Abstract:
Petrographic and chemical studies were made to determine the origin of a band of quartzo-feldspathic rocks enclosed in andesites, basalts and basic tuffs. A primary origin is examined, in which the zone is considered to be acid to intermediate volcanic rocks which were deposited during a normal volcanic cycle; a secondary origin is also considered, in which pervasive alkali-silica metasomatism has replaced basic volcanic flows and tuffs along a shear zone. Gradational features observed in the quartzo-feldspathic band indicate that at least a limited amount of metasomatic transfer of material has occurred. The mechanisms which could have given rise to such a zone include: hydrothermal solutions from a nearby igneous intrusion conducted along a shear zone in the lavas, solphateric alteration, autometamorphism including diagenesis, and dynamic and regional metamorphism.
