STUDY ON THE MAGMATIC PROCESSES OF POST-COLLISIONAL POTASSIC VOLCANIC ROCKS FROM WEST KUNLUN:TAKING THE PULU AND KANGXIWA VOLCANIC ROCKS AS EXAMPLES
The West Kunlun region is located in the northwest margin of the Qinghai-Xizang Plateau.Due to the subduction and collision of the Indian plate,this region has many post-collisional potassic volcanic areas of different sizes.Scholars have conducted many volcanic geology and petro-geochemistry work in the West 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.To reveal the magmatic processes in the West Kunlun region and understand the mechanism of volcanic eruption,we analyzed the whole-rock major elements,structure 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 magma sources of the two volcanic regions are close and their trace element characteristics are similar.Still,their rock types and mineral compositions are significantly different.The Pulu volcanic rocks are mainly trachyandesite and basaltic trachyandesite.The phenocrysts are composed of plagioclase,olivine,clinopyroxene and a small amount of orthopyroxene.The Kangxiwa volcanic rocks are mainly phonotephrite,consisting of clinopyroxene,biotite,and a small amount of olivine and plagioclase.The erosion and zonation of plagioclase,olivine and clinopyroxene were observed under a 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 these two volcanic areas originated from enriched sources.According to the erosion characteristics,zonal composition data,and the concave CSD pattern,we suggest that the magma in Pulu mixed with acidic magma,whereas the magma in Kangxiwa may only mix with the internal magma,resulting in a large amount of melt erosion of phenocrysts.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 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 An values of the core and the rim of the normal zoning plagioclases and the For value of the normal zoning olivines in the Pulu volcanic rocks vary widely,and the feldspars with low An values at the rim are out of balance with the melt.This 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.The mineral-melt equilibrium thermometers show that the equilibrium temperature and pressure of the Pulu volcanic rocks are 1 035-1 218℃,5.1-9.9kbar,respectively,and the corresponding depth is 19.4～37.3km.The equilibrium temperature of Kangxiwa volcanic rocks is 1 154-1 282℃,the equilibrium pressure is 1.2-11.6kbar,and the corresponding depth is 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 West Kunlun region,we provide CSD calculations of the volcanic rocks,reveal the migration and evolution processes of magmas in the crustal magma reservoir,and provide important information for the volcanic activities in the northwest margin of the Qinghai-Xizang Plateau and its surrounding regions.
microstructurecrystal size distributionmagmatic mixingthe Pulu volcanic fieldThe Kangxiwa volcanic fieldthe west Kunlun volcanic region
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