北祁连山俯冲杂岩带的构造演化

TECTONIC EVOLUTION OF SUBDUCTIVE COMPLEX BELTS IN THE NORTH QILIAN MOUNTAINS

北祁连位于华北克拉通西部阿拉善地块与中祁连—柴达木泛地块之间，是我国最具特色的大陆造山带之一。带内发育有震旦纪—中寒武世的裂谷火山岩、晚寒武世—奥陶纪蛇绿岩、中晚奥陶世岛弧火山岩、晚奥陶世弧后拉张盆地火山—沉积岩、志留纪残余海盆相复理石和泥盆纪山间磨拉石等。中间夹两条变质和变形特征不同的加里东期俯冲杂岩带：南带为深层俯冲，北带为浅层俯冲杂岩；这两条杂岩带可能形成于同一俯冲带的不同深度。俯冲杂岩带中岩石学、变质和变形特征揭示北祁连山经历了裂谷—大洋化—俯冲—弧陆碰撞—造山的构造演化历史。

The North Qilian Mountains are located between the Middle Qilian Qaidam Massif and the Alaskan Massif which is the west part of Sin korea Craton. It is one of the most typical continent orogenic belt in China. In this orogenic belt, there are rift volcanic rocks from Sinian to Middle Cambrain, ophiolites from late Cambrain to Ordovician, island arc volcanic rocks from middle to late Ordovician, extensional basin volcanic sedimentary rocks of late Ordovician, residual sea basin flysh of Silurian and Devonian molasse. The ophiolites, which mainly distribute in three belts, show similar geochemical characteristics( except for Dachadaban ophiolite), with the tendency of becoming younger from south to north. In Sunan Qilian region, the middle part of the North Qilian Mountains, two different Caledonian subductive complex belts, namely, the southern deep level subductive complex belt and the northern shallow level subductive complex belt, occur parallelly. The southern belt consists of trench olistostrome and graywacke, ophiolitic blocks,and rift volcanic rocks, etc., with some been transformed into eclogites and high grade blueschists by high pressure metamorphism and distributing as three tectonic slices in the deep level subductive complex belt. Deformation and metamorphic analysis suggest not only a clockwise, retrogressive P T t D path, but also a kinetic variation of plate movements from subduction to strike slip shear to upthrusting. The northern shallow level subductive complex belt consists of graywacke, ophiolite and low grade blueschists, mixed with some island arc volcanic rocks. The P T condition of low grade blueschists is 150～250℃, 0.4～0.7GPa, reflecting high pressure metamorphism at shallow level or anchi surface. The two complex belts were probably formed at different depth of the same subduction zone, and the characteristics of petrology, metamorphism and deformation reveal a tectonic evolution history of transition from rift to oceanization to subduction to arc continent collision to orogenesis.