Petrogenesis And Tectonic Setting Of Granite Porphyry In The Upper Reaches Of Lalinggaoli River, Golmud, Qinghai Province

This paper focuses on the granite porphyris occurring in the upstream region of the Lalinggaoli River,Golmud City,Qinghai Province.In addition to careful petrology study,zircon U-Pb chronology and whole-rock geochemistry have also been carried out to investigate nature of the source region,petrogenesis,genetic type and tectonic background of the granite porphyries for a better understanding of the tectonic evolution of the upstream region of the Gaoli River in the East Kunlun.Zircon 206Pb/238 U ages show a large variation,ranging from 215 to 230 Ma,and can be divied into two clusters,227.2±1.7Ma and 218.7±1.2Ma,among which 218.7±1.2Ma represents the emplacement age of granite porphyries,suggesting they were the productions of the Late Indosinian magmatism.These rocks are high-K calc-alkaline and weak peraluminous,with high differentiation index,and SiO2 and K2 O concentrations.According to geochemical charasteristics,they can be ascribed to highly evolved S-type granites.With respect to trace elements,the samples exhibit moderate total REE contents??REE=121.52×10-6¹45.23×10-6?.Their chondrite-normalized REE patterns are characterized by LREEs enrichment and HREEs depletion,with LREE/HREE varying from 11.08 to 12.72.They display moderately negative Eu anomalies??Eu =0.48⁰.63?and slightly negative Ce anomalies??Ce =0.82⁰.90?.Porphyry granite samples are enriched in K,Rb,Th and U but depleted in Nb,Ta,P and Ti,consistent with crust-derived rocks.The ratios of Al₂O₃/TiO₂ and Rb/Sr for granite porphyry samples vary from 37 to 53 and 0.87 to 1.20,respectively,indicating the origin of lower crust meta-graywacke.Their major and trace elements show the features of post-collision S-type granites.Based on the coeval granitoids features of East Kunlun,regional tectonic setting and tectonic discrimination diagrams,it is envisaged that the formation of granite porphyry is related to the orogenesis of East Kunlun Orogenic Belt and the late-stage extension.During the Early Indosinian,the Paleo-Tethys Ocean was closed and subsequently this region entered into the continent-continent collision stage.The progressive compression induced the thickening of lithosphere.In the Late Indosinian,the regional stress changed from compression to extension,resulting in the delamination of the lower crust,which gave rise to mantle magma upwelling and formed the tunnels for fluids immigration.The input of high temperature fluids could cause the partial melting of greywacke within lower crust,forming granite porphyries in this region.