Mixing Of The Shanwang Alkaline And Tholeiitic Magmas:Evidence From Whole Rock Compositions And CSD

The Cenozoic basalts exposed in Shandong Province are highly alkalic basalts,slightly alkalic basalts and transitional basalts.The compositional characteristics of transitional basalts are between highly alkalic basalts and slightly alkalic basalts.Previous studies have focused on the interpretation of the genetic mechanism and source lithology of the alkaline basalts in Shandong Province,with only a qualitative description of the rock structure and less mention of tholeiitic basalts.In this paper,Shanwang basalts of Yaoshan of Shan-Wang in Linqu-Changle area are selected as the research object,and it is proposed that there are alkali basalts along the slightly alkalic basalts,tholeiite and transitional basalts in Shanwang Yaoshan.Different from the former transitional basalts classified by highly alkalic basalts and slightly alkalic basalts,the compositional characteristics of shanwangyaoshan transitional basalts are between slightly alkalic basalts and tholeiites.In this paper,combined with quantitative textural analysis and whole rock compositions,the genetic mechanism of Shanwang Yaoshan transitional basalt is revealed.Shanwang Yaoshan basalts belong to the slightly alkalic basalts defineed by predecessors.In terms of major elements,they are high SiO2(43.7-48.5 wt%),low P2O5(0.36-0.71 wt%),and low TiO2(1.82-2.62 wt%).In terms of major elements,they are high SiO2(43.7-48.5 wt%),low P2O5(0.36-0.71 wt%),and low TiO2(1.82-2.62 wt%).The distribution curve of alkalic basalt has the highest distribution curve,tholeiite has the lowest distribution curve,transitional basalt is located in the middle,which shows negative K and Pb anomalies,positive Sr anomalies.Generally,Rb to Ta increases,Ta to Lu decreases.Shanwang transitional basalt are low MgO(4.2-5.7 wt%),high SiO2(46.9-47.7 wt%),and high Al2O3(17.6-18.0 wt%),low Ni(52-84ppm),low Cr(55-100ppm),incompatible element ratios La / Yb(16.5-18.22),Sm / Yb(3.5-3.7),La / Sm(4.64-4.93)are all between alkaline and tholeiite.In terms of rock structure,the transitional basalt is seriate inequigranular texture,the size of plagioclase is 0.1-4mm,tholeiite is of porphyritic structure,and the size of plagioclase as matrix is less than 1.2mm.Quantitative texture analysis shows that the CSD of plagioclase of transition basalt is bending and continuous F-CSD,and plagioclase of tholeiite is S-CSD.The alkalic basaltic magma is continuously supplied to the magma chamber of tholeiitic magma,resulting in the multi-stage coarsening of plagioclase and the fractional crystallization of olivine and clinopyroxene,and finally forming the shanwangyaoshan transitional basalt.