中亚造山带东段西拉木伦构造带的性质与演化：来自变形和低温热年代学的约束

The evolution of the Xar Moron tectonic belt in the eastern Central Asian Orogenic Belt: Constraints from evidences of deformation and low-temperature thermochronology

关键词:  中亚造山带  西拉木伦构造带  蛇绿混杂岩  低温热年代学  双井片岩

Key words:  Central Asian Orogenic Belt (CAOB)  Xar Moron tectonic belt  ophiolitic mélange  low-temperature thermochronology  Shuangjing schist

摘要:

中亚造山带东段何时与何地关闭，从俯冲到关闭的过程以及随后的陆内演化又经历了什么主要事件，目前还存在不同认识。中亚造山带东段林西地区的蛇绿混杂岩及其周围地区的区域地质调查表明，以杏树洼蛇绿混杂岩和双井片岩为代表的西拉木伦河构造带是一个晚古生代的增生楔，在该混杂岩带中发育了典型的岩块被包裹在基质中的构造。该楔体被中、晚二叠世克德河砾岩所覆盖。增生楔中最早的近东西向构造代表了向南俯冲阶段的变形，随后继续经历向北的逆冲推覆，卷入了中、晚二叠世地层，形成了碰撞期的变形；在晚二叠世末期—三叠纪早期，蛇绿混杂岩以及上覆的克德河砾岩又经历了区域性的强烈的右行韧性剪切，并发生应变分解。晚二叠世区域性的右行韧性剪切在中亚造山带南缘普遍发育，代表了中亚造山带已经全部进入陆内环境。双井片岩也经历了与蛇绿混杂岩类似的变形事件，在增生楔下部经历变质作用，并在碰撞期抬升至地表，晚期为区域性的右行剪切。同时，结合锆石与磷灰石低温热年代学测试表明，双井片岩和蛇绿混杂岩共同经历了中、晚侏罗世源自北侧蒙古-阿霍茨克大洋关闭导致的近南北向挤压、早白垩世期间遍及东亚的区域性伸展以及晚白垩世短暂的构造反转事件。

Abstract:

It is controversial that when and where the eastern part of the Central Asian Orogenic Belt was closed. The regional geological survey results of the ophiolitic mélanges and surrounding assemblages in Linxi area in eastern part of the Central Asian Orogenic Belt show that the Xar Moron tectonic belt, represented by Xingshuwa ophiolitic mélange and Shuangjing schist, is a late Paleozoic accretionary wedge in which the typical block and matrix structures are developed. The accretionary wedge, covered by the Middle to Late Permian Kedehe conglomerate rocks, has experienced three stages of deformation:including the earliest deformation indicated by the EW-trending structures formed during the southward subduction stage, the northward thrusting deformation in which the Middle and Late Permian strata were involved, and the deformation in the collision stage. From the Late Permian to the Early Triassic, the ophiolitic mélanges and overlying Kedehe conglomerate rocks underwent strong regional dextral ductile shearing and strain partition. The Late Permian regional dextral ductile shearing zone developed on the southern margin of the Central Asian Orogenic Belt, indicating that the Central Asian Orogenic Belt had already entered the intracontinental stage by that time. The Shuangjing schist experienced deformations similar to the ophiolitic mélanges, and the lower part was metamorphosed and uplifted to the surface during the collision period. Combined with zircon and apatite low-temperature thermochronologyical date, it is conclude that the Shuangjing schist and the ophiolite mélanges have experienced NS-trending compression caused by the closure of the northern Mongol-Okhotsk Ocean in the Middle to Late Jurassic, and the regional extension occurred throughout East Asia during the Early Cretaceous and the short-term tectonic inversion occurred in the Late Cretaceous.