Reiner Klemd á Michael BroÈ cker

Fluid in¯uence on mineral reactions in ultrahigh-pressure granulites:a case study in the SÂ nie_znik Mts. (West Sudetes, Poland)

Received: 15 June 1998 /Accepted: 2 March 1999

Abstract Small tectonic slices of undeformed eclogitesand ultrahigh-pressure granulites occur in three tec-tonic units of the S nie _znik Mts. (SW Poland). Ultrahigh-pressure granulite/eclogite transitions with peak meta-morphic conditions between 21 and 28 kbar at 800 to1000 °C occur only in the Zøote unit. ConventionalU-Pb multigrain analyses of zircons from a ma®cgranulite provided 207Pb/206Pb ages between 360 to369 Ma which are interpreted to approximate timing oforiginal crystallisation from a melt. Di�usion kineticsand the restricted availability of a ¯uid phase mainlycontrolled the conversion from granulite to eclogite,although some bulk-chemical di�erences were also rec-ognised. The ultrahigh-pressure granulites from theZøote unit exclusively contain H2O-rich inclusions withvariable salinities which distinguishes them from high-temperature (HT)-granulites world-wide. This is also incontrast to the ¯uid regime (H2O-N2-CO2) recognised inthe lower-temperature eclogites (600±800 °C) from theclosely associated MieÎdzygo rze and S nie _znik units. Thevariation in ¯uid composition between the lower-tem-perature eclogites and ultrahigh-pressure granulites onthe one hand and ultrahigh-pressure granulites and HT-granulites on the other hand probably indicates con-trasting P-T-t paths as a result of di�erent tectonicenvironments.

Introduction

Metamorphic ¯uids play an important role in high-grademetamorphic processes, since reactions involving a free¯uid phase cannot be used to constrain metamorphicP-T conditions without knowing the ¯uid composition.Fluid inclusions in metamorphic minerals may preservethe composition and/or physical properties of such ¯uids(e.g. Crawford and Hollister 1986; Touret 1981, 1992;Andersen et al. 1993). In combination with mineralogi-cal constraints and textural data, ¯uid inclusions are animportant tool that gives insight into high-grade ¯uid-rock interaction. Many studies have been undertaken onhigh-grade metamorphic rocks, in granulites as well ason eclogites. In general, HT-granulites are dominated byCO2-rich ¯uids (e.g. Touret 1981, 1992; Coolen 1982;Lamb et al. 1987; van Reenen and Hollister 1988; El-vevold and Andersen 1993; Herms and Schenk 1998).Some of them clearly originated during prograde orpeak metamorphism (e.g. Vry and Brown 1991;Srikantappa et al. 1992; Herms and Schenk 1998).However, the importance of high-salinity brines in HT-granulites has been outlined by several authors (e.g.Aranovich et al. 1987; Newton 1995; Aranovich andNewton 1998; Fonarev et al. 1998). On the other hand,¯uid inclusion studies of eclogite-facies rocks indicate adi�erent ¯uid regime consisting of aqueous ¯uids withvariable salinities, and N2 � CO2 (Andersen et al. 1989,1993; Klemd 1989; Klemd et al. 1992, 1994; Selverstoneet al. 1992; Giaramita and Sorensen 1994; Philippotet al. 1995; Perchuk 1995). This apparent contrast in¯uid regimes of granulite- and eclogite-facies rocks issupported by recent studies in the Western Gneiss Re-gion (WGR) of Norway, where Grenvillian granuliteswere subjected to eclogite-facies metamorphism duringthe Caledonian orogeny 600 Ma later (Austrheim 1990).Metastable HT-granulites still contain high-densityCO2-rich inclusions without evidence of free H2O, while¯uids in eclogitised rocks and high-pressure granulitesare dominated by H2O and N2 � CO2. Furthermore,

Contrib Mineral Petrol (1999) 136: 358±373 Ó Springer-Verlag 1999

R. Klemd1 (&)UniversitaÈ t Bremen, FB-Geowissenschaften,Postfach 330440, D-28334 Bremen, Germany

M. BroÈ ckerUniversitaÈ t MuÈ nster, Institut fuÈ r Mineralogie,Zentrallaboratorium fuÈ r Geochronologie,Corrensstr.24, D-48149 MuÈ nster, Germany

Present address:1Mineralogisches Institut, UniversitaÈ t WuÈ rzburg,Am Hubland, D-97074 WuÈ rzburg, Germany

Editorial responsibility: J. Touret

the di�erence in ¯uid regimes between HT-granulitesand eclogite-facies rocks is probably related to di�erenttectonic settings (Andersen et al. 1993).

In order further to explore the ¯uid regimes associ-ated with very high pressure rocks, we have studiedgranulites and eclogites from the S nie _znik Mts. (Fig. 2),where two types of ultrahigh-pressure metamorphicrocks occur: (1) eclogites associated with amphibolite-facies gneisses (S nie _znik- and MieÎdzygo rze units); (2)granulites interbedded with eclogite layers and domains(Zøote unit). In a previous study, the ¯uid regime in theS nie _znik- and MieÎdzygo rze units was documented(Klemd et al. 1995). Our present contribution covers theintimate association of eclogites, ma®c and felsic gran-ulites of the Zøote unit, which was ®rst described inmeticulous detail by Smulikowski (1967). We shall showthat this rock association was subjected almost simul-taneously to P-T conditions of at least 21 to 28 kbar at800 to 1000 °C, in contrast to the eclogites and granu-lites of the Western Gneiss Region (WGR) in Norway.Only limited amounts of H2O, and no CO2 were presentduring high- to ultrahigh-pressure metamorphism of thestudied ultrahigh-pressure granulites which is supportedby the fact that several prograde ``frozen in'' net transferreactions survived ultrahigh-pressure metamorphism.This contradicts ¯uid investigations in HT-granuliteterrains world-wide, but is in accordance with eclogite-facies ¯uid regimes.

Geological setting of the SÂ nie_znik Mts.

The West Sudetes are located along the north-eastern margin of theBohemian Massif (Fig. 1). This tectonically very complex regionencompasses a number of structural-stratigraphic domains, one ofthem being the Orlica-S nie _znik dome which hosts the S ie _znik Mts.(Fig. 1). Ages and deformation histories of this complex are stillunder discussion (e.g. _Zelaz niewicz et al. 1995), as is the relation-ship to the Saxothuringian and Moldanubian Zones (e.g. Alek-sandrowski 1993; _Zelaz niewicz and Franke 1993; Oliver et al.1993).

Eclogites occur in at least three tectonically bounded subunits(Fig. 2): (1) the S nie _znik unit; (2) the MieÎdzygo rze unit; (3) theZøote unit (e.g. Bakun-Czubarow 1992; BroÈ cker and Klemd 1996).Eclogites of the S nie _znik and MieÎdzygo rze units occur as lenses andblocks within amphibolite-facies gneisses (Don et al. 1990). In theZøote unit, very small amounts of eclogites occur intimately asso-ciated with ma®c and felsic granulites (Smulikowski 1967; Poubaet al. 1985; Bakun-Czubarow 1991a, b; Kryza et al. 1996).

