Geochronological and isotopic constraints on the Mesoproterozoic Namaqua-Natal Belt: evidence from deep borehole intersections in South Africa

Abstract

Mesoproterozoic lithologies of the Namaqua-Natal Belt are exposed in the west and east of South Africa but the central section of the belt is obscured by younger sedimentary cover. This belt is one of several Grenvillian-aged belts world-wide and is of importance in reconstructions of the Meso- to Neo-Proterozoic supercontinent of Rodinia. Zircons from granite and biotite gneiss in two deep boreholes which penetrated the Phanerozoic cover provide SHRIMP U-Pb dates of 1053+29/-25 Ma and 1134+15/-15 Ma. Sm-Nd model dates for whole-rock samples from these and two other boreholes range from ∼2400 to 1200 Ma, unlike model dates from the eastern (Natal) sector of the Namaqua-Natal Belt which are all younger than ∼1400 Ma. Model dates are similar to those noted in the Bushmanland and Richtersveld Sub-provinces of the western (Namaqua) sector of the Belt. The results are interpreted to indicate that this Proterozoic crust may be an easterly extension of the Bushmanland Sub-province or some equivalent terrane and that there must be a terrane boundary between the eastern-most borehole studied and the surface outcrops further east.