Geochemical Characteristics And Petrogenesis Of The Dashibao Formation Basalts In Shimian,Sichuan

The research on Emeishan basalt has been yielded rich fruits in recent years,however,the Dashibao Formation basalt on the western margin of the Yangtze plate is relatively weak to report,which is mainly concentrated in the Danba and Baoxing areas far from the typical Emeishan basalt.This paper focuses on the petrological and geochemical characteristics of the Dashibao Formation basalt exposed in the Shimian area.Combined with the latest relevant chronological datas,this paper further analyzes the evolution,origin,source properties and the tectonic setting of the magma.Compared with the typical Emeishan basalt,the relationship is briefly discussed between the formation of basalt and the evolution of mantle plume.The Dashibao Formation basalt is distributed in a strip shape in Shimian,Baoxing,Danba,and Jiulong,which thickness shows a thinning trend from the middle to the north and south.Those basalts are combined with basic volcanic lava and basaltic-tuff lava from the seabed that can be roughly divided into six eruption cycles.The interbed of argillite can be founded in those cycles,which demonstrate multi-cycles eruption-overflow of submarine eruption.Those basalts belong to low-titanium basalt with a low content of TiO2(1.58%~2.34%),and also the value of Mg#is 50.56 to 59.67,with high contents of MgO and FeOt,low contents of P2O5 and Al2O3.The petrochemical classification shows that those basalts are mainly tholeiite with a small amount of alkaline basalt.Right deviation type of rare earth distribution patterns reveals obvious enrichment of light rare earth(LREE),light and heavy rare earths are distinctly differentiated,which fractionation is high(LREE/HREE=6.01~7.71,(La/Yb)_N=8.51~12.58).And the weaker Ce and Eu are negatively abnormal;the large ion lithophile elements and high field strength elements are enriched with varying degrees,the compatible elements(Cr,Co,Ni)are depleted,the Sr verifies an obvious negative anomalies,and the elements of Nb,Ta,Hf,Ti are depleted in varying degrees,which demonstrate the same geochemical characteristics to OIB and they are formed in intraplate environment.During the process of magma evolution,basalt had experienced the separation and crystallization of clinopyxene,plagioclase and a small amount of olivine.As the ascending migration process,it was subjected to varying degrees of crustal contamination,which the ratio of incompatible elements has confirmed as La/Nb(0.98 to 2.44),Zr/Nb(4.59 to 17.22)and Th/Nb(0.13 to 0.4).The characteristics of the source region show different degrees of mixing of the multi-terminal components,and tend to be close to the EM-?type OIB endmember that undergoes a partial melting of the lower degree(7%to 12%)of the garnet stable region.Compared with the typical Emeishan basalt in the adjacent area,the petrographic and main trace elements geochemical characteristics are similar to the Emeishan low-titanium basalt.But the source area of the marine Dashibao Formation basalt in the Songpan Ganzi block may have contributed from fluid replacement and enrichment of the subducted plate named the continental lithospheric mantle(SLCM),which indicates that there is lateral heterogeneity in the basalt source area.At the same time,the latest relevant chronological data(260.7±4.3Ma)in the study area is basically consistent with the eruption period of the main basalt in the Emeishan Large Igneous Province,which indicates that the Dashibao Formation basalt in the Shimian area is a product of the mantle plume activity.Based on the latest mantle plume theory model,the formation conditions of the Dashibao Formation basalt in the Shimian area are similar to those of the typical Emeishan basalt.They are both located at the head of the mantle plume,which is the result of the interaction between the Emeishan mantle plume and different parts of the lithosphere.The continental Emeishan basalt and the marine Dashibao formation basalt shaped as the magma spilling after the activity of the Emeishan mantle plume that led to the thinning of the lithosphere,the cracking of the western part of the Yangtze plate and the opening of the Ganzi-Litang ocean basin.