Evaluation of late Permian mafic magmatism in the central Tibetan Plateau as a response to plume-subduction interaction

摘要

An integrated study of the geochronology, mineralogy, geochemistry and Sr-Nd isotope compositions of the upper Permian Yushu mafic rocks in the central Tibetan Plateau (CTP) was conducted to evaluate the interaction between the Emeishan mantle plume and the Paleo-Tethyan subduction system. These mafic rocks can be geochemically subdivided into three groups. Group 1 rocks yielded a weighted mean Pb-206/U-238 age of 258 +/- 2 Ma. They have relatively high TiO2, FeOt, Nb/Y and high epsilon(Nd)(t) values and display oceanic island basalts (OIB)-like rare earth and trace element patterns, with obvious enrichment of Nb and Ta. Group 2 and 3 rocks yield weighted mean Pb-206/U-238 ages of 258 +/- 1 Ma and 257 +/- 1 Ma, respectively. Both have relatively low TiO2, FeOt, Nb/Y and epsilon(Nd)(t) values and island arc tholeiites (IAT)-like rare earth and trace element patterns with obvious depletion of Nb and Ta. However, the Group 2 rocks have relatively low FeOt/MgO ratios, but high MgO, Me, Cr and Ni contents, resembling primitive magmas. They have lower light rare earth element (LREE), Nb, Ti and Zr contents and higher epsilon(Nd)(t) values than the Group 1 rocks. The geochemical and isotopic variations suggest that the Group 1 rocks might be derived from a plume-related mantle source, whereas Group 2 and Group 3 rocks originated from subduction-modified asthenospheric mantle and lithospheric mantle. Moderate degrees of olivine and clinopyroxene fractionation under low oxygen fugacities (fO(2)) appear to be responsible for the Fe-Ti enrichment in the Group I rocks. We propose a model involving plume-subduction interaction to explain the geodynamics and generation of the late Permian mafic magmatism in the CTP. The study region was rifted by the Emeishan mantle plume activity in association with rollback of Longmuco-Shuanghu oceanic lithosphere in late Permian times. Deep melting of the plume source led to the generation of the Group 1 plume-related magmas, whereas shallower melting of the subduction-metasomatized asthenospheric and lithospheric mantle resulted in the generation of the Group 2 and Group 3 subduction-related magmas. (C) 2016 Elsevier B.V. All rights reserved.