Smulikowski (1967), Brueckner et al. (1991) and BroÈ cker andKlemd (1996) interpreted the eclogite-facies metamorphism as an insitu process. In contrast, Don et al. (1990) considered the eclogitesand granulites to be exotic tectonic inclusions within gneisses as thebiotite lineation (L3) is only present in the amphibolitic rims ofeclogite lenses but not in the interior parts. Estimated peak pres-sures in the S nie _znik and MieÎdzygo rze units lie in the coesite sta-bility ®eld with pressures in excess of 27 kbar at temperaturesbetween 700 and 800 °C (BroÈ cker and Klemd 1996). The Zøote unitcontains thin (up to 20 cm) bands of eclogites which are interlay-ered with intermediate to felsic granulites (Smulikowski 1967). Thisinterlayering was interpreted by Bakun-Czubarow (1991b) to be theresult of bimodal volcanism with small contaminations of sedi-mentary material (Bakun-Czubarow 1991b; Brueckner et al. 1991).The peak metamorphic P-T conditions attained in the Zøote unitwere estimated at 800 to 900 °C and at 16 to 22 kbar by Bakun-Czubarow (1991b) and Steltenpohl et al. (1993), while Pouba et al.

Fig. 1 Location of the Orlica-S nie _znik Dome in the centralSudetes after Don et al. (1990)and _Zelaz niewicz et al. (1995).(1 Orlica-S nie _znik Dome, 2low-grade metamorphic rocks,3 Variscan granitoids, 3, 4 Per-mo-Mesozoic cover, 5 Faults:EFZ Elbe Fracture Zone,OFZ Odra Fracture Zone,SMF Sudetic Marginal Fault,RU Ramzovian/MoldanubianThrust. RH Rhenohercynian,SX Saxothuringian, MD Mol-danubian, CR Czech Republic.Square area Fig 2, inlay Bohe-mian massif)

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(1985) reported temperatures of up to 1000 °C. Bakun-Czubarow(1991a) described the presence of former coesite and peak meta-morphic temperatures between 860 and 960 °C, which suggest thatpeak-pressure conditions for the S nie _znik, MieÎdzygo rze and Zøoteunits are not signi®cantly di�erent (BroÈ cker and Klemd 1996).

Using the Sm-Nd method (grt-cpx-whole rock), three eclogitesfrom the S nienik and MieÎdzygo rze units were dated at 341 � 7Ma,337 � 4 Ma, and 324 � 6 Ma (Brueckner et al. 1991). For oneeclogite from the Zøote unit this method yielded an age of352 � 4 Ma.

Sample location and petrography

Due to the extremely poor level of exposure in the Zøote unit,suitable samples could only be collected at Stary Gieraøto w (Fig. 2),where eclogites occurs as 1 to 20 cm thick intercalations and/orpatchy domains within granulites. This outcrop (up to 3 m inlength and 1 m in width) is described in detail by Smulikowski(1967), Smulikowski and Bakun-Czubarow (1973) and Bakun-Czubarow (1991b). They mainly distinguished three rock types: (1)felsic granulites; (2) ma®c granulites with small domains of (3)eclogite. Individual granulite layers range in thickness from severaldecimetres to one centimetre or millimetre. Six samples from thissuccession were studied in detail (Table 1).

The ma®c granulites exhibit the subassemblage garnet,omphacite, quartz, and plagioclase, with biotite, rare white mica asinclusions in omphacite and kyanite as additional phases in varyingmodal proportions (400 points/section; Table 1). Kyanite either

occurs as inclusions in garnet or in the matrix, often showing a thincorona of garnet (Fig. 3). Typical accessories are apatite, rutile,zircon and Fe-oxides. Zoisite, present in most eclogites of the othertectonic units (BroÈ cker and Klemd 1996) is absent in all investi-gated samples. The presence of coesite pseudomorphs is inferredfrom radial fractures around polycrystalline quartz inclusions ingarnet, which were recognised in samples 1107, 1106 and 1041

Fig. 2 Simpli®ed geological map of the S nie_znik Mts. in SW Polandafter Don et al. (1990) and Smulikowski (1967). (I MieÎdzygo rze unit,II S nie_znik unit, III Zøote unit. 1 Mesozoic sediments, 2 S nie_znikaugen gneisses, 3 Gieraøto w gneisses, 4 Stronie-Møynowiec group, 5granulites, 6 Variscan granitoids. SB Stronie S 1aÎskie-Bielice fault, SMStare M�esto fault, SK Stare Kletno fault, RU Ramzovian/Moldan-ubian thrust. Filled dots eclogite localities)

Table 1 Modal mineralogy (vol.%) of the granulites/eclogites ofthe Snieznik Mts. (�present); 1107 contains additional anti-perthite. Abbreviations according to Kretz (1983)

Sample 1041 1041X 1041Xe 1042 1106a 1106a 1107

Grt 24 34 41 35 27 26 12Qtz 29 30 3 40 33 38 45Pl 12 9 ) 5 21 19 31Cpx 32 21 52 18 17 15 +Bt ) 3 2 ) 1 ) +Rt 1 3 2 2 1 2 2Ky 2 ) ) + + + +Kfs ) ) ) ) ) ) 10Phen ) + + ) ) ) )

Fig. 3 Kyanite displaying a slightly kinked cleavage and rutileinclusion (dark, high relief) is surrounded by a garnet corona (Grt)also with rutile inclusions. Omphacite relics (arrows) surround thegarnet coronas. Sample 1106; scale = 0.07 mm; 1 Nicol

Fig. 4 Inclusions of coarse grained polycrystalline quartz and relicticpalisade quartz (arrow) after coesite in garnet. Sample 1041;scale = 0.1 mm; crossed Nicols

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(Fig. 4). Sample 1041X contains a small cm-sized domain of eclo-gite (1041 Xe) with garnet, omphacite, quartz and rutile. This zonegrades into the characteristic granulite assemblage with texturallyprimary plagioclase (Pl 2) and a polygonal-granoblastic texture(Fig. 5a, b). Sample 1107 is an omphacite-free, felsic granulite(granoblastic-polygonal) with matrix K-feldspar and antiperthiticfeldspar inclusions in garnet.

Important microstructural and mineral compositionalcharacteristics

Many details of the petrography and mineral chemistrywere already described in previous studies ofSmulikowski (1967), Smulikowski and Bakun-Czuba-row (1973), Bakun-Czubarow (1991b) and Kryza et al.(1996) and are thus not repeated here. Major composi-tional characteristics of the investigated samples aresummarised in Table 2.

For our interpretation, the following observations areimportant: The jadeite content of omphacite in ma®cgranulites and eclogite varies between 25 and 35 mol%,in some retrograde rims it decreases to 18 mol%. Thesubhedral omphacite in the granulites occurs eitherpoikiloblastic with plagioclase (Pl 1) and biotite inclu-sions, or as spatial vermicular-like intergrowth alwayssurrounding Pl 1 and quartz (Fig. 5c, d). The Pl 1 andquartz inclusions do not show a preferred orientationnor a common extinction thus negating an exsolutionfrom the omphacite host. The Pl 1 displays an An-con-tent between 10 and 14 mol%. The Pl 1 and quartz in-clusions are often intimately intergrown with biotite andphengite, the latter being absent from the matrix. SincePl 1 is formed at the expense of omphacite and quartz, aretrograde formation of Pl 1 and quartz inclusions isunlikely, because both occur in almost equal volumeamounts. Consequently, these inclusions are interpretedto belong to an earlier lower-pressure stage. Almost allrelict Pl 1 is consumed by the formation of inclusion-free, sub- to idioblastic omphacite with straight grainboundaries close to the eclogite domain. The replace-

ment of Pl 1 by inward-directed growth of omphaciteindicates that the central parts of the omphacite maypostdate the rims. Further recrystallisation producedinclusion-free, subidio- to idioblastic omphacite in theeclogite domains (Fig. 5a, b). As already pointed out, inall ma®c granulites some garnet texturally occurs ascoronas around kyanite separating it from omphacite,indicating that this garnet crystallised at the expense ofkyanite and omphacite (Fig. 3). However, it should benoted that the mineral inclusions in omphacite are quitedi�erent than those ones in garnet. Subidioblastic matrixPl 2 occurs in almost all samples and shows a weakzonation with an An-content ranging from 12 to15 mol%. In contact with garnet or garnet coronas theAn-values of plagioclase (Pl 3) are somewhat higher,between 15 and 22 mol%, and the zoning is invertedshowing the highest An-values towards the rim.

