The Relation Of Hannuoba Basalts' Structure And Chemical Multiple Characteristics

Different types of basalts with different compositions often occurred alternately in the profile of Hannuoba basalts.Although their evolution mechanism and cause of formation have been repeatedly discussed by predecessors in the aspect of geochemistry,no relation has been established in terms of their composition and texture,and no unified understanding has been reached so far.In this paper,a basalt profile with a thickness of about 250m in Qingyanggou area of Zhangbei was selected for comprehensive analysis of quantitative rock texture and geochemical.Firstly,according to the length of the long axis of most plagioclase particles?>50%?in the sample,the 17 basalt samples from Qingyanggou were divided into three structural types:fine grained basalts?most plagioclase grains are less than 0.3mm?,coarse graind basalts?most plagioclase grains are more than 1mm?and medium graind basalts?most plagioclase grains are between 0.3-1mm?.Enclaves were present in all samples of fine graind basalts and one sample of coarse graind basalts.It was found that the alkalinity of the three types of samples decreased successively in terms of chemical composition.The fine grained basalts,coarse grained basalts and medium grained basalts were respectively highly alkaline?P₂O₅/TiO₂>0.27,Nb/Y is between 2.66 and 4.03,Sm/Yb is between 5.58 and 11.49?,moderately alkaline?P₂O₅/TiO₂ is between 0.23 and 0.27,Nb/Y is between 1.65 and 2.46,Sm/Yb is between 3.98 and 5.14?and low-alkaline?P₂O₅/TiO₂<0.23,Nb/Y is between 0.83 and 1.48,Sm/Yb is between 3.05 and 4.15?.The basalt profile of Qingyanggou was divided into five stages according to the composition change rule.In different stages,from morning to night,the ratio of major elements and trace elements had the same change of increasing or decreasing?P₂O₅/TiO₂,La/Sm,Ce/Pb ratio decreased,and Ba/Th ratio increased?.A complete production stage is a sequence of fine grained basalts,coarse grained basalts and medium grained basalts.There is a phenomenon of continuous production of each type of sample.According to the isotopic data of Hannuoba basalts analyzed by predecessors and the ratios of trace elements of Hannuoba basalts in this study,the different sources of fine graind basalts and medium graind basalts with large differences in composition were determined.The ratio of trace elements in medium graind basalts varied widely,reflecting the heterogeneous composition in the source region of medium graind basalts.According to the production relationship of the three types of samples in the Qingyanggou basalt profile,that is,coarse graind basalts often appear between fine graind basalts and medium graind basalts.It is inferred that the coarse graind basalts,whose composition are between fine graind basalts and medium graind basalts,were probably formed by crystallization and evolution after the mixing of the two magmas.According to the crystal size distributions?CSDs?,texture coarsening occurred in coarse graind basalts.The average sizes of plagioclase?equal to the volume content weighted average of characteristic length or characteristic length?in the samples are related to the main element and trace element components.The coarser the average size of the sample,the closer its composition is to the fine graind basalts,and the finer the average size,the closer its composition is to the medium graind basalts.