Study On Geological Characteristics And Petrogeochemistry Of The Xinanshan Mafic-ultramafic Intrusion In The Liuyuan Area,Gansu Province

A large number of Permian mafic-ultramafic intrusions are developed in the Beishan Orogenic Belt,NW China.Previous studies mainly focused on the western part of the Beishan Orogenic Belt,with rare concentration on mafic-ultramafic intrusions located in the central and eastern part of the Beishan area.And there are still many different views on the tectonic setting of these intrusions.The Xinanshan is a long lenticular shaped intrusion and located about 8 km southwest of the Liuyuan area and striking along the southeast side of the regional Gubaoquan-Hongliuyuan fault.This paper aims to start with the field geological,petrographic,and mineralogical characteristics of the Xinanshan itrusion,combined with integrated geochemical data to summarize the geological features of the Xinanshan intrusion,discuss the magmatic sequence and reveal the origin of the intrusion.The following understandings have been obtained:1.The Xinanshan intrusion is a mafic-ultramafic complex.According to lithofacies and the contact relationship between them,it can be divided into:gabbro facies,olivinegabbro facies,pyroxenite and peridotite facies,and the latest medium-coarse grained gabbro facies,corresponding to four magma units in turn.The first magmatic subseries comprise the first magmatic unit and the second magmatic subseries consist of the second and third magmatic units.2.The Fo values of olivine from the Xinanshan rock samples range from 78.0 to 85.6,all of which are chrysolite.Calculated results of mineral geothermometer indicate the crystallization temperature of olivine is between 1265?and 1305?.Clinopyroxene from the Xinanshan rock samples are mainly diopside and sahlite,and the crystallization temperature of which ranges from 975?to 1136?.The ratio of Al~?/Al~?is relatively high,with a range of1.42?3.49,indicating that the environmental pressure during the crystallization process is relatively low.3.The mafic-ultramafic rocks in the Xinanshan intrusion are mainly ferro basic-ultrabasic rocks(m/f 1.6?4.8).The olivine gabbro facies,pyroxenite facies and peridotite facies have similar rare earth elements and trace elements compositions,which is different from that of the gabbro facies,indicating that they have undergone different degrees of magmatic differentiation evolution and crustal contamination processes.However,their features are significantly different from that of the mantle plume-related mafic-ultramafic rocks in the Tarim large igneous province,indicating their different magma sources.4.According to the calculation of olivine composition,the Mg O content of parental magma of the Xinanshan intrusion is 13.1%.Combined with the analysis of whole-rock major elements,it is considered that the Xinanshan parental magma belongs to the high-magnesium tholeiitic magma series.The elemental geochemical characteristics show that the Xishanshan magma is derived from the depleted asthenosphere mantle,showing similar features as the island arc basaltic magma and carrying the information about metasomatism of subduction-related fluids.5.The emplacement relationship between the intrusion and stratum and regional geochronological results shows that the Xinanshan intrusion was formed in the Early Permian.Combined with the regional geological background,it is considered that the Xinanshan intrusion was formed in a post-collision extensional environment.Under the mechanism of the demineralization and thinning of the lithospheric mantle,the original magma was formed by decompression melting of the asthenosphere mantle and underwent a certain degree of magmatic differentiation evolution and crustal contamination during the uplifting and emplacement processes.The evolved parental magma rose and formed firstly the gabbro facies,and then the homologous magma at the deeper intruded into the former and subsequently formed the ultrabasic lithofacies.