Isotopic and geochemical evidence for crust-mantle interaction during late Archaean crustal growth

The charnockites and enderbites of the North Marginal Zone of the Limpopo Belt in southern Zimbabwe represent a suite of dry magmas of tonalite-trondhjemite-granodiorite (TTG) affinity which crystallised directly in the lower crust. They were emplaced between 2.6 and 2.7 Ga and were derived from a source which showed remarkable lead isotope homogeneity. Unlike many other lower crustal rocks they have low K/Rb and low K/U ratios and have not experienced depletion of their heat producing elements Th, U, K, and Rb and thus represent an unusual suite of granulites. In this study we have combined the constraints of trace element and isotope geochemistry to characterise the nature of the protolith to the NMZ enderbites and charnockites. We use a combination of high mu-values, calculated lead and strontium isotope initial ratios and old Nd model ages to show that the enderbites and charnockites were derived from a protolith which was, in part, formed more than 500 Ma before magma genesis. We argue from the very high mu-values that this protolith constituted two separate components, an old high-mu, low Sm/Nd crustal component and a younger low-mu high Sm/Nd mantle-derived component, both of which were homogenised by extensive mixing prior to 2.7 Ga. The presence of an older crustal component contrasts with other Archaean TTG suites worldwide which show little evidence for a crustal prehistory. The process of mantle-crust interaction during late Archaean crustal growth in southern Zimbabwe invites comparison with modern convergent margins such as the Andes. Copyright (C) 1997 Elsevier Science Ltd.