黔东北地区南华系大塘坡组冷泉碳酸盐岩及其意义

Characteristics and Significance of the Cold Seep Carbonates from the Datangpo Formation of the Nanhua Series in the Northeast Guizhou

黔东北及毗邻地区新元古界南华系下统铁丝坳组冰碛层(相当于Sturtian冰期)之上,大塘坡组的底部分布一套黑色岩系.在一系列同期形成的断陷盆地近中心区域,黑色岩系下部主要由菱锰矿、白云岩等碳酸盐岩组成.通过对该地区出露的这套碳酸盐岩的野外地质构造、矿物成分、碳、氧、硫稳定同位素特征的研究,发现除碳稳定同位素表现出强烈的负偏移外,硫同位素值表现出明显的正偏移,还发现了大量气孔状构造和泥火山、底辟等典型构造.其特征与现代海底天然气水合物的渗漏、释放和环境突变所形成的冷泉碳酸盐岩机理一致,进而认为这套碳酸盐岩成因是新元古界古海洋的海底天然气水合物渗漏所形成的冷泉碳酸盐岩沉积.该成果拓展了海底天然气水合物研究领域和时空范围,对其所导致的成岩成矿作用、环境评价的分析研究提供了新思路.

In the Northeast Guizhou and its surrounding, a series of black shales, covering the till sheet of Tiesiao Formation which is thought to be formed during the Sturtian, distributes at the bottom of Datangpo Formation, lower Nanhua Series, Neoproterozoic. Generally, the lower part of the black shales is mainly composed of carbonate such as dialogite, dolomite and so on, near the center of a succession of syngenetic fault basins. Through the studies of the carbonate outcrop in this area in terms of the field structure, mineral components and the characteristics of carbonate, oxygen and sulfur isotopes, it is shown that sulfur isotope presents distinct positive excursion in contrast with carbonate isotope that indicates an intense negative excursion. In addition, a mount of stomatal dialogite and significant structures like mud volcano, diapers and so on have been found. Based on the above features, the studied carbonate is supposed to have come into being under the same mechanisms as modern cold venting carbonate on the sea floor via the seep and release of gas hydrate and the environmental mutation. Therefore a simple conclusion is obtained that the carbonate for research is possible to be generated on the ancient ocean floor in the Proterozoic as the deposition of cold venting carbonate, because of the gas hydrate seep. This study widens the research areas for the ocean floor gas hydrate and its spatial and temporal distribution, supplying a new idea for the analysis of the diagenesis, mineralization and environmental assessment.