LA-ICP-MS zircon U-Pb age and geochemistry of Xiatianba A-type granites in Dongchuan, Northeast Yunnan, and their tectonic significance

Geochemical analysis and LA-ICP-MS zircon U-Pb dating of Xiatianba granites of two granitic bodies in Dongchuan of northeast Yuannan were conducted in this study. The general geochemical characteristics of biotite adamellite and porphyraceous moyite are similar to each other, characterized by high SiO2（71.7% on average）and belonging to the peraluminous (A/CNK value between 1.03～1.52) and alkaline series (AI value between 0.95～1.37). They have high ΣREE content with an average ΣREE value of 313×10-6, an average（Ce/Yb）N value of 8.85, an average（La/Yb）N value of 9.54 and an average δEu value of 0.31. The chondirte-normalized REE patterns display right-oblique curves. The large ion lithophile elements, such as Rb, Th, U and K, are relatively enriched, whereas high HFSE such as Nb, Ta, Sr and Ti are relatively depleted, which indicates the existence of residual plagioclase in the granite source area. Calculation also shows that the melt of Xiatianba granites was formed in an intraplate extension environment with temperatures between 724℃ and 786℃. These characteristics show that the Xiatianba granites are typical A-type granite. LA-ICP-MS U-Pb dating of zircon from biotite adamellite yielded a granite age of 801.1±6.6Ma (MSWD=0.84, n=14) and a thermal event age of 762.1±6.2Ma (MSWD=0.72, n=14), which are highly in accordance with the time of breakup of Rodinia supercontinent in the western Yangtze block, and are also consistent with the metallogenic epoch of the Dongchuan copper deposit, suggesting that the magmatism of the Neoproterozoic rift environment affected Dongchuan area and probably triggered hydatogenesis mineralization.