Geochemical Characteristics Of Volcanic Rocks Of Linzizong Group In Xungba Area, Tibet

The Paleogene volcanic rocks in the Linzizong Group are the largest volcanic rock belt on the Tibetan Plateau,and its distribution accounts for more than half of the Gangdese magma belt area.On the basis of the predecessors,this study carrys out field geological surveys and geochemical studies on the Linzizong Group volcanic rocks in Xungba area,which are the westernmost part of the Linzizong Group volcanic rocks in the Gangdese orogenic belt,identified the volcanic apparatus,space distribution,Rock types and lithological associations of Linzong Group volcanic rocks in the study area,obtains the zircon U-Pb isotopic age of Dianzhong formation and Pana formation volcanic rocks,defines the age of the Linzizong volcanic rocks,and analyzes the geochemical characteristics of the volcanic rocks of Dianzhong formation and Pana formation.On these bases,the source area of the volcanic rocks,tectonic environment and petrogenesis of the volcanic rocks of the Linzizong Group in the study area are discussed,and the collision time of the India-Eurasia continent in the study area is preliminarily discussed.There are various types of volcanic rocks in the Linzizong Group in Xungba area.The volcanic rock types in Dianzhong formation include rhyolitic clastic tuff,dacite clastic tuff,andesite,pyroxene andesite,trachyandesite,basaltic trachyandesite,rhyolite and trachyte;Pana formation volcanic rock types include Rhyolitic tuff,dacite fused tuff,trachytic breccia,dacite breccia,trachyte,rhyolite and basalt.Zircon U-Pb harmonic ages of LA-ICP-MS volcanic rocks in Dianzhong formation are 65.8±1.5Ma and 65.62±0.66 Ma,forming in Paleocene;Zircon U-Pb harmonic ages of LA-ICP-MS volcanic rocks in Pana formation are 51.75±0.58 Ma,45.9±6.3Ma and 51.1±1.9Ma,forming in Eocene.The Dianzhong formation basaltic trachyandesite belongs to the potassium basalt series with weak negative Eu anomalies.The REE distribution curve shows the right-leaning enrichment of light REE,enrichment of Rb,Th,Nd,Zr and other elements,loss of Ba,Ta,Nb,Sr,P,Ti,etc.Rhyolite belongs to high potassium calc-alkaline series,negative Eu anomaly is strong,rare earth element distribution curve is characterized by enrichment of light rare earth elements in right inclination.It is characterized by enrichment of Rb,Th,Nd,Zr and Y elements and loss of Ba,Ta,Nb,Sr,P,Ti,etc.Coarse rocks belong to potassium basalt series,showing strong negative Eu anomalies.The distribution curve of rare earth elements is characterized by enrichment of right-leaning light rare earth elements,enrichment of Rb,Th,Nd and Zr elements,and loss of Ba,Nb,Sr,P,Ti and other elements.The trachyte of the Pana Formation belongs to the high-K calc-alkaline series,and shows little Eu anomaly or weak negative Eu anomaly.The distribution curve of rare earth elements is characterized by the right-leaning enrichment of light rare earth elements,enrichment of Rb,Th,La,Ce and Nd,depletion of Ba,Sr and high-field strength elements Nb,P,Ti,etc.The rhyolite belongs to the high-K calc-alkaline series,and shows little Eu anomaly or weak negative Eu anomaly.The distribution curve of rare earth elements is characterized by right-leaning enrichment of light rare earth elements,enrichment of Rb,Th,La,Ce and Nd,and depletion of Ba,Sr and high field strength elements Nb,P and Ti.Most basalts belong to high-K calc-alkaline series,and shows little Eu anomaly or weak negative Eu anomaly.The distribution curve of rare earth elements is characterized by right-leaning enrichment of light rare earth elements,enrichment of La,Ce and large ion lithophile elements Ba,K,loss of Sr and high field strength element Nb.The results show that the main controlling factor of the magma genesis of the volcanic rocks in the Linzizong Group may be the partial melting of mafic materials in the lower crust.The formation of the basaltic trachyandesite,trachyte and parent magma of the Pana Formation in the Dianzhong Formation is participated by mantle-derived materials.The volcanic rocks of Dianzhong Formation have the characteristics of continental margin arc volcanic rocks.The volcanic rocks of Pana Formation show the characteristics of collision-post collision volcanic rocks.Therefore,the volcanic rocks of the Linzizong Group are volcanic rocks produced during the transition from volcanic arc type to collision-post collision and intraplate environment.The time limit for the collision between India and Eurasia may be the same as that of the volcanic rocks in the early years of the Bo Formation.The volcanic rocks in the Linzizong Group are different in the East and West directions.The collision between India and the Asian Continent has the characteristics of the early West and the late east.According to the age of volcanic rocks of the Linzizong Group in the study area,the collision time between India and Eurasia in Xungba area should be about 52~55Ma.