for post-collisional continental tholeiitic magmas ... · intrusion was formerly referred to as...

Ž .Lithos 45 1998 197–222

ž /Isotopic O,Sr, Nd and trace element geochemistry of the Laouniž /layered intrusions Pan-African belt, Hoggar, Algeria : evidence

for post-collisional continental tholeiitic magmas variablycontaminated by continental crust

J.Y. Cottin a,), J.P. Lorand b,1, P. Agrinier c,2, J.L. Bodinier d,3, J.P. Liegeois e,4´a Lab. de Geologie-Petrologie-Geochimie, UMR CNRS 6524, UniÕersite J. Monnet, 23 rue du Dr P. Michelon, F-42023 Saint-Etienne´ ´ ´ ´

Cedex 02, Franceb Lab. de Mineralogie, ESA CNRS 7058, Museum National d’Histoire Naturelle, 61 rue de Buffon, F-75005 Paris, France´ ´

c Lab. Geochimie des isotopes stables, IPGP et UniÕersite Denis Diderot, 2 place Jussieu, F-75251 Paris cedex 05, France´ ´d Inst. des Sciences de la Terre, Geofluides-bassins-eau, URA CNRS D1767, UniÕersite Montpellier II CNRS, case O57, F-34095´

Montpellier cedex, Francee Dept. Geologie, Section de Geologie Isotopique, Musee Royal de l’Afrique Centrale, B-3080 TerÕuren, Belgium´ ´ ´

Received 3 December 1997; accepted 20 May 1998

Abstract

The three layered intrusions studied in the Laouni area have been emplaced within syn-kinematic Pan-African granitesand older metamorphic rocks. They have crystallized at the end of the regional high-temperature metamorphism, but are freefrom metamorphic recrystallization, revealing a post-collisional character. The cumulate piles can be interpreted in terms oftwo magmatic liquid lines of descent: one is tholeiitic and marked by plagioclase–olivine–clinopyroxene cumulatesŽ .troctolites or olivine bearing gabbros , while the other is calc-alkaline and produced orthopyroxene–plagioclase rich

Ž . Ž Ž . .cumulates norites . One intrusion WL West Laouni -troctolitic massif , shows a Lower Banded Zone where olivine-chro-mite orthocumulates are interlayered with orthopyroxene-rich and olivine–plagioclase–clinopyroxene cumulates, whereasthe Upper Massive Zone consists mainly of troctolitic and gabbroic cumulates. The other two massifs are more

Ž .homogeneous: the WL-noritic massif has a calc-alkaline differentiation trend whereas the EL East Laouni –troctoliticmassif has a tholeiitic one. Separated pyroxene and plagioclase display similar incompatible trace element patterns,regardless of the cumulate type. Calculated liquids in equilibrium with the two pyroxenes for both noritic and troctoliticcumulates are characterized by negative Nb, Ta, Zr and Hf anomalies and light REE enrichment inherited from the parental

18 Ž . 87 86 Ž .magmas. Troctolitic cumulates have mantle-derived d O q5 to q6‰ , initial Srr Sr Sr s0.7030 to 0.7054 , ´i NdŽ . 18 Ž . Žq5 to y1 values whereas noritic cumulates are variably enriched in d O q7 to q9‰ , show negative ´ y7 toNd

) Corresponding author. E-mail: [email protected] E-mail: [email protected] E-mail: [email protected] E-mail: [email protected] E-mail: [email protected]

0024-4937r98r$ - see front matter q 1998 Elsevier Science B.V. All rights reserved.Ž .PII: S0024-4937 98 00032-2

( )J.Y. Cottin et al.rLithos 45 1998 197–222198

. Ž .y12 and slightly higher Sr 0.7040–0.7065 . Based on field, isotopic ratios are interpreted as resulting from a depletediŽ 18 .mantle source Sr s0.7030; ´ sq5.1; d Osq5.1‰ having experience short term incompatible element enrichmenti Nd

and variable crustal contamination. The mantle magma was slightly contaminated by an Archaean lower crust in troctoliticcumulates, more strongly and with an additional contamination by an Eburnian upper crust in noritic cumulates. Lower crustinput is recorded mainly by Sr and Nd isotopes and upper crust input by O isotopes. This is probably due to the different

Ž .waterrrock ratios of these two crust types. Assimilation of low amounts -10% of quartz-bearing felsic rocks, comingfrom both lower and upper crust, can explain the rise of SiO activity, the enrichment in 18O and 87Sr and the lowering of2

´ in the noritic cumulates compared to troctolitic ones. The geodynamic model proposed to account for the LaouniNd

tholeiitic magmatism involves a late Pan-African asthenospheric rise due to a rapid lithospheric thinning associated withfunctioning of shear zones, which allowed tholeiitic magmas to reach high crustal levels while experiencing decreasingdegrees of crustal contamination with time. q 1998 Elsevier Science B.V. All rights reserved.

Keywords: Layered ultramafic–mafic intrusions; Continental tholeiites; Crustal contamination; Late Pan-African; Hoggar;Lithospheric thinning

1. Introduction

Post-collisional magmatism comprises a large va-Žriety of magmatic rocks, calk-alkaline granites,

. Ž .granitoids, leucogranites , silicic rhyolitic or pera-Ž .luminous magmas as well as alkaline series A-type

ŽWhalen et al., 1987; Rasmussen et al., 1988;.Sylvester, 1989; Nedelec et al., 1995; Bonin, 1996 .´ ´

Many have isotopic compositions consistent with aŽmantle origin Huppert and Sparks, 1988; England,

.1993 , but post-collisional magmas generated by par-tial melting of a Depleted MORB-type MantleŽ .DMM are scarce. Widespread post-collisional mag-matism developed throughout the Tuareg shield afterthe Pan-African orogeny paroxysm, dated at 600 Ma,and ended with alkaline anorogenic-type magmatismŽ .Liegeois and Black, 1987 . If late Pan-African gran-´

Žites are well known in the Tuareg shield Black et.al., 1994 , some ultramafic–mafic layered intrusions

crystallized from tholeiitic magmas have also beenrelated to the post-collisional magmatism, within the

Žsouthern part of the Tuareg shield Cottin and Lo-.rand, 1990 . These layered intrusions show many

lithological features similar to large layered intru-Ž .sions e.g., Bushveld, Stillwater, Jimberlana , such

as an ultramafic lower banded zone overlain bymafic cumulates.

Two different cumulate suites were recognized inŽ . Ž .the Laouni intrusions Cottin and Lorand, 1990 : 1

Ž .olivine chromite –plagioclase–clinopyroxene cumu-Ž .lates and 2 orthopyroxene–plagioclase cumulates.

Each of these two suites has been ascribed to a liquidŽ .line of descent LLD , respectively tholeiitic and

calk-alkaline. Cumulates from each LLD may beintimately intermingled in a single magma chamber,especially in the lower zone, resulting in consider-able lithological variations between the massifs. It iswell known that tholeiites contaminated by continen-tal crust may generate calk-alkaline LLD’s, becauseof assimilation of SiO -rich materials, increasing2

silica activity, water pressure and oxygen fugacityŽDe Paolo, 1981; Groves and Baker, 1984; Camp-bell, 1985; Gray and Goode, 1989; Stewart and De

.Paolo, 1990 . To see whether crustal contaminationcan pertain to the post-collisional tholeiitic magma-

Žtism of Laouni, a coupled-trace element LILE,. Ž .HFSE, REE and isotope O, Sr, Nd study has been

undertaken on cumulates from the three most impor-tant intrusions. Our aim is to provide mineralogical,geochemical and isotopic constraints on the composi-tions of the magmas involved, their degree of crustalcontamination, and to propose a model to explain thehighly variable lithologies of the Laouni layeredintrusions. This model has important bearing for ourunderstanding of post-collisional magmatism in theTuareg shield.

2. Geological setting

Ž 2 . Ž .The Tuareg shield 500 000 km Hoggar hasbeen interpreted as the result of a complete Wilsoncycle beginning at ca. 800 Ma and ending shortlyafter 600 Ma with the collision of the shield with the

Ž2 Ga old West African craton Black et al., 1979;.Caby et al., 1981 . The Pan-African orogeny in this

area has recently been re-interpreted by Black et al.

( )J.Y. Cottin et al.rLithos 45 1998 197–222 199

Ž .1994 who showed that the Tuareg shield is com-posed of 23 terranes limited by mega-shear zones or

Ž . Žbasal thrusts Fig. 1 . An early collision 760–660.Ma with the East Saharan craton has been identified

Žin the most easterly terranes, especially in Aır SE¨. ŽTuareg shield Bertrand et al., 1986; Caby and

Andreopoulos-Renaud, 1987; Black and Liegeois,´.1993 . This led to the idea of two coupled Pan-Afri-

Ž .can orogenies 750–660 Ma and 650–580 Ma . Theearly Pan-African orogeny induced regional partialmelting of the lower crust in Aır, producing potassic¨

Ž .leucogranite Renatt granite .Post-collisional granitoids related to the late Pan-

African orogeny are abundant in the Tuareg shield.Alkaline to peralkaline plutons, dyke swarms andrhyolitic plateaux are known in the western part of

Žthe shield Liegeois and Black, 1987; Fourcade and´.Javoy, 1985; Hadj Kaddour et al., 1998 whereas, to

the east, these are calc-alkaline and sometimes alka-Žline plutonic rocks ‘Taourirt-type plutons’ 580–520

Ma; Boissonnas, 1973; Moulahoum, 1988; Azzouni-.Sekkal, 1989 . These post-collisional Pan-African

magmatic events are all linked to late movementsŽ .along mega-shear zones Black et al., 1994 .

The Laouni layered intrusions are located in thesouthern part of Central Hoggar, within the Laouni

Ž .terrane Fig. 1 . The Laouni terrane shows bothintensively reworked Archaean and Palaeoprotero-

Ž .zoic 2000 Ma supracrustal sequences that weredeformed and metamorphosed during the Pan-Afri-can orogeny at about 600 Ma. Syn-kinematic Pan-African granitic batholiths have been dated at 630–

Ž . Ž .600 Ma Bertrand et al., 1986 Fig. 1 . Proterozoicmetamorphic rocks are composed of garnet-cordieritegranulites, metagraywackes, calc-silicate rocks, am-phibolites and marbles. With the exception of themetagraywakes, all these rocks show the effect of

Žtwo regional metamorphic events Guiraud et al.,.1996 . The oldest event occurred under the granulite

Ž .facies conditions 8008C and 0.5 to 0.6 GPa- . Theyoungest event produced mineral parageneses char-

Ž .acteristic of the high temperature 6008C , low pres-Ž .sure 0.3 GPa amphibolite facies. This HT-LP meta-

morphism is considered to result from a lithosphericthinning event that reactivated the N–S large strike-slip fault system during the waning stages of the

Ž . ŽPan-African orogeny 580–520 Ma Cottin et al.,.1990; Black et al., 1994 .

