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Chronology,Geochemistry and Petrogenesis for Nyainqentanglha Intermediate-Basic Intrusive Rocks

CHEN Xiao-feng1,2,ZHU Li-xin1,MA Sheng-ming2,XI Ming-jie2,LI Bing2,TENG Xue-jian3 1.Chinese Academy of Geological Sciences,Beijing 100037,China 2.Institute of Geophysical and Geochemical Exploration,Chinese Academy of Geological Sciences, Langfang 065000,Hebei,China 3.Tianjin Institute of Geology and Mineral Resources,Tianjin 300170,China  
Nyainqentanglha Early Eocene intermediate-basic intrusive rocks consist of amphibole gabbro and monzodiorite.The zircon LA-MC-ICPMS U-Pb dating for the amphibole gabbro defines a crystallizationage of(49.92±0.24) Ma.The intermediate-basic intrusive rocks are high-K calc-alkaline to shoshonite series based on the SiO2-K2O diagram.They contain high total REE with enrichment of LREE and LILE(Rb,Th,K),and depletion of HFSE(Nb,Ta,Ti),showing features of magmatic rocks in island arc or active continental margin.They also show relatively high initial 87Sr/86Sr values of 0.708 77-0.713 71,low εNd values from-6.9 to-6.1,old modal age of 1 329-1 995 Ma(TDM),and no correlation for εNd with MgO and Nb/La,which indicates that they come from an enriched mantle metasomatized by ancient continental crust.REE simulation suggests that the forming processes for the intermediate-basic intrusive rocks were controlled by low tomoderate partial melting of the enriched mantle.The dynamic background for the intermediate-basic intrusive rocks are likely linked to breakoff of Neo-Tethyan slab followed by upwelling of asthenospheric materials from the slab window.
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