The Petrogeinesis Of The Permian Xuanwoling Mafic-ultramafic Complex In Xinjiang,China

The large-scale layered mafic-ultramafic complex is a product of massive mantle-derived mafic magmatism,and could to host ultra-large Ni-Cu-PGE magmatic deposit.The large-scale Beishan mafic-ultramafic layered complexes in the northeastern margin of the Tarim Craton contains magmatic copper-nickel sulfide deposits,in which the Xuanwoling mafic-ultramafic complex is characterized by ring-shaped lithofacies distribution with Ni-Cu-PGE sulfide mineralization,The types and mechanisms of contaminated crustal materials confirmed in the magma process,and the contribution of the mantle plume require directive evidences of carbon and noble gas isotopic tracing system.In the present study,we have measured the carbon and noble gas isotopic compositions of volatiles from magmatic minerals including olivine,pyroxene and plagioclase in the Xuanwoling mafic-ultramafic complex,and the source of volatiles of the complex,the type and mechanism of crustal contaminations,and the contribution of mantle plume are discussed.The following main understandings are obtained:1.The ?13CCO2 value of CO2 and CH4 released from the magmatic minerals of the Xuanwoling complex ranged from-24.4 to-6.6‰,and the ?13CCH4 value from-68.4to-18.0‰,which are distributed between the crust and the mantle.The carbon isotope composition of methane homologues showed a normal distribution pattern with carbon number,and some samples showed ?13C reversal pattern between ethane and propane.2.The 3He/4He ratios of the noble gas in the Xuanwoling complex is between0.36 and 1.35 Ra,The 40Ar/36 Ar ratios is between 386.02 and 412.41,3He/4He and40Ar/36 Ar range among the mantle,the crust and the atmosphere values.The20Ne/22 Ne and 21Ne/22 Ne ratios are distributed along the atmospheric Ne mass fractionation line(MFL).The 3He/4He ratios are negatively correlated with the 4He contents,and the40Ar/36 Ar is positively correlated with the40 Ar content,indicating that some of4 Heand 40Ar are radioactive origin.3.The isotopic composition of carbon and noble gas in the fluid components of the Xuanwoling layered complex indicates that there are components derived from mantle,crust and atmosphere.The isotopes of He,Ne and Ar are distributed between the three end-members of the mantle,crust and atmosphere,and the values of ?13CCO2and ?13CCH4 are distributed between the range of the crust(methane oxide)and the mantle,indicating that a large amount of crustal materials and atmospheric components are added to the mantle magmatic fluid components.After deducting the4 He and 40 Ar components of radioactive origin,the calculation of the He and Ar isotopes mixture model shows that the proportions of mantle,crust and atmospheric components are 20.8%,17.5% and 61.7%,respectively.4.A large amount of contaminated crustal materials in the Xuanwoling complex is indicated from sedimentary organic matter.The low ?13CCH4 and the normal distribution of methane homologues indicate that sedimentary organic matter cracking in the source mantle.Low 40Ar/36 Ar and Ne isotope reveal more contribution of atmospheric components.3He/4He and ?13CCO2 indicate the presence of altered oceanic crust and sedimentary components.Therefore,it is inferred that the subducted recycled oceanic crust carried sedimentary organic matter and atmospheric saturated fluid components into the magma source.5.The integrated isotopic composition of carbon and noble gas in the Xuanwoling complex shows that the mantle plume is not obvious contribution in masteries,could provide a heat source for partial melting.A large amount of subductted atmosphere and crustal fluids has overlapped the features of lithospheric mantle source.