| Summary: | Mn-rich layers and interbedded shales from a well exposed natural section of the Northern-Calabrian Unit (Late Jurassic– Early Oligocene) in the surroundings of the Terranova del Pollino village, southern Italy, have been mineralogically and chemically analyzed, in order to reveal the factors controlling their formation. Mn-rich layers are composed of micas/clay minerals, rhodochrosite, siderite, chlorite and quartz whereas shales are formed by micas, clay minerals, chlorite, quartz, and feldspars. The MnO abundances in the Mn-rich layers, which are depleted relatively to the UCC in SiO2, TiO2, Al2O3, Na2O, K2O, and P2O5, are in the range of 11.01 18.41 (wt. %). R-mode Factor analysis indicate that SiO2, Al2O3, TiO2, Na2O and K2O have high positive weights in the first factor (59.8% of the total variance) whereas high negative weights are observed for Fe2O3, MnO, and CaO. This factor accounts for the competition between the terrigeneous component, the authigenic carbonate phases accumulating Mn and Fe which likely formed during paucity of detrital supply. The negative weight of CaO and MnO in this factor, the higher Ca contents in the Mn-rich layers compared to shales, and the lack of calcite, suggest the presence of a mixed Mn–Ca carbonate rather than pure rhodochrosite. It is generally retained that Ca– rhodochrosite precipitates within the pore waters of reducing sediments since neither rhodochrosite nor siderite can form in equilibrium with bottom seawater. Thus the resulting sediment should be a mixing between the detrital component and the authigenic one. Assuming Al2O3 as an index of the detrital component, it is clearly envisaged that in the Al2O3/MnO vs. Al2O3 diagram the carbonate-rich samples fall on the mixing curve having as end members the average shale and the richest MnO sediment. This supports the idea that carbonate-rich samples formed through precipitation of carbonate minerals in the pore waters of the terrigenous detritus accumulating at the sea bottom. Further the REE distribution of ...
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