doi: 10.1111/j.1365-3121.2006.00693.x A two-stage exhumation of the Variscan crust: U–Pb LA-ICP-MS and Rb–Sr ages from Greater , 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 Euge`ne Bataillon, 34 095 Montpellier Cedex 5, ; 3Service ICP-MS, Universite´ de Montpellier II, Place Euge`ne Bataillon, 34 095 Montpellier Cedex 5, France

ABSTRACT The significance and role of major shear zones are paramount to Europe (i.e. South Alpine and Austro-Alpine domains) that understanding continental deformation and the exhumation of suffered crustal thinning during the continental rifting predat- deep crustal levels. LA-ICPMS U–Pb dating of monazites, ing the Tethys opening. Rb–Sr analyses of biotites yield a combined with Rb–Sr analyses of biotites, from an anatectic cooling age of 23.7 ± 1.1 Ma related to the exhumation of the metapelite from Greater Kabylia (Algeria) highlights the history buried Variscan crust during the Miocene as an extrusive slice, of shear zone development and the subsequent exhumation of roughly coeval with the emplacement of nappes, and shortly deep crustal levels in the internal zones of the Maghrebides. followed by lithospheric extension leading to the opening of Monazites give an age of 275.4 ± 4.1 Ma (2r) dating the the Alboran sea. thermal peak coeval with anatexis. This age is identical to those recorded in other crystalline terranes from south-easternmost Terra Nova, 18, 299–307, 2006

Kabylia (Peucat et al., 1996) suggest lain by the Mesozoic to Tertiary Introduction that Alpine events were not negligible sedimentary cover of the Calcareous The Maghrebides are part of the peri- in the Kabylian basement units. In Range capped by allochthonous Mediterranean belt of late Tertiary this study, we present LA-ICP-MS Kabylian flyschs. The lower unit, age that delimits the African and the U–Pb results from monazites and Rb– exposed in two half domes, comprises European plates and runs from the Sr analyses from biotites extracted a continuous tectonic pile, 6–8 km Betico-Rifan arc to Calabria (Fig. 1a, from a major high-temperature crustal thick, of orthogneisses, paragneisses, inset). Classical interpretations (e.g. shear zone from Greater Kabylia. marbles and micaschists affected by Ricou, 1994) consider that they This study was undertaken in order high-temperature syn-metamorphic formed during the 40–25 Ma time to give time constraints on the main ductile deformation and yielding span as a result of underthrusting of high-temperature shearing event that 40Ar/39Ar ages bracketed between 80 the North African margin beneath the affected the crystalline rocks of the and 120 Ma (Monie´et al., 1988). The Alboran plate (Betic-Rif-Kabylies). Kabylian basement and on its possible SABN unit that is dealt with this The inner zones of the Maghrebides reactivation during subsequent events, study exposes another tectono-meta- are represented by the Kabylies, which has implications for unravelling morphic pile showing a normal meta- mainly formed by inliers of crystalline the tectonometamorphic evolution morphic polarity with downward rocks surrounded by Oligo-Miocene through time of this part of the peri- pressure and temperature increase. It and younger Miocene sediments. Pre- Mediterranean fold belt. is in tectonic contact with the Naceria Oligocene reconstructions locate the diatexites in the north. The SABN Kabylies at ‡700 km NNW from their granite has been dated by the U–Pb Geological setting present-day location, along with their zircon conventional method at counterparts in the Betico-Rifan arc Greater Kabylia comprises three ma- 284 ± 3 Ma (Peucat et al., 1996). It and Calabria-Sicily (Lonergan and jor domains: Central Greater Kabylia displays a low-pressure thermal au- White, 1997; Gueguen et al., 1998). (CGK), Eastern Greater Kabylia reole (biotite, andalusite, cordierite, Classical ideas considered that the (EGK) and the Sidi Ali Bou Nab K-feldspar, corundum) formed at Kabylies underwent only slight Alpine (SABN) domain (Saadallah and £3 kbar pressure. Hornfelses were overprint (e.g. Peucat et al., 1996). Caby, 1996) (Fig. 1a,b). In CGK, the progressively sheared downwards and However, 40Ar/39Ar ages of high-tem- Kabylian Detachment Fault is a affected by a distinct synkinematic perature minerals obtained in Greater major low-angle ductile to cataclastic metamorphic overprint portrayed by Kabylia (Monie´et al., 1988) and Rb/ extensional shear zone that sharply the replacement of andalusite by Sr Alpine ages of biotites in Lesser delimits a lower unit of amphibolite staurolite and kyanite. This metamor- facies rocks below, from overlying phic field gradient indicates tempera- Correspondence: Delphine Bosch, Labora- greenschist facies phyllites with 295– ture and pressure increase downward. 40 39 toire de Tectonophysique, Universite´de 315 Ma Ar/ Ar mineral ages (Mon- The deepest rocks exposed on Montpellier II, Place Euge` ne Bataillon, ie´et al., 1988) and non-metamorphic the southern flank of the SABN 34095 Montpellier Cedex 5, France. Tel.: fossiliferous Palaeozoic sediments. ridge below a north-dipping band of +33 4 67 14 32 67; fax: +33 4 67 14 36 03; This upper unit, free of Alpine ductile high-temperature ultramylonites com- e-mail: [email protected] deformation, is unconformably over- prise slightly anatectic metapelites,

