The Geochemical Characteristics Of Permian Picritic And Associated Basalt Flows Of Jianchuan, Western Yunnan

Emeishan basalts widely spread out in the west margin of the Yangzi block.Because of this, most of the Geo-sicence circles are interested in this area currently.Jianchuan is located in the western of Emeishan large igneous province. It is above onthe Jinsha River-Ailaoshan-Red River fault in western Yunnan. On the basis offormers’ research, the paper systematically discusses the characteristics of Emeishanbasalts’petro-geochemistry and magma activation.Jianchuan is located between Lijiang and Binchuan. This region is considered tobe the most strongly of Emeishan plume activity zone, and also a large number ofmaterials are exposed in this region which came from the mantle plume. According tothe content of MgO, Emeishan basalt could be divided into two types: one is high-Mgbasalt volcanic rock（picrites and picritic basalt） and another is lower-Mg basalt.Based on the petrography, these High-Mg volcanics are picrite and picritic basalt. Themainly minerals in the picritic rocks are olivine, clinopyroxene and plagioclase. Thebulk-rock composition is characterized by lower SiO2（39.38wt%～42.82wt%） andhigher MgO （14.86%～19.86wt%）. It shows the characteristics of OIB from thechondrite-normalized REE patterns and primitive mantle-normalized spidergrams ofPermian picrites and basalts. It is rich in LILE （Rb, Ba, Th, and U） and light rare earthelements to prove its mantle plume activity. And this is the evidence to the mantleplume. The low ratio of La/Ta and La/Sm （15.45～20.66and3.09～4.38）, indicatesthat the samples are rarely contaminated by the crust materials. According to therelationships of CaO/Al₂O₃with Mg/（Mg+Fe） and the illustration of Harker, wecould know magma experienced the fractional crystallization of olivine and pyroxene.Picritic magmas were probably derived from the high temperature area at the centre ofthe plume （the axis area of the mantle plume）. However, the basaltic magmas wereprobably derived from the edge of the plume. These magmas were mainly in thegarnet stable field. The samples are melted partially under the high temperature andpressure. And from this to speculate that the magmas are originated fromasthenosphere. From the research of this area and combining the previous studies of others, the type of magma is related to the mantle plume. It is speculated that theplume center is located near Jianchuan. The distance from Jianchuan to Lijiang is verynear and ZHANG ZhaoChong speculates the center of the plume located near Lijiang.Therefore, the conclusion is consistent to Zhang’s opinion.