藏北羌塘中部日湾茶卡地区花岗闪长岩LA-ICP-MS锆石U-Pb年龄与地球化学特征

报道了藏北羌塘中部日湾茶卡地区花岗闪长岩体锆石U-Pb年龄和岩石地球化学特征。该花岗闪长岩位于龙木错—双湖—澜沧江板块缝合带西段。岩石样品中的锆石具有较好的晶形,可见典型的岩浆振荡环带结构,具有较高的Th/U值,为典型的岩浆成因锆石。样品锆石LA-ICP-MS技术测得的锆石年龄为211.0Ma±1.8Ma,表明其时代为晚三叠世。全岩地球化学分析表明,该花岗闪长岩具有高Si O2、Al2O3,富碱,低TFe2O3、Ti O2的特征,A/CNK值为0.97~1.04,为准铝质—弱过铝质岩石。稀土元素配分模式显示,配分曲线右倾,伴随较弱的Eu负异常;微量元素蛛网图表现出Nb、Ta、Ti、P亏损和Rb、Th、U、K、Pb富集的特征;样品点在构造环境判别图解上均落入碰撞型花岗岩区。初步分析,其岩浆来源可能为幔源岩浆底侵使古老地壳发生熔融。此碰撞型花岗闪长岩对龙木错—双湖—澜沧江洋碰撞的时限提供了一定的岩石学约束,为龙木错—双湖—澜沧江洋的演化提供了基础地质资料。

LA-ICP-MS zircon U-Pb age and geochemical characteristics of the granodiorite in the Riwanchaka area, central Qiangtang

This paper reports new zircon U-Pb age and whole-rock major and trace element data of the granodiorite body in the Riwanchaka area, central Qiangtang, northern Tibetan Plateau. The granodiorite lies in the western part of the Lungmu Co-Shuanghu-Lancang River suture zone. Its zircons show no euhedral crystals and have clear oscillatory zones as well as high Th/U ratios, suggesting a magmatic origin. Zircon LA-ICP-MS dating of the granodiorite yielded a weighted mean age of 211.0Ma±1.8Ma, implying that the crystallization of the granodiorite body took place in late Triassic. Petrological and geochemical studies show that it is characterized by high silicon and aluminum, abundant alkali, low TFe2O3 and TiO2, with A/CNK being 0.97-1.04. Hence the granodiorite body belongs to aluminous-peraluminous series. The REE model is characterized by enrichment of light REE with negative Eu anomaly. In addition, it is enriched in Rb, Th, U, K, Pb with negative anomalies of Nb, Ta, Ti, P. These features suggest that the granodiorite has the characteristics of collision volcanics. A preliminary analysis shows that it was probably generated from partial melting of ancient crust through underplating of the mantle-derived magma. The collision-type granodiorite provides important petrologic constraints for the Lungmu Co-Shuanghu-Lancang River Ocean and the data for the study of the evolution of the Lungmu Co-Shuanghu-Lancang River Ocean.