Pb/Pb, SmNd and RbSr geochronology in the Archean Craton of Zimbabwe

Abstract

Pb isotope data for greenstone belt volcanic units, plutons and gneisses of the Archean Craton of Zimbabwe are presented together with SmNd isotopic analyses and model Nd mantle derivation (t_DM) ages for selected samples. Pb/Pb whole-rock isochrons yield well-determined dates for the Cardiff Hill rhyolite of the Shamva-Harare greenstone belt (2659₋₃₉⁺³⁸ Ma), the Somabula tonalite (2752₋₅₂⁺⁵⁰ Ma), the Gwenoro Dam migmatitic gneisses (2705_∂3⁺⁶⁰ Ma), and for various suites from the Chingezi tonalite (from 2874±32 to 2686₋₉₄⁺⁸⁸ Ma).

Comparison of geochronological results from this study with those of earlier work (mainly RbSr whole-rock dating) shows some significant discordances, and their possible causes are discussed with regard to time-integrated Th/U ratios and the geological settings of the relevant rock units.

The Chingezi and Sesombi plutons show good agreement between RbSr and Pb/Pb whole-rock isochron dates and also display a limited range of Th/U ratios, appropriate to a purely igneous differentiation history. Early to mid-Archean gneisses show large ranges of Th/U ratios, probably the results of U disturbances during metamorphism. In these rocks Pb/Pb dates may be older or younger than the corresponding RbSr dates, but t_DM model ages generally agree with the older of the isochron results.

The behaviour of the RbSr and UPb whole-rock systems during metamorphism may depend critically on the nature of the fluid phase evolved. CO₂-rich fluids appear to be implicated in U enrichment of early Archean gneisses at the Shabanie Mine. It is argued that CO₂-rich fluids may cause disturbance of the UPb system without resetting the RbSr system, while a H₂O-rich fluid phase could have the reverse effect.