The Petrogenesis Of Permian Layered Luodong Mafic-ultramafic Instrusion,Xinjiang,China: Evidences From C And He-Ne-Ar Isotopes

The layered mafic-ultramafic intrusion records the large-scale magmatic events and the formation of magmatic deposits in the mantle.Many Permian layered mafic-ultramafic intrusions occurred in the Beishan area of Xinjiang,weatern China,and host magmatic copper-nickel sulfide deposits.For example,the Poyi mafic-ultramafic intrusion contains a large copper-nickel sulfide deposit,which is related to the magmatism of mantle plume.However,the Luodong mafic-ultramafic intrusion hosts a magnetite deposit.The metallogenic potential of the magmatic vanadium-titanium-iron deposit and the contribution of the mantle plume in the mineralization process need further constraints.In this paper,noble gasand carbon isotopic compositions had been analyed for olivine,pyroxene and plagioclase separates in Luodong mafic-ultramafic intrusion,and the sources of volatile in the Luodong intrusion are determined.The mixing mechanism of the crustal components and the contribution of the mantle plume had been discussed.The main conclusions are as followings:?1?The?¹³C_CO2O2 of the volatiles in the magma minerals of the Luodong mafic-ultramafic intrusion is-18.4^-0.3‰,the?¹³C_CH4H4 is-48.4^-21.7‰,the?¹³C_C2H6is-33.0^-23.0‰,and the?¹³C_C3H83H8 is-32.0 to-23.3‰,?¹³C_C4H104H10 is-30.8 to-22.3‰.The?¹³C_CO2O2 and?¹³C_CH4H4 values are plotted in the ranges of organic matter thermogenic,crust and mantle components.The carbon isotopic composition of methane homologues showed a positive sequence distribution with the increase of carbon number.Some olivine and pyroxene samples exhibited the reverse distribution of propane and butane released at 500?and 700?.?2?The ⁴He content in Luodong intrusion ranged from 0.11 to 67.01×10^-6cm³.STP/g?STP-Standard temperature and pressure conditions,The same below?,the content of ²⁰Ne=0.36³2.41×10^-66 cm³/g,and the content of ⁴⁰Ar=95.16⁷56.64×10^-66 cm³/g.The ³He/⁴He ratios varies between 0.43 and 1.23Ra,the ²⁰Ne/²²Ne ratios varies from 8.60 to 11.78,the ²¹Ne/²²Ne ratios ranges from 0.032 to 0.045,and the ⁴⁰Ar/³⁶Ar ratio is 286.34.²97.71.?3?The noble gas isotope composition of the Luodong intrusion is characterized by the mixing of the atmosphere,the crust and the lithospheric mantle:³He/⁴He?0.43¹.23Ra?is located the ranges of mantle and the crust,²⁰Ne/²²Ne?8.60¹1.78?and ²¹Ne/²²Ne?0.032⁰.045?are mainly distributed along the continental crustal evolution line?CC?,and ⁴⁰Ar/³⁶Ar?286.34²97.71?is close to the atmospheric value.?4?The ³He/⁴He and ⁴⁰Ar/³⁶Ar,?¹³C_CO2O2 and?¹³C_CH4H4 of the Luodong intrusion indicated that the Luodong magma originated from the deep mantle source,and mixed by atmospheric component and crust end-elements.The partial reverse patterns of?¹³C of methane homologues are observed in some olivine single minerals at high temperature.Indicating that Luodong magma is derived from partial melting of lithospheric mantle.?5?The presence of crust,sedimentary organic matter and atmospheric components in the mantle-derived magma of Luodong intrusion:the lower ³He/⁴He and ⁴⁰Ar/³⁶Ar ratios may be the addition of recycled oceanic crust material during the subduction processes.The atmosphere and crustal components may be added during the process of ascending and recirculating material in the magma source area to the crust,and the contribution of the mantle plume is not obvious.