臼齿碳酸盐岩: 解密前寒武纪海洋化学性质的钥匙*

臼齿构造(Molar tooth structure,MTS)是指主要由微米级微亮晶方解石充填的、具有肠状褶皱等特殊形态和结构的前寒武纪沉积构造。具有臼齿构造的这类前寒武纪碳酸盐岩称为臼齿碳酸盐岩(Molar tooth carbonate,MTC)。MTS具有全球分布、时代限定、形态多样、由微米级微亮晶方解石组成及早期成岩等基本特征,该认识早已得到学界广泛认同。在系统归纳MTC的地理分布、发育时限、宏观形态和微观组构的基础上,再次总结性地剖析了物理作用、生物作用和生物-地球化学作用3类主要成因假说的基本论点及其局限性。长期以来,作为一种大量出现于中—新元古代、具全球性分布的特殊碳酸盐岩类或沉积构造,研究关注的焦点和热点是MTS形成与消失的机制。尽管MTS成因还有争议,但其所具有的早期成岩特征,使越来越多的研究者将其作为前寒武纪海洋地球化学环境重建的重要研究对象。MTC的兴衰与前寒武纪海洋化学性质演化和前寒武纪一系列全球性事件密不可分。在此基础上提出MTC进一步研究的方向:一是探索MTC事件与相关全球性事件(如叠层石、冰川事件等)相互响应、促进或消长的关系,MTS的兴衰如何协同Columbia和Rodinia超大陆的聚散过程; 二是从MTS微亮晶方解石微观组构出发,采用行之有效的地球化学方法,将MTC的形成、演化、更替与前寒武纪海洋性质以及大气圈、生物圈的演化相结合,进一步完善MTC成因理论,为前寒武纪诸多重大科学问题的研究提供参考。

Molar tooth carbonate: a key to understand geochemistry characteristics of the Precambrian ocean

Molar tooth structure(MTS)refers to a series of peculiar, ptygmatically folded and spar-filled cracks in fine-grained carbonates of the Precambrian. Particular Precambrian carbonate with MTS is called the Molar tooth carbonate(MTC). Consensuses have been achieved that the MTS is globally distributed and composed of micro-sized sparry calcite,developed during a specific time limit,early diagenetic product,and has diversified morphologies. Based on a systematical summary of the palaeogeographical distribution,development time limit,macro morphology,and the micro fabric of MTS,this paper systematically analyzes the basic arguments and limitations of the three hypotheses of MTC origin, i.e.,the physical origin,biological origin,and the biogeochemical origin hypothesis. As a globally distributed special carbonate that only appeared during the Meso-and Neoproterozoic,the focuses and hot spots of the MTC researches are the formation and disappear mechanisms. Although there are still controversies about the formation mechanism,the early diagenetic characteristics of the MTC made a growing number of geologists regard the MTC as an important recorder to reestablish the geochemical properties of the Precambrian ocean. The flourishing and(or)dismissal of the MTC has a close relationship with the evolution of paleocean properties,the supercontinent cycles,and a series of global geological events. Further researches about the MTC can go in two aspects: one is to explore the reciprocal response,promotion or inhibition relationship between the MTC and global events such as stromatolites,glacial events, etc.,and how to couple the MTC events to the assembly and breakup processes of the Columbia and Rodinia supercontinents;the other is the microscopic fabric of the microspar in the MTS, select adopt effective geochemical methods,in order to combine the researches of the occurrence,evolution,and cessation of the MTC with the evolution and changes of atmosphere,biosphere,and the Precambrian ocean properties,and finally improve the theory of the MTC formation mechanism. All these researches will help and provide clues to solve several major Precambrian scientific problems.