GEOLOGY OF LOS OYAMELES OBSIDIAN SOURCE

The Cantonac rhyolite flow, in the state of Puebla, Mexcio, is the Oyameles/Zaragoza obsidian source, which has been studied from an archaeological perspective because it is widely believed that for more than 1600 years, people in nearby Cantona, supported themselves by mining, processing and trading obsidian from this source. The flow was a product of a post-Xaltipan eruptive event, and was emplaced on the rim of the resulting Los Humeros Caldera. The flow can thus be dated at between 0.46 and 0.36 ma, according to previous workers. Detailed geological study is currently underway, to provide a better understanding concerning the economic pattern of ancient Cantona. This paper addresses spatial, petrographical and chemical characteristics of Los Oyameles obsidian source. Detailed geological mapping was conducted to determine the extent of the rhyolite flow and its contained obsidian. Features such as flow banding, phenocryst content, presence of spherulites, glassiness and vescicularity were recorded during mapping. High quality obsidian, defined as non-altered, non-vescicular, consisting of nearly 100% glass that can break into fragments with sharp edges was the focus of the study. Mapping results show that the rhyolite covers about 44 square kilometers in area with thickness varies from centimeters to more than 10 meters, but less than 10% of the volume consists of such high quality obsidian. Representative hand-specimens and thin sections from different locations with different macroscopic petrography have been carefully examined. One sample has about 1% hornblende as the major phenocrysts, and all other porphyritic samples have quartz and sanidine as their dominant phenocryts. Thin sections of banded obsidian samples show microphenocryst concentrations make up most flow bands. X-Ray Fluorescence (XRF) of projectile points and Instrumental Neutron Activation Analysis (INAA) of both obsidian and other volcanic materials from the volcano are carried out in order to determine the chemical variability of the rhyolite.