Basic Characteristics And Petrogenesis Of Emeishan Basalts In Dafang Area,Guizhou Province

ELIP is divided into inner,middle and outer parts.The Emeishan basalts in Dafang area,Guizhou province is located in the ELIP.As an important part of Emeishan basalts,it attracts many scholars.However,the researches on outer parts of Emeishan basalts in Guizhou Province are few.This article is based on fieldwork,throughstuding the rock characteristics,mineralogical characteristics and geochemical characteristics of the Emeishan basaltsin Dafang area.It does a main contrast with the adjacent Shuicheng basalts analyzes the rock characteristics,mineralogy and geochemical characteristics of Emeishan basalts in Dafang area,and discusses the characteristics of the magma source,the temperature and pressure of rock formation and the tectonic background of basalts.Through the spatial and temporal distribution,rock type and lithology characteristics of Emeishan basalts in Dafang areaa identified by field geological survey and profiling,the eruption environment,the eruption cycle and the rhythm are studied.even Yingpan and Zhuchang are not far apart,there are obvious differences in the cycle of volcanic eruption,the rhythm of eruption?times?,the accumulation thickness of basalt rocks,and the thickness of the limestone between the basalts,which indicates that the Emeishan basalts of the Permian in the region is not only horizontal but also varies greatly in vertical level,andthe distribution is not very stable.The basalts is dominated by large,thick layers,no pillow lava,in the volcanic aggregates breccia,grey,brown and gray volcanic debris are common,and the partial sedimentary pyroclastic layers of volcanic rock have been produced by existing bivalve fossils,plant fossils and a large number of plant stems.All these characteristics indicate that the basalt in general should be formed in water eruption environment.The analysis of the characteristics of basalt rock type,rock combination and the production characteristics of limestone interlayer in the field profile?PM11?and the high storehouse section?PM38?in the small town of Yatun area?PM11?.The Permian volcano eruption in the study area is divided into three cycles of eruption or eruption cycle,and the third cycle is the main eruption.The three magmatic eruption cycles in the basalts section of ZhuchangXiaotun township in Dafang county can be divided into nine eruptive rhythms;However,the basalts section of Yingfang Gaodian townshiphas only two magma cycles,and the formation of volcanic rocks is a cumulative thickness,whicf can be divided into 14 eruptive rhythms.The rock type mainly has microcrystalline basalt,cryptocrystalline basalt,almond-shaped basalt and basaltic tuff and breccia.The basalts' rock-forming mineralin this area are plagioclase and clinopyroxene,and the secondary minerals are magnetite,ilmenite,Sprite,etc.The basalts have the intersertal texture,intergranular texture,hyalopilitic texture,etc.Amygdala structure develop locally.Through a single mineral electron probe analysis,clinopyroxene is mainly common augite.The group is divided into Wo=37.45⁴0.80,En=36.54⁴2.89,Fs=17.06²3.22;the plagioclase is mainly a long stone and a small quantity of feldspar,and the group is divided into An=41.34 58.71,Ab=38.79 54.29,Or=1.55¹0.06;the metal minerals are mainly titanium ore,followed by magnetite and titanium magnetite.The main volume element characteristics display SiO2 is between 46.42%and 51.28%,the average is 48.70%;TiO₂ is between 4.07% and 4.66%,the average is 4.34%,whichis titanium basalt.The basalts K₂O+Na₂O average is 3.33%,the total alkali content is not high;Most of the rocks Rittmann index is less than 3.3,and the majority between 1.02 and 2.93,the average is 2.15,is calcium alkaline rock;Mg^# is between 37.20 and 51.70,the average is 43.01;m/f is between 0.50 and 0.90,theaverage is 0.64,which is an iron bastic rock.The AFM diagram shows the series of basalt in Laban.The CIPW standard mineral is calculated.The basalt standard mineral combination is mainly: Basalt standard mineral combination,belonging to SiO₂ supersaturated normal rock type.The basalt has an obvious characteristic of rich light rare earth.The degree of fractional distillation of light rare earth is higher,and there are geochemical features similar to the OIB.In the process of magma evolution,the crystallization of the plagioclase and the removal of the plagioclase result in the loss of the Eu in the residual molten pulp,and the trace elements and isotopic features show that the source region of the Emeishan basalts in Dafang area is the enriched mantle related to the upper crust component,Magma is strongly mixed with the material of the earth's crust during the ascent.The geochemical data of rocks and the structure of basalt tectonics show that the formation of basalts in Dafang area are the environment of the continental plate,and its formation is related to the Emeishan mantle plume activity at the end of the maokou formation.Contrasting with the nearby Shuicheng area,the main mineralogy of basalts in Dafangand Shuicheng area are both pyroxite and plagioclase,and the pyroxite is a common whist,the main genus of plagioclase is a long stone,but the Emeishan basalts in Dafang area is not of olivine,while the Shuicheng basalts contain a small amount of olivine?<5%?.According to the estimation of single clinopyroxene thermometer to generous region of Emeishan basaltsin Dafang area formation temperature is between 1110?and 1253?,the average is 1162?,and the formation of Shuicheng basalts,temperature range is between 1131?and 1231?.The formation temperature of basalts in the two regions is not quite different;through the analysis of geochemical characteristics of the earth,we can see the Emeishan basalts in Dafang aera is tholeiite series,and the Shuicheng basalts is alkaline basalt series.The obvious different of Permian basalts chemical properties may be mainly caused by the location,magma source area quality,degree of partial melting,different process of crustal contamination and magmatism.