Mesoproterozoic orogeny in Malawi -the Irumide event

Overview & Synopsis

The Mesoproterozoic Irumide belt is a paired orogen comprising two contrasting segments. The northerly, Irumide Domain, is composed of reworked Palaeoproterozoic crust and Mesoproterozoic igneous rocks. To the south, the South Irumide Domain consists of an assembly of juvenile Mesoproterozoic arc-related crustal blocks. The suture between the two domains is a sharp tectonic contact marked by a sudden change in lithologies, magmatism and isotopic signature. The Irumide Domain has been subdivided into the Mzimba, Kasungu and Mchinji subdomains. Each contains high-grade paragneisses that post-date the ~2 Ga Ubendian orogeny. Two age-sequences are recognized; an older assemblage of paragneisses and quartzites considered to be high-grade metamorphic equivalents of the Muva Supergroup in Zambia, and including the Mafingi Group in Malawi. Detrital zircon data show that the other paragneiss sequence also contains significant contributions of ~1100 Ma source rocks currently unknown from Zambia. These younger paragneisses, together with a suite of granitoid orthogneisses dated at ~1080 Ma, provide evidence for a short-lived Mesoproterozoic active continental margin along the southern edge of the Congo-BaNy (combined Bangweulu Block Nyika Subdomain) Craton. The first phase of the Irumide orogeny, resulting from arc accretion of the Irumide and South Irumide Domains to the Congo Craton, is dated at 1069 ± 9 Ma by local syntectonic anatectic melting in the suture between them. This was followed by closure of the Irumide Ocean and docking of the Proto-Kalahari craton from the south at c. 1045 Ma, which represents the main Irumide tectono-metamorphic event. Voluminous, syn-collision magmatism at c. 1040 Ma resulted from melting and assimilation of Ubendian-age continental crust from the Irumide Domain mixed with mantle-derived juvenile crust of the South Irumide Domain. Structural observations show that the Irumide Domain is sandwiched between South Irumide Domain rocks at the base of the tectonic pile and Congo Craton on top. This suggests that N-directed subduction of the Irumide Ocean beneath the Irumide Domain was followed by south-directed nappe transport during the collision phase. Our revised model of the Irumide Orogeny suggests that the main crustal architecture of southern Africa was established at the end of the Mesoproterozoic and is not just the result of the Neoproterozoic Pan-African, East African orogeny as advanced by some researchers. It implies just limited opening of the Damara-Lufilian-Zambezi basin during the Neoproterozoic.