UHT granulites from the Kaapvaal craton – garnet U-Pb ages record a long period of extreme temperatures in the lower crust in the Archean

Ultrahigh temperature granulite xenoliths are reported from four cretaceous kimberlites on the Kaapvaal craton that are aligned along the NNE axis of the Witwatersrand basin. The granulites consist of garnet, sillimanite, sapphirine, orthopyroxene, plagioclase and quartz in various proportions. Peak metamorphic conditions were estimated from garnet+sapphirine+quartz to lie above 1050°C and pressures between 0.9 – 1.2 GPa (Dawson et al., 1997). Zircons gave ages around 2.72 Ga (Schmitz and Bowring, 2003), the same age as the voluminous Ventersdorp flood basalts.

We determined U-Pb ages and trace element contents simultaneously on garnets by split stream LA ICP MS. Sixteen ages fell between 2.9 and 3.1 Ga (± 0.06 Ga) and two overlap with the zircons. We interpret the data such that the main age group records a prolonged period of UHT metamorphism of ca. 200 Ma. Such an extended duration can be attributed to a thickened crust, high contents of radioactive elements and high thermal advection from the mantle in the Archean (Harley, 2016). Witnesses of high temperature processes during that period are the 3.1 Ga granulite metamorphism at mid-crust level of the Vredefort dome, voluminous K-accentuated granitoid intrusions (3.14 -3.04 Ga), the continuous infilling of the Witwatersrand basin between 2.96 and 2.78 Ga with basaltic magmas at its base and intercalated between its sediments and the final collision of the Kaapvaal East and West Blocks at around 2.9 Ga. The two youngest garnet and the zircon ages may reflect a reestablishment of UHT conditions by the Ventersdorp magmatism.