a two-stage exhumation of the variscan crust: u–pb la-icp-ms and

A two-stage exhumation of the Variscan crust: U–Pb LA-ICP-MSand Rb–Sr ages from Greater Kabylia, Maghrebides

D. Hammor,1,2 D. Bosch,2 R. Caby2 and O. Bruguier3

1Universite Badji-Mokhtar, BP12, El-Hadjar, Annaba 23 000, Algeria; 2Laboratoire de Tectonophysique, Universite de Montpellier II, Place

Eugene Bataillon, 34 095 Montpellier Cedex 5, France; 3Service ICP-MS, Universite de Montpellier II, Place Eugene Bataillon, 34 095

Montpellier Cedex 5, France

Introduction

The Maghrebides are part of the peri-Mediterranean belt of late Tertiaryage that delimits the African and theEuropean plates and runs from theBetico-Rifan arc to Calabria (Fig. 1a,inset). Classical interpretations (e.g.Ricou, 1994) consider that theyformed during the 40–25 Ma timespan as a result of underthrusting ofthe North African margin beneath theAlboran plate (Betic-Rif-Kabylies).The inner zones of the Maghrebidesare represented by the Kabylies,mainly formed by inliers of crystallinerocks surrounded by Oligo-Mioceneand younger Miocene sediments. Pre-Oligocene reconstructions locate theKabylies at ‡700 km NNW from theirpresent-day location, along with theircounterparts in the Betico-Rifan arcand Calabria-Sicily (Lonergan andWhite, 1997; Gueguen et al., 1998).Classical ideas considered that the

Kabylies underwent only slight Alpineoverprint (e.g. Peucat et al., 1996).However, 40Ar/39Ar ages of high-tem-perature minerals obtained in GreaterKabylia (Monie et al., 1988) and Rb/Sr Alpine ages of biotites in Lesser

Kabylia (Peucat et al., 1996) suggestthat Alpine events were not negligiblein the Kabylian basement units. Inthis study, we present LA-ICP-MSU–Pb results from monazites and Rb–Sr analyses from biotites extractedfrom a major high-temperature crustalshear zone from Greater Kabylia.This study was undertaken in orderto give time constraints on the mainhigh-temperature shearing event thataffected the crystalline rocks of theKabylian basement and on its possiblereactivation during subsequent events,which has implications for unravellingthe tectonometamorphic evolutionthrough time of this part of the peri-Mediterranean fold belt.

Geological setting

Greater Kabylia comprises three ma-jor domains: Central Greater Kabylia(CGK), Eastern Greater Kabylia(EGK) and the Sidi Ali Bou Nab(SABN) domain (Saadallah andCaby, 1996) (Fig. 1a,b). In CGK, theKabylian Detachment Fault is amajor low-angle ductile to cataclasticextensional shear zone that sharplydelimits a lower unit of amphibolitefacies rocks below, from overlyinggreenschist facies phyllites with 295–315 Ma 40Ar/39Ar mineral ages (Mon-ie et al., 1988) and non-metamorphicfossiliferous Palaeozoic sediments.This upper unit, free of Alpine ductiledeformation, is unconformably over-

lain by the Mesozoic to Tertiarysedimentary cover of the CalcareousRange capped by allochthonousKabylian flyschs. The lower unit,exposed in two half domes, comprisesa continuous tectonic pile, 6–8 kmthick, of orthogneisses, paragneisses,marbles and micaschists affected byhigh-temperature syn-metamorphicductile deformation and yielding40Ar/39Ar ages bracketed between 80and 120 Ma (Monie et al., 1988). TheSABN unit that is dealt with thisstudy exposes another tectono-meta-morphic pile showing a normal meta-morphic polarity with downwardpressure and temperature increase. Itis in tectonic contact with the Naceriadiatexites in the north. The SABNgranite has been dated by the U–Pbzircon conventional method at284 ± 3 Ma (Peucat et al., 1996). Itdisplays a low-pressure thermal au-reole (biotite, andalusite, cordierite,K-feldspar, corundum) formed at£3 kbar pressure. Hornfelses wereprogressively sheared downwards andaffected by a distinct synkinematicmetamorphic overprint portrayed bythe replacement of andalusite bystaurolite and kyanite. This metamor-phic field gradient indicates tempera-ture and pressure increase downward.The deepest rocks exposed onthe southern flank of the SABNridge below a north-dipping band ofhigh-temperature ultramylonites com-prise slightly anatectic metapelites,

