MICROSTRUCTURAL EVIDENCE LINKING DAMAGE ZONES AND GRANULITE HYDRATION

The infiltration of fluid into an anhydrous rock body can produce hydrating reactions which alter the bulk properties of the body. In large enough domains, this process can have macro-scale geodynamic and rheological implications as bulk density and strength change. However, the mechanics behind this process are not yet well documented in literature. In this study, we use optical and electron beam observations to document the links among brittle damage, fluid infiltration, and viscous localization. In the Adirondack Marcy massif, New York, cm-wide shear zones cutting meta-anorthosite show a partial transition from dry, granulite-facies assemblages (pyroxene, garnet, plagioclase feldspar, and alkali feldspar) to hydrated assemblages (scapolite and amphibole + quartz symplectite). Importantly, the modal abundance of hydrous phases increases with proximity to the shear zone cores, correlating damage zones and fluid access. Microstructural evidence along this gradient indicates brittle deformation facilitating fluid infiltration along grain boundaries, allowing for metasomatic replacement reactions. Additionally, this process appears to weaken the rock through grain-size reduction as replacement occurs in junctions between grains and as symplectic rims. This is supported by topological phase relationships and partially truncated reaction rims suggesting that viscous shear zones localized along planar areas weakened by brittle damage, which has further rheological implications regarding the localization of viscous shear strain.