The Geochemical Characteristics,Magma Sources And Dynamic Background Of Ashikule Volcano Group In West Kunlun

The Ashikule Volcano Group is located in the West Kunlun orogenic belt and on the northwestern margin of Tibetan Plateau,with an average elevation of more than4,800 meters above sea level.There are 14 Cenozoic volcanoes in the basin,and the Ashi Volcano represents the most recent 1951 eruption in China.Earthquakes in the northwestern margin of Tibet Plateau are frequent.Mw7.1,Mw6.2,and Mw6.9earthquakes occurred in the vicinity of the Ashikule volcanic basin in 2008,2012,and2014,respectively.Of these,the Yutian 2008 Mw7.1 Earthquake formed a large-scale rupture zone in Ashikule.In addition,according to the latest seismic tomographic results,the Indian plate and Tarim block have collided beneath the northwestern edge of Tibetan Plateau.Therefore,the Ashikule volcanic area is an ideal research site for studying the Cenozoic volcanic activity in the Tibetan Plateau and Cenozoic tectonics on Tibetan Plateau.In this thesis,systematic lithology and geochemistry of Ashikule volcanic rocks are studied,including analyses of whole-rock major and trace elements and Sr-Nd-Tl isotopes.The characteristics of the magma source area are discussed and the partial melting processes of the mantle source area are analyzed.Fractional crystallization and crustal contamination that may occur during the magma up-immigration are also analyzed.Combining with the latest geophysical researches,we discussed the dynamic background of volcanic activities in the northwestern margin of the Tibetan Plateau.The results show that the Ashikule volcanic rocks are mainly medium-acid rocks and belong to shoshonites.The main lithology is trachyandensite,with some phonotephrite and basanite rocks.Ashikule volcanic rocks are extremely enriched in light rare earth elements?LREE?and large ion lithophilic elements?LILE?,depleted in heavy rare earth elements?HREE?and high field strength elements?HFSE?.The concentrations of LILEs and LREEs are much higher than those of the crust end-members,which may be related to multiple enrichments.The Sr isotopes in the Ashikule volcanic rocks are ⁸⁷Sr/⁸⁶Sr=0.707490-0.710523,and the Nd isotopes are ¹⁴³Nd/¹⁴⁴Nd=0.512265-0.512472.Except for three older samples,the Sr-Nd isotopes of younger samples are extremely restricted,suggesting that the Ashikule volcanic magmas may originate from a homogeneous source and are not affected significantly by the crustal contamination.The Tl isotopes(?²⁰⁵Tl)in the Ashikule volcanic rocks are fractionated,ranging from-2.7 to+6.4.Thallium content is relatively higher than the mantle,ranging from148 to 8259 ppb and the Ce/Tl ratio is from 37 to 2022.The Ce/Tl ratios and Tl isotopic ratios of samples ASKL-7 and 513-11 are much higher than those of other samples,so these two samples may be greatly affected by magmatic degassing.Excluding the effects of surface fluid erosion and weathering,fractional crystallization and crustal assimilation,this study believes that other heavy Tl isotopes may originate from magmatic sources.Based on the above geochemical analysis,we suggest that the Ashikule volcanic magmas are derived from the lithospheric mantle of Tibetan Plateau,which is mainly of composed of olivine,clinopyroxene,orthopyroxene,phlogopite,garnet,etc.In the lithospheric mantle of Tibetan Plateau,there may exist two types of veins.The Sr-Nd isotopes of the two types of vein are the same,but the Tl isotopes are different.The lithospheric mantle veins may be formed by the subduction of Proto-and Paleo Tethys lithospheres from Proterozoic to Triassic,partial melting of which formed the Ashikule volcanos.The latest seismic tomography researches show that in the lower part of the Ashikule volcanic area,the Indian plate and the Tarim block have not yet collided yet,and there is a gap of about 120 km in width in the lithospheric mantle.To the west the Ashikule,the Indian plate and Tarim block have collided and the volcanism in the west is obviously older than the east.Thus,we speculate that due to the convergence of the Indian plate and Tarim block,shear heating provides heat for partial melting of the dykes in the lithosphere of Tibet Plateau,which formed the volcanism on the northwestern margin of Tibetan Plateau.