OPENING AND CLOSING OF A SHORT-LIVED OCEAN BASIN IN LIBYA DURING THE PAN AFRICAN OROGENY

Generation of new juvenile crust versus reworking of the existing lithosphere is one of the crucial aspects to better understand the tectonomagmatic evolution of the continents. Away from the continents, intra-oceanic formation of island arcs and their subsequent accretion to the continental margin is considered as one of the main mechanisms for the generation of new continental crust. Unprecedented zircon U-Pb dating and zircon Hf isotopic data from the basement cuttings obtained from drill-holes beneath the Sirte basin in northern Libya provide evidence for crustal growth during the Pan African tectonism from 600 to 550 Ma; a time span that has been generally considered to record remobilization, deformation, and reworking of existing crust in northern Africa. Sirte basement comprises mafic as well as felsic magmatic rock assemblages; in addition, metasedimentary rocks are also present. Major and trace element data of studied granitoids and basalts show typical island arc subduction-related compositions which are comparable to the modern island arc systems. The zircon U-Pb data reveal that the granitic basement underneath the Sirte basin was exclusively emplaced between ~600 to 550 Ma soon after the final assembly of western Gondwana. Based on restricted U-Pb ages absence of older inheritance, island arc-type geochemistry, radiogenic hafnium isotopic compositions coupled with previous geological and geophysical data, it is suggested that the magmatic suite underneath the Sirte basin in northern and Tibesti in southcentral Libya is a record of addition of juvenile crust instead of remobilization and reworking of existing Paleoproterozoic crust. Sirte-Tibesti region represents an intra-oceanic island arc system formed during the opening and closing of an ocean basin during 600-550 Ma, referred to here as the Sirte-Tibesti Ocean basin.