西藏冈底斯中段桑桑花岗质岩体锆石U-Pb年龄与Hf同位素组成及其对岩石成因和构造演化的制约

以西藏冈底斯中段西侧桑桑花岗质岩体为对象,进行了系统的年代学、元素地球化学和锆石Hf同位素组成研究,据此阐明了岩体成因,并探讨了其构造意义。锆石LA-ICP-MS U-Pb定年表明,桑桑花岗质岩体的成岩年龄为49~54 Ma。化学组成上,岩体具有亚碱、准铝、贫磷的特征(A/NKC1.10,P_2O_50.20%),属钙碱性I型花岗岩类。岩体富Cs、Rb、Ba、Th、U、K、Pb和轻稀土,贫Nb、Ta、P与Ti,表现出弧岩浆岩的地球化学特征。岩体的锆石εHf(t)值变化较大,散布于正值与负值之间(=-4.24~+5.49),指示其形成存在不同来源物质的贡献。综合分析表明,桑桑花岗质岩体起源于初生地壳的部分熔融,但在成岩过程中有古老地壳组分的参与。结合区域地质背景,笔者认为这一古老地壳组分最可能来自印亚碰撞过程中俯冲下插的印度地壳,由此说明印度-欧亚大陆碰撞的起始时间应早于54 Ma。

Zircon U-Pb Ages and Hf Isotopic Compositions of the Sangsang Granitic Pluton in the Middle Segment of the Gangdese Belt: Constraints on the Petrogenesis and Tectonic Evolution

To better understand the petrogenesis and tectonic implications of the Sangsang granitic pluton located in the western part of the middle segment of the Gangdese belt, we have conducted an integrated study of the geochronology, elemental geochemistry, and zircon Hf isotopic compositions of the granitic pluton. Zircon LA-ICP-MS U-Pb dating for the Sangsang granitic rocks yields ages of 49~54 Ma, indicating that they were emplaced during Eocene. Chemically, all the rocks are subalkaline, metaluminous and poor in phosphorus (A/NKC<1.10，P2O5<0.20%), thus can be grouped into calc-alkaline I-type granites. They are enriched in Cs, Rb, Ba, Th, U, K, Pb and LREE, and depleted in Nb, Ta, P, Ti, showing the geochemical characteristics of arc magmatic rocks. The granitic rocks have variable zircon εHf(t) values which are scattered from positive to negative values (=-4.24~ + 5.49), implying that different source components have contributed to magma genesis. The integrated petrologic, elemental and isotopic compositions suggest that the Sangsang granitic rocks were mainly originated by partial melting of juvenile crust, with some ancient crustal materials involved in their petrogenesis. In combination with the regional tectonic setting, the authors suggest that the ancient crustal components were likely derived fromthesubductedIndiacrust,andherebyinferredthattheinitialtimeoftheIndia-Eurasiacollisionshouldbeearlierthan54Ma