of sudoite(Table 1) ie the random occurrence of sudoiteindependent of bulk-rock composition is compa-tible with a lack of equilibrium relations Theserelations collectively imply that sudoite wasintroduced at some time subsequent to theformation of mineral assemblages at peak meta-morphic conditions On the other hand the size ofsudoite packets is similar to that of trioctahedral

chlorite as consistent with formation at conditionsnot much different from those for chlorite

In samples from the Morron de Totana bothsudoite and trioctahedral chlorite were affected byretrograde alteration to smectite The TEM imagesshow sudoite (Fig 11) and trioctahedral chlorite(Fig 10) with interstratified layers of smectite withspacings of 10 AEcirc due to collapse and dehydration inthe TEM environment and even packets of smectite150 AEcirc thick within chlorite crystals These relationsimply that both sudoite and trioctahedral chloritehave been affected by retrograde alteration at thevery low temperatures consistent with smectiteformation although the presence of more subtlealteration features in sudoite of some samplessuggests that it was less susceptible to alteration

We therefore conclude that at least three episodesof phyllosilicate formation can be recognized inrocks from the Sierra Espuna-area Trioctahedralchlorite was presumably produced at the highestmetamorphicdiagenetic grade in equilibrium withparagonite and phengite Subsequently sudoite wasintroduced but at P-T conditions which could nothave been much different than those of peakmetamorphic grade Finally at very low-T condi-tions hydrothermal fluids caused partial alterationof trioctahedral chlorite and sudoite to smectite

Retrograde products formed by the reaction offluids with mineral phases generated during anearlier prograde event were described previouslyby Nieto et al (1994) and Zhao et al (1999) Thosereactions occurred on a regional scale but similarreactions have been observed on a more localizedscale The fluid activity associated with a majorfault zone is a common source for these reactions(eg Jiang et al 1990) The structure of SierraEspuna (Fig 1) is based on imbrication processesthat superimpose units that have subsequently beenaffected by faults and folds and that have under-gone significant uplift at least 800 m from the lateMiocene to the present (Sanz de Galdeano et al2001) Given this evolution it is easy to justify theretrograde processes that usually seem to be relatedwith tectonic events during which fluid flow andfluidrock ratios are enhanced (Merriman amp Peacor1999) Moreover according to Nieto et al (1994)the uplift of the Betic Cordillera would furnish theintroduction of phreatic water into the systemproducing a reactive environment adequate for thegenesis of smectite Geological conditions aretherefore consistent with post-peak metamorphicconditions as implied by the TEM observations

20 I Abad et al

The introduction of sudoite as a major phaseduring metamorphism implies the possibility ofadditional overprints on a simple prograde assem-blage the evidence for which might be subtle TheNa-rich micas at all grades have compositionswhich either fall within the solvus or are too Na-rich to correspond to the limb of the solvus In thehighest-grade rocks such micas coexist withmuscovite Those relations are incompatible withchemical equilibrium despite the fact that relationsfor muscovite and other minerals imply an approachto chemical equilibrium Furthermore there is aclose association of Na-rich micas with sudoite (egFig 7) Those relations imply the possibility thatNa-micas were introduced concomitant withsudoite P Mata (pers comm) has shown forexample that overprinting of an epizonal assem-blage by hydrothermal solutions resulted in theintroduction of both an Al-rich variety of chlorite(cookeite) and paragonite in pelites of the CamerosBasin Spain The textural relations such as cross-cutting textures which are so striking for sudoitewere not observed for Na-rich micas Neverthelessthe possibility of such overprinting must beconsidered

In spite of all these prograde-retrogradeprocesses in the Los Molinos Unit a smallnumber of homogeneous phases coexist in chemicalequilibrium and the fine-grained metamorphicphyllosilicates are clearly parallel to cleavage witha typical metamorphic aspect Finally all thetextural and compositional changes that haveoperated in the Espuna area during the metamorphicevolution proceed toward a simpler system repre-senting both textural and chemical equilibriumwhich is characteristic of greenschist-facies condi-tions

Roles of detrital and authigenic phyllosilicatesduring slaty cleavage development

Van der Pluijm et al (1998) focused on therespective roles of the phyllosilicates of detritalorigin and those formed during diagenesis andmetamorphism and observed that detrital phyllosi-licates remain preferentially oriented parallel tobedding in pelites with slaty cleavage whereas fine-grained metamorphic phyllosilicates are orientedparallel to cleavage

Although it is usually difficult to determine thepreferred orientations of phyllosilicates in TEMimages as the grade increases better-aligned

packets occur in the cleavage orientation Thetexture of the least metamorphosed sample inwhich even incipient slaty cleavage is absent isreminiscent of the typical textural relation ofsmectites described in the literature The illite-smectite mixed-layers conserve the textural featuresof possible previous smectite such as anastomosingfringes curved and discontinuous with variations ininterplanar spacing d001 These features indicatethat the minimum energy conditions to initiatemechanical transformations have not been reachedin the lower-grade samples as postulated by Ho etal (1996) since slaty cleavage develops as a resultof interactive mechanical deformation and ther-mally driven dissolution and crystallizationprocesses (Knipe 1981) In contrast in the LosMolinos Unit large crystals of chlorite and whitemica (gt1000 AEcirc ) occur which are of metamorphicorigin but with no detrital grains (Fig 3a)Phyllosilicates are preferentially oriented parallelto slaty cleavage in response to increasedtemperature and tectonic strain (Merriman ampPeacor 1999) The oriented microfabrics and slatycleavage well developed in the rocks of this unitmust be a product of a higher thermal and strainenergy environment reached in the deepest part ofthe sequence As pointed out by Van der Pluijm etal (1998) a continuous range of orientations isevidence for mechanical rotation of grains whereasthe lack of such transitional orientations impliesthat cleavage-parallel phyllosilicates originatedthrough dissolution of bedding-parallel and crystal-lization in the slaty cleavage orientation Asobserved in this study for the Intermediate Unitsgrains with orientations which are transitional frombedding-parallel to slaty cleavage orientation areonly rarely observed Cleavage formation is there-fore inferred to have occurred entirely or nearlyentirely through dissolution and crystallization

ACKNOWLEDGMENTS

We thank MM Abad Ortega from the Centro deInstrumentacio n Cient otilde fica of the Universidad deGranada for her help with HRTEM and I Nieto forher essential help with sample preparation Financialsupport was supplied by Research Project nordm BT 2000-0582 FPI research grant to IA both of the SpanishMinistry of Science and Technology Research GroupRNM-0179 of the Junta de Andaluc otildeacutea and to DRP byNSF grant EAR-9814391

Prograde and retrograde metamorphism 21

REFERENCES

Ahn JH amp Peacor DR (1986) Transmission andanalytical electron microscopy of the smectite-to-illite transition Clays and Clay Minerals 34165 ndash179

Azanon JM amp Crespo-Blanc A (2000) Exhumationduring a continental collision inferred from thetectonometamorphic evolution of the AlpujarrideComplex in the central Betics (Alboran Domain SESpain) Tectonics 19 549 ndash565

Bevins RE Robinson D amp Rowbotham G (1991)Compositional variations in mafic phyllosilicatesfrom regional low-grade metabasites and applicationof the chlorite geother mometer Journal ofMetamorphic Geology 9 711 ndash721

Cliff G amp Lorimer GW (1975) The quantitativeanalysis of thin specimens Journal of Microscopy 103 203 ndash207

Crowley SF (1991) Diagenetic modification of detritalmuscovite an example from the Great LimestoneCyclothem (Carboniferous) of Co Durham UKClay Minerals 26 91 ndash103

Dong H amp Peacor DR (1996) TEM observations ofcoherent stacking relations in smectite and illite ofshales evidence for MacEwan crystallites anddominance of 2M1 polytypes Clays and ClayMinerals 44 257 ndash275

Essene E amp Peacor DR (1995) Clay mineral thermo-metry a critical perspective Clays and ClayMinerals 43 540 ndash553

Fransolet AM amp Bourguignon P (1978) Dioctahedralchlorite in quartz veins from Ardennes BelgiumThe Canadian Mineralogist 16 365 ndash373

Fransolet AM amp Schreyer W (1984) Sudoite ditrioctahedral chlorite a stable low-temperature phasein the system MgO-Al2O3-SiO2-H2O Contributionsto Mineralogy and Petrology 86 409 ndash417

Frey M (1987) Very low-grade metamorphism of clasticsedimentary rocks Pp 9 ndash58 in Low TemperatureMetamorphism (M Frey editor) Blackie GlasgowUK

Giorgetti G Goffe B Memmi I amp Nieto F (1998)Metamorphic evolution of Verrucano metasedimentsin northern Apennines new petrological constraintsEuropean Journal of Mineralogy 10 1295 ndash1308

Guidotti CV amp Sassi FP (1986) Classification andcorrelation of metamorphic facies series by means ofmuscovite b0 data from low grade metapelites NeuesJahrbuch fur Mineralogie Abhandlungen 153363 ndash380

Guidotti CV Yates MG Dyar MD amp Taylor ME(1994) Petrogenetic implications of the Fe3+ contentof muscovi te in pel it ic sch is ts AmericanMineralogist 79 793 ndash795

Guthrie GD amp Veblen DR (1989) High resolutionelectron microscopy of mixed-layer illitesmectite

Computer simulations Clays and Clay Minerals 371 ndash11

Ho N Peacor DR amp Van der Pluijm BA (1996)Contrasting roles of detrital and authigenic phyllo-silicates during slaty cleavage development Journalof Structural Geology 18 615 ndash623

Jiang WT Peacor DR Merriman RJ amp Roberts B(1990) Transmission and analytical electron micro-scopic study of mixed layer illitesmectite formed asan apparent replacement product of diagenetic illiteClays and Clay Minerals 38 449 ndash468

Jiang WT Peacor DR amp Buseck PR (1994) Chloritegeothermometry Contamination and apparent octa-hedral vacancies Clays and Clay Minerals 42593 ndash605

Kisch HJ (1991) Development of slaty cleavage anddegree of very-low-grade metamorphism a reviewJournal of Metamorphic Geology 9 735 ndash750

Knipe JR (1981) The interaction of deformation andmetamorphi sm in slates Tectonophysics 78249 ndash272

Kretz R (1983) Symbols for rock-forming mineralsAmerican Mineralogist 68 277 ndash279

Li G Peacor DR Merriman RJ amp Roberts B (1994a)The diagenetic to low grade metamorphism evolu-tion of matrix white mica in the system muscovite-paragonite in a mudrock from Central Wales UKClays and Clay Minerals 42 369 ndash381

Li G Peacor DR Merriman RJ Roberts B amp Vander Pluijm BA (1994b) TEM and AEM constraintson the origin and significance of chlorite-mica stacksin slates an example from Central Wales UKJournal of Structural Geology 16 1139 ndash1157

Livi KJT Veblen DR Ferry JM amp Frey M (1997)Evolution of 21 layered silicates in low-grademetamorphosed Liass ic sha les of CentralSwitzerland Journal of Metamorphic Geology 15323 ndash344

Lonergan L (1991) Structural evolution of the SierraEspuna Betic Cordillera SE Spain PhD thesisOxford University UK

Lonergan L Platt JP amp Gallagher L (1994) TheInternal-External Zone Boundary in the eastern BeticCordillera SE Spain Journal of Structural Geology16 175 ndash188

Makel GH (1981) Differences in tectonic evolution ofsuperimposed Malaguide and Alpujarride tectonicunits in the Espuna area (Betic Cordilleras Spain)Geology in Mijnbouw 60 203 ndash208

Makel GH (1985) The geology of the MalaguideComplex and its bearing on the geodynamicevolution of the Betic-Rif orogen (southern Spainand northern Morocco) GUA papers of Geology Ser1 22 263 pp

Makel GH amp Rondeel HE (1979) Differences instratigraphy and metamorphism between superposedMalaguide and Alpujarride units in the Espuna area

22 I Abad et al

by the continuity of such fringes with openinterlayer spaces produced by contraction due to

dehydration The sequence on average is that ofR4 mixed-layering ie a single smectite-likeinterlayer between four or more illite-like inter-layers Although some packets with more than fourlayers of illite are present repeats of four illite-likelayers predominate as shown by the 50 AEcirc

periodicity of SAED patterns (Fig 9 inset) Theillite-smectite (I-S) has typical 1Md polytypism Theobservation of smectite interstratified with illite isin accord with the changes observed in XRDdiagrams after EG treatment (Fig 2) In additionAEM analyses of areas similar to that of Fig 9have relatively high Si and minor Ca contentsfurther confirming the mixed layering

Some packets which give rise to fringes with10 AEcirc spacing produce SAED patterns typical oftwo-layer polytypism The contrast of all fringes isrelatively constant Such packets have thicknessestypical of diagenetic clays and two orders ofmagnitude smaller than typical detrital grains Thecompositions of such grains (see below) haveinterlayer-cation contents typical of illite They aretherefore inferred to be illite which formed duringlate diagenesis and coexisting with the R4 I-S

Sample Es-86 is composed mainly of detritalgrains of quartz randomly-oriented grains ofchlorite and white mica all immersed in aphyllosilicate-rich fine-grained matrix At theTEM scale chlorite is the most common phyllosi-licate in the matrix As in sample Es-5 trioctahe-dral chlorite and sudoite coexist There areinterstratified packets of 10 AEcirc layers only a fewlayers thick (lt150 AEcirc ) which are quickly damaged

FIG 11 Di-trioctahedral chlorite (sudoite) very similarin appearance to the trioctahedral chlorite shown inFig 10 7 AEcirc layers are marked by an arrow and 10 AEcirc

layers are present in the poorly resolved area in thecentre of the image (see analysis in Table 6 Es-862)

(Morron de Totana Unit)

FIG 12 Low-magnification image illustrating illite-like packets The inset SAED pattern shows only a smallnumber of diffuse poorly defined reflections The packets are 100 ndash200 AEcirc thick (Morron de Totana Unit)

12 I Abad et al

under the electron beam (Figs 10 and 11) TheAEM analyses of such material show an enhancedSi content and the presence of small concentrationsof alkali cations These data are consistent withretrograde alteration of chlorite as observed byNieto et al (1994) They differ however from theresults of Nieto et al (1994) and those describedabove for sample Es-5 in that both tri-trioctahedraland di-trioctahedral chlorite are affected In TEMimages strain features in chlorites such as contrastnormal to the layers or microfolds were observed(Fig 10) Mica is less abundant than sudoiteoccurring in packets 100 ndash400 AEcirc thick The SAEDpatterns exhibit 2M polytypism but with diffuse-ness parallel to c in 0kl rows At low magnifica-tion subparallel and slightly curved illite- or illite-smectite-like packets are present they coalescehave abundant layer terminations and show intensecontrasts (Fig 12)

Sample Es-95 is a very fine-grained rock madeup of large detrital grains of quartz in a fine-grained matrix of phyllosilicates mainly whitemicas and opaque minerals Some biotite grainswere observed as verified by AEM data Biotitewas observed only in separated grains on holey

C-coated Cu grids so it is not possible to describethe textural relations for it Authigenic biotite isinconsistent with the low grade of this samplehowever and it is therefore inferred to be of detritalorigin The AEM analyses show deviations frompure biotite however as consistent with alterationto chlorite the usual process in detrital biotite inrocks undergoing diagenesis Lattice-fringe imagesof ion-milled samples show that individual micapackets are very thin with no more than 10 layersisolated and dispersed in a matrix consisting ofmaterial which gives no SAED pattern Thesepackets are wavy discontinuous and with anasto-mosing layers (Fig 13) Despite these texturalfeatures SAED data indicate that there are 2Mand 1Md polytypes

For all samples prismatic crystals of Ti-oxidedispersed Fe-oxyhydroxides zircon and tourmalineare common accessory minerals

Chemical characterization of phyllosilicates

Dioctahedral mica Table 3 shows the composi-tion of K-rich dioctahedral micas of authigenic ormetamorphic origin corresponding to the samples

FIG 13 General aspect of sample Es-95 The image shows arrays of wavy discontinuous and anastomosingpackets of illite and smectite (Morron de Totana Unit)

Prograde and retrograde metamorphism 13

selected from each unit The Yechar and LosMolinos units are characterized by compositionsapproaching those of evolved micas with a sum ofinterlayer cations slightly less than 1 apfu typicalof phengite or muscovite rather than authigenic

illite In contrast the La Santa and Morron deTotana units show heterogeneous mica composi-tions and display deficiencies in interlayer cations(Fig 14a) This last feature is more evident in theMorron de Totana analyses where the illitic

TABLE 3 Representative chemical compositions for K-rich dioctahedral micas normalized to O10(OH)2

Si IVAl VIAl Fe Mg Ti VIS K Na S inter

Los Molinos Unit (1)Es-10213 337 063 178 010 008 002 198 075 008 085Es-10221 308 092 195 005 001 001 202 082 009 091Es-10231 319 081 191 005 003 001 200 077 012 090Es-10232 314 086 188 006 007 001 202 072 021 093

Yechar Unit (1)Es-5311 332 068 163 015 022 002 204 090 004 093Es-5312 310 090 180 003 013 004 201 089 010 099Es-5322 338 062 170 012 015 003 200 086 001 086Es-5323 319 081 176 016 012 003 206 087 002 089

Yechar Unit (2)Es-53 1 317 083 163 021 026 000 210 084 000 084Es-53 2 320 080 165 011 027 003 207 087 000 087Es-53 3 317 083 155 025 024 002 206 093 000 093Es-53 4 328 072 160 020 024 000 203 092 000 092Es-53 5 326 074 164 019 016 000 199 096 000 096Es-53 6 326 074 158 020 028 000 206 089 000 089Es-53 7 319 081 154 025 026 000 205 097 000 097Es-53 8 325 075 166 016 025 000 206 085 000 085Es-53 9 324 076 167 019 019 000 205 085 000 085

La Santa Unit (2)Es-2 1 311 089 157 019 032 002 210 088 005 094Es-2 3 336 064 148 020 043 001 212 072 003 075Es-2 6 313 087 156 024 029 001 210 075 017 092Es-2 7 313 087 155 024 029 001 209 084 011 095Es-2 8 331 069 157 015 032 000 205 087 004 092Es-2 10 328 072 160 018 027 003 208 068 008 075Es-5 4 303 097 174 015 023 001 213 073 009 082Es-5 10 301 099 189 006 015 000 210 065 021 086Es-5 7 307 093 159 033 016 000 208 075 018 093Es-5 8rsquo 301 099 160 031 025 000 216 067 016 083

Morron de Totana Unit (2) (analyses corresponding to R4 Ill-Sm may also be present see text)Es-77 13 327 073 172 013 027 000 212 067 000 067Es-77 15 316 084 195 003 013 000 211 063 000 063Es-77 16 328 072 182 010 015 000 207 068 000 068Es-77 17 338 062 147 032 019 000 199 093 000 093Es-86 9 334 066 160 017 034 000 210 072 000 072Es-86 10 357 043 151 012 046 000 209 065 000 065Es-86 11 353 047 156 012 036 000 203 077 000 077Es-86 13 324 076 152 025 035 000 211 084 000 084Es-86 14 324 076 173 013 019 000 205 084 000 084Es- 95 2 323 077 158 016 019 004 198 103 000 103Es- 95 5 336 064 169 013 025 000 207 070 000 070Es- 95 6 336 064 148 027 033 000 209 078 000 078

(1) SEMEDX analyses (2) TEMAEM analyses

14 I Abad et al

character is consistent with the presence of the R4mixed-layer illite-smectite described above Most ofthe micas are phengitic in composition (Fig 14ab)with an Fe(Fe+Mg) ratio that varies over the range02 ndash085 However the detrital-like grains aresimilar to those of end-member muscovite (seebelow detrital muscovite vs metamorphic musco-vite or Table 5) with much lower Fe and Mgcontents Data plotted in Fig 14bc confirm theassumption of a significant ferrimuscovitic substitu-tion (Guidotti et al 1994) Some of the K-micaanalysed in the Los Molinos and La Santa unitswere found to have significant Na contents (up to02 apfu) Although Ti is unusual in dioctahedralmicas it has been measured in several analyses

(lt005 apfu) The most evident feature in theplots is the scattering of data in the La Santa andMorron de Totana units ie where diageneticconditions are not exceeded In contrast theYechar Unit analyses define a narrow field in thediagrams and the Los Molinos data are even closerto the theoretical muscovite composition (Table 3)

Typical compositions of micas intermediate tomuscovite and paragonite called Na-K micas in thispaper are shown in Table 4 The compositions arequite variable even in the same sample Whencompared with analyses of K-rich dioctahedralmicas (Table 3) and paragonite analyses are seento span the entire range from muscovite toparagonite Some analyses (eg Es-5 9) fall within

FIG 14 Plots of values of chemical compositions of K-rich dioctahedral phyllosilicates solid circles and linesindicate values for theoretical muscovite and corresponding exchange vectors respectively

Prograde and retrograde metamorphism 15

the limits of the solvus between muscovite andparagonite the limbs of that solvus having theapproximate compositions ms60pg40 and ms20pg80

(Li et al 1994a)Detrital muscovite vs metamorphic muscovite

Detrital-like micas of sample Es-95 the lowest-grade one are large enough to be analysed(Table 5) by electron microprobe (EMPA) Thesedata have been compared with the AEM data ofother samples Compositions of detrital-like grainsof the lowest-grade sample and the detrital micas ofthe anchizonal-grade sample Es-53 (Fig 15) arevery similar and close to that of end-membermuscovite These data suggest that the detrital micahas a common source for the different units thatcomprise Sierra Espuna Figure 15 also comparesthe data from authigenic micas of the samesamples which are clearly more phengitic (Fe+Mg= 015 ndash075 apfu) than the detrital ones (Fe+Mg= 01 ndash02 apfu)

Chlorite Trioctahedral and di-trioctahedral(sudoite) chlorites have been analysed (Table 6)The former corresponding to the species clino-chlore is common in most of the samples unlikesudoite which is absent from most (Table 1)Sudoite was identified because the sum ofoctahedral cations is nearly equal to 5 apfu andthe Si and Al contents are higher than intrioctahedral chlorite (Fig 16a) A slight contam-ination by interlayer cations (K and Na) has beendetected therefore the formulae were recalculatedin order to eliminate those impurities (seeAnalytical Methods) Although total Fe has beenconsidered as Fe2+ according to Fransolet ampBourguignon (1978) the Fe3+ content in sudoitemay be greater than the Fe2+ content As severalanalyses have an octahedral sum slightly higherthan the ideal value of 5 apfu this may indicate atrioctahedral substitution andor the presence ofsignificant Fe3+ No Ti or Mn were detected The

Fe(Fe+Mg) ratios do not define a clear trendthroughout the sequence (Fig 16b) nevertheless itis much greater in chlorite in the Morron de Totanaanalyses falling in the characteristic range ofdiagenetic chlorites (Li et al 1994b) There is anincrease in Mg content (from 15 to 4 apfu)concomitant with increase in grade as compatiblewith some other sequences (eg Bevins et al1991)

D I S C U S S I O N

Dioctahedral phyllosilicates chemicalcompositions IC values and polytypes

On the basis of their large size homogeneity ofindividual grains and well-crystallized aspectdetrital dioctahedral mica grains were identified inseveral samples with the exception of those ofgreenschist facies This confirms the idea of the

TABLE 4 Chemical compositions for Na-K micas normalized to O10(OH)2

Si IVAl VIAl Fe Mg Ti VIS Ca K Na S inter

Es-5 14 313 087 190 004 009 002 204 000 034 048 083Es-5 15 321 079 176 009 037 000 222 000 015 038 053Es-5 4 318 082 187 004 019 000 209 000 013 061 074Es-5 12 329 071 172 008 029 000 208 000 039 036 076Es-5 13 323 077 179 005 028 000 212 000 029 041 069Es-5 9 296 104 163 019 029 000 211 000 048 056 104Es-5 8 292 108 156 043 008 001 209 003 014 080 096

FIG 15 Plot of Si vs Al contents of detrital andauthigenic micas of samples Es-95 and Es-53 Solidsymbols = detrital micas open symbols = authigenic

micas

16 I Abad et al

relative stability of muscovite in most diageneticenvironments (Crowley 1991) The compositions ofthe detrital grains of samples from different unitsare similar The lack of heterogeneity which isexpected for mica from a complex source implies acommon source area for all samples

The IC values define a trend from diagenetic toanchizonal conditions but with smaller IC values inbulk-rock fractions consistent with the contributionof detrital mica The coexistence of mature micawith lower-grade I-S or illite in the sub-greenschistfacies rocks shows that those rocks have not

TABLE 5 EMPA data for detrital-like crystals of mica normalized to O10(OH)2

Si IVAl VIAl Fe Mg Ti VIS Ca K Na K+Na+Ca

Es-95z11 313 087 193 005 004 001 203 000 065 022 087Es-95z11 314 086 190 006 007 001 204 000 068 019 087Es-95z1 2 314 086 191 005 009 001 206 000 070 006 077Es-95z2 1 314 086 176 012 010 003 201 002 095 004 101Es-95z2 2 317 083 177 012 011 003 202 000 095 004 099Es-95z4 1 305 095 188 005 007 004 204 000 081 009 089Es-95z4 2 311 089 187 006 007 004 204 000 078 009 086

TABLE 6 Representative chemical compositions for chlorite normalized to O10(OH)8

Si IVAl VIAl Fe Mg VIS FeFe+Mg

Yechar UnitEs53-1 273 127 112 121 376 609 024Es53-4 287 113 125 070 400 596 015Es53-12 303 097 130 146 307 583 032Es53-13 277 123 125 122 354 601 026Es53-15 307 093 146 094 323 564 023Es53-42 271 129 152 160 294 606 035

La Santa UnitEs2-4 319 081 147 148 273 569 035Es2-5 365 035 183 118 225 526 034Es5-1 354 046 293 023 172 488 012Es5-5 361 039 292 021 178 490 010Es5-10 309 091 262 028 221 511 011Es5-16 310 090 197 112 234 543 032Es5-17 350 050 252 076 170 499 031Es-5 10rsquo 364 036 213 097 202 512 032Es-5 11 291 109 143 136 314 593 030Es-5 12 382 018 229 087 179 495 033Es-5 13 316 084 302 021 172 494 011Es-5 16rsquo 346 054 341 013 101 455 011Es-5 17rsquo 300 100 282 033 194 509 014Es-5 18 285 115 285 026 204 515 011

Morron de Totana UnitEs-86 2 377 023 253 088 149 490 037Es-86 4 284 116 166 230 181 578 056Es-86 5 350 050 191 192 151 534 056Es-86 6 366 034 189 191 149 529 056Es-86 7 315 085 129 273 176 578 061

indicates sudoite analyses

Prograde and retrograde metamorphism 17

reached stable chemical or textural equilibrium atleast at the mm scale In addition the lack ofchemical equilibrium among diagenetic I-S andillite at the nm scale is demonstrated by their highlyvariable Fe+Mg contents (006ndash075 apfu) Theaverages of the sum of interlayer cations are closeto 1 apfu in the Los Molinos and Yechar units asconsistent with mica but values of 06 apfucorresponding to I-S or illitic compositions are alsopresent in the Morron de Totana Unit Except forsamples Es-102 and Es-53 the variable composi-tions represent a metastable state with composi-tions of micas determined by a combination ofphengitic ferrimuscovitic and illitic vectors asshown by Fig 14ab

Paragonite is generally considered an indexmineral for the anchizone (Frey 1987) but Li etal (1994a) have shown that paragonite coexistswith K-rich mica in grades as low as that of the latediagenetic zone for a prograde sequence Samplesin which muscovite paragonite and intermediateNa-K mica coexist as in sample Es-5 areconsistent with metastability both because morethan two such micas coexist and because theircompositions are incompatible with the solvus inthe muscovite-paragonite system (see below) Whilethe 2M polytype typical of metamorphic micas is

the most common in these samples the 1Md

polytype also occurs in some diagenetic samplesparticularly those from the Morron de Totana UnitThe 1Md polytype is metastable relative to the 2Mpolytype and the coexistence of both polytypesfurther demonstrates a lack of equilibrium at the nmscale Dong amp Peacor (1996) showed that thedisorder noted in the diagenetic phyllosilicates bySAED is a consequence of the general high entropyof phases formed at very low temperatures thatproduces for example stacking defects separatingpackets with no more than 10 layers These packetsprogressively thicken with increasing metamorphicgrade and the 2M polytype then becomes the mostcommon polytype in authigenic micas Thenm-scale size of the lower-grade clays and theregular increase in size with increasing gradereflect the lack of textural equilibrium in the low-grade samples

A wide range of sample characteristics thusimply that the lower-grade samples are in statesof metastable equilibrium those factors includingcoexistence of mature mica with I-S and illitevariable chemical compositions incompatiblecompositions of white micas small crystal sizesdefect-rich states and polytypism Such dataemphasize the notion of lsquoreaction progressrsquo relative

FIG 16 (a) Plot of Al vs Fe+Mg contents of chlorites illustrating the contrast in composition between tri-trioctahedral clinochlore or chamosite and di-trioctahedral sudoite (b) plot of Si vs Fe(Fe+Mg) ratio of

chlorites showing an absence of compositional trends throughout the sampled sequence

18 I Abad et al

to Ostwald-step rule processes metastable dioctahe-dral clay systems undergoing reactions toward thestate of stable chemical and textural equilibriumapproached in greenschist-facies samples (Essene ampPeacor 1995)

Prograde evolution based on TEM datacorrelation with the Basque-Cantabriansequence (Northern Spain)

With increasing grade TEM data confirm thatcrystal thickness increases crystal defect densitydecreases compositions become less variable andmicrotextural relations are simpler (eg Peacor1992) This trend is consistent with three well-characterized low-grade settings the Gulf Coastsequence (Ahn amp Peacor 1986) and the WelshBasin (Merriman et al 1990) both extensionalbasins and the Helvetic Alps (Livi et al 1997)The latter is a fold-and-thrust belt with similarseries of prograde transitions to those observed inSierra Espuna in which pelites have passed througha sequence of metastable states in the lithologicalsequence mudstone and shale slate and phyllite

The mudstones of the Morron de Totana Unit arecharacterized by the presence of illite-smectite R4mixed-layers mica packets of no more than 10 ndash40layers and the occurrence of 1Md polytypism TheLa Santa Unit samples represent a step in theprograde evolution with thicker packets than in theprevious samples (200 ndash700 AEcirc ) illitic or phengiticcompositions the absence of illite-smectite mixedlayers as well as the presence of discreteintermediate Na-K micas and paragonite Althoughthere is a tendency towards non-preferred orienta-tion of crystals it is not so evident as in the Morronde Totana samples In the Yechar Unit (anchizone)and more clearly in the Los Molinos phyllites(greenschist facies) the microtextural relations areless complex the compositions are less variablewith phengite and trioctahedral chlorite crystalthickness are greater (gt400 AEcirc in the Yechar Unitand gt1000 AEcirc in the Los Molinos Unit) with thepackets being defect-free and with a very crystallineaspect This description has features in commonwith the three well-characterized low-grade settingspreviously noted Only the mineral assemblagesvary somewhat presumably as a function of thedifferences in bulk-rock composition

A detailed comparison with the Basque-Cantabrian Basin (N Spain) shows that the lowest-grade conditions of Sierra Espuna overlap most of

those of that sequence which is characterized bypassive burial diagenesis (8000 m thick) (Nieto etal 1996) Only in the deepest sample of theBasque-Cantabrian sequence is illite-smectite mixedlayering absent Figure 12 of this study is verysimilar to Fig 7 in Nieto et al (1996) and bothimages correspond to diagenetic samples fromdifferent settings nevertheless they illustratesimilar mica packets 100 ndash200 AEcirc thick withparallel-to-subparallel boundaries the orientationof which is inferred to represent beddingTherefore there is a parallelism between thesesequences down to the scale of individual clay-mineral grains even though they developed underdifferent conditions with stress presumably playinga significant role in the Sierra Espuna units Thetexture of the least metamorphosed sample (Fig 13)is reminiscent of the typical textural relations ofsmectite described by Nieto et al (1996) (seeFig 4) Although it corresponds to illite-richinterstratified illite-smectite the clay mineralgrains conserved the grain size and anastomosingdiscontinuous array of original smectite layers

