内蒙古西乌旗地区大石寨组火山岩时代及地球化学特征

内蒙古西乌旗地区大石寨组火山岩时代及地球化学特征

Age and geochemical characteristics of the Dashizai Formation volcanic rocks in Xi Ujimqin Banner,Inner Mongolia

The Dashizai Formation volcanic rocks in Xi Ujimqin Banner area consist mainly of rhyolite. LA-ICP-MS zircon U-Pb dating and whole-rock major, trace and rare earth elements analyses show that the age of the rhyolite in this area is 280.3Ma±1.4Ma, implying early Permian. Dashizhai volcanic rocks belong to high potassium peraluminous calc-alkaline volcanic rocks, and the content of SiO2, K2O and Al2O3 is high. Chondrite-normalized REE patterns are generally right-oblique with the enrichment of the LREE, Eu exhibits negative anomalies, there exists no significant loss of HREE, Rb, Th, U, K and other LILE elements show significant enrichment, Ba exhibits weak losses, HFSE exhibit obvious Ta, Sr, P and Ti losses, and Zr, Hf shows relative enrichment. It is held that the rhyolite of this area was probably formed by fractional crystallization of basalt, andesite and other basic volcanic rocks, and the tectonic environment for the formation of the rhyolite in this area was continental marginal arc.