桐柏-大别南缘花山蛇绿混杂岩构造属性及其对东古特提斯洋演化的启示

Huashan ophiolitic melange is the eastward extension of Mianlue suture zone, southern Tongbai-Dabie Mountains. Complex petrotectonic assemblage and mafic dykes are widespread in the Huashan ophiolite zone. Comprehensive studies on these mafic dykes are expected to be critical for our understanding on the existing time and mantle feature of Mianlue Ocean and evolution of east Paleo-Tethys. However, documented works including the stratigraphic age and tectonic deformation remains unclear, especially high precision geochronology and isotope geochemistry are relatively absent. In this study, the structural deformation is the dominant ideology. Based on detailed field geological investigation and mirco-structural research, using high precision geochronology zircon LA-ICP-MS U-Pb dating, Ar-Ar tectonic chronology, whole-rock major- and trace elements as well as Sr-Nd isotopes, zircon Hf isotopes and paleontology, aiming to get tectonic setting, accurate age, geochemical data and the nature of mantle source of Huashan ophilitic melange to discuss its relationship with the oceanic mantle of the east Paleo-Tethys. Accordingly, combined with previously documented works of Tongbai-Dabie Mountains, the processes of Tongbai-Dabie orogenic belt and tectonic evolution during the Early Paleozoic to the Early Mesozoic will be discussed. Furthermore, this study will provide new evidence for the evolution of the Dabie orogenic belt and east Paleo-Tethys.

桐柏-大别山南缘花山蛇绿混杂岩是勉略构造带的东延，发育复杂的岩石构造组合，并出露大量基性岩墙和岩脉，这些将对认识勉略洋的存在时间、地幔源区性质及其东古特提斯洋演化提供重要信息，然而它们地层时代、构造变形特征目前仍不明确，尤其是基性岩的高精度年代学和同位素地球化学相对薄弱。本研究以花山蛇绿混杂岩带为研究对象，以构造变形为主线，在宏观露头观察和详细的显微构造研究的基础上，采用高精度锆石微区LA-ICP-MS定年技术、Ar-Ar构造年代学、全岩Sr-Nd同位素和锆石Hf同位素分析，厘定花山蛇绿混杂岩构造属性，为蛇绿岩混杂岩的形成年龄提供可靠的证据，并示踪蛇绿混杂岩地幔源区性质及其与东古特提斯洋幔的关系，探讨早古生代-早中生代桐柏-大别造山带南缘造山作用过程与构造演化，由此来追溯东古特提斯洋北缘构造演化过程。

为认识桐柏-大别南缘花山蛇绿混杂岩的物质组成、存在时间、大地构造背景、地幔源区性质及其构造演化，本项目以构造精细解析和辉长-辉绿岩为重点进行研究，在对构造剖面与关键地质体的精细解析的基础上，采用高精度锆石微区LA-ICP-MS定年、锆石微区SIMS定年、Ar-Ar构造年代学、全岩主微量稀土、全岩Sr-Nd同位素和锆石Hf同位素分析，将花山蛇绿混杂岩解体为由不同时期、不同沉积环境和构造背景形成的岩石类型组成，主要包括岛弧和弧后盆地两大体系的物质，初步确定大洪山地区残存一个新元古代早期弧盆系。岛弧主要由钙碱性I型花岗岩、辉长-辉绿岩、玄武岩、英安岩及酸性火山碎屑岩组成，锆石U-Pb定年获得了岛弧的形成时代为876-840Ma。弧后盆地主要由浊积岩、镁铁质岩块、辉绿岩、玄武岩、打鼓石群残留碎块组成，锆石U-Pb定年获得了弧后盆地的时代为824-800Ma。结合全岩Sr-Nd、锆石Hf同位素地球化学研究表明新元古代桐柏-大别南缘发生过洋内俯冲、洋陆俯冲、弧后盆地拉伸、弧后盆地关闭四大构造演化阶段，以莲沱组角度不整合覆盖在浊积岩之上代表造山事件的结束。在本项目执行三年的研究中，发表学术论文2篇，正撰写论文2篇，待发表科普专著1部。本项目的科学意义在于为桐柏-大别南缘花山蛇绿混杂岩提供了可靠的年龄证据，并示踪了蛇绿混杂岩地幔源区性质，提出了四阶段造山演化过程，探讨了新元古代扬子周缘造山过程与构造演化，为超大陆聚散过程研究提供重要地质信息。