Despite the similarities between the twosequences the chemical behaviour of the micasare clearly different since the grade correspondingto the phengitic compositions of the Yechar Unit isnot reached in the Basque-Cantabrian Basin wherethe deepest and highest-grade sample has illiticcompositions equivalent to those of the La SantaUnit That is in a passive burial metamorphiccontext as in the Basque-Cantabrian Basin thefactors responsible for increasing the rate ofchemical reactions are less significant than in thesequences that were affected by tectonic stressTectonic stress introduces defects which in turnincreases the rate of dissolution of strained grainsthus increasing the rate of dissolution-crystallizationreactions typical of clay minerals Perhaps moreimportantly regional tectonic stress is known tocause mobilization of fluids (eg Oliver 1986) Thepresence and activity of such fluids is a prime causeof increased rates of dissolution ion diffusion andcrystallization Tectonic activity thus may have atwo-fold effect in increasing rate of clay-mineralreactions The phengite component may in part be ameasure of the reaction progress depending on thereaction kinetics in very low-grade rocksAccording to Merriman amp Peacor (1999) whereheat flow is low as in accretionary settingsphengite contents are typically high perhapsbecause there is insufficient thermal energy to

Prograde and retrograde metamorphism 19

cause equilibration of the phengite component ofneoformed illite by contrast with extensional basinswhere this component is typically small

Coexistence of trioctahedral chlorite andsudoite Implications for multiple metamorphicevents

In metamorphic rocks the coexistence of mineralsthat formed at different times and conditions on aPTt path ie not in equilibrium is a commonlyobserved feature where the metamorphic history iscomplex Gross differences in the grain size ofcoexisting phases has long been recognized bymetamorphic petrologists as an indicator of non-equilibrium mineral assemblages Historically muchless attention has been paid to such relations in sub-greenschist-facies rocks because prior to the intro-duction of SEM+TEM techniques their small grainsize prevented observations of textural relationsStudy of such rocks eg by XRD provides littleinformation on textural relations beyond determina-tion of degree of preferred orientation

Study by XRD of the samples showed thatsudoite and trioctahedral chlorite coexist If theyhad formed at the same time and under the sameP-T conditions their compositions would define thelimbs of a solvus The TEM images show howeverthat wherever they are in contact the sudoitetransects the packets of trioctahedral chlorite (egFig 8) Furthermore as illustrated in Fig 8 thetrioctahedral chlorite is commonly deformed anddisplays strain contrast in TEM images Thesetextural relations suggest that sudoite was intro-duced subsequent to the formation of trioctahedralchlorite Similar observations were made byGiorgetti et al (1998) who concluded that texturesin Verrucano metasediments showed that sudoitepost-dated chloritoid pyrophyllite and trioctahedralchlorite ie sudoite is in disequilibrium with thelatter minerals and formed as a retrograde phase Inrocks of the Sierra Espuna area differences in bulk-rock composition are small (Table 2) and do notcorrespond to the presence or absence of sudoite(Table 1) ie the random occurrence of sudoiteindependent of bulk-rock composition is compa-tible with a lack of equilibrium relations Theserelations collectively imply that sudoite wasintroduced at some time subsequent to theformation of mineral assemblages at peak meta-morphic conditions On the other hand the size ofsudoite packets is similar to that of trioctahedral

chlorite as consistent with formation at conditionsnot much different from those for chlorite

In samples from the Morron de Totana bothsudoite and trioctahedral chlorite were affected byretrograde alteration to smectite The TEM imagesshow sudoite (Fig 11) and trioctahedral chlorite(Fig 10) with interstratified layers of smectite withspacings of 10 AEcirc due to collapse and dehydration inthe TEM environment and even packets of smectite150 AEcirc thick within chlorite crystals These relationsimply that both sudoite and trioctahedral chloritehave been affected by retrograde alteration at thevery low temperatures consistent with smectiteformation although the presence of more subtlealteration features in sudoite of some samplessuggests that it was less susceptible to alteration

We therefore conclude that at least three episodesof phyllosilicate formation can be recognized inrocks from the Sierra Espuna-area Trioctahedralchlorite was presumably produced at the highestmetamorphicdiagenetic grade in equilibrium withparagonite and phengite Subsequently sudoite wasintroduced but at P-T conditions which could nothave been much different than those of peakmetamorphic grade Finally at very low-T condi-tions hydrothermal fluids caused partial alterationof trioctahedral chlorite and sudoite to smectite

Retrograde products formed by the reaction offluids with mineral phases generated during anearlier prograde event were described previouslyby Nieto et al (1994) and Zhao et al (1999) Thosereactions occurred on a regional scale but similarreactions have been observed on a more localizedscale The fluid activity associated with a majorfault zone is a common source for these reactions(eg Jiang et al 1990) The structure of SierraEspuna (Fig 1) is based on imbrication processesthat superimpose units that have subsequently beenaffected by faults and folds and that have under-gone significant uplift at least 800 m from the lateMiocene to the present (Sanz de Galdeano et al2001) Given this evolution it is easy to justify theretrograde processes that usually seem to be relatedwith tectonic events during which fluid flow andfluidrock ratios are enhanced (Merriman amp Peacor1999) Moreover according to Nieto et al (1994)the uplift of the Betic Cordillera would furnish theintroduction of phreatic water into the systemproducing a reactive environment adequate for thegenesis of smectite Geological conditions aretherefore consistent with post-peak metamorphicconditions as implied by the TEM observations

20 I Abad et al

The introduction of sudoite as a major phaseduring metamorphism implies the possibility ofadditional overprints on a simple prograde assem-blage the evidence for which might be subtle TheNa-rich micas at all grades have compositionswhich either fall within the solvus or are too Na-rich to correspond to the limb of the solvus In thehighest-grade rocks such micas coexist withmuscovite Those relations are incompatible withchemical equilibrium despite the fact that relationsfor muscovite and other minerals imply an approachto chemical equilibrium Furthermore there is aclose association of Na-rich micas with sudoite (egFig 7) Those relations imply the possibility thatNa-micas were introduced concomitant withsudoite P Mata (pers comm) has shown forexample that overprinting of an epizonal assem-blage by hydrothermal solutions resulted in theintroduction of both an Al-rich variety of chlorite(cookeite) and paragonite in pelites of the CamerosBasin Spain The textural relations such as cross-cutting textures which are so striking for sudoitewere not observed for Na-rich micas Neverthelessthe possibility of such overprinting must beconsidered

In spite of all these prograde-retrogradeprocesses in the Los Molinos Unit a smallnumber of homogeneous phases coexist in chemicalequilibrium and the fine-grained metamorphicphyllosilicates are clearly parallel to cleavage witha typical metamorphic aspect Finally all thetextural and compositional changes that haveoperated in the Espuna area during the metamorphicevolution proceed toward a simpler system repre-senting both textural and chemical equilibriumwhich is characteristic of greenschist-facies condi-tions

Roles of detrital and authigenic phyllosilicatesduring slaty cleavage development

Van der Pluijm et al (1998) focused on therespective roles of the phyllosilicates of detritalorigin and those formed during diagenesis andmetamorphism and observed that detrital phyllosi-licates remain preferentially oriented parallel tobedding in pelites with slaty cleavage whereas fine-grained metamorphic phyllosilicates are orientedparallel to cleavage

Although it is usually difficult to determine thepreferred orientations of phyllosilicates in TEMimages as the grade increases better-aligned

packets occur in the cleavage orientation Thetexture of the least metamorphosed sample inwhich even incipient slaty cleavage is absent isreminiscent of the typical textural relation ofsmectites described in the literature The illite-smectite mixed-layers conserve the textural featuresof possible previous smectite such as anastomosingfringes curved and discontinuous with variations ininterplanar spacing d001 These features indicatethat the minimum energy conditions to initiatemechanical transformations have not been reachedin the lower-grade samples as postulated by Ho etal (1996) since slaty cleavage develops as a resultof interactive mechanical deformation and ther-mally driven dissolution and crystallizationprocesses (Knipe 1981) In contrast in the LosMolinos Unit large crystals of chlorite and whitemica (gt1000 AEcirc ) occur which are of metamorphicorigin but with no detrital grains (Fig 3a)Phyllosilicates are preferentially oriented parallelto slaty cleavage in response to increasedtemperature and tectonic strain (Merriman ampPeacor 1999) The oriented microfabrics and slatycleavage well developed in the rocks of this unitmust be a product of a higher thermal and strainenergy environment reached in the deepest part ofthe sequence As pointed out by Van der Pluijm etal (1998) a continuous range of orientations isevidence for mechanical rotation of grains whereasthe lack of such transitional orientations impliesthat cleavage-parallel phyllosilicates originatedthrough dissolution of bedding-parallel and crystal-lization in the slaty cleavage orientation Asobserved in this study for the Intermediate Unitsgrains with orientations which are transitional frombedding-parallel to slaty cleavage orientation areonly rarely observed Cleavage formation is there-fore inferred to have occurred entirely or nearlyentirely through dissolution and crystallization

ACKNOWLEDGMENTS

We thank MM Abad Ortega from the Centro deInstrumentacio n Cient otilde fica of the Universidad deGranada for her help with HRTEM and I Nieto forher essential help with sample preparation Financialsupport was supplied by Research Project nordm BT 2000-0582 FPI research grant to IA both of the SpanishMinistry of Science and Technology Research GroupRNM-0179 of the Junta de Andaluc otildeacutea and to DRP byNSF grant EAR-9814391

Prograde and retrograde metamorphism 21

REFERENCES

Ahn JH amp Peacor DR (1986) Transmission andanalytical electron microscopy of the smectite-to-illite transition Clays and Clay Minerals 34165 ndash179

Azanon JM amp Crespo-Blanc A (2000) Exhumationduring a continental collision inferred from thetectonometamorphic evolution of the AlpujarrideComplex in the central Betics (Alboran Domain SESpain) Tectonics 19 549 ndash565

Bevins RE Robinson D amp Rowbotham G (1991)Compositional variations in mafic phyllosilicatesfrom regional low-grade metabasites and applicationof the chlorite geother mometer Journal ofMetamorphic Geology 9 711 ndash721

Cliff G amp Lorimer GW (1975) The quantitativeanalysis of thin specimens Journal of Microscopy 103 203 ndash207

Crowley SF (1991) Diagenetic modification of detritalmuscovite an example from the Great LimestoneCyclothem (Carboniferous) of Co Durham UKClay Minerals 26 91 ndash103

Dong H amp Peacor DR (1996) TEM observations ofcoherent stacking relations in smectite and illite ofshales evidence for MacEwan crystallites anddominance of 2M1 polytypes Clays and ClayMinerals 44 257 ndash275

Essene E amp Peacor DR (1995) Clay mineral thermo-metry a critical perspective Clays and ClayMinerals 43 540 ndash553

Fransolet AM amp Bourguignon P (1978) Dioctahedralchlorite in quartz veins from Ardennes BelgiumThe Canadian Mineralogist 16 365 ndash373

Fransolet AM amp Schreyer W (1984) Sudoite ditrioctahedral chlorite a stable low-temperature phasein the system MgO-Al2O3-SiO2-H2O Contributionsto Mineralogy and Petrology 86 409 ndash417

Frey M (1987) Very low-grade metamorphism of clasticsedimentary rocks Pp 9 ndash58 in Low TemperatureMetamorphism (M Frey editor) Blackie GlasgowUK

Giorgetti G Goffe B Memmi I amp Nieto F (1998)Metamorphic evolution of Verrucano metasedimentsin northern Apennines new petrological constraintsEuropean Journal of Mineralogy 10 1295 ndash1308

Guidotti CV amp Sassi FP (1986) Classification andcorrelation of metamorphic facies series by means ofmuscovite b0 data from low grade metapelites NeuesJahrbuch fur Mineralogie Abhandlungen 153363 ndash380

Guidotti CV Yates MG Dyar MD amp Taylor ME(1994) Petrogenetic implications of the Fe3+ contentof muscovi te in pel it ic sch is ts AmericanMineralogist 79 793 ndash795

Guthrie GD amp Veblen DR (1989) High resolutionelectron microscopy of mixed-layer illitesmectite

Computer simulations Clays and Clay Minerals 371 ndash11

Ho N Peacor DR amp Van der Pluijm BA (1996)Contrasting roles of detrital and authigenic phyllo-silicates during slaty cleavage development Journalof Structural Geology 18 615 ndash623

Jiang WT Peacor DR Merriman RJ amp Roberts B(1990) Transmission and analytical electron micro-scopic study of mixed layer illitesmectite formed asan apparent replacement product of diagenetic illiteClays and Clay Minerals 38 449 ndash468

Jiang WT Peacor DR amp Buseck PR (1994) Chloritegeothermometry Contamination and apparent octa-hedral vacancies Clays and Clay Minerals 42593 ndash605

Kisch HJ (1991) Development of slaty cleavage anddegree of very-low-grade metamorphism a reviewJournal of Metamorphic Geology 9 735 ndash750

Knipe JR (1981) The interaction of deformation andmetamorphi sm in slates Tectonophysics 78249 ndash272

Kretz R (1983) Symbols for rock-forming mineralsAmerican Mineralogist 68 277 ndash279

Li G Peacor DR Merriman RJ amp Roberts B (1994a)The diagenetic to low grade metamorphism evolu-tion of matrix white mica in the system muscovite-paragonite in a mudrock from Central Wales UKClays and Clay Minerals 42 369 ndash381

Li G Peacor DR Merriman RJ Roberts B amp Vander Pluijm BA (1994b) TEM and AEM constraintson the origin and significance of chlorite-mica stacksin slates an example from Central Wales UKJournal of Structural Geology 16 1139 ndash1157

Livi KJT Veblen DR Ferry JM amp Frey M (1997)Evolution of 21 layered silicates in low-grademetamorphosed Liass ic sha les of CentralSwitzerland Journal of Metamorphic Geology 15323 ndash344

Lonergan L (1991) Structural evolution of the SierraEspuna Betic Cordillera SE Spain PhD thesisOxford University UK

Lonergan L Platt JP amp Gallagher L (1994) TheInternal-External Zone Boundary in the eastern BeticCordillera SE Spain Journal of Structural Geology16 175 ndash188

Makel GH (1981) Differences in tectonic evolution ofsuperimposed Malaguide and Alpujarride tectonicunits in the Espuna area (Betic Cordilleras Spain)Geology in Mijnbouw 60 203 ndash208

Makel GH (1985) The geology of the MalaguideComplex and its bearing on the geodynamicevolution of the Betic-Rif orogen (southern Spainand northern Morocco) GUA papers of Geology Ser1 22 263 pp

Makel GH amp Rondeel HE (1979) Differences instratigraphy and metamorphism between superposedMalaguide and Alpujarride units in the Espuna area

22 I Abad et al

under the electron beam (Figs 10 and 11) TheAEM analyses of such material show an enhancedSi content and the presence of small concentrationsof alkali cations These data are consistent withretrograde alteration of chlorite as observed byNieto et al (1994) They differ however from theresults of Nieto et al (1994) and those describedabove for sample Es-5 in that both tri-trioctahedraland di-trioctahedral chlorite are affected In TEMimages strain features in chlorites such as contrastnormal to the layers or microfolds were observed(Fig 10) Mica is less abundant than sudoiteoccurring in packets 100 ndash400 AEcirc thick The SAEDpatterns exhibit 2M polytypism but with diffuse-ness parallel to c in 0kl rows At low magnifica-tion subparallel and slightly curved illite- or illite-smectite-like packets are present they coalescehave abundant layer terminations and show intensecontrasts (Fig 12)

Sample Es-95 is a very fine-grained rock madeup of large detrital grains of quartz in a fine-grained matrix of phyllosilicates mainly whitemicas and opaque minerals Some biotite grainswere observed as verified by AEM data Biotitewas observed only in separated grains on holey

C-coated Cu grids so it is not possible to describethe textural relations for it Authigenic biotite isinconsistent with the low grade of this samplehowever and it is therefore inferred to be of detritalorigin The AEM analyses show deviations frompure biotite however as consistent with alterationto chlorite the usual process in detrital biotite inrocks undergoing diagenesis Lattice-fringe imagesof ion-milled samples show that individual micapackets are very thin with no more than 10 layersisolated and dispersed in a matrix consisting ofmaterial which gives no SAED pattern Thesepackets are wavy discontinuous and with anasto-mosing layers (Fig 13) Despite these texturalfeatures SAED data indicate that there are 2Mand 1Md polytypes

For all samples prismatic crystals of Ti-oxidedispersed Fe-oxyhydroxides zircon and tourmalineare common accessory minerals

Chemical characterization of phyllosilicates

Dioctahedral mica Table 3 shows the composi-tion of K-rich dioctahedral micas of authigenic ormetamorphic origin corresponding to the samples

FIG 13 General aspect of sample Es-95 The image shows arrays of wavy discontinuous and anastomosingpackets of illite and smectite (Morron de Totana Unit)

Prograde and retrograde metamorphism 13

selected from each unit The Yechar and LosMolinos units are characterized by compositionsapproaching those of evolved micas with a sum ofinterlayer cations slightly less than 1 apfu typicalof phengite or muscovite rather than authigenic

illite In contrast the La Santa and Morron deTotana units show heterogeneous mica composi-tions and display deficiencies in interlayer cations(Fig 14a) This last feature is more evident in theMorron de Totana analyses where the illitic

TABLE 3 Representative chemical compositions for K-rich dioctahedral micas normalized to O10(OH)2

Si IVAl VIAl Fe Mg Ti VIS K Na S inter

Los Molinos Unit (1)Es-10213 337 063 178 010 008 002 198 075 008 085Es-10221 308 092 195 005 001 001 202 082 009 091Es-10231 319 081 191 005 003 001 200 077 012 090Es-10232 314 086 188 006 007 001 202 072 021 093

Yechar Unit (1)Es-5311 332 068 163 015 022 002 204 090 004 093Es-5312 310 090 180 003 013 004 201 089 010 099Es-5322 338 062 170 012 015 003 200 086 001 086Es-5323 319 081 176 016 012 003 206 087 002 089

Yechar Unit (2)Es-53 1 317 083 163 021 026 000 210 084 000 084Es-53 2 320 080 165 011 027 003 207 087 000 087Es-53 3 317 083 155 025 024 002 206 093 000 093Es-53 4 328 072 160 020 024 000 203 092 000 092Es-53 5 326 074 164 019 016 000 199 096 000 096Es-53 6 326 074 158 020 028 000 206 089 000 089Es-53 7 319 081 154 025 026 000 205 097 000 097Es-53 8 325 075 166 016 025 000 206 085 000 085Es-53 9 324 076 167 019 019 000 205 085 000 085

La Santa Unit (2)Es-2 1 311 089 157 019 032 002 210 088 005 094Es-2 3 336 064 148 020 043 001 212 072 003 075Es-2 6 313 087 156 024 029 001 210 075 017 092Es-2 7 313 087 155 024 029 001 209 084 011 095Es-2 8 331 069 157 015 032 000 205 087 004 092Es-2 10 328 072 160 018 027 003 208 068 008 075Es-5 4 303 097 174 015 023 001 213 073 009 082Es-5 10 301 099 189 006 015 000 210 065 021 086Es-5 7 307 093 159 033 016 000 208 075 018 093Es-5 8rsquo 301 099 160 031 025 000 216 067 016 083

Morron de Totana Unit (2) (analyses corresponding to R4 Ill-Sm may also be present see text)Es-77 13 327 073 172 013 027 000 212 067 000 067Es-77 15 316 084 195 003 013 000 211 063 000 063Es-77 16 328 072 182 010 015 000 207 068 000 068Es-77 17 338 062 147 032 019 000 199 093 000 093Es-86 9 334 066 160 017 034 000 210 072 000 072Es-86 10 357 043 151 012 046 000 209 065 000 065Es-86 11 353 047 156 012 036 000 203 077 000 077Es-86 13 324 076 152 025 035 000 211 084 000 084Es-86 14 324 076 173 013 019 000 205 084 000 084Es- 95 2 323 077 158 016 019 004 198 103 000 103Es- 95 5 336 064 169 013 025 000 207 070 000 070Es- 95 6 336 064 148 027 033 000 209 078 000 078

(1) SEMEDX analyses (2) TEMAEM analyses

14 I Abad et al

character is consistent with the presence of the R4mixed-layer illite-smectite described above Most ofthe micas are phengitic in composition (Fig 14ab)with an Fe(Fe+Mg) ratio that varies over the range02 ndash085 However the detrital-like grains aresimilar to those of end-member muscovite (seebelow detrital muscovite vs metamorphic musco-vite or Table 5) with much lower Fe and Mgcontents Data plotted in Fig 14bc confirm theassumption of a significant ferrimuscovitic substitu-tion (Guidotti et al 1994) Some of the K-micaanalysed in the Los Molinos and La Santa unitswere found to have significant Na contents (up to02 apfu) Although Ti is unusual in dioctahedralmicas it has been measured in several analyses

(lt005 apfu) The most evident feature in theplots is the scattering of data in the La Santa andMorron de Totana units ie where diageneticconditions are not exceeded In contrast theYechar Unit analyses define a narrow field in thediagrams and the Los Molinos data are even closerto the theoretical muscovite composition (Table 3)

Typical compositions of micas intermediate tomuscovite and paragonite called Na-K micas in thispaper are shown in Table 4 The compositions arequite variable even in the same sample Whencompared with analyses of K-rich dioctahedralmicas (Table 3) and paragonite analyses are seento span the entire range from muscovite toparagonite Some analyses (eg Es-5 9) fall within

FIG 14 Plots of values of chemical compositions of K-rich dioctahedral phyllosilicates solid circles and linesindicate values for theoretical muscovite and corresponding exchange vectors respectively

Prograde and retrograde metamorphism 15

the limits of the solvus between muscovite andparagonite the limbs of that solvus having theapproximate compositions ms60pg40 and ms20pg80

(Li et al 1994a)Detrital muscovite vs metamorphic muscovite

Detrital-like micas of sample Es-95 the lowest-grade one are large enough to be analysed(Table 5) by electron microprobe (EMPA) Thesedata have been compared with the AEM data ofother samples Compositions of detrital-like grainsof the lowest-grade sample and the detrital micas ofthe anchizonal-grade sample Es-53 (Fig 15) arevery similar and close to that of end-membermuscovite These data suggest that the detrital micahas a common source for the different units thatcomprise Sierra Espuna Figure 15 also comparesthe data from authigenic micas of the samesamples which are clearly more phengitic (Fe+Mg= 015 ndash075 apfu) than the detrital ones (Fe+Mg= 01 ndash02 apfu)

Chlorite Trioctahedral and di-trioctahedral(sudoite) chlorites have been analysed (Table 6)The former corresponding to the species clino-chlore is common in most of the samples unlikesudoite which is absent from most (Table 1)Sudoite was identified because the sum ofoctahedral cations is nearly equal to 5 apfu andthe Si and Al contents are higher than intrioctahedral chlorite (Fig 16a) A slight contam-ination by interlayer cations (K and Na) has beendetected therefore the formulae were recalculatedin order to eliminate those impurities (seeAnalytical Methods) Although total Fe has beenconsidered as Fe2+ according to Fransolet ampBourguignon (1978) the Fe3+ content in sudoitemay be greater than the Fe2+ content As severalanalyses have an octahedral sum slightly higherthan the ideal value of 5 apfu this may indicate atrioctahedral substitution andor the presence ofsignificant Fe3+ No Ti or Mn were detected The

Fe(Fe+Mg) ratios do not define a clear trendthroughout the sequence (Fig 16b) nevertheless itis much greater in chlorite in the Morron de Totanaanalyses falling in the characteristic range ofdiagenetic chlorites (Li et al 1994b) There is anincrease in Mg content (from 15 to 4 apfu)concomitant with increase in grade as compatiblewith some other sequences (eg Bevins et al1991)

D I S C U S S I O N

Dioctahedral phyllosilicates chemicalcompositions IC values and polytypes

On the basis of their large size homogeneity ofindividual grains and well-crystallized aspectdetrital dioctahedral mica grains were identified inseveral samples with the exception of those ofgreenschist facies This confirms the idea of the

TABLE 4 Chemical compositions for Na-K micas normalized to O10(OH)2

Si IVAl VIAl Fe Mg Ti VIS Ca K Na S inter

Es-5 14 313 087 190 004 009 002 204 000 034 048 083Es-5 15 321 079 176 009 037 000 222 000 015 038 053Es-5 4 318 082 187 004 019 000 209 000 013 061 074Es-5 12 329 071 172 008 029 000 208 000 039 036 076Es-5 13 323 077 179 005 028 000 212 000 029 041 069Es-5 9 296 104 163 019 029 000 211 000 048 056 104Es-5 8 292 108 156 043 008 001 209 003 014 080 096

FIG 15 Plot of Si vs Al contents of detrital andauthigenic micas of samples Es-95 and Es-53 Solidsymbols = detrital micas open symbols = authigenic

micas

16 I Abad et al

relative stability of muscovite in most diageneticenvironments (Crowley 1991) The compositions ofthe detrital grains of samples from different unitsare similar The lack of heterogeneity which isexpected for mica from a complex source implies acommon source area for all samples

The IC values define a trend from diagenetic toanchizonal conditions but with smaller IC values inbulk-rock fractions consistent with the contributionof detrital mica The coexistence of mature micawith lower-grade I-S or illite in the sub-greenschistfacies rocks shows that those rocks have not

TABLE 5 EMPA data for detrital-like crystals of mica normalized to O10(OH)2

Si IVAl VIAl Fe Mg Ti VIS Ca K Na K+Na+Ca

Es-95z11 313 087 193 005 004 001 203 000 065 022 087Es-95z11 314 086 190 006 007 001 204 000 068 019 087Es-95z1 2 314 086 191 005 009 001 206 000 070 006 077Es-95z2 1 314 086 176 012 010 003 201 002 095 004 101Es-95z2 2 317 083 177 012 011 003 202 000 095 004 099Es-95z4 1 305 095 188 005 007 004 204 000 081 009 089Es-95z4 2 311 089 187 006 007 004 204 000 078 009 086

TABLE 6 Representative chemical compositions for chlorite normalized to O10(OH)8

Si IVAl VIAl Fe Mg VIS FeFe+Mg

Yechar UnitEs53-1 273 127 112 121 376 609 024Es53-4 287 113 125 070 400 596 015Es53-12 303 097 130 146 307 583 032Es53-13 277 123 125 122 354 601 026Es53-15 307 093 146 094 323 564 023Es53-42 271 129 152 160 294 606 035

La Santa UnitEs2-4 319 081 147 148 273 569 035Es2-5 365 035 183 118 225 526 034Es5-1 354 046 293 023 172 488 012Es5-5 361 039 292 021 178 490 010Es5-10 309 091 262 028 221 511 011Es5-16 310 090 197 112 234 543 032Es5-17 350 050 252 076 170 499 031Es-5 10rsquo 364 036 213 097 202 512 032Es-5 11 291 109 143 136 314 593 030Es-5 12 382 018 229 087 179 495 033Es-5 13 316 084 302 021 172 494 011Es-5 16rsquo 346 054 341 013 101 455 011Es-5 17rsquo 300 100 282 033 194 509 014Es-5 18 285 115 285 026 204 515 011

Morron de Totana UnitEs-86 2 377 023 253 088 149 490 037Es-86 4 284 116 166 230 181 578 056Es-86 5 350 050 191 192 151 534 056Es-86 6 366 034 189 191 149 529 056Es-86 7 315 085 129 273 176 578 061

indicates sudoite analyses

Prograde and retrograde metamorphism 17

reached stable chemical or textural equilibrium atleast at the mm scale In addition the lack ofchemical equilibrium among diagenetic I-S andillite at the nm scale is demonstrated by their highlyvariable Fe+Mg contents (006ndash075 apfu) Theaverages of the sum of interlayer cations are closeto 1 apfu in the Los Molinos and Yechar units asconsistent with mica but values of 06 apfucorresponding to I-S or illitic compositions are alsopresent in the Morron de Totana Unit Except forsamples Es-102 and Es-53 the variable composi-tions represent a metastable state with composi-tions of micas determined by a combination ofphengitic ferrimuscovitic and illitic vectors asshown by Fig 14ab

Paragonite is generally considered an indexmineral for the anchizone (Frey 1987) but Li etal (1994a) have shown that paragonite coexistswith K-rich mica in grades as low as that of the latediagenetic zone for a prograde sequence Samplesin which muscovite paragonite and intermediateNa-K mica coexist as in sample Es-5 areconsistent with metastability both because morethan two such micas coexist and because theircompositions are incompatible with the solvus inthe muscovite-paragonite system (see below) Whilethe 2M polytype typical of metamorphic micas is

the most common in these samples the 1Md

polytype also occurs in some diagenetic samplesparticularly those from the Morron de Totana UnitThe 1Md polytype is metastable relative to the 2Mpolytype and the coexistence of both polytypesfurther demonstrates a lack of equilibrium at the nmscale Dong amp Peacor (1996) showed that thedisorder noted in the diagenetic phyllosilicates bySAED is a consequence of the general high entropyof phases formed at very low temperatures thatproduces for example stacking defects separatingpackets with no more than 10 layers These packetsprogressively thicken with increasing metamorphicgrade and the 2M polytype then becomes the mostcommon polytype in authigenic micas Thenm-scale size of the lower-grade clays and theregular increase in size with increasing gradereflect the lack of textural equilibrium in the low-grade samples

A wide range of sample characteristics thusimply that the lower-grade samples are in statesof metastable equilibrium those factors includingcoexistence of mature mica with I-S and illitevariable chemical compositions incompatiblecompositions of white micas small crystal sizesdefect-rich states and polytypism Such dataemphasize the notion of lsquoreaction progressrsquo relative

FIG 16 (a) Plot of Al vs Fe+Mg contents of chlorites illustrating the contrast in composition between tri-trioctahedral clinochlore or chamosite and di-trioctahedral sudoite (b) plot of Si vs Fe(Fe+Mg) ratio of

chlorites showing an absence of compositional trends throughout the sampled sequence

18 I Abad et al

to Ostwald-step rule processes metastable dioctahe-dral clay systems undergoing reactions toward thestate of stable chemical and textural equilibriumapproached in greenschist-facies samples (Essene ampPeacor 1995)

Prograde evolution based on TEM datacorrelation with the Basque-Cantabriansequence (Northern Spain)

With increasing grade TEM data confirm thatcrystal thickness increases crystal defect densitydecreases compositions become less variable andmicrotextural relations are simpler (eg Peacor1992) This trend is consistent with three well-characterized low-grade settings the Gulf Coastsequence (Ahn amp Peacor 1986) and the WelshBasin (Merriman et al 1990) both extensionalbasins and the Helvetic Alps (Livi et al 1997)The latter is a fold-and-thrust belt with similarseries of prograde transitions to those observed inSierra Espuna in which pelites have passed througha sequence of metastable states in the lithologicalsequence mudstone and shale slate and phyllite

The mudstones of the Morron de Totana Unit arecharacterized by the presence of illite-smectite R4mixed-layers mica packets of no more than 10 ndash40layers and the occurrence of 1Md polytypism TheLa Santa Unit samples represent a step in theprograde evolution with thicker packets than in theprevious samples (200 ndash700 AEcirc ) illitic or phengiticcompositions the absence of illite-smectite mixedlayers as well as the presence of discreteintermediate Na-K micas and paragonite Althoughthere is a tendency towards non-preferred orienta-tion of crystals it is not so evident as in the Morronde Totana samples In the Yechar Unit (anchizone)and more clearly in the Los Molinos phyllites(greenschist facies) the microtextural relations areless complex the compositions are less variablewith phengite and trioctahedral chlorite crystalthickness are greater (gt400 AEcirc in the Yechar Unitand gt1000 AEcirc in the Los Molinos Unit) with thepackets being defect-free and with a very crystallineaspect This description has features in commonwith the three well-characterized low-grade settingspreviously noted Only the mineral assemblagesvary somewhat presumably as a function of thedifferences in bulk-rock composition