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l IBERIC ta st n ru ine c PENINSULA nt 4 00 s o (a) tic d c e de t B ten rus A Naceria ex nic c ocea Alboran sea Lesser Greater bylia Tizi Ouzou Ka SABN Rif 500 Km Massif Ighil Bouzrou

CENTRAL GREATER KABYLIA

EASTERN GREATER KABYLIA

36 30 BOGHNI BASIN 36 30

4 00 A' 0510

Upper unit Lower unit High-grade metamorphics Kabylian detachment fault Miocene to Pliocene rocks Calcareous range Foliation undifferentiated Major main Miocene Flysch Paleozoic series Lineation normal fault Sidi Ali Bou Nab Units Lineation trajectories Phyllites (including blastomylonite)

(b)

Central Kabylia Dome Calcareous Range A Sidi Ali Bou Nab A’ Massif Cataclastic fault NW Mylonitic/cataclasitic Boghni SE front (KDF) Basin (21 Ma)

0 0

5 km

Sidi Ali Bou Nab granitoids Lower Unit Upper Unit Saravalian Naciria Low-P Low-P molasse diatexites hornfelses Micaschist Molasse

Orthogneiss Phyllites 0 RT-95 Anatectic paragneiss and marbles

Granite gneiss 5 km Garnet-kyanite Blastomylonite mylonitic metapelites

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

300 2006 Blackwell Publishing Ltd Terra Nova, Vol 18, No. 5, 299–307 D. Hammor et al. • A two-stage exhumation of the Variscan crust ...... calc-silicate gneisses, garnet amphibo- synkinematic peak thermal conditions 100 mg of whole-rock sample and lites, rare pyroxenites, and meta- (700–740 C) and partial melting took 10 mg of biotite were dissolved in a pegmatites. The transtensional place towards the boundary between HF/HNO3 mixture at 120 C. After character of the mylonites deduced kyanite and sillimanite stability fields conversion to chlorides, aliquots of the from syn- to late-metamorphic shear and were followed by pervasive high- samples were spiked with 87Rb and criteria accounts for the exhumation temperature extensional shearing. A 84Sr. Rb and Sr were separated by of metamorphic rocks from a depth of cooling path towards the boundary conventional cation-exchange proce- about 30 km, as well as for consider- between kyanite–sillimanite stability dures. Isotopic measurements were able syn-metamorphic thinning of the fields is thus suggested. Minute white made on a VG Sector multi-collector former tectonic pile which is now less mica is rarely observed along some mass spectrometer at the University of than 3 km thick. Biotite and musco- finer grain ribbons and documents a Montpellier II. An average 87Sr/86Sr vite yielded 40Ar/39Ar plateau ages final stage of negligible synkinematic isotopic ratio of 0.710246 ± 20 (2r) around 25–30 Ma (Monie´et al., 1984, low-temperature retrogression. was measured for NBS 987 (n ¼ 2). 1988) and the high-grade mylonites Blanks were lower than 50 pg for Rb that delimit the base of the SABN and Sr and no blank correction was Analytical techniques granite have been tentatively inter- made. preted as having assisted Miocene For U–Pb analyses, monazite grains exhumation of the middle crust (Sa- were mounted in epoxy resin with Geochronology adallah and Caby, 1996). chips of a standard material (Manang- The rock analysed for geochrono- otry crystal of Poitrasson et al., 2000) Forty-six spots were performed on 17 logical purposes (RT-95) is an anatec- and grounded down to half their grains and the results are reported in tic graphitic metapelite displaying a thickness to expose internal structures. Table 1. The monazite crystals have protomylonitic fabric (Fig. 2a). The Data were acquired at the University rounded to irregular shapes (see banding is defined by alternating bio- of Montpellier II using a VG Plasma- Fig. 3) although euhedral grains also tite-rich restitic layers containing quad II turbo ICP-MS coupled with a occur in the leucosomes. Back-scat- clasts of kyanite and garnet up to Geolas (Microlas) automated plat- tered electron imaging indicates that 1.5 cm in diameter, and