The observed textural features indicate that ompha-cite crystallised at the expense of pre-eclogite-facies Pl 1.The recrystallised subidioblastic omphacites are gener-ally unzoned, while the xenoblastic inclusion-richomphacites display unsystematic zoning patterns. Onrare occasions, an uniform increase in Na at the expenseof Ca towards the fringed Pl 1 domain can be observed.The homogenisation of the subidioblastic omphacitesindicates that volume di�usion was enhanced duringrecrystallisation and grain boundary migration. Apartfrom the thin eclogite domains where mineral reactionswere completed, we interpret the textural relations be-tween omphacite and Pl 1 inclusions to represent anincomplete prograde omphacite crystallisation. Conse-quently, the main question arises why the eclogite-forming reactions were completed in the eclogitedomains and not in the surrounding coronitic granulites.

Bulk chemical di�erences allow for three composi-tional garnet groups to be distinguished: (a) almandine-rich garnets (felsic granulite) with the compositionalrange Alm57±63Grs14±30Prp10±20; (b) garnets with inter-mediate almandine content (ma®c granulite) in the rangeAlm50±55Grs23±33Prp13±25; (c) almandine-poor garnets

Table 2 Condensed analyses of minerals used in the geothermobarometric calculation. Micas based on 22 oxygens, clinopyroxene on 6oxygens. Abbreviations according to Kretz (1983)

Matrix garnet Matrix biotite Inclusions in garnet Pl 2

Grt Bt Phen Feldspar Omphacite

XFe XMg XCa XMn Fe2+ Mg Fe2+ Mg XAn XJd Fe2+ Mg

Granulites1106 (rim) 0.52 0.21 0.27 0.01 2.14 2.96 ± ± 0.13 0.18 0.16 0.481106 (core) 0.53 0.22 0.24 0.01 ± ± ± ± 0.12 0.34 0.15 0.361041 (rim) 0.45 0.28 0.26 0.01 1.61 3.51 ± ± 0.18 0.35 0.17 0.491041 (core) 0.44 0.24 0.31 0.01 1.65 3.41 0.45 0.68 0.12 0.35 0.17 0.451107 (rim) 0.57 0.20 0.21 0.01 ± ± ± ± 0.08a ± ± ±1107 (core) 0.62 0.08 0.28 0.02 ± ± ± ± ± ± ± ±

Eclogite1041Xe (rim) 0.49 0.24 0.25 0.01 ± ± ± ± ± 0.22 0.16 0.491041Xe (core) 0.48 0.23 0.20 0.01 1.88 3.11 0.27 0.47 ± 0.32 0.16 0.39

a Inclusion in garnet

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with almost equal proportions of grossular and pyrope(ma®c granulite and eclogite) which display the com-positional range Alm43±50Grs22±32Prp22-30. The garnetsfrom the eclogite domains have often a lower pyropecontent than those from the ma®c granulites (Table 2).This overall pyrope decrease is a result of the increasedmodal amounts of garnet and omphacite in the eclogitedomains. The garnet in sample 1107 displays an exten-sive prograde zoning pattern on a millimetre scale,which is revealed by a smooth decrease in the pyrope/almandine ratios and an increase of the grossular andspessartine content from rim to core domain (Fig. 6a).Similar zoning patterns for the ma®c granulites wereobserved by Bakun-Czubarow (1991 b) and Kryza et al.

(1996). However, the garnet coronas around kyanite inthe eclogitised samples (1041, 1106) usually display noor just a weak zonation as is shown by the decreasinggrossular content and an increasing almandine and py-rope content towards the rims adjacent to Pl 3. The Fe/(Fe + Mg) ratio remains almost constant or slightlydecreases towards the rim (Fig. 6b). This implies only asmall temperature change during corona growth of thegarnet. Some inclusion-free garnets of the eclogites andma®c granulites display a similar zonation towards theouter rim, while the core domain is largely homogeneous(Fig. 6c). In some instances the outermost rims of garnetin contact with biotite show weakly developed reversedzoning pro®les with decreasing pyrope/almandine ratiosand increasing grossular contents (Fig. 6d). This patternis considered to evidence limited re-equilibration as aresult of Mg-Fe exchange between garnet and biotiteduring exhumation. No zonation pattern was recognisedregarding the spessartine component.

The preservation of some growth zoning in the gar-nets of the felsic granulite is very important for the un-derstanding of the reaction kinetics during the P-Tevolution of these rocks, since intergranular di�usion(causes the homogenisation of the garnet composition)usually starts at temperatures of around 750 °C (Spear1988). It is furthermore unusual for the growth zonationin the garnets in the ultrahigh-pressure granulites to bepreserved better than in the garnets of eclogites from the

Fig. 5 a Eclogite domain consisting exclusively of inclusion-freegarnet (grey) and omphacite (dark) with rare kelyphitic rims (black).Only locally omphacite contains vermicular quartz and plagioclaseinclusions (arrows). Sample 1041 Xe; scale = 1.4 mm; 1 Nicol.b Eclogite-textured ma®c granulite with Pl 2 and quartz (light, lowrelief) and biotite (arrows), which occurs in vein-like structuresbetween omphacite (dark) and garnet (grey). Sample 1041;scale = 1.4 mm; 1 Nicol. c Subidioblastic omphacite (dark) with thinkelyphitic rims (black), and few vermicular Pl 1 and quartz inclusions(arrows). Omphacite and garnet (grey) occur intimately intergrownwith Pl 2 and quartz (light). Sample 1041; scale = 0.4 mm; 1 Nicol.d Poikiloblastic, anhedral omphacite (dark) with vermicular Pl 1 andquartz inclusions (light) as well as anhedral, round inclusion-freegarnet (grey) are surrounded by subidioblastic matrix quartz and Pl 2(light). Sample 1106; scale = 1.4 mm; 1 Nicol

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other tectonic units, although these rocks display muchlower temperatures when compared to the granulites(see below).

Antiperthitic feldspars mostly occur as small inclu-sions in the rim zone of garnet and only rarely within thematrix of the felsic granulite. Potassium-feldspar onthe other hand occurs mostly intergrown with Pl 2. Theantiperthite comprises regular rod- to spindle-likeK-feldspar exsolution lamellae (<20 lm wide). To de-termine the bulk composition of the antiperthite, grainswith homogeneous exsolution lamellae were analysedwith a broad electron beam. The resulting alkali feldsparcomposition was Ab65.4Or26.2An8.4. Only homoge-neous albite (Ab86±89Or1±2An10±12) next to K-feldspar(Ab12±15Or85±88An0.4±0.6) occurred as inclusions in sin-gular garnets. Obviously albite and K-feldspar inclusions

are breakdown products during cooling of the initialternary feldspar. Their composition is similar to those ofthe matrix K-feldspar (Ab9±12Or88±91An0.1±0.6) and Pl 2(Ab84±88Or2±3An10±14) suggesting that these feldsparswere also formed during the ternary feldspar breakdown.

