西藏聂荣微陆块早侏罗世中期花岗岩及其包体的岩浆混合成因:锆石LA-ICP-MS U-Pb定年和Hf同位素证据

Tracing magma mixing genesis of the middle Early-Jurassic host granites and enclaves in Nyainrong microcontinent, Tibet from zircon LA-ICP-MS U-Pb dating and Hf isotopes

西藏早侏罗世聂荣岩体中偏基性包体与酸性寄主岩的岩浆源区性质和岩石成因以及相互关系尚未得到很好约束,直接限制了对聂荣微陆块在早侏罗世特提斯构造岩浆演化中的作用的认识。为探讨这一问题,本文对采自聂荣地区的一对花岗岩及其闪长岩包体样品进行了锆石U-Pb定年和原位Hf同位素分析。寄主花岗岩的结晶年龄为185·1±1·5Ma,闪长岩包体的结晶年龄为183·6±1·1Ma,指示酸性岩浆和基性岩浆同时侵位。寄主花岗岩的锆石εHf(t)值介于-17·8^-0·9,其Hf同位素地壳模式年龄变化于1·3~2·4Ga,闪长岩包体的锆石εHf(t)值和Hf同位素地壳模式年龄值分别分布于-11·9^-2·9和1·4~2·0Ga,均表现出很大的变化范围。同时于~185Ma结晶的两种岩浆锆石Hf同位素的不均一性和继承锆石的出现,指示了聂荣微陆块早侏罗世中期发生了古老基底深熔或重熔的熔体和富集岩石圈地幔来源的岩浆间的混合,之后再与围岩混染的岩浆作用过程。

The study on the magma source regions and genesis of the Early Jurassic mafic enclaves and host granitoids from Nyainrong pluton as well as their correlations, is important in understanding the role of Nyainrong microcontinent during the Early Jurassic Tethyan tectonomagmatic evolution. This paper present in situ zircon U-Pb and Hf-isotopic data of a pair of granite and dioritic enclave from Nyainrong. The zircon LA-ICP-MS U-Pb age of the dioritic enclave （183.6±1.1Ma） is identical to that of the host granite （185.1±1.5Ma）, implying that the mafic and felsic magmas were coeval magmatic events. The host granite is characterized by negative zircon εHf（t） values （-17.8^-0.9） and old Hf crustal model ages （1.3~2.4Ga）, and the enclave has heterogeneous zircon εHf（t） values （-11.9^-2.9）, with Hf crustal model ages of 1.4~2.0Ga. This indicating that both crustal and mantle sources have contributed to their origin. Hf isotopic variations in magmatic zircons combine with the presence of the inherited zircons from our samples indicates that mixing of mature crust-derived felsic magma with enriched lithosphere mantle-derived mafic magma, coupled with assimilation of wall rocks, is most likely occurred in the Nyainrong microcontinent in the middle Early Jurassic.