CONSTRAINTS ON ARCHEAN CRUSTAL EVOLUTION OF THE ZIMBABWE CRATON - A U-PB ZIRCON, SM-ND AND PB-PB WHOLE-ROCK ISOTOPE STUDY

The U-Pb ages of zircons from seven felsic volcanic and plutonic rocks from northern Zimbabwe combined with field data and Pb-Pb and Sm-Nd w hole-rock isotope data, constrain the timespan of development of the H arare-Shamva granite-greenstone terrain and establish the relative inv olvement of juvenile mantle-derived and reworked crustal material. Bas ement-cover field relationships and isotope and geochemical data demon strate that the greenstones were deposited onto 3.2-2.8 Ga basement gn eisses, in ensialic, continental basins. Geodynamic models for the gen eration of the areally extensive bimodal magmatic products and growth of the pre-existing crustal nucleus consistent with our interpretation s are rift-related: (1) intracontinental rifting related to mantle plu me activity or; (2) rifting in a back-are environment related to a mar ginal volcanic are. The data, in conjunction with field evidence, do n ot indicate the presence and accretion of an older (ca. 2.70 Ga) and a younger (ca. 2.65 Ga) greenstone sequence in the Harare part of the g reenstone belt, as was recently postulated on the basis of SHRIMP zirc on ages. Zircon ages for basal felsic volcanics (2715 +/- 15 Ma) and a subvolcanic porphyry (2672 +/- 12 Ma) constrain the initiation and te rmination of deposition of the greenstone sequence. The timespan of de position of the Upper Bulawayan part of the greenstone sequence corres ponds well with radiometric ages for Upper Bulawayan green-stones in t he central and southern part of the craton and supports the concept of craton-wide lithostratigraphic correlations for the late Archaean in Zimbabwe. Zircon ages for a syn-tectonic gneiss (2667 +/- 4 Ma) and a late syn-tectonic intrusive granodiorite (2664 +/- 15 Ma) pin-point th e age of deformation of the greenstone sequence and compare well with a Pb-Pb age of shear zone related gold mineralization (2659 +/- 13 Ma) associated with the latter intrusive phase. The intimate timing relat ion of greenstone deformation and granitoid emplacement, but also the metamorphic evidence for a thermal effect of the batholiths on the sur rounding greenstone belts, and the structural and strain patterns in t he greenstone sequence around and adjacent to the batholiths, imply th at the intrusion of the granitoids had a significant influence on the tectono-thermal evolution of the greenstone belt. Prolonged magmatic a ctivity is indicated by the zircon ages of small, post-tectonic pluton s intrusive into the green-stone belt, with a mineralized granodiorite dated at 2649 +/- 6 Ma and an unmineralized tonalite at 2618 +/- 6 Ma . The 2601 +/- 14 Ma crystallization age of an ''external'' Chilimanzi -type granite agrees well with existing radiometric ages for similar g ranites within the southern part of the craton, demonstrating a craton -wide event and heralding cratonization. The similarity between U-Pb z ircon ages and T-DM model ages (2.65-2.62 Ga) and the positive epsilon Nd-T values (+3 to +2) for the late syn-tectonic and post-tectonic in trusive plutons within the greenstone belt indicate magmatism was deri ved directly from the mantle or by anatexis of lower crustal sources, with very short crustal residence times, and minor contamination with older crust. The rather high model mu(1) values (8.2-8. 6) are unlikel y to indicate the involvement of significant amounts of older crust an d may be inherited from a high U/Pb mantle source, as was suggested by previous workers for the Archaean mantle beneath Southern Africa. The older T-DM ages for the felsic volcanics (3.0-2.8 Ga) and the porphyr ies (2.8-2.7 Ga) suggest that these felsic magmas were derived by part ial melting of a source that was extracted from the mantle ca. 200 Ma prior to volcanism or may indicate interaction between depleted mantle -derived melts and older crustal material.