The Mineralogical Study On The Mafic And Ultramafic Pluton In Wengeqi,guyang County,Inner Mongolia

Wengeqi mafic and ultramafic pluton,which has complex geological background and intense tectonic movements,is located in the Chengde-Guyang fault belt of the North China Craton.As is known,Wengeqi is a typical area of the Fluid Minerals Assemblages.The paper mainly studies and analyses the minerals,such as biotite,apatite,pyroxene and amphibole,on the mineralogical and geochemical level.As a result,it can be concluded as the following:1.The main rock types here are pyroxenite,biotite-pyroxenite, amphibole-pyroxenite,amphibololite.And it needs to be stressed that the special one,biotitite,can be composed of biotite with the content—90%.2.The fine biotite(d<1mm)of the biotite-pyroxenite belongs to magnesian biotite,and the big shaped one(d>10mm)of the biotitite to phlogopite.That is to say,the bigger the biotite is,the more magnesium it has.The low concentration value of Fe²⁺/(Fe²⁺+Mg²⁺),high enriched LILE,depleted HFSE and the low REE proves that the biotite is influenced by the fluid and belongs to hydrothermal biotite.3.As an important accessory mineral,the content of apatite can be up to 10%.Some apatite occurs as scattered grains in the pyroxene,amphibole and biotite,and the others as clustered grain in the contact zone,forming the intergranular apatite.Both kinds are fluorapatite,and the latter is the late-stage crystallization phase.The characteristics of apatite are enriched in LREE,with extremely weak Euanomaly,which demonstrates the crystallization process occurs in the environment of high fo₂.These facts prove the participation and impact of the fluid.4.The pyroxene,specificly diopside,is characterized by high Si O₂ and Ca O,low K₂O,Ti O₂ and Al₂O₃,enriched K,Sr,Ti,REE,depleted Nb,Rb,Ba,Th and U.The fact that the diopside has high level of LREE,compared with HREE,reflects the crystallization process is related to the fluid.5.The amphibole is mainly in coarsed grained level.The classification diagram shows the amphibole is assigned to the Calcic Group,more accurately to Ferroan Pargasite and Fea-Par-Hbl.According to the related research,the fluid affects and participates in the forming of the amphibole.6.The calcite mainly cuts the crystals of amphibole and pyroxene as veins,with straight boundaries.7.The mineralogical and geochemical data of the biotite,amphibole,apatite and pyroxene proves the participation of the fluid,whose source may be related to subduction orogenic environment.