The Petrology And Litho-geochemistry And Geo-dynamic Significance Of Emishan Basalt In Mts Emei Of Sichuan Province

The Permian Emeishan basalts,which have been widely exposed in Sichuan,Yunnan,and Guizhou provinces,have attracted the attention of scholars at home and abroad since the 1970s.In particular,since the mantle theory of Emeishan basalts proposed,many scholars have conducted a lot of researches on Emeishan basalts from the perspectives of petrology,geochemistry and geophysics.Even though the research on the basalt in the Emei area,which is the concept of the Emeishan basalts,is relatively weak.Based on the field geological survey,this paper studies the petrology and geochemistry of the basalt in the Emei area,and discusses the origin,evolution characteristics of the magma and the tectonic setting of this area.In addition,the mineral resources associated with the Emeishan basalts are abundant.Through the collection and research of predecessor data,summerised the deposits which related with Emeishan basalts,and discussed the ore-forming methods of the Emeishan basalt native copper deposit,and accodingly divided some of the metallogenic prospects of this deposit.Through field geological surveys,it was found that the basalt joints in the Emei area are developed with columnar joints ranging in thickness from 200 to 400 m,which is the product of magmatic eruption in terrestrial environments.According to the characteristics of basalt production and distribution,the basalt in this area can be divided into three cycles of eruption and nine periods of overflow.In the early stage of the cycle,fine-grained basalt basalts are developed.The microcrystalline basalt develops in the middle stage of the cycle,and the cycle develops in the late stage.Stomatal-amygdalal basalt,in which the basalts in the area are mainly porphyritic basalts and cryptocrystalline basalts..The basalt types in this area include three series of microcrystalline basalt,stomata-almond basalt and porphyritic basalt.They are further classified under the microscope.Basalt includes series of metabasalites,porphyritic basalts,and mottled basalts.Basalt is the main type.The mineral composition is plagioclase,variable pyroxene,?titanium-rich?ordinary pyroxene,containing a small amount of dark minerals such as idling petrochemical olivine and titanium magnetite.According to the analysis of main trace elements in rocks,the main elements of basalt in this area are SiO₂=43.70⁵4.40%with an average of 49.01%;TiO₂=2.42⁴.29%with an average of 3.67%;Na₂O=1.78⁴.97%with an average of2.74%,K₂O=0.57².28%,with an average of 1.28%;Al2O3=7.50¹6.10%,with an average of 12.86%;MgO=3.09⁵.71%,with an average of 4.42%,and Mg^#=0.31⁰.47,with an average of 0.39.The AFM plot shows that the basalt type is mainly tholeiite with a small amount of calc-alkaline basalt.The basalt trace elements in this area are generally enriched with high field strength elements Th,U,Ta,Nb,Zr,Hf,etc.,deplete P,and Ce;enrich large-ion lithophilic elements Ba,loss K,Sr,Rb,etc.The range of Rb and Ba in the lithophile elements is relatively large,and the original mantle standardization is similar to OIB.Rare-earth elements enriched in light rare earths depleted heavy rare earths,and the normalized rare earths distribution pattern of spherulite rocks was similar to OIB in right-dip type,with weak Eu and Ce anomalies.The ratio of strong incompatible elements reflects that the magma source area is similar to the EMI OIB and EMII OIB.The La/Yb-Sm/Yb diagram shows that the magma originated mainly from partial melting of the garnet dihedral peridotite.The Sm-Sm/Yb diagram shows that the magma originated mainly from The garnet dihedral peridotite partially melts at a low degree of about 3 to 7%.The characteristics of the main elements,rare earth elements and trace elements of basaltic rock in the emei mountains indicate that the basalt formation in the region is formed in the internal environment of the plate,which is related to the role of mantle plume,and the basaltic magma originates mainly from the enrichment mantle garnet dihedral peridotite.The magma is in the process of evolution.It is subjected to a low degree of contamination and undergoes isolated crystallization of plagioclase,clinopyroxene,apatite,and the like.Compared with the basalts in the Ertan,Yuexi and Longzhoushan areas of Panxi,it is found that the basalts of the basalts in the Emei area and Panxi area have similarities.Based on the results of this study and previous research on Panxi Rock,we found that the Panxi Rock area possesses the characteristics of the continental rift and mantle plume,combined with the tectonic evolution characteristics,dating data,and petrology of the area.It is believed that the Panxi Rock area is the result of the joint action of plate subduction and mantle upwelling.The Panxi rift was not further developed into an ocean basin because of the collapse,and the basalt in Emei area is the product of this background.The mineral resources associated with the Emeishan basalt are abundant,mainly including vanadium-titanium magnetite,copper-nickel-platinum magma sulfide deposits directly related to magmatism,and basalt native copper ore,lead-zinc polymetallic ore,and micro-fines indirectly related to magmatism.Pulse invasive gold ore and other minerals.However,the basalt native copper deposits are weak in prospecting,combined with regional geological data,geophysical chemical characteristics,and previous research,the summary of their metallogenic methods is considered,and it is believed that in the Ganluo-Meigu-Zuojiao area of Sichuan,the Xuanwei area and Lijiang area in Yunnan,and the Liupanshui area and Weining in Guizhou have the potential to search for basalt native copper deposits.