内蒙古西乌旗地区大石寨组火山岩时代及地球化学特征

西乌旗地区大石寨组火山岩以流纹岩为主,LA-ICP-MS锆石U-Pb同位素测定结果及全岩主量、微量和稀土元素分析结果表明,本区流纹岩的年龄为280.3Ma±1.4Ma,形成于早二叠世;大石寨组火山岩属高钾过铝质钙碱性火山岩,其中SiO2、K2O、Al2O3含量均较高。稀土元素配分曲线总体向右缓倾斜,轻稀土元素相对于重稀土元素富集,Eu呈明显的负异常,重稀土元素亏损不明显。Rb、Th、U、K等大离子亲石元素富集明显,Ba表现为弱亏损,高场强元素具有明显的Ta、Sr、P及Ti亏损,Zr、Hf相对富集。研究认为,该地区流纹岩可能是由玄武岩、安山岩等中基性火山岩分离结晶作用形成的,且形成的构造环境为大陆边缘弧。

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.