Rifting of the Kaapvaal Craton during the early Paleoproterozoic: Evidence from magmatism in the western Transvaal subbasin (South Africa)

Abstract

The Neoarchean to Paleoproterozoic (ca. 2.65–2.06 Ga) Transvaal Supergroup of the Kaapvaal Craton is dominated by clastic and chemical sedimentary rocks, with few strata-bound igneous units. Two ca. 2.2 Ga igneous units in its upper part (Pretoria Group), the Machadodorp Member of the Silverton Formation and the Mashishing dyke swarm, have so far only been recognized in the eastern part of the Transvaal subbasin. As these units are near-contemporaneous with an early Paleoproterozoic geochemical excursion to higher Th/Nb ratios compared to both older and younger igneous units, they probably mark a change in the geodynamic evolution of the craton. In the present contribution, we provide new data on the igneous units within the Silverton Formation (one lava and 5 sills) from a borehole (BB11) in the south-western part of the Transvaal subbasin. Two of these igneous units are particularly interesting because they show strong geochemical similarities to the Machadodorp Member and Mashishing dyke swarm, which are thus more spatially extensive than previously thought. In combination with existing data, our results further show that the Machadodorp Member corresponds to two separate, possibly not co-magmatic, igneous events, as documented by low- and high-Th basalts. The low-Th lavas show flat mantle-normalized REE and multi-element patterns, along with depleted Nd and Hf isotopic signatures, which are unusual among Precambrian magmatic rocks in the craton. This signature can be explained by partial melting of a depleted spinel peridotite incorporated into the subcratonic lithospheric mantle (SCLM) by a prior subduction event. A failed rifting event can have triggered melting of such a shallow source in a cratonic context. Even though comparable in age, the high-Th ca. 2.2 Ga Mashishing dyke swarm comes from a very different source, likely a part of the SCLM that had experienced subduction-related fluid metasomatism around the Archean – Proterozoic transition. About 150 million years later, the Bushveld Complex possibly exploited the same rifting structures during its intrusion.