High to ultra-high temperature metamorphism from the Unango subdomain, Malawi

This work is part the GEMMAP project (Geological Mapping and Mineral Assessment of Malawi) conducted between 2016 and 2020 in Malawi to provide a national coverage of geological maps at 1:100 000 scale. The Unango Subdomain in southern Malawi is part of the Mesoproterozoic South Irumide Domain. Most of the Unango Subdomain was strongly reworked during the Neoproterozoic Pan African orogeny that led to the assembly of Gondwana. Our study focusses of the northwestern part of the Unango subdomain where it is possible to observe a mid- to lower crustal sequence with a well exposed contact between it and the overlying Lilongwe Subdomain. The Unango subdomain is characterized by various granulite facies lithodemic units (charnockitic gneisses, sillimanite garnet gneiss, garnet-pyroxene granulitic gneisses, marble, meta-anorthosite, syenite and syenitic orthogneisses and rare quartzite and calcsilicates) with intense penetrative deformation. Pressure-temperature estimates consistently show peak conditions around 1.0 Gpa and 860°C (orthopyroxene-garnet bearing gneiss; sillimanite-garnet gneiss and garnet-clinopyroxene metabasite) using thermodynamic modelling with Perple_X. These conditions are consistent with the observed partial melting (both at the outcrop scale and as melt inclusion entrapped within garnet). We also present the results of LA-ICP-MS U-Pb geochronology on zircon that constrain the age of the magmatism and high-temperature metamorphism. The magmatic rocks (syenite and syenite orthogneisses) were emplaced between 582 and 500 Ma, during the second phase of the East African orogen. Metamorphism is constrained between 551 and 545 Ma. The combination of alkaline magmatism and structural observation indicates a long-lasting extensional setting of ca. 100 Ma, in the Unango SD with possible intermittent thrusting event at ca. 570 Ma. The normal metamorphic gradient to the overlying Lilongwe SD suggests the interpretation of the Unango SD as a metamorphic dome.