西藏拉萨地体南缘汤白地区始新世辉绿岩脉——新特提斯洋壳断离的证据

拉萨地体南缘汤白地区广泛分布新生代的辉绿岩脉。为探讨该辉绿岩脉形成时代、岩石成因和地质意义,对其开展了详细的岩相学、地球化学、锆石U-Pb年代学及Hf同位素研究。LA-ICP-MS锆石U-Pb定年结果显示辉绿岩脉的结晶年龄为(54±1)Ma,表明其形成于早始新世。微量元素地球化学特征显示富集大离子亲石元素(LILEs:如Rb、Sr和Ba),亏损高场强元素(HFSEs:如Nb、Ta和Ti)。与典型的弧岩浆岩、区域上叶巴组和桑日岩群中的玄武岩相比具有较高的Nb、TiO_2和Zr含量,在微量元素构造环境判别图解中显示出板内玄武岩地球化学属性。微量元素地球化学特征结合锆石Hf同位素表明岩浆源区除被俯冲板片释放的流体交代的岩石圈富集地幔外,还有软流圈亏损地幔物质加入。汤白辉绿岩脉侵入年龄与区域上林子宗群火山活动峰期接近(52 Ma)。同时结合岩石成因及构造背景,作者认为汤白辉绿岩脉是54～52 Ma新特提斯洋壳断离诱发岩浆作用的产物。根据最新大陆碰撞及板片断离的三维数值模型,暗示了印度板块与欧亚大陆碰撞的起始时间为65 Ma或者更早。

Eocene diabase dikes in the Tangbai area, southern margin of Lhasa terrane, Tibet: Evidence for the slab breakoff of the Neo-Tethys Ocean

Diabase dikes are extensively distributed in the Tangbai area on the southern margin of the Lhasa terrane. In order to discuss their formation age, genesis and tectonic implications, the authors investigated their petrography, geochemistry, zircon U-Pb geochronology and Hf isotope. Zircon U-Pb dating yielded an age of (54±1) Ma for the Tangbai diabase dikes, indicating that they were formed in the Early Eocene. The trace elements are characterized by enrichment of LILEs (such as Rb, Sr and Ba) and depletion of HFSEs (such as Nb, Ta and Ti). Compared with typical arc magmas, Sangri Group basalts and Yeba Formation basalts in this area, the Tangbai diabase dikes have higher values of Nb, TiO₂ and Zr. Trace element tectonic discrimination diagrams show that Tangbai diabase dikes fall in intraplate basalts field, and show geochemical affinities with intraplate magmatism. The race element geochemical characteristics and zircon Hf isotopic data suggest that the diabase dikes were likely derived from enriched lithospheric mantle which had been metasomatized by slab-derived fluids during previous subductions, and mixed with upwelling ashospheric mantle. The intrusion age of Tangbai diabase dikes was close to the peak period (52 Ma) of Linzizong volcanic activity. Combined with their genesis and tectonic setting, the authors hold that the formation of the Tangbai diabase dikes was related to slab break-off of the northward subduction of the Neo-Tethyan slab ca. 54~52 Ma in age. In addition, according to the latest 3-D numerical models of continental collision and slab break-off, it is shown that the onset of India-Eurasia continental collision should have occurred at 65 Ma or earlier.