Geochemical Characteristics And Significance Of Dianzhong Volcanic Rocks In The Cuoqin Area Tibet, China

Cuoqin volcanic basin is located on north of the western part of Gangdese magma-tic belt, cenozoic volcanic activities of the study area is so strong that formed L-inzizong Group volcanic rocks with a thickness of over1000m. its petrological and geochemical characteristics of which indicate that they contains abandant information of the transition from the subduction mechanism to continental collisionmechanism.This paper studies petrology, rare earths and trace geochemistry, isotope geochemistry in the lower part of the Dianzhong of Linzizong group of volcanicrocks,explores the characteristics of the process of magmatism, ans reveals characteristics of the geological tectonic environment and source area. On the basis of previous studies, the results of this thesis were also compared to the Dianzhong formation in Linzhou basin,and to that distributed in other different of regions Gandese blet,in order to find the Geochemical, evol-ute characteristics change among these volcanic rocks,explorethe similarities and differences between western and the eastern section the Gangdese magmatic belt of the of in the evolution process.Studies have shown, Dianzhong volcanic rocks mainly consist of andesite,dacite, rhyolite and andesitic, dacitic and rhyolitic pyroclastic lava in Cuoqin are-a.The unconformity between the Dianzhong volcanic rocks and underlying stratais a large-scale regional one.Across the unconformable surface, there are changesin rock assemblages, paleogeographic outlook and so on. Therefore, this unconf-ormity represents a major tectonic event. volcanic rocks of SiO2content of53.50to80.92percent for the medium-acidic rocks, σ <3.3, K2O more than3%, the majority of K2O/Na2O value between0.65to6.44，Dianzhong volcanic rocksbelong to the peraluminous high-potassium calc-alkaline and peraluminous calc-alkaline series. The that of characteristics of REE show that the fractionation of light REE is stronger than that of heavy REE, the level of Heavy REE patterns. The volcanic rocks have significantnegative Eu, the anomaly δEu value of0.14～0.89, the average δEu is0.58, and the basically no pronounced Ce anomaly. REE geochemical characteristics of the study ar-ea between the continental margin arc volcanic rocks and syn-collisional volcanic rocks.The Dianzhong volcanic rocks show that the large ion lithophile elements relatively en-riched in Sr, relatively depleted in Ba, and high field strength elements relative enrich-ment of Th, and relatively depleted in Ta,Zr, Hf, U range is large, showing typical arc-related of geochemical characteristics. Isotopic characteristics of Dianzhong volcanicrocks show that the volcanic rocks in the initial87Sr/86Sr ratio is closed, the range is0.710352to0.713423, have higher⁸⁷Sr/⁸⁶Sr ratios; the the¹⁴³Nd/¹⁴⁴Nd the ratio of initial changes in0.522293⁰.512407, have low¹⁴³Nd/¹⁴⁴Nd ratios. The Dianzhong volca-nic rocks Shows that source material from the enriched mantle, the source region are subduction and recycling of continental crust mixed with pyrolite, the source region is controlled in the Gangdese crustal basement.Comprehensive studies have shown that the Dianzhong volcanic rocks ofpotassium and aluminum content is increased and the continental crust composition ofthe magma source is an increasing trend from eastern to western part of the Gangdesemagmatic belt.From the age of the rocks, there are certain differences in the form oftime between eastern and western part of Dianzhong volcanic rocks,namely magmaticactivity in the western part of Dianzhong volcanic rocks began in the LateCretaceous,which is18Ma earlier than the eastern part.After, the magmatic activitygradually went eastward, and continued until the Paleocene, which reflects volcanicactivity of Dianzhong volcanic rocks in Cuoqin area has lasted for a long time.In terms of Gangdese magmatic belt,the collision between the Indian contin-ental plate and the Asian plate may occur from the western section, after the c-ollision gradually closed eastward, with oblique collision and diachronous charac-teristics.Dianzhong volcanic rocks of this District are rich in potassium and alu-minum, and have high initial Sr isotope ratios. All these geochemical characteri-stics also show that the thickness of the crust in western part of the Gangdesemagmatic belt may be thicker than that in the eastern part. The western part ofCuoqin basin is further away from the subduction zone than the eastern sectionof the Linzhou basin and closer to the mainland side. Also reflects, the closure and disappearance of Tethys ocean basin experienced a complex and lengthy historicalprocess.