The evolution of the Archaean continental crust of Northern Zimbabwe

Dougherty-Page, Jon Stanley (1994). The evolution of the Archaean continental crust of Northern Zimbabwe. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.0000d65d

Abstract

Granitoid clasts preserved in Late Archaean conglomerates indicate the presence of continental crust in Northern Zimbabwe prior to the"" 2.7 to "" 2.6Ga "event" which terminated with the stabilisation of the Zimbabwe Craton. The "Kober Technique" (Kober, 1986, 1987) of direct thermal ionisation of zircons has been set up in order to investigate the geochronological record preserved in such clasts. Conglomerates were sampled from two localities, Shamva, within the central part of Northern Zimbabwe, and Chinhoyi, at thenorth-western boundary of the craton. The results from both localities demonstrate the presence of continental crust in Northern Zimbabwe with along and complex history prior to the Late Archaean "event". The minimum age of continental crust in the Shamva region is 3.34 Ga (Sm-Nd model age),with further episodes of granitiod intrusion indicated by zircon crystallisation at 3/197 ± 10 Ma, 2,925 ± 10 Ma, and 2,800 ± 20 Ma (Pb-Pb zircon). The Chinhoyi region has a shorter, simpler history, with the earliest recorded continental crust at 2,875 ± 3 Ma and later intrusions of granitoids at 2/800 ± 20 Ma, and2,720 ± 6 Ma (Pb-Pb zircon).

Chemically, the early crust was dominated by sodic, Tonalite Trondhjemite-Granodiorite granitoids, whose formation may be modelled by the partial melting of metabasalts with residual hornblende and/or garnet. By contrast, the granitoids formed during the Late Archaean "event" which culminated in the stabilisation of the craton, dominantly follow calc-alkalinetrends, and their formation may be modelled by the fractionation of basaltic magmas (combined with assimilation- of pre-existing continental material) or intra-crustal remelting. This major switch in the origins (and hence chemistry) of granitoids may be attributed to mantle plume activity, the onset of which is recorded by the presence of greens tone belt volcanics derived from anomalously hot mantle, dated at' 2,713 ± 15 Ma (U-Pb zircon Jelsma, 1993).