Abstract:With volcanic rocks of the Mesozoic Qingshan Group in the Xidong area in western Shangdong province as study objective, we carried out systematical analysis of elementisotope geochemical analysis on the volcanic rocks to understand source area of magmatic rocks and their petrogenesis. This study will expand the geochemical features of rock sequences in this area. SiO2 content of Qingshan Group volcanic rocks ranges from 528% to 598%, suggesting they belong to calcalkaline series. The rock assemblage consists of basaltic trachyandesite to trachyandesite. SiO2 shows negative relation to MgO, TFe2O3, MnO and TiO2, but positive to Al2O3 and Na2O. Large ion lithophile elements such as Rb, Ba, Th, U and K are enriched while high field strength elements such as Nb, Ta and Ti are depleted. The total REE ranges from 100×10-6 to 187×10-6, with (La/Yb)N =82～111 and no significant Eu anomaly (Eu=09~102). The volcanic rocks have homogeneous and enriched SrNd isotopic compositions with (87Sr/86Sr)t ratios of 07047～07050 and Nd(t) values of -165～-142. Nd and Sr isotopic compositions of Qingshan Group volcanic rocks in the Xidong area show no distinct negative relation to SiO2, Nb/Ta and La/Nb has no negative relation, and Nb and Th shows positive relation. All these features indicate that crust contamination had much influence on chemical compositions of magma of Qingshan Group volcanic rocks during the uprising and evolution process of magma. The fact that the volcanic rocks have higher MgO content than melt resulted from partial melting of basalt indicates that forming process of intermediatemafic volcanic rocks of the Qingshan Group was probably related to interaction between continental crust and mantle rock series. Integrated analysis indicates that the volocanic rocks of Mesozoic Qingshan Group in the Xidong area were the emplacement of partially melting continental lower curst materials onto the Earth surface, which were firstly metasomatized with upper mantle rocks in the mantle environment due to delamination.
