广东二叠-三叠系充填格架、物源示踪及印支期构造属性限定

The relationship between Indosinian structure patterns, magmatism and sedimentation in the South China Block and the peripheral orogenic events has been a hot topic in the past decades. Recent studies in the Permian-Triassic succession of the Youjiang Basin and Shiwandashan Basin show that during Indosinian, the South China Block was located in a passive continental environment and affected by the Paleo-Tethys subduction zone, whereas some scholars proposed it was a retro-arc foreland basin associated to the flat-slab subduction of the Paleo-Pacific along the southeastern margin of the South China Block. The Guangdong Province, located at the overlapped area of two subduction zones, is one of the best windows to study the tectonic attribute of the South China. But it has been not paid enough attention, such as the Permian to Triassic stratigraphic framework and its material sources now are still not clear. In order to resolve this problem, this project will be based on the previous study, to carry out stratigraphic framework study in two typical sedimentary sections in the Kaiping and Meixian areas and massive paleocurrent analysis to determine the direction of these sediments. Samples will be collected for detrital zircon U-Pb age and in-situ Hf isotope analysis to know its provenance and the characteristics of source rocks. This information, combined with those of the Youjiang-Shiwandashan basins and geology of adjacent areas, will help us to identify the similarities and differences among basins in the South China. This study can help us to understand the sedimentary response and its intrinsic link to the Paleo-Tethys or Paleo-Pacific subduction event, and to provide a scientific basis for the restriction of Indosinian tectonic attribute of the South China.

印支期华南内部构造、岩浆及沉积作用与周缘造山事件的相互关系一直是地学界研究的热点。近年来研究证明，印支期云开大山西侧右江-十万大山盆地的演化与古特提斯地质事件密切相关，但位于东侧的沉积盆地却可能受控于古特提斯或古太平洋俯冲造山事件。广东作为以上两构造域的交汇区，区内沉积盆地的演化是研究华南印支期构造属性的最佳窗口。但目前对该区域与印支事件关系密切的二叠-三叠系充填格架及其物质来源的不明确，限制了对其演化特征的解析。本项目将在前期研究基础上，对开平及梅县地区代表性沉积剖面的地层充填格架及区域上古水流方向展开研究，并对其重要层位的碎屑岩开展碎屑锆石U-Pb年代学和Lu-Hf同位素组成分析，示踪其物源方向与源区特征。通过与右江-十万大山盆地及周缘已有研究资料对比，分析广东二叠-三叠纪盆地沉积响应与古特提斯或古太平洋俯冲造山事件的耦合关系，为约束广东乃至华南印支期构造属性提供科学依据。

华南大陆地处西太平洋、特提斯洋及华北地块碰撞夹持的构造背景之中，多期次的构造运动及陆内改造使得华南内部地质现象极为复杂，导致其包括广西运动及印支运动的动力机制、地质构造背景等科学问题存在广泛争议。探究华南沉积盆地的沉积响应与广西及印支造山事件的盆-山耦合关系，是理解和恢复华南构造演变历史的重要途径之一。本项目针对华南南部寒武-三叠纪代表性沉积剖面开展了详细的野外露头观察，并对重要的砂岩样品开展了包括矿物组成、碎屑锆石U-Pb年代学及Lu-Hf同位素组成的室内测试分析。分析结果显示，广东晚二叠世-三叠纪地层中普遍存在400-350Ma和300-260Ma两期锆石；结合右江、凭祥、十万大山以及福建永安、永定等沉积盆地的已有数据，我们确定了华南南部实际上广泛存在400-350Ma和300-260Ma两期碎屑锆石，指出过去在华南南缘及其外围应该存在对应的两期区域性岩浆活动事件，其成因应与特提斯洋的俯冲消亡有关。此外，我们还对项目计划中没有涉及的早古生代地层展开同方法下的同步研究。研究数据显示，钦州-防城港地区寒武纪、奥陶纪及志留纪样品的碎屑锆石年龄组成基本相似，其年龄主要由930-990 Ma，1600-1800 Ma及2400-2600 Ma组成，但在中泥盆世发生明显改变；结合古水流、岩相学及其它野外证据，识别出早古生代造山前期，钦州-防城港及其两侧地区的物质来源主要来自东冈瓦纳大陆北缘/华夏东部；造山后期，研究地区的源区由于云开大山的隆升呈现多样化；研究结果还进一步确定了钦州-防城港地区的华南地块属性。综上研究，本项目研究进一步证明了华南南部广西期及印支期沉积、造山运动的动力起源更倾向受控于陆内改造及特提斯洋的演化。