A SHRIMP U-Pb study of zircons from the layered sequence of the Great Dyke, Zimbabwe, and a granitoid anatectic dyke

摘要

This paper reports the first SHRIMP zircon U-Pb study of rocks from the Great Dyke layered intrusion of Zimbabwe. Zircons were extracted from a pegmatoidal websterite within the layered succession and also a granitoid dyke intrusive into the ultramafic sequence, which is interpreted as an anatectic melt derived from the wall rocks. The anatectic dyke yielded a single population of zircons. The analyses for the websterite and the dyke yielded statistically indistinguishable ~(207)Pb/~(206)Pb crystallisation ages of 2579 ± 3 and 2574 ± 7 Ma, respectively. These ages are approximately 120 million years older than the previously accepted Rb-Sr age of the Great Dyke and agree with other recently determined conventional U-Pb, Pb-Pb and Sm-Nd isochron ages. As the anatectic dyke did not contain xenocrystic zircons, derivation from the older country rock granitodis, which in this area yield and ~(207)Pb/~(206)Pb age of 2647 ± 9 Ma, must have taken place by complete melting of the country rocks. Confirmation of this older age has important implications for the Archaean crustal history of Zimbabwe. The Chilimanzi granitoid suite, ca. 2600 Ma in age, constitutes nearly 50% of the currently exposed surface area of the craton and is the last tectono-thermal event before complete stabilisation of the craton which must have occurred at least by the time the Great Dyke was emplaced, that is, over a period of some 20-25 million years.