ABSTRACT

The significance and role of major shear zones are paramount tounderstanding continental deformation and the exhumation ofdeep crustal levels. LA-ICPMS U–Pb dating of monazites,combined with Rb–Sr analyses of biotites, from an anatecticmetapelite from Greater Kabylia (Algeria) highlights the historyof shear zone development and the subsequent exhumation ofdeep crustal levels in the internal zones of the Maghrebides.Monazites give an age of 275.4 ± 4.1 Ma (2r) dating thethermal peak coeval with anatexis. This age is identical to thoserecorded in other crystalline terranes from south-easternmost

Europe (i.e. South Alpine and Austro-Alpine domains) thatsuffered crustal thinning during the continental rifting predat-ing the Tethys opening. Rb–Sr analyses of biotites yield acooling age of 23.7 ± 1.1 Ma related to the exhumation of theburied Variscan crust during the Miocene as an extrusive slice,roughly coeval with the emplacement of nappes, and shortlyfollowed by lithospheric extension leading to the opening ofthe Alboran sea.

Terra Nova, 18, 299–307, 2006

Correspondence: Delphine Bosch, Labora-

toire de Tectonophysique, Universite de

Montpellier II, Place Eugene Bataillon,

34095 Montpellier Cedex 5, France. Tel.:

+33 4 67 14 32 67; fax: +33 4 67 14 36 03;

e-mail: [email protected]

� 2006 Blackwell Publishing Ltd 299

doi: 10.1111/j.1365-3121.2006.00693.x

0

SEMylonitic/cataclasiticfront (KDF)

Cataclastic fault

5 km

Calcareous Range

BoghniBasin (21 Ma)

Sidi Ali Bou NabMassif

Central Kabylia Dome

NW

0

5 km

Naciria Low-Pdiatexites

Garnet-kyanitemylonitic metapelites

Sidi Ali Bou Nabgranitoids

Saravalianmolasse Low-P

hornfelses

RT-95

Lower Unit Upper Unit

Micaschist

Orthogneiss

Anatectic paragneissand marbles

Granite gneiss

Molasse

Phyllites

Blastomylonite

0

CENTRAL GREATER KABYLIA

Miocene to Pliocene rocks

Flysch

Calcareous range

Paleozoic series

Phyllites

High-grade metamorphicsundifferentiated

Sidi Ali Bou Nab Units(including blastomylonite)

Foliation

Lineation

Kabylian detachment fault

Major main Miocenenormal fault

Lineation trajectories

Upper unit

EASTERN GREATER KABYLIA

36 30

0 5 10

A

Lower unit

Ighil Bouzrou

Tizi Ouzou

Naceria

4 00

A'

BOGHNI BASIN

4 00

SABNMassif

36 30

oceanic crust

500 Km

Alboran sea

Betics

Kabylia

extended continental

crustIBERICPENINSULA

Rif

Sardinia

Greater Lesser

A A’

(a)

(b)

Fig. 1 (a) Simplified geological sketch map of the Greater Kabylia Massif showing the main lithostratigraphic units (modified fromSaadallah and Caby, 1996). Inset shows the peri-Mediterranean Belt of Late Tertiary age. (b) Interpretative cross-sections of theSidi Ali Bou Nab massif showing the location of the studied sample (RT-95).

A two-stage exhumation of the Variscan crust • D. Hammor et al. Terra Nova, Vol 18, No. 5, 299–307

.............................................................................................................................................................

300 � 2006 Blackwell Publishing Ltd

calc-silicate gneisses, garnet amphibo-lites, rare pyroxenites, and meta-pegmatites. The transtensionalcharacter of the mylonites deducedfrom syn- to late-metamorphic shearcriteria accounts for the exhumationof metamorphic rocks from a depth ofabout 30 km, as well as for consider-able syn-metamorphic thinning of theformer tectonic pile which is now lessthan 3 km thick. Biotite and musco-vite yielded 40Ar/39Ar plateau agesaround 25–30 Ma (Monie et al., 1984,1988) and the high-grade mylonitesthat delimit the base of the SABNgranite have been tentatively inter-preted as having assisted Mioceneexhumation of the middle crust (Sa-adallah and Caby, 1996).The rock analysed for geochrono-

