Petrogenesis Of Mafic-ultramafic Intrusions In Bijiashan Belt, Beishan Area, Xinjiang, P.R. China

The mafic-ultramafic intrusions are well developed in northeastern Tarim plate, inBeishan, Xinjiang, such as the Luodong intrusion, Pobei intrusion and Bijiashan belt. ThePobei intrusion, nearly200km~2, is one of the largest mafic-ultramafic layered intrusions inChina. The Bijiashan belt, about90km in length and nearly1000km~2, is composed of theHongshishan intrusion, Xuanwoling intrusion, Bijiashandong intrusion and so on. Mostintrusions are typical layered intrusions. This paper selects the Hongshishan, Xuanwoling andBijiashandong intrusion of the Bijiashan belt as a key object of study. With the petrology,mineralogy and geochemistry of the intrusions, the paper explored the origin oftexture-structure of the layered intrusions, magmatic evolution and the nature of the primarymagma, magma source and mantle dynamics mechanism. The main cognitions gained fromthe research are as follows:The Hongshishan, Xuanwoling, and Bijiashandong intrusions are layered intrusions withcumulate texture and cumulate layering. Rock types are mainly dunite, troctolite,plagioclase-bearing wehrlite, olivine gabbro, gabbro-lecuogabbro, and anorthosite. In addition,the Hongshishan intrusion has a small amount of olivine pyroxenite and pyroxenite and theXuanwoling intrusion has olivine norite, olivine gabbronorite and gabbronorite. There arediverse types of textures, which is mainly ortho-cumulatic texture, and other texture are less,such as adcumulatic texture, mesocumulatic texture, poikilitic texture, corona texture andcorroded texture.The Hongshishan intrusion and Xuanwoling intrusion develop rhythmic layers andcumulate cycles, which represent the product of multiple magmas pulses. The origins ofcumulate textures and cumulate layers can be attributed to the following aspects:(a) Diffusiveconvection in magma chamber has effects on the formation of the rhythmic layer;(b)Nucleation rate fluctuations and growth rate fluctuations, the oscillatory change of pressureand the supercooling process in subsolidus are also important to form rhythmic layer, inaddition, the pressure change also makes the corroded texture extensively developed;(c)Gravitational differentiation plays an important role in the formation of cumulate structurewhen the system is mainly composed of liquids；(d)In the consolidation process, compactionplays an important role to the formation of adcumulate structure, the formation of the foliationof plagioclase crystals and crystal deformation.Ultramafic rocks of the Hongshishan, Xuanwoling and Bijiashandong intrusion belong totholeiite series, and mafic ones are mostly calc-alkalic series. The geochemical characteristics and the evolutionary trend of Nd and Sr isotopic composition, fully demonstrate that thecontamination increased as magma evolving. At the same time, contamination promotedmagmatic differentiation and brought about the transformation of geochemical series fromtholeiitic series to calc-alkaline series.The primary magmas of mafic-ultramafic intrusions of the Bijiashan belt have picriticmagma and high-Mg tholeiitic magma. The primary magmas of the Hongshishan andXuanwoling intrusion are high-Mg (MgO~14.00%) picritic ones with high temperature(~1350°C); the primary magma of the Bijiashandong intrusion is high-Mg tholeiitic magma(MgO~11.22%) with the temperature to1280°C. And the magmas have mainly undergonefractional crystallization of olivine and plagioclase, and a small amout of orthopyroxene andclinopyroxene in the magmatic evolution. The εNd(t) and εSr(t) values of weak contaminationrocks are+5.2~+7.5,-11.66~-2.9respectively, belonging to the range of oceanic island basalts,so its source should be oceanic island-type mantle source, and the material of magmaticsource should be garnet-pyroxenolite.Through studies on the temporal relationships, geochemical characteristics and thedegree of magmatic evolution between Bijiashan intrusive rocks and volcanic rocks webelieve that the Bijiashan mafic-ultramafic intrusive rocks and the volcanics are derived fromthe same magma, the numerous cumulations form intrusive rocks after magma entered presentchamber, whereas the evolved magma overflowed along the faulted zones or the upperboundary of magma chamber.The Permian mantle-derived igneous rocks in northeastern Tarim Plate, should be a partof the Tarim large igneous province (LIP) and the products of interaction between mantleplume and lithosphere: the Xuanwoling, Hongshishan and Luodong intrusions are products ofpartial melting of plume axis, mafic layered series of Pobei intrusion and Bijiashan intrusionsare derived from partial melting of lithosphere mantle induced by mantle plume upwelling,and Liuyuan basalt belt is the products of asthenosphere mantle partial melting.