Quartz-kyanite pods in the Tusas Mountains, northern New Mexico: A sheared and metamorphosed fossil hydrothermal system in the Vadito Group metarhyolite
— Mary C. Simmons, Karl E. Karlstrom, Michael W. Williams, and Toti E. Larson

Lenses and pods or knobs of quartz-kyanite schist occur within a stratiform, map-scale white mica schist horizon in the 1.7 Ga. Vadito Group metarhyolite in the Tusas Mountains of northern New Mexico. Unusually high Al-bulk compositions in the large quartz-kyanite pods are likely the result multiple alteration processes that operated during the long history of volcanism and four episodes of deformation and metamorphism. Geochemical data collected in sampling traverses across the Vadito Group metarhyolite through the quartz-kyanite pods and sericite schist layer, combined with mineral textures and map patterns, provided the means to evaluate the origin and tectonic evolution of these unusual rocks. Geochemical trends from the sampling traverses show symmetrical depletion of Ca, Na, K, Fe, and enrichment of Si toward the centers of the quartz-kyanite pods, indicating alteration typical of shear zones and hydrothermal zones, but atypical of weathering profiles. Map patterns, geochemistry and mineral textures suggest the aligned quartz-kyanite pods may be the metamorphosed remnants of a hydrothermal alteration zone that was active during Vadito Group volcanism. We envision that hot acidic fluids migrated along fractures, altering rhyolite host rocks, and producing lenses and zones of clay and other minerals including pyrophyllite, chloritoid and possibly andalusite and staurolite. Weakened hydrothermally altered zones (represented by the quartz-kyanite pods) were linked together into a map-scale white-mica-rich S₁ shear zone during D₁ metamorphism, causing additional meta-somatic leaching of mobile elements (e.g. Ca, Na, Fe, K) that enhanced the concentration of silica and aluminum in this zone. Quartz-kyanite oxygen isotope thermometry on two distinct varieties of kyanite, produced ð18O values of 7.0 and 7.5 per mil, corresponding to temperatures of formation of 530°C and 590°C, and correlate with peak metamorphic conditions. Light ð18O values may indicate an early hydrothermal system that acted on these rocks prior to metamorphism. We hypothesize a P-T path for the quartz-kyanite pods that began with sub-surface (1-2 km) hydrothermal alteration of Vadito Group metarhyolite and the production of kaolinite at temperatures less than <300°C, and ended with peak metamorphism within the kyanite stability field during D₂, at conditions of around 500ºC and 400 MPa.