CAMBRIAN BIMODAL VOLCANIC AND HYPABYSSAL INTRUSIVE LITHOFACIES DEVELOPED ALONG A MAJOR RIFT-BOUNDING FAULT IN THE SOUTHERN OKLAHOMA AULACOGEN: RESULTS OF NEW MAPPING IN THE WEST TIMBERED HILLS, ARBUCKLE MOUNTAINS

Cambrian igneous rocks in the Wichita and Arbuckle Mountains in southwestern and south-central Oklahoma provide windows into a mostly buried bimodal igneous province emplaced during rifting within the Southern Oklahoma Aulacogen (SOA). The most extensive outcrops of the igneous rocks (~10 km²) in the Arbuckle Mountains occur in the West Timbered Hills (WTH). These rocks have not been studied in detail before but are of interest because they were emplaced < 1 km from one of the main rift faults along the northern margin of the SOA. Our new mapping reveals the presence of seven rhyolite flows in the WTH that have A-type compositions and generally show a consistent vertical zonation, with originally glassy flow-banded margins that pass inward to massive felsitic zones, in which tridymite needles (now inverted to quartz) increase in size inward, consistent with emplacement at temperatures > 870º C. The simple vertical zonation suggests the rhyolites may represent remnants of laterally extensive flows similar to those found in other A-type provinces.

Microgranite and diabase intrusions cutting the rhyolites make up ~30 % of the WTH igneous outcrop. Microgranite bodies up to 2.7 km across contain heterogeneously developed internal vesicle-rich zones, implying emplacement in pulses. Tholeiitic and alkaline diabase intrusions form numerous dikes, sills and plug-like bodies, indicating that basalt magmas from separate sources were emplaced within a restricted area in this part of the SOA. Masses of igneous breccia up to 1.4 km across cut the felsic rocks and contain a chaotic mixture of felsic and mafic clasts set within a pyroclastic matrix, in which basaltic lapilli and ash show features characteristic of pyroclasts formed by phreatomagmatic eruptions. We interpret the breccias to represent diatremes that fed small maar volcanoes at higher levels and were generated by explosive subsurface interactions between groundwater and rising basalt magma. Similar diatremes have not been documented from other parts of the SOA, nor has such an abundance of hypabyssal intrusions. Both features probably reflect proximity to the rift-bounding fault, which would have provided pathways allowing diverse batches of magma to reach shallow crustal levels and likely also produced a hydrologic setting favoring explosive phreatomagmatism.