CAMBRIAN A-TYPE RHYOLITE LAVA FLOWS AND HYPABYSSAL INTRUSIONS IN THE EAST TIMBERED HILLS, ARBUCKLE MOUNTAINS, SOUTHERN OKLAHOMA

Early Cambrian rifting in southern Oklahoma was associated with emplacement of voluminous bimodal igneous rocks. The upper part of the igneous rift fill consists of the A-type Carlton Rhyolite Group, which is widespread in the subsurface but is only locally exposed in the Wichita and Arbuckle Mountains of SW and south-central Oklahoma. Rhyolites in the Wichitas are the subject of ongoing work, but those in the Arbuckles have not previously been examined in detail.

Rhyolites in the East Timbered Hills in the Arbuckles were emplaced near the northern margin of the rift zone. Our new mapping shows the extrusive sequence exposed there to consist of two lava flows separated by a 60-m-thick interval of distal silicic ash-fall tuff and reworked volcaniclastic rocks. Abundant hypabyssal rhyolite intrusions (typically microgranophyric) cut the extrusive rocks, suggesting proximity to a source vent. The original dimensions of the flows are unknown because they are truncated by faults and partly hidden by younger strata. The lower flow is > 300 m thick but can only be traced laterally for 700 m. It is pervasively flow banded and originally contained abundant glass, based on the presence of relict perlitic texture throughout the flow. These features suggest that the lower unit represents part of a typical rhyolite dome or short, thick lava flow. The upper flow can be traced for 3.5 km and is > 600 m thick. In contrast to the lower flow, it consists largely of felsitic, columnar-jointed rhyolite in which randomly oriented tridymite needles (replaced by quartz) steadily increase in size inwards in the flow, indicating a simple, uniform cooling history after the lava came to rest. We infer that this upper unit represents part of a much more extensive flow, perhaps comparable to “flood rhyolites” documented from other A-type provinces. The great thickness of the flow may be due to ponding of the lava along the rift margin.

Geochemical studies show that both flows represent homogeneous magma batches with little internal compositional variation. The flows also show close geochemical similarities, indicating derivation from a common magma reservoir. Most of the rhyolite intrusions have markedly different trace-element contents and represent small-volume, highly fractionated batches of magma petrogenetically unrelated to the flows.