quartzofeld- form housing a 193 nm Compex 102 most grains show a homogeneous spathic ribbons containing antiperth- laser from LambdaPhysik (Go¨ ttingen, internal structure suggesting a simple itic plagioclase clasts, considered as Germany). Data were acquired in the growth history (Fig. 3a), but some sheared leucosomes. Kyanite is ob- peak jumping mode similar to the often exhibit zones of different bright- served as minute synkinematic prisms procedure described in Bruguier et al. ness, where dark zones (possibly low in the matrix and as polycrystalline (2001) using an energy density of Th) replace homogeneous brighter clasts (Fig. 2e) interpreted as pseudo- 15 J cm)2 at a frequency of 5 Hz and parts (Fig. 3b). These dark zones are morphs after andalusite, as described a spot size of 26 lm. 232Th was not irregularly distributed, suggesting that from Lesser Kabylia basement (Mad- measured during the course of this bulk diffusion was not the mechanism joub et al., 1997). Sillimanite needles study as the high Th concentrations responsible for their formation. As are common along some mylonitic resulted in a detector saturation. It was they are preferentially, but not exclu- bands and also occur as inclusions in therefore not possible to assess the sively, located in fractured parts of the all minerals (Fig. 2d) and at grain reliability of the 232Th–208Pb system crystals, they are interpreted as reflect- boundaries. No clear microstructural for the measured monazites. For ing modification of the original com- relationships can be determined be- instrumental mass bias and Pb–U position during recrystallization tween syn-kinematic matrix kyanite fractionation, measured standards processes that may have been en- and fibrolite. Monazite occurs in the were averaged to give the respective hanced by fluid flows. matrix or as inclusions in various bias factors and their associated Reported on a Terra-Wasserburg minerals (Fig. 2f) and in leucosomes errors, which were propagated with diagram (Fig. 4) most data points where it is occasionally euhedral the analytical errors of each unknown. locate close to, or on Concordia at (Fig. 2g). Pairs of primary garnet For Pb–U ratios, this typically resulted around 280 Ma. Some points are cores and primary biotite inclusions in a 2–5% precision (1r RSD%) after markedly younger, suggesting that (Fig. 2c) give temperatures around all corrections have been made which, they have suffered U–Pb disturbances. 700 C, whereas secondary biotite in this age range, translates to a Grain 20, for example, yields a hetero- and garnet overgrowth (Fig. 2d) yield 5–20 Ma uncertainty (see Table 1). In geneous age distribution with temperatures of about 740 C (Ferry the course of this study, 16 analyses 206Pb/238U ages ranging from c. 140 and Spear, 1978). Pressure estimates of the Manangotry monazite yielded to 240 Ma. This is interpreted as using the garnet–plagioclase–kyanite– a 207Pb/206Pb weighted mean of reflecting post-crystallization distur- quartz geobarometer (Hodges and 0.05862 ± 0.00019 (2r) correspond- bances, which are tentatively related Spear, 1982) give 1 GPa for peak ing to an age of 553.0 ± 7.1 Ma. to the dark BSE (Back Scattered pressure in agreement with the occur- This is in good agreement with the Electron) replacement zones observed rence of rutile and ilmenite coexisting EMP (Electron Micro Probe) (557 ± in some crystals. This is consistent with kyanite (Bohlen et al., 1983). 20 Ma, Montel et al., 1996) reference with a younging of measured ages These estimates and petrological con- age. Ages quoted below were calcula- associated with these zones. Younger siderations indicate that after the ted using the Isoplot program of ages present in other analyses (10-4, emplacement of the SABN pluton, Ludwig (1999). For Rb–Sr analyses 11 and 18) are considered as outliers,

