Study On The Characteristics And Formation Environment Of The "Mung Bean Rock" In The Middle Triassic In The Upper Yangtze Region

Mung bean rock denotes a type of clay stone widely produced in the early and middle Triassic strata in the southern area of China.Featured by stable horizon,being thin,and wide distribution,it is a typical marker on the stratum.It marks the regional tectonic movement and volcanic eruption activity at the end of Early Triassic,and deserves further exploitation and utilization.In the paper,backed up by the rock and geochemical features of mung bean rocks from Longmen cave section of Mount Emei and utilizing the data of mung bean rocks from other sections of the upper Yangtze region,the differences of the rock and geochemical features of mung bean rocks in different regions are compared in detail with the environment where they take shape analyzed.By means of the thin section identification,SEM observation and energy spectrum analysis,the mung bean rock of Longmen cave mainly comprises illite,chalcedony and calcite.In accordance to the classification of pyroclastic rock,it is a kind of pyroclastic sedimentary rock,denominated"siliceous pea illite clay rock".As shown by the whole rock geochemical analysis,in the mung bean rock of Longmen cave,potassium,sodium and lithium are much richer than light rare earth and Ce,and Eu is poor relatively.The mung bean rock is made up of clay minerals and siliceous particles mainly.Concerning clay minerals from different regions,there are different types and combinations.Compared with common clay stone,the mung bean rock is relatively high in Si O₂,Mg O and K₂O,and low in Ca O,Na₂O and Al₂O₃;compared with acid igneous rock,lower in Si O₂ and higher in Mg O and K₂O;compared with neutral and basic igneous rocks,higher in Si O₂ and K₂O,and lower in Na₂O,Al₂O₃ and Mg OLILEs in the mung bean rocks are comparatively rich and the REE distribution pattern in most areas is light REE enrichment.But there are different mung bean rocks in different areas.For instance,the mung bean rocks in the areas of Chongqing City and Sichuan Province,like Longmen cave,Tongliang,Wenquan Town etc.are high in Li while those in Dazhai and Longguang Town-—typical areas in Guizhou Province are low in Li.As a type of hydromica clay rock resulting from the transportation,deposition and alteration of volcanic ash,the original magma composition of mung bean rock must have an effect on its features of rock and mineral,and geochemistry.It is suggested in the study that the parent rocks of Longmen cave mung bean and those of the other areas of upper Yangtze should be classified as the subalkaline rhyolite dacite of the quartz andesite—the andesite series.Based on the study result of the structural environment of the rudimentary magma of mung bean,it is suggested that the tectonic settings of mung bean rock in the upper Yangtze region are chiefly continental margin arc and evolved oceanic arc.As the acknowledged study result shows,the formation of mung bean rock in the upper Yangtze region is probably a result of the tectonic activity of island arc and continental margin of Youjiang Basin,and on the other hand,the volcanic ash just came from Youjiang Basin.During the Middle Triassic,many sedimentary environments were in the upper Yangtze area.The mung bean rocks in Longmen cave,Tongliang,Beibei and Dafang areas of Chongqing was situated in the limited platform environment;those in Wenquan Town of Yubei,were in the shelf environment;those in Hubei,in the Chaoping lagoon environment;those in Longguang Town and Dazhai Town of Guizhou,in the open platform environment;those in Xixiang and Zhenba of Shanxi,in the deep-water shelf environment.Mung bean rocks in all environments are rich in Si O2 and Al2O3,and poor in MnO,Na₂O,P₂O₅ and TiO₂.According to the geochemical data of mung bean rock,the sedimentary environment of mung bean rock is analyzed.As shown by the study result,the redox condition of the deposit of Longmen cave mung bean rock was subject to the sub reduction environment,and the paleo salinity of the water body was in the brackish water environment,and the salinity changed from the brackish state to the salty gradually,and afterwards,it was the opposite process.In the deposition of the mung bean rocks in the upper Yangtze region,the oxidation-reduction conditions were mainly subject to sub oxidation and sub reduction,and the paleo salinity of the water body was generally brackish and salty,but there were different degrees of salinization in different areas above.