Formation Age, Petrogenesis and Geological Significance of Late Jurassic Granodiorites in Kayico Area, Northern Tibet

The widely distributed Middle-Late Jurassic magmatic rocks in south margin of Southern Qiangtang block display complex geochemical compositions, providing an ideal research object for the tectonic evolution of Bangong-Nujiang Suture Zone. In this paper, zircon U-Pb dating, whole rock geochemistry and zircon Lu-Hf isotopes of Kayico granodiorites were analyzed. The investigated granodiorites yielded zircon ages of 158 Ma, coeval with the regional Middle-Late Jurassic magmatic rocks. Geochemically, the granodiorites were characterized by high SiO₂ and total alkalis (Na₂O+K₂O) contents, but low MgO and TiO₂ contents, with depletion in Eu、Sr、Ba, suggesting a geochemical affinity with calc alkaline I-type granites. Combined with the regional research data, it is thus concluded that these granodiorites were derived by partial melting of the ancient metaigneous lower crust, followed by vary degree of crystallization differentiation. Our research favor that the Middle-Late Jurassic magmatic rocks in Southern Qiangtang block were generated in an arc setting during the northward subduction of Bangong-Nujiang oceanic lithosphere. Furthermore, considering the preexisting geochemical data, the granitic rocks of Kayico area can be divided into two groups of calc-alkaline I-type granites and highly fractionated granites, which were derived by the different end-members of the crystal mush process within the shallow crust.