中元古代晚期浅海高能沉积环境中的海绿石:以天津蓟县剖面铁岭组为例

海绿石是一种富钾、富铁的含水层状铝硅酸盐矿物,在沉积学领域常被作为一种普遍的指相矿物。多年研究的结果表明,现代海绿石主要形成在慢速、弱还原的较深水环境中,而且还可以作为"凝缩段"的识别标志之一。天津蓟县剖面中元古界铁岭组第二段灰岩中的海绿石,产在高能叠层石岩礁之中,主要以胶体形式富集在叠层石和均一石的边界上,代表较为典型的原地海绿石;较高的氧化钾含量(大于8%)而显示出高成熟海绿石的特点。很明显,铁岭组二段灰岩中的原地高成熟海绿石,不但不能作为"凝缩段"的识别标志,而且也不是长时间地层间断的产物。由于形成在正常高能浅海环境,而且处于中元古代末期,与现代沉积中的海绿石存在较大的差异,可能代表了中元古代末期的正常浅海还处于含氧量不够充分的弱还原状态,最终使铁岭组灰岩中的海绿石成为前寒武纪海绿石产出的一个典型代表,也间接的表明了在漫长的地质历史演变过程中海绿石产出的多样性特点。

Glauconites formed in the high-energy shallow-marine environment of the late Mesoproterozoic: a case study from Tieling Formation at Jixian section in Tianjin, North China

Glauconite is a kind of hydrous-layered aluminium silicate mineral, and this kind of mineral is frequently used as a facies-indicative mineral. Numerous studies on glauconites formation indicate that modern glauconites are mainly formed in the deeper sedimentary environment, which tends to slow down the sedimentation rates and can be further used as a discerning mark for the condensed section in sequence-stratigraphic divisions. Glauconites developed in limestones of the stromatolitic bioherm in the second member of the Mesoproterozoic Tieling Formation at the Jixian section in Tianjin City of North China, are mainly rich in the boundary between stromatolites and the leiolites as a fashion of colloids, which reflect the features of the autochthonous glauconite that is formed in high-energy shallow sedimentary environment. Also, the high content of K2O (>8%) shows that these glauconites can be grouped with the high-evolved glauconites. Peculiarities of these autochthonous glauconites, i.e., the special forming sedimentary environment that is marked by the high-energy shallow water, the particular occurrence form as a fashion of colloids in the boundary between stromatolites and the leiolites, the older forming times that is in the late Mesoproterozoic, indicate that these glauconites are strongly different from the modern glauconites. Sedimentologically, glauconites from the Mesoproterozoic Tieling Formation at the Jixian section reflect that the normally high-energy shallow water is in the condition of the slightly reduced state resulted from the low-oxygen levels in the air of the late Mesoproterozoic. Obviously, glauconites in the stromatolitic limestones of the Mesoproterozoic Tieling Formation cannot be used as a discerning mark of the condensed section in sequence stratigraphy, and cannot be grouped into the product formed in the long term hiatus. Therefore, glauconites in the Tieling Formation are the representative of the Precambrian, which indirectly reflect the diversity of the glauconitic forming environment in the long evolving geological history.