拉萨地块南部得明顶地区叶巴组火山岩LA-ICP-MS锆石U-Pb年龄、地球化学特征及其成因

作为拉萨地块南部形成时代最早的火山岩，叶巴组火山岩对于确定新特提斯洋的俯冲过程及印度欧亚大陆碰撞前的演化等具有重要意义。前人的研究结果显示，叶巴组火山岩是一套岩性由基性到酸性连续的、具有岛弧地球化学特征的火山岩。目前在叶巴组火山岩中只有酸性岩获得了精确的锆石U-Pb年龄，显示其形成时代为早侏罗世。然而对叶巴组中基性火山岩目前尚无精确的年代学报道。另外，有关叶巴组火山岩形成的动力学背景，目前还存在争议。对拉萨地块南部得明顶地区的叶巴组火山岩进行了锆石U-Pb年龄和地球化学研究，获得其锆石U-Pb年龄为188.8±1.8Ma，表明与酸性火山岩形成时代一致。得明顶地区叶巴组火山岩地球化学数据显示其具有类似于岛弧火山岩的特征，富集大离子亲石元素、亏损高场强元素、轻重稀土元素分异明显，部分样品具有Sr及Eu(δEu=0.75~0.83)的略微异常。结合前人的研究成果认为，叶巴组火山岩很可能形成于大陆边缘弧，其形成与新特提斯洋的北向俯冲有紧密的联系。

LA-ICP-MS zircon U-Pb geochronology,geochemical characteristics and genetic study of Yeba Formation lavas in Demingding area,southern Tibet

The Yeba Formation lava, generally considered to be the earliest volcanic rock in southern Tibet, is important for understanding the subduction of the Neo-Tethys Ocean and the evolution of the Tibet before India-Asia collision. Previously published zircon U-Pb dating results for the Yeba Formation felsic rocks indicated that they were developed during Early-Middle Jurassic. However, so far, the precise ages of intermediate-basic rocks are absent. Meanwhile, the geodynamic setting for the generation of the Yeba Formation lavas remains controversial. The result of LA-ICP-MS zircon U-Pb dating of andesite of the Yeba Formation from Demingding area is 188.8 ± 1.8 Ma, which is consistent with the formation age of acidic rocks. This suggests that the Yeba Formation lavas was mainly developed during Early-Middle Jurassic. The geochemical data of Yeba Formation lavas in Demingding area show an affinity to arc magma, such as enrichment of large ionic lithosphere elements (e.g., Rb, U) and light rare earth elements (e.g., 3.3<(La/Yb)N<9.9), and depletion of high field strength elements (e.g., Nb, Ta, and Ti), Sr and Eu (δEu= 0.75~0.83). The new data provided by this study, combined with previous results, indicate that the generation of the Yeba Formation lavas was probably in a continental arc setting that was related to the northward subduction of the Neo-Tethys Ocean during Early-Middle Jurassic.