A detailed comparison with the Basque-Cantabrian Basin (N Spain) shows that the lowest-grade conditions of Sierra Espuna overlap most of

those of that sequence which is characterized bypassive burial diagenesis (8000 m thick) (Nieto etal 1996) Only in the deepest sample of theBasque-Cantabrian sequence is illite-smectite mixedlayering absent Figure 12 of this study is verysimilar to Fig 7 in Nieto et al (1996) and bothimages correspond to diagenetic samples fromdifferent settings nevertheless they illustratesimilar mica packets 100 ndash200 AEcirc thick withparallel-to-subparallel boundaries the orientationof which is inferred to represent beddingTherefore there is a parallelism between thesesequences down to the scale of individual clay-mineral grains even though they developed underdifferent conditions with stress presumably playinga significant role in the Sierra Espuna units Thetexture of the least metamorphosed sample (Fig 13)is reminiscent of the typical textural relations ofsmectite described by Nieto et al (1996) (seeFig 4) Although it corresponds to illite-richinterstratified illite-smectite the clay mineralgrains conserved the grain size and anastomosingdiscontinuous array of original smectite layers

Despite the similarities between the twosequences the chemical behaviour of the micasare clearly different since the grade correspondingto the phengitic compositions of the Yechar Unit isnot reached in the Basque-Cantabrian Basin wherethe deepest and highest-grade sample has illiticcompositions equivalent to those of the La SantaUnit That is in a passive burial metamorphiccontext as in the Basque-Cantabrian Basin thefactors responsible for increasing the rate ofchemical reactions are less significant than in thesequences that were affected by tectonic stressTectonic stress introduces defects which in turnincreases the rate of dissolution of strained grainsthus increasing the rate of dissolution-crystallizationreactions typical of clay minerals Perhaps moreimportantly regional tectonic stress is known tocause mobilization of fluids (eg Oliver 1986) Thepresence and activity of such fluids is a prime causeof increased rates of dissolution ion diffusion andcrystallization Tectonic activity thus may have atwo-fold effect in increasing rate of clay-mineralreactions The phengite component may in part be ameasure of the reaction progress depending on thereaction kinetics in very low-grade rocksAccording to Merriman amp Peacor (1999) whereheat flow is low as in accretionary settingsphengite contents are typically high perhapsbecause there is insufficient thermal energy to

Prograde and retrograde metamorphism 19

cause equilibration of the phengite component ofneoformed illite by contrast with extensional basinswhere this component is typically small

Coexistence of trioctahedral chlorite andsudoite Implications for multiple metamorphicevents

In metamorphic rocks the coexistence of mineralsthat formed at different times and conditions on aPTt path ie not in equilibrium is a commonlyobserved feature where the metamorphic history iscomplex Gross differences in the grain size ofcoexisting phases has long been recognized bymetamorphic petrologists as an indicator of non-equilibrium mineral assemblages Historically muchless attention has been paid to such relations in sub-greenschist-facies rocks because prior to the intro-duction of SEM+TEM techniques their small grainsize prevented observations of textural relationsStudy of such rocks eg by XRD provides littleinformation on textural relations beyond determina-tion of degree of preferred orientation

Study by XRD of the samples showed thatsudoite and trioctahedral chlorite coexist If theyhad formed at the same time and under the sameP-T conditions their compositions would define thelimbs of a solvus The TEM images show howeverthat wherever they are in contact the sudoitetransects the packets of trioctahedral chlorite (egFig 8) Furthermore as illustrated in Fig 8 thetrioctahedral chlorite is commonly deformed anddisplays strain contrast in TEM images Thesetextural relations suggest that sudoite was intro-duced subsequent to the formation of trioctahedralchlorite Similar observations were made byGiorgetti et al (1998) who concluded that texturesin Verrucano metasediments showed that sudoitepost-dated chloritoid pyrophyllite and trioctahedralchlorite ie sudoite is in disequilibrium with thelatter minerals and formed as a retrograde phase Inrocks of the Sierra Espuna area differences in bulk-rock composition are small (Table 2) and do notcorrespond to the presence or absence of sudoite(Table 1) ie the random occurrence of sudoiteindependent of bulk-rock composition is compa-tible with a lack of equilibrium relations Theserelations collectively imply that sudoite wasintroduced at some time subsequent to theformation of mineral assemblages at peak meta-morphic conditions On the other hand the size ofsudoite packets is similar to that of trioctahedral

chlorite as consistent with formation at conditionsnot much different from those for chlorite

In samples from the Morron de Totana bothsudoite and trioctahedral chlorite were affected byretrograde alteration to smectite The TEM imagesshow sudoite (Fig 11) and trioctahedral chlorite(Fig 10) with interstratified layers of smectite withspacings of 10 AEcirc due to collapse and dehydration inthe TEM environment and even packets of smectite150 AEcirc thick within chlorite crystals These relationsimply that both sudoite and trioctahedral chloritehave been affected by retrograde alteration at thevery low temperatures consistent with smectiteformation although the presence of more subtlealteration features in sudoite of some samplessuggests that it was less susceptible to alteration

We therefore conclude that at least three episodesof phyllosilicate formation can be recognized inrocks from the Sierra Espuna-area Trioctahedralchlorite was presumably produced at the highestmetamorphicdiagenetic grade in equilibrium withparagonite and phengite Subsequently sudoite wasintroduced but at P-T conditions which could nothave been much different than those of peakmetamorphic grade Finally at very low-T condi-tions hydrothermal fluids caused partial alterationof trioctahedral chlorite and sudoite to smectite

Retrograde products formed by the reaction offluids with mineral phases generated during anearlier prograde event were described previouslyby Nieto et al (1994) and Zhao et al (1999) Thosereactions occurred on a regional scale but similarreactions have been observed on a more localizedscale The fluid activity associated with a majorfault zone is a common source for these reactions(eg Jiang et al 1990) The structure of SierraEspuna (Fig 1) is based on imbrication processesthat superimpose units that have subsequently beenaffected by faults and folds and that have under-gone significant uplift at least 800 m from the lateMiocene to the present (Sanz de Galdeano et al2001) Given this evolution it is easy to justify theretrograde processes that usually seem to be relatedwith tectonic events during which fluid flow andfluidrock ratios are enhanced (Merriman amp Peacor1999) Moreover according to Nieto et al (1994)the uplift of the Betic Cordillera would furnish theintroduction of phreatic water into the systemproducing a reactive environment adequate for thegenesis of smectite Geological conditions aretherefore consistent with post-peak metamorphicconditions as implied by the TEM observations

20 I Abad et al

The introduction of sudoite as a major phaseduring metamorphism implies the possibility ofadditional overprints on a simple prograde assem-blage the evidence for which might be subtle TheNa-rich micas at all grades have compositionswhich either fall within the solvus or are too Na-rich to correspond to the limb of the solvus In thehighest-grade rocks such micas coexist withmuscovite Those relations are incompatible withchemical equilibrium despite the fact that relationsfor muscovite and other minerals imply an approachto chemical equilibrium Furthermore there is aclose association of Na-rich micas with sudoite (egFig 7) Those relations imply the possibility thatNa-micas were introduced concomitant withsudoite P Mata (pers comm) has shown forexample that overprinting of an epizonal assem-blage by hydrothermal solutions resulted in theintroduction of both an Al-rich variety of chlorite(cookeite) and paragonite in pelites of the CamerosBasin Spain The textural relations such as cross-cutting textures which are so striking for sudoitewere not observed for Na-rich micas Neverthelessthe possibility of such overprinting must beconsidered

In spite of all these prograde-retrogradeprocesses in the Los Molinos Unit a smallnumber of homogeneous phases coexist in chemicalequilibrium and the fine-grained metamorphicphyllosilicates are clearly parallel to cleavage witha typical metamorphic aspect Finally all thetextural and compositional changes that haveoperated in the Espuna area during the metamorphicevolution proceed toward a simpler system repre-senting both textural and chemical equilibriumwhich is characteristic of greenschist-facies condi-tions

Roles of detrital and authigenic phyllosilicatesduring slaty cleavage development

Van der Pluijm et al (1998) focused on therespective roles of the phyllosilicates of detritalorigin and those formed during diagenesis andmetamorphism and observed that detrital phyllosi-licates remain preferentially oriented parallel tobedding in pelites with slaty cleavage whereas fine-grained metamorphic phyllosilicates are orientedparallel to cleavage

Although it is usually difficult to determine thepreferred orientations of phyllosilicates in TEMimages as the grade increases better-aligned

packets occur in the cleavage orientation Thetexture of the least metamorphosed sample inwhich even incipient slaty cleavage is absent isreminiscent of the typical textural relation ofsmectites described in the literature The illite-smectite mixed-layers conserve the textural featuresof possible previous smectite such as anastomosingfringes curved and discontinuous with variations ininterplanar spacing d001 These features indicatethat the minimum energy conditions to initiatemechanical transformations have not been reachedin the lower-grade samples as postulated by Ho etal (1996) since slaty cleavage develops as a resultof interactive mechanical deformation and ther-mally driven dissolution and crystallizationprocesses (Knipe 1981) In contrast in the LosMolinos Unit large crystals of chlorite and whitemica (gt1000 AEcirc ) occur which are of metamorphicorigin but with no detrital grains (Fig 3a)Phyllosilicates are preferentially oriented parallelto slaty cleavage in response to increasedtemperature and tectonic strain (Merriman ampPeacor 1999) The oriented microfabrics and slatycleavage well developed in the rocks of this unitmust be a product of a higher thermal and strainenergy environment reached in the deepest part ofthe sequence As pointed out by Van der Pluijm etal (1998) a continuous range of orientations isevidence for mechanical rotation of grains whereasthe lack of such transitional orientations impliesthat cleavage-parallel phyllosilicates originatedthrough dissolution of bedding-parallel and crystal-lization in the slaty cleavage orientation Asobserved in this study for the Intermediate Unitsgrains with orientations which are transitional frombedding-parallel to slaty cleavage orientation areonly rarely observed Cleavage formation is there-fore inferred to have occurred entirely or nearlyentirely through dissolution and crystallization

ACKNOWLEDGMENTS

We thank MM Abad Ortega from the Centro deInstrumentacio n Cient otilde fica of the Universidad deGranada for her help with HRTEM and I Nieto forher essential help with sample preparation Financialsupport was supplied by Research Project nordm BT 2000-0582 FPI research grant to IA both of the SpanishMinistry of Science and Technology Research GroupRNM-0179 of the Junta de Andaluc otildeacutea and to DRP byNSF grant EAR-9814391

Prograde and retrograde metamorphism 21

REFERENCES

Ahn JH amp Peacor DR (1986) Transmission andanalytical electron microscopy of the smectite-to-illite transition Clays and Clay Minerals 34165 ndash179

Azanon JM amp Crespo-Blanc A (2000) Exhumationduring a continental collision inferred from thetectonometamorphic evolution of the AlpujarrideComplex in the central Betics (Alboran Domain SESpain) Tectonics 19 549 ndash565

Bevins RE Robinson D amp Rowbotham G (1991)Compositional variations in mafic phyllosilicatesfrom regional low-grade metabasites and applicationof the chlorite geother mometer Journal ofMetamorphic Geology 9 711 ndash721

Cliff G amp Lorimer GW (1975) The quantitativeanalysis of thin specimens Journal of Microscopy 103 203 ndash207

Crowley SF (1991) Diagenetic modification of detritalmuscovite an example from the Great LimestoneCyclothem (Carboniferous) of Co Durham UKClay Minerals 26 91 ndash103

Dong H amp Peacor DR (1996) TEM observations ofcoherent stacking relations in smectite and illite ofshales evidence for MacEwan crystallites anddominance of 2M1 polytypes Clays and ClayMinerals 44 257 ndash275

Essene E amp Peacor DR (1995) Clay mineral thermo-metry a critical perspective Clays and ClayMinerals 43 540 ndash553

Fransolet AM amp Bourguignon P (1978) Dioctahedralchlorite in quartz veins from Ardennes BelgiumThe Canadian Mineralogist 16 365 ndash373

Fransolet AM amp Schreyer W (1984) Sudoite ditrioctahedral chlorite a stable low-temperature phasein the system MgO-Al2O3-SiO2-H2O Contributionsto Mineralogy and Petrology 86 409 ndash417

Frey M (1987) Very low-grade metamorphism of clasticsedimentary rocks Pp 9 ndash58 in Low TemperatureMetamorphism (M Frey editor) Blackie GlasgowUK

Giorgetti G Goffe B Memmi I amp Nieto F (1998)Metamorphic evolution of Verrucano metasedimentsin northern Apennines new petrological constraintsEuropean Journal of Mineralogy 10 1295 ndash1308

Guidotti CV amp Sassi FP (1986) Classification andcorrelation of metamorphic facies series by means ofmuscovite b0 data from low grade metapelites NeuesJahrbuch fur Mineralogie Abhandlungen 153363 ndash380

Guidotti CV Yates MG Dyar MD amp Taylor ME(1994) Petrogenetic implications of the Fe3+ contentof muscovi te in pel it ic sch is ts AmericanMineralogist 79 793 ndash795

Guthrie GD amp Veblen DR (1989) High resolutionelectron microscopy of mixed-layer illitesmectite

Computer simulations Clays and Clay Minerals 371 ndash11

Ho N Peacor DR amp Van der Pluijm BA (1996)Contrasting roles of detrital and authigenic phyllo-silicates during slaty cleavage development Journalof Structural Geology 18 615 ndash623

Jiang WT Peacor DR Merriman RJ amp Roberts B(1990) Transmission and analytical electron micro-scopic study of mixed layer illitesmectite formed asan apparent replacement product of diagenetic illiteClays and Clay Minerals 38 449 ndash468

Jiang WT Peacor DR amp Buseck PR (1994) Chloritegeothermometry Contamination and apparent octa-hedral vacancies Clays and Clay Minerals 42593 ndash605

Kisch HJ (1991) Development of slaty cleavage anddegree of very-low-grade metamorphism a reviewJournal of Metamorphic Geology 9 735 ndash750

Knipe JR (1981) The interaction of deformation andmetamorphi sm in slates Tectonophysics 78249 ndash272

Kretz R (1983) Symbols for rock-forming mineralsAmerican Mineralogist 68 277 ndash279

Li G Peacor DR Merriman RJ amp Roberts B (1994a)The diagenetic to low grade metamorphism evolu-tion of matrix white mica in the system muscovite-paragonite in a mudrock from Central Wales UKClays and Clay Minerals 42 369 ndash381

Li G Peacor DR Merriman RJ Roberts B amp Vander Pluijm BA (1994b) TEM and AEM constraintson the origin and significance of chlorite-mica stacksin slates an example from Central Wales UKJournal of Structural Geology 16 1139 ndash1157

Livi KJT Veblen DR Ferry JM amp Frey M (1997)Evolution of 21 layered silicates in low-grademetamorphosed Liass ic sha les of CentralSwitzerland Journal of Metamorphic Geology 15323 ndash344

Lonergan L (1991) Structural evolution of the SierraEspuna Betic Cordillera SE Spain PhD thesisOxford University UK

Lonergan L Platt JP amp Gallagher L (1994) TheInternal-External Zone Boundary in the eastern BeticCordillera SE Spain Journal of Structural Geology16 175 ndash188

Makel GH (1981) Differences in tectonic evolution ofsuperimposed Malaguide and Alpujarride tectonicunits in the Espuna area (Betic Cordilleras Spain)Geology in Mijnbouw 60 203 ndash208

Makel GH (1985) The geology of the MalaguideComplex and its bearing on the geodynamicevolution of the Betic-Rif orogen (southern Spainand northern Morocco) GUA papers of Geology Ser1 22 263 pp

Makel GH amp Rondeel HE (1979) Differences instratigraphy and metamorphism between superposedMalaguide and Alpujarride units in the Espuna area

22 I Abad et al

selected from each unit The Yechar and LosMolinos units are characterized by compositionsapproaching those of evolved micas with a sum ofinterlayer cations slightly less than 1 apfu typicalof phengite or muscovite rather than authigenic

illite In contrast the La Santa and Morron deTotana units show heterogeneous mica composi-tions and display deficiencies in interlayer cations(Fig 14a) This last feature is more evident in theMorron de Totana analyses where the illitic

TABLE 3 Representative chemical compositions for K-rich dioctahedral micas normalized to O10(OH)2

Si IVAl VIAl Fe Mg Ti VIS K Na S inter

Los Molinos Unit (1)Es-10213 337 063 178 010 008 002 198 075 008 085Es-10221 308 092 195 005 001 001 202 082 009 091Es-10231 319 081 191 005 003 001 200 077 012 090Es-10232 314 086 188 006 007 001 202 072 021 093

Yechar Unit (1)Es-5311 332 068 163 015 022 002 204 090 004 093Es-5312 310 090 180 003 013 004 201 089 010 099Es-5322 338 062 170 012 015 003 200 086 001 086Es-5323 319 081 176 016 012 003 206 087 002 089

Yechar Unit (2)Es-53 1 317 083 163 021 026 000 210 084 000 084Es-53 2 320 080 165 011 027 003 207 087 000 087Es-53 3 317 083 155 025 024 002 206 093 000 093Es-53 4 328 072 160 020 024 000 203 092 000 092Es-53 5 326 074 164 019 016 000 199 096 000 096Es-53 6 326 074 158 020 028 000 206 089 000 089Es-53 7 319 081 154 025 026 000 205 097 000 097Es-53 8 325 075 166 016 025 000 206 085 000 085Es-53 9 324 076 167 019 019 000 205 085 000 085

La Santa Unit (2)Es-2 1 311 089 157 019 032 002 210 088 005 094Es-2 3 336 064 148 020 043 001 212 072 003 075Es-2 6 313 087 156 024 029 001 210 075 017 092Es-2 7 313 087 155 024 029 001 209 084 011 095Es-2 8 331 069 157 015 032 000 205 087 004 092Es-2 10 328 072 160 018 027 003 208 068 008 075Es-5 4 303 097 174 015 023 001 213 073 009 082Es-5 10 301 099 189 006 015 000 210 065 021 086Es-5 7 307 093 159 033 016 000 208 075 018 093Es-5 8rsquo 301 099 160 031 025 000 216 067 016 083

Morron de Totana Unit (2) (analyses corresponding to R4 Ill-Sm may also be present see text)Es-77 13 327 073 172 013 027 000 212 067 000 067Es-77 15 316 084 195 003 013 000 211 063 000 063Es-77 16 328 072 182 010 015 000 207 068 000 068Es-77 17 338 062 147 032 019 000 199 093 000 093Es-86 9 334 066 160 017 034 000 210 072 000 072Es-86 10 357 043 151 012 046 000 209 065 000 065Es-86 11 353 047 156 012 036 000 203 077 000 077Es-86 13 324 076 152 025 035 000 211 084 000 084Es-86 14 324 076 173 013 019 000 205 084 000 084Es- 95 2 323 077 158 016 019 004 198 103 000 103Es- 95 5 336 064 169 013 025 000 207 070 000 070Es- 95 6 336 064 148 027 033 000 209 078 000 078

(1) SEMEDX analyses (2) TEMAEM analyses

14 I Abad et al

character is consistent with the presence of the R4mixed-layer illite-smectite described above Most ofthe micas are phengitic in composition (Fig 14ab)with an Fe(Fe+Mg) ratio that varies over the range02 ndash085 However the detrital-like grains aresimilar to those of end-member muscovite (seebelow detrital muscovite vs metamorphic musco-vite or Table 5) with much lower Fe and Mgcontents Data plotted in Fig 14bc confirm theassumption of a significant ferrimuscovitic substitu-tion (Guidotti et al 1994) Some of the K-micaanalysed in the Los Molinos and La Santa unitswere found to have significant Na contents (up to02 apfu) Although Ti is unusual in dioctahedralmicas it has been measured in several analyses

(lt005 apfu) The most evident feature in theplots is the scattering of data in the La Santa andMorron de Totana units ie where diageneticconditions are not exceeded In contrast theYechar Unit analyses define a narrow field in thediagrams and the Los Molinos data are even closerto the theoretical muscovite composition (Table 3)

Typical compositions of micas intermediate tomuscovite and paragonite called Na-K micas in thispaper are shown in Table 4 The compositions arequite variable even in the same sample Whencompared with analyses of K-rich dioctahedralmicas (Table 3) and paragonite analyses are seento span the entire range from muscovite toparagonite Some analyses (eg Es-5 9) fall within

FIG 14 Plots of values of chemical compositions of K-rich dioctahedral phyllosilicates solid circles and linesindicate values for theoretical muscovite and corresponding exchange vectors respectively

Prograde and retrograde metamorphism 15

the limits of the solvus between muscovite andparagonite the limbs of that solvus having theapproximate compositions ms60pg40 and ms20pg80

(Li et al 1994a)Detrital muscovite vs metamorphic muscovite

Detrital-like micas of sample Es-95 the lowest-grade one are large enough to be analysed(Table 5) by electron microprobe (EMPA) Thesedata have been compared with the AEM data ofother samples Compositions of detrital-like grainsof the lowest-grade sample and the detrital micas ofthe anchizonal-grade sample Es-53 (Fig 15) arevery similar and close to that of end-membermuscovite These data suggest that the detrital micahas a common source for the different units thatcomprise Sierra Espuna Figure 15 also comparesthe data from authigenic micas of the samesamples which are clearly more phengitic (Fe+Mg= 015 ndash075 apfu) than the detrital ones (Fe+Mg= 01 ndash02 apfu)

Chlorite Trioctahedral and di-trioctahedral(sudoite) chlorites have been analysed (Table 6)The former corresponding to the species clino-chlore is common in most of the samples unlikesudoite which is absent from most (Table 1)Sudoite was identified because the sum ofoctahedral cations is nearly equal to 5 apfu andthe Si and Al contents are higher than intrioctahedral chlorite (Fig 16a) A slight contam-ination by interlayer cations (K and Na) has beendetected therefore the formulae were recalculatedin order to eliminate those impurities (seeAnalytical Methods) Although total Fe has beenconsidered as Fe2+ according to Fransolet ampBourguignon (1978) the Fe3+ content in sudoitemay be greater than the Fe2+ content As severalanalyses have an octahedral sum slightly higherthan the ideal value of 5 apfu this may indicate atrioctahedral substitution andor the presence ofsignificant Fe3+ No Ti or Mn were detected The

Fe(Fe+Mg) ratios do not define a clear trendthroughout the sequence (Fig 16b) nevertheless itis much greater in chlorite in the Morron de Totanaanalyses falling in the characteristic range ofdiagenetic chlorites (Li et al 1994b) There is anincrease in Mg content (from 15 to 4 apfu)concomitant with increase in grade as compatiblewith some other sequences (eg Bevins et al1991)

D I S C U S S I O N

Dioctahedral phyllosilicates chemicalcompositions IC values and polytypes

On the basis of their large size homogeneity ofindividual grains and well-crystallized aspectdetrital dioctahedral mica grains were identified inseveral samples with the exception of those ofgreenschist facies This confirms the idea of the

TABLE 4 Chemical compositions for Na-K micas normalized to O10(OH)2

Si IVAl VIAl Fe Mg Ti VIS Ca K Na S inter

Es-5 14 313 087 190 004 009 002 204 000 034 048 083Es-5 15 321 079 176 009 037 000 222 000 015 038 053Es-5 4 318 082 187 004 019 000 209 000 013 061 074Es-5 12 329 071 172 008 029 000 208 000 039 036 076Es-5 13 323 077 179 005 028 000 212 000 029 041 069Es-5 9 296 104 163 019 029 000 211 000 048 056 104Es-5 8 292 108 156 043 008 001 209 003 014 080 096

FIG 15 Plot of Si vs Al contents of detrital andauthigenic micas of samples Es-95 and Es-53 Solidsymbols = detrital micas open symbols = authigenic

micas

16 I Abad et al

relative stability of muscovite in most diageneticenvironments (Crowley 1991) The compositions ofthe detrital grains of samples from different unitsare similar The lack of heterogeneity which isexpected for mica from a complex source implies acommon source area for all samples

The IC values define a trend from diagenetic toanchizonal conditions but with smaller IC values inbulk-rock fractions consistent with the contributionof detrital mica The coexistence of mature micawith lower-grade I-S or illite in the sub-greenschistfacies rocks shows that those rocks have not

TABLE 5 EMPA data for detrital-like crystals of mica normalized to O10(OH)2

Si IVAl VIAl Fe Mg Ti VIS Ca K Na K+Na+Ca

Es-95z11 313 087 193 005 004 001 203 000 065 022 087Es-95z11 314 086 190 006 007 001 204 000 068 019 087Es-95z1 2 314 086 191 005 009 001 206 000 070 006 077Es-95z2 1 314 086 176 012 010 003 201 002 095 004 101Es-95z2 2 317 083 177 012 011 003 202 000 095 004 099Es-95z4 1 305 095 188 005 007 004 204 000 081 009 089Es-95z4 2 311 089 187 006 007 004 204 000 078 009 086

TABLE 6 Representative chemical compositions for chlorite normalized to O10(OH)8

Si IVAl VIAl Fe Mg VIS FeFe+Mg

Yechar UnitEs53-1 273 127 112 121 376 609 024Es53-4 287 113 125 070 400 596 015Es53-12 303 097 130 146 307 583 032Es53-13 277 123 125 122 354 601 026Es53-15 307 093 146 094 323 564 023Es53-42 271 129 152 160 294 606 035

La Santa UnitEs2-4 319 081 147 148 273 569 035Es2-5 365 035 183 118 225 526 034Es5-1 354 046 293 023 172 488 012Es5-5 361 039 292 021 178 490 010Es5-10 309 091 262 028 221 511 011Es5-16 310 090 197 112 234 543 032Es5-17 350 050 252 076 170 499 031Es-5 10rsquo 364 036 213 097 202 512 032Es-5 11 291 109 143 136 314 593 030Es-5 12 382 018 229 087 179 495 033Es-5 13 316 084 302 021 172 494 011Es-5 16rsquo 346 054 341 013 101 455 011Es-5 17rsquo 300 100 282 033 194 509 014Es-5 18 285 115 285 026 204 515 011

Morron de Totana UnitEs-86 2 377 023 253 088 149 490 037Es-86 4 284 116 166 230 181 578 056Es-86 5 350 050 191 192 151 534 056Es-86 6 366 034 189 191 149 529 056Es-86 7 315 085 129 273 176 578 061

indicates sudoite analyses

Prograde and retrograde metamorphism 17

reached stable chemical or textural equilibrium atleast at the mm scale In addition the lack ofchemical equilibrium among diagenetic I-S andillite at the nm scale is demonstrated by their highlyvariable Fe+Mg contents (006ndash075 apfu) Theaverages of the sum of interlayer cations are closeto 1 apfu in the Los Molinos and Yechar units asconsistent with mica but values of 06 apfucorresponding to I-S or illitic compositions are alsopresent in the Morron de Totana Unit Except forsamples Es-102 and Es-53 the variable composi-tions represent a metastable state with composi-tions of micas determined by a combination ofphengitic ferrimuscovitic and illitic vectors asshown by Fig 14ab

Paragonite is generally considered an indexmineral for the anchizone (Frey 1987) but Li etal (1994a) have shown that paragonite coexistswith K-rich mica in grades as low as that of the latediagenetic zone for a prograde sequence Samplesin which muscovite paragonite and intermediateNa-K mica coexist as in sample Es-5 areconsistent with metastability both because morethan two such micas coexist and because theircompositions are incompatible with the solvus inthe muscovite-paragonite system (see below) Whilethe 2M polytype typical of metamorphic micas is

the most common in these samples the 1Md

polytype also occurs in some diagenetic samplesparticularly those from the Morron de Totana UnitThe 1Md polytype is metastable relative to the 2Mpolytype and the coexistence of both polytypesfurther demonstrates a lack of equilibrium at the nmscale Dong amp Peacor (1996) showed that thedisorder noted in the diagenetic phyllosilicates bySAED is a consequence of the general high entropyof phases formed at very low temperatures thatproduces for example stacking defects separatingpackets with no more than 10 layers These packetsprogressively thicken with increasing metamorphicgrade and the 2M polytype then becomes the mostcommon polytype in authigenic micas Thenm-scale size of the lower-grade clays and theregular increase in size with increasing gradereflect the lack of textural equilibrium in the low-grade samples

A wide range of sample characteristics thusimply that the lower-grade samples are in statesof metastable equilibrium those factors includingcoexistence of mature mica with I-S and illitevariable chemical compositions incompatiblecompositions of white micas small crystal sizesdefect-rich states and polytypism Such dataemphasize the notion of lsquoreaction progressrsquo relative

FIG 16 (a) Plot of Al vs Fe+Mg contents of chlorites illustrating the contrast in composition between tri-trioctahedral clinochlore or chamosite and di-trioctahedral sudoite (b) plot of Si vs Fe(Fe+Mg) ratio of

chlorites showing an absence of compositional trends throughout the sampled sequence

18 I Abad et al

to Ostwald-step rule processes metastable dioctahe-dral clay systems undergoing reactions toward thestate of stable chemical and textural equilibriumapproached in greenschist-facies samples (Essene ampPeacor 1995)

Prograde evolution based on TEM datacorrelation with the Basque-Cantabriansequence (Northern Spain)

With increasing grade TEM data confirm thatcrystal thickness increases crystal defect densitydecreases compositions become less variable andmicrotextural relations are simpler (eg Peacor1992) This trend is consistent with three well-characterized low-grade settings the Gulf Coastsequence (Ahn amp Peacor 1986) and the WelshBasin (Merriman et al 1990) both extensionalbasins and the Helvetic Alps (Livi et al 1997)The latter is a fold-and-thrust belt with similarseries of prograde transitions to those observed inSierra Espuna in which pelites have passed througha sequence of metastable states in the lithologicalsequence mudstone and shale slate and phyllite

The mudstones of the Morron de Totana Unit arecharacterized by the presence of illite-smectite R4mixed-layers mica packets of no more than 10 ndash40layers and the occurrence of 1Md polytypism TheLa Santa Unit samples represent a step in theprograde evolution with thicker packets than in theprevious samples (200 ndash700 AEcirc ) illitic or phengiticcompositions the absence of illite-smectite mixedlayers as well as the presence of discreteintermediate Na-K micas and paragonite Althoughthere is a tendency towards non-preferred orienta-tion of crystals it is not so evident as in the Morronde Totana samples In the Yechar Unit (anchizone)and more clearly in the Los Molinos phyllites(greenschist facies) the microtextural relations areless complex the compositions are less variablewith phengite and trioctahedral chlorite crystalthickness are greater (gt400 AEcirc in the Yechar Unitand gt1000 AEcirc in the Los Molinos Unit) with thepackets being defect-free and with a very crystallineaspect This description has features in commonwith the three well-characterized low-grade settingspreviously noted Only the mineral assemblagesvary somewhat presumably as a function of thedifferences in bulk-rock composition

A detailed comparison with the Basque-Cantabrian Basin (N Spain) shows that the lowest-grade conditions of Sierra Espuna overlap most of

those of that sequence which is characterized bypassive burial diagenesis (8000 m thick) (Nieto etal 1996) Only in the deepest sample of theBasque-Cantabrian sequence is illite-smectite mixedlayering absent Figure 12 of this study is verysimilar to Fig 7 in Nieto et al (1996) and bothimages correspond to diagenetic samples fromdifferent settings nevertheless they illustratesimilar mica packets 100 ndash200 AEcirc thick withparallel-to-subparallel boundaries the orientationof which is inferred to represent beddingTherefore there is a parallelism between thesesequences down to the scale of individual clay-mineral grains even though they developed underdifferent conditions with stress presumably playinga significant role in the Sierra Espuna units Thetexture of the least metamorphosed sample (Fig 13)is reminiscent of the typical textural relations ofsmectite described by Nieto et al (1996) (seeFig 4) Although it corresponds to illite-richinterstratified illite-smectite the clay mineralgrains conserved the grain size and anastomosingdiscontinuous array of original smectite layers

