Doctoral Degrees (Earth Sciences)

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Browsing Doctoral Degrees (Earth Sciences) by Author "Hall, Duncan James"

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    The processes of melt segregation, magma ascent and pluton emplacement in the continental crust of the Damara Belt, Namibia
    (Stellenbosch : Stellenbosch University, 2016-03) Hall, Duncan James; Kisters, Alexander F. M.; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
    ENGLISH ABSTRACT: Theoretical models of meso- and macroscale granitic melt/magma transport commonly invoke a prominent role for dilatant fracturing of the wall rocks, but supporting field evidence is in many cases only scantly presented. This dissertation is a compilation of three case studies that integrate some of the theoretical models with field-based aspects from the obliquely exposed south Central Zone magmatic arc of the Damara Belt in Namibia. Chapter 3 describes interconnected leucogranite networks that accommodate the transport of melt at the outcrop-scale in homogeneous, high-grade near-source gneisses in the western parts of the south Central Zone. The net-structured leucogranite networks include small intrafolial stringers, larger shear-band-hosted veins and still larger fracture-hosted leucogranite sheets, which display a size-based hierarchical distribution that results from the self-organisation of the near-source melt transport pathways. The development of the networks is controlled intrinsically by variations in the rate of melt supply. During periods of elevated melt supply and under conditions of low differential stress, melt-induced extensional fracturing generates hydraulic gradients that result in the formation of melt sheets which represent potential far-field ascent conduits (dykes sensu lato). However, the orientations of the sheets are controlled by wall-rock structures and regional and local stress fields, resulting in this case in the preservation and eventual crystallisation of shallowly-dipping leucogranite sheets that were unfavourably orientated to accommodate buoyancy-driven melt/magma drainage. Chapter 4 describes the actual drainage of magma along steeply-dipping fracture conduits initiated in well-layered upper amphibolite-facies basement gneisses ~30 km SE of the outcrops presented in chapter 1. Here, the moderately-dipping gneisses contain large-scale leucogranite networks that include deca- to hecto metre-scale subvertical leucogranite lenses interpreted to document the near-source accumulation of magma necessary for efficient far-field ascent. Field relationships also document the drainage of the subvertical lenses at critical lengths and volumes in excess of 100 m and ~2.4×105 m3 respectively. These values as well as the field relationships are broadly consistent with theoretical models of magma transport along mobilised (self-propagating) hydrofractures. Self-propagating hydrofractures propagate at their tips and close simultaneously along their tails, leaving behind only very subtle and easily overprinted evidence, which may account for the elusiveness of granite conduits in the mid-crust. This work presents the first field evidence of large-scale magma drainage along self-propagating hydrofractures and has important consequences for the subsequent transport of magma through and eventual incremental emplacement within the subsolidus crust. Chapter 5 examines in more detail the implications of fracture-controlled magma ascent for granite pluton emplacement in lower amphibolite-facies wall rocks ~80 km northeast of the near-source features presented in chapters 3 and 4. Three closely-spaced tabular granitoid plutons that were emplaced roughly contemporaneously at widely-varying structural levels are investigated. Each of the plutons demonstrates the effect of mechanical contrasts in the wall-rock that favour fracture arrest rather than ascent, particularly rigidity and rheological contrasts. However, contrary to the theoretical models, it is shown that rigidity contrasts which cause fracture arrest need not be associated with lithological contacts and the role of increasing differential stress during the fracture-controlled ascent of magma towards the brittle-to-ductile transition is highlighted. The three chapters outlined above highlight the importance of fracture-controlled magma transport at all scales in the Damara Belt and substantiate some of the theoretical concepts, but also highlight prominent discrepancies between the models and natural systems.