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(a) (b)

Mnz Qtz

Grt Bt Ky

Pl Mnz 0.2 mm

Qtz (d) Bt1

Ky Sil 1 mm Grt1

(c) Bt2 Grt2 Bt1

Grt Pl 0.25 mm

Ky (f) Grt2

Qtz Mnz Ky 1 mm Bt1 Bt2

(e) Ky 0.5 mm

(g) Ky Ky

Mnz Qtz Ky Qtz Grt 0.2 mm 0.2 mm

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

Table 1 U–Pb LA-ICP-MS results for monazites from the RT-95 metapelite (SABN massif, Greater Kabylia, Algeria). Apparent ages (Ma) Sample no. 208Pb/206Pb 238U/206Pb ± (1r) 207Pb/206Pb ± (1r) 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.

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not included in the age calculation. RT-95 (a) The remaining analyses yield a 206Pb/238U weighted mean age of 292±11 Ma 275.4 ± 4.1 Ma (MSWD, mean square of weighted deviates ¼ 1.4) and fall on a mixing line between the calculated age and the common lead 283±13 Ma composition estimated from Stacey and Kramers (1975). This is taken as our best estimate for the age of the main monazite growth event. In order 282±7 Ma to characterize the low temperature 50 m evolution of the studied sample, Rb– Sr analyses were performed (Table 2). In the Rb–Sr isochron diagram (Fig. 5), the biotite fraction and the RT-95 (b) whole rock yielded an early Miocene 270±7 Ma age of 23.7 ± 1.1 Ma, identical to Ar/ Ar biotite ages (Monie´et al., 1984, 254±10 Ma 1988) obtained on rocks from other lithologies of the SABN massif. 275±14 Ma

Discussion The 275.4 ± 4.1 Ma monazite age is c. 10 Ma younger than, but broadly 262±12 Ma similar to the maximum age of 286±19 Ma emplacement (284 ± 3 Ma) of the SABN granite (Peucat et al., 1996). 50 m According to its microstructural sites, it is likely that monazite formed dur- Fig. 3 Scanning electron microscope (BSE) images of monazite grains from the RT-95 ing prograde metamorphism through metapelite. Quoted ages are ±1r. (a) Homogeneous elongated bright (high Th) grain metamorphic reactions consuming (17) with no fractures and detectable age differences. (b) Rounded grain (#8) showing precursor minerals such as apatite, dark, irregularly shaped, domains concentrated in the fractured part of the grain. xenotime or allanite (e.g. Smith and Barreiro, 1990) and continued under anatectic 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 accepted closure temperature for Pb in monaz- ite (Copeland et al., 1988), and the 275 Ma Permian age could reflect a cooling event or, given the robustness of the U–Pb system in monazite (Bin- gen and Van Breemen, 1998; Montel et al., 2000; Bosch et al., 2002), its prograde growth until anatectic con- ditions. The similarity in age between monazites from the studied metapelite and zircons from a kyanite metapegm- atite (273 ± 6 Ma) emplaced during the first stages of mylonitization (Pe- ucat and Bossie` re, 1991) suggests that the monazite date the high-tempera- ture extensional shear that affected the crustal section of the SABN domain after crystallization of the SABN plu- ton. This age compares well with Fig. 4 Terra-Wasserburg diagram for monazites from the RT-95 metapelite. Crosses similar values obtained on granitoids are 1r error. and gabbros from outermost domains