logical purposes (RT-95) is an anatec-tic graphitic metapelite displaying aprotomylonitic fabric (Fig. 2a). Thebanding is defined by alternating bio-tite-rich restitic layers containingclasts of kyanite and garnet up to1.5 cm in diameter, and quartzofeld-spathic ribbons containing antiperth-itic plagioclase clasts, considered assheared leucosomes. Kyanite is ob-served as minute synkinematic prismsin the matrix and as polycrystallineclasts (Fig. 2e) interpreted as pseudo-morphs after andalusite, as describedfrom Lesser Kabylia basement (Mad-joub et al., 1997). Sillimanite needlesare common along some myloniticbands and also occur as inclusions inall minerals (Fig. 2d) and at grainboundaries. No clear microstructuralrelationships can be determined be-tween syn-kinematic matrix kyaniteand fibrolite. Monazite occurs in thematrix or as inclusions in variousminerals (Fig. 2f) and in leucosomeswhere it is occasionally euhedral(Fig. 2g). Pairs of primary garnetcores and primary biotite inclusions(Fig. 2c) give temperatures around700 �C, whereas secondary biotiteand garnet overgrowth (Fig. 2d) yieldtemperatures of about 740 �C (Ferryand Spear, 1978). Pressure estimatesusing the garnet–plagioclase–kyanite–quartz geobarometer (Hodges andSpear, 1982) give 1 GPa for peakpressure in agreement with the occur-rence of rutile and ilmenite coexistingwith kyanite (Bohlen et al., 1983).These estimates and petrological con-siderations indicate that after theemplacement of the SABN pluton,

synkinematic peak thermal conditions(700–740 �C) and partial melting tookplace towards the boundary betweenkyanite and sillimanite stability fieldsand were followed by pervasive high-temperature extensional shearing. Acooling path towards the boundarybetween kyanite–sillimanite stabilityfields is thus suggested. Minute whitemica is rarely observed along somefiner grain ribbons and documents afinal stage of negligible synkinematiclow-temperature retrogression.

Analytical techniques

For U–Pb analyses, monazite grainswere mounted in epoxy resin withchips of a standard material (Manang-otry crystal of Poitrasson et al., 2000)and grounded down to half theirthickness to expose internal structures.Data were acquired at the Universityof Montpellier II using a VG Plasma-quad II turbo ICP-MS coupled with aGeolas (Microlas) automated plat-form housing a 193 nm Compex 102laser from LambdaPhysik (Gottingen,Germany). Data were acquired in thepeak jumping mode similar to theprocedure described in Bruguier et al.(2001) using an energy density of15 J cm)2 at a frequency of 5 Hz anda spot size of 26 lm. 232Th was notmeasured during the course of thisstudy as the high Th concentrationsresulted in a detector saturation. It wastherefore not possible to assess thereliability of the 232Th–208Pb systemfor the measured monazites. Forinstrumental mass bias and Pb–Ufractionation, measured standardswere averaged to give the respectivebias factors and their associatederrors, which were propagated withthe analytical errors of each unknown.For Pb–U ratios, this typically resultedin a 2–5% precision (1r RSD%) afterall corrections have been made which,in this age range, translates to a5–20 Ma uncertainty (see Table 1). Inthe course of this study, 16 analysesof the Manangotry monazite yieldeda 207Pb/206Pb weighted mean of0.05862 ± 0.00019 (2r) correspond-ing to an age of 553.0 ± 7.1 Ma.This is in good agreement with theEMP (Electron Micro Probe) (557 ±20 Ma, Montel et al., 1996) referenceage. Ages quoted below were calcula-ted using the Isoplot program ofLudwig (1999). For Rb–Sr analyses

100 mg of whole-rock sample and10 mg of biotite were dissolved in aHF/HNO3 mixture at 120 �C. Afterconversion to chlorides, aliquots of thesamples were spiked with 87Rb and84Sr. Rb and Sr were separated byconventional cation-exchange proce-dures. Isotopic measurements weremade on a VG Sector multi-collectormass spectrometer at the University ofMontpellier II. An average 87Sr/86Srisotopic ratio of 0.710246 ± 20 (2r)was measured for NBS 987 (n ¼ 2).Blanks were lower than 50 pg for Rband Sr and no blank correction wasmade.

