羌塘盆地藏夏河组砂岩地球化学特征及意义

Geochemical Characteristics and Signatures of the Sandstones from Zangxiahe Formation in Qiangtang Basin

藏夏河组是指发育于北羌塘盆地北部的一套砂、泥质互层的复理石相沉积地层,其时代为晚三叠世诺利期.由于其形成时间正是羌塘地体与可可西里-松潘地体沿金沙江缝合带进行碰撞、缝合的时期,因此该套地层对于探讨晚三叠世藏北地质演化具有重要的指示意义.通过对北羌塘盆地多色梁一带藏夏河组砂岩进行X射线衍射,全岩常量、稀土和微量元素分析,以及锆石裂变径迹等方法分析该套砂岩的物源、沉积构造背景,进而探讨晚三叠世羌塘盆地的性质.研究结果表明,其岩石类型为杂砂岩,物源主要为长英质火成物质,另有少量古地壳再循环物质加入,沉积大地构造背景为活动大陆边缘与大陆岛弧.结合前人研究资料,推断晚三叠世北羌塘盆地的性质可能为前陆盆地,藏夏河组为一套造山前或同造山期形成的复理石沉积建造.

The Qiangtang basin is located between the Jinshajiang suture （JSS） and Bangong-Nujiang suture. It is an important petroliferous basin in the northern Tibet Plateau, China. It is formed by the stacking of many basins of different stages, prop- erties and generations. There have always been a lot of debate about the Late Triassic property of Qiangtang basin, so the Late Triassic Zangxiahe Formation is selected as the research subject in this paper. The Zangxiahe Formation, which is deposited on the northern margin of the Qiangtang basin, is a very thick flysch （~3 000 m） with sandstone and shale sediments. It is an very important indicator for the Late Triassic property of Qiangtang basin. The X-ray diffraction （XRD）, major compositions, rare earth elements and trace elements of the whole rocks and the fission tracks of clastic zircon of the sandstones from Zangxia- he Formation in Qiangtang basin are studied in this paper, which reveals that the tectonic settings of source rock area belong to active continental margin and continental island arc. Combined with the results of previous studies, it is concluded that the main source area is JSS and the major source rocks are the felsic rocks. The felsic rocks are likely the S-COLG which intruded into the JSS in Middle Triassic. The deposition time of the Late Triassic Zangxiahe Formation is the same as the formation time of JSS. So the Late Triassic Qiangtang basin is a peripheral foreland basin.