FORT SILL RHYOLITE: LARGEST KNOWN LAVA FLOW IN THE CAMBRIAN RHYOLITIC VOLCANIC FIELD OF SOUTHERN OKLAHOMA

Cambrian rift-related A-type rhyolites occupy an area of > 40,000² in the subsurface of southern Oklahoma and adjacent parts of Texas and form scattered outcrops in the Wichita and Arbuckle Mountains in Oklahoma. Associated sheet granites intrude lower parts of the rhyolite succession. In the Wichitas, ~30 different rhyolite flow units have been recognized but typically can only be traced laterally at most a few km because of limited outcrop. The most extensive rhyolite exposures occur in the Fort Sill area, where two flows are present. The Davidson metarhyolite (Dmr) was contact metamorphosed during granite intrusion and be traced ~9 km along strike before being cut out by granite. The overlying Fort Sill rhyolite (FSr) is separated from the Dmr by rhyolitic volcaniclastic rocks and quartzose sandstones deposited on an irregular erosional surface cut into the Dmr. Our mapping shows that the FSr flow can be traced ~18 km before going under cover or being truncated by intrusive granite and is > 190 m thick. This is the longest flow so far documented in the southern Oklahoma rhyolitic volcanic field.

Flow banding extending down to the sub-millimeter scale is variably developed within the FSr flow and is pervasive near the base and top, where thin felsitic bands alternate with originally glassy bands exhibiting relict perlitic texture. Pockets of flow breccia occur near the flow top, and peperite is locally present near the base of the flow, recording interaction between unconsolidated sediment and lava. Many flow bands contain randomly oriented tridymite needles (now inverted to quartz), inferred to have formed during early devitrification as the flow cooled from magmatic temperatures. We infer that the FSr is a remnant of an originally more extensive lava flow, comparable to laterally extensive flow units documented from other A-type felsic provinces.

Seventeen samples from the FSr flow exhibit significant ranges in major and trace element contents (including elements that are resistant to alteration), indicating the flow tapped a heterogeneous magma batch during eruption. On plots employing elements resistant to alteration (e.g., TiO₂ vs P₂O₅, TiO₂ vs Nb), FSr samples define a separate group from the Dmr and other rhyolites exposed in the Wichitas, implying that the FSr was derived from a separate magma reservoir.