Abstract
A lithosphere-scale extensional shear zone juxtaposes an underlying sub-continental peridotite body and overlying migmatitic paragneisses of the Filali unit in the Beni Bousera massif (Internal Rif, Morocco). Three stages are recognized in the metamorphic evolution of the aluminous paragneiss, marked by the chemical zoning of garnet porphyroblasts and the evolution of associated mineral assemblages characterized by the presence of kyanite and rutile (M1), sillimanite, k-feldspar and melt (M2), and cordierite (M3). Phase-equilibrium modeling (pseudosections) and multi-equilibrium thermobarometry point to P-T conditions of 7 kbar 750 degrees C and 3.5 kbar 685 degrees C for the M2 and M3 stages, respectively. M1 conditions of 9.3 kbar 660 degrees C were inferred using modeling after the reintegration of melt lost during M2 into the bulk composition. Published geochronological data suggest a Variscan age (250-340 Ma) for the M1 event, whereas M2 and M3 are Oligo-Miocene and related to the Alpine orogeny. The recorded sub-isothermal decompression is related to significant crustal attenuation in the Oligo-Miocene and is responsible for the juxtaposition of the hot asthenospheric mantle and the crustal units, causing the melting of the paragneiss. The exhumation of the gneisses by crustal extension is associated with the westward retreat of an Alpine subduction (slab rollback).