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分布于甘肃北山北亚带西缘(双沟山—马鞍山北一带)的泥盆纪火山岩主要为一套浅变质的安山岩、英安岩、流纹岩,局部见少量玄武岩。岩石硅碱成分显示为钙碱性系列,钠质类型;稀土总量为(65.48×10-6～196.69×10-6),属轻稀土富集型,稀土配分型式为右倾曲线;微量元素配分型式类似于岛弧及活动大陆边缘特征,具明显的LlLE富集和Nb、Ti等的亏损;同时,从基性到中性熔岩的εNd(t)均为正值(6.46～6.59),暗示其源区类似亏损地幔源区。Y-Nb和Rb-(Y+Nb)构造判别图解显示本区中酸性火山岩形成于岛弧环境。综合分析火山岩的岩石学、地球化学特征,并结合区域构造背景,认为该套火山岩产于活动大陆边缘岛弧带环境,属俯冲型火山作用,形成于俯冲造山带的构造背景。
The Devonian volcanic rocks located in the western margin of the Beishan Northern Belt of Gansu (Shuanggou Mountain - Ma’anshan Bei area) are mainly composed of a series of super-metamorphic andesite, dacite and rhyolite, with few basalts locally. The contents of soda and siliceous rocks show as calc-alkaline series and sodium type. The total amount of rare earths is (65.48 × 10-6 ~ 196.69 × 10-6) The patterns are similar to those of the island arc and active continental margin, with obvious enrichment of LlLE and depletion of Nb, Ti, etc. At the same time, εNd (t) from basic to neutral lava are positive (6.46-6.59) Its source area is similar to the depleted mantle source area. Y-Nb and Rb- (Y + Nb) tectonic discrimination diagrams show that the acidic volcanic rocks formed in the island arc environment. Based on a comprehensive analysis of the petrological and geochemical characteristics of volcanic rocks and the regional tectonic setting, it is believed that this volcanic rock is produced in the active continental marginal island arc environment and belongs to the subduction volcanism and formed in the tectonic setting of the subducted orogenic belt.