Geochemical studies of Archaean granitoid rocks in the Southeastern Kaapvaal Province: implications for crustal development

Abstract

Recently completed mapping of the Archaean granitoid terrain immediately to the south of Swaziland and extending into northern Natal reveals that tonalitic-trondhjemitic-granodioritic (TTG) lithologies are dominant. The first recognized generation of TTG magmas is preserved as complexly deformed layered gneisses comparable in both mineralogy and chemistry to the bimodal suite in the Ancient Gneiss Complex of Swaziland. These TTG gneisses are characterized by heavy rare-earth element (REE) and Y depletion and strongly fractioned REE patterns, having either no or a small positive Eu anomaly. The most widespread TTG magmas are represented by the Anhalt Granitoid Suite that builds a major, tabular batholith. Rb-Sr and Pb isotopic data indicate that the Anhalt Granitoid Suite was emplaced at ∼3.25 Ga. This age suggests emplacement of the suite was contemporaneous with the intrusion of the Kaap Valley and Stolzburg plutons into the northwestern flank of the Barberton Sequence, 100 km to the North. It is proposed that the Anhalt Granitoid Suite was generated as a consequence of thrusting directed N or NW associated with crustal shortening during the initial stages of continental collision that ultimately lead to the closure of an oceanic basin in which the Onverwacht Group of the Barberton Sequence accumulated. Final stages of collision were marked at ∼3.1 Ga by the intrusion of tabular, potassic granite batholiths, the main locus of emplacement of which lies to N and NW of that of the Anhalt Granitoid Suite. Subsequent to the establishment of thick continental crust as a result of repeated granitoid plutonism prior to ∼ 3.1 Ga, an extensive depositional basin developed in which the Pongola Sequence accumulated. Dominantly potassic granitoids having geochemical characteristics of Phanerozoic with-in plate granites intrude the Pongola Sequence as (i) gneiss domes, (ii) a tabular batholith and (iii) sharply discordant, megacrystic plutons. The source of these granitoid magmas is uncertain but partial melting of the pre-Pongola sialic basement is considered to be probable.