Despite the similarities between the twosequences the chemical behaviour of the micasare clearly different since the grade correspondingto the phengitic compositions of the Yechar Unit isnot reached in the Basque-Cantabrian Basin wherethe deepest and highest-grade sample has illiticcompositions equivalent to those of the La SantaUnit That is in a passive burial metamorphiccontext as in the Basque-Cantabrian Basin thefactors responsible for increasing the rate ofchemical reactions are less significant than in thesequences that were affected by tectonic stressTectonic stress introduces defects which in turnincreases the rate of dissolution of strained grainsthus increasing the rate of dissolution-crystallizationreactions typical of clay minerals Perhaps moreimportantly regional tectonic stress is known tocause mobilization of fluids (eg Oliver 1986) Thepresence and activity of such fluids is a prime causeof increased rates of dissolution ion diffusion andcrystallization Tectonic activity thus may have atwo-fold effect in increasing rate of clay-mineralreactions The phengite component may in part be ameasure of the reaction progress depending on thereaction kinetics in very low-grade rocksAccording to Merriman amp Peacor (1999) whereheat flow is low as in accretionary settingsphengite contents are typically high perhapsbecause there is insufficient thermal energy to

Prograde and retrograde metamorphism 19

cause equilibration of the phengite component ofneoformed illite by contrast with extensional basinswhere this component is typically small

Coexistence of trioctahedral chlorite andsudoite Implications for multiple metamorphicevents

In metamorphic rocks the coexistence of mineralsthat formed at different times and conditions on aPTt path ie not in equilibrium is a commonlyobserved feature where the metamorphic history iscomplex Gross differences in the grain size ofcoexisting phases has long been recognized bymetamorphic petrologists as an indicator of non-equilibrium mineral assemblages Historically muchless attention has been paid to such relations in sub-greenschist-facies rocks because prior to the intro-duction of SEM+TEM techniques their small grainsize prevented observations of textural relationsStudy of such rocks eg by XRD provides littleinformation on textural relations beyond determina-tion of degree of preferred orientation

Study by XRD of the samples showed thatsudoite and trioctahedral chlorite coexist If theyhad formed at the same time and under the sameP-T conditions their compositions would define thelimbs of a solvus The TEM images show howeverthat wherever they are in contact the sudoitetransects the packets of trioctahedral chlorite (egFig 8) Furthermore as illustrated in Fig 8 thetrioctahedral chlorite is commonly deformed anddisplays strain contrast in TEM images Thesetextural relations suggest that sudoite was intro-duced subsequent to the formation of trioctahedralchlorite Similar observations were made byGiorgetti et al (1998) who concluded that texturesin Verrucano metasediments showed that sudoitepost-dated chloritoid pyrophyllite and trioctahedralchlorite ie sudoite is in disequilibrium with thelatter minerals and formed as a retrograde phase Inrocks of the Sierra Espuna area differences in bulk-rock composition are small (Table 2) and do notcorrespond to the presence or absence of sudoite(Table 1) ie the random occurrence of sudoiteindependent of bulk-rock composition is compa-tible with a lack of equilibrium relations Theserelations collectively imply that sudoite wasintroduced at some time subsequent to theformation of mineral assemblages at peak meta-morphic conditions On the other hand the size ofsudoite packets is similar to that of trioctahedral

chlorite as consistent with formation at conditionsnot much different from those for chlorite

In samples from the Morron de Totana bothsudoite and trioctahedral chlorite were affected byretrograde alteration to smectite The TEM imagesshow sudoite (Fig 11) and trioctahedral chlorite(Fig 10) with interstratified layers of smectite withspacings of 10 AEcirc due to collapse and dehydration inthe TEM environment and even packets of smectite150 AEcirc thick within chlorite crystals These relationsimply that both sudoite and trioctahedral chloritehave been affected by retrograde alteration at thevery low temperatures consistent with smectiteformation although the presence of more subtlealteration features in sudoite of some samplessuggests that it was less susceptible to alteration

We therefore conclude that at least three episodesof phyllosilicate formation can be recognized inrocks from the Sierra Espuna-area Trioctahedralchlorite was presumably produced at the highestmetamorphicdiagenetic grade in equilibrium withparagonite and phengite Subsequently sudoite wasintroduced but at P-T conditions which could nothave been much different than those of peakmetamorphic grade Finally at very low-T condi-tions hydrothermal fluids caused partial alterationof trioctahedral chlorite and sudoite to smectite

Retrograde products formed by the reaction offluids with mineral phases generated during anearlier prograde event were described previouslyby Nieto et al (1994) and Zhao et al (1999) Thosereactions occurred on a regional scale but similarreactions have been observed on a more localizedscale The fluid activity associated with a majorfault zone is a common source for these reactions(eg Jiang et al 1990) The structure of SierraEspuna (Fig 1) is based on imbrication processesthat superimpose units that have subsequently beenaffected by faults and folds and that have under-gone significant uplift at least 800 m from the lateMiocene to the present (Sanz de Galdeano et al2001) Given this evolution it is easy to justify theretrograde processes that usually seem to be relatedwith tectonic events during which fluid flow andfluidrock ratios are enhanced (Merriman amp Peacor1999) Moreover according to Nieto et al (1994)the uplift of the Betic Cordillera would furnish theintroduction of phreatic water into the systemproducing a reactive environment adequate for thegenesis of smectite Geological conditions aretherefore consistent with post-peak metamorphicconditions as implied by the TEM observations

20 I Abad et al

The introduction of sudoite as a major phaseduring metamorphism implies the possibility ofadditional overprints on a simple prograde assem-blage the evidence for which might be subtle TheNa-rich micas at all grades have compositionswhich either fall within the solvus or are too Na-rich to correspond to the limb of the solvus In thehighest-grade rocks such micas coexist withmuscovite Those relations are incompatible withchemical equilibrium despite the fact that relationsfor muscovite and other minerals imply an approachto chemical equilibrium Furthermore there is aclose association of Na-rich micas with sudoite (egFig 7) Those relations imply the possibility thatNa-micas were introduced concomitant withsudoite P Mata (pers comm) has shown forexample that overprinting of an epizonal assem-blage by hydrothermal solutions resulted in theintroduction of both an Al-rich variety of chlorite(cookeite) and paragonite in pelites of the CamerosBasin Spain The textural relations such as cross-cutting textures which are so striking for sudoitewere not observed for Na-rich micas Neverthelessthe possibility of such overprinting must beconsidered

In spite of all these prograde-retrogradeprocesses in the Los Molinos Unit a smallnumber of homogeneous phases coexist in chemicalequilibrium and the fine-grained metamorphicphyllosilicates are clearly parallel to cleavage witha typical metamorphic aspect Finally all thetextural and compositional changes that haveoperated in the Espuna area during the metamorphicevolution proceed toward a simpler system repre-senting both textural and chemical equilibriumwhich is characteristic of greenschist-facies condi-tions

Roles of detrital and authigenic phyllosilicatesduring slaty cleavage development

Van der Pluijm et al (1998) focused on therespective roles of the phyllosilicates of detritalorigin and those formed during diagenesis andmetamorphism and observed that detrital phyllosi-licates remain preferentially oriented parallel tobedding in pelites with slaty cleavage whereas fine-grained metamorphic phyllosilicates are orientedparallel to cleavage

Although it is usually difficult to determine thepreferred orientations of phyllosilicates in TEMimages as the grade increases better-aligned

packets occur in the cleavage orientation Thetexture of the least metamorphosed sample inwhich even incipient slaty cleavage is absent isreminiscent of the typical textural relation ofsmectites described in the literature The illite-smectite mixed-layers conserve the textural featuresof possible previous smectite such as anastomosingfringes curved and discontinuous with variations ininterplanar spacing d001 These features indicatethat the minimum energy conditions to initiatemechanical transformations have not been reachedin the lower-grade samples as postulated by Ho etal (1996) since slaty cleavage develops as a resultof interactive mechanical deformation and ther-mally driven dissolution and crystallizationprocesses (Knipe 1981) In contrast in the LosMolinos Unit large crystals of chlorite and whitemica (gt1000 AEcirc ) occur which are of metamorphicorigin but with no detrital grains (Fig 3a)Phyllosilicates are preferentially oriented parallelto slaty cleavage in response to increasedtemperature and tectonic strain (Merriman ampPeacor 1999) The oriented microfabrics and slatycleavage well developed in the rocks of this unitmust be a product of a higher thermal and strainenergy environment reached in the deepest part ofthe sequence As pointed out by Van der Pluijm etal (1998) a continuous range of orientations isevidence for mechanical rotation of grains whereasthe lack of such transitional orientations impliesthat cleavage-parallel phyllosilicates originatedthrough dissolution of bedding-parallel and crystal-lization in the slaty cleavage orientation Asobserved in this study for the Intermediate Unitsgrains with orientations which are transitional frombedding-parallel to slaty cleavage orientation areonly rarely observed Cleavage formation is there-fore inferred to have occurred entirely or nearlyentirely through dissolution and crystallization

ACKNOWLEDGMENTS

We thank MM Abad Ortega from the Centro deInstrumentacio n Cient otilde fica of the Universidad deGranada for her help with HRTEM and I Nieto forher essential help with sample preparation Financialsupport was supplied by Research Project nordm BT 2000-0582 FPI research grant to IA both of the SpanishMinistry of Science and Technology Research GroupRNM-0179 of the Junta de Andaluc otildeacutea and to DRP byNSF grant EAR-9814391

Prograde and retrograde metamorphism 21

REFERENCES

Ahn JH amp Peacor DR (1986) Transmission andanalytical electron microscopy of the smectite-to-illite transition Clays and Clay Minerals 34165 ndash179

Azanon JM amp Crespo-Blanc A (2000) Exhumationduring a continental collision inferred from thetectonometamorphic evolution of the AlpujarrideComplex in the central Betics (Alboran Domain SESpain) Tectonics 19 549 ndash565

Bevins RE Robinson D amp Rowbotham G (1991)Compositional variations in mafic phyllosilicatesfrom regional low-grade metabasites and applicationof the chlorite geother mometer Journal ofMetamorphic Geology 9 711 ndash721

Cliff G amp Lorimer GW (1975) The quantitativeanalysis of thin specimens Journal of Microscopy 103 203 ndash207

Crowley SF (1991) Diagenetic modification of detritalmuscovite an example from the Great LimestoneCyclothem (Carboniferous) of Co Durham UKClay Minerals 26 91 ndash103

Dong H amp Peacor DR (1996) TEM observations ofcoherent stacking relations in smectite and illite ofshales evidence for MacEwan crystallites anddominance of 2M1 polytypes Clays and ClayMinerals 44 257 ndash275

Essene E amp Peacor DR (1995) Clay mineral thermo-metry a critical perspective Clays and ClayMinerals 43 540 ndash553

Fransolet AM amp Bourguignon P (1978) Dioctahedralchlorite in quartz veins from Ardennes BelgiumThe Canadian Mineralogist 16 365 ndash373

Fransolet AM amp Schreyer W (1984) Sudoite ditrioctahedral chlorite a stable low-temperature phasein the system MgO-Al2O3-SiO2-H2O Contributionsto Mineralogy and Petrology 86 409 ndash417

Frey M (1987) Very low-grade metamorphism of clasticsedimentary rocks Pp 9 ndash58 in Low TemperatureMetamorphism (M Frey editor) Blackie GlasgowUK

Giorgetti G Goffe B Memmi I amp Nieto F (1998)Metamorphic evolution of Verrucano metasedimentsin northern Apennines new petrological constraintsEuropean Journal of Mineralogy 10 1295 ndash1308

Guidotti CV amp Sassi FP (1986) Classification andcorrelation of metamorphic facies series by means ofmuscovite b0 data from low grade metapelites NeuesJahrbuch fur Mineralogie Abhandlungen 153363 ndash380

Guidotti CV Yates MG Dyar MD amp Taylor ME(1994) Petrogenetic implications of the Fe3+ contentof muscovi te in pel it ic sch is ts AmericanMineralogist 79 793 ndash795

Guthrie GD amp Veblen DR (1989) High resolutionelectron microscopy of mixed-layer illitesmectite

Computer simulations Clays and Clay Minerals 371 ndash11

Ho N Peacor DR amp Van der Pluijm BA (1996)Contrasting roles of detrital and authigenic phyllo-silicates during slaty cleavage development Journalof Structural Geology 18 615 ndash623

Jiang WT Peacor DR Merriman RJ amp Roberts B(1990) Transmission and analytical electron micro-scopic study of mixed layer illitesmectite formed asan apparent replacement product of diagenetic illiteClays and Clay Minerals 38 449 ndash468

Jiang WT Peacor DR amp Buseck PR (1994) Chloritegeothermometry Contamination and apparent octa-hedral vacancies Clays and Clay Minerals 42593 ndash605

Kisch HJ (1991) Development of slaty cleavage anddegree of very-low-grade metamorphism a reviewJournal of Metamorphic Geology 9 735 ndash750

Knipe JR (1981) The interaction of deformation andmetamorphi sm in slates Tectonophysics 78249 ndash272

Kretz R (1983) Symbols for rock-forming mineralsAmerican Mineralogist 68 277 ndash279

Li G Peacor DR Merriman RJ amp Roberts B (1994a)The diagenetic to low grade metamorphism evolu-tion of matrix white mica in the system muscovite-paragonite in a mudrock from Central Wales UKClays and Clay Minerals 42 369 ndash381

Li G Peacor DR Merriman RJ Roberts B amp Vander Pluijm BA (1994b) TEM and AEM constraintson the origin and significance of chlorite-mica stacksin slates an example from Central Wales UKJournal of Structural Geology 16 1139 ndash1157

Livi KJT Veblen DR Ferry JM amp Frey M (1997)Evolution of 21 layered silicates in low-grademetamorphosed Liass ic sha les of CentralSwitzerland Journal of Metamorphic Geology 15323 ndash344

Lonergan L (1991) Structural evolution of the SierraEspuna Betic Cordillera SE Spain PhD thesisOxford University UK

Lonergan L Platt JP amp Gallagher L (1994) TheInternal-External Zone Boundary in the eastern BeticCordillera SE Spain Journal of Structural Geology16 175 ndash188

Makel GH (1981) Differences in tectonic evolution ofsuperimposed Malaguide and Alpujarride tectonicunits in the Espuna area (Betic Cordilleras Spain)Geology in Mijnbouw 60 203 ndash208

Makel GH (1985) The geology of the MalaguideComplex and its bearing on the geodynamicevolution of the Betic-Rif orogen (southern Spainand northern Morocco) GUA papers of Geology Ser1 22 263 pp

Makel GH amp Rondeel HE (1979) Differences instratigraphy and metamorphism between superposedMalaguide and Alpujarride units in the Espuna area

22 I Abad et al

character is consistent with the presence of the R4mixed-layer illite-smectite described above Most ofthe micas are phengitic in composition (Fig 14ab)with an Fe(Fe+Mg) ratio that varies over the range02 ndash085 However the detrital-like grains aresimilar to those of end-member muscovite (seebelow detrital muscovite vs metamorphic musco-vite or Table 5) with much lower Fe and Mgcontents Data plotted in Fig 14bc confirm theassumption of a significant ferrimuscovitic substitu-tion (Guidotti et al 1994) Some of the K-micaanalysed in the Los Molinos and La Santa unitswere found to have significant Na contents (up to02 apfu) Although Ti is unusual in dioctahedralmicas it has been measured in several analyses

(lt005 apfu) The most evident feature in theplots is the scattering of data in the La Santa andMorron de Totana units ie where diageneticconditions are not exceeded In contrast theYechar Unit analyses define a narrow field in thediagrams and the Los Molinos data are even closerto the theoretical muscovite composition (Table 3)

Typical compositions of micas intermediate tomuscovite and paragonite called Na-K micas in thispaper are shown in Table 4 The compositions arequite variable even in the same sample Whencompared with analyses of K-rich dioctahedralmicas (Table 3) and paragonite analyses are seento span the entire range from muscovite toparagonite Some analyses (eg Es-5 9) fall within

FIG 14 Plots of values of chemical compositions of K-rich dioctahedral phyllosilicates solid circles and linesindicate values for theoretical muscovite and corresponding exchange vectors respectively

Prograde and retrograde metamorphism 15

the limits of the solvus between muscovite andparagonite the limbs of that solvus having theapproximate compositions ms60pg40 and ms20pg80

(Li et al 1994a)Detrital muscovite vs metamorphic muscovite

Detrital-like micas of sample Es-95 the lowest-grade one are large enough to be analysed(Table 5) by electron microprobe (EMPA) Thesedata have been compared with the AEM data ofother samples Compositions of detrital-like grainsof the lowest-grade sample and the detrital micas ofthe anchizonal-grade sample Es-53 (Fig 15) arevery similar and close to that of end-membermuscovite These data suggest that the detrital micahas a common source for the different units thatcomprise Sierra Espuna Figure 15 also comparesthe data from authigenic micas of the samesamples which are clearly more phengitic (Fe+Mg= 015 ndash075 apfu) than the detrital ones (Fe+Mg= 01 ndash02 apfu)

Chlorite Trioctahedral and di-trioctahedral(sudoite) chlorites have been analysed (Table 6)The former corresponding to the species clino-chlore is common in most of the samples unlikesudoite which is absent from most (Table 1)Sudoite was identified because the sum ofoctahedral cations is nearly equal to 5 apfu andthe Si and Al contents are higher than intrioctahedral chlorite (Fig 16a) A slight contam-ination by interlayer cations (K and Na) has beendetected therefore the formulae were recalculatedin order to eliminate those impurities (seeAnalytical Methods) Although total Fe has beenconsidered as Fe2+ according to Fransolet ampBourguignon (1978) the Fe3+ content in sudoitemay be greater than the Fe2+ content As severalanalyses have an octahedral sum slightly higherthan the ideal value of 5 apfu this may indicate atrioctahedral substitution andor the presence ofsignificant Fe3+ No Ti or Mn were detected The

Fe(Fe+Mg) ratios do not define a clear trendthroughout the sequence (Fig 16b) nevertheless itis much greater in chlorite in the Morron de Totanaanalyses falling in the characteristic range ofdiagenetic chlorites (Li et al 1994b) There is anincrease in Mg content (from 15 to 4 apfu)concomitant with increase in grade as compatiblewith some other sequences (eg Bevins et al1991)

D I S C U S S I O N

Dioctahedral phyllosilicates chemicalcompositions IC values and polytypes

On the basis of their large size homogeneity ofindividual grains and well-crystallized aspectdetrital dioctahedral mica grains were identified inseveral samples with the exception of those ofgreenschist facies This confirms the idea of the

TABLE 4 Chemical compositions for Na-K micas normalized to O10(OH)2

Si IVAl VIAl Fe Mg Ti VIS Ca K Na S inter

Es-5 14 313 087 190 004 009 002 204 000 034 048 083Es-5 15 321 079 176 009 037 000 222 000 015 038 053Es-5 4 318 082 187 004 019 000 209 000 013 061 074Es-5 12 329 071 172 008 029 000 208 000 039 036 076Es-5 13 323 077 179 005 028 000 212 000 029 041 069Es-5 9 296 104 163 019 029 000 211 000 048 056 104Es-5 8 292 108 156 043 008 001 209 003 014 080 096

FIG 15 Plot of Si vs Al contents of detrital andauthigenic micas of samples Es-95 and Es-53 Solidsymbols = detrital micas open symbols = authigenic

micas

16 I Abad et al

relative stability of muscovite in most diageneticenvironments (Crowley 1991) The compositions ofthe detrital grains of samples from different unitsare similar The lack of heterogeneity which isexpected for mica from a complex source implies acommon source area for all samples

The IC values define a trend from diagenetic toanchizonal conditions but with smaller IC values inbulk-rock fractions consistent with the contributionof detrital mica The coexistence of mature micawith lower-grade I-S or illite in the sub-greenschistfacies rocks shows that those rocks have not

TABLE 5 EMPA data for detrital-like crystals of mica normalized to O10(OH)2

Si IVAl VIAl Fe Mg Ti VIS Ca K Na K+Na+Ca

Es-95z11 313 087 193 005 004 001 203 000 065 022 087Es-95z11 314 086 190 006 007 001 204 000 068 019 087Es-95z1 2 314 086 191 005 009 001 206 000 070 006 077Es-95z2 1 314 086 176 012 010 003 201 002 095 004 101Es-95z2 2 317 083 177 012 011 003 202 000 095 004 099Es-95z4 1 305 095 188 005 007 004 204 000 081 009 089Es-95z4 2 311 089 187 006 007 004 204 000 078 009 086

TABLE 6 Representative chemical compositions for chlorite normalized to O10(OH)8

Si IVAl VIAl Fe Mg VIS FeFe+Mg

Yechar UnitEs53-1 273 127 112 121 376 609 024Es53-4 287 113 125 070 400 596 015Es53-12 303 097 130 146 307 583 032Es53-13 277 123 125 122 354 601 026Es53-15 307 093 146 094 323 564 023Es53-42 271 129 152 160 294 606 035

La Santa UnitEs2-4 319 081 147 148 273 569 035Es2-5 365 035 183 118 225 526 034Es5-1 354 046 293 023 172 488 012Es5-5 361 039 292 021 178 490 010Es5-10 309 091 262 028 221 511 011Es5-16 310 090 197 112 234 543 032Es5-17 350 050 252 076 170 499 031Es-5 10rsquo 364 036 213 097 202 512 032Es-5 11 291 109 143 136 314 593 030Es-5 12 382 018 229 087 179 495 033Es-5 13 316 084 302 021 172 494 011Es-5 16rsquo 346 054 341 013 101 455 011Es-5 17rsquo 300 100 282 033 194 509 014Es-5 18 285 115 285 026 204 515 011

Morron de Totana UnitEs-86 2 377 023 253 088 149 490 037Es-86 4 284 116 166 230 181 578 056Es-86 5 350 050 191 192 151 534 056Es-86 6 366 034 189 191 149 529 056Es-86 7 315 085 129 273 176 578 061

indicates sudoite analyses

Prograde and retrograde metamorphism 17

reached stable chemical or textural equilibrium atleast at the mm scale In addition the lack ofchemical equilibrium among diagenetic I-S andillite at the nm scale is demonstrated by their highlyvariable Fe+Mg contents (006ndash075 apfu) Theaverages of the sum of interlayer cations are closeto 1 apfu in the Los Molinos and Yechar units asconsistent with mica but values of 06 apfucorresponding to I-S or illitic compositions are alsopresent in the Morron de Totana Unit Except forsamples Es-102 and Es-53 the variable composi-tions represent a metastable state with composi-tions of micas determined by a combination ofphengitic ferrimuscovitic and illitic vectors asshown by Fig 14ab

Paragonite is generally considered an indexmineral for the anchizone (Frey 1987) but Li etal (1994a) have shown that paragonite coexistswith K-rich mica in grades as low as that of the latediagenetic zone for a prograde sequence Samplesin which muscovite paragonite and intermediateNa-K mica coexist as in sample Es-5 areconsistent with metastability both because morethan two such micas coexist and because theircompositions are incompatible with the solvus inthe muscovite-paragonite system (see below) Whilethe 2M polytype typical of metamorphic micas is

the most common in these samples the 1Md

polytype also occurs in some diagenetic samplesparticularly those from the Morron de Totana UnitThe 1Md polytype is metastable relative to the 2Mpolytype and the coexistence of both polytypesfurther demonstrates a lack of equilibrium at the nmscale Dong amp Peacor (1996) showed that thedisorder noted in the diagenetic phyllosilicates bySAED is a consequence of the general high entropyof phases formed at very low temperatures thatproduces for example stacking defects separatingpackets with no more than 10 layers These packetsprogressively thicken with increasing metamorphicgrade and the 2M polytype then becomes the mostcommon polytype in authigenic micas Thenm-scale size of the lower-grade clays and theregular increase in size with increasing gradereflect the lack of textural equilibrium in the low-grade samples

A wide range of sample characteristics thusimply that the lower-grade samples are in statesof metastable equilibrium those factors includingcoexistence of mature mica with I-S and illitevariable chemical compositions incompatiblecompositions of white micas small crystal sizesdefect-rich states and polytypism Such dataemphasize the notion of lsquoreaction progressrsquo relative

FIG 16 (a) Plot of Al vs Fe+Mg contents of chlorites illustrating the contrast in composition between tri-trioctahedral clinochlore or chamosite and di-trioctahedral sudoite (b) plot of Si vs Fe(Fe+Mg) ratio of

chlorites showing an absence of compositional trends throughout the sampled sequence

18 I Abad et al

to Ostwald-step rule processes metastable dioctahe-dral clay systems undergoing reactions toward thestate of stable chemical and textural equilibriumapproached in greenschist-facies samples (Essene ampPeacor 1995)

Prograde evolution based on TEM datacorrelation with the Basque-Cantabriansequence (Northern Spain)

With increasing grade TEM data confirm thatcrystal thickness increases crystal defect densitydecreases compositions become less variable andmicrotextural relations are simpler (eg Peacor1992) This trend is consistent with three well-characterized low-grade settings the Gulf Coastsequence (Ahn amp Peacor 1986) and the WelshBasin (Merriman et al 1990) both extensionalbasins and the Helvetic Alps (Livi et al 1997)The latter is a fold-and-thrust belt with similarseries of prograde transitions to those observed inSierra Espuna in which pelites have passed througha sequence of metastable states in the lithologicalsequence mudstone and shale slate and phyllite

The mudstones of the Morron de Totana Unit arecharacterized by the presence of illite-smectite R4mixed-layers mica packets of no more than 10 ndash40layers and the occurrence of 1Md polytypism TheLa Santa Unit samples represent a step in theprograde evolution with thicker packets than in theprevious samples (200 ndash700 AEcirc ) illitic or phengiticcompositions the absence of illite-smectite mixedlayers as well as the presence of discreteintermediate Na-K micas and paragonite Althoughthere is a tendency towards non-preferred orienta-tion of crystals it is not so evident as in the Morronde Totana samples In the Yechar Unit (anchizone)and more clearly in the Los Molinos phyllites(greenschist facies) the microtextural relations areless complex the compositions are less variablewith phengite and trioctahedral chlorite crystalthickness are greater (gt400 AEcirc in the Yechar Unitand gt1000 AEcirc in the Los Molinos Unit) with thepackets being defect-free and with a very crystallineaspect This description has features in commonwith the three well-characterized low-grade settingspreviously noted Only the mineral assemblagesvary somewhat presumably as a function of thedifferences in bulk-rock composition

A detailed comparison with the Basque-Cantabrian Basin (N Spain) shows that the lowest-grade conditions of Sierra Espuna overlap most of

those of that sequence which is characterized bypassive burial diagenesis (8000 m thick) (Nieto etal 1996) Only in the deepest sample of theBasque-Cantabrian sequence is illite-smectite mixedlayering absent Figure 12 of this study is verysimilar to Fig 7 in Nieto et al (1996) and bothimages correspond to diagenetic samples fromdifferent settings nevertheless they illustratesimilar mica packets 100 ndash200 AEcirc thick withparallel-to-subparallel boundaries the orientationof which is inferred to represent beddingTherefore there is a parallelism between thesesequences down to the scale of individual clay-mineral grains even though they developed underdifferent conditions with stress presumably playinga significant role in the Sierra Espuna units Thetexture of the least metamorphosed sample (Fig 13)is reminiscent of the typical textural relations ofsmectite described by Nieto et al (1996) (seeFig 4) Although it corresponds to illite-richinterstratified illite-smectite the clay mineralgrains conserved the grain size and anastomosingdiscontinuous array of original smectite layers

Despite the similarities between the twosequences the chemical behaviour of the micasare clearly different since the grade correspondingto the phengitic compositions of the Yechar Unit isnot reached in the Basque-Cantabrian Basin wherethe deepest and highest-grade sample has illiticcompositions equivalent to those of the La SantaUnit That is in a passive burial metamorphiccontext as in the Basque-Cantabrian Basin thefactors responsible for increasing the rate ofchemical reactions are less significant than in thesequences that were affected by tectonic stressTectonic stress introduces defects which in turnincreases the rate of dissolution of strained grainsthus increasing the rate of dissolution-crystallizationreactions typical of clay minerals Perhaps moreimportantly regional tectonic stress is known tocause mobilization of fluids (eg Oliver 1986) Thepresence and activity of such fluids is a prime causeof increased rates of dissolution ion diffusion andcrystallization Tectonic activity thus may have atwo-fold effect in increasing rate of clay-mineralreactions The phengite component may in part be ameasure of the reaction progress depending on thereaction kinetics in very low-grade rocksAccording to Merriman amp Peacor (1999) whereheat flow is low as in accretionary settingsphengite contents are typically high perhapsbecause there is insufficient thermal energy to

Prograde and retrograde metamorphism 19

cause equilibration of the phengite component ofneoformed illite by contrast with extensional basinswhere this component is typically small

Coexistence of trioctahedral chlorite andsudoite Implications for multiple metamorphicevents

In metamorphic rocks the coexistence of mineralsthat formed at different times and conditions on aPTt path ie not in equilibrium is a commonlyobserved feature where the metamorphic history iscomplex Gross differences in the grain size ofcoexisting phases has long been recognized bymetamorphic petrologists as an indicator of non-equilibrium mineral assemblages Historically muchless attention has been paid to such relations in sub-greenschist-facies rocks because prior to the intro-duction of SEM+TEM techniques their small grainsize prevented observations of textural relationsStudy of such rocks eg by XRD provides littleinformation on textural relations beyond determina-tion of degree of preferred orientation

Study by XRD of the samples showed thatsudoite and trioctahedral chlorite coexist If theyhad formed at the same time and under the sameP-T conditions their compositions would define thelimbs of a solvus The TEM images show howeverthat wherever they are in contact the sudoitetransects the packets of trioctahedral chlorite (egFig 8) Furthermore as illustrated in Fig 8 thetrioctahedral chlorite is commonly deformed anddisplays strain contrast in TEM images Thesetextural relations suggest that sudoite was intro-duced subsequent to the formation of trioctahedralchlorite Similar observations were made byGiorgetti et al (1998) who concluded that texturesin Verrucano metasediments showed that sudoitepost-dated chloritoid pyrophyllite and trioctahedralchlorite ie sudoite is in disequilibrium with thelatter minerals and formed as a retrograde phase Inrocks of the Sierra Espuna area differences in bulk-rock composition are small (Table 2) and do notcorrespond to the presence or absence of sudoite(Table 1) ie the random occurrence of sudoiteindependent of bulk-rock composition is compa-tible with a lack of equilibrium relations Theserelations collectively imply that sudoite wasintroduced at some time subsequent to theformation of mineral assemblages at peak meta-morphic conditions On the other hand the size ofsudoite packets is similar to that of trioctahedral

chlorite as consistent with formation at conditionsnot much different from those for chlorite

In samples from the Morron de Totana bothsudoite and trioctahedral chlorite were affected byretrograde alteration to smectite The TEM imagesshow sudoite (Fig 11) and trioctahedral chlorite(Fig 10) with interstratified layers of smectite withspacings of 10 AEcirc due to collapse and dehydration inthe TEM environment and even packets of smectite150 AEcirc thick within chlorite crystals These relationsimply that both sudoite and trioctahedral chloritehave been affected by retrograde alteration at thevery low temperatures consistent with smectiteformation although the presence of more subtlealteration features in sudoite of some samplessuggests that it was less susceptible to alteration

We therefore conclude that at least three episodesof phyllosilicate formation can be recognized inrocks from the Sierra Espuna-area Trioctahedralchlorite was presumably produced at the highestmetamorphicdiagenetic grade in equilibrium withparagonite and phengite Subsequently sudoite wasintroduced but at P-T conditions which could nothave been much different than those of peakmetamorphic grade Finally at very low-T condi-tions hydrothermal fluids caused partial alterationof trioctahedral chlorite and sudoite to smectite

