The Cambrian metamorphic granites in the Jili area, Baxoi, Xizang:U-Pb dating, geochemical signatures and geological significance

The Cambrian metamorphic granites in the Jili area, Baxoi, Xizang consist dominantly of metamorphic monzogranites and metamorphic granodiorites. The new findings of the metamorphic granites in this study will be of significance in the understanding and reconstruction of the tectonic evolution of the proto-Tethys. The cathodoluminescence images of the zircons indicate the magmatic origin of the metamorphic granites in this area. The LA-ICP-MS zircon U-Pb dating data gives the ages of 503.7±4.7 Ma for the schistositized metamorphic monzogranites, and 494.7±3.4 Ma for the metamorphic granodiorites, implying that the metamorphic granites were initiated during the Cambrian. Geochemically, the metamorphic monzogranites have the SiO₂ contents of 69.87% to 79.89%, Al₂O₃ contents of 12.36% to 14.82%, Na₂O contents of 2.54% to 7.16%, K₂O contents of 0.15% to 5.95%, and K₂O/Na₂O ratios of 0.02 to 2.34, whereas the metamorphic granodiorites have the SiO₂ contents of 66.63% to 70.15%, Al₂O₃ contents of 14.66% to 15.41%, Na₂O contents of 3.60% to 5.63%, K₂O contents of 0.77% to 2.78%, and K₂O/Na₂O ratios of 0.14 to 0.77, indicating that the above-mentioned metamorphic granites belong to the calc-alkaline to alkaline peraluminous granites. In the Rb vs. (Y+Nb) and Rb vs. (Yb+Ta) discrimination diagrams for the tectonic interpretation, all the granites samples are projected into the volcanic arc granite field, suggesting the continental-margin volcanic arc environments. It follows that the metamorphic granites in the Jili area were initiated in the stages of the Kaqiong micro-continent during the breakup of the Gondwana land, and the timing of the proto-Tethys may be traced back to the Cambrian.