TRACE-ELEMENT CONSTRAINTS ON THE PETROGENESIS OF A-TYPE RHYOLITES EMPLACED DURING CAMBRIAN RIFTING IN THE SOUTHERN OKLAHOMA AULACOGEN

The southern Oklahoma aulacogen contains a classic bimodal igneous assemblage emplaced during late Neoproterozoic(?)-Early Cambrian intracontinental rifting associated with opening of Iapetus along the southern Laurentian craton margin. Voluminous A-type felsic volcanic rocks (Carlton Rhyolite) and coeval subvolcanic sheet granites were the last major igneous units to be emplaced in the rift. The rhyolites have a subsurface extent ≥ 40,000 km² but are exposed only locally in the Wichita and Arbuckle Mountains. In spite of the large extent of the volcanic field (comparable to the Snake River Plain), detailed trace-element studies of the rhyolites are lacking. We present geochemical data for ~50 samples of the rhyolites where they are best exposed in the Wichita Mountains, in stratigraphic sequences up to ~2 km thick in four main outcrop areas as much as 45 km apart (Zodletone Mtn., 2 or more flows; Bally Mtn., 9 flows; Blue Creek Canyon, 6 flows; Fort Sill, 2 flows). Individual flows are up to 400 m thick but typically can only be traced a few km along strike before trending beneath Permian and Quaternary cover. One exception, the main Fort Sill rhyolite, has a lateral extent ≥ 19 km.

The rhyolites are characterized by low CaO and relatively high FeO/MgO, typical of A-type felsic rocks; silica typically ranges from 71 to 77.8 wt. % (volatile-free). Variations in major and trace elements indicate fractionation of pyroxene, plagioclase, K-feldspar, Ti-magnetite, apatite and zircon, consistent with observed mineral phases. In various plots using Ti, Zr, Nb, Th and Y, the rhyolites occupy three distinct groups, which require derivation from different reservoirs with distinct trace-element compositions. Flows belonging to more than one trace-element group are intercalated in each outcrop area, indicating that superimposed flows in some cases tapped different magma reservoirs. In plots of Ce/Nb or Ba/La vs Y/Nb (after Eby, 1990, 1992), the rhyolite data define a trend from ocean-island basalt (OIB) compositions to fields for felsic magmas that have interacted with continental crust. We infer that the southern Oklahoma felsic magmas were derived primarily from an OIB-like source, either by fractionation of parental basaltic magma or by partial melting of a mafic underplate, with variable contributions from older crust.