Extensive magmatism and metamorphism at ca. 3.2 Ga in the eastern Kaapvaal Craton

Abstract

The timing of the emergence of modern-style plate tectonics on Earth is of fundamental importance in understanding the thermal and compositional history of the planet. Although the magmatism and metamorphism in the eastern Kaapvaal Craton was thought by some as geological records for mid-Archean subduction at 3.2 Ga, their petrogenesis was fiercely debated. To reveal the nature of the 3.2 Ga magmatism and metamorphism in the eastern Kaapvaal Craton, here we provide a comprehensive zircon and monazite U-Pb geochronological investigation, coupled with in situ trace element and Hf-O isotopic analyses, for the magmatic and metamorphic rocks from the Barberton granitoid-greenstone terrane in South Africa and Ancient Gneiss Complex in Swaziland. In spite of deriving from potentially different sources as revealed by different lithology, nine magmatic rocks sampled from different plutons have consistent zircon/monazite SIMS 207Pb/206Pb ages of 3240–3220 Ma. Distinct zircon grains, including inherited magmatic and metamorphic, are identified in the metamorphic rocks by detailed CL images and U-Pb geochronological, trace elemental and Hf-O isotopic analyses. The metamorphic zircons give consistent 207Pb/206Pb ages of 3240–3220 Ma. One granitic gneiss and a metapelite give monazite SIMS 207Pb/206Pb dates of 3230.6 ± 2.7 Ma and 3223.1 ± 2.4 Ma, respectively, which are consistent with the results of metamorphic zircon and interpreted as the age of metamorphism. Most metamorphic samples give Ti-in-zircon temperatures of 650–740 °C and one sample yields Y-in-monazite temperatures of 918–966 °C, which imply that the rocks of AGC were universally metamorphosed to upper amphibolite facies, and locally reaching high to ultrahigh temperature (HT-UHT) granulite facies (>900 °C) at 3240–3220 Ma. This study, in combination with previous results, suggests widespread distribution for the 3.2 Ga magmatism and metamorphism in the eastern Kaapvaal Craton without asymmetry in the thermal structure, which is quite different from those of the modern subduction zones. The synchronous occurrence of the metamorphism and magmatism in the eastern Kaapvaal Craton at 3.2 Ga was interpreted as the results of partial convective overturn of the crust.