叠层石中的海绿石化和黄铁矿化:以天津蓟县中元古界铁岭组为例

Glauconitization and pyritization of stromatolites: A case of the Mesoproterozoic Tieling Formation at Jixian section,Tianjin,North China

作为微生物碳酸盐岩的主要类型之一,叠层石是微生物席的主要建造物已成为共识。天津蓟县中元古界铁岭组二段叠层石生物礁灰岩发育,其中的细粒叠层石被前人解释为微生物席捕获碳酸盐泥的微生物建造物,使得其既不同于现代叠层石,也不同于显生宙尤其是寒武纪的叠层石。更为特殊的是,这些叠层石中的海绿石和黄铁矿代表着2种特殊的矿化作用,其中研究区普遍产出的黄铁矿,作为硫酸盐还原细菌的产物,是了解古代微生物的窗口;而发育在高能浅海的海绿石,产出环境不同于现代海绿石,不能作为慢速沉积环境的指示矿物,亦不具有沉积间断的地质意义。2种矿化作用表明铁岭组叠层石是由沉淀作用而非捕获碳酸盐泥形成,这为了解中元古代叠层石的形成和特征提供了一些有益的线索。

As the main type of microbial carbonates,it has become the consensus that stromatolites are mainly constructed by microbial mats. The stromatolitic bioherm of the Second Member of the Tieling Formation in Jixian section,Tianjin,are made of fine-grained stromatolites,interpreted as a microbial structure formed by trapping carbonate muds on microbial mats; they are different from the modern stromatolites and most of the Phanerozoic stromatolites. Particularly,the glauconite and pyrite preserved in the stromatolites represent two types of special mineralization. As a result of sulfate-reducing bacteria,pyrite commonly produced in the study area,is a window to understand the ancient microorganisms. Glauconite that is developed in the high-energy shallow sea is different from the modern glauconite in the productionenvironment; it may not be used as the indicator mineral of the slow depositional rate,nor has the geological significance of sedimentary discontinuity. The two types of mineralization directly suggest that the Tieling stromatolites are formed by precipitation,rather than the capture of carbonate muds. This study provides some useful clues for understanding the characteristics and formation of the Mesoproterozoic stromatolites.