扬子北缘中元古代神农架群和打鼓石群的沉积序列、构造属性及对比研究

The Shennongjia and Dagushi groups, located in the adjacent region between the northern Yangtze Block and the Qinling Orogenic Belt, bear pivotal implications for the Columbia breakup and subsequent Rodinia assembly during the Mesoproterozoic. However, their depositional time, basins nature, and tectonic significance are still topics of hotly debated. On the basis of field investigations, zircon U-Pb dating will be performed for the key volcanic rocks and clastic sedimentary rocks to define the exact stratigraphic ages; integrated sedimentological analyses such as facies and microfacies, depositional environments as well as sequence evolution will be carried out to confirm the stratigraphic successions and basin attributes; whole-rock geochemistry and zircon Hf-O isotope data will be obtained from these volcanic rocks for revealing their petrogenesis and tectonic setting. Finally, the tectonic significance and relationships of the Shennongjia and Dagushi groups will be explored. Our new results, in combination with documented data from adjacent regions, will provide new insights into the Mesoproterozoic stratigraphic column of China and sedimentary-tectonic history of the Yangtze Block. This study also holds a key to unravel the reconstruction of South China in the Columbia and Rodinia supercontinents.

神农架群和打鼓石群分布于扬子陆块北缘，与秦岭造山带南部毗邻，是中元古代Columbia和Rodinia超大陆演化转折期的关键记录，但其沉积时限、盆地性质及大地构造意义仍存在较大争论。本项目拟在野外调研的基础上，选择关键层位的火山岩夹层和碎屑沉积岩进行锆石U-Pb年代学研究，以限定其确切沉积时限；对它们所代表的中元古代中期沉积盆地的充填序列开展沉积相与微相、沉积环境、序列演化研究，以厘清地层序列、解析沉积盆地性质；同时，对其中发育的火山岩开展全岩主、微量元素及锆石Hf-O同位素示踪研究，揭示其岩石成因和形成的构造环境；最后，对神农架群和打鼓石群进行综合对比研究，探讨其块体属性及关系。为填补我国中元古代“待建纪”（1400-1100Ma）地层空白、准确认识扬子陆块中元古代沉积-构造演化及其与超大陆聚合-裂解的响应关系提供依据。

神农架群和打鼓石群分布于扬子陆块北缘，与秦岭造山带南部毗邻，是中元古代重要的地层单元。本项目厘定了神农架群和打鼓石群的沉积时限；基本厘清了神农架群和打鼓石群沉积序列的演化特征，揭示了沉积盆地性质；分析了神农架群火山岩的岩石成因及动力学机制，结合沉积学研究，明确了神农架群和打鼓石群形成的大地构造背景；同时与扬子陆块西南缘的相关地层进行了对比研究，探讨了整个扬子中元古代沉积盆地的演化过程及其与超大陆聚合-裂解的响应关系。另外，发现了扬子陆块北缘、西北缘古元古代岩浆岩，并对其岩石成因和形成的构造背景进行了研究。 .研究表明，神农架群和打鼓石群的沉积时限分别为980~1300 Ma和950~1040 Ma，打鼓石群仅相当于神农架群的上部，它们的沉积物源均主要来自周围太古代至中元古代的岩体。神农架群和打鼓石群均经历了滨岸、碳酸盐岩台地和浅海陆棚相的沉积变化，只是沉积序列演化特征有差异；神农架群玄武岩显示大陆拉张带玄武岩的特征，而非岛弧环境，其母岩浆来自岩石圈地幔；综合沉积学和地球化学研究成果，我们认为神农架群和打鼓石群形成于被动大陆边缘盆地。扬子陆块西南缘河口群和东川群的沉积时代分别为1.75~1.40 Ga和1.75~1.35 Ga；东川群的沉积序列较为完整地记录了从陆内裂谷到被动大陆边缘的演化过程，而河口群形成于夭折裂谷，神农架群和打鼓石群应该是被动大陆边缘阶段在扬子陆块北缘形成的沉积产物，可能是Columbia超大陆最终裂解在扬子陆块的响应。在完成拟定的研究目标外，本项目又报道了扬子陆块北缘钟祥地区~2040 Ma和~1950 Ma花岗岩，揭示了扬子陆块在2.04~1.95 Ga期间经历了由俯冲向碰撞的构造体制转换；发现了扬子陆块西北缘碑坝地区~2090 Ma马元花岗闪长岩和~1760 Ma白玉钾长花岗岩，它们分别形成于岩浆弧和陆内裂谷环境，为进一步认识扬子陆块前寒武纪构造演化提供了新的资料。