A multielement geochronologic study of the Great Dyke, Zimbabwe: significance of the robust and reset ages

Abstract

New Sm–Nd, U–Pb, and Pb–Pb age determinations indicate that the Great Dyke of Zimbabwe, an elongate intrusion of mafic and ultramafic rocks some 550 km long and between 3 and 10 km wide, is over 100 Ma older than previously believed based on Rb–Sr ages. The intrusion was emplaced as a series of subchambers with similar stratigraphy, comprising a lower ultramafic sequence with cyclic layering of dunite or harzburgite grading upwards into bronzitite, the top sections of which include Pt-enriched sulfide zones, and an upper mafic sequence of pyroxenites capped by olivine gabbro and gabbronorite. The Sm–Nd method has yielded a combined mineral/whole-rock isochron of 2586±16 Ma and ε Nd( t) of +1.1 for samples from the Darwendale, Sebakwe, and Wedza Subchambers as well as the satellite East Dyke. This isochron age is in excellent agreement with the U–Pb age for three concordant rutile fractions extracted from a feldspathic pyroxenite of the Selukwe Subchamber with an error-weighted mean at 2587±8 Ma. Two zircon fractions from the same feldspathic pyroxenite sample as the rutile are discordant, and although not well constrained, suggest Pb loss from the zircons at ca. 830 Ma. This may be related to the onset of the widespread and diachronous Pan-African tectonothermal event in southern Africa. Whole–rock samples and clinopyroxene and plagioclase separates from a Darwendale Subchamber drill core yielded a 207Pb/ 204Pb vs. 206Pb/ 204Pb isochron age of 2596±14 Ma, which is in agreement with the Sm–Nd isochron and the rutile U–Pb crystallization age. This new age information shows that emplacement of the Great Dyke and its satellite dikes closely followed the amalgamation of the Kaapvaal and Zimbabwe Cratons, and was contemporaneous with emplacement of the youngest of the trondhjemite–tonalite–granodiorite granitoid suite in the Zimbabwe Craton. Assuming that amalgamation of the Kaapvaal and Zimbabwe Cratons was largely by NNW-directed convergence, it follows that the source of the Great Dyke was asthenospheric mantle hydrated and enriched in incompatible elements by subduction processes. Isochrons of 206Pb/ 204Pb vs. 238U/ 204Pb and 207Pb/ 204Pb vs. 235U/ 204Pb yield ages with large errors, but well constrained initial Pb ratios ( 206Pb/ 204Pb = 14.15±0.30 and 207Pb/ 204Pb = 15.04±0.06). Assuming a two-stage model for common lead evolution, this result yields a μ value of 9.5. Along with the calculated initial Sr and Nd isotopic compositions, these data are consistent with derivation of the Great Dyke magmas by large volume melting of a mantle that has been hydrated and enriched by subduction. While a small amount of crustal contamination of magma derived from depleted mantle could produce the composition of the Great Dyke, the uniformity of initial ratios between subchambers supports the notion of enrichment in incompatible elements being an intrinsic characteristic of the mantle source.