西藏南木林县秦马弄地区二长花岗岩体岩石成因

以出露在西藏南木林县秦马弄地区二长花岗岩体为研究对象，从野外地质调查、岩石学、LA-ICP-MS锆石U-Pb定年、元素和Lu-Hf同位素地球化学等方面对其进行了分析研究。结果显示：秦马弄地区二长花岗岩体类似于A型花岗岩，形成于古近纪(E1)，其LA-ICP-MS锆石U-Pb年龄为(62.0±0.5)~(63.0±0.5) Ma；岩体形成的温度较高(锆石饱和温度为745~805 ℃，平均766 ℃)；岩石富SiO2(72.6%~75.0%)、Na2O(2.86%~3.59%)和K2O(4.12%~5.25%)，贫CaO(0.72%~1.39%)和MgO(0.26%~0.36%)，高FeOT/MgO值(7.26~10.3)，属高钾钙碱性系列，弱过铝质；岩石具明显的负Eu异常(Eu/Eu*=0.40~0.70)，普遍表现出Ba、Sr、P、Ti元素的亏损和Rb、Th、K、Zr、Hf元素的富集。εHf(t)值为-4.7~-1.8，其两阶段地壳Hf模式年龄tDM2为(1.1~1.3) Ga。秦马弄A型花岗岩体是冈底斯基底地壳物质部分熔融的岩浆产物，形成于古近纪印度板块与欧亚板块俯冲碰撞引起的总体为岛弧背景下的局部拉张构造环境。

Petrogenesis of the monzonitic granite of the Qinmalong District,Nanmulin County,Tibet, China.

In this paper, we performed comprehensive analyses on the monzonitic granite outcropped in the Qinmalong District, Nanmulin County, Tibet including field geological survey, petrology, LA-ICP-MS zircon U-Pb dating, elemental and Lu-Hf isotope geochemistry. The results showed that the monzonitic granite from the Qinmalong District belongs to the A-type granite and formed in the Paleogene (E1). The LA-ICP-MS zircon U-Pb age of the granite was 62.0±0.5 to 63.0±0.5 Ma. The weakly peraluminous monzonitic granite formed in high temperature (the saturation temperature of zircon was 745805 ℃, averaged at 766 ℃) and belongs to the high-potassium-calcium-alkaline series. It was enriched in SiO2 (72.6%74.97%), Na2O (2.86%3.59%) and K2O (4.12%5.25%) but depleted in CaO (0.72%1.39%) and MgO (0.26%0.36%). In addition, the granite had high FeOT/MgO value (7.2610.30) and obvious negative Eu anomalies (Eu/Eu*=0.400.70), depleted in Ba, Sr, P and Ti but enriched in Rb, Th, K, Zr and Hf element. The εHf(t) value was -4.7-1.8. The cursts two-stage Hf model age (tDM2) was 1.11.3 Ga. Our study indicates that the A-type granite originates from partial melting of the crustal materials in the Gangdese basement and formed in a tectonic setting of localized stretching in arc environment in response to the subduction and collision between the Indian and Eurasian Plates in the Paleogene.