Geochemical Characteristics Study Of The Permian Basalts At Dafang Area In Guizhou Province

Emeishan basalt which located in Yunnan, Guizhou, Sichuan provinces of southwest China is the only accepted large igneous provinces by geologists. Its causes are related to mantle plume activity. Since the mantle plume theory is put forward, the research of the emeishan basalt has entered a new period. Predecessors has do a lot of research about the emeishan basalt in Yunnan and Sichuan provinces, but to the the Guizhou basalt wich in the east of the emeishan large igneous province is relatively weak. Dafang county is located in bijie city, northwest of guizhou province.,which has a complete set of basalt outcropped. In this paper, on the base of measured section, I have studied the petrology characteristics, geochemical characteristics of Dafang basalt and compared with the basalt in adjacent region. At the same time I have analyzed the generous source region features of the basalt and the reasons for the formation and its relationship with the emeishan mantle plume. Results and progress have been obtained.The lithology characteristics and the law of eruption of the Dafang basalt was fouded out through field geological survey and section detailed. The main rocks stypes of Dafang basalt are microcrystalline basalt, stomatal-amygdaloidal basalt, altered the Ann tuff and volcanic agglomerate breccia. Layers of limestone wih thick layer of lava can be seen, according to the limestone sandwich the Dafang basalt can be divided three eruption phases and its eruption environment is the sea-land alternation. Its formation process is give priority to overflow and eruption second.The characteristics of the main elements show that the Dafang basalt has the content of Si O2 is 46.42 to 51.28 percent and the average content is 48.77 percent and the average content of K2 O + Na2 O is 3.12 percent, the total alkali content is low, which are belong to continental tholeiite series.The average content of Ti O2 is 4.24%, thus Dafang basalts are high-Ti O2(HT) type lavas. The rare earth elements display that the LREE/HREE of Dafang basalt is 7.50-9.31, with an average of 8.06, so it has obvious enrichment of light rare earth which shows that the light rare earth high has degree of fractionation. The obvious Ce, Eu negative anomaly on the rare earth elements diagram explain the basalt has undergone extensive fractionation of plagioclase. Trace elements and isotope characteristics apply the basalt EM Ⅱ enriched mantle and the magma stem from garnet stability region which was strongly contaminated by the crustal material in the process of formation.Compared with the neighboring basalt of Shuicheng, the basalt in Dafang area has lower alkali content and higher content of Ti O2 which proves the two are belong to different rock series. Through the analysis can be known that inconsistent source area,different degree of partial melting and different assimilation contamination degree are the reasons of the different geochemical properties between the Dafang and Shuicheng basalts. Dafang basalt comes from EM Ⅱ enriched mantle and Shuicheng is belongs to EMⅠenriched mantle, but the basalt in Dafang has been strongly contaminationed by the crustal in the process of formation.Rock geochemical characteristics show that Dafang basalt is related to the emeishan mantle plume which is a part of the it and arised to the garnet stability field. Because of lower temperature, the thicker lithosphere, the smaller degree of partial melting and far away from the mantle plume axis, the basalt has affected less by the mantle plume. Thus the basalt in Dafang is formed by low degree partial melting in the edge of the plume.