Age,Tectonic Environment Of The Basic Volcanic Series Of The Huili Group,Southwest China And Their Tectono-Paleogeographic Implications

Huili group outcrops is located at the southwest edge of Yangtze Old-land,the existing research reveals that the middle part is the natural west extension of Jiangnan Orogenic Belt,which is an important node to analyze the collision evolution process of the Yangtze Old-land and Cathaysia Old-land.According to the transformation of rock system,the group is divided into lower sedimentary rocks and upper volcanic rocks.The great change must be caused by the tectonic transformation,but the starting time and nature of this tectonic environment transformation are not determined.Luoniegou section develops two rock system: the lower part is fine sandstone with plenty of soft sedimentary deformation,which can be compared to the top of Fengshanying Formation;The upper part is basaltic rock,which can be compared to the bottom of Tianbaoshan Formation.The author studied the characteristics of soft sedimentary deformation of the lower fine sandstone and the lithology,lithofacies,forming epoch,magma source area,tectonic environment of the upper basaltic rocks,which make the tectonic environment and tectonic evolution process of Huili Group determined.The following conclusions are drawn from the study:1.The lower fine sandstone should be the top of the Fengshanying Formation,which is formed in the shallow coastal environment of the passive continental margin.The tectonic activity was enhanced in this period,and the earthquake caused the development of a large number of soft sedimentary deformation structures in the fine sandstone,which is at the end of the passive continental margin stage.2.The basaltic rock should be the bottom of the Tianbaoshan Formation,1126.1±9.9 should represent the oldest age of the Tianbaoshan Formation,and this region had occured in a massive granite intusion events at 1051 ±13 Ma.The Lu-Hf isotope analysis shows that the basaltic rock derived from depleted mantle,but it is mixed with the crust material,and the granite is formed by the partial melting of the ancient crust.3.According to geochemical analysis,the tectonic environment should be the late stage of rift valley in the passive continental margin.The facies analysis also shows that it is formed in the littoral Marine environment of the continental margin,which is consistent with the geochemical analysis results.4.The southwest margin of Yangtze Old-land changed to rift valley at a time of1126.1±9.9,and Huili Group is the product of the convergent of Yangtze Old-land and Cathaysia Old-land.