VOLCANOLOGY OF CAMBRIAN RIFT-RELATED RHYOLITES IN THE EAST TIMBERED HILLS, ARBUCKLE MOUNTAINS, SOUTHERN OKLAHOMA

The Carlton Rhyolite Group forms part of a Lower Cambrian bimodal igneous assemblage emplaced during formation of a major intracontinental rift in southern Oklahoma. The rhyolites, which have A-type compositions, occupy an area of ~ 40,000 km² in the subsurface, with outcrops in the Wichita Mtns. of SW Oklahoma and in the East and West Timbered Hills in the Arbuckle Mtns. ~ 130 km to the SE. Rhyolites in the Wichitas were emplaced as thick cooling units without obvious pyroclastic textures and may represent parts of extensive sheet-like lava flows erupted non-explosively. Rhyolite flows in the Arbuckles have not been studied in detail previously.

Our new mapping in the East Timbered Hills, close to the northern margin of the rift, shows that two rhyolite flows make up most of the volcanic sequence in that area. The total extent and thickness of the flows are unknown because they are truncated by faulting and partly hidden by younger cover. The largest exposed flow can be traced for 3.6 km and is > 300 m thick. It contains feldspar and quartz phenocrysts typically set in a massive felsitic groundmass with randomly oriented to radiating tridymite crystals (now inverted to quartz). Well-developed columnar jointing is perpendicular to the flow base, and delicate flow lamination with local flow breccia occurs in the lower few meters. We infer that this unit is a remnant of a more extensive lava flow of the same type as present in the Wichitas.

The upper flow is separated from the lower flow by volcaniclastic rocks several meters thick that include planar-laminated rhyolitic tuff, tuffaceous mudstone, and massively bedded, polymict conglomerate. The lower rhyolite flow is aphyric and shows flow banding and abundant spherulites throughout; perlitic texture and lithophysae also occur. Open to isoclinal flow folds deform the flow banding, and flow breccia ~ 100 m thick is present in the upper part of the flow. This flow can only be traced ~ 700 m but is > 400 m thick. A complex series of hypabyssal felsic intrusions form irregular contacts with the two main flow units and can be divided into four types based on phenocryst content. The abundance of these subvolcanic intrusions suggests that the study area may be close to a rhyolite source vent. Geochemical studies are in progress to determine petrogenetic relations between the different rhyolite units.