WATER CONTENT IN QUARTZ AND SHEAR INTIATION OF A SMALL MYLONITE IN THE WESTERN ADIRONDACKS

Water content in and around a previously characterized mylonitic shear zone located in the western Adirondacks is being studied by FTIR spectroscopy. The shear zone (strike 163°, dip 55°E) is located in exposures along the banks of the Moose River, approximately 25.3 km east of Lyons Falls, NY. The shear zone has a thickness of about 15 cm and is within a pegmatite in granitic gneiss. General lithology of this area is granitic gneiss, amphibolite and metasedimentary rock (unit CG of Whitney et al. 2002). The pegmatite contains quartz, feldspar and amphibole. We previously interpreted the deformation mechanisms for quartz grains in the shear zone as dislocation creep and possibly diffusion creep or grain boundary sliding. It is hypothesized that strain localization (shear zone formation) was caused by water in the pegmatitic quartz. Basis for this comes from the known hydrolytic weakening effect of water in quartz and that pegmatites as well as hornblende are normally associated with water.

Infrared spectral analysis was carried out using a Nicolet micro-FTIR on three populations of quartz such that measurements were gathered from mylonitic quartz, pegmatitic quartz and gneissic quartz. Aperture size for the IR beam was no larger than 56 mm by 50 mm. Results show mylonitic quartz actually contained the most water with a range of 58 to 1440 H:10⁶ Si (avg 412 H:10⁶ Si). Quartz from the pegmatite contained much less water: 30-45 H:10⁶ Si (avg 36 H:10⁶ Si) and the gneiss contained 30 to 1120 H:10⁶ Si (avg 256 H:10⁶ Si). These data demonstrate there is no excess water in the pegmatitic quartz and thus water within the pegmatitic quartz is not the cause of strain localization. If water weakening is not the cause of shear zone initiation then perhaps further study will show shear initiation as a result of reaction weakening due to the production of new, finer grained phases during retrograde metamorphism.