徐州—淮南地区新元古代臼齿碳酸盐岩成因探讨

臼齿碳酸盐岩是一种发育在中—新元古代，由微亮晶方解石组成的复杂褶皱构造，其成因一百多年来一直是个谜。通过对徐州-淮南地区新元古代臼齿碳酸盐岩特征、形成环境、分布时限研究，探讨了臼齿碳酸盐岩成因。该区臼齿碳酸盐岩发育在台地缓坡沉积体系中，可作为潮下浅水环境的标志，其形态在微层序中的分布反映沉积环境，具有重要的环境意义。建立了臼齿碳酸盐岩微相环境模式。快速石化作用形成均匀、等粒微亮晶方解石，是臼齿碳酸盐岩显著特征。现代类似于发育臼齿碳酸盐岩潮下环境没有这样的快速石化条件，它要求更高的超过饱和CaCO3沉淀。臼齿碳酸盐岩在本区分布时限介于850～720Ma间，在Sturtian冰期之前消失，与全球其他地区臼齿碳酸盐岩消失的时限一致，Sturtian冰期改变了海洋化学性质，是臼齿碳酸盐岩消失的根本原因。Sturtian冰期对海洋化学性质的影响，可能是揭开臼齿碳酸盐岩成因机理的一个重要新途径。

Origin of the Neoproterozoic Molar-tooth Carbonates in the Xuzhou-Huainan Area

Molar-tooth carbonates are intricately crumpled, microsparry calcite fissure fills formed during the Mesoproterozoic-Neoproterozoic. This article studied the sedimentary environment, petrological characteristics and age limit of distribution of molar-tooth carbonates in the Xuzhou-Huainan Area, and discussed its origin. Molar-tooth carbonates mostly occur in micritic limestone, marls, calcisilicic micrite, micrites and marls bearing silt in strata of the Xuzhou-Huainan area. The petrological characteristics of molar-tooth carbonates indicate their formation and development in a sedimentary system of slow ramp in a stable craton, and represent a shallow-water environment of subtide. According to their relationship with neighboring rocks, molar-tooth carbonates are divided into four types, perpendicular, handing-over, disorderly and parallel, and six types of combination types. The classification has an environmental implication and reflects respectively different carbonate sedimentary environments. Based on the cycle sequences and the microfacies units of the molar-tooth types, microfacies environment diagrams of the molar-tooth carbonate types are established. Neoproterozoic molar-tooth carbonates of this area are composed of microsparite calcite of homogenous, uniform, equal-axial and polygonal forms obviously different from its surrounding matrix in composition, texture and mineralogy. The formation of the molar-tooth carbonates needs rapid lithification not observed in modern subtide environments, and would seem, to require higher level of CaCO3 supersaturation than that in today's oceans. They developed during 850- 720 Ma and disappeared before the Sturtian glaciation in the Xuzhou-Huainan area. Molar-tooth carbonates disappeared suddenly before 700 Ma because the Sturtian glaciation changed the chemistry of the oceans. A decrease in CaCO3 saturation and increase in the concentration of precipitation in the mid-Neoproterozoic seawater might have contributed to the disappearance of molar-tooth carbonates.