CAMBRIAN FELSIC ROCKS EMPLACED DURING VOLUMINOUS BIMODAL MAGMATISM IN THE SOUTHERN OKLAHOMA AULACOGEN: AN OVERVIEW

Cambrian rifting along the SE Laurentian margin preceded birth of the southern Iapetus Ocean and included formation of the Southern Oklahoma aulacogen (SOA) striking into the craton interior. Geophysical and basement-well data indicate that ~210,000 km³ of mafic rocks were emplaced within the SOA at this time, along with at least 40,000 km³ of A-type felsic rocks at higher crustal levels. These include abundant lavas inferred at least partly to represent erosional remnants of flood rhyolites, together with a range of granites that in many cases are sheet-like intrusions. No evidence for calderas exists, and the rhyolites likely were erupted from fissures. In the Wichita Mtns., where the best outcrops occur, our ongoing high-precision U-Pb zircon dating shows that emplacement of the felsic rocks occurred at 530.98 ± 0.15 to 530.23 ± 0.14 Ma and was preceded by emplacement of a major layered mafic complex at 532.49 ± 0.15 Ma, as well as smaller, unrelated gabbros at 532.04 ± 0.12 Ma. In the Arbuckle Mtns. ~100 km to the east, two rhyolite flows have yielded ages of 539.20 ± 0.15 and 539.48 ± 0.16 Ma and are part of a complex igneous assemblage including broadly coeval microgranite and diabase intrusions. Rhyolites in the subsurface are intercalated with large amounts of basaltic lavas wherever the volcanic succession has been penetrated by drilling. Widespread diabases, not yet dated, intrude all other igneous units in the SOA, showing that influx of mantle-derived mafic magmas occurred throughout its igneous evolution.

Previously available isotopic data show that the felsic and mafic rocks have comparable initial Sr and Nd initial ratios reflecting derivation from subcontinental lithospheric mantle as well as ocean-island-basalt-type asthenosphere. These data suggest an origin for the felsic rocks related to partial melting of underplated basalt or by fractional crystallization from mafic parental magmas (if a plausible way can be found to bridge the Daly gap). Extensive immobile trace-element data from the rhyolites define several distinct groups suggesting derivation from separate sources or magma reservoirs. The greatest differences in these groups occur in the Arbuckles, possibly because this area lies close to a major rift-bounding fault that channeled upward transport of small, separate magma batches.