西藏山南地区上三叠统复理石郎杰学群岩性分布样式及其意义

Lithological Distribution of the Upper Triassic Flysch Langjiexue Group in Shannan Region,Southern Tibet: Implications to Provenance and Environment

对西藏山南地区雅鲁藏布江以南的上三叠统复理石郎杰学群进行了野外地质调查,完成了14条地质路线调查和65个点的观测,重点统计了露头观测点陆源碎屑岩各种岩性的厚度。根据不同岩性(反映粒度大小)的厚度统计对每个观察点上的砂岩与(粉砂质)板岩厚度的比例进行了计算,将砂岩/板岩比值投点到平面图上,编绘出不同层位的砂岩/板岩比值等值线图。结果表明,绝大部分层位的砂岩/板岩比值呈现由北向南变小的变化,反映了向南粒度变细的趋势;这一变化趋势与过去发表的由北而南古水流格局所反映的物源来源具有相同涵义,进一步支持并夯实了郎杰学群系北部来源而非印度大陆(特提斯喜马拉雅)来源的科学认识。物源来源和东西部砂岩/板岩比值的大小差异还暗示郎杰学群至少有两个主流物源方向和两个叠合的海底扇沉积体系。

The Upper Triassic flysch Langjiexue Group is widespread in southern Tibet. It outcrops south to the river Yarlung Zangbo and east to Bailang county, confined by three thrusts： Naidong-Sangri-Langxian Fault in north, Cuo- na-Milin Fault in east, and Renbu-Zhangda-Longzi Fault in south. A suggestion of the coeval Nieru Formation belong- ing to the Langjiexue Group may have the southern boundary fault southward extended to the Renbu-Zhangda-Longzi Fault. The Upper Triassic Langjiexue Group has been classified as either a part of the northern deep sea Tethys Hima- layas or a component of the melange Xiukang Group accretionary prism within the Yarlung Zangbo suture since it was nominated. Recent geological investigation on paleocurrent flow direction and provenance analysis of clastic com- position and geochemistry demonstrated a different viewpoint from the past, in which lithological distribution has not been established. A geological field investigation of the Upper Triassic flysch Langjiexue Group was in this work carried out in Shannan region, southern Tibet, south to the river Yarlung Zangbo, during which lithology and thickness of variety of terrigenous rocks were described and calculated at sixty-five outcrop observations within fourteen geological traversal profiles in almost N-S direction. Then the data of the thickness were processed as follow ： 1 ） calculating the ratio of sandstone versus slate for every observation response to clastic grain size distribution and source, a method for purpose of recovery of sedimentary dispersal, paleocurrent system based on the principle of finer grain size if more fur- ther transportation; 2） projecting ratio value of each observation on the scaled geological map; and 3） editing equal ratio values and draw by isoline on the map for the same formation. It is noted that sandstone in the statistic of thick- ness, and slate covers the silty and non-silty slate. The sum of thickness is over 50 m, in general 80 m - 200 m, ateach observation. All the data of each observation were optimal by excluding of stratal duplicate. This is because of statistics that could be repeated at two even more observations due to folding and thrusting, etc. for overturn of strata. To avoid this shortage, relative positions of observations were examined by balance section for each profile, and results show no du- plicates of observed places happened to the observed places in this work. The observation of lithology shows there are almost no conglomerates in the Upper Triassic flysch Langjiexue Group, but it is mainly composed of low grade metamorphosed flysch sandstone and slate facies. The strata are com- prised by alteration of sandstone/siltstone beds and dark gray to blackish （hemi-） pelagic shale. Sandstones are domi- nated by medium to fine feldspar / lithic （quartz） sandstone or greywacke with angular - subangular roundness and variable sorting. Lithologies are arranged in an upward fining Bouma sequence, in which graded beddings, parallel cross-beddings, horizontal, and ripples, are common. The sedimentary strata are metamorphosed into variable degree, which is increasing eastward and toward the metamorphic core complex, with the center formed by the Cenozoic granit- ic pluton. Strong folding and thrusting together with metamorphism and lack of fossils makes the sequences stratigra- phically difficult to restore. However, former limited biostratigraphic evidences suggest that the group is of Norian- Rhaetian age. Isolines of lithological ratios display that ratio values of sandstone vs. slate decrease from north to south in majori- ty of formations： 1 ） 0.12 to 0.01 of the slaty Nieru Formation in Nagarze, 2.4 to 0.6 near Xuesa village, Lhunze, and 25.0 extremely down to 2.0 between Xuesa and Zhari villages, Lhunze, indicating an discrepancy between east and west of Shannan region and likely development of channels under water; 2） 1.0 to 0.2 of the Songre Formation in western Shannan, and 0.6 to 0.1 in mid Shannan, in which ratios are bigger in western than in eastern Shannan re- gion; 3） 1.89 to 0.22 of the Jiangxiong Formation in western Shannan, 2.0 to 0.4 in mid Shannan, and 0.8 to 0.4 in eastern Shannan, with few exceptances, and it is also bigger in western than in eastern Shannan region; 4） 1.5 to 0. 5-0.8 of the Jiedexiu Formation southeastward and southwestward in Gonggar and Zhanang. Those results above indi- cate a total tendency of finer and finer grain southward, implicating southward direction of terrigenous matters. This observation is quite consistent with the previously published paleocurrent data, supporting and enforcing the scientific understanding of other origination of the Langjiexue Group instead of the paleo-Indian continent （Tethys Himalaya） in south. According to lithology and the variation （tendency, isoline shape, and pattern） of ratio values of sandstone vs. slate from east to west, it is proposed that the Langjiexue Group documents the sediments of middle and outer （lobe） submarine fan without inner fan, and there are at least two even more coalescing submarine fan systems during the Late Triassic in Shannan area. Recognition of possible amount and superposition of fans remains much work in future.