Retrograde products formed by the reaction offluids with mineral phases generated during anearlier prograde event were described previouslyby Nieto et al (1994) and Zhao et al (1999) Thosereactions occurred on a regional scale but similarreactions have been observed on a more localizedscale The fluid activity associated with a majorfault zone is a common source for these reactions(eg Jiang et al 1990) The structure of SierraEspuna (Fig 1) is based on imbrication processesthat superimpose units that have subsequently beenaffected by faults and folds and that have under-gone significant uplift at least 800 m from the lateMiocene to the present (Sanz de Galdeano et al2001) Given this evolution it is easy to justify theretrograde processes that usually seem to be relatedwith tectonic events during which fluid flow andfluidrock ratios are enhanced (Merriman amp Peacor1999) Moreover according to Nieto et al (1994)the uplift of the Betic Cordillera would furnish theintroduction of phreatic water into the systemproducing a reactive environment adequate for thegenesis of smectite Geological conditions aretherefore consistent with post-peak metamorphicconditions as implied by the TEM observations

20 I Abad et al

The introduction of sudoite as a major phaseduring metamorphism implies the possibility ofadditional overprints on a simple prograde assem-blage the evidence for which might be subtle TheNa-rich micas at all grades have compositionswhich either fall within the solvus or are too Na-rich to correspond to the limb of the solvus In thehighest-grade rocks such micas coexist withmuscovite Those relations are incompatible withchemical equilibrium despite the fact that relationsfor muscovite and other minerals imply an approachto chemical equilibrium Furthermore there is aclose association of Na-rich micas with sudoite (egFig 7) Those relations imply the possibility thatNa-micas were introduced concomitant withsudoite P Mata (pers comm) has shown forexample that overprinting of an epizonal assem-blage by hydrothermal solutions resulted in theintroduction of both an Al-rich variety of chlorite(cookeite) and paragonite in pelites of the CamerosBasin Spain The textural relations such as cross-cutting textures which are so striking for sudoitewere not observed for Na-rich micas Neverthelessthe possibility of such overprinting must beconsidered

In spite of all these prograde-retrogradeprocesses in the Los Molinos Unit a smallnumber of homogeneous phases coexist in chemicalequilibrium and the fine-grained metamorphicphyllosilicates are clearly parallel to cleavage witha typical metamorphic aspect Finally all thetextural and compositional changes that haveoperated in the Espuna area during the metamorphicevolution proceed toward a simpler system repre-senting both textural and chemical equilibriumwhich is characteristic of greenschist-facies condi-tions

Roles of detrital and authigenic phyllosilicatesduring slaty cleavage development

Van der Pluijm et al (1998) focused on therespective roles of the phyllosilicates of detritalorigin and those formed during diagenesis andmetamorphism and observed that detrital phyllosi-licates remain preferentially oriented parallel tobedding in pelites with slaty cleavage whereas fine-grained metamorphic phyllosilicates are orientedparallel to cleavage

Although it is usually difficult to determine thepreferred orientations of phyllosilicates in TEMimages as the grade increases better-aligned

packets occur in the cleavage orientation Thetexture of the least metamorphosed sample inwhich even incipient slaty cleavage is absent isreminiscent of the typical textural relation ofsmectites described in the literature The illite-smectite mixed-layers conserve the textural featuresof possible previous smectite such as anastomosingfringes curved and discontinuous with variations ininterplanar spacing d001 These features indicatethat the minimum energy conditions to initiatemechanical transformations have not been reachedin the lower-grade samples as postulated by Ho etal (1996) since slaty cleavage develops as a resultof interactive mechanical deformation and ther-mally driven dissolution and crystallizationprocesses (Knipe 1981) In contrast in the LosMolinos Unit large crystals of chlorite and whitemica (gt1000 AEcirc ) occur which are of metamorphicorigin but with no detrital grains (Fig 3a)Phyllosilicates are preferentially oriented parallelto slaty cleavage in response to increasedtemperature and tectonic strain (Merriman ampPeacor 1999) The oriented microfabrics and slatycleavage well developed in the rocks of this unitmust be a product of a higher thermal and strainenergy environment reached in the deepest part ofthe sequence As pointed out by Van der Pluijm etal (1998) a continuous range of orientations isevidence for mechanical rotation of grains whereasthe lack of such transitional orientations impliesthat cleavage-parallel phyllosilicates originatedthrough dissolution of bedding-parallel and crystal-lization in the slaty cleavage orientation Asobserved in this study for the Intermediate Unitsgrains with orientations which are transitional frombedding-parallel to slaty cleavage orientation areonly rarely observed Cleavage formation is there-fore inferred to have occurred entirely or nearlyentirely through dissolution and crystallization

ACKNOWLEDGMENTS

We thank MM Abad Ortega from the Centro deInstrumentacio n Cient otilde fica of the Universidad deGranada for her help with HRTEM and I Nieto forher essential help with sample preparation Financialsupport was supplied by Research Project nordm BT 2000-0582 FPI research grant to IA both of the SpanishMinistry of Science and Technology Research GroupRNM-0179 of the Junta de Andaluc otildeacutea and to DRP byNSF grant EAR-9814391

Prograde and retrograde metamorphism 21

REFERENCES

Ahn JH amp Peacor DR (1986) Transmission andanalytical electron microscopy of the smectite-to-illite transition Clays and Clay Minerals 34165 ndash179

Azanon JM amp Crespo-Blanc A (2000) Exhumationduring a continental collision inferred from thetectonometamorphic evolution of the AlpujarrideComplex in the central Betics (Alboran Domain SESpain) Tectonics 19 549 ndash565

Bevins RE Robinson D amp Rowbotham G (1991)Compositional variations in mafic phyllosilicatesfrom regional low-grade metabasites and applicationof the chlorite geother mometer Journal ofMetamorphic Geology 9 711 ndash721

Cliff G amp Lorimer GW (1975) The quantitativeanalysis of thin specimens Journal of Microscopy 103 203 ndash207

Crowley SF (1991) Diagenetic modification of detritalmuscovite an example from the Great LimestoneCyclothem (Carboniferous) of Co Durham UKClay Minerals 26 91 ndash103

Dong H amp Peacor DR (1996) TEM observations ofcoherent stacking relations in smectite and illite ofshales evidence for MacEwan crystallites anddominance of 2M1 polytypes Clays and ClayMinerals 44 257 ndash275

Essene E amp Peacor DR (1995) Clay mineral thermo-metry a critical perspective Clays and ClayMinerals 43 540 ndash553

Fransolet AM amp Bourguignon P (1978) Dioctahedralchlorite in quartz veins from Ardennes BelgiumThe Canadian Mineralogist 16 365 ndash373

Fransolet AM amp Schreyer W (1984) Sudoite ditrioctahedral chlorite a stable low-temperature phasein the system MgO-Al2O3-SiO2-H2O Contributionsto Mineralogy and Petrology 86 409 ndash417

Frey M (1987) Very low-grade metamorphism of clasticsedimentary rocks Pp 9 ndash58 in Low TemperatureMetamorphism (M Frey editor) Blackie GlasgowUK

Giorgetti G Goffe B Memmi I amp Nieto F (1998)Metamorphic evolution of Verrucano metasedimentsin northern Apennines new petrological constraintsEuropean Journal of Mineralogy 10 1295 ndash1308

Guidotti CV amp Sassi FP (1986) Classification andcorrelation of metamorphic facies series by means ofmuscovite b0 data from low grade metapelites NeuesJahrbuch fur Mineralogie Abhandlungen 153363 ndash380

Guidotti CV Yates MG Dyar MD amp Taylor ME(1994) Petrogenetic implications of the Fe3+ contentof muscovi te in pel it ic sch is ts AmericanMineralogist 79 793 ndash795

Guthrie GD amp Veblen DR (1989) High resolutionelectron microscopy of mixed-layer illitesmectite

Computer simulations Clays and Clay Minerals 371 ndash11

Ho N Peacor DR amp Van der Pluijm BA (1996)Contrasting roles of detrital and authigenic phyllo-silicates during slaty cleavage development Journalof Structural Geology 18 615 ndash623

Jiang WT Peacor DR Merriman RJ amp Roberts B(1990) Transmission and analytical electron micro-scopic study of mixed layer illitesmectite formed asan apparent replacement product of diagenetic illiteClays and Clay Minerals 38 449 ndash468

Jiang WT Peacor DR amp Buseck PR (1994) Chloritegeothermometry Contamination and apparent octa-hedral vacancies Clays and Clay Minerals 42593 ndash605

Kisch HJ (1991) Development of slaty cleavage anddegree of very-low-grade metamorphism a reviewJournal of Metamorphic Geology 9 735 ndash750

Knipe JR (1981) The interaction of deformation andmetamorphi sm in slates Tectonophysics 78249 ndash272

Kretz R (1983) Symbols for rock-forming mineralsAmerican Mineralogist 68 277 ndash279

Li G Peacor DR Merriman RJ amp Roberts B (1994a)The diagenetic to low grade metamorphism evolu-tion of matrix white mica in the system muscovite-paragonite in a mudrock from Central Wales UKClays and Clay Minerals 42 369 ndash381

Li G Peacor DR Merriman RJ Roberts B amp Vander Pluijm BA (1994b) TEM and AEM constraintson the origin and significance of chlorite-mica stacksin slates an example from Central Wales UKJournal of Structural Geology 16 1139 ndash1157

Livi KJT Veblen DR Ferry JM amp Frey M (1997)Evolution of 21 layered silicates in low-grademetamorphosed Liass ic sha les of CentralSwitzerland Journal of Metamorphic Geology 15323 ndash344

Lonergan L (1991) Structural evolution of the SierraEspuna Betic Cordillera SE Spain PhD thesisOxford University UK

Lonergan L Platt JP amp Gallagher L (1994) TheInternal-External Zone Boundary in the eastern BeticCordillera SE Spain Journal of Structural Geology16 175 ndash188

Makel GH (1981) Differences in tectonic evolution ofsuperimposed Malaguide and Alpujarride tectonicunits in the Espuna area (Betic Cordilleras Spain)Geology in Mijnbouw 60 203 ndash208

Makel GH (1985) The geology of the MalaguideComplex and its bearing on the geodynamicevolution of the Betic-Rif orogen (southern Spainand northern Morocco) GUA papers of Geology Ser1 22 263 pp

Makel GH amp Rondeel HE (1979) Differences instratigraphy and metamorphism between superposedMalaguide and Alpujarride units in the Espuna area

22 I Abad et al

the limits of the solvus between muscovite andparagonite the limbs of that solvus having theapproximate compositions ms60pg40 and ms20pg80

(Li et al 1994a)Detrital muscovite vs metamorphic muscovite

Detrital-like micas of sample Es-95 the lowest-grade one are large enough to be analysed(Table 5) by electron microprobe (EMPA) Thesedata have been compared with the AEM data ofother samples Compositions of detrital-like grainsof the lowest-grade sample and the detrital micas ofthe anchizonal-grade sample Es-53 (Fig 15) arevery similar and close to that of end-membermuscovite These data suggest that the detrital micahas a common source for the different units thatcomprise Sierra Espuna Figure 15 also comparesthe data from authigenic micas of the samesamples which are clearly more phengitic (Fe+Mg= 015 ndash075 apfu) than the detrital ones (Fe+Mg= 01 ndash02 apfu)

Chlorite Trioctahedral and di-trioctahedral(sudoite) chlorites have been analysed (Table 6)The former corresponding to the species clino-chlore is common in most of the samples unlikesudoite which is absent from most (Table 1)Sudoite was identified because the sum ofoctahedral cations is nearly equal to 5 apfu andthe Si and Al contents are higher than intrioctahedral chlorite (Fig 16a) A slight contam-ination by interlayer cations (K and Na) has beendetected therefore the formulae were recalculatedin order to eliminate those impurities (seeAnalytical Methods) Although total Fe has beenconsidered as Fe2+ according to Fransolet ampBourguignon (1978) the Fe3+ content in sudoitemay be greater than the Fe2+ content As severalanalyses have an octahedral sum slightly higherthan the ideal value of 5 apfu this may indicate atrioctahedral substitution andor the presence ofsignificant Fe3+ No Ti or Mn were detected The

Fe(Fe+Mg) ratios do not define a clear trendthroughout the sequence (Fig 16b) nevertheless itis much greater in chlorite in the Morron de Totanaanalyses falling in the characteristic range ofdiagenetic chlorites (Li et al 1994b) There is anincrease in Mg content (from 15 to 4 apfu)concomitant with increase in grade as compatiblewith some other sequences (eg Bevins et al1991)

D I S C U S S I O N

Dioctahedral phyllosilicates chemicalcompositions IC values and polytypes

On the basis of their large size homogeneity ofindividual grains and well-crystallized aspectdetrital dioctahedral mica grains were identified inseveral samples with the exception of those ofgreenschist facies This confirms the idea of the

TABLE 4 Chemical compositions for Na-K micas normalized to O10(OH)2

Si IVAl VIAl Fe Mg Ti VIS Ca K Na S inter

Es-5 14 313 087 190 004 009 002 204 000 034 048 083Es-5 15 321 079 176 009 037 000 222 000 015 038 053Es-5 4 318 082 187 004 019 000 209 000 013 061 074Es-5 12 329 071 172 008 029 000 208 000 039 036 076Es-5 13 323 077 179 005 028 000 212 000 029 041 069Es-5 9 296 104 163 019 029 000 211 000 048 056 104Es-5 8 292 108 156 043 008 001 209 003 014 080 096

FIG 15 Plot of Si vs Al contents of detrital andauthigenic micas of samples Es-95 and Es-53 Solidsymbols = detrital micas open symbols = authigenic

micas

16 I Abad et al

relative stability of muscovite in most diageneticenvironments (Crowley 1991) The compositions ofthe detrital grains of samples from different unitsare similar The lack of heterogeneity which isexpected for mica from a complex source implies acommon source area for all samples

The IC values define a trend from diagenetic toanchizonal conditions but with smaller IC values inbulk-rock fractions consistent with the contributionof detrital mica The coexistence of mature micawith lower-grade I-S or illite in the sub-greenschistfacies rocks shows that those rocks have not

TABLE 5 EMPA data for detrital-like crystals of mica normalized to O10(OH)2

Si IVAl VIAl Fe Mg Ti VIS Ca K Na K+Na+Ca

Es-95z11 313 087 193 005 004 001 203 000 065 022 087Es-95z11 314 086 190 006 007 001 204 000 068 019 087Es-95z1 2 314 086 191 005 009 001 206 000 070 006 077Es-95z2 1 314 086 176 012 010 003 201 002 095 004 101Es-95z2 2 317 083 177 012 011 003 202 000 095 004 099Es-95z4 1 305 095 188 005 007 004 204 000 081 009 089Es-95z4 2 311 089 187 006 007 004 204 000 078 009 086

TABLE 6 Representative chemical compositions for chlorite normalized to O10(OH)8

Si IVAl VIAl Fe Mg VIS FeFe+Mg

Yechar UnitEs53-1 273 127 112 121 376 609 024Es53-4 287 113 125 070 400 596 015Es53-12 303 097 130 146 307 583 032Es53-13 277 123 125 122 354 601 026Es53-15 307 093 146 094 323 564 023Es53-42 271 129 152 160 294 606 035

La Santa UnitEs2-4 319 081 147 148 273 569 035Es2-5 365 035 183 118 225 526 034Es5-1 354 046 293 023 172 488 012Es5-5 361 039 292 021 178 490 010Es5-10 309 091 262 028 221 511 011Es5-16 310 090 197 112 234 543 032Es5-17 350 050 252 076 170 499 031Es-5 10rsquo 364 036 213 097 202 512 032Es-5 11 291 109 143 136 314 593 030Es-5 12 382 018 229 087 179 495 033Es-5 13 316 084 302 021 172 494 011Es-5 16rsquo 346 054 341 013 101 455 011Es-5 17rsquo 300 100 282 033 194 509 014Es-5 18 285 115 285 026 204 515 011

Morron de Totana UnitEs-86 2 377 023 253 088 149 490 037Es-86 4 284 116 166 230 181 578 056Es-86 5 350 050 191 192 151 534 056Es-86 6 366 034 189 191 149 529 056Es-86 7 315 085 129 273 176 578 061

indicates sudoite analyses

Prograde and retrograde metamorphism 17

reached stable chemical or textural equilibrium atleast at the mm scale In addition the lack ofchemical equilibrium among diagenetic I-S andillite at the nm scale is demonstrated by their highlyvariable Fe+Mg contents (006ndash075 apfu) Theaverages of the sum of interlayer cations are closeto 1 apfu in the Los Molinos and Yechar units asconsistent with mica but values of 06 apfucorresponding to I-S or illitic compositions are alsopresent in the Morron de Totana Unit Except forsamples Es-102 and Es-53 the variable composi-tions represent a metastable state with composi-tions of micas determined by a combination ofphengitic ferrimuscovitic and illitic vectors asshown by Fig 14ab

Paragonite is generally considered an indexmineral for the anchizone (Frey 1987) but Li etal (1994a) have shown that paragonite coexistswith K-rich mica in grades as low as that of the latediagenetic zone for a prograde sequence Samplesin which muscovite paragonite and intermediateNa-K mica coexist as in sample Es-5 areconsistent with metastability both because morethan two such micas coexist and because theircompositions are incompatible with the solvus inthe muscovite-paragonite system (see below) Whilethe 2M polytype typical of metamorphic micas is

the most common in these samples the 1Md

polytype also occurs in some diagenetic samplesparticularly those from the Morron de Totana UnitThe 1Md polytype is metastable relative to the 2Mpolytype and the coexistence of both polytypesfurther demonstrates a lack of equilibrium at the nmscale Dong amp Peacor (1996) showed that thedisorder noted in the diagenetic phyllosilicates bySAED is a consequence of the general high entropyof phases formed at very low temperatures thatproduces for example stacking defects separatingpackets with no more than 10 layers These packetsprogressively thicken with increasing metamorphicgrade and the 2M polytype then becomes the mostcommon polytype in authigenic micas Thenm-scale size of the lower-grade clays and theregular increase in size with increasing gradereflect the lack of textural equilibrium in the low-grade samples

A wide range of sample characteristics thusimply that the lower-grade samples are in statesof metastable equilibrium those factors includingcoexistence of mature mica with I-S and illitevariable chemical compositions incompatiblecompositions of white micas small crystal sizesdefect-rich states and polytypism Such dataemphasize the notion of lsquoreaction progressrsquo relative

FIG 16 (a) Plot of Al vs Fe+Mg contents of chlorites illustrating the contrast in composition between tri-trioctahedral clinochlore or chamosite and di-trioctahedral sudoite (b) plot of Si vs Fe(Fe+Mg) ratio of

chlorites showing an absence of compositional trends throughout the sampled sequence

18 I Abad et al

to Ostwald-step rule processes metastable dioctahe-dral clay systems undergoing reactions toward thestate of stable chemical and textural equilibriumapproached in greenschist-facies samples (Essene ampPeacor 1995)

Prograde evolution based on TEM datacorrelation with the Basque-Cantabriansequence (Northern Spain)

With increasing grade TEM data confirm thatcrystal thickness increases crystal defect densitydecreases compositions become less variable andmicrotextural relations are simpler (eg Peacor1992) This trend is consistent with three well-characterized low-grade settings the Gulf Coastsequence (Ahn amp Peacor 1986) and the WelshBasin (Merriman et al 1990) both extensionalbasins and the Helvetic Alps (Livi et al 1997)The latter is a fold-and-thrust belt with similarseries of prograde transitions to those observed inSierra Espuna in which pelites have passed througha sequence of metastable states in the lithologicalsequence mudstone and shale slate and phyllite

The mudstones of the Morron de Totana Unit arecharacterized by the presence of illite-smectite R4mixed-layers mica packets of no more than 10 ndash40layers and the occurrence of 1Md polytypism TheLa Santa Unit samples represent a step in theprograde evolution with thicker packets than in theprevious samples (200 ndash700 AEcirc ) illitic or phengiticcompositions the absence of illite-smectite mixedlayers as well as the presence of discreteintermediate Na-K micas and paragonite Althoughthere is a tendency towards non-preferred orienta-tion of crystals it is not so evident as in the Morronde Totana samples In the Yechar Unit (anchizone)and more clearly in the Los Molinos phyllites(greenschist facies) the microtextural relations areless complex the compositions are less variablewith phengite and trioctahedral chlorite crystalthickness are greater (gt400 AEcirc in the Yechar Unitand gt1000 AEcirc in the Los Molinos Unit) with thepackets being defect-free and with a very crystallineaspect This description has features in commonwith the three well-characterized low-grade settingspreviously noted Only the mineral assemblagesvary somewhat presumably as a function of thedifferences in bulk-rock composition

A detailed comparison with the Basque-Cantabrian Basin (N Spain) shows that the lowest-grade conditions of Sierra Espuna overlap most of

those of that sequence which is characterized bypassive burial diagenesis (8000 m thick) (Nieto etal 1996) Only in the deepest sample of theBasque-Cantabrian sequence is illite-smectite mixedlayering absent Figure 12 of this study is verysimilar to Fig 7 in Nieto et al (1996) and bothimages correspond to diagenetic samples fromdifferent settings nevertheless they illustratesimilar mica packets 100 ndash200 AEcirc thick withparallel-to-subparallel boundaries the orientationof which is inferred to represent beddingTherefore there is a parallelism between thesesequences down to the scale of individual clay-mineral grains even though they developed underdifferent conditions with stress presumably playinga significant role in the Sierra Espuna units Thetexture of the least metamorphosed sample (Fig 13)is reminiscent of the typical textural relations ofsmectite described by Nieto et al (1996) (seeFig 4) Although it corresponds to illite-richinterstratified illite-smectite the clay mineralgrains conserved the grain size and anastomosingdiscontinuous array of original smectite layers

Despite the similarities between the twosequences the chemical behaviour of the micasare clearly different since the grade correspondingto the phengitic compositions of the Yechar Unit isnot reached in the Basque-Cantabrian Basin wherethe deepest and highest-grade sample has illiticcompositions equivalent to those of the La SantaUnit That is in a passive burial metamorphiccontext as in the Basque-Cantabrian Basin thefactors responsible for increasing the rate ofchemical reactions are less significant than in thesequences that were affected by tectonic stressTectonic stress introduces defects which in turnincreases the rate of dissolution of strained grainsthus increasing the rate of dissolution-crystallizationreactions typical of clay minerals Perhaps moreimportantly regional tectonic stress is known tocause mobilization of fluids (eg Oliver 1986) Thepresence and activity of such fluids is a prime causeof increased rates of dissolution ion diffusion andcrystallization Tectonic activity thus may have atwo-fold effect in increasing rate of clay-mineralreactions The phengite component may in part be ameasure of the reaction progress depending on thereaction kinetics in very low-grade rocksAccording to Merriman amp Peacor (1999) whereheat flow is low as in accretionary settingsphengite contents are typically high perhapsbecause there is insufficient thermal energy to

Prograde and retrograde metamorphism 19

cause equilibration of the phengite component ofneoformed illite by contrast with extensional basinswhere this component is typically small

Coexistence of trioctahedral chlorite andsudoite Implications for multiple metamorphicevents

In metamorphic rocks the coexistence of mineralsthat formed at different times and conditions on aPTt path ie not in equilibrium is a commonlyobserved feature where the metamorphic history iscomplex Gross differences in the grain size ofcoexisting phases has long been recognized bymetamorphic petrologists as an indicator of non-equilibrium mineral assemblages Historically muchless attention has been paid to such relations in sub-greenschist-facies rocks because prior to the intro-duction of SEM+TEM techniques their small grainsize prevented observations of textural relationsStudy of such rocks eg by XRD provides littleinformation on textural relations beyond determina-tion of degree of preferred orientation

Study by XRD of the samples showed thatsudoite and trioctahedral chlorite coexist If theyhad formed at the same time and under the sameP-T conditions their compositions would define thelimbs of a solvus The TEM images show howeverthat wherever they are in contact the sudoitetransects the packets of trioctahedral chlorite (egFig 8) Furthermore as illustrated in Fig 8 thetrioctahedral chlorite is commonly deformed anddisplays strain contrast in TEM images Thesetextural relations suggest that sudoite was intro-duced subsequent to the formation of trioctahedralchlorite Similar observations were made byGiorgetti et al (1998) who concluded that texturesin Verrucano metasediments showed that sudoitepost-dated chloritoid pyrophyllite and trioctahedralchlorite ie sudoite is in disequilibrium with thelatter minerals and formed as a retrograde phase Inrocks of the Sierra Espuna area differences in bulk-rock composition are small (Table 2) and do notcorrespond to the presence or absence of sudoite(Table 1) ie the random occurrence of sudoiteindependent of bulk-rock composition is compa-tible with a lack of equilibrium relations Theserelations collectively imply that sudoite wasintroduced at some time subsequent to theformation of mineral assemblages at peak meta-morphic conditions On the other hand the size ofsudoite packets is similar to that of trioctahedral

chlorite as consistent with formation at conditionsnot much different from those for chlorite

In samples from the Morron de Totana bothsudoite and trioctahedral chlorite were affected byretrograde alteration to smectite The TEM imagesshow sudoite (Fig 11) and trioctahedral chlorite(Fig 10) with interstratified layers of smectite withspacings of 10 AEcirc due to collapse and dehydration inthe TEM environment and even packets of smectite150 AEcirc thick within chlorite crystals These relationsimply that both sudoite and trioctahedral chloritehave been affected by retrograde alteration at thevery low temperatures consistent with smectiteformation although the presence of more subtlealteration features in sudoite of some samplessuggests that it was less susceptible to alteration

We therefore conclude that at least three episodesof phyllosilicate formation can be recognized inrocks from the Sierra Espuna-area Trioctahedralchlorite was presumably produced at the highestmetamorphicdiagenetic grade in equilibrium withparagonite and phengite Subsequently sudoite wasintroduced but at P-T conditions which could nothave been much different than those of peakmetamorphic grade Finally at very low-T condi-tions hydrothermal fluids caused partial alterationof trioctahedral chlorite and sudoite to smectite

Retrograde products formed by the reaction offluids with mineral phases generated during anearlier prograde event were described previouslyby Nieto et al (1994) and Zhao et al (1999) Thosereactions occurred on a regional scale but similarreactions have been observed on a more localizedscale The fluid activity associated with a majorfault zone is a common source for these reactions(eg Jiang et al 1990) The structure of SierraEspuna (Fig 1) is based on imbrication processesthat superimpose units that have subsequently beenaffected by faults and folds and that have under-gone significant uplift at least 800 m from the lateMiocene to the present (Sanz de Galdeano et al2001) Given this evolution it is easy to justify theretrograde processes that usually seem to be relatedwith tectonic events during which fluid flow andfluidrock ratios are enhanced (Merriman amp Peacor1999) Moreover according to Nieto et al (1994)the uplift of the Betic Cordillera would furnish theintroduction of phreatic water into the systemproducing a reactive environment adequate for thegenesis of smectite Geological conditions aretherefore consistent with post-peak metamorphicconditions as implied by the TEM observations

20 I Abad et al

The introduction of sudoite as a major phaseduring metamorphism implies the possibility ofadditional overprints on a simple prograde assem-blage the evidence for which might be subtle TheNa-rich micas at all grades have compositionswhich either fall within the solvus or are too Na-rich to correspond to the limb of the solvus In thehighest-grade rocks such micas coexist withmuscovite Those relations are incompatible withchemical equilibrium despite the fact that relationsfor muscovite and other minerals imply an approachto chemical equilibrium Furthermore there is aclose association of Na-rich micas with sudoite (egFig 7) Those relations imply the possibility thatNa-micas were introduced concomitant withsudoite P Mata (pers comm) has shown forexample that overprinting of an epizonal assem-blage by hydrothermal solutions resulted in theintroduction of both an Al-rich variety of chlorite(cookeite) and paragonite in pelites of the CamerosBasin Spain The textural relations such as cross-cutting textures which are so striking for sudoitewere not observed for Na-rich micas Neverthelessthe possibility of such overprinting must beconsidered

In spite of all these prograde-retrogradeprocesses in the Los Molinos Unit a smallnumber of homogeneous phases coexist in chemicalequilibrium and the fine-grained metamorphicphyllosilicates are clearly parallel to cleavage witha typical metamorphic aspect Finally all thetextural and compositional changes that haveoperated in the Espuna area during the metamorphicevolution proceed toward a simpler system repre-senting both textural and chemical equilibriumwhich is characteristic of greenschist-facies condi-tions

Roles of detrital and authigenic phyllosilicatesduring slaty cleavage development

Van der Pluijm et al (1998) focused on therespective roles of the phyllosilicates of detritalorigin and those formed during diagenesis andmetamorphism and observed that detrital phyllosi-licates remain preferentially oriented parallel tobedding in pelites with slaty cleavage whereas fine-grained metamorphic phyllosilicates are orientedparallel to cleavage

Although it is usually difficult to determine thepreferred orientations of phyllosilicates in TEMimages as the grade increases better-aligned

packets occur in the cleavage orientation Thetexture of the least metamorphosed sample inwhich even incipient slaty cleavage is absent isreminiscent of the typical textural relation ofsmectites described in the literature The illite-smectite mixed-layers conserve the textural featuresof possible previous smectite such as anastomosingfringes curved and discontinuous with variations ininterplanar spacing d001 These features indicatethat the minimum energy conditions to initiatemechanical transformations have not been reachedin the lower-grade samples as postulated by Ho etal (1996) since slaty cleavage develops as a resultof interactive mechanical deformation and ther-mally driven dissolution and crystallizationprocesses (Knipe 1981) In contrast in the LosMolinos Unit large crystals of chlorite and whitemica (gt1000 AEcirc ) occur which are of metamorphicorigin but with no detrital grains (Fig 3a)Phyllosilicates are preferentially oriented parallelto slaty cleavage in response to increasedtemperature and tectonic strain (Merriman ampPeacor 1999) The oriented microfabrics and slatycleavage well developed in the rocks of this unitmust be a product of a higher thermal and strainenergy environment reached in the deepest part ofthe sequence As pointed out by Van der Pluijm etal (1998) a continuous range of orientations isevidence for mechanical rotation of grains whereasthe lack of such transitional orientations impliesthat cleavage-parallel phyllosilicates originatedthrough dissolution of bedding-parallel and crystal-lization in the slaty cleavage orientation Asobserved in this study for the Intermediate Unitsgrains with orientations which are transitional frombedding-parallel to slaty cleavage orientation areonly rarely observed Cleavage formation is there-fore inferred to have occurred entirely or nearlyentirely through dissolution and crystallization

ACKNOWLEDGMENTS

We thank MM Abad Ortega from the Centro deInstrumentacio n Cient otilde fica of the Universidad deGranada for her help with HRTEM and I Nieto forher essential help with sample preparation Financialsupport was supplied by Research Project nordm BT 2000-0582 FPI research grant to IA both of the SpanishMinistry of Science and Technology Research GroupRNM-0179 of the Junta de Andaluc otildeacutea and to DRP byNSF grant EAR-9814391

Prograde and retrograde metamorphism 21

REFERENCES

Ahn JH amp Peacor DR (1986) Transmission andanalytical electron microscopy of the smectite-to-illite transition Clays and Clay Minerals 34165 ndash179

Azanon JM amp Crespo-Blanc A (2000) Exhumationduring a continental collision inferred from thetectonometamorphic evolution of the AlpujarrideComplex in the central Betics (Alboran Domain SESpain) Tectonics 19 549 ndash565

Bevins RE Robinson D amp Rowbotham G (1991)Compositional variations in mafic phyllosilicatesfrom regional low-grade metabasites and applicationof the chlorite geother mometer Journal ofMetamorphic Geology 9 711 ndash721

Cliff G amp Lorimer GW (1975) The quantitativeanalysis of thin specimens Journal of Microscopy 103 203 ndash207

Crowley SF (1991) Diagenetic modification of detritalmuscovite an example from the Great LimestoneCyclothem (Carboniferous) of Co Durham UKClay Minerals 26 91 ndash103

Dong H amp Peacor DR (1996) TEM observations ofcoherent stacking relations in smectite and illite ofshales evidence for MacEwan crystallites anddominance of 2M1 polytypes Clays and ClayMinerals 44 257 ndash275

