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

The study of geological structural setting of Changdu 2 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-con tinental 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 agraben-horst ancient landform, abimodal volcanic-sedimentary assemblage, and amagmatism with extension factors. Volcanic activities are mostly seena long Xiangcheng, forming the so-called lag-arcmagma. However, lots of geological facts indicate that, after these activities, this is not an extrusive process all the time but a stretching process after ashort period of extrusion. Aiming at this problem, we collected and analyzed a large number of data and found that the volcan icrocks had a bimodal feature. Ba sed on the geochem istry diagram, we believe that the Changdu-Simao structure was in a stretching environment in late Triassic period. So this a reabelongs to an intra-continental riftenvironment.