GEOCHEMISTRY AND PETROGENESIS OF EXTENSIVE A-TYPE, PARTLY PERALKALINE CAMBRIAN RHYOLITES EMPLACED DURING CAMBRIAN RIFTING WITHIN THE SOUTHERN OKLAHOMA AULACOGEN (SOA)

Cambrian rifting associated with opening of the Iapetus Ocean resulted in emplacement of ~250,000 km³ of bimodal igneous rock within the SOA. Most of the SOA is deeply buried, but the Arbuckle Mountains expose parts of the bimodal assemblage that were emplaced directly along the northern rift flank; outcrops in the Wichita Mountains farther west provide views of igneous rocks farther into the rift. Geophysical data indicate that much of the igneous assemblage in the subsurface is mafic, but the uppermost part consists of voluminous rhyolites intercalated with tholeiitic and transitional to alkaline basaltic lavas; numerous sheet granites and smaller diabase dikes and sills intrude the volcanic pile. The rhyolites represent ~40,000 km³ of felsic magma erupted in a relatively narrow time frame in the early Cambrian.

Both the rhyolites and granites have clear A-type signatures, and some of the granites are also peralkaline (Na₂O + K₂O > Al₂O₃ in mole proportions), consistent with the presence of aegirine and alkalic amphibole. The rhyolites are more strongly altered, and mafic silicate phases are completely replaced by low-T secondary minerals. None of the rhyolites are now alumina-undersaturated, but ratios of immobile trace elements resistant to alteration (Nb/Y, Zr/Ti) suggest that many of the rhyolites were peralkaline prior to alkali loss during alteration. Isotopic data (Hogan et al., 1995) show that the felsic and mafic rocks have comparable juvenile Sr and Nd initial ratios, arguing that the felsic magmas were generated by fractionation from mafic precursors and/or partial melting of mafic rocks emplaced at deeper levels. Our more extensive trace-element data for the rhyolites suggest that the felsic magmas were derived from OIB-type sources, with varying amounts of crustal assimilation. Immobile trace-element contents for the rhyolites define several distinct groups that are inferred to reflect derivation from separate sources or magma reservoirs. The greatest differences between these groups, and the most fractionated rhyolites (with pronounced negative Eu anomalies and Zr contents as high as 2000 ppm), occur in the Arbuckles, suggesting that proximity to the rift-bounding fault provided pathways for diverse rhyolite magma batches to rise to shallow crustal levels.