Mesoproterozoic orogeny in Malawi, Part 2: the South Irumide Domain and Irumide synthesis

The Mesoproterozoic Irumide belt is a paired orogen with two contrasting segments. In this second paper, we present field, petrographic, U-Pb zircon and whole-rock major and trace element geochemical data on the South Irumide Domain. The northerly, Irumide Domain (IRD), the subject of the first paper, is composed of reworked Palaeoproterozoic crust and Mesoproterozoic igneous rocks along the SE margin of the Congo Craton. To the south, the South Irumide Domain (SID) 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 lithology, magmatism and isotopic signature. In Malawi, the SID has been subdivided into the Lilongwe (west), Unango (east) and Nampula (south) subdomains (SDs). Each is marked by distinct supracrustal rock assemblages and metamorphic conditions but similar calc-alkaline plutonism emplaced in several pulses between 1180 and 1020 Ma. In the SID Nd TDM 2 model ages between 1.13 and 1.47 Ga demonstrate the predominantly juvenile origin of the magmas and supracrustal rocks. Structural observations reveal tectonic stacking of the Unango and Lilongwe subdomains below the IRD, arguing for northward-directed subduction of the SID below the Congo Craton. Arc-accretion has been dated by local anatectic melts in the suture between the IRD and SID at 1067 ± 8 Ma. In the south, the Nampula SD lies in turn below the Unango SD along the Lurio Shear Zone. The continuation of the Nampula SD SE of Proto-Kalahari implies that the craton was part of the Mesoproterozoic orogeny. Arc-continent collision is associated with the main tectono-metamorphic event at 1050 ± 5 Ma. It overlaps with the peak of voluminous syn-to post-collisional plutonism in both the IRD and SID at about 1045 Ma. The difference in geochemistry of the igneous rocks indicating collision (IRD) and subduction (SID) signatures is attributed to the origin of the magmas from different source regions. The former are derived from melting of Paleoproterozoic crust whereas the latter originated from juvenile Mesoproterozoic metasomatized mantle and island arc crust. A later stage of the Mesoproterozoic orogeny, dated by anatectic granites in the SID at 1026 ± 15 Ma, is possibly related to the accretion of the Nampula SD along the southern margin of proto-Kalahari and the northern parts of the SID. This implies that the SID comprises two distinct branches: a northern branch (Lilongwe and Unango SDs and coeval terranes to the west) running between the Congo and Protokalahari cratons and a southern branch (Nampula SD, possibly linking with the Maud belt of East Antarctica and thence westwards into the Namaqua-Natal belt) along the eastern and southern margins of Protokalahari. The Tete anorthosite-gabbro Suite (including the Tengani anorthosite in the Nampula SD in the far south of Malawi, dated at 1008 ± 39 Ma) is associated with orogenic uplift and collapse in the final stages of the Irumide orogeny. Variable reworking by the Pan-African East African orogeny at c. 0.5 Ga is apparent in all domains, broadly decreasing in intensity the NE.