The chemical and carbon isotopic compositions of volatiles in Cenozoic high-potassic basalts in western Qinling, China and their mantle geodynamic implications
西秦岭新生代高钾质玄武岩流体组成及其地幔动力学意义

India-Asia continental plate subduction and collision lead to widespread Cenozoic potassic-ultrapotassic magmatism in the Tibet which provides a valuable indicator of the composition and nature of the deep mantle beneath the Tibet,The Cenozoic mantle xenolith bearing high-potassic basalts(7.1～23Ma) distributed in western Qinling,western China are a window to probe the mantle composition and geodynamics beneath north-eastern margin of Tibet Plateau,The chemical and carbon isotopic compositions of volatiles in olivine(Olv)and plagioclase(Pl)phenocrysts and matrix(Mtx)of Cenozoic high-potassic basalts in western Qinling have been determined by stepwise heating mass spectrometer to reveal volatile sources and geodynamic implications under the system of continental plate subduction and collision,The results showed that the volatiles in high-potassic basalts were released by stage at temperature intervals of 200～400℃,400～800℃ and 800～1200℃,and are characterized by dominant H2O with minor CO2 and SO2,as well as detectable He,The olivine phenocrysts show higher release temperature interval(900～1200℃)of main volatiles than those in mantle xenoliths in eastern China and ultramafic intrusion(400～800℃) in other regions,and are mainly composed of SO2(68.72mm3/g)and CO2(59.46mm3/g)etc,oxidized volatiles,The contents of H2O and CO2 increased gradually from olivine phenocrysts to plagioclase phenocryst and matrix,Olivine phenocrysts display relatively lighter δ13CCO2(-26.21‰～-20.85‰,av.-23.32‰)and δ13CCH4(-42.35‰～-38.17 ‰,av.-40.03‰)than matrix(-16.43‰～-11.67‰,av.-13.22‰ for CO2,-44.22‰～-34.03‰,av.-39.70‰ for CH4),which show the δ13C features of crust with thermal cracking component of organic matters,The volatiles in primary magma are mainly composed of SO2,N2 and CO2,and could be derived from a deep mixed mantle reservoir and developed in high fO2 environment,They could be mixed by coexisting carbonatite magma or assimilated by the local Carboniferous carbonate which magma penetrated,and had been added by a large quantity of H2O and CO2 during magma ascent and evolution,The chemical and carbon isotopic compositions of volatiles suggested that a recycled crustal component derived from the devolatilization of subducted oceanic plate or sedimentary rocks,which could be related to paleo-Tethyan oceanic plate or Northern China plate with Yangtze plate subduction and collision under the system of India-Asia collision.