西藏冈底斯岩带曲水杂岩体锆石LA-ICP-MS U-Pb年龄及其地质意义

Zircon U-Pb dating for the Quxu granitic complex in the Gangdise belt,Tibet,and its geological significance

曲水杂岩体位于冈底斯构造-岩浆岩带东段南缘,其岩浆活动与雅鲁藏布江新特提斯洋壳向北俯冲、消减以及印度与欧亚板块碰撞息息相关。本文以曲水县-昌果乡广泛分布的中酸性花岗岩体为研究对象,进行了系统的LA-ICP-MS锆石U-Pb年代学和岩石地球化学研究。结果表明,曲水杂岩体由3期时代和规模不同的花岗质岩体构成,其LA-ICP-MS锆石U-Pb年龄分别为95.2±1.0~88.5±1.0Ma、65.2±0.6Ma和48.5±0.5~43.3±0.7Ma;岩石地球化学研究表明,晚白垩世和古新世花岗岩以中性-中酸性为主,属钙碱性系列,具中铝特征,A/CNK比值小于1.1,属于I型花岗岩,是玄武质下地壳部分熔融产物,指示其形成于特提斯洋壳俯冲过程的岛弧构造环境。始新世花岗岩以高钾钙碱性系列为主,并出现钾玄岩系列,具偏铝-过铝质特征,指示岩浆上侵过程中遭受了不同程度的地壳物质混染,其形成于印度-欧亚板块强烈碰撞的构造环境。

The Quxu granitic complex is located at the southern margin of the eastern Gangdise tectonic-magmatic belt.Magmatic activities of the Quxu granitic complex were closely related to the northward subduction of the Neotethys and the Indo-Asian collision.In this paper,a systematic study of LA-ICP-MS zircon U-Pb geochronology and petrochemistry have been carried out on the intermediate-acidic granites widely distributed in the Changguo village,east of Quxu county.The results show that the Quxu complex is characterized by three stages of granitoids with different ages and scales.The zircon U-Pb ages of LA-ICP-MS are 95.2±1.0~88.5±1.0Ma,65.2±0.6Ma and 48.5±0.5~43.3±0.7Ma,respectively.The results of petrochemistry show that the late Cretaceous and Paleocene granites are mainly neutral to intermediate-acidic,calc-alkaline series,with the characteristics of medium aluminum,A/CNK less than 1.1,belonging to I-type granites,which are the products of partial melting of basaltic lower crust formed in the arc tectonic setting during the subduction of the Tethyan oceanic crust.The Eocene granites are mainly high-Kcalc-alkaline series as well as shoshonitic series and are metaluminous to peraluminous,which indicate that the magma was formed in the collisional setting of India-Eurasia plates and mixed with crustal materials at different degrees during the magma underplating processes.