Extremely depleted lithospheric mantle and diamonds beneath the southern Zimbabwe Craton

Inclusion-bearing diamonds, mantle xenoliths, and kimberlite concentrates from the Cambrian-aged Murowa and Sese kimberlites have been studied to characterise the nature of the lithospheric mantle beneath the southern Zimbabwe Craton. The diamonds are mostly octahedral, moderately rich in nitrogen with moderate to high aggregation, and contain mainly dunite-harzburgite mineral inclusions. Similarly, dunite xenoliths predominate over harzburgite and lherzolite and carry olivines with Mg/Mg + Fe (Mg#) values of 0.92-0.95, spanning the average signatures for Kaapvaal Craton peridotites. Eclogitic xenoliths are extremely rare, in contrast to the Kaapvaal mantle lithosphere. The Zimbabwe mantle assemblage has been only slightly affected by later silicic metasomatism and re-fertilisation with re-introduction of pyroxenes in contrast to the Kaapvaal and many cratonic lithospheric blocks elsewhere where strong metasomatism and re-fertilisation is widespread. Pyroxene, garnet and spinel thermobarometry suggests an ambient 40 mW m(-2) geotherm, with the lithosphere extending down to 210 km at the time of kimberlite eruption. Whole rock peridotite Re-Os isotope analyses yield T-RD model ages of 2.7 to 2.9 Ga, providing minimum estimates of the time of melt depletion, are slightly younger in age than the basement greenstone formation. These model ages coincide with the mean TRD age of >200 analyses of Kaapvaal Craton peridotites, whereas the average Re-Os model age for the Zimbabwe peridotites is 3.2 Ga. The Os data and low Yb-n/Lu-n ratios suggest a model whereby thick lithospheric mantle was stabilised during the early stages of crustal development by shallow peridotite melting required for formation of residues with sufficiently high Cr/Al to stabilise chromite which then transforms to low Ca, high Cr garnet. Sulphide inclusions in diamond produce minimum TRD model ages of 3.4 Ga indicating that parts of the lithosphere were present at the earliest stages of crust formation in this area. Published mineral analyses for mantle xenoliths, diamonds and concentrate macrocrysts from the Venetia, River Ranch, Mwenezi and Chingwizi pipes which intrude the Limpopo Mobile Belt show strongly depleted peridotitic mantle signatures matching with the mantle beneath southern Zimbabwe. Published seismic velocity imagery suggests the presence of a deep lithospheric mantle keel beneath the Limpopo Mobile Belt today and Re-Os analyses of peridotite xenoliths from Venetia have previously yielded an Archean age. The Limpopo Mobile Belt is therefore interpreted as a product of thin skin crustal tectonics with the underlying lithospheric mantle linked to that of the southern Zimbabwe Craton and unaffected by the overlying crustal events. (C) 2009 Elsevier B.V. All rights reserved.