Essene E amp Peacor DR (1995) Clay mineral thermo-metry a critical perspective Clays and ClayMinerals 43 540 ndash553

Fransolet AM amp Bourguignon P (1978) Dioctahedralchlorite in quartz veins from Ardennes BelgiumThe Canadian Mineralogist 16 365 ndash373

Fransolet AM amp Schreyer W (1984) Sudoite ditrioctahedral chlorite a stable low-temperature phasein the system MgO-Al2O3-SiO2-H2O Contributionsto Mineralogy and Petrology 86 409 ndash417

Frey M (1987) Very low-grade metamorphism of clasticsedimentary rocks Pp 9 ndash58 in Low TemperatureMetamorphism (M Frey editor) Blackie GlasgowUK

Giorgetti G Goffe B Memmi I amp Nieto F (1998)Metamorphic evolution of Verrucano metasedimentsin northern Apennines new petrological constraintsEuropean Journal of Mineralogy 10 1295 ndash1308

Guidotti CV amp Sassi FP (1986) Classification andcorrelation of metamorphic facies series by means ofmuscovite b0 data from low grade metapelites NeuesJahrbuch fur Mineralogie Abhandlungen 153363 ndash380

Guidotti CV Yates MG Dyar MD amp Taylor ME(1994) Petrogenetic implications of the Fe3+ contentof muscovi te in pel it ic sch is ts AmericanMineralogist 79 793 ndash795

Guthrie GD amp Veblen DR (1989) High resolutionelectron microscopy of mixed-layer illitesmectite

Computer simulations Clays and Clay Minerals 371 ndash11

Ho N Peacor DR amp Van der Pluijm BA (1996)Contrasting roles of detrital and authigenic phyllo-silicates during slaty cleavage development Journalof Structural Geology 18 615 ndash623

Jiang WT Peacor DR Merriman RJ amp Roberts B(1990) Transmission and analytical electron micro-scopic study of mixed layer illitesmectite formed asan apparent replacement product of diagenetic illiteClays and Clay Minerals 38 449 ndash468

Jiang WT Peacor DR amp Buseck PR (1994) Chloritegeothermometry Contamination and apparent octa-hedral vacancies Clays and Clay Minerals 42593 ndash605

Kisch HJ (1991) Development of slaty cleavage anddegree of very-low-grade metamorphism a reviewJournal of Metamorphic Geology 9 735 ndash750

Knipe JR (1981) The interaction of deformation andmetamorphi sm in slates Tectonophysics 78249 ndash272

Kretz R (1983) Symbols for rock-forming mineralsAmerican Mineralogist 68 277 ndash279

Li G Peacor DR Merriman RJ amp Roberts B (1994a)The diagenetic to low grade metamorphism evolu-tion of matrix white mica in the system muscovite-paragonite in a mudrock from Central Wales UKClays and Clay Minerals 42 369 ndash381

Li G Peacor DR Merriman RJ Roberts B amp Vander Pluijm BA (1994b) TEM and AEM constraintson the origin and significance of chlorite-mica stacksin slates an example from Central Wales UKJournal of Structural Geology 16 1139 ndash1157

Livi KJT Veblen DR Ferry JM amp Frey M (1997)Evolution of 21 layered silicates in low-grademetamorphosed Liass ic sha les of CentralSwitzerland Journal of Metamorphic Geology 15323 ndash344

Lonergan L (1991) Structural evolution of the SierraEspuna Betic Cordillera SE Spain PhD thesisOxford University UK

Lonergan L Platt JP amp Gallagher L (1994) TheInternal-External Zone Boundary in the eastern BeticCordillera SE Spain Journal of Structural Geology16 175 ndash188

Makel GH (1981) Differences in tectonic evolution ofsuperimposed Malaguide and Alpujarride tectonicunits in the Espuna area (Betic Cordilleras Spain)Geology in Mijnbouw 60 203 ndash208

Makel GH (1985) The geology of the MalaguideComplex and its bearing on the geodynamicevolution of the Betic-Rif orogen (southern Spainand northern Morocco) GUA papers of Geology Ser1 22 263 pp

Makel GH amp Rondeel HE (1979) Differences instratigraphy and metamorphism between superposedMalaguide and Alpujarride units in the Espuna area

22 I Abad et al

relative stability of muscovite in most diageneticenvironments (Crowley 1991) The compositions ofthe detrital grains of samples from different unitsare similar The lack of heterogeneity which isexpected for mica from a complex source implies acommon source area for all samples

The IC values define a trend from diagenetic toanchizonal conditions but with smaller IC values inbulk-rock fractions consistent with the contributionof detrital mica The coexistence of mature micawith lower-grade I-S or illite in the sub-greenschistfacies rocks shows that those rocks have not

TABLE 5 EMPA data for detrital-like crystals of mica normalized to O10(OH)2

Si IVAl VIAl Fe Mg Ti VIS Ca K Na K+Na+Ca

Es-95z11 313 087 193 005 004 001 203 000 065 022 087Es-95z11 314 086 190 006 007 001 204 000 068 019 087Es-95z1 2 314 086 191 005 009 001 206 000 070 006 077Es-95z2 1 314 086 176 012 010 003 201 002 095 004 101Es-95z2 2 317 083 177 012 011 003 202 000 095 004 099Es-95z4 1 305 095 188 005 007 004 204 000 081 009 089Es-95z4 2 311 089 187 006 007 004 204 000 078 009 086

TABLE 6 Representative chemical compositions for chlorite normalized to O10(OH)8

Si IVAl VIAl Fe Mg VIS FeFe+Mg

Yechar UnitEs53-1 273 127 112 121 376 609 024Es53-4 287 113 125 070 400 596 015Es53-12 303 097 130 146 307 583 032Es53-13 277 123 125 122 354 601 026Es53-15 307 093 146 094 323 564 023Es53-42 271 129 152 160 294 606 035

La Santa UnitEs2-4 319 081 147 148 273 569 035Es2-5 365 035 183 118 225 526 034Es5-1 354 046 293 023 172 488 012Es5-5 361 039 292 021 178 490 010Es5-10 309 091 262 028 221 511 011Es5-16 310 090 197 112 234 543 032Es5-17 350 050 252 076 170 499 031Es-5 10rsquo 364 036 213 097 202 512 032Es-5 11 291 109 143 136 314 593 030Es-5 12 382 018 229 087 179 495 033Es-5 13 316 084 302 021 172 494 011Es-5 16rsquo 346 054 341 013 101 455 011Es-5 17rsquo 300 100 282 033 194 509 014Es-5 18 285 115 285 026 204 515 011

Morron de Totana UnitEs-86 2 377 023 253 088 149 490 037Es-86 4 284 116 166 230 181 578 056Es-86 5 350 050 191 192 151 534 056Es-86 6 366 034 189 191 149 529 056Es-86 7 315 085 129 273 176 578 061

indicates sudoite analyses

Prograde and retrograde metamorphism 17

reached stable chemical or textural equilibrium atleast at the mm scale In addition the lack ofchemical equilibrium among diagenetic I-S andillite at the nm scale is demonstrated by their highlyvariable Fe+Mg contents (006ndash075 apfu) Theaverages of the sum of interlayer cations are closeto 1 apfu in the Los Molinos and Yechar units asconsistent with mica but values of 06 apfucorresponding to I-S or illitic compositions are alsopresent in the Morron de Totana Unit Except forsamples Es-102 and Es-53 the variable composi-tions represent a metastable state with composi-tions of micas determined by a combination ofphengitic ferrimuscovitic and illitic vectors asshown by Fig 14ab

Paragonite is generally considered an indexmineral for the anchizone (Frey 1987) but Li etal (1994a) have shown that paragonite coexistswith K-rich mica in grades as low as that of the latediagenetic zone for a prograde sequence Samplesin which muscovite paragonite and intermediateNa-K mica coexist as in sample Es-5 areconsistent with metastability both because morethan two such micas coexist and because theircompositions are incompatible with the solvus inthe muscovite-paragonite system (see below) Whilethe 2M polytype typical of metamorphic micas is

the most common in these samples the 1Md

polytype also occurs in some diagenetic samplesparticularly those from the Morron de Totana UnitThe 1Md polytype is metastable relative to the 2Mpolytype and the coexistence of both polytypesfurther demonstrates a lack of equilibrium at the nmscale Dong amp Peacor (1996) showed that thedisorder noted in the diagenetic phyllosilicates bySAED is a consequence of the general high entropyof phases formed at very low temperatures thatproduces for example stacking defects separatingpackets with no more than 10 layers These packetsprogressively thicken with increasing metamorphicgrade and the 2M polytype then becomes the mostcommon polytype in authigenic micas Thenm-scale size of the lower-grade clays and theregular increase in size with increasing gradereflect the lack of textural equilibrium in the low-grade samples

A wide range of sample characteristics thusimply that the lower-grade samples are in statesof metastable equilibrium those factors includingcoexistence of mature mica with I-S and illitevariable chemical compositions incompatiblecompositions of white micas small crystal sizesdefect-rich states and polytypism Such dataemphasize the notion of lsquoreaction progressrsquo relative

FIG 16 (a) Plot of Al vs Fe+Mg contents of chlorites illustrating the contrast in composition between tri-trioctahedral clinochlore or chamosite and di-trioctahedral sudoite (b) plot of Si vs Fe(Fe+Mg) ratio of

chlorites showing an absence of compositional trends throughout the sampled sequence

18 I Abad et al

to Ostwald-step rule processes metastable dioctahe-dral clay systems undergoing reactions toward thestate of stable chemical and textural equilibriumapproached in greenschist-facies samples (Essene ampPeacor 1995)

Prograde evolution based on TEM datacorrelation with the Basque-Cantabriansequence (Northern Spain)

With increasing grade TEM data confirm thatcrystal thickness increases crystal defect densitydecreases compositions become less variable andmicrotextural relations are simpler (eg Peacor1992) This trend is consistent with three well-characterized low-grade settings the Gulf Coastsequence (Ahn amp Peacor 1986) and the WelshBasin (Merriman et al 1990) both extensionalbasins and the Helvetic Alps (Livi et al 1997)The latter is a fold-and-thrust belt with similarseries of prograde transitions to those observed inSierra Espuna in which pelites have passed througha sequence of metastable states in the lithologicalsequence mudstone and shale slate and phyllite

The mudstones of the Morron de Totana Unit arecharacterized by the presence of illite-smectite R4mixed-layers mica packets of no more than 10 ndash40layers and the occurrence of 1Md polytypism TheLa Santa Unit samples represent a step in theprograde evolution with thicker packets than in theprevious samples (200 ndash700 AEcirc ) illitic or phengiticcompositions the absence of illite-smectite mixedlayers as well as the presence of discreteintermediate Na-K micas and paragonite Althoughthere is a tendency towards non-preferred orienta-tion of crystals it is not so evident as in the Morronde Totana samples In the Yechar Unit (anchizone)and more clearly in the Los Molinos phyllites(greenschist facies) the microtextural relations areless complex the compositions are less variablewith phengite and trioctahedral chlorite crystalthickness are greater (gt400 AEcirc in the Yechar Unitand gt1000 AEcirc in the Los Molinos Unit) with thepackets being defect-free and with a very crystallineaspect This description has features in commonwith the three well-characterized low-grade settingspreviously noted Only the mineral assemblagesvary somewhat presumably as a function of thedifferences in bulk-rock composition

A detailed comparison with the Basque-Cantabrian Basin (N Spain) shows that the lowest-grade conditions of Sierra Espuna overlap most of

those of that sequence which is characterized bypassive burial diagenesis (8000 m thick) (Nieto etal 1996) Only in the deepest sample of theBasque-Cantabrian sequence is illite-smectite mixedlayering absent Figure 12 of this study is verysimilar to Fig 7 in Nieto et al (1996) and bothimages correspond to diagenetic samples fromdifferent settings nevertheless they illustratesimilar mica packets 100 ndash200 AEcirc thick withparallel-to-subparallel boundaries the orientationof which is inferred to represent beddingTherefore there is a parallelism between thesesequences down to the scale of individual clay-mineral grains even though they developed underdifferent conditions with stress presumably playinga significant role in the Sierra Espuna units Thetexture of the least metamorphosed sample (Fig 13)is reminiscent of the typical textural relations ofsmectite described by Nieto et al (1996) (seeFig 4) Although it corresponds to illite-richinterstratified illite-smectite the clay mineralgrains conserved the grain size and anastomosingdiscontinuous array of original smectite layers

Despite the similarities between the twosequences the chemical behaviour of the micasare clearly different since the grade correspondingto the phengitic compositions of the Yechar Unit isnot reached in the Basque-Cantabrian Basin wherethe deepest and highest-grade sample has illiticcompositions equivalent to those of the La SantaUnit That is in a passive burial metamorphiccontext as in the Basque-Cantabrian Basin thefactors responsible for increasing the rate ofchemical reactions are less significant than in thesequences that were affected by tectonic stressTectonic stress introduces defects which in turnincreases the rate of dissolution of strained grainsthus increasing the rate of dissolution-crystallizationreactions typical of clay minerals Perhaps moreimportantly regional tectonic stress is known tocause mobilization of fluids (eg Oliver 1986) Thepresence and activity of such fluids is a prime causeof increased rates of dissolution ion diffusion andcrystallization Tectonic activity thus may have atwo-fold effect in increasing rate of clay-mineralreactions The phengite component may in part be ameasure of the reaction progress depending on thereaction kinetics in very low-grade rocksAccording to Merriman amp Peacor (1999) whereheat flow is low as in accretionary settingsphengite contents are typically high perhapsbecause there is insufficient thermal energy to

Prograde and retrograde metamorphism 19

cause equilibration of the phengite component ofneoformed illite by contrast with extensional basinswhere this component is typically small

Coexistence of trioctahedral chlorite andsudoite Implications for multiple metamorphicevents

In metamorphic rocks the coexistence of mineralsthat formed at different times and conditions on aPTt path ie not in equilibrium is a commonlyobserved feature where the metamorphic history iscomplex Gross differences in the grain size ofcoexisting phases has long been recognized bymetamorphic petrologists as an indicator of non-equilibrium mineral assemblages Historically muchless attention has been paid to such relations in sub-greenschist-facies rocks because prior to the intro-duction of SEM+TEM techniques their small grainsize prevented observations of textural relationsStudy of such rocks eg by XRD provides littleinformation on textural relations beyond determina-tion of degree of preferred orientation

Study by XRD of the samples showed thatsudoite and trioctahedral chlorite coexist If theyhad formed at the same time and under the sameP-T conditions their compositions would define thelimbs of a solvus The TEM images show howeverthat wherever they are in contact the sudoitetransects the packets of trioctahedral chlorite (egFig 8) Furthermore as illustrated in Fig 8 thetrioctahedral chlorite is commonly deformed anddisplays strain contrast in TEM images Thesetextural relations suggest that sudoite was intro-duced subsequent to the formation of trioctahedralchlorite Similar observations were made byGiorgetti et al (1998) who concluded that texturesin Verrucano metasediments showed that sudoitepost-dated chloritoid pyrophyllite and trioctahedralchlorite ie sudoite is in disequilibrium with thelatter minerals and formed as a retrograde phase Inrocks of the Sierra Espuna area differences in bulk-rock composition are small (Table 2) and do notcorrespond to the presence or absence of sudoite(Table 1) ie the random occurrence of sudoiteindependent of bulk-rock composition is compa-tible with a lack of equilibrium relations Theserelations collectively imply that sudoite wasintroduced at some time subsequent to theformation of mineral assemblages at peak meta-morphic conditions On the other hand the size ofsudoite packets is similar to that of trioctahedral

chlorite as consistent with formation at conditionsnot much different from those for chlorite

In samples from the Morron de Totana bothsudoite and trioctahedral chlorite were affected byretrograde alteration to smectite The TEM imagesshow sudoite (Fig 11) and trioctahedral chlorite(Fig 10) with interstratified layers of smectite withspacings of 10 AEcirc due to collapse and dehydration inthe TEM environment and even packets of smectite150 AEcirc thick within chlorite crystals These relationsimply that both sudoite and trioctahedral chloritehave been affected by retrograde alteration at thevery low temperatures consistent with smectiteformation although the presence of more subtlealteration features in sudoite of some samplessuggests that it was less susceptible to alteration

We therefore conclude that at least three episodesof phyllosilicate formation can be recognized inrocks from the Sierra Espuna-area Trioctahedralchlorite was presumably produced at the highestmetamorphicdiagenetic grade in equilibrium withparagonite and phengite Subsequently sudoite wasintroduced but at P-T conditions which could nothave been much different than those of peakmetamorphic grade Finally at very low-T condi-tions hydrothermal fluids caused partial alterationof trioctahedral chlorite and sudoite to smectite

Retrograde products formed by the reaction offluids with mineral phases generated during anearlier prograde event were described previouslyby Nieto et al (1994) and Zhao et al (1999) Thosereactions occurred on a regional scale but similarreactions have been observed on a more localizedscale The fluid activity associated with a majorfault zone is a common source for these reactions(eg Jiang et al 1990) The structure of SierraEspuna (Fig 1) is based on imbrication processesthat superimpose units that have subsequently beenaffected by faults and folds and that have under-gone significant uplift at least 800 m from the lateMiocene to the present (Sanz de Galdeano et al2001) Given this evolution it is easy to justify theretrograde processes that usually seem to be relatedwith tectonic events during which fluid flow andfluidrock ratios are enhanced (Merriman amp Peacor1999) Moreover according to Nieto et al (1994)the uplift of the Betic Cordillera would furnish theintroduction of phreatic water into the systemproducing a reactive environment adequate for thegenesis of smectite Geological conditions aretherefore consistent with post-peak metamorphicconditions as implied by the TEM observations

20 I Abad et al

The introduction of sudoite as a major phaseduring metamorphism implies the possibility ofadditional overprints on a simple prograde assem-blage the evidence for which might be subtle TheNa-rich micas at all grades have compositionswhich either fall within the solvus or are too Na-rich to correspond to the limb of the solvus In thehighest-grade rocks such micas coexist withmuscovite Those relations are incompatible withchemical equilibrium despite the fact that relationsfor muscovite and other minerals imply an approachto chemical equilibrium Furthermore there is aclose association of Na-rich micas with sudoite (egFig 7) Those relations imply the possibility thatNa-micas were introduced concomitant withsudoite P Mata (pers comm) has shown forexample that overprinting of an epizonal assem-blage by hydrothermal solutions resulted in theintroduction of both an Al-rich variety of chlorite(cookeite) and paragonite in pelites of the CamerosBasin Spain The textural relations such as cross-cutting textures which are so striking for sudoitewere not observed for Na-rich micas Neverthelessthe possibility of such overprinting must beconsidered

In spite of all these prograde-retrogradeprocesses in the Los Molinos Unit a smallnumber of homogeneous phases coexist in chemicalequilibrium and the fine-grained metamorphicphyllosilicates are clearly parallel to cleavage witha typical metamorphic aspect Finally all thetextural and compositional changes that haveoperated in the Espuna area during the metamorphicevolution proceed toward a simpler system repre-senting both textural and chemical equilibriumwhich is characteristic of greenschist-facies condi-tions

Roles of detrital and authigenic phyllosilicatesduring slaty cleavage development

Van der Pluijm et al (1998) focused on therespective roles of the phyllosilicates of detritalorigin and those formed during diagenesis andmetamorphism and observed that detrital phyllosi-licates remain preferentially oriented parallel tobedding in pelites with slaty cleavage whereas fine-grained metamorphic phyllosilicates are orientedparallel to cleavage

Although it is usually difficult to determine thepreferred orientations of phyllosilicates in TEMimages as the grade increases better-aligned

packets occur in the cleavage orientation Thetexture of the least metamorphosed sample inwhich even incipient slaty cleavage is absent isreminiscent of the typical textural relation ofsmectites described in the literature The illite-smectite mixed-layers conserve the textural featuresof possible previous smectite such as anastomosingfringes curved and discontinuous with variations ininterplanar spacing d001 These features indicatethat the minimum energy conditions to initiatemechanical transformations have not been reachedin the lower-grade samples as postulated by Ho etal (1996) since slaty cleavage develops as a resultof interactive mechanical deformation and ther-mally driven dissolution and crystallizationprocesses (Knipe 1981) In contrast in the LosMolinos Unit large crystals of chlorite and whitemica (gt1000 AEcirc ) occur which are of metamorphicorigin but with no detrital grains (Fig 3a)Phyllosilicates are preferentially oriented parallelto slaty cleavage in response to increasedtemperature and tectonic strain (Merriman ampPeacor 1999) The oriented microfabrics and slatycleavage well developed in the rocks of this unitmust be a product of a higher thermal and strainenergy environment reached in the deepest part ofthe sequence As pointed out by Van der Pluijm etal (1998) a continuous range of orientations isevidence for mechanical rotation of grains whereasthe lack of such transitional orientations impliesthat cleavage-parallel phyllosilicates originatedthrough dissolution of bedding-parallel and crystal-lization in the slaty cleavage orientation Asobserved in this study for the Intermediate Unitsgrains with orientations which are transitional frombedding-parallel to slaty cleavage orientation areonly rarely observed Cleavage formation is there-fore inferred to have occurred entirely or nearlyentirely through dissolution and crystallization

ACKNOWLEDGMENTS

We thank MM Abad Ortega from the Centro deInstrumentacio n Cient otilde fica of the Universidad deGranada for her help with HRTEM and I Nieto forher essential help with sample preparation Financialsupport was supplied by Research Project nordm BT 2000-0582 FPI research grant to IA both of the SpanishMinistry of Science and Technology Research GroupRNM-0179 of the Junta de Andaluc otildeacutea and to DRP byNSF grant EAR-9814391

Prograde and retrograde metamorphism 21

REFERENCES

Ahn JH amp Peacor DR (1986) Transmission andanalytical electron microscopy of the smectite-to-illite transition Clays and Clay Minerals 34165 ndash179

Azanon JM amp Crespo-Blanc A (2000) Exhumationduring a continental collision inferred from thetectonometamorphic evolution of the AlpujarrideComplex in the central Betics (Alboran Domain SESpain) Tectonics 19 549 ndash565

Bevins RE Robinson D amp Rowbotham G (1991)Compositional variations in mafic phyllosilicatesfrom regional low-grade metabasites and applicationof the chlorite geother mometer Journal ofMetamorphic Geology 9 711 ndash721

Cliff G amp Lorimer GW (1975) The quantitativeanalysis of thin specimens Journal of Microscopy 103 203 ndash207

Crowley SF (1991) Diagenetic modification of detritalmuscovite an example from the Great LimestoneCyclothem (Carboniferous) of Co Durham UKClay Minerals 26 91 ndash103

Dong H amp Peacor DR (1996) TEM observations ofcoherent stacking relations in smectite and illite ofshales evidence for MacEwan crystallites anddominance of 2M1 polytypes Clays and ClayMinerals 44 257 ndash275

Essene E amp Peacor DR (1995) Clay mineral thermo-metry a critical perspective Clays and ClayMinerals 43 540 ndash553

Fransolet AM amp Bourguignon P (1978) Dioctahedralchlorite in quartz veins from Ardennes BelgiumThe Canadian Mineralogist 16 365 ndash373

Fransolet AM amp Schreyer W (1984) Sudoite ditrioctahedral chlorite a stable low-temperature phasein the system MgO-Al2O3-SiO2-H2O Contributionsto Mineralogy and Petrology 86 409 ndash417

Frey M (1987) Very low-grade metamorphism of clasticsedimentary rocks Pp 9 ndash58 in Low TemperatureMetamorphism (M Frey editor) Blackie GlasgowUK

Giorgetti G Goffe B Memmi I amp Nieto F (1998)Metamorphic evolution of Verrucano metasedimentsin northern Apennines new petrological constraintsEuropean Journal of Mineralogy 10 1295 ndash1308

Guidotti CV amp Sassi FP (1986) Classification andcorrelation of metamorphic facies series by means ofmuscovite b0 data from low grade metapelites NeuesJahrbuch fur Mineralogie Abhandlungen 153363 ndash380

Guidotti CV Yates MG Dyar MD amp Taylor ME(1994) Petrogenetic implications of the Fe3+ contentof muscovi te in pel it ic sch is ts AmericanMineralogist 79 793 ndash795

Guthrie GD amp Veblen DR (1989) High resolutionelectron microscopy of mixed-layer illitesmectite

Computer simulations Clays and Clay Minerals 371 ndash11

Ho N Peacor DR amp Van der Pluijm BA (1996)Contrasting roles of detrital and authigenic phyllo-silicates during slaty cleavage development Journalof Structural Geology 18 615 ndash623

Jiang WT Peacor DR Merriman RJ amp Roberts B(1990) Transmission and analytical electron micro-scopic study of mixed layer illitesmectite formed asan apparent replacement product of diagenetic illiteClays and Clay Minerals 38 449 ndash468

Jiang WT Peacor DR amp Buseck PR (1994) Chloritegeothermometry Contamination and apparent octa-hedral vacancies Clays and Clay Minerals 42593 ndash605

Kisch HJ (1991) Development of slaty cleavage anddegree of very-low-grade metamorphism a reviewJournal of Metamorphic Geology 9 735 ndash750

Knipe JR (1981) The interaction of deformation andmetamorphi sm in slates Tectonophysics 78249 ndash272

Kretz R (1983) Symbols for rock-forming mineralsAmerican Mineralogist 68 277 ndash279

Li G Peacor DR Merriman RJ amp Roberts B (1994a)The diagenetic to low grade metamorphism evolu-tion of matrix white mica in the system muscovite-paragonite in a mudrock from Central Wales UKClays and Clay Minerals 42 369 ndash381

Li G Peacor DR Merriman RJ Roberts B amp Vander Pluijm BA (1994b) TEM and AEM constraintson the origin and significance of chlorite-mica stacksin slates an example from Central Wales UKJournal of Structural Geology 16 1139 ndash1157

Livi KJT Veblen DR Ferry JM amp Frey M (1997)Evolution of 21 layered silicates in low-grademetamorphosed Liass ic sha les of CentralSwitzerland Journal of Metamorphic Geology 15323 ndash344

Lonergan L (1991) Structural evolution of the SierraEspuna Betic Cordillera SE Spain PhD thesisOxford University UK

Lonergan L Platt JP amp Gallagher L (1994) TheInternal-External Zone Boundary in the eastern BeticCordillera SE Spain Journal of Structural Geology16 175 ndash188

Makel GH (1981) Differences in tectonic evolution ofsuperimposed Malaguide and Alpujarride tectonicunits in the Espuna area (Betic Cordilleras Spain)Geology in Mijnbouw 60 203 ndash208

Makel GH (1985) The geology of the MalaguideComplex and its bearing on the geodynamicevolution of the Betic-Rif orogen (southern Spainand northern Morocco) GUA papers of Geology Ser1 22 263 pp

Makel GH amp Rondeel HE (1979) Differences instratigraphy and metamorphism between superposedMalaguide and Alpujarride units in the Espuna area

22 I Abad et al

reached stable chemical or textural equilibrium atleast at the mm scale In addition the lack ofchemical equilibrium among diagenetic I-S andillite at the nm scale is demonstrated by their highlyvariable Fe+Mg contents (006ndash075 apfu) Theaverages of the sum of interlayer cations are closeto 1 apfu in the Los Molinos and Yechar units asconsistent with mica but values of 06 apfucorresponding to I-S or illitic compositions are alsopresent in the Morron de Totana Unit Except forsamples Es-102 and Es-53 the variable composi-tions represent a metastable state with composi-tions of micas determined by a combination ofphengitic ferrimuscovitic and illitic vectors asshown by Fig 14ab

Paragonite is generally considered an indexmineral for the anchizone (Frey 1987) but Li etal (1994a) have shown that paragonite coexistswith K-rich mica in grades as low as that of the latediagenetic zone for a prograde sequence Samplesin which muscovite paragonite and intermediateNa-K mica coexist as in sample Es-5 areconsistent with metastability both because morethan two such micas coexist and because theircompositions are incompatible with the solvus inthe muscovite-paragonite system (see below) Whilethe 2M polytype typical of metamorphic micas is

the most common in these samples the 1Md

polytype also occurs in some diagenetic samplesparticularly those from the Morron de Totana UnitThe 1Md polytype is metastable relative to the 2Mpolytype and the coexistence of both polytypesfurther demonstrates a lack of equilibrium at the nmscale Dong amp Peacor (1996) showed that thedisorder noted in the diagenetic phyllosilicates bySAED is a consequence of the general high entropyof phases formed at very low temperatures thatproduces for example stacking defects separatingpackets with no more than 10 layers These packetsprogressively thicken with increasing metamorphicgrade and the 2M polytype then becomes the mostcommon polytype in authigenic micas Thenm-scale size of the lower-grade clays and theregular increase in size with increasing gradereflect the lack of textural equilibrium in the low-grade samples

A wide range of sample characteristics thusimply that the lower-grade samples are in statesof metastable equilibrium those factors includingcoexistence of mature mica with I-S and illitevariable chemical compositions incompatiblecompositions of white micas small crystal sizesdefect-rich states and polytypism Such dataemphasize the notion of lsquoreaction progressrsquo relative

FIG 16 (a) Plot of Al vs Fe+Mg contents of chlorites illustrating the contrast in composition between tri-trioctahedral clinochlore or chamosite and di-trioctahedral sudoite (b) plot of Si vs Fe(Fe+Mg) ratio of

chlorites showing an absence of compositional trends throughout the sampled sequence

18 I Abad et al

to Ostwald-step rule processes metastable dioctahe-dral clay systems undergoing reactions toward thestate of stable chemical and textural equilibriumapproached in greenschist-facies samples (Essene ampPeacor 1995)

Prograde evolution based on TEM datacorrelation with the Basque-Cantabriansequence (Northern Spain)

With increasing grade TEM data confirm thatcrystal thickness increases crystal defect densitydecreases compositions become less variable andmicrotextural relations are simpler (eg Peacor1992) This trend is consistent with three well-characterized low-grade settings the Gulf Coastsequence (Ahn amp Peacor 1986) and the WelshBasin (Merriman et al 1990) both extensionalbasins and the Helvetic Alps (Livi et al 1997)The latter is a fold-and-thrust belt with similarseries of prograde transitions to those observed inSierra Espuna in which pelites have passed througha sequence of metastable states in the lithologicalsequence mudstone and shale slate and phyllite

The mudstones of the Morron de Totana Unit arecharacterized by the presence of illite-smectite R4mixed-layers mica packets of no more than 10 ndash40layers and the occurrence of 1Md polytypism TheLa Santa Unit samples represent a step in theprograde evolution with thicker packets than in theprevious samples (200 ndash700 AEcirc ) illitic or phengiticcompositions the absence of illite-smectite mixedlayers as well as the presence of discreteintermediate Na-K micas and paragonite Althoughthere is a tendency towards non-preferred orienta-tion of crystals it is not so evident as in the Morronde Totana samples In the Yechar Unit (anchizone)and more clearly in the Los Molinos phyllites(greenschist facies) the microtextural relations areless complex the compositions are less variablewith phengite and trioctahedral chlorite crystalthickness are greater (gt400 AEcirc in the Yechar Unitand gt1000 AEcirc in the Los Molinos Unit) with thepackets being defect-free and with a very crystallineaspect This description has features in commonwith the three well-characterized low-grade settingspreviously noted Only the mineral assemblagesvary somewhat presumably as a function of thedifferences in bulk-rock composition

A detailed comparison with the Basque-Cantabrian Basin (N Spain) shows that the lowest-grade conditions of Sierra Espuna overlap most of

those of that sequence which is characterized bypassive burial diagenesis (8000 m thick) (Nieto etal 1996) Only in the deepest sample of theBasque-Cantabrian sequence is illite-smectite mixedlayering absent Figure 12 of this study is verysimilar to Fig 7 in Nieto et al (1996) and bothimages correspond to diagenetic samples fromdifferent settings nevertheless they illustratesimilar mica packets 100 ndash200 AEcirc thick withparallel-to-subparallel boundaries the orientationof which is inferred to represent beddingTherefore there is a parallelism between thesesequences down to the scale of individual clay-mineral grains even though they developed underdifferent conditions with stress presumably playinga significant role in the Sierra Espuna units Thetexture of the least metamorphosed sample (Fig 13)is reminiscent of the typical textural relations ofsmectite described by Nieto et al (1996) (seeFig 4) Although it corresponds to illite-richinterstratified illite-smectite the clay mineralgrains conserved the grain size and anastomosingdiscontinuous array of original smectite layers

