Petrogenesis Of Cenozoic Volcanic Rocks From Tengchong, SE Margin Of The Tibetan Plateau:Constraints From The Petrogeochemical And Volatile Compositions

The Tengchong Quaternary volcanic field is unique representative volcano and geothermal fields with large quality of emission of mantle-derived volatiles in the Tibetan Plateau, southwestern China. It is currently a emission chanel of mantle-derived volatiles. The volatile compositions in the Quaternary volcanic rocks have not been studied systemically. The Cenozoic volcanic rocks from Tengchong were collected for analysis of major elements, trace elements, volatiles composition and carbon isotopes to reveal the nature and environmental conditions of magmatism, and the origin of volatiles. The main conclusions are summarized below:(1) The Cenozoic volcanic rocks in Tengchong are mainly alkaline-subalkaline series, including trachybasalt, basaltic trachyandesite and trachyteandesite. They show low Mg#values (from46.52-54.35%), and belong to low-Ti volcanic series. The chondrite-normalized REE patterns are enriched in LREE with various δEu values (0.51to1.44). The primitive mantle normalized trace element patterns are enriched in large-ion-lithophile elements and light rare-earth elements, and display depleted P and rich in Pb.(2) The volatiles in Tengchong Quaternary volcanic rocks are mainly composed of H2O (av.-95vol.%) with minor CO2, N2and O2. The chemical compositions of volatiles vary with different volcanic fields. Those in Laoguipo and Dayingshan volcanic fields is characterized by high content of CO2, whereas those in Maanshan is by CO and N2, except for H2O.(3) The carbon isotopic compositions of volatiles in Tengchong Quaternary volcanic rocks show varing δ13CCH4(-27.05‰--7.47‰) and δ13CCo2(43.59%o--25.92%o) values, which is a mixture feature of mantle and crust components. The δ13C value of CO2become lighter with decrease of C content. The δ13C values of CH4, C2H6, C3H8and C4H10etc. alkane hydrocarbon volatiles show a normal carbon isotopic distribution pattern among CH4-C4H10hydrocarbons with partial reversal carbon isotopic distribution pattern between C2H6and C3H8, which suggest the alkane hydrocarbon volatiles trapped in magma mainly originates from decomposition of oceanic organic matter.(4) The petrogeochemical and volatile geochemical characteristics of the Tengchong volcanic rocks indicated that the magma sources could be the EMI type mantle, which were contaminated by the fliuds from subducted plate and sediment. The dehydration process of hydrous minerals and degassing of volatiles led to the abundant H2O, N2and O2. The δ13C values of alkane hydrocarbon volatiles suggestted it can originated from subducted plate and sediment.(5) The mantle magma had experienced the fractional crystallization of plagioclase, olivine, pyroxene and apatite during their evolution process. The degassing of CO2could be occurred in the magma rising stage. Tengchong volcanic rocks could been formed in the continental-rift environment.