西藏冈底斯东部然乌地区早白垩世岩浆混合作用：锆石SHRIM PU-Pb年龄和Hf同位素证据

Early Cretaceous magma mixing in Ranwu Area of Eastern Gangdese,Tibet:Evidence from zircon SHRIMP U-Pb age and Hf isotopic composition

目前对西藏冈底斯带早白垩世大规模岩浆作用的岩石成因以及冈底斯带不同构造单元的东延仍存在不同看法。为探讨这些问题,文中对冈底斯带东部地区然乌岩体中的闪长岩脉进行了锆石SHRIM PU-Pb定年和锆石Hf同位素分析。结果表明:然乌岩体中闪长岩脉的锆石SHRIM PU-Pb年龄为(114.2±0.9)Ma,与二长花岗岩为同期侵位。然乌闪长岩脉具有不均一的锆石Hf同位素组成,其εHf(t)值介于-4.2～+4.9,对应的Hf同位素地壳模式年龄为0.85～1.44Ga。闪长岩脉的全岩εNd(t)值为-4.7,Nd同位素两阶段模式年龄(TDM2)为1.29Ga,与锆石Hf同位素模式年龄一致。然乌地区同期发生的闪长质岩浆和花岗质岩浆侵位以及不均一的锆石Hf同位素组成,很可能表明然乌地区大约在115Ma发生了重要的岩浆混合作用。结合锆石Hf同位素地壳模式年龄的区域性对比,我们认为,与北冈底斯带相比,然乌地区同中冈底斯带之间具有更好的可对比性。

The petrogenesis of the extensive Early Cretaceous magmatism in Gangdese Belt and how the tectonic units correlate to the east have long been subjects of debate. To explore these issues, we performed zircon SHRIMP U-Pb dating and zircon Hf isotopic determination on a dioritic dike from the Ranwu Pluton in Eastern Gangdese, Tibet. Zircons from a diorite sample yield a crystallization age of （114.2±0. 9） /Via, identical to the monzogranite of the Ranwu Pluton. Zircon Hf isotopic compositions of the diorite sample are heterogeneous with εHf （t） values ranging from 4.2 to ＋4. 9, corresponding to the Hf isotopic crustal model ages （TDM^C） of 0. 85-1. 44 Ga. The dioritic sample has εNd （t） value of -4. 7 with two-stage model age （TDM2） of 1. 29 Ga, within the range of the zircon Hf isotope model age. The contemporaneous dioritic dike （and enclave） and the host monzogranite, along with the heterogeneous Hf isotopic compositions indicate the presence of an important magma mixing in Ranwu Area at - 115 Ma. This finding, in combination with the Hf isotopic crustal model ages recently reported in other places in the Gangdese Belt, suggest that the Ranwu Area could be correlated with the Middle Gangdese Belt rather than the Northern Gangdese Belt as previously thought.