SHRIMP zircon age constraints on Mesoarchean crustal development in the Vredefort dome, central Kaapvaal Craton, South Africa

Abstract

SHRIMP (sensitive high-resolution ion microprobe) U-Pb zircon dating of the Archean basement rocks in the core of the Vredefort dome, South Africa, is integrated with results of lithological and structural mapping and geochemical data to assess the tectonic evolution of the central parts of the Kaapvaal Craton. Recent tectonic models have envisaged the rocks in the core of the dome as part of a much larger, >3200 Ma, Archean cratonic shield (Kaapvaal Shield) that attained stability during partial melting of its early crust and emplacement of voluminous, Mesoarchean, crustally derived granitoid bodies between 3200 and 3100 Ma. Our studies have indicated, however, that the oldest sialic rocks (tonalite-trondhjemite-granodiorite-TTG) in the core of the dome were only formed between 3120 and 3100 Ma, probably in an island arc setting, and that they were accreted onto the craton shortly thereafter. Tectonic accretion of the TTG rocks led to crustal thickening, partial melting, and emplacement of 3100-3080 Ma granites and granodiorites at midcrustal levels under high-grade metamorphic conditions, slightly later than the main pulse of granitoid magmatism in the Kaapvaal Shield. This event was followed almost immediately by collapse of the thickened crust, leading to rapid exhumation of the midcrustal rocks prior to the deposition of rift-related volcanic and sedimentary supracrustal strata at ca. 3074 Ma. The Mesoarchean tectonomagmatic activity in the core of the dome was terminated with emplacement of 3068 Ma aplite dikes.