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Table 2 Strontium and rubidium iso- (Monie´et al., 1984, 1988). This age 24 Ma). In the present case, it can topic analyses for biotite and whole rock falls within the main Rb–Sr biotite however be speculated that thermal from the RT-95 metapelite. Rb/Sr ratios whole-rock age group (22–26 Ma) conditions prevailing during this event are considered precise to about ±2%. defined by Peucat et al. (1996) for were above 300–350 C, but did not basement rocks of the Lesser Kabylia. reach 500–550 C. This is in agree- Sample name Whole rock Biotite Collectively all aforementioned ages ment with the preservation of some Rb (ppm) 106.13 145.16 plead for regional cooling, down to c. Late Hercynian Rb–Sr muscovite ages Sr (ppm) 155.87 31.24 350 C (Dahl, 1996) during the Late in the Kabylies (Peucat et al., 1996). 87 86 Sr/ Sr 0.72278 0.72654 Oligocene–Early Miocene, which cor- This is also consistent with partial ±(2r) 0.00002 0.00001 responds to the main phase of thrust- rejuvenation of monazite under relat- 87 86 Rb/ Sr 1.93 13.14 ing in the internal zone of the ively low temperature conditions, Maghrebides (Madjoub et al., 1997), either during the waning stages of shortly followed by global extension metamorphism (e.g. Lanzirotti and of the Palaeo-European Variscan belt in the western Mediterranean realm Hanson, 1995) or linked with fluid such as the Western and Central (Gelabert et al., 2002; Mauffret et al., circulations (Townsend et al., 2000). (Thoni and Jagoutz, 1992; Bertrand 2004). Do the combination of U–Pb Most U–Pb lower intercepts from et al., 2000; Mu¨ ntener et al., 2000; Permian ages and 40Ar/39Ar late- Permian occurrences preserved in the Mayer et al., 2000), Calabria (Graess- Alpine ages imply a two-stage evolu- Alpine and peri-Mediterranean areas ner et al., 2000) and (Paquette tion? Or is it simply related to a long- are Mesozoic in age, as do many Rb–Sr et al., 2003). These ages are within the lived burial of the Variscan crust and 40Ar/39Ar mineral ages (Costa and range of the last late-orogenic mag- allowing continuous Ar diffusion up Maluski, 1988; Gebauer, 1993; Monie´ matic pulse of the Variscan Belt of to the Miocene? et al., 1994). In the southern Alps, Europe (Schaltegger, 1997). It is thus The LA-ICP-MS results indicate long-lived burial of the hot Variscan inferred that high-temperature crys- that some monazite grains have suf- crust from the Ivrea Zone was followed talline rocks from the Kabylian base- fered U–Pb disturbances, which is in by crustal attenuation from Triassic to ment represent an analogue of the agreement with a two-stage model. Late Jurassic times (Zingg et al., 1990; north-western part of Adria that This suggests that the Rb–Sr biotite Schmid, 1993). Triassic reheating recorded pervasive Permian magma- age is more likely related to a regional resulted from asthenosphere upwelling tism related to lithospheric thinning cooling subsequent to a reheating (Snoke et al., 1999) and is well docu- leading to the opening of the Tethyan event that affected the U–Pb systems mented by zircon and monazite over- oceanic domain (Stampfli et al., 2001). of the discordant monazites. Deter- growths and/or nucleation in the The Rb–Sr age of biotite mining the age(s) of these U–Pb dis- granulites from the Ivrea Zone (Vavra (24 ± 1 Ma) is in good agreement turbances is not possible with the and Schaltegger, 1999; Vavra et al., with other published Ar ages (ranging present data set, but can be bracketed 1999). A similar evolution also took from 25 to 30 Ma) measured on meta- by the Rb–Sr biotite age (24 ± 1 Ma) place in Calabria (Graessner et al., morphic minerals (biotite and musco- and the 206Pb/238U age of the young- 2000). At variance with the south vite) from rocks of the SABN domain est discordant monazite (142 ± Alpine block where thermal and de- compressional pulses linked to exten- sion led to Permo-Mesozoic crustal thinning leading to continental break 0.727 up and opening of the Neothethys RT 95 (Stampfli et al., 2001), Mesozoic heat- Biotite ing periods in Greater Kabylia are more discrete and are only recorded 0.726 through some mineral ages. Creta- ceous ages (c. 128 Ma) have been interpreted as dating a shearing event,

