New Mexico Geological Society Annual Spring Meeting — Abstracts

Large quartz-kyanite schist pods of unusual bulk composition enclosed by shells of sericite schist occur within the 1.7 Ga Vadito Group metarhyolite in northern New Mexico. These pods are discontinuous, lenticular, symmetrically zoned, and are stratiform within a map-scale sericitic horizon. Previous studies have not resolved whether the high-Al bulk compositions of the quartz-kyanite pods was the result of weathering, hydrothermal alteration, or shearing. Geochemical differences between the quartz-kyanite/sericite schist pods and the sericite-rich layer that connects them suggest more than one alteration process. This study uses geochemical, structural, and metamorphic data to evaluate the origin and tectonic evolution of the quartz-kyanite rocks.

Geochemical data from sampling traverses, mineral textures and map patterns indicate that the quartz-kyanite pods obtained their unusual (high AI, low K, Na, Ca, Fe) compositions through hydrothermal alteration associated with volcanism. Geochemical profiles are symmetrical, with depletion of Ca, Na, K, Fe, and enrichment of Si toward the center of the alteration zone. Higher K and Fe compositions in the sericite-rich layer that connects the pods suggests a different alteration process. Truncation of stratigraphic map units, grain-size reduction, S-C fabrics, and asymmetric porphyroblasts suggest that this second alteration process was related to a top-to-the-south shearing episode (D₁) along a bedding-subparallel zone before D₂ (N-vergent) deformation produced map-scale folds.

Microstructural studies show that kyanite is an early (S₁) metamorphic mineral produced prior to shearing of the previously altered volcanic rock, shown by alignment and grain-size reduction of kyanite within the earliest fabric (S₁). Subsequent metamorphism and shearing may have enhanced the concentration of silica and aluminum in this zone, and linked the pods of altered rock into a map-scale sericite-rich (S₁) shear zone. Other minerals that formed early in the deformational history of these rocks include staurolite, chloritoid, paragonite and albite, indicating peak P-T conditions of up to ~550°, ~5 kbar for S₁ fabrics.