The source of Early Paleozoic bentonites in the western margin of the Upper Yangtze and its indicative significance for the Yichang uplift
Yichang uplift is a phased evolutionary movement of the Guangxi Movement in South China,but the time node of its evolution in the Upper Yangtze region is still unclear,and several layers of bentonites were widely deposited in the Upper Yangtze region during this period.This paper studies the lithostratigraphy and chronology of the bentonites of the Jiancaogou Formation-Longmaxi Formation of the Late Ordovician-Early Silurian in northeastern Yunnan,It defines the starting and ending times of Yichang uplift on the western margin of the Upper Yangtze region as 445±2Ma and 441±2Ma,respectively,indicating that the basin mountain transition of the Late Ordovician passive continental margin basin in South China to the foreland basin was initially completed at this stage.Zircon trace elements graphically show that it was formed in the orogenic background related to collision and compression.The geochemical characteristics show that its SiO2 content is 46.32％～54.64％,and it has low TiO2,and the trace elements are rich in Rb,Ba,Th and U,but deficient in Ti and P.The diagram of Nb/Y-SiO2 shows that its protoliths are intermediate-basic volcanic rocks(basalt and andesite),and the diagram of structural background discrimination shows that it was formed.The early stage is mainly volcanic arc environment,and the late stage is intraplate environment.The Hf isotope characteristics further show that the early bentonite has mantle-derived materials,which records the subduction and subduction process of the ocean crust.Therefore,it is considered that the Early Paleozoic bentonites originated from the subduction-collision orogenic environment under the background of the convergence of the Yangtze plate and the Cathaysian plate,and it is the product of the transformation from arc-land collision to land-land collision from the Yangtze plate to the Cathaysian plate.
BentonitesYichang upliftZircon U-Pb datingWestern margin of upper Yangtze
万方数据
维普
