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 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 Euge`ne Bataillon, 34 095 Montpellier Cedex 5, France; 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,