藏南江孜县白沙地区宗卓混杂岩：岩石组成与物源区分析

Petrologic and Provenance Analysis of the Zongzhuo Mélange in Baisha Area,Gyangzê,Southern Tibet

藏南宗卓混杂岩是印度-欧亚大陆早期碰撞过程的重要地质记录。本文对江孜县白沙地区宗卓混杂岩进行了详细的野外和室内岩石学研究。该混杂岩由岩块和＂基质＂组成,岩块岩石类型有砂岩、硅质岩、玄武岩、砾岩等,＂基质＂以未变质的黑色页岩、泥岩为主。砂岩碎屑统计显示,其碎屑成分为火山岩屑和沉积岩屑,与日喀则弧前盆地类似。砂岩碎屑锆石年代学研究表明,其年龄峰主要分布在77~129 Ma,与冈底斯弧的年龄值相同。物源区分析说明,宗卓混杂岩内的大量砂岩物源来自于缝合带北侧的冈底斯火山弧,是在印度板块与欧亚板块碰撞背景下形成的一套碰撞混杂岩。

The Zongzhuo mélange in southern Tibet is an important geological record of the initial collision process of the Indian-Eurasian continents.Detailed field investigation,petrologic study and detrital zircon geochronologic analysis for sandstones were carried out on the Zongzhuo mélange in the Baisha area,Gyangzê county to better understanding of its provenance and tectonic setting.The Zongzhuo mélange consists of blocks and unmetamorphic shale and mudstone ＂matrix＂.The blocks are several centimeters to tens of meters in size,and contain sandstone,cherts,basalt,and conglomerate,etc.,while ＂matrix＂ is dominantly black shale and mudstone.Statistic result of the sandstone debris shows that compositions of the debris are composed mainly of volcaniclastic and sedimentary grains,with minor quartzes and feldspars,and similar to sandstones from the Xigazê forearc basin.U-Pb dating of detrital zircons from Sandstone blocks masses yielded ages clustering at ~77~129 Ma,which are consistent to age of the Gangdese arc.Provenance analysis indicates that voluminous sandstone occurring in the Zongzhuo mélange originated from Gangdese volcanic arc in the northern side of the suture zone,and the Zongzhuo mélange is one suite of the collisional mélange under the geological setting of collision between Indian plate and Eurasian plate.