Geochronology

Forty-six spots were performed on 17grains and the results are reported inTable 1. The monazite crystals haverounded to irregular shapes (seeFig. 3) although euhedral grains alsooccur in the leucosomes. Back-scat-tered electron imaging indicates thatmost grains show a homogeneousinternal structure suggesting a simplegrowth history (Fig. 3a), but someoften exhibit zones of different bright-ness, where dark zones (possibly lowTh) replace homogeneous brighterparts (Fig. 3b). These dark zones areirregularly distributed, suggesting thatbulk diffusion was not the mechanismresponsible for their formation. Asthey are preferentially, but not exclu-sively, located in fractured parts of thecrystals, they are interpreted as reflect-ing modification of the original com-position during recrystallizationprocesses that may have been en-hanced by fluid flows.Reported on a Terra-Wasserburg

diagram (Fig. 4) most data pointslocate close to, or on Concordia ataround 280 Ma. Some points aremarkedly younger, suggesting thatthey have suffered U–Pb disturbances.Grain 20, for example, yields a hetero-geneous age distribution with206Pb/238U ages ranging from c. 140to 240 Ma. This is interpreted asreflecting post-crystallization distur-bances, which are tentatively relatedto the dark BSE (Back ScatteredElectron) replacement zones observedin some crystals. This is consistentwith a younging of measured agesassociated with these zones. Youngerages present in other analyses (10-4,11 and 18) are considered as outliers,

Terra Nova, Vol 18, No. 5, 299–307 D. Hammor et al. • A two-stage exhumation of the Variscan crust

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Pl

Pl

Mnz

Mnz

Qtz

Qtz

Qtz

Qtz

Qtz

Bt

Grt1

Grt2

Grt2

Grt

Grt

Ky

Ky

Ky

Ky

Ky

Bt1

Bt2

Sil

Bt1

Mnz

BtBtBt

MnzKy

Ky

BtBt

Ky

Bt2

(a)

(c)

(e)

(b)

(d)

(f)

(g)

1 mm

1 mm

0.2 mm

0.25 mm

0.5 mm

0.2 mm0.2 mm

Bt1

Grt


.............................................................................................................................................................


Table 1 U–Pb LA-ICP-MS results for monazites from the RT-95 metapelite (SABN massif, Greater Kabylia, Algeria).

Sample no. 208Pb/206Pb 238U/206Pb ± (1r) 207Pb/206Pb ± (1r)

Apparent ages (Ma)

206Pb/238U ± (1r)

