Geochemical Characters And Petrogenesis Of Hongshishan Mafic-ultramafic Layered Intrusion In Beishan District, Xinjiang

Hongshishan mafic-ultramafic layed intrusion lies in the Beishan zone in Xinjiang, northeast of the Tarim plate.The Beishan zone is one of the most important mafic-ultramafic layed complex zone in China.lt is reported that almost hundreds of such intrusions has been found in this area, while the Hongshishan intrusion is one of the most representative mafic-ultramafic intrusions. According to systematically research the petrography, crystal chemistry, rare earth elements, trace elements and Sr, Nd, Pb isotopic geochemistry of Hongshishan intrusion, it's demonstrateed its magmatic process, magma nature and mantle source district features in this paper. Based on such researches,we get some initial understanding as follows:1. In the Hongshishan mafic-ultramafic layed intrusion, there are two phases:peridotitic facies and gabbroic.facies,which are products of different stages of comagmatic rock.The facies units of the peridotitic facies are dunite,clinopyxene peridotite,peridotite,feldspathic peridotite,troctolite,while the gabbroic facies are olivine gabbro,gabbro,anorthosite. MgO as well as FeOT are rich and CaO,Al2O3 poor in the peridotitic facies,the m/f ratio is 5.74-8.76 and belong to ferruginous-magnesium ultrabasic rocks,while CaO,Al2O3 are rich and MgO,FeOT are poor in the gabbroic facies,the m/f ratio is 1.81-4.56 and belong to feruginous ultrabasic rocks. The peridotitic facies have a relation with sulfide metallization,while the gabbroic facies with magnetite metallization. The intrusion is a chonolith,forming in the Early Permian.2.The Hongshishan intrusion is a mafic-ultramafic layered intrusion with cumulate bedding well development.After researchs on two cores,at least four cycles can be classified in the Hongshishan intrusion,and every cycle can be representative for a magma injection.The cumulate rhythmicities are obvious in every cycles,and a complete cumulate rhythmicity should be dunite+clinopyroxene peridotite+ feldspathic peridotite+ troctolite+ olivine gabbro+gabbro+anorthosite.3.The orthocumulate texture is the primary texture in the Hongshishan intrusion. The cumulate minerals is olivine,spinel, plagioclase,magnetite,and the intercumulate mainerals is primaryly clinopyroxene.Petrography and geochemistry characters shou that the Hongshishan intrusion had undergone fractional crystallizition of olivine,spinel,plagioclase,clinopyroxene and magnetite in the magma evolution,with crystallization order of olivine +Cr-spinel→olivine+Cr-spinel+clinopyroxene→olivine+Cr-spinel+ plagioclase→olivine+ plagioclase+ clinopyroxene→plagioclase+ clinopyroxene+ magnetite.4. The concentrations of rare earth elements and trace elements are very low with 1 to 10 times for that of the primitive mantle.The REE patterns are flattened,with positive Eu anomalies to different extent.The Large Ion Lithpophile Elements (LILE) are enriched,while TiO2,P2O5,Na2O,K2O as well as high field-strength elments(HFSE) depleted in the Hongshishan intrusion. The characters of Nd,Sr,Pb isotopes show that the magmatic source belong to the depleted mantle (DM),and it was locally inflenced by contamination with the wall rocks.5. The primary magma of the Hongshishan intrusion belong to Mg-enriched and high-temperature picritic magma,which forms in the axle center of mantle plume under fraction melting.It is a conclusive evidence to mantle plume of northeast Tarim Plate.