Geochemistry and geochronology of the Caima granites in the western part of the Bangong Lake-Nujiang metallogenic zone, Xizang

The caima granites are delineated as important metallogenic rock masses in the Rutog-Dobza magmatic arc zone in the western part of the Bangong Lake-Nujiang metallogenic zone, Xizang. The zircon LA-ICP-MS and U-Pb isotopic values of the medium-grained biotite adamellite were determined for the Caima granites, indicating an age of 165.1±1.5 Ma (n=17, MSWD=0.86). The whole-rock Rb-Sr isotopic ages are dated at 163.5±2 Ma (n=5). The analytical results suggest that the ages of the Caima granites should be traced back to the Middle Jurassic (163 to 165 Ma) rather than the formerly dated the Early Cretaceous. The Caima granites belonging to the high-K calc-alkaline series have the following characteristics:the enrichment of LREE, ΣREE=109.5×10^-6-225.2×10^-6, LREE/HREE ratios=2.37-7.77, depletion of Eu, highly depletion of Ba, Nb, P and Ti, enrichment of Th, U and Pb, and slightly depletion of Zr. It is inferred that the Caima granites may be the products of the northward subduction of the Bangong Lake-Nujiang oceanic crust and originated from partly melting of the mantle above the subduction zone and the melting and mixing of the crustal matter.