Implications of whole-rock PbPb and zircon evaporation dates for the early metamorphic history of the Kasaï craton, Southern Zaïre

Abstract

A whole-rock PbPb date of 3021 + 47–49 Ma, found for granodioritic granitoid occurring within biotite gneiss in the Sandoa-Kapanga region of the Kasaï Craton, southern Zaïre, in reflected in single zircon evaporation data (centered around 3014 ± 2.3 Ma) obtained from the biotite gneiss itself. Other zircon evaporation dates found in the biotite gneiss range from ca. 3.0 Ga to 3.09 Ga and are interpreted as minimum ages of discordant zircon phases overprinted during one or more metamorphic events. Anomalously high Pb initial ratios characterise the granitoid and consequently its ca. 3.02 Ga age, taken together with the corresponding zircon date, is considered to reflect a metamorphic event during which anatectic melting took place to produce the granodioritic magma. During the same event, overgrowths formed on older zircon cores. The ca. 3.02 Ga date is the oldest yet recognised in the southern Kasaï Craton and a correlation is suggested with the D₂ metamorphic event of this region. Additionally a prehistory back to at least 3.09 Ga is indicated by the other zircon dates in the biotite gneiss.