首页|High-and low-Mg adakitic rocks in southern Tibet: Implication for the crustal thickening and geodynamic process in the late Cretaceous
High-and low-Mg adakitic rocks in southern Tibet: Implication for the crustal thickening and geodynamic process in the late Cretaceous
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The history of crustal thickening is critical for comprehending the evolution process of the Tibetan plateau. The genetic relationship and spatiotemporal distribution of adakitic rocks with high Sr/Y, (La/Yb)N ratios may impose fundamental constraints on specific geodynamic and crustal thickening processes. The Songka intrusive suite consists of adakitic rocks with high-Mg adakite (monzonites), low-Mg adakitic rocks (quartz monzonites), and gabbroic-diorites. Their geochemical characteristics, pedogenesis, and geodynamic implication is discussed in this paper. Songka high-Mg adakites formed when subducting Neo-Tethyan oceanic slab melts were assimilated by mantle peridotite at elevated temperatures (average Ti-in-zircon temperatures of sample SK1319 are 830 °C). Low-Mg adakitic rocks exhibit markedly different geochemical characteristics than those formed in the juvenile thickened lower crust. The least-squares mass balance and trace element simulations indicate that Songka low-Mg adakitic samples were formed at a degree of 60-65% fractional crystallization of melts resembling Songka high-Mg adakites. Songka gabbroic diorites are formed in the metasomatic mande wedge. The genetic relationship between these rocks indicates that the crust of die southern Tibet did not appear to thicken obviously during the early stage of late Cretaceous (100-90 Ma). The intensive emplacement of late Cretaceous high-Mg adakites (Mg# >50) and mafic rocks (SiO2 < 52%) in the southern Lhasa terrane around 95-90 Ma, together with the slow convergence velocity of the Indian-Asian continent, can be explained by die response of rapid crustal thickening and Neo-Tethyan slab rollback at 95-90 Ma.
Research Center of Continental Dynamics, College of Earth Science and Engineering, Shandong University of Science and Technology, Qingdao 266590, China
National Astronomical Observatories, Beijing 100101, China
State Key Laboratory of Geological Processes and Mineral Resources, School of Earth Science and Resources, China University of Geosciences, Beijing 100083, China
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