西藏赛卓花岗闪长岩锆石U-Pb年龄、地球化学特征及构造意义

Geochemistry and zircon U-Pb dating of the Saizhuo granodiorite in Tibet and their tectonic significance

对西藏日松地区赛卓花岗闪长岩体进行锆石U-Pb年代学和地球化学研究。赛卓花岗闪长岩体形成于早白垩世,锆石U-Pb年龄为106.0±1.0 Ma。岩体铝饱和指数为0.96~1.02,K2O/Na2O为1.06~1.30,CIPW标准矿物中绝大多数样品含紫苏辉石标准分子,部分样品含透辉石标准分子,岩石中发育大量暗色微粒包体,属于准铝质—弱过铝质、高钾钙碱性系列I型花岗岩;稀土元素总量为(97.84~143.44)×10-6,轻稀土元素明显富集,δEu为0.63~0.73,具中等负Eu异常;富集Rb、Th、U,亏损K、Ba、Sr及高场强元素Nb、P、Ti。结合区域地质背景,认为赛卓花岗闪长岩形成于岛弧构造环境,与班公湖—怒江特提斯洋岩石圈向南俯冲有关,为研究班公湖—怒江结合带构造演化提供新证据。

This paper reports chronological and geochemical studies on the Saizhuo granodiorite in the Risong area,western Tibet.Zircon U-Pb dating yields an age of 106.0±1.0 Ma,indicating that the Saizhuo granodiorite emplaced in Early Cretaceous.The granodiorite has A/CNK ratios of 0.96-1.02 and K2O/Na2O ratios of 1.06-1.30.Compared with the CIPW compositions,most samples contain hypersthene and part of samples contains diopside.A large amount of mafic microgranular enclaves occur within granodiorites,suggesting that the Saizhuo granodiorite belongs to metaluminous to weakly peraluminous high-K calc-alkaline series I-type granitoid.The rocks have a total REE content of（97.84-143.44）×10^-6 and distinct enrichment of LREE,showing a distinct right-oblique feature.The negative Eu anomalies are obvious,withδEu values of 0.63-0.73.They are also enriched in Rb,Th and U,but depleted in K,Ba,Sr,Nb,P and Ti.Considering regional geology of the Risong area,the Saizhuo granodiorite is inferred to form in an island arc setting related to the southward subduction of the Bangonghu-Nujiang oceanic crust,which provides new evidence for the tectonic evolution of the Bangonghu-Nujiang Tethys Belt.