安徽宿州新元古界望山组臼齿碳酸盐岩特征及成因讨论

CHARACTERISTICS AND ORIGIN OF THE MOLAR-TOOTH CARBONATES FROM THE NEOPROTEROZOIC WANGSHAN FORMATION IN SUZHOU,ANHUI PROVINCE

臼齿构造(Molar-tooth structures)在全球中—新元古代浅水碳酸盐岩地层中广泛发育,其特殊形态及矿物组成、时空分布特征及缺乏现代类比物都使其成因成为难解之谜。近年来,气体扩散-迁移模型提出的臼齿裂隙形成及微亮晶方解石的快速充填机制引起了广泛讨论。本文对安徽省宿州市沟后剖面望山组上部地层中发育的臼齿碳酸盐岩开展了岩石学和碳同位素分析。结果显示,望山组臼齿碳酸盐岩中臼齿及围岩的δ13C值分布范围具有一致性,二者不具有系统性差别。中—新元古代广泛分布于浅海地区的硫化楔可能为臼齿碳酸盐岩的发育创造了条件,望山组叠层石碳酸盐岩与臼齿碳酸盐岩在地层中的交替出现可能指示二者形成时浅海水体不同的氧化还原状态。

Molar-tooth structures are widespread in the Meso-Neoproterozoic shallow water carbonates around the world,whose formation mechanism,however,has remained open to debate due to their bizarre morphology,mineral composition,and spatial-temporal distribution pattern,as well as the absence of modern analogue.The generation of cracks and quick infilling with calcispar precipitation proposed by"gas bubble expansion model"has been widely discussed in recent years.In this study we carried out petrographic observations and carbon isotopic analysis on the Molar-tooth carbonates collected from the Neoproterozoic Wangshan Formation at the Gouhou section,Suzhou,Anhui Province.Our results show that theδ13C values of the Molar-tooth microspar are similar to those of their host carbonate rocks.Shallow water euxinic conditions common in the Meso-and Neoproterozoic may provide a prerequisite environment for the formation of the Molar-tooth carbonates,the change of bottom water redox states may account for alternate appearances of stromatolitic carbonates and Molar-tooth carbonates in the upper Wangshan Formation.