Spatial, temporal and geochemical evolution of Oligo-Miocene granitoid magmatism in western Anatolia, Turkey

摘要

Western Anatolia (Turkey) experienced widespread Cenozoic magmatism after the collision between the Sakarya (SC) and Anatolide-Tauride continental blocks (ATP) in the pre-middle Eocene. Voluminous granitic magmas were generated and emplaced into the crystalline basement rocks of the Rhodope (RM) and Sakarya continent to the north and Anatolide-Tauride Platform to the south of the ~E-W-trending Izmir-Ankara suture zone (IASZ) during the late Oligocene-middle Miocene. We report here a comprehensive geochronological (combined zircon U-Pb and ~(40)Ar- ~(39)Ar dating) and geochemical (major and trace element geochemistry, and Sr-Nd isotopes) dataset from the Oligo-Miocene granitoids in order to evaluate the nature and the spatial-temporal distribution of the Cenozoic magmatism in the Aegean extensional province. Zircon SHRIMP U-Pb dating of these plutons yields ages between 19.48±0.29 and 23.94±0.31Ma as the timing of their emplacement, whereas ~(39)Ar/ ~(40)Ar dating of biotite separates from these plutons reveals cooling ages of 18.9±0.1-24.8±0.1Ma. Regardless of the lithological make-up of the collided blocks, the RMG, SCG and NATPG granitoids that were emplaced into the RM, SC and ATP, respectively, show similar major and trace element and Sr-Nd isotopic compositions, indicating common mantle melt sources and magmatic evolutionary trends. The isotopic signatures and trace element characteristics of these granitoids indicate that both lithospheric and asthenospheric mantle melts appear to have contributed to source region of the RMG, SCG and NATPG magmas. The compositional variations observed in these granitoids are interpreted as a result of open-system processes (AFC and/or MASH) rather than a reflection of different compositions of crustal lithologies through which RMG and SCG, ATPG magmas migrated. On the other hand, the SATPG with crustal signatures stronger than the other groups may have been produced by crustal melting or significant contributions from the ATP crystalline basement. The isotopic compositions and cooling age relationships of western Anatolian granitoids indicate an increasing crustal signature from 24 to 18Ma coinciding with crustal exhumation (Kazdag and Menderes core complexes) and extension in western Anatolia. Asthenospheric upwelling caused by partial delamination or convective thinning of lithosphere led to underplating of mantle-derived magmas that provided melts and heat to induce partial melting of sub-continental lithospheric mantle. Stalling of mantle-derived melts in the crust triggered open system processes in separate magma chambers, resulting in the production of granitic magmas. This inferred melt source and magma evolution readily explains the I-type granitoid nature of most late Oligocene to middle Miocene plutons in western Anatolia regardless of their temporal and spatial position. The widespread early to middle Cenozoic magmatism caused thermal weakening and played a significant role for the initiation of synconvergent extension, exhumation and thinning in the hinterland of a young Tethyan orogen in western Anatolia and the broader Aegean region.