Shallow intrusion conditions in a flood-basalt province : the story from dykes and a sill offshoot along the contact between a vent complex and country rock : Coombs Hills, Ferrar Province, Antarctica

Emilie B. M. Guégan (Emilie Benedicte Montaine), 1983-

The Middle Jurassic Kirkpatrick flood basalts and comagmatic Ferrar intrusions exposed in the Transantarctic Mountains represent a major pulse of tholeiitic magmatism, and together constitute the Ferrar Large Igneous Province of Antarctica. At Coombs Hills, the exposed Ferrar intrusions consist of numerous dykes and sills representing a complex plumbing system. This work examines the mode of dyke propagation and magma flow within different dykes present within, and on the edge of, a vent complex, enriching our understanding of magma transport and shallow intrusion conditions within Large Igneous Provinces generally. A tentative scaling relationship analysis has been applied to the dyke population of Coombs Hills from study of aerial photographs. This preliminary study shows a heterogeneous distribution of dykes, both spatially and in terms of orientation, within the area. The length-frequency distribution of dykes follows a power-law distribution, but the physical significance of these observations is not yet known. Field mapping, plus detailed observation and description of dykes' geometry in the field produced one of the main datasets of this study. Dykes exposed at Coombs Hills are highly segmented. The mechanisms inferred for segmentation suggests either a system of preexisting segmented country-rock fractures in some areas, or variations in the orientation or intensity of the principal stresses along the propagation paths. En-echelon arrays of dyke segments are considered to have formed by breakdown of a parent crack in response to spatial or temporal rotations of the remote principal stresses, and mostly show a sense of dextral offset in the field. Bridging structures and en-echelon dykes array reflect a predominantly vertical flow within the area. There are differences in the inferred flow patterns and dyke geometry between the dykes that intrude the well r) . layered but isotropic Beacon sandstone versus dykes intruding the poorly stratified ---------~ Mawson lapilli tuff. These differences may reflect a meandering flow in the poorly stratified Mawson compared to a more straightforward one within the sandstone, and may reflect the poor consolidation inferred for the recently deposited Mawson at the ---~-----~ time of dyke intrusion. The structural analysis reveals two distinct sets of dyke trends, subperpendicular to each other. The country rock fractures in the area present the same geomorphologic aspect as the dykes' walls and their orientations follow similar subperpendicular trends as those identified for the dykes. The fractures are found in prolongation of dykes and are here interpreted to have formed during an early stage of dyke propagation and in the absence of significant stresses other than the magmatically induced ones. The characteristic sub-perpendicular dyke and fracture trends observed suggest that the "background" stress field was an effectively triaxial stress field with strains and fractures generated only by vertical movement and pressure on the host rock. Anisotropy of Magnetic Susceptibility (AMS) analyses have been done on 80 samples collected from the field area. This analysis reveals the importance, when investigating the flow direction within volcanic rocks of considering and removing the magnetic remanent component from the bulk magnetic susceptibility measured on the sample. Unfortunately the degree of anisotropy measured for most samples was consistently very low and did not allow determination of extensive and characteristic flow directions within dykes and within the whole area. The AMS analysis, however, still suggest a sub-horizontal flow direction within the massive basaltic cliff of the area, suggesting it to be a major (more than 300 m wide) sill; this is interesting, because the inner, western contact of the sill with the vent complex is subvertical. The petrographical and geochemical observations indicate that the intrusions are basaltic andesite in terms of T AS classification, with little variation among samples. The geochemical results suggest the dykes intruded the Beacon sandstone first, before minor fractional crystallisation reduced the compatible element abundance in the magma that was subsequently emplaced as the massive sill (basaltic cliff) intrusion. The model of emplacement proposed for Coombs Hills intrusion is comparable to the observed system exposed at Mt Grant where both vertical and horizontal intrusive sheets, as well as inclined ones, are linked together as record of a complex magma distribution system.

v, 188 page + appendices; A4, 1 map (~A2), CD

2006Guegan