Despite the similarities between the twosequences the chemical behaviour of the micasare clearly different since the grade correspondingto the phengitic compositions of the Yechar Unit isnot reached in the Basque-Cantabrian Basin wherethe deepest and highest-grade sample has illiticcompositions equivalent to those of the La SantaUnit That is in a passive burial metamorphiccontext as in the Basque-Cantabrian Basin thefactors responsible for increasing the rate ofchemical reactions are less significant than in thesequences that were affected by tectonic stressTectonic stress introduces defects which in turnincreases the rate of dissolution of strained grainsthus increasing the rate of dissolution-crystallizationreactions typical of clay minerals Perhaps moreimportantly regional tectonic stress is known tocause mobilization of fluids (eg Oliver 1986) Thepresence and activity of such fluids is a prime causeof increased rates of dissolution ion diffusion andcrystallization Tectonic activity thus may have atwo-fold effect in increasing rate of clay-mineralreactions The phengite component may in part be ameasure of the reaction progress depending on thereaction kinetics in very low-grade rocksAccording to Merriman amp Peacor (1999) whereheat flow is low as in accretionary settingsphengite contents are typically high perhapsbecause there is insufficient thermal energy to

Prograde and retrograde metamorphism 19

cause equilibration of the phengite component ofneoformed illite by contrast with extensional basinswhere this component is typically small

Coexistence of trioctahedral chlorite andsudoite Implications for multiple metamorphicevents

In metamorphic rocks the coexistence of mineralsthat formed at different times and conditions on aPTt path ie not in equilibrium is a commonlyobserved feature where the metamorphic history iscomplex Gross differences in the grain size ofcoexisting phases has long been recognized bymetamorphic petrologists as an indicator of non-equilibrium mineral assemblages Historically muchless attention has been paid to such relations in sub-greenschist-facies rocks because prior to the intro-duction of SEM+TEM techniques their small grainsize prevented observations of textural relationsStudy of such rocks eg by XRD provides littleinformation on textural relations beyond determina-tion of degree of preferred orientation

Study by XRD of the samples showed thatsudoite and trioctahedral chlorite coexist If theyhad formed at the same time and under the sameP-T conditions their compositions would define thelimbs of a solvus The TEM images show howeverthat wherever they are in contact the sudoitetransects the packets of trioctahedral chlorite (egFig 8) Furthermore as illustrated in Fig 8 thetrioctahedral chlorite is commonly deformed anddisplays strain contrast in TEM images Thesetextural relations suggest that sudoite was intro-duced subsequent to the formation of trioctahedralchlorite Similar observations were made byGiorgetti et al (1998) who concluded that texturesin Verrucano metasediments showed that sudoitepost-dated chloritoid pyrophyllite and trioctahedralchlorite ie sudoite is in disequilibrium with thelatter minerals and formed as a retrograde phase Inrocks of the Sierra Espuna area differences in bulk-rock composition are small (Table 2) and do notcorrespond to the presence or absence of sudoite(Table 1) ie the random occurrence of sudoiteindependent of bulk-rock composition is compa-tible with a lack of equilibrium relations Theserelations collectively imply that sudoite wasintroduced at some time subsequent to theformation of mineral assemblages at peak meta-morphic conditions On the other hand the size ofsudoite packets is similar to that of trioctahedral

chlorite as consistent with formation at conditionsnot much different from those for chlorite

In samples from the Morron de Totana bothsudoite and trioctahedral chlorite were affected byretrograde alteration to smectite The TEM imagesshow sudoite (Fig 11) and trioctahedral chlorite(Fig 10) with interstratified layers of smectite withspacings of 10 AEcirc due to collapse and dehydration inthe TEM environment and even packets of smectite150 AEcirc thick within chlorite crystals These relationsimply that both sudoite and trioctahedral chloritehave been affected by retrograde alteration at thevery low temperatures consistent with smectiteformation although the presence of more subtlealteration features in sudoite of some samplessuggests that it was less susceptible to alteration

We therefore conclude that at least three episodesof phyllosilicate formation can be recognized inrocks from the Sierra Espuna-area Trioctahedralchlorite was presumably produced at the highestmetamorphicdiagenetic grade in equilibrium withparagonite and phengite Subsequently sudoite wasintroduced but at P-T conditions which could nothave been much different than those of peakmetamorphic grade Finally at very low-T condi-tions hydrothermal fluids caused partial alterationof trioctahedral chlorite and sudoite to smectite

Retrograde products formed by the reaction offluids with mineral phases generated during anearlier prograde event were described previouslyby Nieto et al (1994) and Zhao et al (1999) Thosereactions occurred on a regional scale but similarreactions have been observed on a more localizedscale The fluid activity associated with a majorfault zone is a common source for these reactions(eg Jiang et al 1990) The structure of SierraEspuna (Fig 1) is based on imbrication processesthat superimpose units that have subsequently beenaffected by faults and folds and that have under-gone significant uplift at least 800 m from the lateMiocene to the present (Sanz de Galdeano et al2001) Given this evolution it is easy to justify theretrograde processes that usually seem to be relatedwith tectonic events during which fluid flow andfluidrock ratios are enhanced (Merriman amp Peacor1999) Moreover according to Nieto et al (1994)the uplift of the Betic Cordillera would furnish theintroduction of phreatic water into the systemproducing a reactive environment adequate for thegenesis of smectite Geological conditions aretherefore consistent with post-peak metamorphicconditions as implied by the TEM observations

20 I Abad et al

The introduction of sudoite as a major phaseduring metamorphism implies the possibility ofadditional overprints on a simple prograde assem-blage the evidence for which might be subtle TheNa-rich micas at all grades have compositionswhich either fall within the solvus or are too Na-rich to correspond to the limb of the solvus In thehighest-grade rocks such micas coexist withmuscovite Those relations are incompatible withchemical equilibrium despite the fact that relationsfor muscovite and other minerals imply an approachto chemical equilibrium Furthermore there is aclose association of Na-rich micas with sudoite (egFig 7) Those relations imply the possibility thatNa-micas were introduced concomitant withsudoite P Mata (pers comm) has shown forexample that overprinting of an epizonal assem-blage by hydrothermal solutions resulted in theintroduction of both an Al-rich variety of chlorite(cookeite) and paragonite in pelites of the CamerosBasin Spain The textural relations such as cross-cutting textures which are so striking for sudoitewere not observed for Na-rich micas Neverthelessthe possibility of such overprinting must beconsidered

In spite of all these prograde-retrogradeprocesses in the Los Molinos Unit a smallnumber of homogeneous phases coexist in chemicalequilibrium and the fine-grained metamorphicphyllosilicates are clearly parallel to cleavage witha typical metamorphic aspect Finally all thetextural and compositional changes that haveoperated in the Espuna area during the metamorphicevolution proceed toward a simpler system repre-senting both textural and chemical equilibriumwhich is characteristic of greenschist-facies condi-tions

Roles of detrital and authigenic phyllosilicatesduring slaty cleavage development

Van der Pluijm et al (1998) focused on therespective roles of the phyllosilicates of detritalorigin and those formed during diagenesis andmetamorphism and observed that detrital phyllosi-licates remain preferentially oriented parallel tobedding in pelites with slaty cleavage whereas fine-grained metamorphic phyllosilicates are orientedparallel to cleavage

Although it is usually difficult to determine thepreferred orientations of phyllosilicates in TEMimages as the grade increases better-aligned

packets occur in the cleavage orientation Thetexture of the least metamorphosed sample inwhich even incipient slaty cleavage is absent isreminiscent of the typical textural relation ofsmectites described in the literature The illite-smectite mixed-layers conserve the textural featuresof possible previous smectite such as anastomosingfringes curved and discontinuous with variations ininterplanar spacing d001 These features indicatethat the minimum energy conditions to initiatemechanical transformations have not been reachedin the lower-grade samples as postulated by Ho etal (1996) since slaty cleavage develops as a resultof interactive mechanical deformation and ther-mally driven dissolution and crystallizationprocesses (Knipe 1981) In contrast in the LosMolinos Unit large crystals of chlorite and whitemica (gt1000 AEcirc ) occur which are of metamorphicorigin but with no detrital grains (Fig 3a)Phyllosilicates are preferentially oriented parallelto slaty cleavage in response to increasedtemperature and tectonic strain (Merriman ampPeacor 1999) The oriented microfabrics and slatycleavage well developed in the rocks of this unitmust be a product of a higher thermal and strainenergy environment reached in the deepest part ofthe sequence As pointed out by Van der Pluijm etal (1998) a continuous range of orientations isevidence for mechanical rotation of grains whereasthe lack of such transitional orientations impliesthat cleavage-parallel phyllosilicates originatedthrough dissolution of bedding-parallel and crystal-lization in the slaty cleavage orientation Asobserved in this study for the Intermediate Unitsgrains with orientations which are transitional frombedding-parallel to slaty cleavage orientation areonly rarely observed Cleavage formation is there-fore inferred to have occurred entirely or nearlyentirely through dissolution and crystallization

ACKNOWLEDGMENTS

We thank MM Abad Ortega from the Centro deInstrumentacio n Cient otilde fica of the Universidad deGranada for her help with HRTEM and I Nieto forher essential help with sample preparation Financialsupport was supplied by Research Project nordm BT 2000-0582 FPI research grant to IA both of the SpanishMinistry of Science and Technology Research GroupRNM-0179 of the Junta de Andaluc otildeacutea and to DRP byNSF grant EAR-9814391

Prograde and retrograde metamorphism 21

REFERENCES

Ahn JH amp Peacor DR (1986) Transmission andanalytical electron microscopy of the smectite-to-illite transition Clays and Clay Minerals 34165 ndash179

Azanon JM amp Crespo-Blanc A (2000) Exhumationduring a continental collision inferred from thetectonometamorphic evolution of the AlpujarrideComplex in the central Betics (Alboran Domain SESpain) Tectonics 19 549 ndash565

Bevins RE Robinson D amp Rowbotham G (1991)Compositional variations in mafic phyllosilicatesfrom regional low-grade metabasites and applicationof the chlorite geother mometer Journal ofMetamorphic Geology 9 711 ndash721

Cliff G amp Lorimer GW (1975) The quantitativeanalysis of thin specimens Journal of Microscopy 103 203 ndash207

Crowley SF (1991) Diagenetic modification of detritalmuscovite an example from the Great LimestoneCyclothem (Carboniferous) of Co Durham UKClay Minerals 26 91 ndash103

Dong H amp Peacor DR (1996) TEM observations ofcoherent stacking relations in smectite and illite ofshales evidence for MacEwan crystallites anddominance of 2M1 polytypes Clays and ClayMinerals 44 257 ndash275

Essene E amp Peacor DR (1995) Clay mineral thermo-metry a critical perspective Clays and ClayMinerals 43 540 ndash553

Fransolet AM amp Bourguignon P (1978) Dioctahedralchlorite in quartz veins from Ardennes BelgiumThe Canadian Mineralogist 16 365 ndash373

Fransolet AM amp Schreyer W (1984) Sudoite ditrioctahedral chlorite a stable low-temperature phasein the system MgO-Al2O3-SiO2-H2O Contributionsto Mineralogy and Petrology 86 409 ndash417

Frey M (1987) Very low-grade metamorphism of clasticsedimentary rocks Pp 9 ndash58 in Low TemperatureMetamorphism (M Frey editor) Blackie GlasgowUK

Giorgetti G Goffe B Memmi I amp Nieto F (1998)Metamorphic evolution of Verrucano metasedimentsin northern Apennines new petrological constraintsEuropean Journal of Mineralogy 10 1295 ndash1308

Guidotti CV amp Sassi FP (1986) Classification andcorrelation of metamorphic facies series by means ofmuscovite b0 data from low grade metapelites NeuesJahrbuch fur Mineralogie Abhandlungen 153363 ndash380

Guidotti CV Yates MG Dyar MD amp Taylor ME(1994) Petrogenetic implications of the Fe3+ contentof muscovi te in pel it ic sch is ts AmericanMineralogist 79 793 ndash795

Guthrie GD amp Veblen DR (1989) High resolutionelectron microscopy of mixed-layer illitesmectite

Computer simulations Clays and Clay Minerals 371 ndash11

Ho N Peacor DR amp Van der Pluijm BA (1996)Contrasting roles of detrital and authigenic phyllo-silicates during slaty cleavage development Journalof Structural Geology 18 615 ndash623

Jiang WT Peacor DR Merriman RJ amp Roberts B(1990) Transmission and analytical electron micro-scopic study of mixed layer illitesmectite formed asan apparent replacement product of diagenetic illiteClays and Clay Minerals 38 449 ndash468

Jiang WT Peacor DR amp Buseck PR (1994) Chloritegeothermometry Contamination and apparent octa-hedral vacancies Clays and Clay Minerals 42593 ndash605

Kisch HJ (1991) Development of slaty cleavage anddegree of very-low-grade metamorphism a reviewJournal of Metamorphic Geology 9 735 ndash750

Knipe JR (1981) The interaction of deformation andmetamorphi sm in slates Tectonophysics 78249 ndash272

Kretz R (1983) Symbols for rock-forming mineralsAmerican Mineralogist 68 277 ndash279

Li G Peacor DR Merriman RJ amp Roberts B (1994a)The diagenetic to low grade metamorphism evolu-tion of matrix white mica in the system muscovite-paragonite in a mudrock from Central Wales UKClays and Clay Minerals 42 369 ndash381

Li G Peacor DR Merriman RJ Roberts B amp Vander Pluijm BA (1994b) TEM and AEM constraintson the origin and significance of chlorite-mica stacksin slates an example from Central Wales UKJournal of Structural Geology 16 1139 ndash1157

Livi KJT Veblen DR Ferry JM amp Frey M (1997)Evolution of 21 layered silicates in low-grademetamorphosed Liass ic sha les of CentralSwitzerland Journal of Metamorphic Geology 15323 ndash344

Lonergan L (1991) Structural evolution of the SierraEspuna Betic Cordillera SE Spain PhD thesisOxford University UK

Lonergan L Platt JP amp Gallagher L (1994) TheInternal-External Zone Boundary in the eastern BeticCordillera SE Spain Journal of Structural Geology16 175 ndash188

Makel GH (1981) Differences in tectonic evolution ofsuperimposed Malaguide and Alpujarride tectonicunits in the Espuna area (Betic Cordilleras Spain)Geology in Mijnbouw 60 203 ndash208

Makel GH (1985) The geology of the MalaguideComplex and its bearing on the geodynamicevolution of the Betic-Rif orogen (southern Spainand northern Morocco) GUA papers of Geology Ser1 22 263 pp

Makel GH amp Rondeel HE (1979) Differences instratigraphy and metamorphism between superposedMalaguide and Alpujarride units in the Espuna area

22 I Abad et al

to Ostwald-step rule processes metastable dioctahe-dral clay systems undergoing reactions toward thestate of stable chemical and textural equilibriumapproached in greenschist-facies samples (Essene ampPeacor 1995)

Prograde evolution based on TEM datacorrelation with the Basque-Cantabriansequence (Northern Spain)

With increasing grade TEM data confirm thatcrystal thickness increases crystal defect densitydecreases compositions become less variable andmicrotextural relations are simpler (eg Peacor1992) This trend is consistent with three well-characterized low-grade settings the Gulf Coastsequence (Ahn amp Peacor 1986) and the WelshBasin (Merriman et al 1990) both extensionalbasins and the Helvetic Alps (Livi et al 1997)The latter is a fold-and-thrust belt with similarseries of prograde transitions to those observed inSierra Espuna in which pelites have passed througha sequence of metastable states in the lithologicalsequence mudstone and shale slate and phyllite

The mudstones of the Morron de Totana Unit arecharacterized by the presence of illite-smectite R4mixed-layers mica packets of no more than 10 ndash40layers and the occurrence of 1Md polytypism TheLa Santa Unit samples represent a step in theprograde evolution with thicker packets than in theprevious samples (200 ndash700 AEcirc ) illitic or phengiticcompositions the absence of illite-smectite mixedlayers as well as the presence of discreteintermediate Na-K micas and paragonite Althoughthere is a tendency towards non-preferred orienta-tion of crystals it is not so evident as in the Morronde Totana samples In the Yechar Unit (anchizone)and more clearly in the Los Molinos phyllites(greenschist facies) the microtextural relations areless complex the compositions are less variablewith phengite and trioctahedral chlorite crystalthickness are greater (gt400 AEcirc in the Yechar Unitand gt1000 AEcirc in the Los Molinos Unit) with thepackets being defect-free and with a very crystallineaspect This description has features in commonwith the three well-characterized low-grade settingspreviously noted Only the mineral assemblagesvary somewhat presumably as a function of thedifferences in bulk-rock composition

A detailed comparison with the Basque-Cantabrian Basin (N Spain) shows that the lowest-grade conditions of Sierra Espuna overlap most of

those of that sequence which is characterized bypassive burial diagenesis (8000 m thick) (Nieto etal 1996) Only in the deepest sample of theBasque-Cantabrian sequence is illite-smectite mixedlayering absent Figure 12 of this study is verysimilar to Fig 7 in Nieto et al (1996) and bothimages correspond to diagenetic samples fromdifferent settings nevertheless they illustratesimilar mica packets 100 ndash200 AEcirc thick withparallel-to-subparallel boundaries the orientationof which is inferred to represent beddingTherefore there is a parallelism between thesesequences down to the scale of individual clay-mineral grains even though they developed underdifferent conditions with stress presumably playinga significant role in the Sierra Espuna units Thetexture of the least metamorphosed sample (Fig 13)is reminiscent of the typical textural relations ofsmectite described by Nieto et al (1996) (seeFig 4) Although it corresponds to illite-richinterstratified illite-smectite the clay mineralgrains conserved the grain size and anastomosingdiscontinuous array of original smectite layers

Despite the similarities between the twosequences the chemical behaviour of the micasare clearly different since the grade correspondingto the phengitic compositions of the Yechar Unit isnot reached in the Basque-Cantabrian Basin wherethe deepest and highest-grade sample has illiticcompositions equivalent to those of the La SantaUnit That is in a passive burial metamorphiccontext as in the Basque-Cantabrian Basin thefactors responsible for increasing the rate ofchemical reactions are less significant than in thesequences that were affected by tectonic stressTectonic stress introduces defects which in turnincreases the rate of dissolution of strained grainsthus increasing the rate of dissolution-crystallizationreactions typical of clay minerals Perhaps moreimportantly regional tectonic stress is known tocause mobilization of fluids (eg Oliver 1986) Thepresence and activity of such fluids is a prime causeof increased rates of dissolution ion diffusion andcrystallization Tectonic activity thus may have atwo-fold effect in increasing rate of clay-mineralreactions The phengite component may in part be ameasure of the reaction progress depending on thereaction kinetics in very low-grade rocksAccording to Merriman amp Peacor (1999) whereheat flow is low as in accretionary settingsphengite contents are typically high perhapsbecause there is insufficient thermal energy to

Prograde and retrograde metamorphism 19

cause equilibration of the phengite component ofneoformed illite by contrast with extensional basinswhere this component is typically small

Coexistence of trioctahedral chlorite andsudoite Implications for multiple metamorphicevents

In metamorphic rocks the coexistence of mineralsthat formed at different times and conditions on aPTt path ie not in equilibrium is a commonlyobserved feature where the metamorphic history iscomplex Gross differences in the grain size ofcoexisting phases has long been recognized bymetamorphic petrologists as an indicator of non-equilibrium mineral assemblages Historically muchless attention has been paid to such relations in sub-greenschist-facies rocks because prior to the intro-duction of SEM+TEM techniques their small grainsize prevented observations of textural relationsStudy of such rocks eg by XRD provides littleinformation on textural relations beyond determina-tion of degree of preferred orientation

Study by XRD of the samples showed thatsudoite and trioctahedral chlorite coexist If theyhad formed at the same time and under the sameP-T conditions their compositions would define thelimbs of a solvus The TEM images show howeverthat wherever they are in contact the sudoitetransects the packets of trioctahedral chlorite (egFig 8) Furthermore as illustrated in Fig 8 thetrioctahedral chlorite is commonly deformed anddisplays strain contrast in TEM images Thesetextural relations suggest that sudoite was intro-duced subsequent to the formation of trioctahedralchlorite Similar observations were made byGiorgetti et al (1998) who concluded that texturesin Verrucano metasediments showed that sudoitepost-dated chloritoid pyrophyllite and trioctahedralchlorite ie sudoite is in disequilibrium with thelatter minerals and formed as a retrograde phase Inrocks of the Sierra Espuna area differences in bulk-rock composition are small (Table 2) and do notcorrespond to the presence or absence of sudoite(Table 1) ie the random occurrence of sudoiteindependent of bulk-rock composition is compa-tible with a lack of equilibrium relations Theserelations collectively imply that sudoite wasintroduced at some time subsequent to theformation of mineral assemblages at peak meta-morphic conditions On the other hand the size ofsudoite packets is similar to that of trioctahedral

chlorite as consistent with formation at conditionsnot much different from those for chlorite

In samples from the Morron de Totana bothsudoite and trioctahedral chlorite were affected byretrograde alteration to smectite The TEM imagesshow sudoite (Fig 11) and trioctahedral chlorite(Fig 10) with interstratified layers of smectite withspacings of 10 AEcirc due to collapse and dehydration inthe TEM environment and even packets of smectite150 AEcirc thick within chlorite crystals These relationsimply that both sudoite and trioctahedral chloritehave been affected by retrograde alteration at thevery low temperatures consistent with smectiteformation although the presence of more subtlealteration features in sudoite of some samplessuggests that it was less susceptible to alteration

We therefore conclude that at least three episodesof phyllosilicate formation can be recognized inrocks from the Sierra Espuna-area Trioctahedralchlorite was presumably produced at the highestmetamorphicdiagenetic grade in equilibrium withparagonite and phengite Subsequently sudoite wasintroduced but at P-T conditions which could nothave been much different than those of peakmetamorphic grade Finally at very low-T condi-tions hydrothermal fluids caused partial alterationof trioctahedral chlorite and sudoite to smectite

Retrograde products formed by the reaction offluids with mineral phases generated during anearlier prograde event were described previouslyby Nieto et al (1994) and Zhao et al (1999) Thosereactions occurred on a regional scale but similarreactions have been observed on a more localizedscale The fluid activity associated with a majorfault zone is a common source for these reactions(eg Jiang et al 1990) The structure of SierraEspuna (Fig 1) is based on imbrication processesthat superimpose units that have subsequently beenaffected by faults and folds and that have under-gone significant uplift at least 800 m from the lateMiocene to the present (Sanz de Galdeano et al2001) Given this evolution it is easy to justify theretrograde processes that usually seem to be relatedwith tectonic events during which fluid flow andfluidrock ratios are enhanced (Merriman amp Peacor1999) Moreover according to Nieto et al (1994)the uplift of the Betic Cordillera would furnish theintroduction of phreatic water into the systemproducing a reactive environment adequate for thegenesis of smectite Geological conditions aretherefore consistent with post-peak metamorphicconditions as implied by the TEM observations

20 I Abad et al

The introduction of sudoite as a major phaseduring metamorphism implies the possibility ofadditional overprints on a simple prograde assem-blage the evidence for which might be subtle TheNa-rich micas at all grades have compositionswhich either fall within the solvus or are too Na-rich to correspond to the limb of the solvus In thehighest-grade rocks such micas coexist withmuscovite Those relations are incompatible withchemical equilibrium despite the fact that relationsfor muscovite and other minerals imply an approachto chemical equilibrium Furthermore there is aclose association of Na-rich micas with sudoite (egFig 7) Those relations imply the possibility thatNa-micas were introduced concomitant withsudoite P Mata (pers comm) has shown forexample that overprinting of an epizonal assem-blage by hydrothermal solutions resulted in theintroduction of both an Al-rich variety of chlorite(cookeite) and paragonite in pelites of the CamerosBasin Spain The textural relations such as cross-cutting textures which are so striking for sudoitewere not observed for Na-rich micas Neverthelessthe possibility of such overprinting must beconsidered

In spite of all these prograde-retrogradeprocesses in the Los Molinos Unit a smallnumber of homogeneous phases coexist in chemicalequilibrium and the fine-grained metamorphicphyllosilicates are clearly parallel to cleavage witha typical metamorphic aspect Finally all thetextural and compositional changes that haveoperated in the Espuna area during the metamorphicevolution proceed toward a simpler system repre-senting both textural and chemical equilibriumwhich is characteristic of greenschist-facies condi-tions

Roles of detrital and authigenic phyllosilicatesduring slaty cleavage development

Van der Pluijm et al (1998) focused on therespective roles of the phyllosilicates of detritalorigin and those formed during diagenesis andmetamorphism and observed that detrital phyllosi-licates remain preferentially oriented parallel tobedding in pelites with slaty cleavage whereas fine-grained metamorphic phyllosilicates are orientedparallel to cleavage

Although it is usually difficult to determine thepreferred orientations of phyllosilicates in TEMimages as the grade increases better-aligned

packets occur in the cleavage orientation Thetexture of the least metamorphosed sample inwhich even incipient slaty cleavage is absent isreminiscent of the typical textural relation ofsmectites described in the literature The illite-smectite mixed-layers conserve the textural featuresof possible previous smectite such as anastomosingfringes curved and discontinuous with variations ininterplanar spacing d001 These features indicatethat the minimum energy conditions to initiatemechanical transformations have not been reachedin the lower-grade samples as postulated by Ho etal (1996) since slaty cleavage develops as a resultof interactive mechanical deformation and ther-mally driven dissolution and crystallizationprocesses (Knipe 1981) In contrast in the LosMolinos Unit large crystals of chlorite and whitemica (gt1000 AEcirc ) occur which are of metamorphicorigin but with no detrital grains (Fig 3a)Phyllosilicates are preferentially oriented parallelto slaty cleavage in response to increasedtemperature and tectonic strain (Merriman ampPeacor 1999) The oriented microfabrics and slatycleavage well developed in the rocks of this unitmust be a product of a higher thermal and strainenergy environment reached in the deepest part ofthe sequence As pointed out by Van der Pluijm etal (1998) a continuous range of orientations isevidence for mechanical rotation of grains whereasthe lack of such transitional orientations impliesthat cleavage-parallel phyllosilicates originatedthrough dissolution of bedding-parallel and crystal-lization in the slaty cleavage orientation Asobserved in this study for the Intermediate Unitsgrains with orientations which are transitional frombedding-parallel to slaty cleavage orientation areonly rarely observed Cleavage formation is there-fore inferred to have occurred entirely or nearlyentirely through dissolution and crystallization

ACKNOWLEDGMENTS

We thank MM Abad Ortega from the Centro deInstrumentacio n Cient otilde fica of the Universidad deGranada for her help with HRTEM and I Nieto forher essential help with sample preparation Financialsupport was supplied by Research Project nordm BT 2000-0582 FPI research grant to IA both of the SpanishMinistry of Science and Technology Research GroupRNM-0179 of the Junta de Andaluc otildeacutea and to DRP byNSF grant EAR-9814391

Prograde and retrograde metamorphism 21

REFERENCES

Ahn JH amp Peacor DR (1986) Transmission andanalytical electron microscopy of the smectite-to-illite transition Clays and Clay Minerals 34165 ndash179

Azanon JM amp Crespo-Blanc A (2000) Exhumationduring a continental collision inferred from thetectonometamorphic evolution of the AlpujarrideComplex in the central Betics (Alboran Domain SESpain) Tectonics 19 549 ndash565

Bevins RE Robinson D amp Rowbotham G (1991)Compositional variations in mafic phyllosilicatesfrom regional low-grade metabasites and applicationof the chlorite geother mometer Journal ofMetamorphic Geology 9 711 ndash721

Cliff G amp Lorimer GW (1975) The quantitativeanalysis of thin specimens Journal of Microscopy 103 203 ndash207

Crowley SF (1991) Diagenetic modification of detritalmuscovite an example from the Great LimestoneCyclothem (Carboniferous) of Co Durham UKClay Minerals 26 91 ndash103

Dong H amp Peacor DR (1996) TEM observations ofcoherent stacking relations in smectite and illite ofshales evidence for MacEwan crystallites anddominance of 2M1 polytypes Clays and ClayMinerals 44 257 ndash275

Essene E amp Peacor DR (1995) Clay mineral thermo-metry a critical perspective Clays and ClayMinerals 43 540 ndash553

Fransolet AM amp Bourguignon P (1978) Dioctahedralchlorite in quartz veins from Ardennes BelgiumThe Canadian Mineralogist 16 365 ndash373

Fransolet AM amp Schreyer W (1984) Sudoite ditrioctahedral chlorite a stable low-temperature phasein the system MgO-Al2O3-SiO2-H2O Contributionsto Mineralogy and Petrology 86 409 ndash417

Frey M (1987) Very low-grade metamorphism of clasticsedimentary rocks Pp 9 ndash58 in Low TemperatureMetamorphism (M Frey editor) Blackie GlasgowUK

Giorgetti G Goffe B Memmi I amp Nieto F (1998)Metamorphic evolution of Verrucano metasedimentsin northern Apennines new petrological constraintsEuropean Journal of Mineralogy 10 1295 ndash1308

Guidotti CV amp Sassi FP (1986) Classification andcorrelation of metamorphic facies series by means ofmuscovite b0 data from low grade metapelites NeuesJahrbuch fur Mineralogie Abhandlungen 153363 ndash380

Guidotti CV Yates MG Dyar MD amp Taylor ME(1994) Petrogenetic implications of the Fe3+ contentof muscovi te in pel it ic sch is ts AmericanMineralogist 79 793 ndash795

Guthrie GD amp Veblen DR (1989) High resolutionelectron microscopy of mixed-layer illitesmectite

Computer simulations Clays and Clay Minerals 371 ndash11

Ho N Peacor DR amp Van der Pluijm BA (1996)Contrasting roles of detrital and authigenic phyllo-silicates during slaty cleavage development Journalof Structural Geology 18 615 ndash623

Jiang WT Peacor DR Merriman RJ amp Roberts B(1990) Transmission and analytical electron micro-scopic study of mixed layer illitesmectite formed asan apparent replacement product of diagenetic illiteClays and Clay Minerals 38 449 ndash468

Jiang WT Peacor DR amp Buseck PR (1994) Chloritegeothermometry Contamination and apparent octa-hedral vacancies Clays and Clay Minerals 42593 ndash605

Kisch HJ (1991) Development of slaty cleavage anddegree of very-low-grade metamorphism a reviewJournal of Metamorphic Geology 9 735 ndash750

Knipe JR (1981) The interaction of deformation andmetamorphi sm in slates Tectonophysics 78249 ndash272

Kretz R (1983) Symbols for rock-forming mineralsAmerican Mineralogist 68 277 ndash279

Li G Peacor DR Merriman RJ amp Roberts B (1994a)The diagenetic to low grade metamorphism evolu-tion of matrix white mica in the system muscovite-paragonite in a mudrock from Central Wales UKClays and Clay Minerals 42 369 ndash381

Li G Peacor DR Merriman RJ Roberts B amp Vander Pluijm BA (1994b) TEM and AEM constraintson the origin and significance of chlorite-mica stacksin slates an example from Central Wales UKJournal of Structural Geology 16 1139 ndash1157

Livi KJT Veblen DR Ferry JM amp Frey M (1997)Evolution of 21 layered silicates in low-grademetamorphosed Liass ic sha les of CentralSwitzerland Journal of Metamorphic Geology 15323 ndash344

Lonergan L (1991) Structural evolution of the SierraEspuna Betic Cordillera SE Spain PhD thesisOxford University UK

Lonergan L Platt JP amp Gallagher L (1994) TheInternal-External Zone Boundary in the eastern BeticCordillera SE Spain Journal of Structural Geology16 175 ndash188

