东北大兴安岭北段蛇绿岩的时空分布及与区域构造演化关系的研究

Spatiotemporal distribution of ophiolites in the northern Great Xing'an Range and its relationship with the geotectonic evolution of NE China

大兴安岭北段作为中亚造山带东段重要组成单元,该区域的构造背景一直悬而未决,它涉及兴安与松辽地块基底属性、额尔古纳地块亲缘性以及三者之间的拼贴位置、时限和方式等众多科学问题。蛇绿岩作为研究造山带的重要工具之一,不仅是古板块或微地块的重要边界,而且是认识地幔组成和壳幔演化的重要窗口。然而大兴安岭北段蛇绿岩却鲜有报道。根据近年来野外工作及结合前人研究成果,文章探讨了蛇绿岩形成时代及构造环境。研究表明:(1)大兴安岭北段蛇绿岩整体呈北东向展布,但时代跨越较大,主要包括新元古代、早奥陶世—志留纪、石炭纪—二叠纪;(2)大兴安岭北段蛇绿岩均有E-MORB地球化学特征,其中新元古代的蛇绿岩明显亏损高场强元素Nb、Ta,而晚古生代的蛇绿岩则相对富集高场强元素;(3)大兴安岭北段新元古代蛇绿岩可能形成于洋内岛弧环境,记录着前寒武纪陆-弧碰撞信息,而早奥陶世—志留纪蛇绿岩可能与早古生代弧-盆体系演化相关。尽管上述认识为大兴安岭北段古生代的地球动力学背景研究提供了新的依据,但大兴安岭北段蛇绿岩仍存在较多问题亟须解决。

Located in the eastern segment of the Central Asian Orogenic Belt(CAOB),the northern Great Xing’an Range holds answers to many key questions concerning the tectonic setting of the region.Among the controversies is the debate on the basement composition of the Xing’an and Songliao blocks.The key questions are:What is the tectonic affinity of the Erguna Block?Where and how are the three blocks connected to each other?And when exactly were the connections made?As is known,ophiolites not only represent allochthonous fragments of upper-mantle and oceanic crustal rocks,but also is an important tool for studying mantle composition and crustal-mantle evolutionary history.However,ophiolites in the region have not been well studied.In this paper,together with results of previous researchers,we present the following findings:(1)The ophiolites were developed in different eras,including the Neoproterozoic,Early Ordovician-Silurian and Carboniferous-Permian in a NE trending zone;(2)All ophiolites have the geochemical feature of E-MORB,with Neoproterozoic ophiolites depleted in Nb,Ta and Ti while Late Paleozoic ophiolites enriched in high field-strength elements;and(3)Neoproterozoic ophiolites may be related to intraoceanic accretionary complex marking the suture of an arc-continent,whereas Early Paleozoic ophiolites were developed in a back-arc basin setting.Although we have made significant contrition to the understanding of the geodynamic background of the northern Great Xing’an Range,more systematic studies on ophiolites are needed to resolve many outstanding issues.