NEWLY IDENTIFIED ERUPTIVE PHASES OF THE BUTLER HILL GRANITIC SERIES IN THE EASTERN ST. FRANCOIS MOUNTAINS: GEOLOGIC AND GEOCHEMICAL ANALYSIS OF THE RHYOLITIC UNITS EXPOSED IN A ROADCUT AT THE INTERSECTION OF HIGHWAY 67 AND STATE ROAD E NEAR FREDERICKTON, MISSOURI

At the intersection of Highway 67 and State Road E west of Fredericktown, Missouri, four rhyolitic units have been exposed in a relatively new road cut on the west side of Highway 67. From north to south the units are: a black, porphyritic rhyolite that is intruded by a thick (~10m), pink, subvertical, porphyritic rhyolite dike and a thin (<1m), red, subvertical, aphanitic rhyolite dike, and a thick, red, hypabyssal, porphyritic rhyolitic unit. The red rhyolite is unconformably overlain by a Precambrian boulder conglomerate which in turn, is overlain by sandy dolomitic sediments. The red rhyolite intruded the black rhyolite along a northward, steeply dipping contact along which, the red rhyolite exhibits a thin chill zone. The intrusive nature of the red rhyolite may indicate that this unit may be part of a cone sheet related to intrusion of the Slabtown Granite (Butler Hill granitic suite). The red rhyolite contains abundant xenoliths of a black porphyritic rhyolite (geochemically unrelated to the black rhyolite unit) and shows evidence of significant mingling and mixing of the xenolithic material. The black rhyolite flow has uncommon red xenoliths (not geochemically related to the red rhyolite unit) as well as occasional pyrite-rich quartz veins, rare veins of carbonate and isolated, thin veins of galena. The subvertical orientation of the rhyolite dikes that intrude the black rhyolite unit is used here to suggest that there has been minimal tilting of the outcrop. Major and trace element analysis of the rhyolitic units reveals that the Breadtray and Slabtown granites resemble the red rhyolite and black rhyolite respectively. In Harker variation diagrams, the black rhyolite was much lower in silica content but is similar to other major element concentrations. On whole-rock chondrite plots, all of the units are chemically homogenous. This indicates a possible co-magmatic relationship between the granitic units and the eruptive rhyolitic units. The primary difference between the granites and rhyolites is the Eu anomaly. Both the Breadtray granite and red rhyolite show a negative Eu anomaly but the Breadtray granite’s negative anomaly is much deeper. The black rhyolite shows a positive Eu anomaly while the Slabtown granite has no significant Eu anomaly.