Stabilising a craton: the origin and emplacement of the 3.1 Ga Mpuluzi Batholith

The Barberton Greenstone Belt (BGB) and Ancient Gneiss Complex (AGC), located in the eastern part of the Kaapvaal Craton (Swaziland and adjacent South Africa), represent some of the best-preserved and most-studied Early- to Mid-Archaean (3.6–3.2 Ga) crustal remnants. The extensively-studied granite-greenstone belt is surrounded and overlain by several large granitoid bodies (Mpuluzi, Piggs Peak, Nelspruit, and Heerenveen batholiths), all emplaced at ~3.1 Ga, which coincides with the end of TTG magmatism and regional metamorphism in the area. This study focuses on the Mpuluzi batholith as a case study to constrain the prevailing conditions at 3.1 Ga that led to the generation and emplacement of these granitoids, and subsequent cratonisation. The granitoids were emplaced as extensive, km-thick sheets and extend over more than 20,000 km2. Zircon U-Pb ages vary from ~3.16 to ~3.09 Ga; the presence of inherited cores in some grains suggests the involvement of an older crustal component (~3.5 Ga) in the magma generation. This is confirmed by the Hf isotopic data, which show clear indications of mixing between an older crustal component and juvenile material. The Hf isotopic data also provide a good estimate of the age of extraction of the inherited component, with one sample showing a well-defined evolution trend back to ~4.1 Ga. The Hf data also suggest that the juvenile material may have been extracted from the mantle very close to the time of emplacement of the batholith, with the mixing trend intersecting the depleted mantle line in some samples. Whole-rock Sr and Nd isotopic data yield tight isochrons for both systems (MSWD <1), and calculated isochron ages are within error of each other, and of the U-Pb ages (Sr: 3113 ± 84 Ma; Nd: 3027 ± 520 Ma). The close agreement between ages from the three isotopic systems strongly suggests that the whole mass formed and cooled together. The isotopic evidence for mixing between an older crustal component and juvenile material suggests that the mantle may have both provided the heat for melting and contributed material to the magma itself. The variation within the batholith thus represents variable proportions of mixing between these components, and perhaps also a build-up of heat over time. These 3.1 Ga granitoids therefore represent the final stage in the cratonisation of the region; this could represent the “draining” of fusible material from the lower crust, increasing its rigidity and limiting further tectonism.