TEXTURAL ANALYSIS OF LAYERING IN THE DAIS LAYERED MAFIC INTRUSION, ANTARCTICA

The McMurdo Dry Valleys of Antarctica exposes the upper ~3 km of a Jurassic aged, shallow magmatic plumbing system. This system is comprised of a stacked series of sills. The lowest most exposed sill in the system, the Basement Sill, has phenocryst-free upper and lower contacts and in its interior, a laterally constrained cumulate tongue of orthopyroxene and plagioclase. This tongue can be tracked for many kilometers, and in upper Wright Valley the cumulate pile transitions, becoming a layered mafic intrusion (LMI), with alternating plagioclase-rich and pyroxene-rich layers. Because the Dais LMI laterally transitions into non-layered cumulate, there is great potential for using this site as a natural laboratory for better understanding the origin of igneous layering. Presented here are textural analyses across a boundary between layers: a lower plagioclase-rich layer and an upper pyroxene-rich layer. Crystal Size Distributions are used to characterize crystal populations across the layer boundary. CSD’s are non-linear, concave. The lower, plagioclase-rich layer contains a larger population of finer particles, both plagioclase and pyroxene, relative to the overlying layer. The upper, pyroxene-rich layer contains significantly larger pyroxene grains (~3x larger) than the lower plagioclase-rich layer. EBSD analyses are used to identify fabric as well as to characterize crystal deformation. A strong fabric exists in the lower plagioclase-rich layer, with a foliation normal to the vertical axis. Transitioning into the overlying pyroxene-rich layer a much weaker, more random fabric exists. Taken in sum, these observations are not consistent with classical models of layer formation, for example: crystal-liquid segregation in a convecting magma chamber, or in situ crystallization and compaction. Instead, the textural patterns identified are suggestive of layer formation via the process of kinetic sieving—a process whereby particle-particle interactions result in the segregation of particles primarily on the basis of size, with finer particles migrating downwards and larger particles migrating upwards. If granular processing in the cumulate pile is indeed possible, then a wholly separate set of physics is necessary to more fully understand the process of magmatic layering.