Granitoid gneisses of the Morokweng impact structure: Implications for Neoarchaean evolution of the western Kaapvaal craton

Abstract

One of the most enigmatic aspects of the geology of the Kaapvaal craton of southern Africa concerns the age and nature of the rocks involved in the formation of its western parts, which are almost completely obscured beneath extensive Neoarchaean to Cenozoic supracrustal sequences. A 369-m-deep drillhole near the centre of the ~70 km wide Morokweng impact structure intersects a suite of Neoarchaean, calc-alkaline, granitic-granodioritic, trondhjemitic and monzonitic gneisses. The gneisses are LREE-enriched, but most display primitive mantle-like to slightly depleted HREE concentrations suggestive of a mantle source with garnet retention in the restite. The monzonitic gneiss is distinctly more enriched in trace and REE, and particularly LREE, which is attributed to differentiation processes in parent magmas to the granitoids. Microbeam zircon U Pb geochronology indicates an emplacement age of 2922 ± 5 Ma for the oldest granite, with the remaining granitic, granodioritic and monzonitic rocks being emplaced coevally at 2906 ± 6 Ma. Rare xenocrystic cores preserve an inherited >3.3 Ga crustal component. These crystallization ages coincide with the proposed culmination of westward-directed subduction beneath a significant continental fragment – the Kimberley Block - that led to its collision with the proto-Kaapvaal craton (Witwatersrand Block) between 2.93 and 2.88 Ga, and support a magmatic arc setting. Based on the age data, the granitoid gneisses intersected in the M4 drillhole predate ca. 2.88 Ga unfoliated granitoids found in outcrops and other drillcores in the vicinity that have been interpreted as post-orogenic intrusions but which nonetheless show similar geochemical characteristics to the M4 core granitoids. The distinct age of the M4 granitoid gneisses relative to other granitoid rocks in the Morokweng region may reflect the greater exhumation that occurred in the central parts of the 146 Ma Morokweng impact structure relative to its margins. • Granitoid gneisses exposed by the 146 Ma Morokweng impact belong to a 2.90–2.92 Ga suite of calc-alkaline TTG gneisses • Monzonitic gneiss is strongly enriched in REE, suggesting derivation from a residual magma after fractional crystallization • The granitoid gneisses are interpreted as mid-crustal intrusions formed in a magmatic arc setting • Their intrusion marks the climax of collision and amalgamation of the two major crustal blocks constituting the Kaapvaal craton • Collision affected the tectonic style and sedimentation in the Witwatersrand basin, assisting formation of its gold deposits