The ultramafic–mafic intrusions are five smallŽbodies of roughly elliptical shape 15 by 3 km in

.maximum dimensions . The three intrusions investi-gated in the present studies comprise two in West

Ž . Ž .Laouni WL and one in East Laouni EL . The ELintrusion was formerly referred to as Oued Zazir by

Ž .Guiraud et al. 1996 . All three are intrusive intosyn-kinematic granites and Neoproterozoic metamor-phic rocks. Country-rocks and ultramafic–mafic in-trusions are crosscut by post-tectonic albite-topazbearing granitic dykes related to the Taourirt granite

Ž . Ž .magmatism 520"20 Ma Fig. 1 . These field rela-tionships suggest emplacement ages between 600–520 Ma for the intrusions. Such a post-collisional

Ž .age is supported by i the north–south elongation ofthe massifs which is controlled by the large strike-slipfault system reactivated between 580 and 520 Ma,i.e., during the waning stages of the Pan-African

Ž .orogeny ii well-preserved concentric magmaticŽ .structures and iii the absence of any penetrative

Ždeformation and metamorphic recrystallization Cot-.tin and Lorand, 1990 . There is no paragenetic change

in the HT-LP metamorphic paragenesis of themetapelites that occur in the immediate vicinity of

Ž .the three intrusions Guiraud et al., 1996 . It can bededuced from this observation that the Laouni magmachambers opened at shallow depth in the order of 10km, during the HT-LP metamorphism of the crusttriggered by the late Pan-African lithospheric thin-ning event.

3. Lithology of the Laouni layered intrusions

Petrographic studies of the West Laouni intru-Žsions previously referred to as the ‘noritic’ and

Ž .‘troctolitic’ massifs in Cottin and Lorand 1990Žhave already been published Lorand and Cottin,

.1987; Lorand et al., 1987; Cottin and Lorand, 1990 .Ž .On the other hand, the easternmost intrusion EL is

described here for the first time. True chilled mar-gins are lacking but each intrusion is surrounded bya 10- to 50-m thick external ring of amphibolitizedgabbros showing extensive alteration to assemblageswith amphibole, chlorite, epidote, prehnite and cal-

Ž .cite Cottin and Lorand, 1990 . In spite of thisalteration, the ‘border rings’ preserve doleritic tex-tures and their primary magmatic assemblages re-


main recognisable. In the WL-noritic intrusion, theseare orthopyroxene-rich gabbros containing up to 10vol.% Fe–Ti oxides and abundant magmatic horn-blende. In the other two intrusions, the amphiboli-tized ‘border rings’ comprise opx-poor amphibolegabbros. Diorites with intercumulus quartz and bi-

Ž .otite and locally anorthosites An and trond-85Ž .jhemites An are observed in the outermost45 – 20

part of the ‘border-ring’ and they can form metre-thick dykes cross-cutting the granitic country-rocks.

Ultramafic–mafic cumulates define funnel-shapedŽinternal concentric structures Cottin and Lorand,

.1990 . Both the WL-noritic and the WL-troctoliticŽ .intrusions display a Lower Banded Zone LBZ over-Ž .lain by a Main more Massive Zone MMZ . The

modal layering is outlined by ultramafic rocks. TheŽ .LBZ of the WL-noritic intrusion Fig. 2A displays aŽ .meter-thick opx-rich cumulate 70 vol.% sand-

Ž .wiched by thicker up to 20 m olivine–chromiteŽ .cumulate layers Fo , without any transition.83-74

Olivine–chromite cumulates are especially rich inŽ .intercumulus orthopyroxene 5–15 vol.% and parg-

Ž .asite 10 vol.% . The MMZ is composed of orthopy-roxene-rich plagioclase meso- to ad-cumulates. Theirmodal composition ranges from melanorites at thebase to leucogabbronorites at the top of the MMZ,thus reflecting decreasing opx modal contents and anincreasing abundance of plagioclase and clinopyro-

Žxene. In MMZ, orthopyroxene compositions En to80.En indicate that magmatic differentiation operated62

with moderate iron enrichment. Clinopyroxene be-comes a cumulus phase in the upper leucogab-bronorites, whereas magmatic amphibole and Fe–Ti

Žoxides are abundant throughout the MMZ up to 5%.by volume , yet always intercumulus. Taken as a

whole, the WL-noritic MMZ shows modal composi-tions and mineral chemistry typical of cumulates

Žseparated from calc-alkaline LLD i.e., the ‘Bowentrend’, Osborn, 1959; Groves and Baker, 1984 and

.references therein . Moreover, plagioclase is calcic

Ž .An and reversely zoned; its anorthite content80-62

may show abrupt variations which are not correlatedwith pyroxene mg number, a feature generally inter-preted in terms of high and variable water pressureŽ .Thy, 1987 .

Ž .In the WL-troctolitic intrusion, the LBZ Fig. 2Bis thicker and comprises four rhythmic units. The

Ž .lowermost three units start with olivine Fo -88-85

chromite orthocumulates and end up with orthopy-Ž .roxenites En , in which intercumulus plagioclase72

Ž .An can be contained numerous 100 mm-size55

phlogopite inclusions. The olivine–chromite cumu-lates distinguish from equivalent rocks in the WL-noritic intrusion by higher abundances of intercumu-

Ž .lus plagioclase An and lower modal propor-55-65

tions of opx and amphibole. The uppermost rhythmicunit of the LBZ culminates with a meter thickolivine-plagioclase mesocumulate layer which shows5 to 10 vol.% pargasite and normally-zoned cumulus

Ž .plagioclases An . Such layers have not been73-67

observed in the WL-noritic intrusion. Another partic-ularity of the LBZ of the WL-troctolitic intrusion isthe presence of a decametre-sized lens of pegmatiticolivine-poor two pyroxene gabbro which contains 11

Ž .vol% cumulus opx 2H99; Fig. 2B .In the WL-troctolitic intrusion, the MMZ is less

heterogeneous than the LBZ. It is composed ofplagioclase–olivine–clinopyroxene cumulates sho-wing modal and mineral compositional character-istics similar to those of early cumulates separatedby closed-system fractional crystallization from

Žtholeiitic magmas Campbell and Borley, 1974; Thy,.1987; Groves and Baker, 1984 . The MMZ starts

Ž .with plagioclase-rich up to 90% , olivine-poor adcu-mulates alternating with olivine–chromite orthocu-

Ž .mulate layers Transition Zone; Fig. 2B . Small scalevariations in the proportions of plagioclase andolivine define a modal layering. From the bottom tothe top of the MMZ, olivine modal abundance de-

Ž .creases 25 to 0 vol.% while cpx increases; how-

Ž . ŽFig. 1. A Main terranes of the Tuareg shield defined by Black et al., 1994 DjsDjanet, EdsEdembo, AosAouzegueur, BasBarghotŽ . Ž .Aır , As-IssAssode Aır -Issalane, TchsTchillit, TasTazat, ScsSerouenout, Eg-ALsEgere-Aleksod, AzsAzrou-n-Fad, Tes¨ ¨ ´ ´ ´ ´Tefedest, LasLaouni, Isks Iskel, Its In Teidini, ZasTin Zaouatene, TirsTirek, AhsAhnet, Ous In Ouzzal, Ugis Iforas granulitic

. Ž . Ž .unit, TassTassendjanet, KisKidal, TilsTilemsi, TimsTimetrine ; B Detailed map of the Laouni area after Guiraud et al., 1996´Ž .showing the three ultramafic–mafic intrusions studied WLsWestern Laouni; ELsEastern Laouni . The Laouni area shows large volume

Žof syn-kinematic Pan-African granites with Palaeo- and NeoProterozoic metamorphic rocks and scarce late-kinematic granites 580–520.Ma .


Fig. 2. Lithology, modal composition and compositions of the four main anhydrous phases in the three Laouni layered intrusions. ElectronŽ . Žmicroprobe data from Cottin and Lorand 1990 and Lorand and Cottin, in prep. Hydrous phasessamphiboleqbiotite; OPXs

2q.orthopyroxene; CPXsclinopyroxene; mg)s100 MgrMgqFe A: WL-noritic intrusion B: WL-troctolitic intrusion C: EL-troctoliticintrusion.

ever, clinopyroxene becomes a cumulus phase onlyin the uppermost MMZ. Orthopyroxene, hydrous

Ž .phases amphiboleqbiotite and Fe–Ti oxides areŽ .very scarce -1% by volume and systematically

intercumulus. Mafic phases and plagioclase showscarce compositional variations. Plagioclase is

Ž .slightly less calcic ;An than in equivalent rocks65Ž .of the MMZ of the WL-noritic intrusion ;An ,70

Ž .whereas mafic phases olivine and pyroxenes areŽmore magnesian Fo ; En ; mg number cpxs85-72 80-70

. Ž87-80 compared with noritic equivalents Fo ;78

.En ; mg number cpxs68-62 . As commonly78-60Žobserved in the tholeiitic LLD e.g., Wilson et al.,

.1981 and references therein , An content and mgnumber of crystal core vary more regularly in theWL-troctolitic MMZ.

In addition to sharp boundary between each typeof cumulates within a rhythmic unit and between therhythmic units, the LBZ of the WL-troctolitic intru-sion display small scale variations of phase composi-tions that cannot be interpreted by simple models of

Ž .fractional crystallization Fig. 2B . There is no large


compositional difference between olivine-rich cumu-lates from the LBZ and the MMZ rocks. Olivine andpyroxene mg number gently decreases from the bot-

Žtom to the top of the whole cumulate pile Fo at88

the bottom of the LBZ, Fo at the bottom of the85.Transition zone and Fo at the top of the MMZ , a75

feature which can be explained by a fractional crys-tallization trend. By contrast, there are large compo-sitional gaps between olivine-chromite and plagio-clase-olivine-rich cumulates at one hand, and opx-rich cumulates on the other hand. The opx-cumulates

Ž .interlayered with peridotite cumulates 2H104 orŽ .intermingled with them 2H99 , are always much

less magnesian and their orthopyroxene composi-tions are similar to those of the WL-noritic MMZŽ .En .75

The EL-troctolitic intrusion does not show aŽ .Lower Banded Zone Fig. 2C . Its cumulate pile

shows most of the mineralogical features of theMMZ of the WL-troctolitic intrusion, except thatplagioclase-olivine-clinopyroxene cumulates are or-ganised into four roughly layered macro-cyclic unitsŽ .A to D; Fig. 2C . Unit A starts with a twenty meterthick two-pyroxene gabbro in which both cpx andopx are cumulus phases. Opx modal content de-creases towards the top of this unit, in parallel withprogressively increasing olivine contents. The other

Ž .three cyclic units B to D always start with 10-mthick cpx–plagioclase–olivine mesocumulates char-acterized by cm-sized poikilitic clinopyroxene crys-tals. These latter are usually considered to crystallizein magma mixing interfaces in layered intrusionsŽ .Irvine, 1980 . Cumulates are opx-poor olivine gab-bros. Cyclic units B to D display modal variations,

Ži.e., decreasing olivine modal abundances 40 to less. Žthan 10% and increasing cpx modal abundances up.to 50 vol.% from the bottom to the top. Intercumu-

Žlus orthopyroxene, magmatic amphibole pargasite-.hornblende and Fe–Ti oxides are subordinate in

macrocyclic units A to C, yet always present. Theirrelative abundances increase in the uppermost cumu-lates of the D layer. At this level, orthopyroxene isdisseminated in the interstices between plagioclasecrystals but may form coarser crystals that look likecumulus crystals.