The composition of the ternary feldspar is in excellentagreement with the composition of an original ternaryfeldspar (Ab67Or27An6) in similar felsic granulites fromthe Zøote unit as deduced by integrated line scan analysisfrom an anthiperthitic plagioclase (Kryza et al. 1996).Therefore, the ternary feldspar is interpreted to be a partof the peak metamorphic mineral assemblage.

P-T conditions during ultrahigh-pressure metamorphism

The textural-equilibrium mineral assemblage of theeclogite includes garnet-omphacite-rutile-quartz. Thema®c granulites display the peak metamorphic mineral

Fig. 6a±f Compositional zoning patterns in garnets of the ma®cgranulites of the Zøote unit (for details see text)

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assemblage garnet-kyanite-omphacite-quartz-Pl 2, where-as garnet(rim)-Pl 3-omphacite rim (when zoned) belongto a lower-pressure stage. Retrograde mineral phases arebiotite and adjacent garnet (outermost rims). The felsicgranulite displays the textural-equilibrium assemblagegarnet(rim)-kyanite-ternary alkali feldspar-quartz. Ret-rograde phases are Pl 2 and K-feldspar as well as minorbiotite.

Temperature conditions

The onset of prograde omphacite crystallisation proba-bly proceeded somewhere between 500 and 700 °C, as isindicated by the presence of pre-eclogitic biotite, phen-gite (Si up to 3.33 pfu) and Pl 1 inclusions in omphacitein ma®c granulites (see Goldsmith 1982; Spear 1989). Asis outlined below, garnet nucleated at higher P-T con-ditions, which is probably due to the fact that the acti-vation energy for garnet nucleation is much higher thanthat for clinopyroxene (Rubie 1990). This explains thedi�erent mineral inclusions in garnet and omphacite. Toavoid the large uncertainty when considering appropri-ate activity models for omphacite and garnet by usingthermodynamic datasets, we obtained absolute temper-ature estimates for the eclogite and ma®c granulites byMg-Fe exchange equilibrium, using the garnet-clinopy-roxene geothermometers (Ellis and Green 1979; Krogh1988; Fig. 7). All temperatures were calculated using Feafter correction for Fe3+, which was based on charge-balance criteria. Calculated temperatures show a largerange due to the presence of variable amounts of Fe3+

and/or some re-equilibration during retrogression. Thetemperature estimates for the eclogite and ma®c granu-lites were almost identical. Temperatures based on theKrogh calibration are 5 to 60 °C lower than the Ellis and

Green calibration. Using rim-rim compositions ofhomogeneous garnets and unzoned omphacites, the Ellisand Green calibration indicates peak temperatures thatrange between 790±980 °C and 870±1040 °C for as-sumed pressures of 20 and 30 kbar (18 mineral pairs)respectively. However, the Fe/(Fe+Mg) ratio in garnetsalmost stays constant, indicating only a small tempera-ture change during growth of the core and rim of thegarnet. The absence of a prograde growth zonation inmost of the garnets either implies that volume di�usionobliterated all zoning features, or that these garnets grewentirely during the peak of metamorphism at tempera-tures >800 °C.

The two-feldspar geothermometer of Fuhrman andLindsley (1988) was applied to the felsic granulite 1107.The high-temperature ternary feldspar should lie on thesame solvus as a hypothetical coexisting feldspar withequal An-, Ab- and Or-activities of all three componentsat a given temperature and pressure (Kryza et al. 1996;O'Brien et al. 1997). The composition of the hypotheti-cal feldspar on the opposite limb of the solvus is com-position Ab56Or38An6 yielding a minimum temperatureof 990 °C at 27 kbar. Recrystallised matrix K-feldsparand plagioclase which show triple junctions with adja-cent quartz display much lower temperatures between540 and 660 °C at 10 to 15 kbar.

Pressure conditions

The occurrence of possible coesite pseudomorphs asinclusions in garnet and signs for the former presence ofnon-stoichiometric clinopyroxenes (Bakun-Czubarow1991a, 1992) even indicate ultrahigh-pressure conditionsin the SÂ nie _znik Mts. However, it is uncertain at whatspeci®c equilibrium temperature coesite was formed.The coesite pseudomorphs always occur as inclusions ingarnet and, thus, may indicate passage through thecoesite stability before peak-temperature conditionswere reached (also see Bakun-Czubarow 1991a).

During prograde metamorphic conditions at least sixplagioclase breakdown reactions can be proposed foreclogites and ma®c granulites (Bucher and Frey 1994;abbreviations according to Kretz, 1983):

1. Ab = Jd + Qtz;2. An = CaTs + Qtz;3. 3 An = Grs + 2 Ky + Qtz ;4. 3 An + 4 Tr = 3 Prp + 11 Di + 7 Qtz + H2O;5. 3 An + 3 Di = 2 Grs + Prp + 3 Qtz; or6. 4 En + An = Prp + Di + Qtz.

A pressure estimate independent of the H2O-activity canbe deduced from reaction 1 (Holland 1980, 1983). Theplagioclase-breakdown reaction produces a Ca-tscher-maks component which is accommodated by theomphacite (reaction 2). As is shown above, this plagio-clase breakdown reaction is clearly an early progradereaction which only proceeded to completion in theeclogite domains (plagioclase absence). Therefore in the

Fig. 7 P-T estimates for ma®c granulites and eclogites. Samplenumbers in parentheses. See text for detailed description of thedi�erent equilibria and references. (HP high-pressure stage, Si = 3.33phengite barometry)

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absence of plagioclase, the omphaciteJd35-quartz as-semblage indicates only minimum pressures of about20 kbar at 900 °C for the eclogite. Due to the lack ofother pressure sensitive mineral equilibria no furtherpressure estimates are possible for the eclogite. In thema®c granulites omphacite with a jadeite content of35 mol% immediately adjacent to relictic Pl 1 (An14)inclusions indicates equilibrium pressures of 10.5±14.5 kbar (Fig. 7), at temperatures between 500±700 °C.The Si-content of the phengite inclusions in omphaciteindicates minimum pressures between 7 and 11 kbar(Massonne and Schreyer 1987). A pressure of 21.5 kbarat 1000 °C is given by the maximum jadeite content(35 mol%) of omphacite and Pl 2 (An15) for samples1041 and 1106. Further, the plagioclase-garnet-kyanite-quartz (GASP) barometer based on reaction 3 or theplagioclase-clinopyroxene-garnet-quartz (GADS) ba-rometer based on reaction 5 can be used to deducepressures. As discussed in the previous section, homo-geneous garnet cores and homogeneous garnets seem tohave entirely grown during the peak of metamorphismand are interpreted to have been equilibrated with ad-jacent omphacite, kyanite and Pl 2. The internally con-sistent data set of Berman (1988) and mixing modelsoutlined below were used for the calculations. The ac-tivity model of Berman (1990) was applied for garnet,while the mixing model of Fuhrman and Lindsley (1988)was used for plagioclase. The mixing model of Holland(1983) was used for omphacite. Quartz and kyanite ac-tivity are 1. Pressures calculated by the GASP barometerfor samples 1106 and 1041 are 26 and 28 kbar at1000 °C, while pressures derived by the GADS barom-eter are signi®cantly lower, ranging between 21 and23 kbar at the same temperature. Pressures are almostidentical when using the formulation of Newton andHaselton (1981) and Newton and Perkins (1982). TheGASP barometer for the felsic granulite (sample 1107)which contains high-T ternary feldspar but no ompha-cite, reveals a pressure of 27 kbar at 1000 °C usinggarnet rim composition (highest Mg/Fe) and the anort-hite component of the ternary feldspar. The corre-sponding pressure estimates derived by the GASPbarometer for the di�erent samples strongly advocateequilibrium conditions for the chosen mineral assem-blages (Fig. 7).

