Petrogenesis Of The Mafic Layered Intrusion In Pobei Complex, Northeast Tarim Plate

There are more than one hundred Permian mafic-ultramafic intrusions in the Beishanarea, northeastern Tarim plate. West put that in Luodong and Pobei complex east of Lop, afterBijiashan complex zone, the eastern end of Heishan and Guaishishan complex northeast ofLiuyuan town, these intrusions compose a complex belt, which is500km in length and100kmin width. At present, ore bodies have been found in Poyi and Poshi intrusions, but the layeredseries formed during the first stage of the Pobei complex has not been studied systematicallyyet. Based on the situation, this thesis will discuss the magma source, primary magma,assimilation and fractional crystallization by doing researches on petrography, mineragraphy,mineralogy and geochemistry. The achievements obtained as follows:1. The intrusion formed during the first stage of Pobei magmatism is a layered serieswhich is mainly composed of mafic rocks. Resulted from repeated injection of magma, themafic layered intrusion develops monomineral laminas and rhythmic layering. According tolithology, texture and structure, the series can be divided into17rhythmic layers and5cycles.2. The types of the rocks are olivine gabbronorite, gabbro, melagabbro, leucogabbro,anorthosite, clinopyroxenite, spotted gabbro and gabbro-diorite. The rocks are mainlycomposed of plagioclase, olivine, clinopyroxene, orthopyroxene, hornblende and biotite.3. In the series of clinopyroxenite–spotted gabbro–gabbro–gabbro-diorite fromcycleⅠ, clinopyroxenite is tholeiitic, while others are calc-alkaline. They have relatively highconcentration of rare earth and trace elements. The content of rare earth elements is3~110times that of chondrite’s, and the content of most trace elements is0.1~100times that ofprimitive mantle’s. On the other hand, the series of olivine gabbronorite–gabbro–leucogabbro–anorthosite from cycle Ⅱ, Ⅲ, Ⅳ and Ⅴ are calc-alkaline, with lowconcentration of rare earth and trace elements. The content of rare earth elements is1~10times that of chondrite’s, and the content of most trace elements is0.1~2times that ofprimitive mantle’s.4. The magma source is subcontinental lithospheric mantle. The primary magma isbasaltic.5. The magma had gone through assimilation in the process of emplacement. Thecontamination materials are mainly biotite schist, biotite-quartz schist and mica schist of theGudongjing Group, Changchengian System, and marble of the Lower CarboniferousHongliuyuan Formation. 6. The crystallization in the mafic layered series is plagioclase—plagioclase+olivine—plagioclase+olivine+clinopyroxene±orthopyrocene—plagioclase+clinopyroxene±hornblende±biotite. The crystalline temperature of rock-forming minerals is1278℃~945℃. Thecrystallization in the clinopyroxenite is clinopyroxene—clinopyroxene+magnetite—clinopyroxene+magnetite+plagioclase—clinopyroxene+plagioclase+hornblende±magnetite.