LARGE-SCALE CAMBRIAN RHYOLITIC VOLCANISM IN SOUTHERN OKLAHOMA RELATED TO OPENING OF IAPETUS

Denison (1958) and Ham, Denison, and Merritt (1964) showed that limited outcrops of rhyolite in the Wichita and Arbuckle Mountains in southern Oklahoma are parts of a major Cambrian rhyolitic field that can be traced for ~40,000 km² in the subsurface. This work established that the rhyolites and associated sheet granites form part of a bimodal igneous assemblage related to a rift zone that trends at a high angle into the cratonic interior from the ancient continental margin. The volume of felsic rocks within the rift is probably > 40,000 km³. The rifting is now recognized to have occurred during opening of the Iapetus Ocean, either as part of an r-r-r triple junction or within a leaky transform at an offset in the Laurentian margin.

Rhyolites in the Wichita Mountains have silica values of 70-78 % (based on 48 new analyses) and trace-element contents typical of intraplate, A-type felsic rocks, similar to the Wichita sheet granites. The maximum exposed thickness of rhyolite, at Bally Mountain, is > 2 km. Flows at the base and top of this sequence have yielded U-Pb zircon dates of ~534 Ma, similar to zircon dates of ~533-530 Ma reported by other workers for Wichita sheet granites and a rhyolite xenolith within granite. Emplacement of voluminous A-type magmas in the relatively narrow time frame suggested by these data implies rapid influx of heat into the crust and is consistent with models in which Cambrian rifting along the Laurentian margin tapped an underlying mantle thermal anomaly.

Exposures of rhyolite in the Wichitas consist of a series of unusually thick (up to 400 m), tabular flows separated in places by thin tuffs; similar flow units also appear to be present in the Arbuckles. Limited outcrops typically do not allow individual flows to be traced more than a few km. However, each flow has a consistent vertical textural zonation, with originally glassy, flow-banded margins grading into massive felsitic lava, recording uniform cooling of sheet-like flows. We infer that these units formed primarily from large-volume rhyolite lavas emplaced in rapid succession, although some units could also represent homogenized rheomorphic ignimbrites. Similar features are shown by Miocene intraplate A-type rhyolites in the Snake River Plain in Idaho, which may provide an appropriate analogue for Cambrian volcanism in southern Oklahoma.