西秦岭北缘早古生代天水—武山构造带及其构造演化

Early Paleozoic Tianshui-Wushan Tectonic zone of the Northern Margin of West Qinling and its Tectonic Evolution

西秦岭北缘早古生代天水—武山构造带位于甘肃省东部天水地区,主要由寒武纪关子镇-武山蛇绿岩带、晚寒武世—早奥陶世李子园群浅变质活动陆缘沉积-火山岩系、奥陶纪草滩沟群岛弧型火山-沉积岩系以及加里东期岛弧型深成侵入岩体、俯冲-碰撞型花岗岩体等组成。关子镇蛇绿岩中变质基性火山岩属于N-MORB型玄武岩,武山蛇绿岩中变质基性火山岩属于E-MORB型玄武岩,是洋脊型蛇绿岩的重要组成部分,形成时代大致在534～489Ma之间的寒武纪。李子园群火山岩主要形成于岛弧或与岛弧相关的弧前盆地构造环境,草滩沟群火山岩形成于与俯冲作用相关的岛弧环境。关子镇流水沟和百花中基性岩浆杂岩总体形成于中晚奥陶世(471～440Ma)古岛弧构造环境,同时发育加里东期俯冲型(450～456Ma)花岗岩类和碰撞型(438～400Ma)花岗岩类岩浆活动。西秦岭北缘早古生代古洋陆构造格局经历了从洋盆形成-洋壳俯冲消减直至陆-陆碰撞造山的板块构造演化过程。总体构造演化可划分为四个阶段:①晚寒武世古洋盆初始形成阶段;②早奥陶世洋盆初始俯冲阶段;③中晚奥陶世洋壳大规模俯冲与古岛弧发育阶段;④志留纪陆-陆或陆-弧碰撞造山阶段。

Early Paleozoic Tianshui Wushan tectonic zone is located in the Tianshui area in the eastern Gansu province. It consists mainly of Guanzizhen-Wushan ophiolite belt in the Cambrian mesoto epi- metamorphic sedimentary-volcanic rock series at the active continental margin in the Late Cambrian-Early Ordovician Liziyuan Group, island-arc volcanic-sedimentary rock series in the Ordovieian Caotangou Group, plutonic intrusion and subduction collision granitoid in the Caledonian. Metabasic volcanic rocks in the Guanzizhen ophiolite belong to N-MORB, and metabasic volcanic rocks in the Wushan ophiolite belong to E-MORB. They are importance segments of the ocean ridge ophiolite, formed in between 534Ma to 489Ma of Cambrian. Volcanic rocks in the Liziyuan Group mainly formed in forearc basin setting with island-arc or island-arc-related, and volcanic rocks in the Caotangou Group formed in island-arc setting with subduction-related. Intermediate basic igneous complexes of Liushuigou and Baihua in the Guanzizhen formed in paleo-island-arc setting in the Middle-Late Ordovician （471-440Ma）, while had subduction type （450-456Ma） and collision type （438-400Ma） granitoids magmatism in the Caledonian. Early Paleozoic paleo-ocean-continent of structural framework in northern margin of West Qinling had come through evolution of plate tectonics from formation ocean basin to subduction of oceanic crust, until continentalcontinental collision orogeny. The overall tectonic evolution be divided into four stages. ① Paleo-ocean basin initial form stage in the Late Cambrian. ② Ocean basin initial subduction stage in the Early Ordovician. ③ Wholesale subduction and paleo island-arc growth stage in the Middle-Late Ordovician. ④ Continental-continental or continental-arc collision orogeny stage in the Silurian.