The EL-troctolitic cumulate pile is characterizedŽby a narrow range of mineral compositions Fig.

.2C . Like the MMZ of the WL-troctolitic intrusion,

the EL-troctolitic intrusion displays all characteristicfeatures of cumulates separated by a tholeiitic LLD

Žfrom an olivine tholeiite parental liquid i.e., earlycrystallization of moderately calcic plagioclase-Any , cumulus cpx instead of opx, late crystalliza-65-75

tion of Fe–Ti oxides and hydrous minerals andrelative positive correlations between An content ofcore crystals of plagioclase and mg numbers of

. Žolivine and pyroxenes . Compositional reversals i.e.,increasing mg numbers of olivine and pyroxenes and

.An% of core crystals of plagioclase are observedabove each poikilitic cpx-bearing layer which, there-fore, correspond to magma replenishment zones.

The three massifs are crosscut by fine-grainedbasaltic dykes or contain fine-grained patchy zones.Fine-grained rocks show mineralogical features ap-proaching surrounding cumulates. For instance,opx-rich dykes crosscut the LBZ of the WL-noriticmassif while fine-grained rocks in the WL and EL-troctolitic intrusions are olivine-cpx-bearing and con-tain more than 60% plagioclase.

4. Trace element and isotope geochemistry

4.1. Analytical methods

Special attention has been paid to the fine-graineddykes cross-cutting the three massifs since thesesamples could represent chilled compositions of theparental magmas that have fed the Laouni layered

Žintrusions. Four fine-grained samples two dykes-2H297, 2H256- and two patchy zones -2H669 and

.832926- were analyzed with ICP-AES at the CRPGin Nancy for major elements and eight trace ele-

Ž .ments Ba, Rb, Sr, Co, Cr, Ni, V, Cu . Detectionlimits are 0.05 wt.% for major elements and 5 ppm

Ž .for minor elements except Ni, Cr and Cu 10 ppmŽ .Govindaraju and Mevelle, 1987 . Further Rb and Srdetermination have been carried out at the Musee´

Ž .Royal de l’Afrique Centrale MRAC , Tervuren, us-ing high precision XRF for concentrations above 30ppm and isotope dilution for concentrations below

Ž .30 ppm. Be, Ga, HFSE Nb, Zr , highly incompati-Ž .ble elements Th, Y and the rare earth elements

Ž .have been determined either at the CRPG Nancy orŽ .by ICP-MS at the MRAC Table 1 .

Incompatible trace element contents of clinopy-roxene, orthopyroxene and plagioclase separated


Table 1Whole-rock major and trace element compositions of fine-grained rocks and dykes. Major elements, minor and trace elements by ICP-AESŽ . Ž .Nancy ; Rare Earth Elements by ICP-MS Musee Royal de l’Afrique Centrale; Tervuren´

1 2 3 4WL-troctolitic 2H256 WL-noritic 2H297 EL-troctolitic 832926 EL-troctolitic 2H669dyke inside the intrusion intrusive in the LBZ discordant fine grained rock discordant fine grained rock

W%SiO 48.00 47.72 51.85 47.452

TiO 1.33 0.32 0.47 0.42

AL O 16.32 13.63 16.50 17.452 3

FeO 11.24 8.96 7.11 7.84t

MnO 0.17 0.19 0.14 0.13MgO 9.03 13.93 9.01 10.2CaO 11.58 11.58 11.05 11.68Na O 1.35 1.2 2.76 2.052

K O 0.05 0.06 0.32 0.042

P O 0.27 0.16 nd 0.022 5

L.O.I. 0.45 1.97 0.57 1.36TOTAL 99.79 99.72 99.78 98.71MgOrMgOqFeO 0.47 0.63 0.58 0.59t

( )ppmBa 73 108 82 40Be 1.2Co 60 55 73 101Cr 524 575 447 355Cu 59 99 27 79Ga 31 20Nb 7 5 10.92 10.1Ni 159 193 469 168Rb 5 6 10 10Sc 38.5 46Sr 321 524 416 290Th 7 5V 271 177 171 138Y 19.63 9.26 7.86 6.22Zn 75 55Zr 42 18 17.83 20.49La 4.00 2.84 1.58 1.65Ce 10.5 12.51 6.79 8.97Nd 7.77 3.99 3.7 2.6Sm 2.21 1.4 1.31 1.46Eu 1.00 0.56 0.74 0.72Gd 2.87 1.69 1.44 1.53Dy 2.92 1.47 1.34 1.21Er 1.68 0.82 0.71 0.72Yb 1.47 0.77 0.66 0.57Lu 0.22 0.1 0.14 0.22

Norme CIPWQuartz 1.81 0.00 0.00 0.00Orthose 0.29 0.36 1.91 0.24albite 11.59 10.46 23.66 17.94anorthite 38.91 32.59 32.10 36.61diopside 14.21 20.40 18.71 16.15


Ž .Table 1 continued

1 2 3 4WL-troctolitic 2H256 WL-noritic 2H297 EL-troctolitic 832926 EL-troctolitic 2H669dyke inside the intrusion intrusive in the LBZ discordant fine grained rock discordant fine grained rock

Norme CIPWHypersthene 25.80 22.34 18.69 11.44olivine 0.00 10.10 1.11 10.91magnetite 4.16 2.71 2.89 2.85ilmenite 2.56 0.62 0.90 0.79apatite 0.63 0.38 0 0.05

from 12 representative samples in the three intru-Žsions have been determined by ICP-MS Montpel-

.lier using the method described by Ionov et al.Ž .1992 . In the WL-troctolitic intrusions, representa-tive samples from both troctolitic cumulates andopx-rich cumulates were selected. Pure mineralogicalfractions were obtained by a combination of Frantzmagnetic separation and final handpicking under abinocular microscope. Separated minerals were ana-lyzed without leaching. Detection limits are 1.1 ppbŽ . Ž . Ž .Gd , 1 to 3 ppb Pr–Sm–Nd , 10 ppb Ce and 12

Ž .ppb La .The same separated minerals were also analyzed

for oxygen isotopes. Oxygen was extracted fromminerals and whole-rock powders using BrF at5

Ž .6008C Clayton and Mayeda, 1963 and then reactedwith carbon to produce carbon dioxide. Oxygen iso-tope ratios of the CO samples were obtained with a2

Finnigan Mat DELTA E mass spectrometer at theLaboratoire de Geochimie des Isotopes Stables of the´Universite Denis Diderot and Institut de Physique du´Globe de Paris. 18 Or16O ratios are given as d

18 OwŽ18 16 . Ž18 16 .samples Or O sampler Or O SMOW-

x Ž .1 ) 1000 in per mil unit ‰ . Usual reproducibilityof replicate oxygen isotopic analyses was about

Ž . 18"0.1‰ 2s . NBS28 mean d O value was 9.56"

Ž . Ž . 18 180.10‰ 2s , ns12 . D XyY sd OX-d OY isthe oxygen isotope fractionation between phase Xand phase Y.

Sr and Nd isotopic measurements have been car-ried out on a Fisons VG Sector 54 mass spectrome-ter. Repeated measurements of Sr and Nd standardshave shown that between-run error is better than0.00002. The NBS987 standard has given a value for87Srr86Sr of 0.710263"0.000005 during this studyŽ 86 88 .2s on the mean, normalised to Srr Srs0.1194

and the MERCK Nd standard a value for 143Ndr144

ŽNd of 0.512722"0.000006 2s on the mean, nor-146 144 .malised to Ndr Nds0.7219 . Used decay con-

Ž . y11stants Steiger and Jager, 1977 are 1.42=10¨y1 Ž87 . y12 y1 Ž147 .a Rb and 6.54=10 a Sm .

4.2. Fine-grained rocks and dykes

Fine-grained rocks have broadly basaltic bulk-rockŽ .compositions 47–51 wt.% SiO ; Table 1 . Sample2

2H669 displays a major element composition ofŽolivine tholeiite alike that of MORB see composi-

.tion F2-1 in Fisk and Bence, 1980 . Apart from aslightly higher Al O content, sample 832926 is2 3

compositionally similar to feeder dykes of Continen-Ž .tal Flood Basalts Philpotts and Asher, 1993 as

regards major element compositions. Nevertheless,Mg ratios are on average lower than those expectedfor primitive magmas equilibrated with the mantleŽ .-0.60 vs. 070–0.72; Roeder and Emslie, 1970 ,and range from 0.63–0.47.

Primitive mantle-normalized REE patterns showstrong similarities with continental tholeiites, espe-cially the early Mesozoic continental tholeiites from

Ž .Morocco see sample 1127 in Bertrand et al., 1982Ž . wŽ .Fig. 3 . All are LREE-enriched LarYb s1.8–N

x2.5 . Samples 832926, 2H297, 2H256 and 2H669display positive Eu anomalies that reflect the abun-dance of cumulus plagioclase. Then, these fine-grained basaltic dykes cannot represent true liquids.The same samples in extended incompatible traceelement patterns show positive Sr anomalies and low

Žlevels of the less incompatible trace elements Zr-Hf-.Y-HREE . The cumulus plagioclase effect is reduced

in sample 2H256; except for Ba and Th, the mantle-normalized incompatible trace element pattern of this


Ž .Fig. 3. Chondrites-normalized REE A and Primitive-MantleŽ .incompatible trace element B patterns for fine-grained rocks

cross-cutting the three Laouni intrusions, and for comparison,ŽDeccan Trapps sample BAS 258; Lightfoot and Hawkesworth,

. Ž1988 and continental tholeiites from Morocco sample 1127;.Bertrand et al., 1982 . Normalizing values after Sun and Mc-

Ž .Donough 1989 .

sample is the closest to those of continental tholeiitesŽLILE and REE abundances at 10 to 20=primitive

.mantle .

