Precambrian crustal architecture of Malawi: a keystone in SE Africa

The results of a major geological mapping project in Malawi have produced a new model of the Precambrian structural architecture of this small country, with major implications for the whole of central-eastern Africa. In the north, the oldest rocks of the Palaeoproterozoic Ubendian Domain (c. 2150-1950 Ma), the traditional division into the Mbozi, Ufipa and Nyika “terranes” (here termed “subdomains - SD”) is confirmed, but with one important proviso. Geophysical data indicate that the Nyika SD extends westwards without break into the “Bangweulu Block” in northern Zambia, suggesting that they are one and the same and the name “(Bangweulu-Nyika block” (BaNy) is proposed for the combined entity. The extended Nyika SD is considered to be the central zone of the orogen, with the other two being the steeply dipping root zones of nappes that were transported NE over the Tanzanian Craton during the Ubendian collision phase. The Ubendian Domain returns Neoarchaean to Palaeoroterozoic Nd model ages. It is unconformably overlain by the low-grade Mafingi Group metasedimentary rocks, seen as equivalents of parts of the Muva Supergroup in Zambia. The Irumide Domain in central Malawi is separated into the Mzimba, Kasungu and Mchinji subdomains. They are characterized by Nd model ages between 1.5 and2.7 Ga, no Ubendian magmatism, voluminous collision-related granitoid magmatism between 1070-1030 Ma and medium to high-grade metamorphism. The South Irumide Domain in southern Malawi is subdivided into the Lilongwe, Unango and Nampula subdomains, characterized by Nd model ages between 1.1-1.5 Ga, indicating generation in a juvenile island arc geodynamic setting with little evidence of older crustal involvement. The Irumide orogeny is marked by tectonic stacking at ∼1070 Ma with polyphase reactivation at the end of the main collision at ∼1020 Ma and overprinting during the Pan-African event at ∼560 Ma. Structural younging of the Irumide thrust stack implies subduction of the South Irumide Domain to the north below the Irumide Domain, with the latter constituting an active continental margin SE of the Congo-BaNy craton. The main collision phase and prograde metamorphism between 1050-1020 Ma are interpreted as evidence of collision between the Congo-BaNy-Tanzania Craton (upper plate), and the subducting Proto-Kalahari Craton margin. The three fundamental Proterozoic domains of Malawi were intruded by several polyphase Neoproterozoic (Tonian) alkaline igneous plutons (Nyassa Suite) between ∼850 and 700 Ma, implying that the domains formed a contiguous block of continental crust in Rodinia at c. 1000 Ma. The Nyassa Suite represents a failed continental rift associated with Rodinia break-up between c. 700-640 Ma. To the west of Malawi, this event was manifested in a confined oceanic basin between the Kalahari and Congo cratons and represented by the Zambezi-Lufilian-Damara belts. This basin closed between 590 and 520 Ma resulting in the re-amalgamation of Congo and Kalahari during the later phases of the Pan-African orogeny and variable tectonic (shear zones), magmatic (Blantyre Suite) and metamorphic effects in Malawi increasing in intensity towards the south.