In summary, geothermobarometric evaluation formatrix assemblages of the eclogites and granulites re-veals peak metamorphic conditions between c. 21 to28 kbar at 800 to 1000 °C.

Post eclogite-facies conditions

The investigated granulites and associated eclogite do-mains from the Zøote unit have undergone remarkablylittle retrograde re-equilibration during post eclogite-facies conditions. Almost no retrograde overprint isdisplayed by the breakdown of primary silicates in allsamples. In some ma®c granulites matrix biotite appears

to be in textural equilibrium with garnet and omphacite,however, it frequently occurs as vein-like intergrowthsalong the grain boundaries of these minerals (Fig. 5b).This suggests that this biotite formed during retrogradeconditions through the consumption of garnet. Al-though no ternary feldspar and/or K-feldspar werefound in this rock type, the biotite may be a breakdownproduct of garnet and former potassium-rich feldspar.

In rare cases, omphacite displays a thin retrograderim. The jadeite content decreases to 18 mol% indicat-ing pressures of 8±12 kbar at temperatures between 500and 740 °C. Steltenpohl et al. (1993) calculated tem-peratures of 630 � 50 °C at pressures of 11 � 1.5 kbarfor secondary garnet-biotite and garnet + hornblende+ plagioclase + quartz assemblages in the granulites.Application of the garnet-biotite geothermometer(Perchuk and Lavrent'eva 1983) for garnet rims incontact with biotite in samples 1041Xe and 1041 re-vealed somewhat higher temperatures of 710 � 45 at10 kbar. The biotite composition was not corrected forFe3+.

BroÈ cker and Klemd (1996) calculated various P-Tpoints re¯ecting the di�erent stages of recrystallisationduring the exhumation process of eclogites from theS nie _znik and MieÎdzygo rze units, which range between4 and 11 kbar at 600 and 650 °C.

Timing of metamorphism

For geochronology (U-Pb, Sm-Nd) we have selected twoma®c granulites (samples 1106 and 1041). Isotope ana-lyses were carried out at the ``Zentrallaboratorium fuÈ rGeochronologie'' in MuÈ nster. Laboratory methods anddetails of data evaluation are described in BroÈ cker et al.(1998) and Witt-Eickschen and Kramm (1998). Analyt-ical results are listed in Tables 3 and 4.

Previous work by Brueckner et al. (1991) yielded aSm-Nd (Grt-Cpx-WR) age of 352 � 4 Ma for aneclogite from Stary Gieraøto w. For ma®c granulites, weobtained broadly similar Sm-Nd garnet ± whole rockages (341 � 10 Ma and 343 � 11 Ma), with high er-rors due to the low spread in 147Sm/144Nd ratios(Table 3). The closure temperature for the Sm-Nd systemin garnet is estimated at c. 600 °C (Mezger et al. 1992).Thus, the Sm-Nd garnet ages most likely correspond toa post-peak metamorphic stage of the cooling path, butstill at eclogite-facies conditions. By use of the U-Pbzircon method, we have attempted to constrain timing ofpeak metamorphic conditions, because the high isotopicclosure temperature of zircon (>900 °C, e.g. Mezgerand Krogstad 1997) closely corresponds to the estimatedmaximum metamorphic temperatures (800±1000 °C).Conventional U-Pb multigrain dating of zircon wasapplied to six unabraded size fractions of sample 1106.The studied zircon population consists mostly of pris-matic grains with rounded terminations and very smallamounts of rounded zircons (Fig. 8a±d). The morpho-logical characteristics most likely are related to various

365

degrees of zircon dissolution during high-grade meta-morphism (e.g. KroÈ ner et al. 1998). All grains wereclear, without visible inclusions. Cathodoluminescence(CL) studies provided no indication for magmaticzonation or the presence of inherited cores; however,the grains studied are not homogeneous but exhibitirregular shaped patches with light and dark grey CLcolours.

The uranium concentrations of the four grain sizefractions >100 lm are little di�erent and range between183±209 ppm; the smaller size fractions (80±62 lm and<40 lm) have higher uranium contents of 305 ppm and370 ppm, respectively (Table 4). All fractions are dis-cordant and plot very close to the concordia curve nearto a poorly de®ned upper intercept around 370 Ma(Fig. 9). The small spread in 206Pb/238U and 207Pb/235Udoes not allow to calculate a geological meaningfuldiscordia chord. As a result, an upper concordia inter-cept cannot clearly be established, but a date of c. 370 to375 Ma is broadly constrained. The 207Pb/206Pb agesrange between 360 to 369 Ma. Since internal structuresand morphology are typical for zircons modi®ed duringhigh-grade metamorphism, we interpret the zircon datesas best approximation for primary crystallisation from amelt, which took place either before or during earlystages of high-grade metamorphism. Due to the fact thatunambiguous indications for newly grown metamorphiczircons were not found, timing of peak metamorphicconditions could not be constrained. In order to ®nd theanswer to this question, a more detailed U-Pb zircon

study, using both conventional dissolution and the va-pour-digestion technique, is currently in progress.

Fluid inclusions

Fluid inclusions were examined using the U.S. Geological Surveygas ¯ow heating/freezing stage (for description see Roedder 1984)at the UniversitaÈ t Bremen. A detailed description of the micro-thermometry and calibration procedure is given in Klemd (1989)and Klemd et al. (1992). Densities and isochores for the H2O-saltinclusions were calculated using the equation of state for H2O-NaCl (Zhang and Frantz 1987). Salinities were determined by ice-melting depressions using the experimental data of Potter et al.(1978). Raman microspectroscopy on representative ¯uid inclu-sions as performed at the University of GoÈ ttingen. A detailed de-scription of the method used is given in Klemd et al. (1992).

Only 120 ¯uid inclusions were found in 15 thick sec-tions of the samples 1041, 1041X, 1041Xe, 1107 and 1106from Stary Gieraøto w, although more than 40 thick sec-tions were investigated. They exclusively occur in matrixquartz. Despite an intensive search no inclusions werefound in omphacite and garnet. The inclusions are usu-ally £12 lm in diameter, four have a size of up to 24 lm.The shape of all inclusions ranges from irregular tospherical, and negative crystal shapes also occur. Allinclusions are aqueous, simple two-phase inclusionsconsisting of an aqueous liquid and a vapour bubble.They mostly occur along healed fractures which may ormay not crosscut grain boundaries irrespective of the¯uid composition. The ¯uid inclusions are extremely rareand no fracture intersections are present. It was therefore

Table 3 Sm-Nd analytical re-sults for whole rock samplesand garnet from Stary Gier-aøto w

Sample Type Sm (ppm) Nd (ppm) 147Sm/144Nda 143Nd/144Ndb Age (Ma, 2r)c

1106 Whole rock 6.003 24.22 0.1498 0.512322 (12) 341 � 10Whole rockd 6.674 27.22 0.1482 0.512329 (12)Garnet 9.313 10.32 0.5457 0.513210 (19)

