Regresar

A mineralogical, chemical and isotopic investigation of shales from the barberton greenstone belt, South Africa, to constrain source materials and post-deposition evolution

Abstract:

Major, trace and rare-earth elements were determined, together with their Rb-Sr, Sm-Nd and Pb-Pb isotopic systems, on shales from the ∼3.5 to 3.1 Ga old Barberton Greenstone Belt in South Africa, to constrain their source materials and post-depositional evolution, on the basis of successive potential alteration impacts. A progressive change in the chemical composition has been identified from a dominant ultramafic-mafic (such as a komatiite end-member) source for the Fig Tree Group to a progressively felsic-plutonic (such as a granite end-member) provenance for the Moodies Group. The SiO2, K2O, U, Rb, Ba, and Sr contents, and the Zr/Y ratio increase upwards the stratigraphy, while the MgO, Fe2O3 contents, and the Sm/Nd and Cr/Zr ratios decrease. Both groups of rocks yield also high concentrations of Cr and Ni in comparison to other Archean shales. The shales of the Fig Tree Group yield varied REE patterns with average LaN/YbN ratios of 6.6 ± 1.9 (2∞), while those of the Moodies Group yield more fractionated REE patterns with LaN/YbN ratios of 8.8 ± 4.6. An upward increase in the GdN/YbN, Th/Sc, La/Sc and La/Yb ratios, and a decrease in the Sm/Nd ratio, all reflect a change from an initial relatively undifferentiated mafic uppercrustal provenance to a more differentiated felsic source. The Rb-Sr and Pb-Pb isotopic ages point to a tectono-thermal episode at ∼2.7 to ∼2.6 Ga in the Fig Tree shales, while those of the Moodies Group are less constrained and point to a younger activity at about ∼2.3 to ∼2.2 Ga. The depleted mantle Sm-Nd model ages (TDM) are widely scattered between ∼3.85 and ∼3.02 Ga for the two groups of shales, suggesting mixing of materials from both mafic and felsic origins. The Rb-Sr and Pb-Pb ages resulted from extensive fluid migrations during tectono-thermal episodes that disturbed and even reset completely the examined isotopic systems. Some high ϵNd values for the sedimentary rocks at the time of deposition reflect an initial heterogeneity of the Sm-Nd chronometer.