Eo to Paleoarchean metamafic-ultramafic rocks from the central portion of the Rio Grande do Norte Domain, Borborema Province, northeast Brazil: The oldest South American platform rocks

摘要

Metamorphosed mafic-ultramafic rocks from the central portion of the Rio Grande do Norte Domain of the Borborema Province (BP), northeastern Brazil, have been studied here with the objective of gaining a better understanding of the Archean evolution of this portion of the BP. The study area is underlain by gneissic-migmatitic basement as well as biotite schists and paragneisses of the Serido and Jucurutu formations of the Serid6 Group. The mafic-ultramafic rocks (serpentinites, amphibole-schists and amphibolites) occur in lenses for 30 km along strike following the NNE regional foliation. The serpentinites occur as relatively small exposures of elongated bodies within paragneisses of the Jucurutu Formation and orthogneisses of the Caico Complex. They consist of xenoblastic serpentine (75-99%) that has replaced the primary ferromagnesian minerals of the protolith. The amphibole schists and amphibolites are whitish gray to green, depending on the dominant mineral phase, tremolite, talc or actinolite. The mafic-ultramafic rocks present high MgO, Ni and Cr2O3 contents and very low TiO2, Na2O and K2O contents, being compositionally similar to mafic-ultramafic rocks from other Archean greenstone belts worldwide. U-Pb (SHRIMP) dating of zircons returned concordia ages of 3526 +/- 5 Ma (MSWD = 0.0084) for the serpentinites of the Serra Verde Mine and 3747 +/- 12 Ma (MSWD = 9.8) for serpentinites from the Oiticica Mine. The petrographic and geochemical characteristics as well as the Eo to Paleoarchean ages of the studied rocks are significant as the basement of this portion of the BP is dominantly of Paleoproterozoic age. It is envisaged that these results will lead to further debate and a greater understanding of crustal evolution of the BP. We propose that these mafic-ultramafic rocks are part of a dismembered Archean greenstone belt, which present the oldest ages so far determined for the South American Platform.