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Paleoenvironments in Meso-Neoproterozoic carbonates of the Mbuji-Mayi Supergroup (Democratic Republic of Congo) - Microfacies analysis combined with C-O-Sr isotopes, major-trace elements and REE+Y distributions

par Delpomdor, Franck ;Blanpied, Christian;Virgone, Aurélien;Préat, Alain
Référence Journal of African earth sciences, 88, page (72-100)
Publication Publié, 2013-12

Article révisé par les pairs

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Résumé :

The Meso- and Neoproterozoic Mbuji-Mayi Supergroup (1155Ma to ca. 800Ma) was deposited in the SE-NW trending siliciclastic-carbonate failed-rift in the Sankuru-Mbuji-Mayi-Lomami-Lovoy Basin. Drillcore- and outcrop-derived microfacies, isotope (C, O and Sr) compositions of carbonates and REE+Y distributions are integrated to unravel the paleoenvironmental and chemical conditions prevailing during deposition and alteration (or contamination) of the Mbuji-Mayi carbonates. The carbonate succession (BIe subgroup and BIIa to BIIe subgroups), composed of 11 microfacies (MF), records the evolution of a marine ramp submitted to evaporation, with basinal and low-energy outer-ramp environments (MF1-MF5), biohermal mid-ramp (MF6) and restricted tide-dominated lagoon inner-ramp (MF7-MF9) settings, overlain by lacustrine (MF10) and sabkha (MF11) environments. The ramp margin is characterized by thick stacks of stromatolitic bioherms. δ13C and δ18O relationships in the Mbuji-Mayi carbonates allow discrimination between meteoric (δ13C: -7.5‰ to +0.0‰, δ18O: -7.0‰ to -1.0‰) and burial lithifications (δ13C: -1.5‰ to +0.0‰, δ18O: -15.1‰ to -7.0‰), that overprinted a primary marine signal (δ13C: -1.5‰ to +2.0‰, δ18O: -3.0‰ to +0.5‰) partially preserved in the subgroups. Unaltered pristine signals are found in the Mbuji-Mayi carbonates with 87Sr/86Sr ratios (0.7065-0.7082) similar to those of the marine-preserved strontium signatures of the early Neoproterozoic oceans. The PAAS-normalized REE+Y distributions indicate that the BIe carbonates were altered by Fe-oxide-rich hydrothermal fluids. BIIb and BIIe carbonates exhibit uniform light REE depleted patterns suggesting inputs of detrital river material whereas a marine seawater, highlighted by the REE+Y distributions is preserved in BIIc and BIId carbonates. The pattern of carbon, oxygen and strontium isotopic variations in the Mbuji-Mayi carbonates reflects deposition and early diagenesis in variety domains in marine, evaporitic and meteoric conditions. Almost all Mbuji-Mayi carbonates display discrete seawater REE+Y distributions, reflecting influences of particulate and colloidal materials derived from riverine inputs or hydrothermal fluids. Our systematic REE+Y approach allows also to infer the nature of the dolomitization processes operating in each carbonate subgroup, i.e. dolomitization may be attributed to evaporative reflux of groundwater or mixing zones of freshwater lenses. The internal architecture and evolution of the carbonate Mbuji-Mayi succession are similar to many Phanerozoic ramps submitted to sealevel variations, climatic changes and episodic detrital inputs. © 2013 Elsevier Ltd.