THE RED, HIGH-SILICA, RHYOLITIC LAVAS OF THE PRECAMBRIAN ST. FRANCOIS MOUNTAINS, SOUTHEAST MISSOURI

The Precambrian volcanics of the St. Francois Mountains of southeastern Missouri are composed of predominately rhyolitic ash-flow tuffs with lesser amounts of rhyolitic lavas and minor amounts of intermediate to mafic flows and dikes. The red, high-silica, rhyolite lavas form a physically and chemically distinct subset of a larger group of rhyolitic lavas. These lavas occur across the exposed volcanic terrain as flow domes and small flows intercalated with ash-flow tuffs. Examples include the Royal Gorge rhyolite in the Taum Sauk region (western mountains) and a series of flow domes and lava flows in the DesArc NE and Rockpile Mountain quadrangles (southern mountains).

This distinct group of lavas is characterized by their brick-red color, excellent flow-banding in their upper parts, and whole-rock silica values of 75 weight percent or greater. The following microscopic textures are noted: decompression vesicles, lithophysae, spherulites (spherical, fan and axiolitic) as well as polyhedral mosaics, vapor phase crystallization, poikilomosaic texture (snowflake), felsitic texture, secondary vesicle fillings, and alkali feldspar phenocrysts with chessboard albite and quartz phenocrysts with embayments. Although these old rocks have undergone devitrification, hydrothermal alteration and local recrystallization, the original textures and cooling history can still be mostly deciphered.