Isotope and geochemical constraints on Proterozoic crustal evolution in south-eastern Africa

Abstract

Proterozoic exposures south of the imbricate thrust terrane in the eastern sector of the Namaqua-Natal Belt of southern Africa are dominated by granitoid plutons intrusive into older supracrustal gneisses. Many of these granites have geochemical characteristics similar to “A-type” and rapakivi suites, as previously described in Australia and the northern hemisphere. Ages obtained from the Natal units range from ∼1.1 to ∼0.88 Ga. Nd TDM values cluster about ∼1.4 Ga. The oldest granitoids have “volcanic arc” chemical signatures but, subsequent to ∼1.1 Ga, “A-type” granites are common. This suggests a change in tectonic environment or melting process at about this time. Initial 87Sr/86Sr ratios and ϵNd values for the older suites approach values normally associated with a depleted mantle reservoir whilst μ values are similar to those found in “orogen” environments. These data clearly indicate that the Proterozoic crust of south-eastern Africa was not derived by extensive reactivation of Archaean or Early Proterozoic protoliths. Low 87Sr/86Sr ratios in metasedimentary carbonates and ∼1.8 Ga Nd TDM ages for metapelites from the Belt support this model and preclude the formation of the Belt in an intracratonic setting. All the data thus support models involving substantial crustal accretion and differentiation, primarily subsequent to ∼1.5 Ga.