MICROSTRUCTURAL ANALYSIS OF HYPERSOLIDUS FOLIATIONS AT THE ATLANTIS BANK OCEANIC CORE COMPLEX, SOUTHWEST INDIAN RIDGE
In Fabric Type I, subhedral and elongate plagioclase laths define a strong foliation where plagioclase grains display microstructures such as undulose extinction and tapered deformation twins, which are indicative of very minor strain. In contrast, clinopyroxene grains display microstructures indicative of slightly more strain relative to plagioclase, as subgrains are common and systematically oriented perpendicular to foliation. In Fabric Type II, a moderate foliation is defined by subhedral and elongate plagioclase laths that exhibit undulose extinction and deformation twins, and smaller grains that show curved or lobate grain boundaries indicative of recovery through grain boundary mobility and minor recrystallization. Both clinopyroxene and olivine display evidence of minor strain such as undulose extinction, but olivine exhibits rare subgrains that are systematically oriented perpendicular to foliation. Late-stage igneous brown hornblende mantles clinopyroxene grains and displays straight extinction. Fabric Types I and II have strong to moderate foliations, yet the constituent minerals show relatively minor evidence of internal crystal plasticity, suggesting both fabrics formed in the presence of melt. However, the fabrics are differentiated by relative abundances of microstructures indicative of strain and recovery, where Type II fabrics may indicate more intense deformation associated with the onset of strain localization, possibly due to decreasing melt fraction approaching the solidus.