1041 Whole rock 7.243 25.61 0.1710 0.512491 (12) 343 � 11Whole rockd 7.258 25.71 0.1707 0.512479 (13)Garnet 10.22 11.13 0.5551 0.513347 (19)

a 147Sm/144Nd ratios were assigned uncertainties of 0.3%bNumbers in parentheses indicate uncertainties in ratios quoted at the 2rm levelc Ages were calculated using linear least square regression (York 1969)dDuplicates were prepared from a di�erent aliquot of the same whole rock powder. Frequent mea-surement of the LaJolla standard yielded a 143Nd/144Nd ratio of 0.511844 � 30 (n = 24)

Table 4 U-Pb analytical results for zircon of the ma®c granulite 1106 from Stary Gieraøto w

Size(lm)

Weight(mg)

U(ppm)

Pbtotal

(ppm)

206Pba

(ppm)

208Pb/206Pbb

206Pb/204Pbb

206Pb/238Uc 207Pb/235Uc 207Pb/206Pbc Correlationcoe�cient

206Pb/238Ud

207Pb/235Ud

207Pb/206Pbd

250±200 2.154 191 10.6 9.3 0.08752 15880 0.05663 (17) 0.4200 (13) 0.05379 (3) 0.987 355 356 362 � 1200±160 1.434 183 10.2 8.9 0.09316 11741 0.05675 (17) 0.4204 (17) 0.05373 (15) 0.744 356 356 360 � 6160±125 1.500 197 11.0 9.6 0.08782 14057 0.05694 (17) 0.4228 (14) 0.05385 (7) 0.928 357 358 365 � 3125±100 1.109 209 11.5 10.1 0.08722 10568 0.05627 (17) 0.4180 (13) 0.05388 (4) 0.975 353 355 366 � 280±62 0.921 305 16.7 14.7 0.08461 11311 0.05607 (17) 0.4157 (13) 0.05376 (4) 0.974 352 353 361 � 2<40 1.448 370 20.4 18.0 0.08083 23977 0.05641 (17) 0.4196 (13) 0.05395 (2) 0.993 354 356 369 � 1

aRadiogenic lead onlybMeasured ratiosc Corrected for fractionation, spike, blank and initial common lead. Numbers in parentheses indicate uncertainties in ratios quoted at the2 rm leveldAges in Ma (2 r)

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impossible to establish a relative age relationship usingpetrographic observations. All of these inclusions areconsequently regarded as secondary or pseudosecond-ary. Carbondioxide-bearing inclusions were not ob-served. Apart from sample 1107 which exclusivelycontains high-salinity inclusions, all other samples con-tain both low- as well as high-salinity ¯uid inclusions.

The aqueous inclusions constitute two compositionalgroups. The ®rst group (low-salinity inclusions) has ®nalmelting temperatures (ice melting) between )8.9 to

Fig. 8a±d Scanning electron photomicrographs showing zircon mor-phologies in sample 1106: a, b Short-prismatic grains with rounded,recrystallised terminations; c, d irregular rounded grains related tometamorphic corrosions, surface pitting and recrystallisation. In allcases, crystal faces are not (a, c, d) or only poorly preserved (b)

Fig. 9 Concordia diagram for zircons of sample 1106 from StaryGieraøto w. Error ellipse of grain-size fraction A is hidden behind Band not labelled

Fig. 10 Relationship between homogenization temperatures (Th) and®nal melting temperatures (Tmf) of ¯uid inclusions

367

)0.3 °C with a maximum between )5.0 and )0.3 °C(Fig. 10), implying the dissolution of 0.5 to 12.6 wt%NaCl equivalent in the aqueous phase. Clathrate meltingdid not occur. Eutectic melting temperatures alwaysoccur above )23 °C indicating a NaCl-KCl-H2O-domi-nant system. The homogenisation temperatures of those¯uid inclusions range between 100 and 295 °C with amaximum between 150 and 200 °C (Fig. 10). Severalinclusions decrepitated during heating. Calculated den-sities range between 0.72 and 1.02 g/cm3. The meltingtemperatures of the second group (high-salinity inclu-sions) range between )20.7 and )9.3 °C (Fig. 10) cor-responding to 13.5 to 22.8 wt% NaCl equivalent.Eutectic melting <)50 °C occurs at much lower tem-peratures than observed for the ®rst group. This suggeststhat, apart from Na+ and K+, divalent ions such asCa2+ and Mg2+ are also present. Cotectic and clathratemelting were not observed. Homogenisation tempera-tures (Th) between 100 and 290 °C are essentially thesame as for the ®rst group (Fig. 10). Densities rangebetween 0.92 and 1.08 g/cm3. For both groups of in-clusions homogenisation is always into the liquid phase.Raman spectroscopy on representative inclusions didnot reveal the presence of minor gases such as CO2 and/or N2. The variation of homogenisation and meltingtemperatures in the low-salinity and high-salinity inclu-sion group could be due to autodecrepitation duringexhumation conditions of at least two generations of¯uid inclusions, namely low- and high-salinity inclu-sions. Such a process would explain the variation ofmelting and homogenisation temperatures and, hence,the resulting density range. This is supported by the factthat ¯uid inclusions along healed fractures and in clus-ters do not show compositional di�erences, as wouldhave been expected (e.g. Touret 1981). However, modi-®ed ¯uid inclusions of the same shape should reveal acorrelation between Th and inclusion size, since largerinclusions, as a result of pressure di�erences betweeninternal pressure (within the inclusion) and con®ningpressure, re-equilibrate before smaller ones during ex-humation (e.g. Bodnar et al. 1989; Hall et al. 1991;Klemd et al. 1995). Such a correlation was not observedin the here investigated samples. However, it should benoted that recent theoretical ®ndings of KuÈ ster andStoÈ ckhert (1997) seriously challenge this interpretation.They consider decrepitation (failure by fracturing) of¯uid inclusions in natural rocks not to be feasible above300±350 °C and thereby question all former experi-mental results. Also, a preferential loss of H2O duringre-equilibration of the smaller inclusions as described byHall and Sterner (1993) cannot be proved, because thereis no correlation between inclusion size and salinity.Another possible explanation for the salinity range is theentrapment under di�erent P-T conditions of more thantwo generations of ¯uids. The fact that some inclusionsseem to have decrepitated or streched during the post-amphibolite-facies conditions and others not, is proba-bly a result of the individual ¯uid density, geometry aswell as size of inclusions.

Discussion and conclusions

Metamorphic evolution

The eclogite mineral assemblage omphacite-garnet-quartz-rutile indicates temperatures of 800 to 1000 °C atminimum pressures of about 20 kbar. Ma®c granulitescontain the mineral parageneses omphacite-garnet(cores)-kyanite-quartz-rutile-Pl 2 which constrain equi-librium pressures of 20±28 kbar for this temperaturerange. Pressures of at least 27 kbar at 990 °C wereestimated for the felsic granulite with the peak meta-morphic mineral assemblage garnet(rim)-kyanite-anti-perthite-quartz. Pseudomorphs after coesite indicate thateven higher pressures were reached at some stage of themetamorphic evolution. The absence of garnet-break-down reactions to orthopyroxene and sillimanite-bear-ing assemblages indicate an overstepping of suchreactions due to unfavourable reaction kinetics duringrapid isothermal decompression. These features and thelack of melting processes are remarkable when consid-ering temperatures of up to 1000 °C during peak meta-morphism.

