早侏罗世叶巴组与桑日群火山岩：特提斯洋俯冲过程中的陆缘弧与洋内弧？  

Early Jurassic volcanic rocks from the Yeba Formation and Sangri Group: Products of continental marginal arc and intra-oceanic arc during the subduction of Neo-Tethys Ocean?

作　　者：黄丰[1,2] 许继峰[1,3] 陈建林[1,3] 康志强[4] 董彦辉[5] 

机构地区：[1]同位素地球化学国家重点实验室、中国科学院广州地球化学研究所,广州510640 [2]中国科学院大学,北京100049 [3]中国科学院青藏高原地球科学卓越创新中心,北京100101 [4]桂林理工大学地球科学学院,桂林541004 [5]国家海洋局海底科学重点实验室、国家海洋局第二海洋研究所,杭州310012

出　　处：《岩石学报》2015年第7期2089-2100,共12页Acta Petrologica Sinica

基　　金：中国科学院先导专项(XDB03010300);国家973项目(2015CB452602);国际合作项目(IGCP/SIDA-600);国家基金委创新群体项目(41121002);国家自然科学基金项目(40872055;41273039;41373030)联合资助

摘　　要：雅鲁藏布特提斯洋的演化对研究青藏高原的形成具有重要的意义,一般认为广泛分布于拉萨地块南部叶巴组和桑日群火山-沉积岩系是该特提斯洋早期的俯冲岩浆产物。本研究选取了拉萨附近达孜地区叶巴组中的2个中基性火成岩样品进行了锆石LA-ICPMS U-Pb分析,结果显示其年龄分别为188±2Ma和175±2Ma,与已发表的叶巴组中酸性火成岩的形成时代（174~193Ma）一致,已发表的桑日群火山岩的年龄也在相同范围内,因此叶巴组和桑日群火山岩喷发时间主要为早侏罗世。叶巴组和桑日群基性及中酸性岩浆均类似岛弧型火山岩,但前者具有相对高的Nb、Zr含量,Th/Y比值及相对较低的La/Nb比值,呈现出大陆地壳组分增加的趋势,叶巴组火山岩表现为典型的大陆边缘弧特征而桑日群类似于洋内弧火山岩,桑日群火山岩分布于叶巴组南侧,并呈碎片似展布于拉萨地块南部,同时显示了与叶巴组不同的岩性组合,暗示同时代的叶巴组和桑日群火山-沉积岩可能分别代表特提斯洋俯冲过程中的形成的陆缘弧和洋内弧。Studying evolutionary history of the Yarlung-Zangbo Neo-Tethys Ocean can help us to better understand how the Tibetan Plateau formed. The volcanic-sedimentary rocks of Yeba Formation and Sangri Group are widely distributed in southern Lhasa subterrane, which are usually considered to be the early-stage products of the Neo-Tethyan subduction. By dating of the LA-ICPMS U- Pb zircons from Yeba Formation volcanic rocks in Dazi, an andesitic sample and a basaltic sample yield 175 ＋2Ma and 188 ~2Ma, respectively, which are in agreement with published dating results （174 ~ 193Ma）. These dating results indicate that both Yeba Formation and Sangri Group volcanics were formed in the Early Jurassic. Both volcanic rocks in Yeba Formation and Sangri Group show arc-like geochemical characteristics. In the other hand, volcanic rocks from the Yeba Formation show relatively higher Nb and Zr concentrations, Th/Y ratios and relatively lower La/Nb ratios than volcanic rocks of Sangri Group, demonstrating that the former were formed in a continental arc. Meanwhile the volcanic rocks from Sangri Group have geochemical signature similar to those from an intra- oceanic arc. Moreover, the Sangri Group, locating at the south side of the Yeba Formation and southern margin of Lhasa terrane, display igneous association different from the Yeba Formation. These facts hint that the Yeba Formation and Sangri Group represent a continental arc and an intra-oceanic arc, respectively, when the Neo-Tethys Ocean subducted northward below the Lhasa terrane in Early Jurassic.

关 键 词：叶巴组 桑日群 弧火山岩 新特提斯洋 青藏高原 

分 类 号：P588.14[天文地球—岩石学] P597.3[天文地球—地质学]