Zircon SHRIMP U–Pb geochronology, geochemistry,and geological significance of dacite in the Zhesang gold district,southeast Yunnan Province, China

The Late Paleozoic and Mesozoic tectonic framework of the Nanpanjiang Basin has much been disputed. Herein, the middle-acid volcanic rock, dacite,exposed to the Zhesang gold district, southeast Yunnan Province, has been analyzed. The results show that the dacite belongs to a calc-alkaline series, SiO2 contents range from 62.79 to 76.66 wt%. Zircon SHRIMP U–Pb dating of dacite demonstrates that they were formed in the Early Triassic（247.8 ± 1.7 Ma, MSWD = 1.2）. All samples exhibit enrichment in LILE（e.g. Rb, K, Th, and U）, and depletion in HFSE（e.g. Nb, Ta, and Ti）, which has the geochemical affinity of I-type granite. La–La/Sm and La–La/Yb discrimination diagrams show that the partial melting, mainly of the mafic lower crust, of rocks, plays a major role in the formation process. The dacite has low initial 87Sr/86Sr ratios（0.706954 to 0.708589） and negative εNd（t） values（-11.77 to-10.88）. Zircons in dacite have εHf（t） values of-16.2 to-8.3, and the two-stage Hf model ages are 1799–2301 Ma, mostly concentrated between 1800 and 1900 Ma, indicating that the magma source area is the reconstructed ancient lower crust mixed with some mantle materials, and crystal fractionation process underwent in the late stage of magma migration. This study reveals that the arc-volcanic rocks of the Early Triassic in the southern margin of the Nanpanjiang Basin were formed by the subduction of the Late Paleozoic ocean basin within the border region between China and Vietnam.