PB/PB, SM-ND AND RB-SR GEOCHRONOLOGY IN THE ARCHEAN CRATON OF ZIMBABWE

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(-63)+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.