Magmatic Processes Of The Post-collisional Potassic Volcanic Rocks In Western Kunlun

The Western Kunlun region is located in the northwestern margin of the Tibetan Plateau.Due to the subduction of the Indian plate and collision between the Indian plate and the Eurasian plate,there are many post-collisional potassic volcanic areas of different sizes in this region.Researchers have conducted many volcanic geology and petrogeochemistry work in the Western Kunlun volcanic area,mainly focusing on the origin of deep magmas and plate dynamics.However,the magmatic processes of these potassic volcanic rocks are still unclear.In order to reveal the magmatic processes in the Western Kunlun region and understand the mechanism of volcanic eruption,we analyzed the whole-rock major and trace elements,texture and compositions of the phenocrysts,the crystal size distribution(CSD),and magma crystallization temperature and pressure conditions of the Pulu and Kangxiwa volcanic rocks.The results show that the Pulu volcanic rocks are mainly trachyandesite and basaltic trachyandesite.The phenocrysts are composed of olivines,clinopyroxenes,plagioclases and a small amount of orthopyroxenes.The Kangxiwa volcanic rocks are mainly phonotephrites,consisting of clinopyroxenes,micas,and a small amount of olivines and plagioclases.The erosion and zonation of plagioclase,olivine and clinopyroxene were observed under microscope.There are Nb-Ta and Ti negative anomalies in the two regions,with relative enrichment of large ion lithophile elements(LILF)and light rare earth elements(LREE),indicating that the magma source area has the characteristics of island arc magma,which is related to plate subduction.Based on the analysis of previous Sr-Nd datas,we suggest that the magmas from this two vocanic areas originated from enriched sources.The positive correlation of La/Yb-La indicates that the formation of magma in the two regions are controlled by partial melting process.Additionally,the La/Yb ratio of volcanic rocks in the Pulu region is relatively higher than Kangxiwa region,indicating that the degree of partial melting in the area is lower than that in the Kangxiwa region.The CSD curves of the volcanic rocks from Pulu and Kangxiwa volcanic regions are concave upward,indicating that magma mixing may have occurred both in the two volcanic regions.The CSD curves of plagioclase phenocrysts in Pulu trachyandensite kink at the size of 5mm.In Pulu volcanic rocks,the retention time of the smaller size(<5mm)crystal is 190～332a,and that of crystallographic size(>5mm)is 339～860a.The CSD curves of clinopyroxene phenocrysts in Kangxiwa phonotephrites kink at the size of 1.5mm.In Kangxiwa volcanic rocks,the residence time of smaller crystallographic size(<1.5mm)is 5.8～6.4a,and that of larger crystallographic size(>1.5mm)is 9.6～21.2a.The An values of the core and the rim of the normal zoning plagioclases and the Fo value of the normal zoning olivines in the Pulu volcanic rocks vary widely,and the feldspars with low An values at the rim is out of balance with the melt.There indicates that the magma of the Pulu volcanic group mixed with the acidic magma.The Mg~#of the normal zoning clinopyroxenes in the Kangxiwa volcanic rocks has a narrow range,and they are all in balance with the melt.The crystallization pressure at the rim was low,and the decompression caused a large number of resorbed phenocrysts to melt.This indicates that the mixing of phenocrysts with different degrees of melting and erosion may result in upward concave CSD curves of clinopyroxenes in the Kangxiwa volcanic rocks,so the Kangxiwa volcanic rocks may only have internal magma mixing.Meanwhile,most olivines of coarse andesite in the Pulu area was unbalanced with the melt,whereas most olivines of coarse andesite in the Kangxiwa area was balanced with the melt,which also proves that the magma in the Pulu area has mixed with the external magma,while the magma in the Kangxiwa area has not mixed with the external magma.The calculation of mineral-melt equilibrium thermometers show that the equilibrium temperature and pressure of the Pulu volcanic rocks are in the range of1035～1218℃and,5.1～9.9kbar,respectively,and the corresponding depth is between19.4～37.3km.The equilibrium temperature of Kangxiwa volcanic rocks is between1154～1282℃,the equilibrium pressure is between 1.2～11.6kbar,and the corresponding depth is between 4.3～43.7km.The variation range of equilibrium pressure in the Kangxiwa region is large,which may be related to the deep fault zone.In this study,by quantitatively studying the magmatic processes of post-collisional potassic volcanic rocks in the Western Kunlun region,we provide CSD calculations of the volcanic rocks.At the same time,we reveal the migration and evolution processes of magmas in the crustal magma reservoir,and provide important information for the volcanic activities in the northwestern margin of the Tibetan Plateau and its surrounding regions.