矿床地球化学国家重点实验室知识库

The Tengchong–Lianghe tin belt is the most importantpart of the tin metallogenic belt in southwest China’s Sanjiang Metallogenic province. In this district, two A-type granite plutons that are spatially associated with the tin deposits have recently been discovered. These granitoids are dominated by biotite granites with weakly peraluminous to metaluminous compositions, high SiO2 contents (73.3–76.2 wt.%), and high alkali contents (K2O + Na2O = 8.3–9.17 wt.%). Trace element spider diagrams show that these granitoids are also clearly enriched with the large-ion lithophile elements Rb, Th, U, and K, but are markedly depleted in Ba and Sr. They are significantly depleted in the high field-strength elements Nb and Ti, but are enriched with Zr and Hf. These rocks have relatively high zircon saturation temperatures (774–833 °C), high Zr + Nb + Ce + Y contents (272–416 ppm), and 10,000 × Ga/Al ratios (almost N2.7) that are typical of A-type granites. They also have high total rare earth element (REE) contents (174–404 ppm) and relatively flat chondrite-normalized REE patterns with significantly negative Eu anomalies. LA–ICP–MS zircon U–Pb dating results indicate that the emplacement of these two granite plutons occurred during the early Tertiary (52.7 ± 0.3–53.0 ± 0.4 Ma) and the Late Cretaceous (73.3 ± 0.5–73.3 ± 0.5 Ma) periods, respectively. Isotopic compositions show that the granitoids have highly variable (87Sr/86Sr)i ratios (0.7182–0.7457), relatively constant low εNd(t) values (−11.2 to −12.4), and ancient Nd and Hf model ages (1.56–1.88 Ga), suggesting that they were derived from the partial melting of the Paleoproterozoic continental crust. Their zircon δ18O values (6.6–8.5‰) and εHf values (from −8.6 to −11.3) also suggest that these granitoids mainly originated from the middle-lower continental crust consisting of mafic and metasedimentary rocks. Such geochemical characteristics indicate that these two A-type granite plutons were generated by the partial melting of Paleoproterozoic mafic and the metasedimentary basement of the Tengchong block in a post-collisional or syn-collisional extensional setting of the Neo-Tethy anorogenic zone during the Late Cretaceous–Early Cenozoic. The Late Cretaceous granitoids were probably associated with the subduction of the Neo-Tethyan plate under the post-collisional extension of the western Burma and Tengchong microplates’ collision during the Jurassic–Early Cretaceous. The Early Tertiary granitoids might be the syn-collisional product of stretching relaxation during the middle and late main-collisional period (55–41 Ma) between India and Asia.