Mechanisms of Shear Zone Localization and Raman Thermobarometry in Spinel Peridotites from the Alpine Fault, New Zealand

A suite of xenoliths from two localities on South Island, New Zealand, exhibit rare microstructures and provide information on deformation during the early stages of development of the Alpine Fault. The first set of samples has evidence of dislocation creep with subgrains, recrystallized grains, undulose extinction and a lattice preferred orientation. These samples also show evidence of post deformation static grain growth with polygonal grains, 120° triple junctions, and euhedral grains within larger grains of the same phase. Samples from the second locality also show evidence of dislocation creep but with minimal static grain growth. This set also has a population of ultrafine grains (10µm) and millimeter scale mylonitic banding. The ultramylonite bands exhibit phase mixing suggesting grain boundary sliding. Olivine porphyroclasts also contain strings of fine recrystallized grains in the interior of large porphyroclasts, suggesting some grain size reduction via brittle processes. The contrast between these localities and mylonitic banding at the second locality suggests the presence of a narrow-localized shear zone within a zone of weaker deformation. Grain size reduction through brittle deformation would aid in the process of weakening and localization but depth constraints are required to properly interpret these and other microstructures. We find that remnant pressures preserved in spinel inclusions in olivine may be detectable using Raman spectroscopy, thereby serving as a thermobarometer for spinel peridotites. Preliminary results indicate that this a non-trivial undertaking due mainly to compositional variations in spinels and potentially surface variations. Compositional variations will require the determination of each inclusions chemical composition and the use of a single-grain Raman spectrum comparison, as opposed to the grain to grain comparison used for quartz inclusions in garnet. It will also require a new calibration of residual pressure isopleths in pressure-temperature space for each composition. These results demonstrate the viability of developing a reliable thermobarometer for spinel peridotites