A quantitative microstructure and crystallography study addressing the development of the crystal and magnetic fabrics from cumulate rocks in the lower portion of the Dufek Massif Intrusion, Antarctica

Our understanding of cumulate rock formation in intrusive mafic igneous bodies has dramatically changed over the last half century. This thesis examines and constrains the processes occurring during gabbroic cumulate formation in the lower portion of the Dufek Massif Intrusion, Antarctica. We used quantitative microstructural and crystallographic observations to identify (i) the primary cumulate formation processes that form an initial crystal framework such as; crystal accumulation by density currents, crystal rich plumes, and/or in-situ crystal nucleation. (ii) The secondary processes that alter the initial crystal framework and reduce the rocks porosity and include; in-situ crystal overgrowth by porous media convection, mechanical compaction, and viscous compaction along with, (iii) the sub-solidus processes such as; textural re-equilibrium, recrystallization, and mineral exsolution that occur at high temperatures after the rock has solidified.;Electron backscatter diffraction and crystal shape preferred orientation analysis reveals that the cumulates have moderately developed crystal foliations and very weak crystal lineations. The foliations developed early during the primary accumulation of crystals probably due to deposition from density currents or crystal laden plume. Spatial distribution analysis and textural relationships indicate that the major secondary process was porous media convection and consequent in-situ crystal overgrowth; viscous compaction played a limited role in the fabric development. This thesis also examines the relationship of the anisotropy of magnetic susceptibility (AMS) and anisotropy of thermal remanent magnetization (ATRM) to the crystallographic fabric because these magnetic properties are often used as easily measured proxies for rock microstructures. We find the pole to the crystallographic foliation correlates well with the minimum magnetic eigenvector, however, there are no significant mineral lineations that correlate with the maximum magnetic eigenvector.