Emilie Guegan
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Magma-supply dynamics of the Ferrar large igneous province, Coombs Hills, AntarcticaEmilie GueganMSc studentSupervisor
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Context of the research
Flood basalt emplacement results from eruption of dyke swarms. A large sill and numerous dykes of Ferrar Dolerite crop out on the north face of Coombs hills and comprise the field area for this study (Grapes et al. 1974; McClintock and White 2001 ;Ross 2005). Theoretical aspects of magma flow though giant dykes were analysed by Fialko and Rubin (1999). A field-based study of a giant swarm of dykes by Mège and Korme (2004) in western Ethiopia shows the importance of dyke study as a key dataset for understanding how much of a dyke swarm is eruptive, and to better understand the mechanisms of fissure eruption in LIPs.
Propagation conditions for long distance lateral growth of dykes that feed flood basalts (Elliot et al. 1999; Elliot et Fleming 2000) have not yet been constrained, however, and only limited information concerning magma flow within the dykes is available. Antarctica has as a major advantage superb unweathered and unvegetated outcrop areas, very favourable to field study. The proposed research is a systematic study of dyke contact characteristics, fabric, geometry and orientations, in the Ferrar Dolerite province to help interpret the nature of magma flow in the plumbing system of this LIP. Exposed at Coombs Hills is a major diatreme complex surounding by massive magmatic intrsuive bodies as well as thinner ones (dikes) which constitue the main interest of our research.
Objectives
The proposed research is a systematic study of dykes geometry and orientations, in the Ferrar Dolerite province to provide interpretation of magma flow in this Large Igneous Province (LIP), as well as information concerning the past tectonic activity of this area.
This project (1) investigate the mode of magma transport through the upper levels of the plumbing system of a continental flood basalt province, (2) assess the evolving stress regime in the lithosphere during magmatism by looking at the timing of magmatism relative to ocean opening, (3) provide further explanation concerning the mechanical evolution of fracture population, thus improving our current knowledge on mechanics of fissure eruption within a LIP.
Methodology
The field-work have been done during an 6-week expedition in Coombs Hill. The area chosen is of particular interest because (1) it lies at shallow intrusion levels (probably only a few hundred meters below eruptive surface), because (2) it includes both country rock (horizontally layered continental quartzose sandstones) and volcaniclastic rocks of a vent complex (fragmental basalt plus fragmented country rock, and of almost the same age as the dikes, hence potentially not lithified at the time of intrusion), and because (3) these two rock types are separated by a thick inclined sheet of dolerite apparently related to intrusion of the massive Ferrar Dolerite sills, and the temporal and genetic relationship between the dikes and this inclined sheet have yet to be established.
The research programme involved: (1) field observations of dike texture, vesicularity, crystal population, and lithic-fragment content, dike contacts with the country rock, and joint patterns within the dikes, (2) flow direction investigation using Anisotropic Magnetic Susceptibility techniques (AMS). The method principally involves the collection of oriented rock samples followed by the measurement of the orientation at which an applied magnetic field produces the strongest induced magnetic field in the sample. Assuming that we can use AMS to determine magma flow directions in these dikes, it will constrain interpretations of the overall magma distribution story, (3) systematic field investigation as well as aerial photo study of dike orientation patterns and crosscutting relationships between Jurassic Ferrar Dolerite dikes, any associated fractures in the area,(4) determination of a fracture-length distribution analysis on the dikes, (5) petrology study, to determine mineralogy and degree of glassiness, which is a good indicator of magma fluidity, (6) A geochemical study of sample collected on different magmatic bodies present at Coombs Hills should reveal if the magma was belonging to distinct events from distinct sources, or not and if the magmas present various degrees of partial melting from the same source and the degree of assimilation/contamination of these magmas.