藏东吉塘地区早古生代花岗岩锆石U-Pb年龄、Hf同位素及其对原特提斯洋演化的启示

以前对藏东吉塘地区的早古生代岩浆事件鲜有报道，在吉塘地区的浪拉山新发现了中—晚奥陶世花岗岩。采用LA-ICP-MS锆石U-Pb测年，获得该花岗岩的206Pb/238U年龄加权平均值为455±2.0 Ma（WSWD=1.15，n=26），属于中—晚奥陶世。岩石地球化学特征表明，该花岗岩具有高SiO2（78.72%~79.48%）、富钾（K2O/Na2O=5.55~10.20），低MgO（0.10%~0.25%）的特征，铝饱和指数A/CNK为1.08~1.26，岩石属于高钾钙碱性强过铝S型花岗岩。岩石稀土元素总体表现为轻稀土元素富集、重稀土元素亏损的特征，具强烈负Eu异常，δEu值为0.35~0.39；在原始地幔标准化微量元素蛛网图上，明显富集Rb、Th、Nd、Pb等元素，亏损Ba、Nb、Sr、P、Ti等元素。结合区域地质资料分析，该岩体是由原特提斯洋俯冲消减引起古老地壳物质部分熔融形成的，是原特提斯洋俯冲消减的岩浆事件响应；说明在中—晚奥陶世吉塘地区存在原特提斯洋的俯冲消减作用。

Zircon U-Pb age and Hf isotope of Early Paleozoic granite from the Jitang area in eastern Tibet and its insight into the evolution of the Proto-Tethys Ocean

There is little report on the Early Paleozoic magmatic events in the Jitang area, eastern Tibet.The Middle-Late Ordovician granites have been discovered in Langla Mountain, Jitang.The weighted average U-Pb age of the granite is 455±2.0 Ma(WSWD=1.15, n=26) by the LA-ICP-MS U-Pb dating of the zircon, indicating that it was emplaced in Middle-Late Ordovician.The geochemical characteristics indicate that the granite has high SiO2(78.72%~79.48%), high potassium(K2O/Na2O=5.55~10.20), low MgO(0.10%~0.25%), with aluminum supersaturated index A/CNK values ranging from 1.08 to 1.26, and the rock is a high-potassium-calcium- alkali-strong over aluminum S-type granite.The REE contents of rocks are enriched in LREE and depleted in HREE, with a negative Eu anomaly and δEu values ranging from 0.35 to 0.39.They are also enriched in LILEs (Rb, Th, Nd and Pb), but depleted in HFSEs (Ba, Nb, Sr, P and Ti).Combined with regional geological data, it is suggested that the granites might have originated from partial melting of ancient crustal materials, caused by the subduction of the Proto-Tethys, and is the response of the magmatic events during subduction of the Proto-Tethys.