乌兰乌拉湖幅地质调查新成果及主要进展

New results and major progress in regional geological survey of the Ulan Ul Lake Sheet

在原划若拉岗日群分布区内新发现混杂有二叠纪外来岩块,在构造透镜体灰岩中见有晚二叠世长兴期有孔虫化石。在晚三叠世苟鲁山克错组中采集到丰富的植物化石,其中以种子蕨纲的属种Hyrcanopteris最多,与唐古拉山结扎群、土门格拉群及藏东甲丕拉组、巴贡组以蕨类Neocalamites,Equisetites为主的晚三叠世植物群有所差异。侏罗系是调查区内分布面积最大、地层系统发育最完整的岩石地层单元,建立了2个生物组合、3个生物组合带、3个组合亚带。建立了一条高精度古地磁控制的地层剖面,为第三系陆相红层沉积时代的确定和对比提供了依据。据新的同位素测年数据和分析结果,确认藏北面积最大的新生代火山熔岩的形成时代为45～40Ma,它源于EMⅡ型富集地慢。发现乌兰乌拉湖构造混杂岩带,为区域构造对比提供了新的依据。

By geological survey, the authors found that the strata originally assigned to the Rola Kangri Group contain Permian exotic blocks, and Late Permian Changxingian foraminifera is present in tectonic lenticular limestones. A large number of fossil plants are present in the Late Triassic Goulushankecuo Formation, in which Hyrcanopteris of Pteridospermopsida is dominant. This is somewhat different from the case with the Gyiza Group and Tumengela Group of the Tanggula Mountains and the Jiapeila Formation and Bagong Formation of eastern Tibet, in which the Late Triassic flora is dominated by Neocalamites and Equisetites of ferns. Jurassic strata are of the widest distribution and have the most complete succession in this area. Two assemblages, three assemblage zones and three assemblage sub-zones have been established. A highly precise paleomagnetic data-controlled stratigraphic section has been constructed, providing a basis for the determination and correlation of the ages of Tertiary terrestrial red beds. New geochronological results of Cenozoic volcanic lavas covering the largest area in northern Tibet confirm its ages ranging from 40.1 to 44.6 Ma. Isotopic and geochemical analysis clearly indicate an enriched EMⅡ mantle source. The discovery of the Ulan Ul Lake tectonic mêlange belt provides a new ground for regional tectonic correlation.