湖南寒武系第三统混积岩特征及沉积环境研究

In prior research，we discovered that the main sediments of the Series 3 in Cambrian were consisted of the carbonatite and the mixed rock and were not the mud-banded limestone in Huayuan and Yongshun of the northwest Hunan. And also, it had been considered that formed in the mixed shelf but not the platform-marginal slope with the help of same other evidence. So the spatial distribution and characteristic of the mixed rock was very important in recognizing the sedimentary environment of the Series 3 in Cambrian. On the existence of the mixed rock in Huayuan and Yongshun, it could be inferred that the mixed rock also widely distributed in the Hunan. Thus, we chose the Hunan which located in he intermediate zone which between Yangtze Block and Cathaysia Block, and research the the Series 3 of Cambrian in depth. Sedimentology and geochemistry such as analysis of microelement and C&O isotope would be utilized in the study of the spatial distribution, rock fabric, mixed frequency and scale. How the sea-level change and paledepth of water influenced the formation of sediments especial mixed rock that would be discussed, the sea-level change in the study area also would be compared with the sea-level change in the earth. The rock association，sedimentary tectonics, section sequence and paleontological characteristics also would be considered in the analysis and the lateral comparison of the sedimentary facies. Above all, the sedimentary environment could be defined and the lithofacies paleography map could draw up. On the basis of the research, we can renewedly set up the evolution of the carbonate platform in the southeast wedge of Upper-Middle Yangtze area, and explain the tectonic relationship between Yangtze Block and Cathaysia Block from the sedimentology.

在前期研究中，课题组发现湘西北花垣、永顺寒武系第三统主要沉积物并非泥质条带灰岩而是由碳酸盐岩与混积岩组成的混积层系。结合其他证据，认为其形成于混积陆棚而非台缘斜坡。混积岩的空间分布及特征在对沉积环境的重新认知中显得尤为重要。鉴于在花垣及永顺寒武系第三统均发育混积岩，推测混积岩在区域上广泛分布。因此，选择横贯华夏陆块与扬子陆块萍乡-郴州一带构造结合带的湖南地区，运用沉积学和微量元素、稳定碳氧同位素等地球化学分析手段，重点研究寒武系第三统混积岩空间分布、岩石学特征、混积频率及规模等沉积特征，探讨海平面变化、古水深对沉积物尤其是混积岩形成的影响，并与全球海平面变化进行对比。结合岩石组合、沉积构造、剖面序列及古生物特征，分析沉积相，进行横向对比，判断寒武纪第三世的沉积环境并编制岩相古地理图。通过研究，重塑寒武纪中上扬子东南缘碳酸盐台地的演化，并为解释扬子陆块与华夏陆块的构造关系提供沉积学证据。

湖南地区寒武系第三统发育大量的混积岩及混积层系，其沉积特征、成因及时空分布，对研究寒武纪中上扬子地区东南缘的沉积环境及碳酸盐台地的形成演化具有重要的意义。通过野外观察、微观岩石学分析、地球化学分析等方法对混积岩时空分布、沉积特征、矿物组分、主微量元素特征等进行了研究，首次系统总结了湖南地区寒武系第三统发育的混积层系类型与组合方式，及其分布的6类不同的岩相分区，并按照成分分类依据划分出4种混积岩类型，分布在湘西北地区及湘中地区，靖州—涟源—隆回—宁乡一线以北的区域。同时，总结出以下认识：1）第三统混积岩形成于受周期性强风浪、风暴作用影响的浅水混积陆棚环境及风暴浪基面以下的深水混积陆棚环境；2）主要成因为机械成因的相缘渐变沉积混合及事件突变沉积混合；3）混积陆棚的存在，表明寒武纪第三世时期，中上扬子东南缘湘西北地区不发育台缘相带，未建立起完整的镶边碳酸盐台地模式；4）主微量稀土元素特征指示湖南寒武系第三统混积岩的碎屑组分可能主要来自与大陆岛弧构造背景有关的环境。此外，湘南地区芙蓉统碳酸盐沉积与碎屑沉积物指状交互过渡沉积的发现指示在寒武纪芙蓉世时期，扬子地块与华夏地块可能处于同一沉积海域。