存在比桑日群弧火山岩更早的新特提斯洋俯冲记录么？

新特提斯洋的演化过程对于青藏高原的形成具有重要的意义,一直以来桑日群火山岩被认为是其俯冲作用的代表性岩浆记录.在拉萨以东发育的叶巴组火山岩,虽被认为早于桑日群的火山岩,但一直缺乏年代学的证据.本次研究选取了叶巴组的样品做了单颗粒锆石的LAM-ICP-MS定年,获得了174Ma左右的年龄,明显老于桑日群.同时,叶巴组火山岩具有与岛弧火成岩类似的地球化学特点：如大离子亲石元素相对富集,中基性火山岩表现明显的高场强元素HFSE的亏损.综合来看,叶巴组火山岩很可能是新特提斯洋向北俯冲作用的产物.如果这一点能够确证的话,新特提斯洋开始俯冲的时代将可以前推至早中侏罗纪或更早.

Is There a Neo-Tethys'' Subduction Record Earlier than arc volcanic rocks in the Sangri Group?

Formation of the Tibetan Plateau is related to evolution of Neo-Tethys.It was thought that volcanic rocks in the Sangri Group were formed in late Jurassic to early Cretaceous,and represented the earliest magmatic record of the Neo-Tethys subduction. However,there is another volcanic suite called Yeba Formation in the east of the Lhasa block.Eruption of Yeba volcanic rocks was thought to be earlier than the Sangri Group,but lack of chronological evidence.Zircons from a sample of Yeba Formation lavas were selected and dated by LAM-ICP-MS,obtaining an age of 174Ma,indicating that Yeba Formation indeed is older than Sangri Group. In addition,lavas of the Yeba Formation show geochemical characteristics similar to volcanic rocks in an arc or active continental margin,such as LILEs' enrichment and HFSEs' depletion.We conclude that northward subduction of the Neo-Tethys resulted in the magmatic eruption of the Yaba Formation in early-middle Jurassic.If it is true,the Neo-tethys subduction probably started in early- middle Jurassic or earlier.