4.3. Incompatible trace element content of separatedminerals

The two pyroxenes and plagioclase have remark-ably homogeneous incompatible trace element pat-

Žterns throughout the three Laouni intrusions Table.2; Fig. 4 . Primitive-mantle normalized patterns are

those of minerals equilibrated with mantle-derivedmagmas. Cpx shows depletion in LILE relative to

Ž .the REE 3 to 6=mantle abundances with Nb, Ta,Sr, Zr and Hf negative anomalies. Opx is one orderof magnitude poorer in incompatible trace elementrelative to cpx. Its primitive mantle-normalized pat-terns display LILE and LREE depletions relative to

Žthe HREE. The less magnesian pyroxenes opx-.2H104, 2H99, 2H342- generally have slightly higher

incompatible trace element contents than Mg-opxŽ . Ž . Ž .2H292 or cpx 833112 related to Mg-cpx 2H85 .On the other hand, plagioclase shows no relationshipbetween An content and incompatible trace elementpattern. Plagioclase patterns display strong HREE

Ž .depletions 0.01–0.1 = primitive mantle , highlyŽvariable LREE enrichments 1-30=primitive man-

.tle and strongly positive Eu, Sr and Ba anomalies.Compared with plagioclases from troctolitic or gab-broic cumulates, those from noritic or opx-rich cu-mulates are distinguished by LREE-enrichments.

Ž .Partition coefficients K calculated from avail-dŽable mineral pairs opx–plagioclase and cpx–

.plagioclase are in ranges generally accepted forequilibration achieved at magmatic temperatureŽ .Kelemen et al., 1993 . Nevertheless, it is not easy touse them for cumulus minerals within plutonic rocksbecause their crystallization is too slow, often indisequilibrium with magma, as attested by zonationor secondary homogenization with badly known in-terstitial trapped melts and because magmatic equili-bration kinetic which control crystal overgrowthcould give very different composition in incompati-ble trace elements. Then analyses on mineral sepa-rates would yield mean compositions, not strictly

Žrelated to the parental liquid composition Cawthorn,.1996; Blundy, 1997; Mathez et al., 1997 . Two

Žsamples from the WL-troctolitic LBZ 2H85 for.plagioclase; 2H104 for both opx and plagioclase are

Ba-enriched, probably because of numerous minutephlogopite inclusions in plagioclase and orthopyrox-

Ž .ene Lorand and Cottin, 1987 .In the three massifs, pyroxenes and plagioclase

display negative Nb-Ta anomalies relative to U andLa in their primitive mantle-normalised trace ele-ment patterns; this is a further geochemical charac-

Žteristic of continental tholeiites Briggs and Mc-.Donough, 1990 . Zr and Hf negative anomalies rela-

tive to LREE are also observed in primitive mantle


Tab

le2

Tra

ceel

emen

tco

mpo

siti

ons

ofse

para

ted

min

eral

s

ppm

CP

XC

PX

OP

XO

PX

OP

XO

PX

OP

XP

lP

lP

lP

lP

lP

lP

lP

lP

lP

lP

l

2H85

83-3

1-2

2H29

22H

342

2H33

52H

1O4

2H99

2H29

22H

342

2H85

2H10

42H

802H

192

2H10

92H

9983

-27-

583

-25-

2583

-31-

2

Sc

102

126.

525

.753

.956

.535

.632

.10.

631.

120.

641.

050.

40.

230.

340.

330.

630.

380.

51R

b0.

110.

460.

20.

510.

750.

30.

420.

791.

070.

430.

571.

131.

260.

470.

481.

650.

331.

77S

r23

.730

.59.

754

.8

35.3

10.9

50.0

078

990

776

071

763

358

349

778

163

852

562

7Z

r19

.78

21.1

66.

000

8.00

012

.38.

000

7.2

0.26

450.

9545

0.34

50.

3105

1.79

40.

391

0.17

250.

1265

0.16

10.

2875

0.11

5N

b0.

099

0.39

20.

061

0.05

60.

114

0.13

90.

10.

070.

056

0.06

20.

076

0.10

90.

081

0.07

80.

032

0.02

90.

050.

059

Ba

2.8

9.5

7.3

12.5

14.8

59.8

13.5

294

170

592

293

117

116

109

188

174

7615

1L

a0.

874

1.71

0.21

0.51

0.73

0.38

0.31

6.35

46.

845

2.32

524

.250

2.57

50.

767

2.05

12.

312

1.44

31.

275

1.44

8C

e3.

155.

320.

50.

981.

970.

910.

678.

1810

.28

3.57

36.3

4.61

0.91

3.51

3.73

2.13

2.45

2.13

Pr

0.71

11.

060.

080.

120.

330.

140.

110.

938

1.24

10.

449

4.48

40.

593

0.13

00.

448

0.46

80.

270

0.33

20.

269

Nd

4.59

6.36

0.43

0.56

1.77

0.71

0.58

2.05

3.25

1.33

12.6

1.86

0.51

1.38

1.37

0.81

1.17

0.8

Sm

1.99

42.

327

0.16

80.

195

0.59

20.

251

0.23

10.

217

0.36

90.

212

0.28

30.

272

0.10

50.

193

0.19

10.

119

0.18

90.

133

Eu

0.75

10.

860

0.07

10.

105

0.29

60.

082

0.12

0.60

81.

146

0.71

90.

612

0.46

60.

216

0.44

10.

632

0.53

0.38

20.

445

Gd

3.08

3.21

0.31

0.38

0.83

0.42

0.42

0.49

0.37

0.57

0.24

0.27

0.18

0.25

0.32

0.27

0.21

0.25

Tb

0.54

70.

537

0.06

90.

086

0.15

80.

087

0.08

90.

015

0.02

0.01

60.

021

0.02

20.

012

0.01

50.

014

0.01

0.01

70.

012

Dy

3.75

3.52

0.6

0.76

1.19

0.71

0.76

0.07

30.

086

0.08

40.

072

0.09

60.

060.

069

0.05

90.

044

0.07

50.

051

Ho

0.76

0.70

0.16

0.2

0.28

0.17

0.2

0.01

20.

012

0.01

40.

008

0.01

60.

011

0.01

10.

009

0.00

70.

011

0.00

8E

r2.

041.

840.

560.

710.

890.

590.

680.

033

0.03

70.

037

0.03

60.

041

0.02

60.

029

0.01

50.

011

0.02

80.

02T

m0.

282

0.25

00.

102

0.12

80.

148

0.09

70.

117

0.00

430.

0048

0.00

470.

0047

0.00

510.

0029

0.00

320.

0023

0.00

250.

004

0.00

3Y

b1.

671.

510.

830.

991.

10.

730.

870.

023

0.02

40.

022

0.02

60.

026

0.01

50.

014

0.01

10.

008

0.01

70.

011

Lu

0.25

80.

232

0.16

20.

188

0.20

30.

135

0.15

90.

005

0.00

50.

0062

0.00

510.

0038

0.00

310.

0027

0.00

250.

0019

0.00

310.

0029

Hf

0.89

0.76

0.24

0.33

0.41

0.27

0.24

0.00

90.

024

0.00

90.

011

0.03

30.

0121

0.00

610.

0061

0.00

680.

0098

0.00

75T

a0.

0052

0.02

250.

0023

0.00

350.

0069

0.00

620.

0043

0.02

0.00

860.

0012

0.00

120.

0019

0.00

440.

0065

0.00

270.

0016

0.00

140.

0019

Pb

0.24

0.22

0.3

0.44

0.47

3.55

0.16

7.94

4.07

3.18

25.0

41.

230.

941.

471.

872.

010.

731.

14T

h0.

034

0.08

10.

026

0.06

30.

097

0.08

60.

044

0.06

0.03

60.

012

0.01

80.

044

0.00

630.

0051

0.00

330.

0032

0.00

860.

0084

U0.

013

0.02

30.

012

0.01

80.

028

0.01

70.

013

0.02

60.

013

0.00

870.

0079

0.02

10.

0046

0.00

650.

0035

0.00

480.

0099

0.00

47


Fig. 4. Primitive mantle normalized incompatible trace element patterns for separated minerals: Asclinopyroxene and orthopyroxene;Ž . Ž .Bsplagioclase. Normalizing values after Sun and McDonough 1989 . See Fig. 2 for sample location .

normalized trace element patterns for cpx and pla-gioclase. By contrast, opx patterns are distinguishedby slight positive Zr–Hf anomalies, a feature whichis consistent with opxrcpx partition coefficients de-termined for HFSE from studies of mantle rocksŽ .e.g., Kelemen et al., 1993; Hart and Dunn, 1993

4.4. Oxygen isotopes

Taken as a whole, oxygen isotopic compositionsof cumulates and fine-grained rocks from the Laouni

Žlayered intrusions spread over a large range Table.3 . These variations correlate well with the two sets

of cumulate compositions identified through petro-graphic studies.

Whether belonging to WL- or EL-intrusions, troc-tolitic cumulates have very homogeneous oxygenisotope compositions. Both whole-rocks and sepa-rated minerals yield d

18 O values clustering withinŽthe compositional range of mantle rocks q5 to

.q6.2‰ . For these samples, the plagioclase-pyrox-ene oxygen isotope fractionations range between 0.14

Ž .to 1.19‰ Fig. 5 . This value indicates that oxygenisotope compositions of troctolitic cumulates werepreserved since the magmatic stage. As calculated by

Ž .Bottinga and Javoy 1975 and observed in unalteredŽ .peridotitic samples e.g., Taylor, 1968; Javoy, 1980 ,

d18 O cannot fractionate by more than 0.5‰ to 1.2‰

between plagioclase and pyroxenes at magmatic tem-Ž .peratures )8008C .


Tab

le3

O,

Sr

and

Nd

isot

opic

rati

osof

the

thre

eL

aoun

iin

trus

ions

1818

1818

8786

8786

147

143

Ech

.d

Od

Od

Od

OR

bS

rR

brS

rS

rrS

r2

sS

rS

rS

mN

dS

mr

Ndr

2s

´´

Ti

520

Ma

i60

0M

aN

dN

dD

M14

414

4N

dN

dP

lO

pxC

pxW

R52

0M

a60

0M

a

WL

-nor

itic

2H27

88.

082.

9151

20.

0164

40.

7046

820.

0000

080.

7045

600.

7045

411.

605.

900.

1640

0.51

2109

0.00

0010

y8.

2y

7.8

2919

2H33

59.

428.

639.

422.

5151

70.

0140

50.

7051

870.

0000

160.

7050

830.

7050

671.

024.

140.

1490

0.51

2114

0.00

0006

y7.

1y

6.6

2190

2H33

98.

733.

0855

40.

0160

80.

7048

630.

0000

170.

7047

440.

7047

252H

292

8.76

7.25

8.76

1.18

129

0.02

646

0.70

4888

0.00

0017

0.70

4692

0.70

4662

2H29

77.

192.

9452

20.

0162

90.

7041

310.

0000

080.

7040

100.

7039

921.

164.

670.

1502

0.51

2151

0.00

0007

y6.

4y

5.9

2142

2H34

29.

629.

439.

622.

5939

40.

0190

20.

7066

680.

0000

100.

7065

270.

7065

050.

552.

280.

1459

0.51

1818

0.00

0010

y12

.6y

12.1

2784

WL

-tro

ctol

itic

2H80

6.34

5.40

2H82

6.06

1.72

427

0.01

165

0.70

5549

0.00

0009

0.70

5463

0.70

5449

2H85

5.52

5.08

1.57

660.

0688

20.

7044

500.

0000

160.

7039

400.

7038

610.

732.

900.

