Geochemical Characteristics And Diagenetic Evolution Of Permian Basalt In Yanyuan Area,Sichuan Province

The basalt of Mount Emei is widely distributed in Yanyuan area,which is an important part of Mount Emei's igneous province.A large number of field geological investigations have been carried out in the basalt group of Mount Emei in Yanyuan area,the representative samples in the area were tested and analyzed,and a large number of data and materials are collected.By studying the petrography,mineralogy and elemental geochemistry of rock samples,the origin of the basalt group of Mount Emei in Yanyuan area is discussed in detail,inversion of tectonic magma evolution in this area,to reveal the magma origin and evolution of rock mass.It provides important theoretical support for basic geological survey and prospecting work in this area.The basalt of Mount Emei basalt formation in Yanyuan area is mainly composed of gray-green compact massive basalt,porphyritic basalt,amygdaloidal basalt and they belong to calc-alkaline series,with a little picrite and tuffaceous sandstone.The first rhyme of these volcanic rocks: it refer to the clear red top and green bottom cooling unit in the lava rocks,the basalt is erupted in sequence of a large pore-amygdaloidal basalt,a compact block basalt not containing or less pore,a small and dense pore-amygdaloidal basalt.From the aspect of the principal element: this area is mainly composed of alkaline series of potassic volcanic rocks and tholeiite series of high potassium basalt.Except for 2 samples TiO2 < 2.8%,the rest samples have high titanium.From the aspect of the trace element: high field strength elements such as La,Pb,Th and U are enriched in this area;depletion in elements such as Nb,Ce and Sr.Rb,Ba,Nd and Sm have obvious fluctuations.Compared with the original mantle,the degree of dispersion is moderate,which is about 5-150 times that of the original mantle.Ce negative anomalies and rocks are generally exposed to chloritization and epidotization,indicating low temperature alteration.The loss of Nb is caused by alteration or crystallization and Nb precipitation.Sr loss is influenced by alteration or mantle metasomatism.Generally speaking,trace elements reflect the characteristics of enriched incompatible elements.From the aspect of rare earth elements: the average content of rare earth elements is 89 times that of chondrites,and the distribution curve is right-handed LREE enrichment mode;(La/Yb)N is 6.13 ~ 15.95,it shows that the degree of fractionation between light and heavy rare earth elements is high;No obvious loss in ?Eu.The basalt of Mount Emei basalt formation in Yanyuan area belong to a intraplate basalt environment,partial melting of 10%-25% originates from asthenospheric mantle or enriched mantle,the magma source area is the garnet peridotite mantle.The magma has undergone a certain degree of separation and crystallization,including apatite,rutile,clinopyroxene,etc.The basalt source area is similar to the enriched mantle,and is not obvious mixed by the crustal materials,the nature of mantle is garnet peridotite.