Geochemical Characterization and Genetic Analysis of Rocks of the Dianzhong Formation in Geda Township Area,South-Central Tibet

The study on the genetic and geochemical characteristics of volcanic rocks of the Cretaceous-Paleogene Dienzhong Formation in central and southern Tibet is of great significance for the establishment of magma evolution sequence in the working area. The volcanic rocks of Dianzhong Formation in Geda area can be roughly divided into dite, andesite and rhyolite. Zircon U-Pb isotopic dating results show that the weighted average age of Pb206/U238 of oligopleural rhyolite and Andesite samples is 65. 5±2. 3Ma (MSWD=2. 0) and 68. 7±3. 1Ma (MSWD=2. 0) , respectively. This indicates that the middle acid volcanic rocks of Dianzhong Formation erupted from Late Cretaceous to Paleocene. Geochemical analysis of rocks shows that the acidic volcanic rocks in Dianzhong Formation are a set of meta-aluminous-peraluminouscalc- alkaline andesite-dacite-rhyolite rock assemblages. Large ion lithophile elements and light rare earth elements are enriched, and high field strength elements are deficient. In the process of magmatic evolution, there is a large amount of crystallization of plagiase or a large amount of refractory residue of plagiase, and there are a lot of continental crust materials in the volcanic rocks of the Dian Zhong Formation. Comprehensive analysis shows that the volcanic rocks of Dianzhong Formation have the chemical characteristics of continental arc volcanic rocks, and may be magmatic products in the transition stage from subduction mechanism to continental collision mechanism. Zircon U-Pb dating, in situ Lu-Hf isotope and whole rock principal and trace analysis were carried out on the acidic volcanic rocks in Dianzhong Formation, Geda Area. The aim is to study the geochemical characteristics and evolutionary origin of rocks in this area. It provides the geochemical data support for the discussion on the division of tectonic evolution stages in the study area since Jurassic.