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

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 k m long and between 3 and 10 km wide, is over 100 Ma older than previously b elieved based on Rb-Sr ages. The intrusion was emplaced as a series of subc hambers with similar stratigraphy, comprising a lower ultramafic sequence w ith cyclic layering of dunite or harzburgite grading upwards into bronzitit e, the top sections of which include Pt-enriched sulfide zones, and an uppe r 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 epsilon(Nd)(t) of +1.1 for samples from the Darwendale, Seba kwe, and Wedza Subchambers as well as the satellite East Dyke. This isochro n age is in excellent agreement with the U-Pb age for three concordant ruti le fractions extracted from a feldspathic pyroxenite of the Selukwe Subcham ber with an error-weighted mean at 2587 +/- 8 Ma. Two zircon fractions from the same feldspathic pyroxenite sample as the rutile are discordant, and a lthough not well constrained, suggest Pb loss from the zircons at ca. 830 M a. This may be related to the onset of the widespread and diachronous Pan-A frican tectonothermal event in southern Africa. Whole-rock samples and clin opyroxene and plagioclase separates from a Darwendale Subchamber drill core yielded a Pb-207/Pb-204 vs. Pb-206/Pb-204 isochron age of 2596 +/- 14 Ma, which is in agreement with the Sm-Nd isochron and the rutile U-Pb, crystall ization age. This new age information shows that emplacement of the Great D yke and its satellite dikes closely followed the amalgamation of the Kaapva al and Zimbabwe Cratons, and was contemporaneous with emplacement of the yo ungest of the trondhjemite-tonalite-granodiorite granitoid suite in the Zim babwe Craton. Assuming that amalgamation of the Kaapvaal and Zimbabwe Crato ns was largely by NNW-directed convergence, it follows that the source of t he Great Dyke was asthenospheric mantle hydrated and enriched in incompatib le elements by subduction processes. Isochrons of Pb-206/Pb-204 vs. U-238/P b-204 and Pb-207/Pb-203 VS. U-235/Pb-204 yield ages with large errors, but well constrained initial Pb ratios (Pb-206/Pb-204 = 14.15 +/- 0.30 and Pb-2 07/Pb-204 = 15.04 +/- 0.06). Assuming a two-stage model for common lead evo lution, this result yields a mu value of 9.5. Along with the calculated ini tial Sr and Nd isotopic compositions, these data are consistent with deriva tion 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 c ontamination of magma derived from depleted mantle could produce the compos ition of the Great Dyke, the uniformity of initial ratios between subchambe rs supports the notion of enrichment in incompatible elements being an intr insic characteristic of the mantle source. (C) 1998 Elsevier Science B.V. A ll rights reserved.