Characteristics And Geological Significance Of Extraneous Rock Block In The Upper Cretaceous Zong Zhuo Formation In The Rongbu Area,Tibet

In this paper,through the field geological survey and the indoor data compilation and comprehensive analysis of the Upper Cretaceous Zongzhuo Formation(K₂z)in the Rongbu area of Tibet,the characteristics of the extraneous rock block of the Zongzhuo Formation in the Upper Cretaceous in the Rongbu area and the distribution and sedimentation of the Zongzhuo Formation were studied.The Zongzhuo Formation is dominated by shallow sea shelf sedimentary strata with the main lithology being shale,mudstone,sandstone and medium-thin layer limestone.Taking limestone as the marker,the Zongzhuo Formation can be divided into the lower member(K₂z¹)and the upper member(K₂z²).The extraneous rock block of Zongzhuo Formation was formed in a tectonic setting where the Indian Plate of the Early Cretaceous began to descend northward and slid beneath the Eurasian plate.Due to plate collisions,the strata deposited previously before the Late Cretaceous were fractured and slipped into the Zongzhuo Formation,and deposited in the formation as extraneous rock block.The rock masses in the Zongzhuo Formation are mainly divided into native rock blocks and foreign rock blocks.The native rock is mainly dominated by sandstone and limestone whereas the foreign rocks mainly include sandstone,limestone,siliceous rock and basalt.Rock masses are identified by thin sections,trace elements,and rare earth elements for source analysis.Combined with the regional geological background and the comprehensive lithologic characteristics of stratigraphic lithology in adjacent areas,it is concluded that the sandstone rocks originate from the Lang jie group(T₃L)of the front edge of the Tethys Oceanic crust and the front area of the Lhasa terrane,and the siliceous rocks originate from Zongzhuo Formation(K₂z)itself,the limestone rock masses originate from the third member of the Nieru Formation(T₃n³),and the basalt rock masses originate from the Sangxiu Formation(J₃-K₁s).The foreign rock was mainly deposited in the middle area of the upper Zongzhuo Formation,no rock appeared in the upper section of the lower Zongzhuo Formation(K₂z¹)and the upper Zongzhuo Formation(K₂z²),which confirms that during the upper Zongzhuo Formation(K₂z²)period,the Indian plate and the Lhasa block were in a period of intense collision.