2 7.4 22.74 0.94 0.0683 0.0053 277 11

3 3.76 22.87 0.62 0.0516 0.0008 276 7

6 7.45 22.41 1.11 0.0508 0.0016 281 14

6-2 8.75 22.53 0.63 0.0542 0.0016 280 8

6-3 4.41 23.42 1.46 0.052 0.0002 270 16

6-4 8.09 22.33 0.74 0.0574 0.0028 282 9

6-5 6.84 21.28 0.63 0.0544 0.0007 296 9

7 3.16 23.58 1.36 0.0531 0.0004 268 15

7-2 3.34 23.84 1.05 0.0606 0.0018 265 11

7-3 3.12 21.42 1.48 0.0702 0.0023 294 20

8 1.32 24.09 1.14 0.0497 0.0004 262 12

8-2 3.36 22.03 1.48 0.0494 0.0001 286 19

8-3 2.96 22.97 1.16 0.0517 0.0011 275 14

8-4 1.38 24.88 1.03 0.0517 0.0008 254 10

8-5 3.76 23.37 0.66 0.052 0.0019 270 7

9 3.32 20.86 0.92 0.0504 0.0013 302 13

9-2 4.05 25.5 1.27 0.051 0.0009 248 12

9-4 3.58 24.21 1.15 0.051 0.0007 261 12

10 2.49 24.34 1.45 0.051 0.0008 260 15

10-2 1.5 24.38 1.32 0.0517 0.0007 259 14

10-3 4.8 24.07 1.04 0.0501 0.0008 262 11

10-4* 8.22 26.62 0.39 0.0505 0.0023 238 3

11* 5.69 26.66 0.86 0.0576 0.003 237 7

11-2 7.1 22.35 0.96 0.0593 0.004 282 12

11-3 7.14 23.82 0.73 0.0541 0.0026 265 8

12 5.95 22.92 0.7 0.0562 0.0015 275 8

12-2 2.75 22.82 1.24 0.0533 0.0006 276 15

13 1.65 23.75 1.19 0.0511 0.0006 266 13

13-2 2.3 23.66 1.14 0.0501 0.0013 267 13

14 2.39 21.65 0.77 0.0492 0.0016 291 10

14-2 3.81 22.15 0.9 0.0491 0.0008 285 11

15 3.5 21.32 0.74 0.0506 0.0004 295 10

15-2 2.32 23.12 0.95 0.053 0.0016 273 11

15-3 2.6 21.48 1.11 0.0506 0.0009 293 15

16 8.89 24.85 1.17 0.0562 0.0038 254 12

16-2 11.71 21.38 1.06 0.068 0.0063 295 14

17 3.81 22.34 0.54 0.0516 0.0009 282 7

17-2 2.53 21.57 0.85 0.0519 0.0009 292 11

17-3 2.69 22.24 1.02 0.0513 0.0012 283 13

18* 3.3 26.24 0.42 0.0608 0.0024 241 4

18-2 2.07 24.43 1.24 0.0573 0.0017 259 13

19 4.72 23.15 1.66 0.0527 0.0014 273 19

19-2 3.87 23.87 1.03 0.054 0.0007 265 11

20* 2.15 44.95 3.93 0.0625 0.0019 142 12

20-2* 2.2 37.29 3.8 0.0568 0.0018 171 17

20-3* 2.85 26.84 2.19 0.0543 0.0016 236 19

*Spot analyses excluded from the age calculation.

Fig. 2 Photomicrographs from the RT-95 sample (symbols of minerals after Kretz, 1983). (a) General aspect of the analysedsample formed by alternating quartz-plagioclase bands and biotite-rich ribbons in which garnet clasts are enclosed. Note thebiotite wings from garnet and the monazite grain in matrix. The lower part of the photograph includes thin biotite ribbons andseveral fresh clasts of polycrystalline kyanite. (b) Xenomorphic monazite in a quartz-plagioclase band and clasts of polycrystallinekyanite. (c) Garnet clast with biotite inclusions adjacent to polycrystalline clasts of plagioclase and kyanite. (d) Garnet displayingtwo stages of growth. The core (Grt1) contains biotite (Bt1) and sillimanite (Sil) inclusions. It is rimmed by a garnet overgrowthrich in kyanite inclusions and displays an external coronitic overgrowth (Grt2) in textural equilibrium with the biotite of thematrix. (e) Polycrystalline kyanite pseudomorph after possible andalusite. (f) Large biotite clast including a monazite grain set upin a fine-grained matrix of biotite 2 and minute acicular kyanite. (g) Euhedral monazite grain in leucosome.


.............................................................................................................................................................


not included in the age calculation.The remaining analyses yield a206Pb/238U weighted mean age of275.4 ± 4.1 Ma (MSWD, meansquare of weighted deviates ¼ 1.4)and fall on a mixing line between thecalculated age and the common leadcomposition estimated from Staceyand Kramers (1975). This is taken asour best estimate for the age of themain monazite growth event. In orderto characterize the low temperatureevolution of the studied sample, Rb–Sr analyses were performed (Table 2).In the Rb–Sr isochron diagram(Fig. 5), the biotite fraction and thewhole rock yielded an early Mioceneage of 23.7 ± 1.1 Ma, identical to Ar/Ar biotite ages (Monie et al., 1984,1988) obtained on rocks from otherlithologies of the SABN massif.

Discussion

The 275.4 ± 4.1 Ma monazite age isc. 10 Ma younger than, but broadlysimilar to the maximum age ofemplacement (284 ± 3 Ma) of theSABN granite (Peucat et al., 1996).According to its microstructural sites,it is likely that monazite formed dur-ing prograde metamorphism throughmetamorphic reactions consumingprecursor minerals such as apatite,xenotime or allanite (e.g. Smith andBarreiro, 1990) and continued underanatectic conditions, as euhedral crys-tals are observed only in leucosomes(Fig. 2g). Peak metamorphic temper-atures (700–740 �C at about 1 GPa)are similar to the classically acceptedclosure temperature for Pb in monaz-ite (Copeland et al., 1988), and the275 Ma Permian age could reflect acooling event or, given the robustnessof the U–Pb system in monazite (Bin-gen and Van Breemen, 1998; Montelet al., 2000; Bosch et al., 2002), itsprograde growth until anatectic con-ditions. The similarity in age betweenmonazites from the studied metapeliteand zircons from a kyanite metapegm-atite (273 ± 6 Ma) emplaced duringthe first stages of mylonitization (Pe-ucat and Bossiere, 1991) suggests thatthe monazite date the high-tempera-ture extensional shear that affected thecrustal section of the SABN domainafter crystallization of the SABN plu-ton. This age compares well withsimilar values obtained on granitoidsand gabbros from outermost domains

