西藏安多地区粗面岩的特征及其锆石SHRIMP U-Pb定年

安多粗面岩位于班公湖-怒江缝合带中段，紧邻安多蛇绿岩带的北侧，出露面积约30km2，堆积厚度约1500m，属陆相火山岩。火山机构面貌保存较完整。岩石类型包括安粗岩、石英安粗岩、粗面岩及少量火山碎屑岩，以粗面岩为主，属钾玄岩系列。岩石地球化学和同位素示踪特征表明岩石属相对高压型的粗面岩，是与大陆碰撞后造山有关的钾质火山岩。岩浆主要来源于加厚的陆壳下部，但也有幔源组分的贡献。粗面岩中锆石SHRIMP U-Pb年龄为79.9Ma±1.9Ma，是粗面质岩浆上侵结晶的年龄，表明安多粗面岩的喷溢时代为晚白垩世， 也暗示班公湖-怒江洋盆安多段早白垩世晚期已经闭合，晚白垩世时期羌塘地块与拉萨地块已成为统一的陆块，并具有加厚的陆壳。

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.