Research Of Charateristics And Petrogensis Of Emeishan Basalt In Makou Area,Northeastern Yunnan

Emeishan Great Igneous Province has always been the focus of scholars at home and abroad,mainly distributed in Sichuan,Yunnan,Guizhou and other places in China.The Makou area in northeastern Yunnan is located in the western margin of the Yangtze landmass.The Emeishan basalt is widely distributed in the area.The basalt outcrop thickness is more than 1446.57 M.Based on the analysis and research of the rock assemblages of field measured sections,combined with the petrographic and geochemical characteristics of Emeishan basalt in Makou area,the characteristics of its magma source area and its petrogenesis are analyzed and discussed.The rock types of Emeishan basalt in Makou are mainly lava rocks and tuff.Volcanic rhythm and cycle characteristics are obvious.The eruption characteristics of lava and tuff are:rhythmic cycle of dense basalt-porphyry basalt-almond basalt-tuff.Lava outcrop thickness is huge,mostly overflow phase eruption,its structure is mainly less porphyry structure,porphyry structure,cryptocrystalline structure and microcrystalline structure.The phenocryst mineral is mainly plagioclase.Geochemical characteristics of Emeishan basalt in Makou area indicate that the SiO₂ content of the basalt in this area is 46.99%⁵1.02%,average content is 48.67%;the TiO₂ content is 3.50%⁵.09%,average value is 4.32%;the Mg^#content is0.40⁰.50,average value is 0.45.Rare earth elements are enriched in light rare earth elements,light and heavy rare earth fractionation is obvious,and the standardized curve of chondrite is steep right inclination.High field strength elements such as Th,U,Ce,Hf,Zr and Sm are enriched in the rocks.K,Nb,P,Sr and Y are slightly deficient.Mg^#is positively correlated with elements such as Cr,V and Ni,but negatively correlated with elements such as Ta,Nb and Co.The ratio of incompatible elements is appropriate to the Ocean Island Basalt?OIB?.The study of diagenetic tectonic setting and magma source area shows that the tectonic environment of Emeishan basalt in Makou is intraplate extensional environment,Ti/Y is between 490.06⁶27.77,indicating that it is high-Ti basalt;The content and ratio of strongly incompatible elements indicate that the magma originated in the stable zone of enriched mantle garnet;the average Nb/U is 25.18,the average value of Ta/U is 1.74,slightly lower than the crust and Mid-Ocean Ridge Basalt?MORB?;the slight Eu negative anomaly of the rock??Eu=0.72⁰.93?indicates that plagioclase separation and crystallization occurred during the magma crystallization process;combined with the basalt loss characteristics of large ion lithophile elements,suggesting that the basalt was contaminated by the crust at the end of magmatic diagenetic process.