Comportamento dos sistemas Sm-Nd e Rb-Sr da seqüência acamadada máfico-ultramáfica Ribeirão dos Motas (arqueano), cráton São Francisco Meridional: evidências de enriquecimento mantélico e fracionamento isotópico

Abstract

The Ribeirão dos Motas layered sequence (SARM) crops out in the southern part of the São Francisco Craton, Brazil. This sequence comprises phaneritic metaultramafic and metamafic rocks, which, although slightly deformed and metamorphosed, retain primary igneous layers. Porphyritic rocks with idiomorphic pyroxene crystals and heteradcumulate and adcumulate textures are also present. Eighteen isotopic analyses were performed in the SARM, comprising rocks with primary (relict) textures, as well as rocks in amphibolite facies and retro-metamorphosed to green-schist facies. Seven samples yield a Sm/Nd isochron age of 2.79 ± 0.30 Ga (MSWD = 1.2 e µNdt = + 0.48), constraining the accretion time of the SARM rocks. The positive µNdt value coupled with the Rb/Sr evidence is consistent with mantle source relatively enriched in Nd and Sr isotopes. Nevertheless, some SARM samples display isotopic fractionation and disturbance, which can be ascribed to the following processes or their combinations: a) mobilization of the incompatible elements due to regional high grade metamorphism; b) isotopic changes during upper amphibolite facies overprint; c) isotopic resetting by low-grade fluids associated to the Cláudio Shear zone, which is located nearby the SARM.