Includes bibliographical references. / Melt migration is an important process in the crust that causes significant mass transport, as well as differentiation and stabilisation of continental crust. Melt migration near the source occurs pervasively, through interconnected networks of melt-bearing structures. This style is restricted to the suprasolidus mid- to lower crust, while focused migration and ascent of magma occurs in isolated dykelike structures under subsolidus conditions, generally in the upper crust where brittle fracturing of rocks can occur. The details of how and when melt migration changes from a pervasive to focused style are poorly understood, particularly the temperature, pressure and deformation conditions which allow the transition to occur. The Nupskapa nunatak, in Dronning Maud Land of East Antarctica, exposes large cliffs that record evidence of multiple episodes of melt movement, in the form of pervasive leucogranite vein networks cross-cut by larger leucogranite dykes. Mineral equilibria modelling with THERMOCALC and comparison of results with previous work indicates that the Nupskapa nunatak records both Grenvillian and Pan-African metamorphism. Coarse-grained peak assemblages in samples from the Nupskapa area record conditions of 820-880 C at 9.5-11.6 kbar, while post-tectonic retrograde assemblages record late Pan-African conditions of 555- 595 C at 3.2{4.8 kbar. These later conditions lie between the wet solidus and the brittle-viscous transition and are inferred to represent the conditions of intrusion for post-tectonic composite dykes. Small-scale leucosomes predominantly lie parallel to the gneissic host rock fabric and define a pervasive network across the Nupskapa cliff. These leucosomes exhibit diffuse feathery boundaries and are inferred to represent in situ melting and melt segregation during M1 granulite facies peak metamorphism. Composite leucogranitic dykes cross-cut both the early leucosome phase and Pan-African shear zones in the field area. These north-trending, subvertical dykes are neariii orthogonal to the gneissic fabric. They are 0.5-2 m wide and spaced ~10-20 m apart but not interconnected except where two dykes coalesce. The dykes show almost no shear displacement, indicating that they formed via tensile fracture. This indicates that their intrusion occurred during extensional or strike-slip deformation, under conditions of low differential stress, probably coupled to high melt pressure. The composite dykes resulted from the far-field transport of melt from a source 5 to 15 km below the Nupskapa outcrop. Although individually they are discrete and focused structures, they are numerous across the field area and closely spaced, so together they do not represent a wholly focused melt transfer system. The style of melt migration displayed by the composite dykes is an example of the transition from pervasive to focused migration, occurring in the mid-crust at subsolidus conditions. This transition involved a network of smaller melt-filled fractures gradually coalescing into larger ones with decreasing depth. If pervasive migration becomes focused via this gradual transition, melt accumulation and mixing need not occur solely in the source or final emplacement structure, but rather occurs throughout transport of the magma.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:uct/oai:localhost:11427/13277 |
| Date | January 2014 |
| Creators | Thomas, Sukey Anna Jay |
| Contributors | Diener, Johann, Fagereng, Ake |
| Publisher | University of Cape Town, Faculty of Science, Department of Geological Sciences |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Master Thesis, Masters, MSc |
| Format | application/pdf |
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