Sr 0.725 responsible for the blastesis of green 86 23.7 ± 1.1 Ma biotite and phengite in gneisses (Cheil-

Sr/ letz et al., 1999). In the SABN domain, 87 0.724 pre-Alpine heating events are not clearly documented and are only sug- gested by low temperature replacement zones affecting monazite grains. This 0.723 Whole rock suggests that, during the Mesozoic, rocks from the middle crust of CGK 87 86 I0 = 0.72213 ± 6 Rb/ Sr and SABN were only slightly rehea- ted as they were still attached to 0481216southern Europe (or possibly formed the AlKaPeCa (Alboran-Kabylia- Fig. 5 Rb–Sr isochron diagram for biotite and whole-rock from the RT-95 Peleritan-Calabria) domain; Loner- metapelite. gan and White, 1997). At the scale

2006 Blackwell Publishing Ltd 305 A two-stage exhumation of the Variscan crust • D. Hammor et al. Terra Nova, Vol 18, No. 5, 299–307 ......

of Northern Africa (Maghrebides + M., 2000. SHRIMP and IDTIMS U–Pb Gelabert, B., Sabat, F. and Rodriguez- Alboran sea + Betics), it is significant zircon ages of the pre-Alpine basement in Perea, A., 2002. A new proposal for the that no record of exposure and erosion the Internal (Savoy and late Cenozoic geodynamic evolution of of high-grade metamorphic rocks prior Piemont). Schweiz. Mineral. Petrol. Mitt., the Western Mediterranean. Terra Nova, to the unconformable Late Oligocene 80, 225–248. 14, 93–100. Bingen, B. and Van Breemen, O., 1998. Graessner, T., Shenk, V., Bro¨ cker, M. and or Miocene sediments can be found. U–Pb monazite ages in amphibolite- to Mezger, K., 2000. Geochronological This suggests that doming, extrusion granulite-facies orthogneiss reflect constraints on the timing of granitoid and tectonic unroofing were the dom- hydrous mineral breakdown reactions: magmatism, metamorphism and post- inant mechanisms leading to exhuma- Sveconorwegian province of SW metamorphic cooling in the Hercynian tion of the Kabylian crystalline rocks at Norway. Contrib. Mineral. Petrol., 132, cross-section of Calabria. J. Metamor- the time of nappe emplacement, 336–353. phic Geol., 18, 409–421. slightly before the opening of the west- Bohlen, S.R., Wall, V.J. and Boettcher, Gueguen, E., Doglioni, C. and Fernandez, ern Mediterranean basin (21–11 Ma; A.L., 1983. Experimental investigations M., 1998. On the post-25 Ma geo- Lonergan and White, 1997). and geological applications of equilibria dynamic evolution of the western Medi- in the system FeO–TiO2–Al2O3–SiO2– terranean. Tectonophysics, 298, 259–269. H2O. Am. Mineral., 68, 1049–1053. 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