CAMBRIAN BASALTIC VENT CONDUITS IN THE WEST TIMBERED HILLS, ARBUCKLE MOUNTAINS, SOUTHERN OKLAHOMA: EVIDENCE FOR EXPLOSIVE PHREATOMAGMATIC ERUPTIONS DURING RIFT-RELATED VOLCANISM

Cambrian rhyolites and basalts are present over an area ≥ 40,000 km² in the subsurface in southern Oklahoma and adjacent parts of Texas and crop out locally in the Wichita and Arbuckle Mountains. These rocks represent the upper part of a bimodal igneous assemblage emplaced within a major intracontinental rift that developed during opening of the Iapetus Ocean to the southeast. New mapping in the West Timbered Hills in the Arbuckles reveals that the volcanic succession and associated hypabyssal intrusions there are penetrated by unusual igneous breccias forming numerous discordant, ovoid or elongate masses ~50 to 1400 m across scattered over an area 6 km². The breccias contain a chaotic mixture of basalt, diabase, rhyolite and microgranite clasts ≤ 1.3 m across set within a finer fragmental matrix of the same rock types. Juvenile, nonvesicular to scoriaceous basaltic ash and lapilli with angular to fluidal shapes within the matrix consist of sideromelane and tachylite glass (now altered). These are characteristic features of basaltic pyroclasts formed by phreatomagmatic eruptions, and some fluidal lapilli are cored by felsic lithic fragments, clearly indicating that basalt magma was involved in breccia formation.

Damage zones ≤ 40 m across occur along contacts between the breccia and older rhyolite and microgranite. The outer parts of these zones show in situ fragmentation, recording explosive shattering of the conduit walls, with the felsic fragments becoming progressively more chaotically mixed with other clast types farther into the breccia masses. We interpret the larger breccia masses to represent vent conduits for maar-type volcanoes generated when rising basalt magma interacted explosively with groundwater in the shallow subsurface; some of the smaller breccia masses may have formed from explosions that did not reach the surface. Tholeiitic basalt and diabase clasts within the breccia have nearly identical geochemical signatures to diabase intrusions that cut the breccia and locally show fluidal, partly fragmented tongues injected into it, indicating they were intruded while the breccia was still unconsolidated. The intrusions thus provide a window into the plumbing system that supplied basaltic magma to the vents at various stages in their activity.