50 m

RT-95

50 m

RT-95

292±11 Ma

283±13 Ma

282±7 Ma

275±14 Ma

286±19 Ma

270±7 Ma

254±10 Ma

262±12 Ma

(a)

(b)

Fig. 3 Scanning electron microscope (BSE) images of monazite grains from the RT-95metapelite. Quoted ages are ±1r. (a) Homogeneous elongated bright (high Th) grain(17) with no fractures and detectable age differences. (b) Rounded grain (#8) showingdark, irregularly shaped, domains concentrated in the fractured part of the grain.

Fig. 4 Terra-Wasserburg diagram for monazites from the RT-95 metapelite. Crossesare 1r error.


.............................................................................................................................................................


of the Palaeo-European Variscan beltsuch as the Western and Central Alps(Thoni and Jagoutz, 1992; Bertrandet al., 2000; Muntener et al., 2000;Mayer et al., 2000), Calabria (Graess-ner et al., 2000) and Corsica (Paquetteet al., 2003). These ages are within therange of the last late-orogenic mag-matic pulse of the Variscan Belt ofEurope (Schaltegger, 1997). It is thusinferred that high-temperature crys-talline rocks from the Kabylian base-ment represent an analogue of thenorth-western part of Adria thatrecorded pervasive Permian magma-tism related to lithospheric thinningleading to the opening of the Tethyanoceanic domain (Stampfli et al., 2001).The Rb–Sr age of biotite

(24 ± 1 Ma) is in good agreementwith other published Ar ages (rangingfrom 25 to 30 Ma) measured on meta-morphic minerals (biotite and musco-vite) from rocks of the SABN domain

(Monie et al., 1984, 1988). This agefalls within the main Rb–Sr biotitewhole-rock age group (22–26 Ma)defined by Peucat et al. (1996) forbasement rocks of the Lesser Kabylia.Collectively all aforementioned agesplead for regional cooling, down to c.350 �C (Dahl, 1996) during the LateOligocene–Early Miocene, which cor-responds to the main phase of thrust-ing in the internal zone of theMaghrebides (Madjoub et al., 1997),shortly followed by global extensionin the western Mediterranean realm(Gelabert et al., 2002; Mauffret et al.,2004). Do the combination of U–PbPermian ages and 40Ar/39Ar late-Alpine ages imply a two-stage evolu-tion? Or is it simply related to a long-lived burial of the Variscan crustallowing continuous Ar diffusion upto the Miocene?The LA-ICP-MS results indicate

that some monazite grains have suf-fered U–Pb disturbances, which is inagreement with a two-stage model.This suggests that the Rb–Sr biotiteage is more likely related to a regionalcooling subsequent to a reheatingevent that affected the U–Pb systemsof the discordant monazites. Deter-mining the age(s) of these U–Pb dis-turbances is not possible with thepresent data set, but can be bracketedby the Rb–Sr biotite age (24 ± 1 Ma)and the 206Pb/238U age of the young-est discordant monazite (142 ±

24 Ma). In the present case, it canhowever be speculated that thermalconditions prevailing during this eventwere above 300–350 �C, but did notreach 500–550 �C. This is in agree-ment with the preservation of someLate Hercynian Rb–Sr muscovite agesin the Kabylies (Peucat et al., 1996).This is also consistent with partialrejuvenation of monazite under relat-ively low temperature conditions,either during the waning stages ofmetamorphism (e.g. Lanzirotti andHanson, 1995) or linked with fluidcirculations (Townsend et al., 2000).Most U–Pb lower intercepts from