1522

0.51

2572

0.00

0008

1.7

2.1

1167

2H10

95.

945.

801.

1343

50.

0075

10.

7040

830.

0000

150.

7040

270.

7040

190.

492.

770.

1070

0.51

2238

0.00

0010

y1.

8y

0.9

1150

2H19

27.

422.

1061

70.

0098

50.

7047

850.

0000

230.

7047

120.

7047

012H

256

7.42

()

LW

-Tro

ctol

itic

opx-

rich

laye

r2H

997.

616.

931.

8660

80.

0088

50.

7044

860.

0000

080.

7044

200.

7044

101.

103.

300.

2016

0.51

2217

0.00

0025

y8.

5y

8.6

y19

042H

104

7.66

6.80

1.22

930.

0379

50.

7050

000.

0000

100.

7047

190.

7046

750.

441.

750.

1521

0.51

2102

0.00

0011

y7.

5y

7.0

2334

EL

-tro

ctol

itic

2H58

45.

930.

6138

40.

0046

00.

7053

920.

0000

080.

7053

580.

7053

532H

588

5.86

0.63

504

0.00

362

0.70

3898

0.00

0008

0.70

3871

0.70

3867

8331

-16.

495.

731.

4823

60.

0181

40.

7040

210.

0000

080.

7038

870.

7038

662H

669

5.10

0.97

309

0.00

908

0.70

3048

0.00

0008

0.70

2981

0.70

2970

0.78

2.63

0.17

940.

5128

310.

0000

154.

95.

195

283

2525

6.34

5.15

1.31

469

0.00

808

0.70

3794

0.00

0007

0.70

3734

0.70

3725

8327

-56.

141.

2030

80.

0112

70.

7039

070.

0000

080.

7038

230.

7038

1183

2926

6.05

0.97

403

0.00

696

0.70

3726

0.00

0008

0.70

3674

0.70

3666

0.59

2.30

0.15

510.

5125

210.

0000

210.

50.

913

532H

543

5.92

0.05

433

0.00

0334

0.70

3766

0.00

0009

0.70

3764

0.70

3763

2H72

42.

2313

.075

60.

0496

00.

7051

850.

0000

100.

7048

170.

7047

612.

8017

0.09

960.

5122

290.

0000

30y

1.5

y0.

510

8983

25-3

35.7

882

0.11

710

0.70

5391

0.00

0009

0.70

4523

0.70

4389

2H63

88.

2148

.784

70.

1664

0.70

6218

0.00

0008

0.70

4985

0.70

4795

5.50

310.

1073

0.51

2143

0.00

0010

y3.

7y

2.8

1288

d18

Oin

per

mil

;R

b,S

r,S

man

dN

din

ppm

.


Ž . 18 18 Ž .Fig. 5. A d O plagioclase vs. d O pyroxenes OPX and CPX . Dotted field indicates isotopic compositions of the upper mantle; dashedŽ . 18lines delineate the pyroxene-plagioclase magmatic fractionation range. B Distribution of d O values of ultramafic–mafic cumulates

Ž .whole rocks .


In the WL-noritic intrusions, whole-rock data andseparated mineral analyses yield similar results. Onlyaltered sample 2H292 slightly deviates from themagmatic fractionation range, due to the high d

18 Oof the plagioclase. Plagioclase exchanges oxygenwith external fluids more rapidly than pyroxene

Ž .Gregory and Taylor, 1986 . Opx–plagioclase dise-quilibrium is therefore likely to be due to the degreeof hydrothermal alteration of this sample. Noriticcumulates are distinctly 18 O-enriched and display a

18 Ž .much wider range of d O q7.2 to 9.5‰ than thetroctolitic cumulates. The data points are clearly

Ž87 86 . Ž . Ž . Ž87 86 . Ž .Fig. 6. Srr Sr vs. 1rSr A ´ vs. Srr Sr B for the three Laouni intrusions.600 Ma Nd 600 Ma 600 Ma


offset from the mantle domain in Fig. 5. Within eachintrusion, oxygen isotopic compositions and mg-numbers are not correlated.

Like noritic cumulates, the two orthopyroxene-richŽcumulates from the WL-troctolitic intrusion 2H99

.and 2H104 have oxygen isotopic compositions inŽ .excess of the mantle range 6.6–7.5‰ . On average,

their values are mid-way between the mantle-likevalues of troctolitic cumulates and the enriched com-positions of the WL-noritic massif. Plagioclase–py-roxene oxygen isotope fractionations also fall withinthe magmatic fractionation domain of Fig. 5.

Oxygen isotopic compositions of fine-grainedrocks corroborate the trends defined by cumulates.Fine-grained samples 2H669 and 832926, which arehosted in the EL-troctolitic massif, have the same

18 Žmantle-like d O as WL-troctolitic cumulates q5-.q6‰ . Sample 2H297 collected in a dyke from the

WL-noritic massif is similarly enriched in d18 O as

opx-rich cumulates. Nevertheless, in both cases, thesefine-grained samples have slightly lower d

18 O thancumulates of similar mineralogy.

No relationship between oxygen isotopic compo-sitions and the magmatic trend has been observed foramphibolitized samples from the external ‘border

Ž .ring’. One of the two samples analysed 2H724 has18 Ž .an anomalously low d O 2.23‰ which probably

reflects intense hydrothermal alteration driven byfluids released during cooling of the intrusion orcoming from the contact between the intrusion and

Ž .country rocks. The second sample 2H638 has high18 O compared to cumulates and fine-grained rocks

Žanalysed in the EL-troctolitic intrusion q8.21‰.vs.q5–q6‰ .

4.5. Strontium and neodymium isotopes

Initial strontium isotope ratios, corrected for 600ŽMa, vary within a relatively wide range 0.7029 to

.0.7065; Table 3 but are not correlated with 1rSrŽ .Fig. 6 . The two sets of cumulates overlap, althoughthe WL and EL troctolitic cumulates, as a mean,have more depleted mantle-like 87Srr86Sr ratiosŽ .0.7030 to 0.7054 than the WL-noritic cumulatesŽ .0.7040–0.7065 . Strontium isotopic compositions offine-grained and dyke rocks reflect those of coexist-ing cumulates. As observed for oxygen isotopes,sample 2H669 from the EL-troctolitic massif has the

87 86 Ž .lowest Srr Sr 0.7029 of all the Laouni cumu-Žlates analysed, while sample 2H297 WL-noritic

. 87 86massif has the lowest Srr Sr from the opx-richcumulates.

´ recalculated at 600 Ma for all cumulatesNdŽ .varies from q5 to y12.25 Table 3 . They discrimi-

Ž .nate Fig. 6B the olivine–plagioclase–clinopyro-xene cumulates which have depleted mantle-like ´ NdŽ .)y1 and the noritic cumulates, characterized by

Ž .lower ´ y5.9 to y12.1 . Orthopyroxene-richNd

cumulates from the LBZ of the WL-troctolitic intru-sion do not differ from noritic cumulates in their ´ NdŽ .y7.0 and y8.6 . The fine-grained and dyke-rocksanalysed are in average less radiogenic than thecorresponding cumulates: this is true for sample

Ž2H669 ´ sq5.1 vs.q1.5 to y3.8 for troctoliticNd. Žcumulates and 2H297 y5.9 vs. y6.4 to y12.6 for

.noritic and opx-rich cumulates . The diorite from theEL-external ring is intermediate between the opx-bearing and opx-free rocks.

5. Discussion

5.1. Petrographical and geochemical eÕidences for asingle primitiÕe continental tholeiitic magma

ŽOn the basis of the amount and Fo-content F.88% of olivine in LBZ orthocumulates, Cottin and

Ž .Lorand 1990 suggested a mantle-derived origin forthe parental magma of Laouni. Oxygen and radio-genic isotope compositions support this origin forplagioclase-olivine-clinopyroxene cumulates. Sample2H669, which is a gabbroic fine-grained rock of theEl-troctolitic massif, is the closest to a depletedupper mantle reservoir for the three ´ , Sr andNd i

d18 O isotopic parameters. By contrast, mg-numbers

of olivine and opx in noritic and opx-rich cumulatesŽ .Fo-83; En-80 are too low to have been equili-brated with mantle-derived primitive magma. Rela-tive to troctolitic cumulates, these rocks show higherSr and d

18 O and lower ´ classically interpreted ini Nd

terms of the variable assimilation of crustal materialsŽby mantle-derived magmas e.g., Taylor, 1980; Al-

.therr et al., 1988; Kempton and Harmon, 1992 .Moreover, the calc-alkaline differentiation trendrecorded by the WL-noritic intrusion is symptomaticof high water pressure and oxygen fugacity, as


demonstrated by calcic plagioclase compositions, rel-ative abundances of hydrous phases and Fe–Ti ox-

Ž .ides e.g., Groves and Baker, 1984 .The question that arises is to know if the Laouni

magma chambers have been fed by one magma thatexperienced various degrees of crustal contaminationor by two different magmas. The ‘single magma’hypothesis is supported by incompatible trace ele-ment compositions of separated minerals, especiallyplagioclase, which displays similar primitive-mantlenormalized patterns, whether separated from noriticor troctolitic cumulates. This conclusion can be re-fined by calculating the trace element content ofliquids in equilibrium with separated minerals usingpublished mineralrliquid partition coefficients. Data

Žavailable in the literature Kelemen et al., 1993; Hart.and Dunn, 1993 allows this procedure to be done

for cpx and opx only. Either calculated from cpxŽ . Ž .troctolitic or opx noritic , the REE patterns areremarkably similar both in shape and elemental

Ž .abundances Fig. 7A . They all display the moderatewŽ . xLREE enrichments LarYb s2–3 of continentalN

tholeiites. Likewise, there is no systematic differenceof extended trace element patterns calculated fromnoritic or troctolitic cumulates. Both are character-

Žized by LILE enrichment 5–20=MORB abun-. Ždances and strong HFSE negative anomalies Fig.

.7B . Laouni calculated liquids reproduce quite wellpatterns of the fine-grained dyke 2H256, which crosscuts the WL-troctolitic massif, and of continental

Ž .tholeiites from worldwide provenances Fig. 7C .The differences between calculated liquids and ana-

Žlyses of natural lavas e.g., a factor 5 for REE.absolute abundances are likely to be due to large

uncertainties in the determination of mineralrliquidpartition coefficients for trace elements and of ki-netic equilibration between cumulus phases and in-

Žterstitial trapped melt Cawthorn, 1996; Blundy,.1997; Mathez et al., 1997 . However, the remarkably

homogeneous trace element compositions of the li-quids in equilibrium with noritic and troctolitic cu-

Ž .mulates argues for 1 a unique LILE-enriched and

HFSE-depleted continental tholeiitic magma beforecrustal contamination and magmatic differentiation;

Ž .and 2 a trace element pattern mostly inherited fromthe mantle source.

