Quartzite (re)crystallization across a contact metamorphic zone

Abstract

Quartz has long piqued the interest of geoscientists because it is a mineral that is stable over a wide range of temperatures and pressures and controls the rheology of Earth’s continental crust. The rheology of quartz is affected by factors such as temperature, pressure, and presence of fluids or other phases. While these variables can be well constrained in experimentally deformed quartz, the behavior of naturally deformed quartz under different conditions is still poorly constrained. A suite of quartzites, naturally deformed by the syntectonic emplacement of the McCartney Mountain Pluton in southwest Montana, was collected and analyzed to evaluate the effect of temperature on quartz recrystallization and mobility of titanium (Ti). A useful method for estimating temperature of these quartz-rich rocks is Titanium in Quartz thermobarometry because the quartz contains rutile inclusions, indicating Ti was present during (re)crystallization of the quartz. Domains of interest were selected on each sample and scanned for electron backscatter diffraction (EBSD) and cathodoluminescence (CL) imaging. CL imaging allows us to qualitatively determine where Ti is concentrated. By linking Ti concentration ([Ti]) to quartz microstructures, we can establish how effectively Ti diffuses under different (re)crystallization conditions. We observed distinct CL features (i.e. concentric zoning, patchy zoning, oscillatory zoning, healed microcracks, and laminations) and interpreted their respective formation (or deformation) mechanisms. Because of the vast array of CL features preserved, these quartzites tell a detailed story of crystallization and recrystallization of quartz, both with and without the presence of hydrothermal fluids. With quantitative trace element analysis, [Ti] can be determined for each of the aforementioned CL features. By combining EBSD, CL imaging and quantitative analysis, semiquantitative [Ti] maps can be created, allowing for the interpretation of [Ti] across the entire domain of interest (DOI) for each sample. Using crosscutting relationships, a detailed thermal history is interpreted for the quartzite suite. The preservation, on the centimeter scale, of different quartz zoning features indicates that quartz underwent dissolution and neocrystallization followed by static or dynamic recrystallization by grain boundary migration at high temperature (~600°C) in the presence of fluids, and that this event likely lasted between 10,000 and 100,000 years according to models for the cooling history near the contact of a shallow pluton. These high-temperature processes were overprinted by brittle deformation that formed relatively Ti-rich healed microcracks.