The Study Of The Characteristics Of Rock Forming Minerals Of The Pobei Mafic-Ultramafic Complex In The Northeastern Margin Of The Tarim Plate

The pobei complex is located in the middle of the beishan riff belt on the northeastern margin of the tarim plate,and on the southern of the baidiwa fault.The complex shapes of rockbasin and exposed nearly 200km~2.The Pobei complex can be divided into 5 units corresponding to 5 types of lithofacies according to the field mapping:olivine facies,olivine gabbro facies,gabbro norite facies,gabbro facies(main part)and fine gabbro facies(only locally distributed in Rock Branch).the complex can also be divided into 2 sub-suites according to mineralization type and magma nature.The first four lithofacies were selected as the research objects,and the following understandings were obtained according to their mineralogical characteristics and differences by observing and summarying by mirror and analysis of mineral composition:1.The mineral composition of the each lithofacies in pobei complex is different,the olivine in olivine facies is chrtsolite-forsterite,in olivine gabbro facies is chrtsolite,in gabbro norite facies is hyalosiderite;the orthopyroxene in olivine facies is bronzite,in gabbro norite facies is bronzite and hypersthene;the compositions of clinopyroxene in each facies are not very different,including diopside and augite;the plagioclase in olivine facies and gabbro facies is labradorite-bytownite,in olivine gabbro facies is bytownite,in gabbro norite facies is labradorite.2.The crystallization temperature of olivine in each facies is different,and it increases with the increase of the Fo value at 1119.6~1333.9?,but the crystallization temperature of clinopyroxene is not quite differentm,all at about 1100?.3.According to the analysis of composition characteristics of major rock forming minerals,the parent magma of the pobei complex is tholeiite series,the MgO content of the parent magma of the olivine facies is 18%,which belongs to the picritic magma;the MgO content of the parent magma of the gabbro facies is 8.61%;the MgO content of the parent magma of the gabbro norite facies is 8.98%.4.The fractional crystallization of olivine,pyroxene and plagioclase generally occur in the pobei complex.The sequence of crystallization in olivine facies is olivine?olivine±orthopyroxene+clinopyroxene?clinopyroxene±orthopyroxene+plagioclase?plagioclase.the sequence of crystallization in olivine gabbro facies is olivine?olivine+clinopyroxene?clinopyroxene+plagioclase?plagioclase.The sequence of crystallization in gabbro norite facies is olivine?olivine+clinopyroxene±orthopyroxene?orthopyroxene±clinopyroxene+plagioclase?clinopyroxene+plagioclase?plagioclase?amphibole.The sequence of crystallization in gabbro facies is plagioclase?clinopyroxene+plagioclase.5.According to the analysis of the characteristics of trace elements,the magma had gone through assimilation in the process of formation.6.The pobei complex is formed in tensioning environment similar to rift,and which may be related to the activity of the mantle plume.