北祁连奥陶纪抠门子组火山岩特征及构造意义

Characteristics and Tectonic Significance of the Volcanic Rock in Ordovician Koumenzi Formation,North Qilian Orogen

青海省祁连县武松塔拉一带发育奥陶纪抠门子组,以构造岩片形式与周围地层呈断层或不整合接触,在抠门子组下段发育一套岩浆喷发序列组成的中-基性火山岩建造,岩石类型以安山岩、玄武岩为主,其次为英安质、安山质及玄武质火山碎屑岩。经过野外调查和岩石化学分析表明:安山岩总体为高铝富钠的碱性火山岩系列,其微量元素和稀土元素分配型式具有岛弧玄武岩特征(VAB);玄武岩总体为次铝富钠碱性火山岩系列,其微量元素和稀土元素分配型式和混染的大洋玄武岩(MORB)特征相似。抠门子组火山岩具有2种截然不同的岩石地球化学特征,构造环境分析具有混染的大洋盆地和岛弧双重属性,这些大洋玄武岩及岛弧火山岩的出现标志着中—晚奥陶世北祁连洋壳的俯冲消减事件,该火山活动为北祁连早古生代构造演化研究提供了依据。

The Ordovician Koumenzi formation ists as a tectonic slice confined by ductile and located in south Wusongtala of Qilian county ex- brittle faults, and enclosed in surrounding strata. The lower unit of this formation is composed of a set of intermediate-mafic volcanic layers bearing a clear magmatic eruption sequence, with lithologies of andesite, basalt and less dacitic, andesitic and basaltic pyroclastics. Field investigation and geochemical analyses indicate that the andesite is high aluminum and sodium alkaline volcanic rock; its trace element and the REE characteristics exhibits oceanic island-arc basalt features （VAB）. Basalt combination in general belongs to poor aluminum and high sodium alkaline basalt series, which bears similarities to contaminated oceanic basalt （MORB） in terms of trace elements and the REE characteristics. The volcanic rocks of Koumenzi formation have two distinct lithogeochemical characteristics. Analysis on the tectonic setting show dual attributes of contaminated ocean basin and island arc. The occurrence of oceanic basalt and island arc volcanic rock implicate subduction event of oceanic crust of north Qilian in Middle-Late Ordovician. The volcanic activity record provides evidence for tectonic evolution of North Qilian in Early Paleozoic.