Friday, July 25, 2014

Geochronology of Meso/NeoProterozoic Carbonate Platforms in Brazil

Meso-Neoproterozoic isotope stratigraphy on carbonates platforms in the Brasilia Belt of Brazil


Alvarenga et al


Carbonate platforms were present worldwide during the Proterozoic Eon, and variations in their C and Sr isotope ratios are commonly used as a correlation tool particularly through the Neoproterozoic, when rapid secular change in marine C and Sr isotope values permit distinction between Cryogenian glacial events. In central Brazil, the late Mesoproterozoic and Neoproterozoic eras are represented by a thick succession of sedimentary rocks that were deposited, and later deformed, along the eastern margin of the São Francisco Craton. These strata are divided into the three major groups: (1) the Paranoá Group, which consists of a succession of sandstone, siltstone, rhythmite, and selected intervals of carbonate, (2) the Macaúbas Group, which consists of a glacial diamictite (Jequitaí Formation), and (3) the Bambuí Group with an important carbonate-bearing succession that includes characteristic “cap carbonate” facies in its lower strata. Carbonate facies of Paranoá and Bambuí Groups typically occur in unconformable contact, and when the diamictite of Jequitaí Formation is absent, it can be difficult to determine the stratigraphic position of these lithological similar groups. Furthermore, uncertainties in the age of the Bambuí Group has lead to several distinct interpretations regarding the age of the Jequitaí glacial diamictites.

We investigated the C, O, and Sr isotopes and chemical composition of carbonate rocks in five measured sections, including both pre- and post-glaciation carbonate successions. The δ13C (‰ pdb) values in the upper Paranoá Group occur in a narrow interval between +0.6 and +3.6, whereas the post-glacial Bambuí Group begins with substantially negative values (as low as −5.7‰) in cap dolomite facies and rises to values up to +11 permil in limestone of the upper Sete Lagoas Formation. Similarly, carbonate rocks of the Paranoá and Bambuí groups are distinct in terms of their 87Sr/86Sr ratio. Generally non-radiogenic ratios between 0.7056 and 0.7068 are recorded in the upper Paranoá Group, and ratios between 0.7074 and 0.7080, occurring within the Bambuí Group.

The stratigraphic pattern of the C and Sr ratios indicates distinctive isotopic characteristics for these two carbonate successions. The isotopic data for the cabonates in the Paranoá Group are consistent with a sedimentation age in upper Mesoproterozoic or lowermost Neoproterozoic, preceding the first Cryogenian glaciation. Carbonate facies and the isotopic data for the lower Bambuí Group suggest a relationship with the second Cryogenian glaciation.

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