Origin and geodynamic significance of ultramafic-mafic complexes in the North Atlantic and Kaapvaal Cratons

The physical manifestations of plate tectonics on the modern Earth are relatively well-understood, but the nature and timing of its onset remains enigmatic, with the geodynamic regime(s) that operated during the Archaean hotly debated. This absence of a consistent geodynamic framework within which regional-scale observations can be placed limits our understanding of Archaean assemblages and associated mineral deposits. To engage with the Archaean geodynamics discussion, this thesis focuses on ultramafic-mafic complexes in the Lewisian Gneiss Complex (LGC) of the North Atlantic Craton and Johannesburg Dome of the Kaapvaal Craton. Globally, such complexes have been the subject of wide-ranging interpretations that have disparate implications for Archaean geodynamic regimes. Throughout this thesis, it is demonstrated that confidently constraining element mobility is of paramount importance when aiming to constrain the origin of Archaean ultramafic rocks, with a variety of geochemical proxies shown to be susceptible to element mobility. Notably, high field strength element anomalies - a geochemical proxy commonly used to fingerprint subduction-related magmatism - are here shown to be highly susceptible to element mobility, with the role of subduction as an Archaean geodynamic process potentially overestimated as a result. Such mobility can, however, be constrained and a primary geochemistry obtained using the integrated approach utilised here, whereby detailed petrography, bulk-rock geochemistry and mineral chemistry are examined using the context provided by rigorous field geology. Using this approach, the ultramafic-mafic complexes in the LGC are here interpreted as recording two temporally and petrogenetically distinct phases of Archaean magmatism. One group of complexes likely represents an early ultramafic-mafic crust that pre-dates the tonalite-trondhjemite-granodiorite (TTG) magmas, while a second group of complexes are interpreted as representing several layered intrusions that were emplaced into TTG. The ultramafic-mafic complexes in the Johannesburg Dome are considered to represent intrusive and extrusive remnants of an Archaean greenstone belt, contradicting a recently proposed hypothesis whereby the complexes are interpreted as fragments of an Archaean ophiolite. When combined with similar opposition to other proposed Archaean ophiolite occurrences in other cratons, this contradiction is potentially significant to the Archaean geodynamics debate, raising questions as to the validity of a > 3.6 Ga onset for modern-style plate tectonics.