Preservation and evolution of quartz phenocrysts in deformed rhyolites from the proterozoic of southwestern North America

Abstract

Euhedral quartz phenocrysts (quartz eyes) are well preserved in ductilely deformed Proterozoic metarhyolites in southwestern North America. In a single sample, the phenocrysts can range from undeformed to highly deformed and recrystallized. Matrix fabrics are extremely similar to those in undeformed ash-flow tuffs. The preservation of the phenocrysts reflects an efficient partitioning of strain from the phenocrysts into the finegrained, wet and possibly glassy matrix. Variations in the degree of phenocryst deformation involve: (1) the chemical and mechanical character of the original phenocryst; (2) the degree to which matrix water has access to the quartz crystal: and (3) crystallographic orientation. Phenocryst-bearing metarhyolites are relatively strain-insensitive, but they contain important kinematic information even in recrystallized rocks where constraints on early structural events are rare. Most phenocrysts have asymmetrical tails, shadows, or deflected matrix foliations that indicate a consistent sense of shear throughout a sample or outcrop. These deformation-induced fabrics are readily distinguishable from primary fabrics in all but the lowest grade samples. In the Tusas Mountains of northern New Mexico and in the Shylock fault zone of central Arizona, quartz-eye metarhyolites have been useful in differentiating early thrusting and shortening events from later syn-metamorphic events that dominate the microstructures and obscure the early tectonic history.