Geochemistry and Genesis of the Late Cretaceous Granodiorite in Nulin, Tibet

There are different understandings of the genesis and geodynamic mechanism for the Late Cretaceous magmatic rocks of the South Gangdese. In this paper, we present geochemistry, zircon U-Pb and Sr-Nd isotopic data for the granodiorite from Nulin area. The results show that the rocks are characterized by high SiO₂(66.62%~67.81%)、Al₂O₃(15.11%~15.66%)、Sr(> 481×10^-6),low Y(≤ 8.13×10^-6)and Yb(≤ 0.73×10^-6) and high Sr/Y ratio(59~111),implying adakitic geochemical features. These rocks are enriched in LREE and depleted in HREE, together with enrichment in large-ion lithophile elements(LILE) and depletion in high field strength elements(HFSE),as well as significant positive Eu. (⁸⁷Sr/⁸⁶Sr)_i=0.704011~0.704244,ε_Nd(t)=3.61~5.75,indicating mantle source of Sr and Nd isotopic characteristics. The results of zircon U-Pb LA-ICP-MS of the granodiorite show that, the magmatic activities was 83Ma and 89Ma respectively. Based on the geological and geochemical analysis, we infer that the Nulin granodiorite may be the product of melting of the thickened juvenile lower crust which was triggered by the subduction of New Tethys ridge.