P-T and Ar–Ar age constraints on low- to high-grade metabasites from the Buhwa Greenstone Belt, southern Africa: Implications for Neoarchean to Paleoproterozoic thermal evolution along the Limpopo Complex–Zimbabwe Craton boundary

Abstract

This study addresses the Neoarchean to Paleoproterozoic thermal evolution from the Buhwa Greenstone Belt (BGB) along the contact zone between the Archean Zimbabwe Craton (ZC) and a granulite terrane of the Northern Marginal Zone (NMZ) of the Limpopo Complex. Available tectonic models suggest a thrusting of the hot NMZ over and onto the cold ZC, which possibly gave rise to local heating of the margin of the craton by a “hot-iron effect”. Petrographic, pressure–temperature data, along with hornblende Ar–Ar ages for metabasites from the BGB were coupled with a discussion of the tectonics of this region. The metabasites typically occur interlayered with metasediments. Four types of metabasites occur, namely, calcic amphibole–epidote schist, amphibolite (garnet-bearing and garnet-free), calcic amphibole–clinopyroxene schist, and mafic granulite, indicating a varying metamorphic grade from epidote–amphibolite facies to granulite facies. The lower-grade calcic amphibole–epidote schists occur in the northwestern margin of the belt, whereas the higher-grade amphibolites and mafic granulites are present along the southeastern margin of the BGB adjacent to the granulite terrane. The increase in metamorphic grade towards the NMZ of the Limpopo Complex is supported by temperature estimates of <500 °C to >700 °C from northwest to south east. The results of our study are consistent with the pervailing tectonic model which considers the NMZ–ZC boundary as a major thrust-sense shear zone dipping southward, and the hot NMZ thrusted over the cold ZC. New hornblende Ar–Ar ages indicated two discrete thermal events recorded in the BGB rocks: 2525 ± 19 Ma from mafic granulite and 2020 ± 8 Ma to 1968 ± 10 Ma from amphibolites. The Neoarchean age probably corresponds to a cooling age after the peak metamorphism, indicating that the NMZ thrusting onto the ZC took place at >2525 Ma. The age of the thrusting of the NMZ is younger than that of the Southern Marginal Zone (SMZ) of the Limpopo Complex onto the Kaapvaal Craton (KC) (ca. 2.72–2.69 Ga). Therefore, the formation of the Limpopo Complex is not because of a simple collision of the KC and the ZC, but the SMZ-KC collision is more than 100 Myr earlier than the NMZ–ZC collision, suggesting multiple collision for the formation of the Limpopo Complex. In contrast, the younger Paleoproterozoic thermal event coincidental with the last high-grade metamorphic event within the Limpopo Complex might correspond to a thermal reactivation along the NMZ–ZC boundary possibly caused by the global ca. 2.0 Ga orogenic events.