QUARTZ-KYANITE PODS IN PROTEROZOIC ROCKS IN NORTHERN NEW MEXICO: SHEAR ZONE FORMATION ALONG AN OLDER HYDROTHERMAL ALTERATION HORIZON

Large quartz-kyanite schist pods of unusual bulk composition occur within the 1.7 Ga Vadito Group metarhyolite in northern New Mexico. Pods are discontinuous, lenticular, symmetrically zoned, and stratiform within a mappable sericitic horizon. Previous studies have not resolved whether the unusual (high Al, low K, Na, Ca, Fe) bulk composition of these pods was the result of weathering, hydrothermal alteration, or shearing.

Symmetrical geochemical profiles (depletion of Ca, Na, K, Fe, and enrichment of Si toward the center of sampling traverses) indicate that hydrothermal alteration associated with volcanism produced the pre-metamorphism, high-Al bulk composition of the quartz-kyanite. Quartz from the quartz-kyanite pods produced d18O values of 10.5 per mil, and values of 9.6 per mil for quartz from the surrounding metarhyolite. The similarity of these unusually light d18O values points to hydrothermal alteration as the most likely process that produced the unusual bulk compositions.

Truncation of stratigraphic map units, grain-size reduction, S-C fabrics, and asymmetric porphyroblasts point to a top-to-the-south shearing episode (D1) along a bedding-subparallel zone before D2 (N-vergent) deformation produced map-scale folds. Alignment and grain-size reduction of kyanite within the earliest fabric (S1) show kyanite to be an early S1 metamorphic mineral, produced prior to shearing. Other S1 minerals include staurolite, paragonite and albite, indicating peak P-T conditions of up to ~600°, ~6 kbar for S1 fabrics. S2 minerals in the quartz-kyanite pods include staurolite, and chloritoid, and in the adjacent pelitic schist layer, staurolite, biotite and garnet, indicating peak temperature for S2 of 575°, and pressure sufficient for kyanite formation.

Quartz-kyanite oxygen isotope thermometry on two distinct varieties of kyanite, produced d18O values of 7.0 and 7.5 per mil, corresponding to temperatures of formation of 530oC and 590oC, respectively. The temperature estimate of 590oC corresponds with the "peak metamorphic" mineral assemblage formed during D1 (1.65 Ga?). The 530oC temperature estimate may be related to D2, and a 1.4 Ga thermal heating of the region.