The ultramafic dikes in the Tarim large igneous province(Tarim LIP), exposed in the Xiaohaizi area in the northwestern Tarim Basin of northwestern China, have porphyritic textures, and the olivine and clinopyroxene are as the major phenocryst phases. The groundmass therein consists of clinopyroxene, plagioclase and Fe-Ti oxides, with the cryptocrystalline texture. The olivine phenocrysts in one typical ultramafic dike have Fo(Mg/(Mg+Fe)) numbers ranging from 73 to 85, which are not in equilibrium with the olivine(Mg# of 89) from the host rock crystalized. Combined with microscope observation, both the olivine and clinopyroxene phenocrysts as well as some Fe-Ti oxides in the ultramafic rock are accounted as cumulates. The liquid(parental magma) composition of SiO_2 of 45.00 wt.%–48.82 wt.%, MgO of 9.93 wt.%– 18.56 wt.%, FeO of 5.85 wt.%–14.17 wt.%, CaO of 7.54 wt.%–11.52 wt.%, Al_2O_3 of 8.70 wt.%–11.62 wt.% and TiO_2 of 0.00 wt.%–3.43 wt.% in the Xiaohaizi ultramafic rock was estimated by mass balance, and the results show a reasonable liquid proportion in the cumulate-bearing ultramafic dike(ca. 45%–60% in the whole rock). The estimated parental magma composition corresponds to a melting temperature of 1 300–1 550 oC, which is equal or higher than those of a normal asthenosphere mantle, supporting the involvement of a mantle plume. Combined with other previous studies, an evolution model for the formation processes of the Xiaohaizi ultramafic dike of the Tarim LIP is proposed.
Tarim Large Igneous Province(TLIP)is the second Late Paleozoic LIPs in China after the recognition of Emeishan LIP,and is a hot research topic in geosciences.On the basis of the analysis of research history about TLIP,this paper summarizes the research result during last twenty years and suggests the key research area in the future.The residual distribution range of TLIP is up to 250000 km2,and the largest residual thickness is 780 m.The eruption of basalt happened during 290–288 Ma and belongs to LIPs magmatic event with fast eruption of magma.The lithological units of the TLIP include basalt,diabase,layered intrusive rock,breccia pipe mica-olivine pyroxenite,olivine pyroxenite,gabbro,ultramafic dyke,quartz syenite,quartz syenite porphyry and bimodal dyke.The basalt and diabase of TLIP exhibit OIB-like trace element patterns and enrichment of LILE and HFSE,and mainly belong to high TiO2series.There is an obvious difference in isotope among the basalt from Keping and the basalt and dibase from the northern Tarim Basin.The basalt from Keping with negative Nd and high REE value derives from enriched mantle,and the diabase and basalt from the northern Tarim Basin with positive Nd and low REE value are related to depleted mantle.The crust uplifting in the Early Permian and the development of picrite and large scale dyke and formation of large scale V-Ti-Magnetite deposit in Wajilitag area support the view that the TLIP is related to mantle plume.The TLIP has a temporal-spatial relationship with Permian basic to ultra-basic igneous rock,which is distributed widely in Central Asia,and they represent a tectono-magmatic event with very important geodynamic setting.This paper also suggests that the deep geological process,the relation with mantle plume,mineralization,the relation with environmental change and biological evolution,and the geodynamics of the TLIP will be the key research topics in the future.
YANG ShuFengCHEN HanLinLI ZiLongLI YinQiYU XingLI DongXuMENG LiFeng