Permian occurrences preserved in theAlpine and peri-Mediterranean areasareMesozoic in age, as domanyRb–Srand 40Ar/39Ar mineral ages (Costa andMaluski, 1988; Gebauer, 1993; Monieet al., 1994). In the southern Alps,long-lived burial of the hot Variscancrust from the Ivrea Zone was followedby crustal attenuation from Triassic toLate Jurassic times (Zingg et al., 1990;Schmid, 1993). Triassic reheatingresulted from asthenosphere upwelling(Snoke et al., 1999) and is well docu-mented by zircon and monazite over-growths and/or nucleation in thegranulites from the Ivrea Zone (Vavraand Schaltegger, 1999; Vavra et al.,1999). A similar evolution also tookplace in Calabria (Graessner et al.,2000). At variance with the southAlpine block where thermal and de-compressional pulses linked to exten-sion led to Permo-Mesozoic crustalthinning leading to continental breakup and opening of the Neothethys(Stampfli et al., 2001), Mesozoic heat-ing periods in Greater Kabylia aremore discrete and are only recordedthrough some mineral ages. Creta-ceous ages (c. 128 Ma) have beeninterpreted as dating a shearing event,responsible for the blastesis of greenbiotite and phengite in gneisses (Cheil-letz et al., 1999). In the SABN domain,pre-Alpine heating events are notclearly documented and are only sug-gested by low temperature replacementzones affecting monazite grains. Thissuggests that, during the Mesozoic,rocks from the middle crust of CGKand SABN were only slightly rehea-ted as they were still attached tosouthern Europe (or possibly formedthe AlKaPeCa (Alboran-Kabylia-Peleritan-Calabria) domain; Loner-gan and White, 1997). At the scale

Table 2 Strontium and rubidium iso-

topic analyses for biotite and whole rock

from the RT-95 metapelite. Rb/Sr ratios

are considered precise to about ±2%.

Sample name Whole rock Biotite

Rb (ppm) 106.13 145.16

Sr (ppm) 155.87 31.2487Sr/86Sr 0.72278 0.72654

± (2r) 0.00002 0.0000187Rb/86Sr 1.93 13.14

0.723

0 4 8 12 16

0.724

0.725

0.726

0.727

87Rb/86Sr

87S

r/86

Sr

RT 95

23.7 ± 1.1 Ma

I0 = 0.72213 ± 6

Biotite

Whole rock

Fig. 5 Rb–Sr isochron diagram for biotite and whole-rock from the RT-95metapelite.


.............................................................................................................................................................


of Northern Africa (Maghrebides +Alboran sea + Betics), it is significantthat no record of exposure and erosionof high-grademetamorphic rocks priorto the unconformable Late Oligoceneor Miocene sediments can be found.This suggests that doming, extrusionand tectonic unroofing were the dom-inant mechanisms leading to exhuma-tion of theKabylian crystalline rocks atthe time of nappe emplacement,slightly before the opening of the west-ern Mediterranean basin (21–11 Ma;Lonergan and White, 1997).

Conclusions

Anatectic metapelites from GreaterKabylia affected by high-temperaturemylonitic deformation reached high-temperature amphibolite facies condi-tions (740 �C at 1 Gpa) at275 ± 4 Ma. This age correlates withsimilar ages from other crustal do-mains from south and south-easternEurope that suffered Permo-Mesozoiccrustal thinning predating the Tethysopening. As a consequence, it is sug-gested that the basement rocks ofGreater Kabylia represent a southerncounterpart of the north-western partof Adria, affected by pervasive Per-mian magmatism and deformation.During Late Oligocene–Early Mio-cene times, exhumation brought deepcrustal units of Greater Kabylia up toshallow levels as indicated by the Rb–Sr biotite age of 23.7 ± 1.1 Ma. Thisevent was synchronous with the mainphase of nappe emplacement wellidentified in Lesser Kabylia (Madjoubet al., 1997) and was followed bydisruption of the Alboran microplateand opening of the Alboran Sea.

Acknowledgements

During the course of this study, the firstauthor (DH) benefited from a CMEPfinancial support and an access to thefacilities of the �Laboratoire de Tectono-physique� (Universite de Montpellier II). B.Galland is thanked for clean laboratorymaintenance, C. Nevado and D. Delmasfor preparation of laser mounts, and C.Grill for SEM imaging. We acknowledgethe constructive criticisms of two anony-mous referees.

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Received 19 January 2006; revised versionaccepted 21 June 2006


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a two-stage exhumation of the variscan crust: u–pb la-icp-ms and

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