仲巴地体的板块亲缘性:来自碎屑锆石U-Pb年代学和Hf同位素的证据

仲巴地体位于青藏高原西南部,其南北两侧均为雅鲁藏布蛇绿岩或蛇绿混杂岩。仲巴地体主要由一套断续出露的前寒武系构造基底、古生代和三叠系地层组成,岩性主要包括石英砂岩、含白云母石英岩、泥质石英粉砂岩,白云母方解石片岩和大理岩等。岩相学揭示这套地层总体上为滨浅海-陆棚-外陆棚环境。碎屑锆石U-Pb年代学和Hf同位素分析表明,锆石年龄出现特征年龄峰值530Ma和950Ma,与西羌塘、特提斯喜马拉雅以及高喜马拉雅地体非常类似,并且950Ma左右的碎屑锆石群具有与上述地体上报道的同期碎屑锆石群相似的εHf(t)值和地壳模式年龄。这种碎屑锆石年龄和Hf同位素组成特征明显区别于拉萨地体,后者以出现约1170Ma的特殊年龄群为特征。基于碎屑锆石特征和沉积学研究,本文认为仲巴地体属于西羌塘-大印度-特提斯喜马拉雅构造体系,与拉萨地体具有不同的板块构造亲缘性。

Tectonic affinity of Zhongba terrane: Evidences from the detrital zircon geochronology and Hf isotopes

In the western Yarlung zangbo suture zone,there are widespread outcrops of Zhongba terrane between southern and northern tectonic mélange.Petrology,detrital zircon geochronology and Hf isotope analysis were carried out on this strata,Zhongba terrane consists of low-grade metamorphic quartz sandstone,muscovite quartzite,muscovite calcite schist,argillaceous quartz siltstone and marble,this sedimentary succession was deposited on the coastal and shallow sea-shelf-outer shelf environments.U-Pb dating of detrital zircons from the quartz sandstone of Zhongba terrane yielded ages clustering at ～ 530Ma and ～ 950Ma,which can be compared with the counterpart from western Qiangtang terrane,Tethyan Himalaya and High Himalaya.They are remarkably similar with a wide but identical range of εHf(t) values of the ca.950Ma age population as well,at the same,the distinctive ca.1170Ma age population which is characteristic of zircons in the Lhasa terrane is absent in samples from Zhongba terrane.Our data suggests that Zhongba terrane were part of the Qiangtang-Greater India-Tethyan Himalaya continental margin system during the Paleozoic and had a probably different source and tectonic evolution with the Lhasa terrane.