CAMBRIAN A-TYPE RHYOLITE LAVAS, BASALTIC PHREATOMAGMATIC VENT CONDUITS AND BIMODAL HYPABYSSAL INTRUSIONS EMPLACED ALONG A RIFT-BOUNDING FAULT IN THE SOUTHERN OKLAHOMA AULACOGEN

Cambrian bimodal volcanic rocks within the Southern Oklahoma aulacogen (SOA) extend for ~40,000 km² in the subsurface but crop out only in limited areas in the Wichita Mountains in SW Oklahoma and the Arbuckle Mountains farther east. Volcanic rocks in the Wichitas were emplaced ~10 km into the rift from its northern margin and consist mostly of tabular rhyolite flows stacked in vertical successions; similar flows in the subsurface are intercalated with large amounts of basalt. Volcanic rocks exposed in the Arbuckles were emplaced as near as 0.5 km from the main normal fault zone defining the northern rift flank, and our detailed mapping reveals a distinctly different lithofacies assemblage in this part of the rift.

Nine separate rhyolite flows are exposed in the Arbuckles and have A-type compositions similar to rhyolites elsewhere in the SOA. The flows can only be traced limited distances (<3.5 km) laterally before being truncated by faults, but most show a consistent vertical zonation of emplacement- and cooling-related features that suggests they are erosional remnants of more extensive lava flows similar to those found in other A-type provinces. The largest flow in the Arbuckles is >600 m thick and is the thickest flow so far documented in the SOA, suggesting that the flow ponded against the northern rift flank. In contrast to outcrops in the Wichitas, hypabyssal intrusions cutting the rhyolite lavas are abundant in the Arbuckles. They comprise several geochemically distinct types of microgranite and rhyolite (including the most fractionated examples so far found in the SOA), as well as diabase dikes, sills and plugs with both tholeiitic and alkaline compositions. Also present are diatremes up to 1.4 km across that are interpreted to represent conduits for small, maar-type phreatomagmatic volcanoes and contain mafic and felsic lithic clasts intermixed with variably vesicular juvenile basaltic pyroclasts. We infer that formation of the diatremes and the wide compositional range of the hypabyssal intrusions in this part of the SOA resulted from proximity to the rift-bounding fault zone, which created favorable pathways for diverse small magma batches to rise to shallow crustal levels and also facilitated downward circulation of groundwater to drive explosive subsurface phreatomagmatic interactions.