Concerning major elements, it is necessary torecall that dykes and fine-grained rocks hosted byWL- and EL-troctolitic cumulates show low mg-numbers and strong cumulus plagioclase effect. Evenfor sample 2H669 olivine fractionation at depth isnecessary to explain mg-numbers of 0.59. Neverthe-less, its composition probably provides the bestavailable estimate of the primitive continental tholei-ite magma that was involved in the Laouni intru-sions. Olivine tholeiite compositions experimentally

Žinvestigated in the literature at low pressure ;0.2. Ž .GPa and low H O content -0.2 wt.% reproduce2

the order of crystallization of cumulus phases inŽtroctolitic cumulates quite well i.e., chromite–

olivine; chromite–olivine–plagioclase; olivine–plagioclase–clinopyroxene; plagioclase–clinopyro-

. Žxene–orthopyroxene e.g., Fisk and Bence, 1980;Groves and Baker, 1984; Nicholls et al., 1986;

.Michael and Chase, 1987 .

5.2. Isotopic constraints for interaction between aprimitiÕe continental tholeiitic magma and both lowercrust and upper crust

It is important to recall that formation of thealtered ‘border ring’ is observed both in the WL-noritic and EL-troctolitic intrusions, regardless oftheir lithology. This is the best evidence that directassimilation of country rocks cannot explain the twoLLD’s recorded by the Laouni intrusions. Accordingto petrological constraints, crustal contaminationstarted very early in the history of the Laouni magmachambers. Indeed, the close mineralogical similaritybetween the altered ‘border ring’, cumulates andfine-grained rocks in the WL-noritic intrusion canonly be explained by pre-emplacement deep crustalcontamination. Moreover, pyroxene-plagioclase oxy-

Ž . Ž .Fig. 7. Chondrites-normalized REE A and incompatible trace element B patterns for calculated liquids in equilibrium with orthopyrox-Ž . Ženes and clinopyroxenes. Patterns of fine-grained rock 2H256 and continental tholeiites Morocco; Bertrand et al., 1982 and Deccan

. Ž .Trapps; Lightfoot and Hawkesworth, 1988 are given for comparison C . For symbols see Fig. 3. Normalizing values after Sun andŽ .McDonough 1989 .


gen isotope fractionations were inherited from mag-matic temperatures in both noritic and troctoliticcumulates.

In order to verify this assumption, oxygen, stron-tium and neodymium isotopes have been determinedin some country rocks hosting the Laouni layeredintrusions, i.e., three samples of syn-kinematic gran-ites, one migmatitic granite, a granulitic orthopyrox-ene–cordierite–biotite-bearing metavolcanite and a

Ž .metagraywacke sample Table 4 .For deeper crust, other crustal components identi-

fied in the Tuareg shield and suspected in the LaouniŽ .terrane Black et al., 1994 were used. The Assod-Is-

Ž .salane terrane Aır, Fig. 1 has been intruded at ca.¨666 Ma by a widespread parauthochtonous potassic

Ž .leucogranite Renatt granite , in response to a post-collisional crustal collapse following a lithospheric

Ž .delamination Liegeois et al., 1994 . Largest and´cleanest zones of this granite have yielded strongly

Žnegative ´ values y20 to y25; T model agesNd DM.in the 2500–3100 Ma range for moderately radio-

Ž .genic Sr initial ratios around 0.710 , interpreted asrepresenting its source, an Archaean granulitic lower

Ž .crust Fig. 8A . In most areas, this granite showsimportant interactions with its Eburnian amphi-

Žbolite-facies country-rocks numerous partly digested.xenoliths, migmatitic contacts, . . . . Samples from

these areas show important variations in Nd and Srinitial isotopic ratios, going up to y14 for ´ NdŽ .lowering T to 1500 Ma and up to 0.780 in Sr ,DM i

Ž .with all intermediate values Fig. 8A . This has beeninterpreted as the result of the observed interactions

Žwith the Eburnian upper crust ‘upper’ meaning iso-topically and geochemically undepleted crust, even if

.located in amphibolite facies . Through lower crustmelting and important interactions with upper crustduring emplacement, the Renatt granites give then aconstrained average of both these crusts in the Tu-

Žareg shield for Sr and Nd isotopic ratios Liegeois et´.al., 1994 . To see whether Renatt-type leucogranites

were involved in the contamination process of theLaouni magma, two Renatt samples with Sr and Ndisotopic compositions of the Archaean lower crusthave been analysed for oxygen isotopes. In addition,oxygen isotopic ratios have been determined in a

Ž .metagreywacke 2H40-0 which has Nd and Sr iso-topic compositions similar to the average for Ebur-

Ž .nian Renatt upper crust Table 4; Fig. 8A .

These end-member isotopic compositions demon-strate that the range of isotopic composition of theLaouni layered intrusions cannot be attributed to a

Žsimple mixing process between two reservoirs Fig..8 . Nd and Sr isotopes point to mixing between a

Ž .mantle-derived continental tholeiitic magma 2H669Ž .and a Renatt-type lower crust Fig. 8A . Oxygen

isotopic compositions of the noritic cumulates re-Ž .quire the participation of the upper crust Fig. 8B,C ,

here fixed up by metagreywackes, similar in Sr andNd isotopic compositions to the Eburnian upper crust

Ž .in Aır Fig. 8A . An upper crust effect on Sr isotopes¨Žcan be envisaged but it had to be very slight Fig.

.8B . Country-rocks in the immediate vicinity of theŽ .Laouni complexes 2H348; 222.9 cannot explain the

Laouni Sr, Nd, O isotopic compositions except forŽ .the dioritic ring dyke 2H638 . This rock acquired

Ž .most likely its isotopic composition Fig. 8A-Cthrough strong interaction with country-rock duringemplacement, which modified also its mineralogy.Despite this important interaction, that the analyzedsample come from a ring around a troctolitic massifis obvious.

The decoupling between Sr–Nd radiogenic iso-topes and oxygen isotopes can be ascribed to differ-ences in H O content and temperature between lower2

and upper crust. O-isotopes are less sensitive than Srand Nd isotopes to deep crustal contamination be-cause of the almost anhydrous nature of the lowercrust and its d

18 O not much higher than that of theŽ .mantle Taylor, 1980; this study . This is shown by

samples 2H82 and 2H584, which define a Sr enrich-i18 Ž .ment at constant d O Fig. 8B . When en route

towards the surface, the mantle magma already con-taminated by the lower crust met the wet upper crust;it rapidly reached high d

18 O values due to a highoxygen waterrrock ratio. By contrast, it did notsignificantly influence the Sr and ´ because ofi Nd

much lower Sr and Nd waterrrock ratios.This decoupling between stable and radiogenic

isotopes, and the variety of contaminants involved,makes it difficult to estimate the amount of assimi-lated material using assimilation–fractionation mod-

Ž .els AFC . Taking oxygen for example, the amountof assimilated material can be estimated by a crudem ass balance calculation based on theoxygen isotope conservation law: d

18 O smagma18 Ž .d O X q 1yX =initialmagma initial magma initial magma


Tab

le4

O,

Sr

and

Nd

isot

opic

rati

osof

coun

try-

rock

s86

8786

147

144

143

144

1887

Ech

.d

OR

bS

rR

brS

rS

rrS

r2

sS

rS

rS

mN

dS

mr

Nd

Ndr

Nd

2s

´´

Ti

520

Ma

i60

0M

aN

dN

dD

M

WR

520

Ma

600

Ma

Syn-

kine

mat

icgr

anit

es2H

157

7.93

2H42

310

.05

2H42

49.

19

Mig

mat

icgr

anit

e22

2.9

10.4

478

.662

60.

3633

70.

7085

820.

0000

180.

7058

890.

7054

734.

8123

.90.

1218

0.51

2225

0.00

0006

-3.1

-2.3

1353

Cd-

Gt

gran

ulit

e2H

348

12.6

116

316

0.14

692

0.71

0017

0.00

0011

0.70

8928

0.70

8760

5.03

26.8

0.11

370.

5121

170.

0000

06-4

.7-3

.814

09

()

Met

agra

ywac

keE

burn

ian

med

ium

crus

tre

pres

enta

tiÕe

2H40

-011

.56

80.2

31.0

7.58

00.

8324

750.

0000

120.

7762

970.

7676

173.

1017

.70.

1059

0.51

1695

0.00

0004

-12.

4-1

1.4

1904

Ž.

Ren

att

leuc

ogra

nite

Arc

haea

nlo

wer

crus

tre

pres

enta

tive

Sr

´i

666

Ma

Nd,

666

Ma

BL

N30

8)7.

6424

439

18.3

770.

8896

810.

0000

110.

7150

083.

6012

.40.

1755

0.51

1423

0.00

0024

-22.

5-

BL

N14

4)8.

1928

354

15.4

700.

8567

400.

0000

300.

7097

003.

8118

.70.

1231

0.51

1178

0.00

0008

-22.

731

87

18Ž

.)

Exc

ept

dO

,da

tafr

omL

iege

ois

etal

.19

94.

´d

18O

inpe

rm

il;

Rb,

Sr,

Sm

and

Nd

inpp

m.


Fig. 8. Identification of potential crustal contaminants for theŽ . 18Laouni layered intrusions through ´ vs. Sr A ; d O vs. SrNd i i

Ž . 18 Ž . ŽB and ´ vs. d O C country rocks-migmatitic and syn-Nd

kinematic granites, metagraywackes and Gt–Cd granuliticŽ . Ž .metavolcanites this study ; Renatt granites Liegeois et al., 1994 .´

d18 O . The result is that noritic cumu-assimilatedmaterial

lates with d18 O around q9 require more than 35%

assimilation of an upper crustal material with d18 O

of q12 into a mantle-derived olivine tholeiite withaverage d

18 Osq5.5. This value has to be com-pared with major element data. Because parentalmagma composition for the noritic massif is lacking,the liquid in equilibrium with norite 2H342 has beenrecomputed using least square calculation with avail-able mineralrliquid partition coefficients for major

Ž .elements Thy and Xenophontos, 1991 , modal andŽ .phase compositions Cottin and Lorand, 1990 . The

Žcalculated composition SiO s52.25%; Al O s2 2 3

14.96%; FeO s 16.71%; MgO s 4.46%; CaO st.7.74%; Na Os2.71% is close to a quartz norma-2

tive tholeiite, except for the FeO rMgO ratio, higher,t

not unlike those of some Continental Flood BasaltŽFeeder dykes see composition 6 in Philpotts and

.Asher, 1993 . This composition makes assimilationof crustal felsic material containing 60–70 wt.%SiO necessary if the mass balance estimate based2

on oxygen isotopes is correct. Metagreywackes orRenatt-type upper crustal granitoids have suchsiliceous compositions, by contrast to garnet–

Žcordierite–metavolcanites 2H348: 47.7 wt.% SiO ;2.Guiraud et al., 1996 .

