扬子西南缘早新元古代构造体制转换期的沉积学响应研究

The early Neoproterozoic Era was a crucial period that the Rodinia supercontinent transformed from its final convergence to the initial breakup. The available data indicated that the southwestern Yangtze Block experienced an obvious tectonic transition during the early Neoproterozoic. However, the characteristics of this tectonic transition remains a hotly debated issue whether the southwestern Yangtze Block transformed from a passive continental margin to an active continental margin, or from a collision orogenic setting to an intracontinental extension setting? As revealed by the preliminary study, the Liubatang, Dayingpan, and Shuangshuijing formations unconformably overlaid the Kunyang Group and underlain the Chengjiang Formation as well as their equivalents in the southwestern Yangtze Block, and their depositional time can be roughly defined as ca. 930~830 Ma. They bear the crucial information about the regional tectonic transition during the early Neoproterozoic. This study proposes to conduct systematic field investigations and sedimentological, structural, geochronological, and geochemical studies on these strata, so as to constrain their accurate depositional ages and the nature of their bounding surfaces. By using facies and provenance analyses as the key methodology, we also plan to clarify the sedimentary sequence, depositional environment evolution as well as the provenance characteristics of these strata. Through a comprehensive comparison with those available data of the underlying Kunyang Group and overlying Chengjiang Formation, this study aims to reconstruct the early Neoproterozoic sedimentary evolution process in the southwestern Yangtze Block, and also to constrain the nature of the regional tectonic transition. This project will provide new sedimentary evidence for the relationship between the Meso-Neoproterozoic tectonic history of South China and the assembly and breakup of the Rodinia supercontinent.

早新元古代是Rodinia超大陆由最终聚合向初始裂解转换的关键时期。已有资料揭示扬子西南缘早新元古代经历了一次明显的构造体制转换，但其性质仍然存在争论，例如是由被动大陆边缘转换为活动大陆边缘，还是由碰撞造山转换为陆内伸展？初步研究表明，扬子西南缘不整合于昆阳群和澄江组及其相当地层之间的柳坝塘组、大营盘组和双水井组的沉积时限大致为930~830 Ma，蕴含了早新元古代区域构造体制转换的关键信息。本项目拟在野外调查的基础上，通过沉积学、构造地质学、同位素年代学及地球化学综合研究，确定其地层精确时限及顶底不整合界面成因，阐明其沉积相、沉积环境及序列演化，揭示其沉积物源特征。通过与下伏昆阳群、上覆澄江组等相关地层的研究资料进行全面对比，重建扬子西南缘早新元古代沉积演化过程，进而限定区域构造体制转换的性质。本项目将为深入理解华南中—新元古代构造演化与Rodinia超大陆聚合—裂解的关系提供新的依据。

以扬子西南缘不整合于昆阳群和澄江组之间的柳坝塘组及其相当地层为主要研究对象，开展了详细的沉积学、构造地质学、同位素年代学及全岩地球化学等研究。本项目取得的主要认识包括：1）柳坝塘组及其相当地层的沉积时限为900-850Ma，其底部界面属于构造隆升侵蚀形成的不整合面，而顶部界面则属于伸展不整合；2）柳坝塘组底部为洪积扇相-冲积扇相，下部为蒸发岩相-滨湖相-（半）深湖相，上部为浊流相和水下扇相，顶部则为内陆棚相-外陆棚相；3）柳坝塘组下部的碎屑锆石年龄主峰为2030-2080Ma、1830Ma和1560-1590Ma，而上部为1610-1630Ma和1200Ma，结合沉积学证据，确定柳坝塘组物源主要为下伏昆阳群（特别是美党组）；4）昆阳群记录了由陆内裂谷发展为被动大陆边缘的过程，随后发生了长达约30Ma的构造隆升，柳坝塘组形成于断陷盆地陡坡型扇三角洲环境，而澄江组则响应于华南裂谷盆地的广泛发育，这一沉积过程与Rodinia超大陆的聚合和裂解有关；5）首次识别出扬子西南缘的早前寒武纪结晶基底-撮科杂岩，并解析了太古宙-古元古代构造热事件的时空演化。项目执行期间，共在国内外主流地学期刊发表论文22篇，其中包括国外SCI论文14篇，并培养博士和硕士研究生5人。