Eruption Stages And Evolution Process Of Basalt In Penglaimiaoshan

Miaoshan basalt is a small volume of Penglai volcanic basalt?There are a lot of researches on Penglai basalt by predecessors,but most of them focus on the geochemical composition,and the study of Penglai basalt profile only rests on the large-scale profile observed in the field,while the causing of iddingsite alteration of olivine in the basalt and the study of the small-scale profile is relatively less,the change rule of chemical composition and Textures between different periods from morning till night and the relationship between them are not clear.Therefore,the spatial-temporal evolution of Miaoshan basalts is limited by the heterogeneity of rock texture and geochemistry and the relationship between them.According to the presence or absence of pores and loess layer in the field Miaoshan basalt profile,the 21 m thick of basalt section is divided into two eruption period,and according to the change of porosity content,black lava slag and red weathering layer in the profile,the Second stages are divided into five small eruption stages.Through the petrographic observation,crystal size distribution analysis and geochemical composition analysis of Miaoshan basalt,it is found that the particle size of olivine phenocryst and matrix magnetite in Miaoshan basalt pgase of the eruption I is smaller than that in pgase of the eruption II,and the trace element ratio of Sm / Yb,Nb / Y,Ba / Th in period I is smaller than that in period II,and the trace element ratio of La/ Sm is larger than that in period II,which is obviously distinguishable.Therefore,it is speculated that Miaoshan basalt period I and period II come from different magma chambers.According to the classical semi logarithmic curve,the residence time of magma chamber in period I is 35 years,and the residence time of magma chamber in period II is 52-59 years.In the second eruption period of time,the particle of magnetite varies greatly(the particle of magnetite at the top is smaller than that at the bottom).In combination with the change of porosity content in the field profile(the porosity content at the top is smaller than that at the bottom),it is considered that when the magma at the bottom ofthe second period erupts to the ground,the bubbles in the magma are not discharged,so there are many volatilities,resulting in the decrease of the undercooling of the magma,and the magnetite in the magma continues to grow,while the top of the porosity content is less,less volatile,leading to the rapid cooling and crystallization of magnetite in lava.In terms of geochemical composition,the Sm / Yb,Nb / Y,Nb / U and other trace element ratios at the top of period II are larger than those at the bottom,and the La / Nb,Ce / Pb ratios are smaller than those at the bottom.It is considered that the melting degree of Miaoshan basalt source area which is not contaminated by the crust is not uniform or the material in the source area itself is heterogeneous.At the same time,the particle size of magnetite from the bottom to the top of the second period is smaller,the Sm / Yb,Nb / Y,Nb / U trace element ratio is larger,and the La /Nb,Ce/ Pb trace element ratio is smaller,while the size of olivine is similar between different stages of the second stage.It is considered that the composition of magma melt has no effect on the early olivine phenocryst,but it will have a certain effect on the crystallization process of the matrix magnetite in the late magma,or there is a close relationship between them.Both the magma composition and the undercooling of magma can influence the particle size change of matrix magnetite,so the change of porosity content in Miaoshan basalt section can represent the change rule of magma composition to some extent.