A Palaeoproterozoic dolomite (Vempalle Formation, Cuddapah Basin, India) showing Phanerozoic-type dolomitisation
A Palaeoproterozoic dolomite (Vempalle Formation, Cuddapah Basin, India) showing Phanerozoic-type dolomitisation
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DOI：
10.1016/j.precamres.2019.04.013
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发表时间：
2019-07
影响因子：
3.8
通讯作者：
A. Banerjee;M. Słowakiewicz;T. Majumder;Sayani Khan;S. Patranabis‐Deb;M. Tucker;D. Saha
中科院分区：
文献类型：
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作者：
A. Banerjee;M. Słowakiewicz;T. Majumder;Sayani Khan;S. Patranabis‐Deb;M. Tucker;D. Saha
古元古代Vempalle组的Cuddapah盆地，印度，显着增加了我们的理解前寒武纪海洋碳酸盐系统的演变和地球的早期海洋的氧化还原状态。对新开凿的运河段中1000米长暴露的样品进行的相-微相-成岩-地球化学检查表明，10-15%的前体石灰岩仍以残留的钙晶灰岩斑块和含方解石晶石胶结物的卵状体的形式保存在Vempalle组中。保存原始放射状和同心状结构以及放射状裂缝的卵状岩被认为最初是以方解石形式沉淀的，方解石可能是低镁的。在某些地方，保存的方解石晶石，这是部分取代组构破坏白云岩，显示I型方解石双纹层。岩相学观察表明，Vempalle组白云岩形成通过非常早期的降水，在叠层石保存微生物细丝，以及通过织物破坏性石化过程中浅至中度埋藏。Vempalle组白云岩的特征是泥晶白云岩晶体，这表明石灰泥和泥晶方解石从过饱和的富镁钙溶液中快速早期石化，可能在近地表或浅埋过程中。Vempalle组白云岩的Na、Sr含量沿沿着亏损，δ 18 O值为负值，表明白云岩在埋藏和进一步交代过程中发生了重结晶。白云岩δ 13 C值为-0.5至2‰，可能是继承的原始海洋值。地球化学指标（微量元素和稀土）表明，Cuddapah盆地海水和酸化流体缺氧和含铁，但不是富氧。地球化学分析还表明，埋藏成岩流体可能来自1.9- 2.0Ga大陆裂谷作用的富Eu海水。这种可能的海洋化学与元古代盛行的缺氧、含铁和极高镁/钙的“白云石海洋”形成对比。与典型的前寒武纪白云岩相比，Vempalle白云岩显示出更多的类似于碳酸盐岩化的古生代台地碳酸盐岩，其结构和埋藏白云岩胶结物保存完好。
The Palaeoproterozoic Vempalle Formation of the Cuddapah Basin, India, significantly adds to our understanding of the evolution of Precambrian marine carbonate systems and the redox state of the Earth’s early oceans. A facies-microfacies-diagenetic-geochemical examination of samples from a ∼1000-m long exposure in a freshly-cut canal section shows that 10–15% of precursor limestone is still preserved in the Vempalle Formation in the form of remnant patches of calcimicrite and ooids with calcite spar cement. The ooids, preserving primary radial and concentric fabrics and radial fractures, are considered to have been originally precipitated as calcite, which may have been low-Mg. In places the preserved calcite spar, that is partially replaced by fabric-destructive dolomite, shows Type I calcite twin lamellae. Petrographic observations demonstrate that Vempalle Formation dolomite formed through very early precipitation, which in stromatolites preserved microbial filaments, as well as through fabric-destructive dolomitization during shallow to moderate burial. Vempalle Formation dolomite is characterized by micritic dolomite crystals which suggest rapid early dolomitization of lime mud and micritic calcite from a supersaturated Mg-Ca-rich solution, probably near-surface or during shallow burial. Depletion of Na and Sr contents of Vempalle Formation dolomite along with negative δ18O values indicate dolomite recrystallisation during burial and further replacement. Dolomite δ13C values of −0.5 to 2‰ are likely inherited original marine values. Geochemical proxies (trace elements and rare earths) imply that Cuddapah Basin seawater and dolomitizing fluids were anoxic and ferruginous but not euxinic. Geochemical analyses also indicate that the burial diagenetic fluids evolved from Eu-enriched seawater that probably resulted from continental rifting around 1.9–2.0 Ga. This probable ocean chemistry is in contrast with the anoxic, ferruginous and extremely high Mg/Ca “dolomite oceans” that prevailed during Proterozoic time. The Vempalle dolomite shows more similarities with dolomitised Phanerozoic platform carbonates than typical Precambrian dolomite with its well-preserved textures and burial dolospar cements.