Apart from samples 2H82 and 2H584 Fig. 8B,isotopic compositions of troctolitic cumulates arguefor a very low percentage of lower crust assimilationin their parental magma. Absence of upper crustcontamination in the troctolitic cumulates may bedue to shorter residence time in the crust, or tocooler or less chemically reactive crust. It is wellknown that the barrier to assimilation of crustalmaterial in basaltic magma is thermal: the higher thetemperature of the wall rock, the greater the amount

Ž .of the assimilation Campbell, 1985 . In the WL-troctolitic intrusion, opx-rich cumulates are overlainby troctolitic and gabbroic cumulates unprovided or

Ž .poorer in intercumulus opx Fig. 2B . This impliesthat the youngest magma batches, characterized byhigher Mg number and ´ and by lower d

18 O andNd

Sr , in the TZ and MMZ, were less contaminatedi

than the oldest ones, probably because of a decreas-ing reactivity of the crust. To phrase it another way,the first batches created a refractory halo in theupper crust that might have played a shield role forthe latest magma fluxes. The extreme case is repre-


sented by dyke rocks, which systematically displayless contaminated isotopic signatures than host cu-mulates. This shielding effect occurred in lower and

Župper crusts noritic cumulates are more contami-.nated by both crusts but was more efficient in the

upper crust: the troctolitic rocks are slightly contami-nated by lower crust, not by upper crust. A maincause is probably the thermal state of the crust. Suchpulsations between a lower and an upper chamberare likely to be related to tectonics.

5.3. A geodynamic model to explain petrologicalfeatures of the Laouni layered intrusions

Trace element and isotopic compositions of theleast contaminated fine-grained rocks constrain theorigin of Laouni continental tholeiites in a depletedmantle source having experienced short-term enrich-ments in incompatible trace elements. A short-termenrichment relative to partial melting andror meta-somatic component with low RbrSr and highSmrNd are necessary to explain why sample 2H669preserves Sr–Nd isotopic compositions close to the

Ž .Depleted MORB Mantle DMM end-member, inas-much as its ´ may be a less-than estimate due to aNd

possible assimilation of very small percentages ofdeep crustal contaminant. Isotopic values of this

Ž .sample Sr s0.7030 and ´ sq5.1 are close toi NdŽthe PREMA Prevalent Mantle; Zindler and Hart,

.1986 composition suggested to be the source of thesubcontemporaneous post-collisional alkaline–peral-kaline granite and gabbro dyke swarm of the Tin

ŽZebane area in western Hoggar Sr s0.7028 andi.´ sq6.2; Hadj Kaddour et al., 1998 . Incompati-Nd

ble trace elements-enriched portions of the mantleare preferentially located in lithospheric mantle roots

Ž .of continents Menzies and Hawkesworth, 1997 .This suggests a source at the interface between low-

Žest lithospheric mantle and asthenosphere Black and.Liegeois, 1993 . This implies the coexistence of the´

sources of the tholeiitic and alkaline–peralkaline se-ries in the Tuareg post-collisional setting whose rela-tionships have to be deciphered.

Olivine tholeiites similar to the presumed parentalmagma are generated by about 10% melting at mod-

Ž .erate pressure 1–2 GPa from a lherzolitic mantleŽ .see, for example, Falloon and Green, 1988 . Theinferred source composition and conditions of melt-

ing for the primitive continental tholeiite are goodevidence that the Laouni intrusions developed in apost-collisional setting of lithospheric thinning. Sucha model has been proposed to explain the high-Tlow-P metamorphism that reworked Proterozoic rocks

Ž .in the Laouni area Guiraud et al., 1996 . Fieldrelationships indicate that the opening of the threemagma chambers was contemporaneous with thismetamorphic climax. The reactivation of a N–Sstrike-slip mega shear zone provided a route for themantle-derived magma to reach upper crustal levels.

In addition to variations due to the time of mag-mas intrusions, there is evidence that crustal contam-ination did not operate homogeneously on a regionalscale for the three intrusions studied. The relativeproportions between noritic and troctolitic cumulatesindicate an eastward decrease of crustal contamina-tion which, as discussed above, is likely to be relatedto a sum of parameters such as the thermal state ofthe crust, its heterogeneous composition or magmareplenishment rates. There is little evidence formagma replenishment in the WL-noritic intrusion,which thus possibly crystallized from a unique con-taminated tholeiitic magma batch. On the other hand,the lithology of the EL-troctolitic intrusion indicatesperiodic refilling of the magma chamber by the sameuncontaminated olivine-tholeiitic magma. In theWL-troctolitic massif, it has been suggested on pe-trographic and isotopic grounds that the intimateassociation between olivine-chromite and orthopy-roxene cumulates cannot be interpreted in terms ofsimple fractional crystallization from a single magma.The sympathetic variations between crystallization ofopx, increasing d

18 O and Sr and fall in mg-numbersi

and in ´ support a model of small-scale mixingNd

between resident contaminated tholeiite and moreprimitive magma batches. This mixing process pre-served isotopic heterogeneities on a metre-scale. Allof these isotopic and petrologic heterogeneities areinherent to small-sized magma chambers developedin shortlived regimes of post-collisional transcurrenttectonic settings.

6. Conclusions

The small Laouni layered intrusions within syn-kinematic Pan-African granites have been fed by


post-collisional continental tholeiitic magmas. Theselatter derived from one lithosphericrasthenospheric

Ž 18 87 86mantle source d Osq5‰; Srr Srs0.703;.´ sq5 slightly enriched in incompatible traceNd

elements, as demonstrated by liquid compositionscalculated from separated pyroxenes or compositionsof some fine-grained rocks.

Both calc-alkaline and tholeiitic liquid lines ofdescent have been identified in the Laouni magmachambers. The Laouni intrusions provide an exampleof the general observation that continental tholeiitescrystallizing orthopyroxene first invariably have as-similated deep continental crust, before reaching up-

Žper crustal levels example in feeder dykes Camp-. Žbell, 1985 . Assimilation of both lower Sr–Nd iso-

. Ž .topes and upper crust O isotopes material is re-quired to explain isotopic compositions of noriticcumulates. Troctolitic cumulates crystallized fromolivine tholeiite having assimilated only small per-centages of lower crust.

The Lower Banded Zone of the WL- troctoliticmassif provides mineralogical and geochemical evi-dence for mixing between contaminated and moreprimitive tholeiite in a single magma chamber. Thefact that, troctolitic and gabbroic cumulates overlieopx-rich cumulates, in this massif, suggests thatcrustal contamination preferentially affected the ear-liest magma batches.

The Laouni post-collisional continental tholeiitesprovide strong support to the hypothesis of a generallithospheric thinning event postdating the late-Pan-

Ž .African collision -600 Ma in the Laouni terraneŽ .in Central Hoggar Black et al., 1994 .

Acknowledgements

This study has been made possible through finan-cial supports provided by the French-Algeria cooper-

Ž .ation program 94-MDU-286: CMEP and by theŽ .CNRS UMR 6524 and ESA 7058 . Thanks are also

extended to Russell Black, Jacques Leterrier, MichelGuiraud, Louis Latouche and Prof. J. Fabries who`provided helpful encouragements and lively discus-sions at each stage of this study. Riccardo Tribuzioand J. Richard Wilson are thanked for their detailedand constructive reviews. Finally, we thank friendly

Michel Gregoire et Bertrand Moine for kind help in´several drawings.

References

Altherr, R., Henjes-Kunst, F., Matthews, A., Friedrichsen, H.,Tauber Hensen, B., 1988. O–Sr isotopic variations in Miocenegranitoides from the Aegean: evidence for an origin by com-bined assimilation and fractional crystallization. Contrib. Min-eral. Petrol. 100, 528–541.

Azzouni-Sekkal, A., 1989. Petrologie et Geochimie des granitesde type ‘Taourirt’: un exemple de province magmatique detransition entre les regimes orogeniques et anorogeniques, au´ ´ ´

Ž .Pan-Africain Hoggar-Algerie . PhD thesis, USTHB, Alger,´628 pp.

Bertrand, H., Dostal, J., Dupuy, C., 1982. Geochemistry of EarlyMesozoic tholeiites from Morocco. Earth Planet. Sci. Lett. 58,225–239.

Bertrand, J.M., Michard, A., Boullier, A.M., Dautel, D., 1986.ŽStructure and UrPb geochronology of Central Hoggar Alge-

.ria : a reappraisal of its pan-African evolution. Tectonic 5,955–972.

Boissonnas, J., 1973. Les granites a structure concentrique et`quelques autres granites tardifs de la chaine pan africaine en

Ž .Ahaggar Sahara central, Algerie . PhD thesis, Univ. Paris VI,´CNRS, 662 pp.

Bonin, B., 1996. A-type granite ring complexes: mantle originthrough crustal filters and the anorthosite-Rapakivi magma-

Ž .tism connection. In: Demaiffe, D. Ed. , Petrology and Geo-chemistry of magmatic suites of rocks in the continental andoceanic crusts. A volume dedicated to Pr. J. Michot, ULB andMRAC., pp. 201–218.

Black, R., Liegeois, J.P., 1993. Cratons, mobile belts, alkaline´rocks and continental lithospheric mantle. The Pan-africantestimony. J. Geol. Soc. Lond. 150, 89–98.

Black, R., Caby, R., Moussine-Pouchkine, A., Bayer, R., Bertrand,J.M.L., Boullier, A.M., Fabre, J., Lesquer, A., 1979. Evidencefor late Precambrian plate tectonics in West Africa. Nature278, 223–227.

Black, R., Liegeois, J.P., Latouche, L., Caby, R., Bertrand, J.M.,´1994. Pan-African displaced terranes in the Tuareg shieldŽ .central Sahara . Geology 22, 641–644.

Blundy, J., 1997. Experimental study of a Kiglapait marginal rockand implications for trace element partitioning in layeredintrusions. Chem. Geol. 141, 73–92.

Bottinga, Y., Javoy, M., 1975. Oxygen isotope partitioning amongthe minerals in igneous and metamorphic rocks. Rev. Geo-phys. Space Phys. 13, 401–418.

Briggs, R.M., McDonough, W.F., 1990. Contemporaneous con-vergent margin and intraplate magmatism, North Island, NewZealand. J. Petrol. 31, 813–851.

Caby, R., Andreopoulos-Renaud, U., 1987. Le Hoggar oriental,bloc cratonise a 730 Ma. dans la chaine Pan-Africaine du nord´ `du continent africain. Precamb. Res. 36, 335–344.

Caby, R, Bertrand, J.M.L., Black, R., 1981. Pan-African ocean


closure and continental collision in the Hoggar-iforas segment,Ž .Central Sahara. In: Kroner, A. Ed. , Precambrian Plate Tec-¨

tonics, pp. 407–433.Campbell, I.H., 1985. The difference between oceanic and conti-

nental tholeiites: a fluid dynamic explanation. Contrib. Min-eral. Petrol. 91, 37–43.

Campbell, I.H., Borley, G.D., 1974. The geochemistry of pyro-xenes from the lower layered series of the Jimberlana intru-sion, Western Australia. Contrib. Mineral. Petrol. 47, 281–297.

