西藏尼玛县塔色普勒地区去申拉组火山岩的发现及其地质意义

The discovery of Qushenla volcanic rocks in Tasepule area of Nyima Country,Tibet,and its geological significance

对塔色普勒地区新发现的去申拉组火山岩进行了系统的地球化学和年龄研究。根据主量元素特征,将区内火山岩划分为富镁安山岩、英安岩和高分异流纹岩3种类型。富镁安山岩具有较高的MgO含量和Mg#值,是俯冲板片熔体与地幔楔相互作用的产物;英安岩与高分异流纹岩具有同源岩浆的特征,富集大离子亲石元素Rb、Th、U、K、Pb,亏损Nb、Ta、Ti、Sr等元素,并具有较高的Rb/Sr值,可能是地壳部分熔融经历了不同分异作用形成的。对流纹岩进行LA-ICP-MS锆石U-Pb定年,获得103Ma±1Ma和107Ma±1Ma的年龄值,显示区内去申拉组火山岩形成于早白垩世晚期。研究认为,区内去申拉组火山岩形成的地球动力学背景是早白垩世晚期班公湖-怒江洋壳向南俯冲过程中发生的板片断离,塔色普勒地区班公湖-怒江洋的陆-陆碰撞时间在107Ma左右。

This paper reports new whole-rock major and trace element data and zircon U-Pb dating results of the newly discov- ered Qushenla volcanic rocks in Tasepule area, Nyima County, Tibet. According to the features of major elements, the authors grouped volcanic rocks into three types, i.e., magnesian andesites, dacite and highly differentiated rhyolite. The magnesian andesites produced by the interaction between the subducted slab reek and mantle wedge are rich in MgO and Mg#. Dacite and highly dif- ferentiated rhyolites are enriched in Ikb, Th, U, K, Pb and depleted in Nb, Ta, Ti, and have high Rb/Sr ratios, which shows that they might be the product of partial melting of the crust which experienced different degrees of differentiation and evolution. The dating of zircons from two rhyolitic samples by LA-ICP-MS yielded ages of 103Ma＋lMa and 107Ma±1Ma, which indicates that the Qushenla volcanic rocks in the study area were formed in late Early Cretaceous. It is considered that the plate fracturing that happened during the process of southward subducdon of Bangong Co-Nujiang Ocean at about ll0Ma caused the upwelling of mantle source materials, which was the geodynamic background for the formation of the Qushenla volcanic rocks. The result ob- tained by the authors suggests that the continent-continent collision of Bangong Co-Nujiang Ocean should occur at about 107Ma in Tasepule area.