Fluid-controlled grain boundary migration and switch in slip systems in a high strain, high temperature contact aureole, California, USA

Abstract

Within the highly strained aureole surrounding the Eureka Valley–Joshua Flat–Beer Creek (EJB) composite pluton of eastern California, an inversion in microstructures and crystallographic preferred orientations (CPOs) exists with distance from the contact. An inner aureole (<250m from the contact) consists of quartzites that are interbedded with marbles and calc-silicates. These quartzites are incompletely recrystallized. Most grain boundaries have migrated, although it is clear that grain boundary migration (GBM) is not extensive. Multiple data sets indicate that temperatures of deformation were above 650°C. CPOs are indicative of <a> slip in quartz. Within the outer aureole (250m to 1500m from the contact), quartzites are interbedded with pelitic schist and are completely recrystallized and microstructures are indicative of extensive GBM. CPOs are indicative of prism [c] slip. Oxygen isotope ratios in the inner aureole are only slightly shifted from their original values. Oxygen isotopes from the outer aureole are shifted more, which is consistent with equilibration with locally derived fluids. We suggest that recrystallization in the outer aureole was aided by pore water, water derived from fluid inclusions, and water generated by prograde reactions in the schists. The pore fluids in the inner aureole were also probably initially water-rich. However, during prograde reactions in the intervening calc-silicate rocks, and perhaps more importantly, between calcite cement and quartz in the quartzites, the pore fluid composition in the inner aureole changed to become dominated by CO2, which acted as a non-wetting phase and decreased the fugacity of water slowing grain boundary mobility. Low water fugacity also suppressed the activity of prism [c] slip. Therefore, we propose that dry conditions or a grain boundary fluid with a significant non-wetting component (CO2) can result in apparent temperatures of deformation that are more than 100°C lower than the real temperatures of deformation.