The Pl 1, phengite, quartz, and biotite inclusions inomphacite are interpreted as relic phases related to anearly stage of the prograde metamorphic evolution. As isindicated by the petrographical and mineral chemicalevidence, the garnet coronas around kyanite in the ma®cgranulite are the result of the prograde reaction:3 Di + 2 Ky = Grt + 2 Qtz close to peak metamor-phic conditions (Fig. 11). In accordance with the pro-grade garnet zoning in the felsic granulite, this implies aclockwise P-T path (Fig. 12), which is characterised bythe presence of kyanite and the absence of sillimanite.

Fig. 11 Schreinemakers grid depicting a CMAS invariant point andrelated univariant reaction curves relevant to the reaction texturespreserved in the ma®c granulites. The generalised prograde P-T pathis shown by the large arrow. The diagram was constructed using thethermodynamic dataset of Berman (1988) and calculated mineralphase activities (sample 1041)

368

This is typical for a continent-continent collision setting,not for subduction or collision-magmatism tectonics(Bucher and Frey 1994).

A considerable problem concerns the postulated sta-bility of plagioclase and antiperthite at pressures be-tween 21 and 28 kbar in the ma®c and felsic granulites.A major constraint concerning the plagioclase stability isthe bulk rock chemistry of the igneous precursor rock(Green and Ringwood 1967). Plagioclase (Ab-rich) maybe stable up to 29 kbar at 1000 °C in eclogite-faciesrocks with low Ca/Na ratios, high SiO2- and Al2O3-contents, and high Na2O-contents (Green and Ring-wood 1967; Ringwood 1975). The ma®c granulites of theZøote unit have a lower Ca/Na ratio and Na2O-contentas well as higher SiO2-content than the associated eclo-gites, while the composition of the felsic granulites issimilar to rhyodacite (Smulikowski 1967; Bakun-Czubarow 1991b). Therefore, we assume that the per-sistence of matrix Pl 2 and ternary feldspar in thegranulites during ultrahigh-P-T conditions is mainly dueto bulk chemical di�erences compared to the eclogitedomains.

Fluid inclusions

Maximum and minimum isochores were constructed forthe highest- and lowest-density aqueous inclusions(Fig. 12). The isochores of the highest densities of bothgroups pass through the P-T ®eld of the estimated am-phibolite-facies conditions and through or just below thepeak P-T estimate. There are at least three possible ex-planations: (1) the high-density inclusions are unmodi®edrelics of ¯uids that were present during or close to the peakof metamorphism; (2) the high-density inclusions weretrapped or retrapped at amphibolite-facies conditions;(3) most high-density inclusions were trapped after am-phibolite-facies conditions. We regard the second optionas the most probable one, because ¯uids at peak tem-peratures (800±1000 °C) (option 1) should reveal a lowH2O-activity. Otherwise, the granulites and eclogite do-mains should show some evidence for partial melting.Further, a low H2O-activity explains the presence of thealmost completely anhydrous ultrahigh-pressure mineralassemblage of the Zøote unit. Another cause of a lowH2O-activity would be the dilution of H2O by a freenon-aqueous ¯uid phase such as CO2 and/or N2. ManyHT-granulites world-wide contain high-density CO2 �N2-¯uid inclusions thus implying the development ofanhydrous mineral assemblages by means of dehydrationcaused by in®ltration of a CO2 � N2-rich ¯uid phase(e.g. for extensive discussion and reference list see Touret1992, 1993 and ``Introduction'' chapter). However, al-most all of those granulites show peak-metamorphicconditions between 750±900 °C at 4 ±12 kbar and manyfurthermore show a P-T-t path suggesting magmaticaccretion. The very high pressure granulites of the Zøoteunit on the other hand were subjected to much highereclogite-facies conditions in the range of 800 to 1000 °Cat 21 to 28 kbar and display a clockwise P-T-t path (seeabove). Eclogite-facies metamorphism is typically ac-companied by a ¯uid regime which is dominated by H2Oand in places by N2 � CO2 (e.g. Touret 1992; Andersenet al. 1993). This is in accordance with H2O-N2-CO2-rich¯uid inclusions observed in the lower-temperature (600±800 °C) eclogites in the S nie _znik and MieÎdzygo rze unit(Klemd et al. 1995), and with the fact that all observed¯uid inclusions in the granulites and eclogites of the Zøoteunit are aqueous. Furthermore, some H2O-rich ¯uidmust have been present locally in the recrystallisedeclogite domains where volume di�usion and thus grainboundary di�usion were more e�cient. It is howeverhighly unlikely that those ¯uid inclusions, if formed atultrahigh-pressure conditions, would have survived theisothermal decompression during exhumation, and thesubsequent static recrystallisation of the quartz grainswhich occurred during amphibolite-facies conditions attemperatures as low as 550 to 660 °C. Densities of ¯uidinclusions which were trapped or retrapped during re-crystallisation will probably re¯ect those P-T conditions(e.g. Johnson and Hollister 1995).

Considering the anhydrous mineral assemblage of theinvestigated rocks, a ¯uid phase causing the stabilisation

Fig. 12 P-T path for eclogites and ma®c granulites from theSÂ nie_znik Mts.. Generalized exhumation path of the ultrahigh-pressure rocks from the Zøote unit after Steltenpohl et al. (1993).H2O-isochores for minimum and maximum density (g/cm3) ofhigh-salinity (continuous line) and low-salinity (interrupted line)inclusion ¯uids. HP = high-pressure stage, as taken from Fig. 9[1 Jd18 + Qz = Ab10, 2 amphibolite-facies conditions after Stelten-pohl et al. (1993), and Borkowska (1996)]. For further referencesand details see text

369

of retrograde biotite must have had an external source ofderivation. However, ¯uid inclusions in microfracturesare very rare, which indicates that only a few micro-cracks acted as conduits for ¯uid transport. This is dis-tinctively di�erent to the behaviour of the country-rockgneisses which responded to ¯uid in®ltration by partialmelting (Fischer 1935; Smulikowski 1967). Although it ishighly unlikely that a considerable amount of ¯uid wasable to in®ltrate the ultrahigh-pressure rocks at post-amphibolite-facies conditions (option 3), there is nounambiguous evidence that all in®ltration occurredduring amphibolite-facies conditions. However, if someof the ¯uid inclusions were trapped or retrapped atamphibolite-facies conditions (P = 9.5±12.5 kbar, T =550±750 °C) their preservation would require an exhu-mation of the host rocks almost parallel to the highestdensity inclusions (Fig. 12), otherwise those inclusionswould stretch or re-equilibrate as a result of internalover- or underpressure (e.g. Bodnar et al. 1989; Touret1992; Giaramita and Sorensen 1994).

In summary, the presence of a discrete and free ¯uidphase during eclogite-facies metamorphism cannot un-equivocally be substantiated by our ¯uid inclusionstudy. The absence of any CO2 and/or N2, which wouldlower the H2O-activity, suggests that only limitedamounts of H2O were present during high- to ultrahigh-pressure metamorphism. We assume that the preserva-tion of local pre-eclogitic reaction textures in the ma®cgranulites as well as prograde growth zonations of gar-nets in the felsic granulites is largely controlled by lim-ited volume di�usion. The preservation of pre-eclogiticreaction textures and prograde-zonation patterns ingarnet (sample 1107), instead of a homogenisation byintragranular di�usion, indicates kinetic barriers topolymorphic inversion in response to pressure variations(Rubie and Thompson 1984). The anhydrous ultrahigh-pressure rocks escaped deformation and subsequentpervasive ¯uid in®ltration. Interface kinetics and di�u-sion were too sluggish for net-transfer reactions toproceed, thereby promoting the preservation of the pre-eclogite and eclogite-facies mineral assemblages. How-ever, some aqueous ¯uid must have been present in themore recrystallised domains, since volume di�usion wasmuch more e�cient here than in the associated domainsof disequilibrium. This is also supported by the presenceof exclusively H2O rich inclusions in the ultrahigh-pressure rocks of the Zøote unit. Textural observationsindicate that large ¯uid activity gradients persistedduring high-pressure metamorphism which suggests alocal ¯uid bu�ering as well as short-distance transportduring eclogite-facies metamorphism. Furthermore, ourresults suggest limited ¯uid in®ltration during amphib-olite-facies conditions.