Cawthorn, R.G., 1996. Models for incompatible trace-elementabundances in cumulus minerals and their application to pla-gioclase and pyroxenes in the Bushveld Complex. Contrib.Mineral. Petrol. 123, 109–115.

Clayton, R.N., Mayeda, T.K., 1963. The use of Bromine pentaflu-oride in the extraction of oxygen from oxides and silicates forisotopic analysis. Geochim. Cosmochim. Acta 27, 43–52.

Cottin, J.Y., Lorand, J.P., 1990. Nouvelles donnees petrologiques´ ´sur les massifs ultramafiques–mafiques de la region de Laouni´Ž .Hoggar central, Algerie . Bull. Soc. Geol. Fr. 8, 525–536.´ ´

Cottin, J.Y., Guiraud, M., Lorand, J.P., 1990. Le magmatisme etle metamorphisme pan-africains temoins d’amincissement´ ´

Ž .crustal dans la region de Laouni Hoggar Central, Algrie ?.´Ž .C.R. Acad. Sci. Paris 311, 1345–1351, Serie II .´

De Paolo, D.J., 1981. Trace element and isotopic effects ofcombined wallrock assimilation and fractional crystallization.Earth Planet. Sci. Lett. 53, 189–202.

England, P., 1993. Convective removal of thermal boundary layerof thickened continental lithosphere: a brief summary of causesand consequences with special reference to the Cenozoictectonics of the Tibetan Plateau and surrounding regions.Tectonophysics 223, 67–73.

Falloon, T.J., Green, D.H., 1988. Anhydrous partial melting ofperidotite from 8 to 35 kb and the petrogenesis of MORB. In:

Ž .Menzies, M.A., Cox, K.G. Eds. , Oceanic and ContinentalLithosphere; Similarities and Differences, J. Petrol., Spec.Lithosphere Issue, pp. 379–414.

Fisk, M.R., Bence, A.E., 1980. Experimental crystallisation ofchrome spinel in Famous basalt 527-1. Earth Planet. Sci. Lett.48, 111–123.

Fourcade, S., Javoy, M., 1985. Preliminary investigations of18 Or16O and DrH composition in rhyo-ignimbrites in the In

Ž .Hibaou In Zize magmatic center, central Ahaggar, Algeria.Contrib. Mineral. Petrol. 89, 285–295.

Govindaraju, K., Mevelle, G., 1987. Fully automated dissolutionand separation methods for inductively coupled plasma atomicemission spectrometry rock analysis. J. Anat. At Spectrom. 2,615–621.

Gray, C.M., Goode, A.D.T., 1989. The Kalka layered intrusion,Central Australia. A strontium isotopic history of contamina-tion and magma dynamics. Contrib. Mineral. Petrol. 103,35–43.

Gregory, R.T., Taylor, H.P., 1986. Non-equilibrium, metasomatic18 Or16O effects in upper mantle mineral assemblages. Con-trib. Mineral. Petrol. 93, 124–135.

Groves, T.L., Baker, M.B., 1984. Phase equilibrium controls onthe tholeiitic versus Calc-alkaline differentiation trends. J.Geophys. Res. 89, 3253–3274.

Guiraud, M., Powell, R., Cottin, J.Y., 1996. Hydration of orthopy-roxene–cordierite-bearing assemblages at Laouni, CentralHoggar, Algeria. J. Metam. Geol. 14, 467–476.

Hadj Kaddour, Z., Demaiffe, D., Liegeois, J.P., Caby, R., 1998.´The alkaline–peralkaline granitic post-collisional Tin Zebane

Ž .dyke swarm Pan-African Tuareg shield, Algeria : prevalentmantle signature and late agpaitic differentiation. Lithos, thisvolume.

Hart, S.R., Dunn, T., 1993. Experimental cpxrmelt partitioning of24 elements. Earth Planet. Sci. Lett. 113, 1–8.

Huppert, H.E., Sparks, R.S.J., 1988. The generation of granitesmagmas by intrusion of basalt into the continental crust. J.Petrol. 28, 599–624.

Ionov, D.A., Savoyant, L., Dupuy, C., 1992. Application of theICP-MS technique to trace-element analysis of peridotites andtheir minerals. Geostand. Newslett. 16, 311–315.

Irvine, T.N., 1980. Magmatic infiltration metasomatism, double-diffusive fractional crystallisation, and adcumulus growth inthe Muskox intrusion and other layered intrusions. In: Har-

Ž .graves, R.B. Ed. , Physics of Magmatic Processes. Princeton,NJ Univ. Press, pp. 325–383.

Javoy, M., 1980. 18 Or16O and DrH ratios in high temperatureperidotites. Int. Coll. C.N.R.S., Grenoble 272, 279–287.

Kempton, P.D., Harmon, R.S., 1992. Oxygen isotope evidence forlarge-scale hybridization of the lower crust during magmaticunderplating. Geochim. Cosmochim. Acta 56, 971–986.

Kelemen, T.D., Schimizu, N., Dunn, T., 1993. Relative depletionof Niobium in some arc magmas and the continental crust:partitioning of K, Nb, La and Ce during meltrrock reaction inthe upper mantle. Earth Planet. Sci. Lett. 120, 111–134.

Liegeois, J.P., Black, R., 1987. Alkaline magmatism subsequent´to collision in the Pan-african belt of the Adrar des IforasŽ . Ž .Mali . In: Fitton, J.G., Upton, B.J.G. Eds. , Alkaline IgneousRocks. Geol. Soc. London Spec. Publ., 30, pp. 381–401.

Liegeois, J.P., Black, R., Navez, J., Latouche, L., 1994. Early and´late Pan-African orogenies in the Aır assembly of terranes¨Ž .Tuareg shield, Niger . Precamb. Res. 67, 59–88.

Lightfoot, P., Hawkesworth, C., 1988. Origin of Deccan Traplavas: evidence from combined trace element and Sr-, Nd- andPb-isotope studies. Earth Planet. Sci. Lett. 91, 89–104.

Lorand, J.P., Cottin, J.Y., 1987. Na- Ti- Zr- H O-rich mineral2

inclusions indicating postcumulus chrome–spinel dissolutionand recrystallisation in the Western Laouni mafic intrusionAlgeria. Contrib. Mineral. Petrol. 97, 251–263.

Lorand, J.P., Cottin, J.Y., Parodi, G.C., 1987. Occurence andpetrological significance of loveringite in the Western Laounilayered complex, Southern Hoggar, Algeria. Can. Mineral. 25,683–693.

Mathez, E.A., Hunter, R.H., Kinzler, R., 1997. Petrologic evolu-tion of partially molten cumulate: the Atok section of theBushveld Complex. Contrib. Mineral. Petrol. 129, 20–34.

Menzies, M.A., Hawkesworth, C.J., 1997. Mantle Metasomatism,Academic Press, 472 pp.

Michael, P.J., Chase, R.L., 1987. The influence of primary magmacomposition, H O and pressure on Mid-Ocean Ridge Basalt2

differentiation. Contrib. Mineral. Petrol. 96, 245–263.Moulahoum, O., 1988. Dualite du magmatisme d’age Pan-Afri-´ ˆ


cain: Aspects structuraux et petrologiques des granites subal-´Žcalins et alumineux de la region de Tamanrasset Hoggar´

.Central, Algerie . PhD Thesis, Univ. Nancy I, 145 pp.´Nedelec, A., Stephens, W.E., Fallick, A.E., 1995. The Panafrican´ ´

stratoıd granites of Madagascar: Alkaline magmatism in a¨post-collisional extensional setting. J. Petrol. 36, 1367–1391.

Nicholls, J., Russel, J.K., Stout, M.Z., 1986. Testing magmatichypothesis with thermodynamic modelling. In: Scarfe, C.M.Ž .Ed. , Silicate melts: their properties and structure applied toproblems in geochemistry, petrology, economic geology andplanetary geology. Mineral. Ass. Can., Ottawa, pp. 210–235.

Osborn, E.F., 1959. Role of oxygen pressure in the crystallizationand differentiation of basaltic magmas. Am. J. Sci. 257,609–647.

Philpotts, A.R., Asher, P.M., 1993. Wallrock melting and reactioneffect along the Higganum diabase dyke in Connecticut: con-tamination of a continental flood basalt feeder. J. Petrol. 34,1029–1058.

Rasmussen, E., Neumann, E.R., Andersen, T., Sundvoll, B., Fjerk-ingstad, V., Stabel, A., 1988. Petrogenetic processes associ-ated with intermediate and silicic magmatism in the Oslo rift,southeast Norway. Min. Mag. 52, 293–307.

Roeder, P.L., Emslie, R.F., 1970. Olivine–liquid equilibrium.Contrib. Mineral Petrol. 29, 275–289.

Steiger, R.H., Jager, E., 1977. Subcommission on geochronology:¨convention on the use of decay constant in geo-and cos-mochronology. Earth Planet. Sci. Lett. 36, 359–362.

Stewart, B.W., De Paolo, D.J., 1990. Isotopic studies of processes

in mafic magma chambers: II. The Skaergaard intrusion, EastGreenland. Contrib. Mineral. Petrol. 104, 125–141.

Sun, S.S., McDonough, W.F., 1989. Chemical and isotopic sys-tematics of oceanic basalts: implications for mantle composi-

Ž .tion and processes. In: Saunders, A.D., Norry, M.J. Eds. ,Magmatism in Ocean Basins. Geol. Soc. Lond. Spec. Publ.,42, pp. 313–345.

Sylvester, P.J., 1989. Post-collisional alkaline granites. J. Geol.97, 261–280.

Taylor, H.P. Jr., 1968. The oxygen isotope geochemistry onigneous rocks. Contrib. Mineral. Petrol. 19, 1–71.

Taylor, H.P. Jr., 1980. The effects of assimilation of country rocksby magmas on 18 Or16O and 87Srr86 Sr systematics in igneousrocks. Earth Planet. Sci. Lett. 47, 243–254.

Thy, P., 1987. Petrogenetic implication of mineral crystallizationtrends of Troodos cumulates, Cyprus. Geol. Mag. 124, 1–11.

Thy, P., Xenophontos, C., 1991. Crystallization order and phasechemistry of glassy lavas from the pillow sequence, Troodos,ophiolite, Cyprus. J. Petrol. 83, 150–158.

Whalen, J.B., Currie, K.L., Chappell, B.W., 1987. A-type gran-ites: geochemical characteristics, discrimination and petrogen-esis. Contrib. Mineral. Petrol. 87, 319–327.

Wilson, J.R., Esbensen, K.H., Thy, P., 1981. Igneous petrology ofthe Synorogenic Fongen-Hyllingen layered basic complex,south–central Scandinavian Caledonides. J. Petrol. 22, 584–627.

Zindler, A., Hart, S.R., 1986. Chemical geodynamics. Annu. Rev.Earth Planet. Sci. 14, 493–571.

for post-collisional continental tholeiitic magmas ... · intrusion was formerly referred to as...

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