U-PB AND RB-SR GEOCHRONOLOGICAL EVIDENCE FOR LATE-HERCYNIAN TECTONIC AND ALPINE OVERTHRUSTING IN KABYLIAN METAMORPHIC BASEMENT MASSIFS (NORTHEASTERN ALGERIA)

A geochronological study was carried out on rocks from the ''Petite Ka bylie Massif' in northeastern Algeria to determine the timing of magma tic and metamorphic events affecting a metamorphic complex involved in a major phase of Alpine overthrusting. This type of metamorphic compl ex is known throughout the northwestern African orogenic domain: the M aghrebide Chain (northern Algeria), the Betic Cordilliera (southeaster n Spain) and Calabria (southern Italy). The study, which is mainly foc used on the Petite Kabylie area, is supplemented with the dating of ro cks from a key occurrence of synkinematic granite in the ''Grande Kaby lie Massif'. In Petite Kabylie, an early phase of magmatism at 455 +/- 19 Ma and a second phase in the range 270-280 Ma (278 +/- 3 Ma and 27 3 +/- 6 Ma) are dated using the U-Pb zircon method. A Hercynian metamo rphic event is recorded in detrital zircons dated at 370 +/- 6 Ma. Rb- Sr micas ages are scattered between 280 Ma and 7 Ma, suggesting a mixi ng system or a history of complex cooling between the late Hercynian a nd the Alpine events. The basement here is also characterized by two s uperimposed metamorphic events, the first one of LP and the second of HP type. The earlier event could be around 280 Ma or older (370 Ma?), while the second event is thought to be either late Hercynian, Eoalpin e or even Alpine in age. The petrological and geochronological data ca n best be explained by a continuous evolution of the 280 Ma-old granit e and its associated HP metamorphism taking place over a short time sp an and ending with extensive deformation around 270 Ma ago as suggeste d by the fast cooling of the crust. This interpretation is in agreemen t with the U-Pb zircon age of the synkinematic granite from Grande Kab ylie, dated between 284 +/- 2 Ma and 273 +/- 6 Ma, which corresponds t o a late-Hercynian transtensional regime. The Eoalpine tectono-metamor phic events are not constrained from geochronological data. A low-grad e Alpine event is recorded during the Eocene-Oligocene (40-25 Ma) from Rb-Sr biotite ages, which agrees with the age of basement overthrusti ng. Late stage of low temperature activity continued up to 7-10 Ma as indicated by some biotite ages.