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Maria Clélia Barreto, Federal University of Pernambuco (Brazil)
Valderez Ferreira, Federal University of Pernambuco (Brazil)
Alcides Sial, Federal University of Pernambuco (Brazil)
José Maurício Silva, Federal University of Pernambuco (Brazil)
Cândido Moura, Federal University of Pará (Brazil)
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The isotopic composition of marbles from two localities (Surubim and Caruaru) of the Caroalina-Surubim Complex, Transversal Zone of the Borborema Province (NE Brazil), was used to estimate the age of deposition of their protoliths and to investigate changes in the ocean water composition at that time. The δ13C values of the analyzed samples from Surubim and Caruaru sites vary, respectively, from -2.6 to +3.9‰PDB and from -2.6 to +4.9‰PDB. The highest values (above +3‰) likely result from increased sequestration and burying of organic carbon during extensive crustal growth and agglutination of the Rodinia supercontinent. Values of δ13C lower than +3‰PDB, on the other hand, probably are related to an increased CO2 flux, rich in 12C, during an event of crustal rifting followed by subduction and sequestration of 12C. The 87Sr/86Sr ratios in the Surubim and Caruaru marbles average, respectively, 0.70833 ±0.00029, and 0.70767 ± 0.00046. In light of these results, it is proposed that the protoliths of the marbles were deposited after a period of rifting in the Transversal Zone. In some samples, variations in the C isotopic composition are possibly associated with the presence of graphite as, during closed-system metamorphism, diffusion of 12C-rich CH4 from graphite to calcite promotes reduction in the δ13C values of the latter. Comparing the results with several secular variation curves of C and Sr isotopes of carbonate successions of known ages, a Neoproterozoic sedimentation age of 0.97-0.8 Ga is proposed. The metamorphism occurred under low pressure and intermediate/high temperature (500-700oC) conditions, as indicated by the main paragenesis of the marbles (calcite, dolomite, tremolite, muscovite, diopside, ± quartz).
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