华南埃迪卡拉系陡山沱组盖帽白云岩成因及成岩演化研究

The dolomite problem has long been the debate since its discovery. Deposited at the initial Ediacaran period, cap dolostone is an important type of dolostone as it recorded paleoclimatic, oceanic and biological signatures during the transition from the Cryogenian to the Ediacaran. Dolomitization and a variety of diagenetic alternations that occurred in cap dolostone, however, have the high potential weakening, overprinting and/or changing the signals that can be used to reflect the original paleoenvironmental information recored in the rock. Hence, it is essential to investigate the origin and diagenetic evolutions of the cap dolostone prior to any paleoclimatology/paleoceanography interpretations. This project is designed to study the origin and diagenetic evolutions in cap dolostone by exploring samples from the Doushantuo Formation (Ediacaran) in South China. Cap dolostone samples will be collected from a variety of depositional settings from inner shelf – carbonate platform – shelf margin – slope – basin, individually. The key of this topic is to investigate both stratigraphic and lateral variations in the petrographic and geochemical properties of cap dolostone, which include dolomite crystal structures, high versus low calcium dolomite compositions, dolostone petrographic properties, diagenetic features, carbon and oxygen isotopes, trace elements and REE contents and Sr isotopes. The origin of the Doushantuo cap dolostone will be interpreted based primarily on the spatial variability in the cross section. Diagenetic evolutions of the cap dolostone will be studied to evaluate the effect from dolomitization and diagenesis on the geochemical parameters used for paleoclimatic reconstruction. This project will contribute to our understanding of the dolomite problem, and have important implications for reconstructing paleoclimatology and paleoceanography during the Cryogenian-Ediacaran transition.

白云岩成因是全球地学重大难题。作为记录重大地质转折期气候海洋和生物信息的盖帽白云岩，对其成因及演化的研究尤为重要。白云化及成岩作用会弱化、叠加甚至更改保存在盖帽白云岩中的原始古环境信息，这对利用该组岩石获得古环境海洋气候的可靠性提出了巨大挑战。本课题以华南埃迪卡拉系陡山沱组盖帽白云岩为研究对象，对横贯陆架至盆地沉积体系一系列剖面上的样品进行地质与地化多手段综合研究。从白云石晶体结构、高钙/低钙白云石构成、白云岩岩石学特征、成岩作用与期次、碳氧同位素、主微量元素含量在各剖面上横向与纵向变化趋势入手，结合新生代及合成白云岩最新研究成果，研究盖帽白云岩的成因机制及成岩演化，揭示其对原始沉积环境信息的控制作用，探讨白云化与成岩在古气候古海洋重建中的影响。本课题对前寒武白云岩成因问题具有启示作用，对成冰纪到埃迪卡拉过渡期气候海洋特征有一定指示意义。

新远古代末期埃迪卡拉纪早期形成的盖帽白云岩在全球广泛分布。盖帽白云岩见证了地质历史上最大的雪球地球消融的过程，记录了该地质演化转折期重大的古海洋和古气候信息。利用盖帽白云岩对古环境的恢复具有诸多挑战和争议性，现阶段对该白云岩地层的形成机制和成岩演化仍不明确。本项目以扬子板块西南地区碳酸盐岩台地内部-台地边缘-斜坡-盆地的陡山沱组盖帽白云岩为研究对象，对其矿物岩石学、沉积学、地球化学及晶体学特征开展了综合研究。研究区陡山沱组盖帽白云岩体现出（1）原始沉积结构和沉积构造罕见，大部分为结晶白云岩；（2）白云化程度具有空间差异性，台地内白云化不完全；（3）白云石具有高成熟度（阳离子有序度和化学计量值）；（4）白云石晶体均一化严重，单个白云石晶体往往由无数亚晶构成;（5）晶体缺陷明显，极个别单晶中存在纳米晶；（6）碳氧同位素值不具明显相关性，未见明显δ13C负偏；（7）富铁锰，贫氧化还原敏感元素；（8）稀土元素含量及其配分特征体现海水特征。综合分析认为：（1）盖帽白云岩通过同沉积或沉积浅埋藏白云化作用形成，海水是白云化的关键因素，白云岩的形成不排除微生物的参与；（2）盖帽白云岩的经历了复杂的成岩演化进程，主要包括交结、白云化作用、裂缝及胶结充填、重结晶、晚期白云石交代；（3）盖帽白云岩指示热液活动是海水铁锰及稀土元素的重要来源，埃迪卡拉早期海水仍处于还原态，仅表层海水含有低溶解氧; 盖帽白云岩见证了雪球地球融化过程中浅层海水溶解氧的增加。