贵州松桃大塘坡地区南华纪早期冷泉碳酸盐岩地质地球化学特征

贵州松桃大塘坡地区的南华系是研究古代天然气渗漏及冷泉碳酸盐岩的理想地区之一.该区冷泉碳酸盐岩由两类岩石组成: 一是两界河组含砾砂岩中(Sturtian冰期早期) 的白云岩丘或透镜体; 二是之上的大塘坡组第一段黑色含锰岩系中的似层状或透镜状菱锰矿矿体及少量白云岩透镜体.该地区冷泉碳酸盐岩的碳同位素具明显的负偏特征, 充填在菱锰矿矿石气孔中的沥青碳同位素为-30.98‰, 并具有异常高的硫同位素正值.结合冷泉碳酸盐岩的空间分布、古天然气渗漏构造、菱锰矿的沉积有机质等特征分析, 认为其与现代冷泉碳酸盐岩的特征十分相近.两界河组的白云岩丘是目前所发现的最古老的冷泉碳酸盐岩, 与在它之上的菱锰矿矿体是同一个古天然气渗漏系统中不同时期的冷泉碳酸盐沉积.天然气泄漏形成的冷泉与大塘坡组烃源岩的形成关系密切. 

Geological and Geochemical Characteristics of the Cold Seep Carbonates in the Early Nanhua System in Datangpo,Songtao,Guizhou Province

Nanhua System in Datangpo, Songtao County, Guizhou Province is proposed to be one of the ideal areas to investigate the ancient gas seep and the associated cold spring carbonates. The cold seep carbonates found here occur as two kinds of rocks. One is the dolomite hills or lens in the clastic rocks of Liangjiehe Formation, which is thought to be formed during the early Sturtian glacial stage, and thus to be the oldest cold spring carbonates. The other is represented by the rhodochrosite orebodies occurring as layers and lens in association with some dolomite lens in the black manganese-bearing rocks in the first member in Datangpo Formation. The carbon isotope composition of the cold spring carbonates in the area shows a negative anomaly, and the carbon isotope composition of the asphalt filled in the gas holes in rhodochrosite orebodies is measured to be -30.98‰. The negative carbon isotope values are associated with the positive anomaly of the sulfur isotope composition. The cold seep carbonate rocks identified in this region are comparable with modern counterparts in the characteristics and the distribution of the carbonates, the gas seep structures and the depositional organics in the rhodochrosite orebodies. The dolomite hills and the rhodochrosite orebodies are formed in the same ancient gas seep system during different periods, showing a close relationship with the hydrocarbon source rocks of Datangpo Formation.