祁连地块前寒武纪基性变质岩的原岩属性、变质作用PTt轨迹和构造演化研究

The Qilian Orogen, located in the northeastern part of the Qinghai-Tibet Plateau, at the junction of the North China craton in the northeast, the South China craton in the southeast, and the Tarim craton in the northwest, is a part of the northern boundary zone of the Proto-Tethys Ocean. The Qilian block, located in the Qilian Orogen, is thought to represent a Precambrian basement unit related with the aggregation-breakup of the Rodinia supercontinent and the opening-closure of the Proto-Tethys Ocean. The tectonic affinity and metamorphic evolution of the Qilian block have been controversial, e.g., whether there are material records on the Neoproterozoic mantle plume activity, the basement rocks whether experienced high-pressure metamorphism during the Paleozoic oceanic crust subduction-collision, and the spatial distribution regularities of metamorphic zones in the block. In this study, the mafic metamorphic rocks in the Qilian block were chosen to study their field occurrence, mineralogy, petrology, geochemistry and geochronology. The data allow protolith recovery, metamorphic PTt path modelling, and analysis of tectonic mechanism, leading to the following research goals: 1) establishing the Mesoproterozoic-Neoproterozoic magmatic sequences and the crust-mantle interaction process, which will provide constraints on the aggregation-breakup process of the Rodinia supercontinent; 2) establishing the spatial distribution regularities of metamorphic zones in the Qilian block and dissecting the response to the closure process of the Proto-Tethys Ocean; 3) exploring the tectonic transition mechanism and geodynamic setting of accretion-type orogen.

祁连造山带位于青藏高原东北缘，为华北板块、扬子板块和塔里木板块的接壤带，是原特提斯洋北部边界的组成部分。祁连地块为祁连造山带内的前寒武纪变质基底，其形成和演化可能与Rodinia超大陆的汇聚-裂解和原特提斯洋的扩张-闭合有关。目前对祁连地块前寒武纪基底的构造属性和变质演化存在不同认识，如是否有新元古代地幔柱活动的物质记录、古生代洋-陆俯冲碰撞过程中是否遭受了高压变质作用、空间变质相带的展布规律如何等。项目拟对祁连地块前寒武纪基性变质岩进行详细野外地质调查基础上的矿物学、岩石学、地球化学和年代学研究，通过原岩属性恢复、变质作用PTt轨迹模拟和构造机制分析，实现以下目标：①查明祁连地块中-新元古代岩浆演化序列、壳幔作用过程、及对Rodinia超大陆汇聚-裂解过程的响应；②建立祁连地块基底变质岩系的空间变质相带，剖析对原特提斯洋闭合过程的响应；③探讨增生型造山带各演化阶段的转换机制和动力学背景。

造山带结构与过程是板块构造和大陆动力学研究的重点和热点。造山带内前寒武纪微陆块的基底物质记录了陆块的来源和构造属性，其后的演化又制约了显生宙洋盆的闭合方式和造山作用过程，是研究造山带结构与过程的重要突破口。祁连造山带位于青藏高原东北缘，其形成和演化与元古代全球超大陆汇聚-裂解和原特提斯洋扩张-闭合有关。项目聚焦祁连地块中东段的前寒武纪变质基底湟源群和马衔山群，开展了扎实野外地质工作基础上的矿物学、岩石学、地球化学和年代学研究，通过原岩恢复、变质作用PTt轨迹模拟和构造机制分析，在准确解析其中的基性变质岩的产出特征、原岩属性和变质演化的基础上，进一步厘定了其围岩变碎屑岩系的原岩建造和与其伴生的花岗质片麻岩的岩浆作用，对比了不同岩石单元的变质演化。研究发现，岩群中角闪岩原岩为967-957Ma侵位的拉斑质和钙碱性辉长闪长岩，岩浆来源于俯冲板片交代的尖晶石橄榄岩相地幔部分熔融。花岗片麻岩原岩为963-936Ma侵位的I型和A2型弱过铝质花岗岩，岩浆来源于地壳减压熔融。云母片岩和长英质片麻岩原岩分别为页岩和杂砂岩，最大沉积年龄分别为1317-928Ma和927Ma，其原岩建造经历了长期的沉积过程，指示大洋弧前盆地-大陆弧前盆地-活动陆缘盆地的转换。湟源群和马衔山群中的角闪岩均记录了顺时针的PTt轨迹，峰期变质条件分别为5.6-6.6Kbar/730-770℃/496-489Ma和约5.2Kbar/630℃/451Ma，冷却阶段分别持续到457-428Ma和409-350Ma。认为祁连地块基底为Rodinia超大陆边缘沟-弧-盆体系中形成的火山-沉积岩系及侵入体，经历了初始洋内俯冲（1121-967Ma）、洋-陆俯冲（967-896Ma）和大陆裂谷（<882Ma）阶段；建立了古生代祁连地块北缘的双变质带系统和南缘的弧-陆碰撞系统，分别受到北祁连洋向南俯冲和南祁连洋向北俯冲的影响。