中文摘要：

臼齿（Molar Tooth,简称MT）碳酸盐岩是产于元古代、并具有特殊全球性意义的碳酸盐岩类型之一。MT特有的微亮晶结构、严格的时空分布范围以及百余年来众多的成因解释,一直吸引着国内外地质学家对其进行持续的研究和探讨。作者在前人研究基础上,基于作者近10余年成果积累及新近对MT微观组构的研究进展,提出了若干新认识。MT是由原始晶核及同沉积期增生、加大而形成的微晶集合体;元古代时期属于还原或弱氧化环境,具有较高的碳酸盐饱和度;浅海海底碳酸盐灰泥固结缓慢;这些灰泥中丰富的有机质不断分解产生气体,封闭体系中的气体浓度持续增高、压力增大,从而导致灰泥中裂隙形成;裂隙形成或可借助其他外力作用,并不断扩大或变形、甚至被破坏;在有机质的催化作用下,裂隙内隙间水中迅速析出CaCO3,形成初始晶核;晶核进一步增生加大并组合成微晶集合体（MT）,发生石化作用,充填于原始裂隙中;元古代古海洋的物理、化学条件是MT形成的背景,也提供着沉积物的来源和动力条件,控制着MT的时空发育与分布。

英文摘要：

Molar Tooth（abbreviated MT） carbonate rocks,i.e.,the Proterozoic carbonate rocks with particular texture,are characterized by the enigmatic micro-spar fabric,limitation of temporal and spatial distribution or diverse origin interpretations since the discovery of more than a hundred years,and attract the attention of numerous geologists worldwide.Based on the previous researches and our own new studies on MT microscopic fabric in the recent decade,some new results were proposed in this paper.MT is a stacked equant calcite sparry constructed by aggradated accretion of calcite microspar surrounding an initial nucleus of calcite.The Proterozoic ocean was a deoxidation or suboxidation environment with a high carbonate saturation.As a result,the crystallization and consolidation of lime mud on the bottom of the shallow sea became more slowly.Due to continuous decomposition of organic matter in lime mud,gas was generated.And then,within a closed system,the concentration of the gas was increased to a higher value,resulting in the formation of cracks and voids in the plastic lime mud.Cracks were perhaps generated by some unknown mechanisms and enlarged in size or deformed,destructed as well.By the catalyzed reaction of organic matter,calcites（mainly CaCO3）were precipitated quickly from liquids within voids or cracks,and then the initial nucleus of calcite appeared.Afterwards,a stacked equant calcite sparry was constructed by the aggradated accretion of calcite microspar surrounding an initial nucleus of calcite.Consequently,calcite sparry was solidified and deposited within the cracks.The proposed model for the MT formation indicates that the void-forming and crack-filling represents microscale gas-sediment interactions and the geophysical and geochemical condition of the Proterozoic paleo-ocean plays an important role,which is not only the geological setting constraint but also provides sediment materials and dynamic conditions,and temporally and spatially restricted the development and distribution of MT.