The Crustal Contamination Machanism In The Xiwanggou Mafic Magmatism,East Kunlun Orogenic Belt,China:Constraints From C-He-Ne-Ar Isotopes

The early Paleozoic mafic magmatism developed large-scale Ni-Co sulfide mineralization in the East Kunlun orogenic belt,while the late Paleozoic mafic magmatism developed nickel-cobalt mineralization on a small scale.The Xiwanggou mafic-ultramafic intrusion co-occurs the plutons of the above two stages,and the nickel-cobalt mineralization is weak,which is the key control of sulfur saturation and sulfide melting in the process of nickel-cobalt sulfide mineralization.The ideal rock mass for factor-crust source contamination.The C-He-Ne-Ar isotope tracer system of fluid volatiles in magmatic minerals is an effective means to understand the types and mechanisms of crust-derived contaminants.In this paper,the C-He-Ne-Ar isotopic analysis of magmatic minerals such as olivine,pyroxene and plagioclase in different types of rocks in the Xiwanggou mafic-ultramafic intrusion was carried out to determine the source of the fluid and the source of the crust during the magmatism.types of contaminants and contamination mechanisms.Mainly obtained the following knowledge:1.The?¹³C_CO2 of CO₂ released from magmatic minerals in the Xiwanggou intrusion is between-41.4‰?4.2‰,which is mainly between the carbon isotope composition of mantle and crust,and the carbon isotope composition of methane is between-51.5‰?-5.4‰,located in the Biogenic methane and magma-derived methane carbon isotopic composition range.The distribution patterns of carbon isotope compositions of methane homologues are positive and partially reversed.2.The rare gas isotopes of magmatic minerals in the Xiwanggou intrusion vary greatly.The ~3He/~4He value is between 0.11 and 0.46Ra,which is located between the lithospheric mantle and the crust value.The ~3He/~4He ratio is negatively correlated with the 4He content.;The value of ⁴⁰Ar/³⁶Ar ranges from 296.0 to 1169.7,and there is a weak positive correlation between ⁴⁰Ar/³⁶Ar-⁴⁰Ar;the ratio of ²⁰Ne/²²Ne and ²¹Ne/²²Ne is located in the interval between the radiogenic line and the continental crustal evolution line.3.The carbon and rare gas isotopic compositions of the Xiwanggou intrusion reveal the contribution of atmospheric components and crustal material components in the mantle-derived magma.After deducting radioactive helium and argon,the⁴⁰Ar/³⁶Ar-~3He/~4He compositional characterization shows that a large amount of atmospheric saturated fluid is added.The isotopic mixing model estimates that the contributions of different endmember fluid components are:59.5%of the saturated fluid component of the atmosphere,18.3%of the subducted oceanic crust material,16.6%of the crustal wall rock,and 5.6%of the lithospheric mantle.4.The isotopic composition of carbon and rare gases in the Xiwanggou intrusion reveals that the fluid components originate from the mantle,crust and sedimentary organic matter,and the positive sequence distribution model of the carbon isotope composition of methane homologues indicates that there are sedimentary materials added to the mantle-derived magma;³⁶Ar and ~3He have a positive correlation.The correlation revealed that the magma source area was mixed with crustal materials and atmospheric components.The distribution characteristics of?¹³C_CO2 value and ~3He/~4He ratio shows that the sedimentary organic matter originates from the sedimentary material in the altered oceanic crust and wall rocks.5.The magma source area of the Xiwanggou intrusion is the lithospheric mantle,and the altered oceanic crust components during magmatism may be derived from the devolatilization of the subducted oceanic crust and its atmospheric saturated fluid components,which partially melted in the magma source area During the process,it entered the parent magma,and part of the sedimentary organic matter may have come from the marine sedimentary strata of the surrounding rocks mixed with the magma in the process of upwelling.The proportion of crustal surrounding rocks mixed with the Xiwanggou intrusion is higher than that of the Xiarihamu and Shitoukengde deposits.