Geochemical Character And Petrogenesis Of Luodong Mafic-Ultramafic Layered Intrusion In Beishan Area, Xinjiang, P.R. China

The northeastern of Tarim plate includes Beishan region of Xinjiang and Liuyuan region of western Gansu. It develops a series of regional mafic-ultramafic rock during early Permian and highly concerned by geologists. No.l body and No.10 body in Pobei intrusion have magmatic sulphide deposits; Hongshishan intrusion, Xuanwoling intrusion in Bijiashan zone and Luodong intrusion develops Ni-Cu sulphide mineralization. Currently, Luodong intrusion has carried out a preliminary survey and mining inspection, lacking of comprehensive study for petrogenesis. Therefor, this paper select Luodong intrusion as research object, through systematically research on petrography, crystal chemistry, rare earth elements, trace elements and Sr, Nd, Pb isotopic geochemistry of the intrusion, scientifically demonstrates its magmatic evolution process, primary magma and mantle source nature. Based on these researches we get some understanding as follows:1. Lens shaped Luodong mafic-ultramafic intrusion covers an area of 2.1 km2. It consists of dunite, wehrlite, lherzolite, olivine websterite, peridotite pyroxenite, troctolite, olivine gabbro, gabbro, noritegabbro and leucogabbro. The Luodong intrusion is a layered intrusion with obvious cumulus textures and cumulus layering. The main rock-forming minerals are olivine, pyroxene and plagioclase, the accessoryminerals are Cr-spinel, magnetite and ilmenite.2. The rock chemical compositions in the mass belong to basic and ultrabasic rocks with rich magnesium. Most of the samples' m/f are 3.37-6.29, which belong to iron series. All kinds of rocks are characterized with very low TiO2, K2O, P2O5, rare earth elements and trace elements (∑REE=4.89×10-6～14.58×10-6) content. The REE patterns are flat, with positive Eu anomalies to different extent. The characteristics of petrochemistry do not show strong crustal contamination.3. The study of petrography, petrochemistry and crystal chemistry of minerals suggest that the primary magma of the Luodong intrusion have undergone fractional crystallization or cumulus crystal of olivine and Cr-spinel in the early stage and clinopyxene, orthopyroxene in the later stage. Plagioclase fractionally crystallizated during magma evolution. The direction of magmatic evolution of the iron enrichment, in compliance with tholeiitic magma evolution trend.4. MgO content of Primary magma is 14.7%, it belongs to picritic magma.5.εNd(t)=+7.03～+7.67,εSr(t)=-7.23～-3.05, (206Pb/204Pb)i=18.068～18.414, (207Pb/204Pb)i=15.513～15.629, (208Pb/204Pb)i=38.078～38.409. The intrusion has OIB-like Nd, Sr isotopic compositions and MORB-like Pb isotopic compositions, indicating that magma source is a depleted mantle source.6. OIB-like Nd, Sr isotopic compositions and picritic primary magma indicate that Luodong intrusion has an affinity with mantle plume and it may be formed by partial melting of axis plume. The Luodong intrusion is thus a part of Tarim Large Igneous Province.