Decompressional Heating of the Mahalapye Complex (Limpopo Belt, Botswana): a Response to Palaeoproterozoic Magmatic Underplating?

Abstract

Results of petrological and geochronological investigations indicate that during the Palaeoproterozoic the Mahalapye Complex of the Limpopo Belt experienced a prograde decompression-heating P–T path from 650°C/6·5 kbar to 800°C/5 kbar, followed by a retrograde decompression-cooling P–T path to 650°C/3 kbar. This P–T–time path is constrained by detailed petrographic observations in concert with P–T pseudosection calculations applied to Si-undersaturated metapelitic rocks, U–Pb ages between 2046 ± 9 and 2025 ± 18 Ma obtained by in situ dating of monazite, xenotime and zircon grains from metasediments, and a garnet Lu–Hf isochron age of c. 2040 Ma. Additionally, combined U–Pb and Lu–Hf isotope data from magmatic zircon grains indicate that the inferred metamorphic evolution was accompanied by the intrusion of granitic rocks derived from both crust and mantle sources. Diorites, which intruded the Mahalapye Complex between 2061 ± 6 and 2040 ± 18 Ma (i.e. prior to or during the metamorphic peak) are characterized by εHft values between –0·5 and –3·5, whereas granodiorites and granites with intrusion ages between 2039 ± 9 and 2026 ± 10 Ma have significantly lower εHf2·04Ga between –9·4 and –12·6. These data suggest that the diorites were derived predominantly from a Palaeoproterozoic mantle source, whereas the granites were formed by re-melting of Archaean crust. This history is consistent with a model suggesting that the Mahalapye Complex was underplated and intruded by mantle-derived mafic melts between 2·06 and 2·04 Ga and that these melts caused partial melting of the adjacent continental crust. Formation and ascent of voluminous crustal melts caused heat transfer into the middle and upper crust and led to the observed high-grade metamorphic overprint. In a wider geological context, the inferred magmatic underplating at 2·06–2·04 Ga was caused by mantle melting beneath the Mahalapye Complex, which was related to the formation of the Bushveld Igneous Complex and/or to subduction processes during the Kheis–Magondi Orogeny.