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

Beishan district, in northeastern of Tarim plate, develops a series of regional mafic-ultramafic rock formed the complex zone with the length of 500km and the width of 100km. The mafic-ultramafic complex zone begins from the west with Luodong intrusion and Pobei intrusion in the east Lop Nur, through Bijiashan complex zone, to Heishan intrusion and Guaishishan intrusion in the northeast Liuyuan town. No.1 body and No.10 body in Pobei intrusion have magmatic sulphide deposits, Hongshishan intrusion and Xuanwoling intrusion in Bijiashan zone develops Ni-Cu sulphide mineralization. Currently, Bijiashan complex zone have carried out a preliminary survey and mining inspection, lacking of comprehensive study for petrogenesis. Therefor, this paper select Xuanwoling 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 process, magma nature and mantle source district features. Based on these researches we get some understanding as follows:1. Xuanwoling intrusion is a mafic-ultramafic layered intrusion with cumulate bedding development. The main rock types are dunite, troctolite, olivine norite, melanic olivine gabbro, gabbro norite, gabbro and leucogabbro. The main rock-forming minerals are olivine, chrome spinel, pyroxene, plagioclase and amphibole.2. The m/f values of most rocks range from 3.69 to 6.41, an iron ultrabasic rocks and the total alkali is low with Na2O+K2O=0.46%～2.22%, so the most rocks of Xuanwoling intrusion belong to ferruginous ultrabasic rocks and tholeiitic series. The concentrations of rare earth elements and trace element in rocks are very low with 1 to 10 times for that of the primitive mantle. The rocks are enriched in light rare earth elements and large ion lithophile elements and depleted in high field strength elements relatively, with obvious positive Eu anomaly. Element geochemistry and Nd, Sr, Pb isotope studies show that magma experienced obvious assimilation contamination.The Nd, Sr isotopic compositions of less contaminated rock and element characters suggest that the magma source region was depleted mantle.3. Petrography and geochemistry characters indicate that Xuanwoling intrusion had undergone fractional crystallization of olivine, spinel, plagioclase, orthopyroxene and clinopyroxene in the magma evolution, by crystallization order olivine+Cr-spinel→olivine +Cr-spinel+plagioclase→olivine+plagioclase+orthopyroxene+clinopyroxene→plagioclase+orthopyroxene+clinopyroxene→plagioclase+clinopyroxene. 4. The Fo molecular of olivine in Dunite of Xuanwoling intrusion is up to 90, which is different from the Huangshan mafic-ultramafic zone and Kalatongke intrusion in northern Xinjiang and Jinchuan intrusion in Gansu. Using Fo molecular of olivine to estimates the MgO content of primitive magma, The MgO content of primitive magma of Xuanwoling intrusion is 14.0%, so the magma is a high temperature and high MgO of picritic magma, indicating that Xuanwoling intrusion is partial melting of plume axis, which provide conclusive evidence to mantle plume of northeast Tarim Plate.