Feasibility of total-rock Pb?Pb dating of metamorphosed banded iron formation; the marydale group, Southern Africa

Abstract

Total-rock PbPb data for a banded iron formation unit and total-rock PbPb and RbSr data for a metabasalt unit, both of which are included in the Marydale Group, southern Africa, are presented. The banded iron formation unit was metamorphosed under conditions ranging from greenschist to amphibolite facies. The samples collected from surface outcrop, in contrast to those obtained from borehole core, show evidence of hydration and oxidation attributed to weathering. The preservation in the greenschist-grade banded iron formation samples of an ∼3000-Ma PbPd date, possibly reflecting the time of deposition, is in marked contrast with the PbPb date of ∼2200 Ma obtained for the amphibolite-facies samples. The younger date is interpreted to indicate Pb isotopic homogenisation during medium-grade metamorphism. A PbPb data of ∼2910 Ma, determined for one suite of metabasalt samples, supports this interpretation. The RbSr data for the metabasalt unit scatter and are not amenable to an age interpretation. The PbPb date obtained for the metabasalt and the banded iron formation confirms the profound age differences that exist in the boundary region between the late-Proterozoic Namaqua-Natal Mobile Belt and the Archean Kaapvaal Craton.