Magma Evolution And Ni-Cu-(PGE)mineralization Potential Of The Neoproterozoic Wangjiangshan Layered Intrusion In The Northern Margin Of Yangtze Block

The Wangjiangshan layered intrusion occur in the Neoproterozoic Hannan arc in the northern margin of Yangtze Block,and the area of about 100 km²,rhythmic beds are well growth.Rock assemblages also show some basic characteristics of stratiform complexes of crystalline differentiation origin.From the bottom to the top,the intrusion is composed of ultramafic rocks to intermediate rocks from the bottom to the top.The lower zone is mainly composed of pyroxenite and troctolite,the middle zone is composed of olivine gabbro and gabbro,and the upper zone is made up of gabbro and diorite.This study focuses on olivine noritegabbro,noritegabbro and gabbro in the middle zone.Based on the analysis of major and trace elements in main minerals and in whole rock,the magmatic evolution and metallogenic potential of Wangjiangshan intrusion in the northern margin of Yangtze are further recognized,and the following progress and understanding are obtained:1.The content of the ^IVAl in clinopyroxene is higher than ^VIAl,and most samples from the Wangjiangshan intrusion have Nb/Th ratio of 10,suggesting a subcontinental lithospheric mantle source.And it indicating that the source region was metasomatized by subducted slab-derived melts.The parental magma of the Wangjiangshan instrusion is tholeiitic basalt magma.2.The eutectic temperature of clinopyroxene and orthopyroxene of samples from the middle zone ranges from 901 to 1009°C,calculated by two-pyroxenes thermometer.The pressure calculated by the clinopyroxene geobarometry vary from 4.26 to 6.03 kbar,suggesting an emplacement depth from 12.9 to 18 km.The oxygen fugacity(log₁₀?O₂)of samples from the middle zone vary from-23.63 to-25.77,and was plotted in the stable region of magnetite,indicating the intrusion was formed in an oxidizing environment.3.Rare earth elements and platinum group elements?except Pd?of Wangjiangshan intrusion are basically not affected by hydrothermal alteration.The discriminant map of assimilation and contamination of instrusion shows a weak correlation,and the Nb/U ratio is8.95-41,with an average of 27.Overall,Wangjiangshan rock mass is basically not contaminated by crustal materials.4.The modeling result shows that olivine from olivine gabbronorite have Ni concentrations and Fo values higher than the fractionation line of olivine,indicating that the parent magma didn't reached S saturation.The composition of olivine is mainly controlled by fractional crystallization,and was product of²8%crystal fractionation of the parental magma.In addition,PGE show no depletion relative to Ni and Cu in the primitive mantle normalized patterns,arguing against large-scale sulfide segregation during the magma evolving due to lack of crustal contamination.5.The source area of Wangjiangshan intrusion is mainly composed of spinel lherzolite,derived from the depleted mantle of continental lithosphere metasomatized by subduction fluids.6.The TiO₂ content and percentage of tetrahedral sites occupied by Al?Alz?of clinopyroxene vary from 0.05 to 0.87 wt.%and from 1.40 to 5.85 mol.%,respectively,similar to the composition of the clinopyroxene cumulus in arc environment,and different from those in the rift setting.Moreover,samples from the intrusion have Th/Yb and Nb/Yb ratios of0.11⁰.70 and 1.04².37,respectively,similar to those of island arc basalts.We argue that the tearing and breakoff of subduction oceanic slab resulted in asthenosphere upwelling during the Neoproterozoic long-term subduction beneath the northern margin of Yangtze.This provided a heat source for the partial melting of the metasomatized subcontinental lithospheric mantle and subsequent formation of mafic magma with island arc characteristics,which ascended in an extensional environment to form mafic-ultramafic layered intrusions in the Hannan arc.7.Based on the research of rock assemblage,mineral assemblage,rock type,tectonic environment and magmatic differentiation of Wangjiangshan layered intrusion,we hold that Wangjiangshan layered intrusion has no potential to form copper-nickel sulfide deposits.