Characteristics and zircon SHRIMP U-Pb dating of the Amduo trachyte,Tibet,China

Amduo trachyte is located at the middle segment of the Bangonghu-Nujiang Suture, and adjacent to the northern side of the Amduo ophiolites. The outcrop area of the trachite is about 30km2, with accumulated thickness about 1500m. The features of trachyte show that it is continental volcanic rock with a relatively complete volcanic apparatus. The rock association which falls into shoshonite series includes latite, quartz latite, trachyte and some volcaniclastic rocks, but the trachyte is the major type. The characteristics of the petrochemistry, geochemistry and isotope-tracer measurement together suggest that the trachyte has relatively high pressure forming condition and belongs to potassic rocks fromed during post-orogenic process triggeved by continental collision. The magma generated the trachyte is mostly derived from the lower part of the thickened continental crust, with contribution of the mantle composition somewhere. The zircon SHRIMP U-Pb dating from the trachyte is about (79.9±1.9)Ma, a crystallization age of trachytic magma, which indicates that the Amduo trachyte erupted in the Late Cretaceous, and implies that the Bangonghu-Nujiang oceanic basin had been closed in the late stage of the Early Cretaceous, and the Qiangtang Block and Lhasa Block had been become a united continental block with a thickened continental crust in the Late Cretaceous.