Makel GH (1981) Differences in tectonic evolution ofsuperimposed Malaguide and Alpujarride tectonicunits in the Espuna area (Betic Cordilleras Spain)Geology in Mijnbouw 60 203 ndash208

Makel GH (1985) The geology of the MalaguideComplex and its bearing on the geodynamicevolution of the Betic-Rif orogen (southern Spainand northern Morocco) GUA papers of Geology Ser1 22 263 pp

Makel GH amp Rondeel HE (1979) Differences instratigraphy and metamorphism between superposedMalaguide and Alpujarride units in the Espuna area

22 I Abad et al

cause equilibration of the phengite component ofneoformed illite by contrast with extensional basinswhere this component is typically small

Coexistence of trioctahedral chlorite andsudoite Implications for multiple metamorphicevents

In metamorphic rocks the coexistence of mineralsthat formed at different times and conditions on aPTt path ie not in equilibrium is a commonlyobserved feature where the metamorphic history iscomplex Gross differences in the grain size ofcoexisting phases has long been recognized bymetamorphic petrologists as an indicator of non-equilibrium mineral assemblages Historically muchless attention has been paid to such relations in sub-greenschist-facies rocks because prior to the intro-duction of SEM+TEM techniques their small grainsize prevented observations of textural relationsStudy of such rocks eg by XRD provides littleinformation on textural relations beyond determina-tion of degree of preferred orientation

Study by XRD of the samples showed thatsudoite and trioctahedral chlorite coexist If theyhad formed at the same time and under the sameP-T conditions their compositions would define thelimbs of a solvus The TEM images show howeverthat wherever they are in contact the sudoitetransects the packets of trioctahedral chlorite (egFig 8) Furthermore as illustrated in Fig 8 thetrioctahedral chlorite is commonly deformed anddisplays strain contrast in TEM images Thesetextural relations suggest that sudoite was intro-duced subsequent to the formation of trioctahedralchlorite Similar observations were made byGiorgetti et al (1998) who concluded that texturesin Verrucano metasediments showed that sudoitepost-dated chloritoid pyrophyllite and trioctahedralchlorite ie sudoite is in disequilibrium with thelatter minerals and formed as a retrograde phase Inrocks of the Sierra Espuna area differences in bulk-rock composition are small (Table 2) and do notcorrespond to the presence or absence of sudoite(Table 1) ie the random occurrence of sudoiteindependent of bulk-rock composition is compa-tible with a lack of equilibrium relations Theserelations collectively imply that sudoite wasintroduced at some time subsequent to theformation of mineral assemblages at peak meta-morphic conditions On the other hand the size ofsudoite packets is similar to that of trioctahedral

chlorite as consistent with formation at conditionsnot much different from those for chlorite

In samples from the Morron de Totana bothsudoite and trioctahedral chlorite were affected byretrograde alteration to smectite The TEM imagesshow sudoite (Fig 11) and trioctahedral chlorite(Fig 10) with interstratified layers of smectite withspacings of 10 AEcirc due to collapse and dehydration inthe TEM environment and even packets of smectite150 AEcirc thick within chlorite crystals These relationsimply that both sudoite and trioctahedral chloritehave been affected by retrograde alteration at thevery low temperatures consistent with smectiteformation although the presence of more subtlealteration features in sudoite of some samplessuggests that it was less susceptible to alteration

We therefore conclude that at least three episodesof phyllosilicate formation can be recognized inrocks from the Sierra Espuna-area Trioctahedralchlorite was presumably produced at the highestmetamorphicdiagenetic grade in equilibrium withparagonite and phengite Subsequently sudoite wasintroduced but at P-T conditions which could nothave been much different than those of peakmetamorphic grade Finally at very low-T condi-tions hydrothermal fluids caused partial alterationof trioctahedral chlorite and sudoite to smectite

Retrograde products formed by the reaction offluids with mineral phases generated during anearlier prograde event were described previouslyby Nieto et al (1994) and Zhao et al (1999) Thosereactions occurred on a regional scale but similarreactions have been observed on a more localizedscale The fluid activity associated with a majorfault zone is a common source for these reactions(eg Jiang et al 1990) The structure of SierraEspuna (Fig 1) is based on imbrication processesthat superimpose units that have subsequently beenaffected by faults and folds and that have under-gone significant uplift at least 800 m from the lateMiocene to the present (Sanz de Galdeano et al2001) Given this evolution it is easy to justify theretrograde processes that usually seem to be relatedwith tectonic events during which fluid flow andfluidrock ratios are enhanced (Merriman amp Peacor1999) Moreover according to Nieto et al (1994)the uplift of the Betic Cordillera would furnish theintroduction of phreatic water into the systemproducing a reactive environment adequate for thegenesis of smectite Geological conditions aretherefore consistent with post-peak metamorphicconditions as implied by the TEM observations

20 I Abad et al

The introduction of sudoite as a major phaseduring metamorphism implies the possibility ofadditional overprints on a simple prograde assem-blage the evidence for which might be subtle TheNa-rich micas at all grades have compositionswhich either fall within the solvus or are too Na-rich to correspond to the limb of the solvus In thehighest-grade rocks such micas coexist withmuscovite Those relations are incompatible withchemical equilibrium despite the fact that relationsfor muscovite and other minerals imply an approachto chemical equilibrium Furthermore there is aclose association of Na-rich micas with sudoite (egFig 7) Those relations imply the possibility thatNa-micas were introduced concomitant withsudoite P Mata (pers comm) has shown forexample that overprinting of an epizonal assem-blage by hydrothermal solutions resulted in theintroduction of both an Al-rich variety of chlorite(cookeite) and paragonite in pelites of the CamerosBasin Spain The textural relations such as cross-cutting textures which are so striking for sudoitewere not observed for Na-rich micas Neverthelessthe possibility of such overprinting must beconsidered

In spite of all these prograde-retrogradeprocesses in the Los Molinos Unit a smallnumber of homogeneous phases coexist in chemicalequilibrium and the fine-grained metamorphicphyllosilicates are clearly parallel to cleavage witha typical metamorphic aspect Finally all thetextural and compositional changes that haveoperated in the Espuna area during the metamorphicevolution proceed toward a simpler system repre-senting both textural and chemical equilibriumwhich is characteristic of greenschist-facies condi-tions

Roles of detrital and authigenic phyllosilicatesduring slaty cleavage development

Van der Pluijm et al (1998) focused on therespective roles of the phyllosilicates of detritalorigin and those formed during diagenesis andmetamorphism and observed that detrital phyllosi-licates remain preferentially oriented parallel tobedding in pelites with slaty cleavage whereas fine-grained metamorphic phyllosilicates are orientedparallel to cleavage

Although it is usually difficult to determine thepreferred orientations of phyllosilicates in TEMimages as the grade increases better-aligned

packets occur in the cleavage orientation Thetexture of the least metamorphosed sample inwhich even incipient slaty cleavage is absent isreminiscent of the typical textural relation ofsmectites described in the literature The illite-smectite mixed-layers conserve the textural featuresof possible previous smectite such as anastomosingfringes curved and discontinuous with variations ininterplanar spacing d001 These features indicatethat the minimum energy conditions to initiatemechanical transformations have not been reachedin the lower-grade samples as postulated by Ho etal (1996) since slaty cleavage develops as a resultof interactive mechanical deformation and ther-mally driven dissolution and crystallizationprocesses (Knipe 1981) In contrast in the LosMolinos Unit large crystals of chlorite and whitemica (gt1000 AEcirc ) occur which are of metamorphicorigin but with no detrital grains (Fig 3a)Phyllosilicates are preferentially oriented parallelto slaty cleavage in response to increasedtemperature and tectonic strain (Merriman ampPeacor 1999) The oriented microfabrics and slatycleavage well developed in the rocks of this unitmust be a product of a higher thermal and strainenergy environment reached in the deepest part ofthe sequence As pointed out by Van der Pluijm etal (1998) a continuous range of orientations isevidence for mechanical rotation of grains whereasthe lack of such transitional orientations impliesthat cleavage-parallel phyllosilicates originatedthrough dissolution of bedding-parallel and crystal-lization in the slaty cleavage orientation Asobserved in this study for the Intermediate Unitsgrains with orientations which are transitional frombedding-parallel to slaty cleavage orientation areonly rarely observed Cleavage formation is there-fore inferred to have occurred entirely or nearlyentirely through dissolution and crystallization

ACKNOWLEDGMENTS

We thank MM Abad Ortega from the Centro deInstrumentacio n Cient otilde fica of the Universidad deGranada for her help with HRTEM and I Nieto forher essential help with sample preparation Financialsupport was supplied by Research Project nordm BT 2000-0582 FPI research grant to IA both of the SpanishMinistry of Science and Technology Research GroupRNM-0179 of the Junta de Andaluc otildeacutea and to DRP byNSF grant EAR-9814391

Prograde and retrograde metamorphism 21

REFERENCES

Ahn JH amp Peacor DR (1986) Transmission andanalytical electron microscopy of the smectite-to-illite transition Clays and Clay Minerals 34165 ndash179

Azanon JM amp Crespo-Blanc A (2000) Exhumationduring a continental collision inferred from thetectonometamorphic evolution of the AlpujarrideComplex in the central Betics (Alboran Domain SESpain) Tectonics 19 549 ndash565

Bevins RE Robinson D amp Rowbotham G (1991)Compositional variations in mafic phyllosilicatesfrom regional low-grade metabasites and applicationof the chlorite geother mometer Journal ofMetamorphic Geology 9 711 ndash721

Cliff G amp Lorimer GW (1975) The quantitativeanalysis of thin specimens Journal of Microscopy 103 203 ndash207

Crowley SF (1991) Diagenetic modification of detritalmuscovite an example from the Great LimestoneCyclothem (Carboniferous) of Co Durham UKClay Minerals 26 91 ndash103

Dong H amp Peacor DR (1996) TEM observations ofcoherent stacking relations in smectite and illite ofshales evidence for MacEwan crystallites anddominance of 2M1 polytypes Clays and ClayMinerals 44 257 ndash275

Essene E amp Peacor DR (1995) Clay mineral thermo-metry a critical perspective Clays and ClayMinerals 43 540 ndash553

Fransolet AM amp Bourguignon P (1978) Dioctahedralchlorite in quartz veins from Ardennes BelgiumThe Canadian Mineralogist 16 365 ndash373

Fransolet AM amp Schreyer W (1984) Sudoite ditrioctahedral chlorite a stable low-temperature phasein the system MgO-Al2O3-SiO2-H2O Contributionsto Mineralogy and Petrology 86 409 ndash417

Frey M (1987) Very low-grade metamorphism of clasticsedimentary rocks Pp 9 ndash58 in Low TemperatureMetamorphism (M Frey editor) Blackie GlasgowUK

Giorgetti G Goffe B Memmi I amp Nieto F (1998)Metamorphic evolution of Verrucano metasedimentsin northern Apennines new petrological constraintsEuropean Journal of Mineralogy 10 1295 ndash1308

Guidotti CV amp Sassi FP (1986) Classification andcorrelation of metamorphic facies series by means ofmuscovite b0 data from low grade metapelites NeuesJahrbuch fur Mineralogie Abhandlungen 153363 ndash380

Guidotti CV Yates MG Dyar MD amp Taylor ME(1994) Petrogenetic implications of the Fe3+ contentof muscovi te in pel it ic sch is ts AmericanMineralogist 79 793 ndash795

Guthrie GD amp Veblen DR (1989) High resolutionelectron microscopy of mixed-layer illitesmectite

Computer simulations Clays and Clay Minerals 371 ndash11

Ho N Peacor DR amp Van der Pluijm BA (1996)Contrasting roles of detrital and authigenic phyllo-silicates during slaty cleavage development Journalof Structural Geology 18 615 ndash623

Jiang WT Peacor DR Merriman RJ amp Roberts B(1990) Transmission and analytical electron micro-scopic study of mixed layer illitesmectite formed asan apparent replacement product of diagenetic illiteClays and Clay Minerals 38 449 ndash468

Jiang WT Peacor DR amp Buseck PR (1994) Chloritegeothermometry Contamination and apparent octa-hedral vacancies Clays and Clay Minerals 42593 ndash605

Kisch HJ (1991) Development of slaty cleavage anddegree of very-low-grade metamorphism a reviewJournal of Metamorphic Geology 9 735 ndash750

Knipe JR (1981) The interaction of deformation andmetamorphi sm in slates Tectonophysics 78249 ndash272

Kretz R (1983) Symbols for rock-forming mineralsAmerican Mineralogist 68 277 ndash279

Li G Peacor DR Merriman RJ amp Roberts B (1994a)The diagenetic to low grade metamorphism evolu-tion of matrix white mica in the system muscovite-paragonite in a mudrock from Central Wales UKClays and Clay Minerals 42 369 ndash381

Li G Peacor DR Merriman RJ Roberts B amp Vander Pluijm BA (1994b) TEM and AEM constraintson the origin and significance of chlorite-mica stacksin slates an example from Central Wales UKJournal of Structural Geology 16 1139 ndash1157

Livi KJT Veblen DR Ferry JM amp Frey M (1997)Evolution of 21 layered silicates in low-grademetamorphosed Liass ic sha les of CentralSwitzerland Journal of Metamorphic Geology 15323 ndash344

Lonergan L (1991) Structural evolution of the SierraEspuna Betic Cordillera SE Spain PhD thesisOxford University UK

Lonergan L Platt JP amp Gallagher L (1994) TheInternal-External Zone Boundary in the eastern BeticCordillera SE Spain Journal of Structural Geology16 175 ndash188

Makel GH (1981) Differences in tectonic evolution ofsuperimposed Malaguide and Alpujarride tectonicunits in the Espuna area (Betic Cordilleras Spain)Geology in Mijnbouw 60 203 ndash208

Makel GH (1985) The geology of the MalaguideComplex and its bearing on the geodynamicevolution of the Betic-Rif orogen (southern Spainand northern Morocco) GUA papers of Geology Ser1 22 263 pp

Makel GH amp Rondeel HE (1979) Differences instratigraphy and metamorphism between superposedMalaguide and Alpujarride units in the Espuna area

22 I Abad et al

The introduction of sudoite as a major phaseduring metamorphism implies the possibility ofadditional overprints on a simple prograde assem-blage the evidence for which might be subtle TheNa-rich micas at all grades have compositionswhich either fall within the solvus or are too Na-rich to correspond to the limb of the solvus In thehighest-grade rocks such micas coexist withmuscovite Those relations are incompatible withchemical equilibrium despite the fact that relationsfor muscovite and other minerals imply an approachto chemical equilibrium Furthermore there is aclose association of Na-rich micas with sudoite (egFig 7) Those relations imply the possibility thatNa-micas were introduced concomitant withsudoite P Mata (pers comm) has shown forexample that overprinting of an epizonal assem-blage by hydrothermal solutions resulted in theintroduction of both an Al-rich variety of chlorite(cookeite) and paragonite in pelites of the CamerosBasin Spain The textural relations such as cross-cutting textures which are so striking for sudoitewere not observed for Na-rich micas Neverthelessthe possibility of such overprinting must beconsidered

In spite of all these prograde-retrogradeprocesses in the Los Molinos Unit a smallnumber of homogeneous phases coexist in chemicalequilibrium and the fine-grained metamorphicphyllosilicates are clearly parallel to cleavage witha typical metamorphic aspect Finally all thetextural and compositional changes that haveoperated in the Espuna area during the metamorphicevolution proceed toward a simpler system repre-senting both textural and chemical equilibriumwhich is characteristic of greenschist-facies condi-tions

Roles of detrital and authigenic phyllosilicatesduring slaty cleavage development

Van der Pluijm et al (1998) focused on therespective roles of the phyllosilicates of detritalorigin and those formed during diagenesis andmetamorphism and observed that detrital phyllosi-licates remain preferentially oriented parallel tobedding in pelites with slaty cleavage whereas fine-grained metamorphic phyllosilicates are orientedparallel to cleavage

Although it is usually difficult to determine thepreferred orientations of phyllosilicates in TEMimages as the grade increases better-aligned

packets occur in the cleavage orientation Thetexture of the least metamorphosed sample inwhich even incipient slaty cleavage is absent isreminiscent of the typical textural relation ofsmectites described in the literature The illite-smectite mixed-layers conserve the textural featuresof possible previous smectite such as anastomosingfringes curved and discontinuous with variations ininterplanar spacing d001 These features indicatethat the minimum energy conditions to initiatemechanical transformations have not been reachedin the lower-grade samples as postulated by Ho etal (1996) since slaty cleavage develops as a resultof interactive mechanical deformation and ther-mally driven dissolution and crystallizationprocesses (Knipe 1981) In contrast in the LosMolinos Unit large crystals of chlorite and whitemica (gt1000 AEcirc ) occur which are of metamorphicorigin but with no detrital grains (Fig 3a)Phyllosilicates are preferentially oriented parallelto slaty cleavage in response to increasedtemperature and tectonic strain (Merriman ampPeacor 1999) The oriented microfabrics and slatycleavage well developed in the rocks of this unitmust be a product of a higher thermal and strainenergy environment reached in the deepest part ofthe sequence As pointed out by Van der Pluijm etal (1998) a continuous range of orientations isevidence for mechanical rotation of grains whereasthe lack of such transitional orientations impliesthat cleavage-parallel phyllosilicates originatedthrough dissolution of bedding-parallel and crystal-lization in the slaty cleavage orientation Asobserved in this study for the Intermediate Unitsgrains with orientations which are transitional frombedding-parallel to slaty cleavage orientation areonly rarely observed Cleavage formation is there-fore inferred to have occurred entirely or nearlyentirely through dissolution and crystallization

ACKNOWLEDGMENTS

We thank MM Abad Ortega from the Centro deInstrumentacio n Cient otilde fica of the Universidad deGranada for her help with HRTEM and I Nieto forher essential help with sample preparation Financialsupport was supplied by Research Project nordm BT 2000-0582 FPI research grant to IA both of the SpanishMinistry of Science and Technology Research GroupRNM-0179 of the Junta de Andaluc otildeacutea and to DRP byNSF grant EAR-9814391

Prograde and retrograde metamorphism 21

REFERENCES

Ahn JH amp Peacor DR (1986) Transmission andanalytical electron microscopy of the smectite-to-illite transition Clays and Clay Minerals 34165 ndash179

Azanon JM amp Crespo-Blanc A (2000) Exhumationduring a continental collision inferred from thetectonometamorphic evolution of the AlpujarrideComplex in the central Betics (Alboran Domain SESpain) Tectonics 19 549 ndash565

Bevins RE Robinson D amp Rowbotham G (1991)Compositional variations in mafic phyllosilicatesfrom regional low-grade metabasites and applicationof the chlorite geother mometer Journal ofMetamorphic Geology 9 711 ndash721

Cliff G amp Lorimer GW (1975) The quantitativeanalysis of thin specimens Journal of Microscopy 103 203 ndash207

Crowley SF (1991) Diagenetic modification of detritalmuscovite an example from the Great LimestoneCyclothem (Carboniferous) of Co Durham UKClay Minerals 26 91 ndash103

Dong H amp Peacor DR (1996) TEM observations ofcoherent stacking relations in smectite and illite ofshales evidence for MacEwan crystallites anddominance of 2M1 polytypes Clays and ClayMinerals 44 257 ndash275

Essene E amp Peacor DR (1995) Clay mineral thermo-metry a critical perspective Clays and ClayMinerals 43 540 ndash553

Fransolet AM amp Bourguignon P (1978) Dioctahedralchlorite in quartz veins from Ardennes BelgiumThe Canadian Mineralogist 16 365 ndash373

Fransolet AM amp Schreyer W (1984) Sudoite ditrioctahedral chlorite a stable low-temperature phasein the system MgO-Al2O3-SiO2-H2O Contributionsto Mineralogy and Petrology 86 409 ndash417

Frey M (1987) Very low-grade metamorphism of clasticsedimentary rocks Pp 9 ndash58 in Low TemperatureMetamorphism (M Frey editor) Blackie GlasgowUK

Giorgetti G Goffe B Memmi I amp Nieto F (1998)Metamorphic evolution of Verrucano metasedimentsin northern Apennines new petrological constraintsEuropean Journal of Mineralogy 10 1295 ndash1308

Guidotti CV amp Sassi FP (1986) Classification andcorrelation of metamorphic facies series by means ofmuscovite b0 data from low grade metapelites NeuesJahrbuch fur Mineralogie Abhandlungen 153363 ndash380

Guidotti CV Yates MG Dyar MD amp Taylor ME(1994) Petrogenetic implications of the Fe3+ contentof muscovi te in pel it ic sch is ts AmericanMineralogist 79 793 ndash795

Guthrie GD amp Veblen DR (1989) High resolutionelectron microscopy of mixed-layer illitesmectite

Computer simulations Clays and Clay Minerals 371 ndash11

Ho N Peacor DR amp Van der Pluijm BA (1996)Contrasting roles of detrital and authigenic phyllo-silicates during slaty cleavage development Journalof Structural Geology 18 615 ndash623

Jiang WT Peacor DR Merriman RJ amp Roberts B(1990) Transmission and analytical electron micro-scopic study of mixed layer illitesmectite formed asan apparent replacement product of diagenetic illiteClays and Clay Minerals 38 449 ndash468

Jiang WT Peacor DR amp Buseck PR (1994) Chloritegeothermometry Contamination and apparent octa-hedral vacancies Clays and Clay Minerals 42593 ndash605

Kisch HJ (1991) Development of slaty cleavage anddegree of very-low-grade metamorphism a reviewJournal of Metamorphic Geology 9 735 ndash750

Knipe JR (1981) The interaction of deformation andmetamorphi sm in slates Tectonophysics 78249 ndash272

Kretz R (1983) Symbols for rock-forming mineralsAmerican Mineralogist 68 277 ndash279

Li G Peacor DR Merriman RJ amp Roberts B (1994a)The diagenetic to low grade metamorphism evolu-tion of matrix white mica in the system muscovite-paragonite in a mudrock from Central Wales UKClays and Clay Minerals 42 369 ndash381

Li G Peacor DR Merriman RJ Roberts B amp Vander Pluijm BA (1994b) TEM and AEM constraintson the origin and significance of chlorite-mica stacksin slates an example from Central Wales UKJournal of Structural Geology 16 1139 ndash1157

Livi KJT Veblen DR Ferry JM amp Frey M (1997)Evolution of 21 layered silicates in low-grademetamorphosed Liass ic sha les of CentralSwitzerland Journal of Metamorphic Geology 15323 ndash344

Lonergan L (1991) Structural evolution of the SierraEspuna Betic Cordillera SE Spain PhD thesisOxford University UK

Lonergan L Platt JP amp Gallagher L (1994) TheInternal-External Zone Boundary in the eastern BeticCordillera SE Spain Journal of Structural Geology16 175 ndash188

Makel GH (1981) Differences in tectonic evolution ofsuperimposed Malaguide and Alpujarride tectonicunits in the Espuna area (Betic Cordilleras Spain)Geology in Mijnbouw 60 203 ndash208

Makel GH (1985) The geology of the MalaguideComplex and its bearing on the geodynamicevolution of the Betic-Rif orogen (southern Spainand northern Morocco) GUA papers of Geology Ser1 22 263 pp

Makel GH amp Rondeel HE (1979) Differences instratigraphy and metamorphism between superposedMalaguide and Alpujarride units in the Espuna area

22 I Abad et al

REFERENCES

Ahn JH amp Peacor DR (1986) Transmission andanalytical electron microscopy of the smectite-to-illite transition Clays and Clay Minerals 34165 ndash179

Azanon JM amp Crespo-Blanc A (2000) Exhumationduring a continental collision inferred from thetectonometamorphic evolution of the AlpujarrideComplex in the central Betics (Alboran Domain SESpain) Tectonics 19 549 ndash565

Bevins RE Robinson D amp Rowbotham G (1991)Compositional variations in mafic phyllosilicatesfrom regional low-grade metabasites and applicationof the chlorite geother mometer Journal ofMetamorphic Geology 9 711 ndash721

Cliff G amp Lorimer GW (1975) The quantitativeanalysis of thin specimens Journal of Microscopy 103 203 ndash207

Crowley SF (1991) Diagenetic modification of detritalmuscovite an example from the Great LimestoneCyclothem (Carboniferous) of Co Durham UKClay Minerals 26 91 ndash103

Dong H amp Peacor DR (1996) TEM observations ofcoherent stacking relations in smectite and illite ofshales evidence for MacEwan crystallites anddominance of 2M1 polytypes Clays and ClayMinerals 44 257 ndash275

Essene E amp Peacor DR (1995) Clay mineral thermo-metry a critical perspective Clays and ClayMinerals 43 540 ndash553

Fransolet AM amp Bourguignon P (1978) Dioctahedralchlorite in quartz veins from Ardennes BelgiumThe Canadian Mineralogist 16 365 ndash373

Fransolet AM amp Schreyer W (1984) Sudoite ditrioctahedral chlorite a stable low-temperature phasein the system MgO-Al2O3-SiO2-H2O Contributionsto Mineralogy and Petrology 86 409 ndash417

Frey M (1987) Very low-grade metamorphism of clasticsedimentary rocks Pp 9 ndash58 in Low TemperatureMetamorphism (M Frey editor) Blackie GlasgowUK

Giorgetti G Goffe B Memmi I amp Nieto F (1998)Metamorphic evolution of Verrucano metasedimentsin northern Apennines new petrological constraintsEuropean Journal of Mineralogy 10 1295 ndash1308

Guidotti CV amp Sassi FP (1986) Classification andcorrelation of metamorphic facies series by means ofmuscovite b0 data from low grade metapelites NeuesJahrbuch fur Mineralogie Abhandlungen 153363 ndash380

Guidotti CV Yates MG Dyar MD amp Taylor ME(1994) Petrogenetic implications of the Fe3+ contentof muscovi te in pel it ic sch is ts AmericanMineralogist 79 793 ndash795

Guthrie GD amp Veblen DR (1989) High resolutionelectron microscopy of mixed-layer illitesmectite

Computer simulations Clays and Clay Minerals 371 ndash11

Ho N Peacor DR amp Van der Pluijm BA (1996)Contrasting roles of detrital and authigenic phyllo-silicates during slaty cleavage development Journalof Structural Geology 18 615 ndash623

Jiang WT Peacor DR Merriman RJ amp Roberts B(1990) Transmission and analytical electron micro-scopic study of mixed layer illitesmectite formed asan apparent replacement product of diagenetic illiteClays and Clay Minerals 38 449 ndash468

Jiang WT Peacor DR amp Buseck PR (1994) Chloritegeothermometry Contamination and apparent octa-hedral vacancies Clays and Clay Minerals 42593 ndash605

Kisch HJ (1991) Development of slaty cleavage anddegree of very-low-grade metamorphism a reviewJournal of Metamorphic Geology 9 735 ndash750

Knipe JR (1981) The interaction of deformation andmetamorphi sm in slates Tectonophysics 78249 ndash272

Kretz R (1983) Symbols for rock-forming mineralsAmerican Mineralogist 68 277 ndash279

Li G Peacor DR Merriman RJ amp Roberts B (1994a)The diagenetic to low grade metamorphism evolu-tion of matrix white mica in the system muscovite-paragonite in a mudrock from Central Wales UKClays and Clay Minerals 42 369 ndash381

Li G Peacor DR Merriman RJ Roberts B amp Vander Pluijm BA (1994b) TEM and AEM constraintson the origin and significance of chlorite-mica stacksin slates an example from Central Wales UKJournal of Structural Geology 16 1139 ndash1157

Livi KJT Veblen DR Ferry JM amp Frey M (1997)Evolution of 21 layered silicates in low-grademetamorphosed Liass ic sha les of CentralSwitzerland Journal of Metamorphic Geology 15323 ndash344

Lonergan L (1991) Structural evolution of the SierraEspuna Betic Cordillera SE Spain PhD thesisOxford University UK

Lonergan L Platt JP amp Gallagher L (1994) TheInternal-External Zone Boundary in the eastern BeticCordillera SE Spain Journal of Structural Geology16 175 ndash188

Makel GH (1981) Differences in tectonic evolution ofsuperimposed Malaguide and Alpujarride tectonicunits in the Espuna area (Betic Cordilleras Spain)Geology in Mijnbouw 60 203 ndash208

Makel GH (1985) The geology of the MalaguideComplex and its bearing on the geodynamicevolution of the Betic-Rif orogen (southern Spainand northern Morocco) GUA papers of Geology Ser1 22 263 pp

Makel GH amp Rondeel HE (1979) Differences instratigraphy and metamorphism between superposedMalaguide and Alpujarride units in the Espuna area

22 I Abad et al

(Betic Cordilleras Spain) Estudios Geologicos 35109 ndash117

Mart otilde n-Mart otildeacuten M amp Martotildeacuten-Algarra A (1997) Laestructura del area de Sierra Espuna (ContactoZonas Internas-Externas Sector oriental de laCordillera Betica) Estudios Geologicos 53237 ndash248

Merriman RJ amp Peacor DR (1999) Very low-grademetapelites mineralogy microfabrics and measuringreaction progress Pp 10 ndash60 in Low-GradeMetamorphism (M Frey amp D Robinson editors)Blackwell Science Oxford UK

Merriman RJ amp Roberts B (1985) A survey of whitemica crystallinity and polytypes in pelitic rocks ofSnowdonia and Llyn North Wales MineralogicalMagazine 49 305 ndash319

Merriman RJ Roberts B amp Peacor DR (1990) Atransmission electron microscope study of whitemica crystallite size distribution in a mudstone toslate transitional sequence North Wales UKContributions to Mineralogy and Petrology 10627 ndash40

Nieto F (1997) Chemical composition of metapeliticchlorites X-ray diffraction and optical propertyapproach European Journal of Mineralogy 9829 ndash841

Nieto F Velilla N Peacor DR amp Ortega-Huertas M(1994) Regional retrograde alteration of sub-greens-chist facies chlorite to smectite Contributions toMineralogy and Petrology 115 243 ndash252

Nieto F Ortega-Huertas M Peacor DR amp Arostegui J(1996) Evolution of illitesmectite from earlydiagenesis through incipient metamorphism in sedi-ments of the Basque-Cantabrian Basin Clays andClay Minerals 44 304 ndash323

Oliver J (1986) Fluids expelled tectonically from

orogenic belts their role in hydrocarbon migrationand other geologic phenomena Geology 1499 ndash102

Paquet J (1969) Etude geologique de lrsquoOuest de laprovince de Murcie Bulletin de la Societe geologi-que de France 111 270 pp

Peacor DR (1992) Diagenesis and low-grade meta-morphism of shales and slates Pp 113 ndash140 inMinerals and Reactions at the Atomic ScaleTransmission Electron Microscopy (PR Buseckeditor) Reviews in Mineralogy 27 MineralogicalSociety of America Washington DC

Pouchou JL amp Pichoir F (1985) lsquoPAPrsquo (f) (r) (t)procedure for improved quantitative microanalysisPp 104 ndash106 in Microbeam Analysis (JTArmstrong editor) San Francisco Press SanFrancisco California USA

Sanz de Galdeano C Mart otildeacuten-Mart otildeacuten M amp Estevez A(2001) Unidades tectonicas y estructura del sectormeridional de Sierra Espuna (Cordillera BeticaMurcia) Estudios Geologicos 56 269 ndash278

Taylor SR amp McLennan SM (1985) The ContinentalCrust its Composition and Evolution BlackwellOxford UK 312 pp

Van der Pluijm BA Ho NC Peacor DR amp MerrimanRJ (1998) Contradictions of slate formation re-solved Nature 392 348

Warr LN amp Rice HN (1994) Interlaboratory standar-dization and calibration of clay mineral crystallinityand crystallite size data Journal of MetamorphicGeology 12 141 ndash152

Zhao G Peacor DR amp McDowell SD (1999)lsquoRetrograde diagenesisrsquo of clay minerals in thePrecambrian Freda sandstone Wisconsin Claysand Clay Minerals 47 119 ndash130

Prograde and retrograde metamorphism 23