The Chirwa dome: granite emplacement during late Archaean thrusting along the northeastern margin of the Zimbabwe craton

Abstract

The Chirwa dome in northeast Zimbabwe is situated at the boundary between the Zimbabwe craton and Archaean gneisses of the Zambezi mobile belt. This circular granite intrusion has long been regarded as Archaean granite which was remobilized during Pan-African times and emplaced as a mantled gneiss dome into a Proterozoic metasedimentary sequence. Field mapping, structural work and new zircon dates indicate that the Chirwa dome and surrounding rocks underwent a very different history. The supracrustal sequence was deposited and moderately deformed between ~2613 and ~2601 Ma ago, as bracketed by a provenance date and an intrusive date, respectively. Shortly after deposition, the sequence was thrust westward and juxtaposed against a greenstone terrain of the Zimbabwe craton. This active thrust stack was intruded by successive syntectonic granitoids at ~2601 and ~2593 Ma. The Chirwa dome granite, ~2570 Ma old, represents the last intrusive event before cessation of deformation. Our data indicate that while the central Zimbabwe craton already behaved as a stable crustal block at ~2600 Ma, tectonism was active along its northeastern margin.