前寒武纪“臼齿状构造谜”的一些认识：来自天津蓟县剖面高于庄组的信息

臼齿状构造是一种主要分布在前寒武纪非叠层石碳酸盐岩中的、特殊的“谜”一样的沉积构造，以发育一系列奇形怪状、大大小小的裂缝和裂隙为特征，这些裂缝和裂隙由等粒和均匀的微亮晶方解石充填。臼齿状构造时代分布的特殊性（新太古代至新元古代）以及分布的全球性，使其在显生宙还未发现类似的对等物。对臼齿状构造成因的不同认识使其成为“臼齿状构造谜”。天津蓟县剖面中元古界高于庄组（1600 Ma 至 1400 Ma）第3段隐晶质泥晶灰岩序列中的臼齿状构造，以其特别的沉积特征为了解“臼齿状构造谜”提供了一些有用的信息。这些特征包括：(1)臼齿状构造特别的形态学特征，(2)由微亮晶所充填的臼齿状裂缝具有明显的边界（以富集残余有机质和黄铁矿为特点），(3)臼齿状构造的宿主岩石是不发育叠层石和纹理化构造的隐晶质泥晶灰岩，(4)与臼齿状构造常常共生的极为特别的宏观藻类化石等。因此，臼齿状构造可以解释为一种发育在席底生境中、由一系列复杂的作用过程所形成的、与微生物相关的原生沉积构造。作为前寒武纪最普遍的沉积现象，臼齿状构造为更好地理解复杂多变的前寒武纪碳酸盐岩提供了许多有意义的信息。

Some reviews on the Precambrian molar-tooth enigma: Information from the Gaoyuzhuang Formation at Jixian section in Tianjin

Molar tooth(MT)structure is an enigmatic structure which is mainly distributed in the nonstromatolitic carbonate rocks of the Precambrian. It is characterized by cracks and voids with various shapes and different sizes which are filled with homogeneous and equally sized microspar calcites. Due to its special distribution in the geologic times(from Neoarchean to Neoproterozoic)and its global distribution, there are no similar analogs in the Phanerozoic. Debates on the origin of the molar tooth structure have continued until now and thus make it become the “molar tooth structure enigma”. The molar tooth structure developed in the micrite limestones of the Member 3 of Gaoyuzhuang Formation(from 1600 Ma to 1400 Ma)at Jixian section, Tianjin, has provided some important information for resolving the molar tooth enigma. Their characters include(1)the particular configuration of the MT structures;(2)the obvious interface with rich organic matter and pyrite between the MT ribbon cracks and the host rock;  (3)the host rocks are characterized by nonstromatolitic micrite limestones with few liminites;(4)the special macroscopic fossils of algae which were simultaneously developed with the MT structures. Therefore, the MT structure can be interpreted as a type of the microbial induced primary structures formed by a complex process in a matground environment. As one of the most common sedimentary phenomena,the MT structure provides more significant information for understanding the complex Precambrian carbonate rocks.