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Selective blackening of bioclasts via mixing‐zone aragonite neomorphism in Late Triassic limestone,Zlatibor Mountains,Serbia
Authors:KINGA HIPS  JÁNOS HAAS  MÁRIA VIDÓ  ZSUZSANNA BARNA  DIVNA JOVANOVIĆ  MILAN N. SUDAR  ZOLTÁN SIKLÓSY
Affiliation:1. Geological, Geophysical and Space Science Research Group of Hungarian Academy of Sciences, E?tv?s University, Pázmány s. 1/c, 1117 Budapest, Hungary (E‐mail: hips@ludens.elte.hu);2. E?tv?s Loránd Geophysical Institute, Kolumbusz u. 17–23, 1145 Budapest, Hungary;3. Geological Institute of Hungary, Stefánia u. 14, 1143 Budapest, Hungary;4. Geological Institute of Serbia, Rovinjska St. 12, 11000 Belgrade, Serbia;5. University of Belgrade, Department of Palaeontology, Kameni?ka St. 6, PO Box 227, 11000 Belgrade, Serbia;6. Institute for Geochemical Research, Hungarian Academy of Sciences, Buda?rsi u. 45, 1112 Budapest, Hungary

Associate Editor – Tracy Frank
Abstract:A peculiar facies of the Norian–Rhaetian Dachstein‐type platform carbonates, which contains large amounts of blackened bioclasts and dissolutional cavities filled by cements and internal sediments, occurs in the Zlatibor Mountains, Serbia. Microfacies investigations revealed that the blackened bioclasts are predominantly Solenoporaceae, with a finely crystalline, originally aragonite skeleton of fine cellular structure. Blackening of other bioclasts also occurs subordinately. Solenoporacean‐dominated reefs, developed behind the platform margin patch‐reef tract, were the main source of sand‐sized detritus. The blackened and other non‐blackened bioclasts are incorporated in automicrite cement. Radiaxial fibrous calcite cements in the dissolutional cavities are also black, dark grey or white. Reworked black pebbles were reported from many occurrences of peritidal deposits; in those cases, the blackening took place under pedogenic, meteoric diagenetic conditions. In contrast, in the inner platform deposits of the Ilid?a Limestone, the blackening of bioclasts occurred in a marine–meteoric mixing‐zone, as indicated by petrographic features and geochemical data of the skeleton‐replacing calcite crystals. Attributes of mixing‐zone pore waters were controlled by mixing corrosion, different solubility of carbonate minerals and microbial decomposition of organic matter. In the moderate‐energy inner platform environment, large amounts of microbial organic tissue were accumulated and subsequently decomposed, triggering selective blackening in the course of early, shallow burial diagenesis. The δ18O and δ13C values of the mixing‐zone precipitates and replacive calcite do not produce a linear mixing trend. Variation mainly resulted from microbial decomposition of organic matter that occurred under mixing‐zone conditions. The paragenetic sequence implies cyclic diagenetic conditions that were determined by marine, meteoric and mixing‐zone pore fluids. The diagenetic cycles were controlled by sea‐level fluctuations of moderate amplitude under a semi‐arid to semi‐humid climate.
Keywords:Aragonite replacement  blackening  diagenetic cycles  organic matter  paragenetic sequence  Solenoporaceae
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