西藏那曲地区嘎加组闪长玢岩LA-ICP-MS锆石U-Pb年龄与地球化学特征

Zircon U-Pb ages and geochemical features of the diorite porphyrite in Gajia Formation in the Nagqu area, Tibet

西藏那曲地区的中—晚三叠世嘎加组中存在一套闪长玢岩，对这套闪长玢岩进行了LA-ICP-MS锆石U-Pb同位素测定，获得的年龄为115.4±0.6Ma，属早白垩世，与区域上多尼组和则弄群时代一致。闪长玢岩SiO2含量均值为57.96%，富集Rb、Th元素，亏损Nb、P、Ti元素，与上地壳具有相似的蛛网图曲线。εHf（t）值在2.0～4.1之间，平均值为3.0，岩浆来自新生地壳。结合地球化学和构造环境综合分析，闪长玢岩可能形成于拉萨地块和羌塘地块碰撞环境，115Ma时研究区洋壳俯冲作用已结束，进入陆—陆碰撞的演化阶段，随着羌塘地块与拉萨地块的碰撞，地壳不断加厚，慢慢聚集在浅部岩浆房中的大量岩浆把已经加厚的地壳不断加热，从而引起上覆地壳的部分熔融。

A suite of diorite porphyrites exists in Middle-Late Triassic Gajia Formation of the Nagqu area, Tibet. The authors select-ed samples for zircon U-Pb dating by using LA-ICP-MS system, and obtained an age of 115.4 ± 0.6Ma, belonging to Early Creta-ceous and being consistent with ages of Duoni Formation and Zelong Group. The SiO2 average concentration is 57.96%, with the enrichment of Rb and Th and loss of Nb, P and Ti elements, exhibiting the characteristics of island arc granitoid. The diorite porphy-rite and upper crust have similar spidergrams. The εHf（t） values are in the range of 2.0~4.1, with the mean value of 3.0, suggesting that magma came from new crust. Based on the comprehensive analysis of geochemistry and tectonic environment, the authors hold that the diorite porphyrite formed in a collision environment of the Lhasa block and Qiangtang terrane. At 115Ma, the subduction of the oceanic crust was over, and the evolution entered into the continental collision stage. Due to the collision between the Qiangtang terrane and the Lhasa terrane, the crust thickened, a large quantity of magma concentrated in shallow magmatic chambers and contin-uously heated the thickened crust heating, resulting in the partial melting of the overlying crust.