The tectonic evolution of the southern Congo Craton

Abstract

Constraining the evolution of continents, and the tectonic plates they reside upon, enables geoscientists to understand phenomena such as mantle dynamics, mineral and energy resource distribution, faunal evolution, and climate development. Thus, there is an underlying necessity to have rigorous palaeogeographic models that constrain plate reconfiguration and interaction throughout earth history. The Congo Craton encompasses present-day central Africa, and is comprised of Archean crustal blocks and Proterozoic orogens. The southern margin of this craton acted as a plate boundary from the Palaeoproterozoic to present. However, the evolution of this margin remains largely enigmatic. This thesis interrogates the tectonic evolution Southern Irumide Belt (SIB), a predominantly Mesoproterozoic orogen located along the southern Congo margin, which serves as a vital proxy for understanding the evolution of the Congo Craton. U–Pb dating of detrital zircons from metasedimentary rocks within the Zambian terranes of the SIB identifies Palaeoproterozoic to Mesoproterozoic age populations that are equivalent to those preserved in the Muva Supergroup, found in the Irumide Belt (sensu stricto) of the Congo Craton. This depositional connection between the SIB and Congo Craton prior to the late-Mesoproterozoic is supportive of a tectonic model where the SIB formed on the southern Congo margin. Neoproterozoic sedimentary rocks identified in the Nyimba–Sinda Terrane highlight subsequent extension in the region that is likely a response to rifting. Full-plate topological models suggest that this rifting was a southern extension of the spreading that separated Australia from Laurentia during Rodinia break-up. An isotopic and geochronological investigation of intrusions throughout this region suggest that the SIB formed on an isotopically evolved Palaeoproterozoic basement. This is equivalent to basement in the neighbouring Irumide Belt, and further supports the SIB forming on the southern Congo margin. During the late stages of Gondwana amalgamation, the Congo Craton collided with the Kalahari Craton to form central Gondwana, generating tectono-metamorphic overprints that are displayed in the rocks of the Southern Irumide and Zambezi belts. These overprints record amphibolite to granulite facies mineral assemblages, and include more exotic, high-pressure ‘whiteschist’ assemblages. An activity-composition model was created for yoderite, a key mineral in whiteschists, for use with the pressure–temperature (P–T) modelling software THERMOCALC. Using this model, P–T diagrams were calculated for both a retrogressed whiteschist and metapelite from the region, to constrain the features of the Gondwana forming collision. A thermal gradient of 30–90 °C/kbar was calculated for the whiteschist, consistent with those calculated for peak metamorphism, whereas a gradient of 70–165 °C/kbar was calculated for the metapelite. The different thermal gradients relate to different aspects of the collision. Where the amphibolite facies rocks formed in a compressional setting proximal to the southern Congo margin, the whiteschists instead formed directly at the site of continental collision, marking the suture zone between the Congo and Kalahari Cratons. U–Pb apatite and 40Ar–39Ar muscovite data reveal high-temperature cooling ages spanning the late-Neoproterozoic to Cambrian, relating to cooling after Congo–Kalahari collision. Apatite fission track dating identifies periods of low-temperature cooling during the Carboniferous, Triassic, and Cretaceous, with thermal modelling identifying rapid Cenozoic cooling. These periods are interpreted to relate to periods of exhumation in central Africa, which occurred in response to the wider-scale tectonic processes of Karoo rift basin formation, Gondwana break-up, and the development of the East African Rift System (EARS). These studies provide a framework for understanding the evolution of the Southern Irumide Belt. As a proxy for the evolution of the southern Congo margin, this work serves to constrain palaeogeographic models for the Congo Craton, further elucidating its role within the wider Earth system.