念青唐古拉中基性侵入岩年代学、地球化学及岩石成因

念青唐古拉早始新世中基性侵入岩由角闪辉长岩及二长闪长岩组成,其中角闪辉长岩锆石LA-MC-ICPMS U-Pb年龄为(49.92±0.24)Ma。中基性侵入岩属于高钾钙碱系列至钾玄岩系列;稀土总量较高且轻稀土富集;富集大离子亲石元素(Rb,Th,K),亏损高场强元素(Nb,Ta,Ti),具有弧岩浆岩的微量元素特征。中基性侵入岩具有较高的初始锶((87Sr/86Sr)i为0.708 77～0.713 71)、低的εNd值(-6.9～-6.1)、古老的亏损地幔模式年龄(TDM为1 329～1 995Ma),以及εNd、Nb/La与MgO无明显相关性,表明其源区为受古老地壳交代的富集地幔。稀土元素模拟表明中基性侵入岩的成岩过程受地幔中轻程度部分熔融作用所控制。动力学背景分析表明其与新特提斯洋板片的断离及软流圈物质的上涌密切相关。

Chronology, Geochemistry and Petrogenesis for Nyainqentanglha Intermediate-Basic Intrusive Rocks

Nyainqentanglha Early Eocene intermediate-basic intrusive rocks consist of amphibole gabbro and monzodiorite.The zircon LA-MC-ICPMS U-Pb dating for the amphibole gabbro defines a crystallizationage of(49.92±0.24) Ma.The intermediate-basic intrusive rocks are high-K calc-alkaline to shoshonite series based on the SiO2-K2O diagram.They contain high total REE with enrichment of LREE and LILE(Rb,Th,K),and depletion of HFSE(Nb,Ta,Ti),showing features of magmatic rocks in island arc or active continental margin.They also show relatively high initial 87Sr/86Sr values of 0.708 77-0.713 71,low εNd values from-6.9 to-6.1,old modal age of 1 329-1 995 Ma(TDM),and no correlation for εNd with MgO and Nb/La,which indicates that they come from an enriched mantle metasomatized by ancient continental crust.REE simulation suggests that the forming processes for the intermediate-basic intrusive rocks were controlled by low tomoderate partial melting of the enriched mantle.The dynamic background for the intermediate-basic intrusive rocks are likely linked to breakoff of Neo-Tethyan slab followed by upwelling of asthenospheric materials from the slab window.