大兴安岭中部柴河——蘑菇气地区早白垩世中性火山岩岩石成因及构造背景

Genesis and tectonic significance of the Early Cretaceous intermediate volcanic rocks in Chaihe-Moguqi area,central Da Hinggan Mountains

大兴安岭中部柴河—蘑菇气地区早白垩世中性火山岩岩相学研究表明,其主要岩石组合为安山岩、英安岩、安山质晶屑凝灰熔岩、凝灰角砾熔岩等,LA-ICP-MS锆石U-Pb测年结果显示其形成于140~130Ma之间。岩石主量元素具有富钾钠、富铝,贫镁的特点,属高钾钙碱性岩石。相对富集轻稀土元素、亏损重稀土元素,无明显的Eu异常,微量元素主要富集大离子亲石元素(如Rb、K、LREE)和Ba、Sr元素,亏损高场强元素(Ta、Nb、P、Ti)。地球化学特征指示,这些早白垩世中性火山岩来源于新元古代和古生代期间新增生的下地壳玄武质岩石的部分熔融,并可能受到早先俯冲事件带来的流体的影响,形成于蒙古-鄂霍茨克洋闭合陆壳加厚之后的岩石圈伸展环境。

In this paper, the authors studied the Early Cretaceous intermediate volcanic rocks in Chaihe- Moguqi area, central Da Hinggan Mountains. The intermediate volcanic rocks consist of andesite, dacite, intermediate lava and tuff, formed from -140 to -130Ma. Chemical analyses show that the intermediate volcanic rocks are classified as high-K calc-alkaline volcanic rocks, and are characterized by enrichment of light rare earth elements （LREEs）, large ion lithophile elements （LILEs）, Ba, Sr, and depletion of heavy rare earth elements （HREEs）, high field strength elements （HFSEs）, with no distinct Eu anomalies. All the geochemical characteristics suggest that the Early Cretaceous intermediate volcanic rocks were derived from the partial melting of a juvenile lower crustal basaltic rocks, and might have been affected by the fluid caused by earlier subduction event. The authors hold that the generation of the Early Cretaceous intermediate volcanic rocks was related to the lithospheric extensional setting after the final closure of the Mongol-Okhotsk Ocean and subsequent continental thickening.