Geodynamic consequences

The ultrahigh-pressure rocks from the Zøote unit ap-parently have experienced a di�erent metamorphic

history than the eclogites from the MieÎdzygo rze andS nie _znik units (BroÈ cker and Klemd 1996). The lowerpeak metamorphic temperatures (<800 °C) of the latterindicate that these eclogites are the product of sub-duction, since the oceanic and the continental thermalgradient away from subduction zones imply tempera-tures between 1000 and 1120 °C at about 100 km depth(Hacker and Peacock 1995). The U-Pb zircon dating of ama®c granulite indicates primary crystallisation around365 to 375 Ma, shortly before or during early stages ofmetamorphism. The Sm-Nd mineral-whole rock datafor an eclogite and granulites from Stary Gieraøto wyielded dates between 340 to 350 Ma (Brueckner et al.1991; this study) which are interpreted as a lower timelimit for eclogite-facies metamorphism.

The exhumation of the ultrahigh-pressure rocks fromthe Zøote unit (Fig. 12) which involved simultaneouscooling and decompression, may have occurred after acontinent-continent collision i.e. after the Carboniferouscollision of Gondwana and Baltica. The almost iso-thermal exhumation of the eclogites in the S nie _znik andMieÎdzygo rze units must have occurred after exhumationof the ultrahigh-pressure rocks from the Zøote unit, sincethe eclogites were subducted before collision. This con-clusion is in accord with the assumption that the rangeof Sm-Nd ages from the S nie _znik and MieÎdzygo rze units(341±329 Ma, Brueckner et al. 1991) is related to dif-ferent episodes of exhumation of high-pressure tectonicslabs within a relatively short time span. This model ofsequential uplift, as ®rst suggested by Brueckner et al.(1991), best explains the di�erences in the P-T paths (seeBroÈ cker and Klemd 1996). We regard the S nie _znik Mts.as a collage of several ultrahigh-pressure units whichwere tectonically exhumed and juxtaposed after colli-sional tectonics. The P-T di�erences between eclogite-facies rocks from di�erent locations of the study areaindicate amalgamation of a composite framework oftectonic slabs, derived from various depths.

The importance of contrasting eclogite-facies¯uid regimes

One of the problems when dealing with P and T esti-mates of metamorphic rocks is the presence or absenceof a free ¯uid phase, i.e. the in¯uence of the ¯uid onintergranular di�usion and reaction kinetics (see dis-cussion and references in Rubie 1990; Rubie andThompson 1984). The results of this study are in ac-cordance with ®ndings of Selverstone et al. (1990),Philippot and Selverstone (1991) and Klemd et al.(1992) who presented evidence from ¯uid inclusionstudies indicating ¯uid segregation and limited ¯uidtransport rather than pervasive ¯uid ¯ow during eclo-gite-facies metamorphism. The HT-granulites (750±900 °C, 4±12 kbar), which are dominated by CO2-rich¯uids and in places by high-salinity brines (e.g. Arano-vich et al. 1987; Touret 1992; Andersen et al. 1993;Newton 1995; Fonarev et al. 1998), have a di�erent ¯uid

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regime than the ultrahigh-pressure granulites which arecharacterised by the exclusive presence of H2O-rich in-clusions with variable salinities. The latter display noevidence for N2 and or CO2 which is also the case ineclogite terrains of the Alps and the Caucasus (Philippotand Selverstone 1991; Philippot et al. 1995; Perchuk1995). An important factor controlling the ¯uid com-position is possibly the general tectonic setting. For ex-ample, many HT-granulites are the result of magmaticaccretion (supplying a CO2-rich ¯uid phase) beneathalready existing crust and thus may display counter-clockwise P-T-t paths (see Bohlen 1987; Touret 1993; cf.Santosh and Yoshida 1992). However, the geometry ofprograde P-T-t paths of HP-granulites are commonlyclockwise, as is shown in the present case, thus favouringdi�erent tectonic mechanisms instead of magmatic ac-cretion (e.g. Harley 1989). This consequently explainsthe lack of CO2-rich inclusions and associated side ef-fects such as incipient charnockitisation and K-feldsparmetasomatism (see Newton 1995, for an extensive dis-cussion and references).

The di�erence in ¯uid regimes between eclogite-faciesrocks formed by subduction tectonics and eclogite-faciesrocks formed by continent-continent collision tectonicsis suggested to be a consequence of this concept (An-dersen et al. 1993). In subduction zone environments,for example, besides hydrated oceanic crust large vol-umes of clastic sediments are subducted, thus, repre-senting a major source of nitrogen and water duringdehydration, while rocks from continent-continent col-lision environments may have been thoroughly dehy-drated prior to eclogite-facies metamorphism. Thedi�erences in the ¯uid regimes in di�erent ultrahigh-pressure rocks of the S nie _znik Mts. (S nie _znik andMieÎdzygo rze units versus Zøote unit) support the modelof Andersen et al. (1993). The lower-temperature eclo-gites from the MieÎdzygo rze and S nie _znik units, whichformed as a result of subduction are dominated by aH2O-salt-N2-CO2 ¯uid regime. In contrast, the ultra-high-pressure rocks from the Zøote unit, which are re-lated to continent-continent collision tectonics, aredominated by a H2O-salt ¯uid system. This clearly dis-tinguishes high-pressure (HP)-granulites from HT-granulites which are believed to have been generated bymagmatic accretion. However this di�erence in ¯uidregimes between HP- and HT-granulites should be ver-i®ed by further ¯uid inclusion studies on other ultrahigh-pressure and HP-granulites.

The results of our studies in the SÂ nie _znik Mts.(Klemd et al. 1995; this study) demonstrate the generalpetrogenetic potential of ¯uid inclusion studies inmetamorphic rocks. Besides constraints for ¯uid activi-ties and P-T conditions, an increasing body of evidencesuggests striking di�erences in ¯uid compositions relatedto di�erent tectonic settings and resulting P-T-t paths.As a consequence, ¯uid inclusion studies in high-pres-sure rocks may assist in identi®cation of geodynamicprocesses and thus help to unravel the metamorphicevolution in complex tectonometamorphic settings.

Acknowledgements S. Matthes (WuÈ rzburg) is thanked for supply-ing samples of his collection. Microprobe work bene®ted fromassistance and advice of U. SchuÈ ssler (WuÈ rzburg). Discussions withL. Y. Aranovich (Chicago), G. Franz (Berlin), M. Okrusch(WuÈ rzburg) and L. M. Kriegsman (Sydney) on various aspects ofthe paper are highly appreciated. We are most grateful to H.Austrheim (Oslo) and E. Krogh (TromsoÈ ) for careful reviews of aformer version of the manuscript. Furthermore this paper hasbene®ted from extensive comments on the ®nal version by J. L. R.Touret. Financial support of the Deutsche Forschungsgemeinsc-haft is gratefully acknowledged (Kl -692/4-1).

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