从地球化学角度看微亮晶臼齿碳酸盐岩形成的环境条件——以吉辽地区新元古代微亮晶碳酸盐岩为例

THE ENVIRONMENTAL CONDITIONS OF THE MICROSPARITE (MOLAR TOOTH) CARBONATES OPENED OUT BY GEOCHEMISTRY:AN EXAMPLE FROM THE MICROSPARITE CARBONATES OF NEOPROTEROZOIC IN JI-LIAO REGION, CHINA

近年来随着对前寒武纪研究的进一步开展,中晚元古代地层的一种被称为"臼齿碳酸盐岩[MolarTooth(简称MT)Carbonates]"或"微亮晶碳酸盐岩"或"臼齿构造(MolarToothStructure,简称MTS)"的沉积现象受到了国际地学界的重视。臼齿碳酸盐岩特指由微亮晶方解石组成、具肠状褶皱席(Ptygmaticallyfoldedsheets)构造的元古代碳酸盐岩。臼齿碳酸盐岩在地质历史中扮演着重要的角色,其似乎成了早元古代火山喷发热水沉淀碳酸盐与显生宙带壳化石碳酸盐沉积之间的桥梁,更重要的是在地球生命演化中起着重要的作用。利用现代测试手段和方法对吉辽地区新元古代含臼齿碳酸盐岩地层剖面砂岩的氧化物含量、稀土元素和臼齿碳酸盐岩的碳同位素、氧同位素、锶同位素、生物标志物、能谱进行了分析,从地球化学角度揭示了臼齿碳酸盐岩形成的构造背景、古温度场、形成时限、物质组分和沉积环境特征等,从而印证了沉积学研究成果:吉辽地区的臼齿碳酸盐岩主要发育在新元古代地层中,大约在750～900Ma左右;MT形成于华北地台稳定构造背景之下低纬度、古水温一般在50℃左右的热带气候条件下的内缓坡环境中;MT由微亮晶方解石和基质两部分组成。

Along with the research progress on Precambrian, the Molar tooth carbonates (briefly called MT, or called microsparite carbonates or MT structure) being formed in mid-late Proterozoic become a target of crossing firerecently. The Proterozoic Molar-tooth (MT) carbonate rocks refer to those Meso-to Neoproterozoic (1 700～650 Ma) carbonates with MT structure, being a series of pecular, ptygmatically folded and spar-filled cracks in fine-grained carbonates of Precambrian age, located in the environment of mid-to inner ramp and shallow platform. Like a bridge, MTS connected the inorganic world with the organic one and is closely related to the evolution of the paleo-oceans, the atmosphere and the biosphere. Their development and/or recession are related to the origin of life and the abruption of sedimentary geochemistry of marine carbonates. By using of morden instruments and testing methods adequately in this paper, contents of sandstone oxides and the model curve of REE distribution were measured; the ()^(87)Sr/()^(86)Sr isotopic ratio obtained an accurate value, that is, the age of MT formation is about 750～900 Ma; C and O isotopes of some fresh micrite limestone samples were analyzed; the energy spectrum analysis found that MT consists of microspar calcite mainly, while chemical composition of the matrix is abundantly of peaks Ca, M, Al, Si, K.The geochemical indicators proved that the MT carbonates of Neoproterozoic in the Ji-Liao region developed in the margin of the stable continent, formed in the torrid zone and the paleo-temperatures were about 50℃, the sea water was normal salinity when MT formed during the wanlong period in the southern Jilin, the Yingchengzi period and the Xingmincun period in the eastern Liaoning. the sedimentary environment located in the inner ramp. In summary, it is of important significance for the origin of the MT, also for the paleo-climate, the paleo-environment, ascertaining the age and the stratigraphy division and correlation of the Proterozoic to study the geochemical features of the MT carbonates and its formation environment.