Geochronology And Geochemical Characteristics And Geological Significance Of Linzizong Volcanic Rocks In Western Of Lhasa Block

The Qinghai Tibet Plateau,known as the "roof of the world",is a natural laboratory for studying geodynamics.It is widely concerned by geosciences.The Lhasa block,located in the southern margin of the Qinghai Tibet Plateau,is the frontier of the interaction between the India plate and the Eurasian plate.The widely distributed volcanic rocks of the Linzizong Group recorded the whole evolution process of the subduction and closure of the new Tethys ocean and the India Eurasian collision.The previous studies on the volcanic rocks of the Linzizong group mainly focus on the typical section of the Linzhou basin in the eastern Lhasa area and the areas of Cuomai,Zhunuo and Sang Sang in the central region,and lack of a systematic study on the volcanic rocks of the Linzizong group in the western part of the Lhasa block.In this paper,the detailed petrology,chronology,element and Sr-Nd isotope geochemistry of the Linzizong group,which are exposed to the Shiquan River,Yare,Zhada and Sailipu area in the west of the Lhasa block,are studied and compared with the volcanic rocks in the eastern and central areas of the Lhasa block.The purpose of this study is to reveal its petrogenesis,tectonic background and geological significance.Finally,we get the following understanding:1?Using the high-precision LA-(MC)-ICP-MS zircon U-Pb dating method,we got the crystallization age of Lizizong volcanic rocks in westen of Lhasa block.2 rhyolite samples of Pana Formation in Yare area get the age of 51.3±0.4Ma and 50.1±0.3Ma respectively,which are all formed in the Paleocene Eocene;the age of 2 Dianzhong rhyolite are 70.3±0.6Ma and 70.8±1.1Ma respectively,which are formed in the late Cretaceous.The volcanic rocks of the late Jurassic late Cretaceous Zennong group in the western part of the Lhasa block are redefined as volcanic rocks of the late Cretaceous Eocene Linzizong group.2?The main lithology of the Linzizong group volcanic rocks in the study area is rhyolite,andesite,andesite and a small amount of basaltic andesite.The rock is mainly a set of alalic alkaline calcium alkaline series with high potassium calcium,a small amount of sodium rock,and the rare earth element shows the light rare earth enrichment right model,and the light rare earth in the Pana formation is richer.The distribution of heavy rare earth is relatively flat,and the overall performance of Eu negative anomaly is moderate,but the Dianzhong formation in the study area is more intense than the Eu anomaly in the canonical group,suggesting that the magma separation and crystallization in the study area is stronger than that in the Linzhou area,and the trace elements show that it enriches the large ion stone elements Rb,Th,U etc.,and has a high loss of high field strength.The elements Nb,Ta and Ti have typical characteristics of the island arc volcanic rocks,and the negative anomalies of Ba and Sr also indicate that the magma in the study area may experience the separation and crystallization of plagioclase,and the result of Sr-Nd isotope analysis in the study area shows that the volcanic rock slurry of the Linzizong formation in the western region of Lhasa block is derived from the fertile mantle or lower crust,while the negative Nd values also indicate that magma is contaminated by crustal materials.3?The volcanic rocks of the Linzizong formation in the study area are mainly formed in the tectonic environment of the active continental margin and the trend of the evolution to the internal environment.In the same period,the western part of the Lhasa block is more biased and intracontinental than the eastern and central part of the Lhasa block,which may imply that the India Eurasian collision has the characteristics of the penetrating time.At the same time,the early Dianzhong formation is more inclined to the island arc environment than the late Pana formation.Pana formation is more inclined to the same collision or post collision environment.This shows that the time of the collision between India and Eurasia may be formed after the Dianzhong.4?The volcanic rocks of the Linzizong group in the western part of the Lhasa block are related to the partial melting of the mantle wedge,which cause partial melting is plate fluid,and no sediments or subtractive plates are added to the magma.The Dianzhong formation is formed in the metasomatic enriched mantle mixed crust,and the Pana formation is formed in crustal remelting which caused by upwelling asthenosphere substance after plate dismantling.