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C-isotope composition of early Paleoproterozoic carbonates from the Minas Supergroup and the record of The Lomagundi phenomenon in Brazil

C-ISOTOPE COMPOSITION OF EARLY PALEOPROTEROZOIC CARBONATES FROM THE MINAS SUPERGROUP AND THE RECORD OF THE LOMAGUNDI PHENOMENON IN BRAZIL

ALCIDES N. SIAL1, VALDEREZ P. FERREIRA1 AND ANTONIO W. ROMANO2

1NEG-LABISE, Department of Geology, UFPE, Cx. Postal 7852, 59732-970 Recife, PE, Brazil.

2Institute of Geosciences, UFMG, 30430-120 Belo Horizonte, MG, Brazil.

The Paleoproterozoic d13 Ccarb positive excursion (2.25-2.05Ga; Lomagundi phenomenon) has a global character, but no occurrence in South America has been registered. This study reports, by the first time, this C isotope anomaly in Brazil, in the early Paleoproterozoic, marine carbonates of the Fecho do Funil Fm., Minas Supergroup.

The 2.42Ga-old Gandarela Fm. consists of red carbonate BIF at the base of the sequence (Minas Supergroup), gradually replaced upwards by buff dolomites, and limestones, locally stromatolitic, in light and dark-gray alternating bands. Carbonates display d13 Ccarb from -1.6 to + 0.4º/ooPDB (n=58), the most negative values found in red dolomites in contact with the underlying, finely laminated, Cauê banded iron formations. Gandarela carbonates from the Heargraves quarry yielded d13 Ccarb from -1.4 to - 0.6º/ooPDB (n=28).In the Cercadinho Fm., at the base of the Piracicaba Group, d13 Ccarb varies from +3.3 to + 4.2º/ooPDB (n=10), values decreasing erratically with depth.

The Fecho do Funil dolomites (2.11 ± 0.11 Ga, deformation/metamorphic age) were probably deposited within the time span for the Lomagundi positive excursion age of the Kaapval craton, Africa. The sampled section of this Formation consists, at the base, by fine-grained, stromatolite-rich white and pink dolostones, and then by fine-grained white marble. Stratigraphically collected samples show d13 Ccarb remarkably homogeneous (+6.0 to + 6.5º/ooPDB, n=47). The oxygen isotopes are also fairly constant (-9.7 to - 10.8º/ooPDB) and show a trend which is rather antipathetic to the variation in C isotopes. These high d13 Ccarb carbonates show little scatter, relatively shallow trend on d13 C vs d18 CO diagram and are consistent with low-grade metamorphic decarbonation. The elevated C-isotope values were least reset and probably reflect their protolith composition, rather than subsequent diagenetic or metamorphic processes. This Formation is a proxy, in South America, for the global Lomagundi phenomenon.

There is no evidence for the triad early Paleoproterozoic glacial events of the (2.45 - 2.25Ga interval) recognized in North America. C isotope patterns for carbonates of the Minas Supergroup suggest that the Gandarela Fm. was deposited around 2.4Ga; the moderate C positive anomaly of the Cercadinho carbonates suggests deposition around 2.35 and the Fecho do Funil carbonates were deposited probably around 2.2Ga. — (May 24, 2002).

Publication Dates

  • Publication in this collection
    09 Oct 2002
  • Date of issue
    Sept 2002
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