Geochemistry Discrimination of Late Triassic Volcanic Rocks in Changdu-Simao Tectonic Zone

The study of geological structural setting of Changdu-Simao Tectonic zone(CSTZ) located in East Margin of Qinghai-Tibet,southwest China,is of vital significance for determining the favorable area of ore deposits.By the end of Middle Triassic,the Three-river Composite Terrane had been formed,making the Changdu-Simao Tectonic Zone(CSTZ) go into an intra-continental evolution stage.Our research on geochemical and stratigraphic characteristics of Late Triassic volcanic rocks in the Changdu-Simao Tectonic Zone shows that the volcanic rocks are characterized by a graben-horst ancient landform,a bimodal volcanic-sedimentary assemblage,and a magmatism with extension factors.Volcanic activities are mostly seen along Xiangcheng,forming the so-called lag-arc magma.However,lots of geological facts indicate that,after these activities,this is not an extrusive process all the time but a stretching process after a short period of extrusion.Aiming at this problem,we collected and analyzed a large number of data and found that the volcanic rocks had a bimodal feature.Based on the geochemistry diagram,we believe that the Changdu-Simao structure was in a stretching environment in late Triassic period.So this area belongs to an intra-continental rift environment.