Master of Science / Department of Geology / Matthew E. Brueseke / The Southern Oklahoma rift zone (SOA), which stretches from southern Oklahoma through the Texas panhandle and into Colorado and New Mexico, contains extensive bimodal mafic-silicic magmatism related to the opening of the Iapetus Ocean during the late Precambrian and early Cambrian. Within the SOA, the subsurface in and adjacent to the Arbuckle Mountains in southern Oklahoma contains thick packages of mafic to intermediate lava flows interlayered with thick, extensive rhyolite lava flows and lesser silicic intrusive bodies, which were first described during a 1982 drill test (Hamilton Brothers Turner Falls well) in the region. Well cuttings of these units were collected from that well and three others (Pan-Am Williams D-2, Pan-Am Jarman, Pan-Am Newberry). This study is focused on these mafic-intermediate lava flows, which represent an important stage in the evolution of the SOA and provide insights into the formation and tectonomagmatic evolution of the rift zone. The estimated 210,000 km[superscript]3 of mafic rocks in the SOA were extruded as a result of the break-up of Pannotia and the formation of the failed arm of a three-armed radial rift system. Samples analyzed from the wells plot as basalts to andesites on the TAS diagram of Le Bas et al (1986) and as subalkaline-alkaline basalts to andesite-trachyandesites on the Zr/TiO[subscript]2 vs. Nb/Y diagram of Winchester and Floyd (1977). They are dominantly tholeiitic on multiple discrimination diagrams including those of Miyashiro (1974) and Irvine and Baragar (1971). The lava flows contain traits common with EMI OIB coupled with upper crustal contamination, such as Zr/Nb values ranging from 8 to 10, Ba/Nb values ranging from 10 to 20, and K/Nb values ranging from 300 to 600. Chemostratigraphic comparisons between each well reveal up to five lava flow packages within the larger mafic-intermediate sequence, at least in the vicinity of the sampled wells. When compared with intrusive mafic rocks outcropped in the Wichita Mountains, the SOA lava flows display geochemical traits most similar to those of the Roosevelt Gabbros, suggesting a possible co-genetic relationship. Overall, the whole rock chemical characteristics coupled with comparisons with other large igneous provinces (Columbia River and Oregon Plateaus, East African Rift System) indicate that the SOA lava flows are the result of flood basalt volcanism.

Master of Science / Department of Geology / Matthew Brueseke / The Southern Oklahoma Aulacogen (SOA) produced more than 250,000 km[superscript]3 of Cambrian mafic to silicic magmatism, associated with the opening of the Iapetus Ocean. In the Arbuckle Mountains, oil and gas exploration showed mafic to intermediate volcanic rock interbedded with rhyolite lavas. The first description of these lavas was a result of the 1982 drilling of the Hamilton Brothers Turner Falls well. Cuttings have been collected from this well and five others, and whole rock major and trace element analysis, Sr and Nd isotope analysis, and rare earth element analysis has been completed on these samples. These samples plot primarily as tholeiitic to transitional basalts to andesites. Trace element ratios show Zr/Nb values ranging from 8-10, K/Nb values ranging from 300-600, and Ba/Nb values ranging from 10-20, which overlap with known EM1 OIB values. Applying a conservative age of 535 Ma for these rocks yields [superscript]87Sr/[superscript]86Sr[subscript]i values of 0.703970 to 0.706403 and epsilon Nd values of 1.67 to 3.22, which also fall within the accepted range of EMI values. [superscript]87Sr/[superscript]86Sr[subscript]i increases with wt. % SiO[subscript]2 and K/P, consistent with the generation of evolved compositions via open-system processes. The sample with the least radiogenic Sr isotope ratio, combined with its trace element ratios is most consistent with an EM1-type source. These results, coupled with existing isotope and trace element constraints from regionally exposed dikes and plutonic rocks that crop out in the Wichita Mts., give better insight into understanding what tectonic model (lower-mantle derived hotspot or extension of the lithosphere) drove the magmatic production of the SOA. The results are more consistent with a lower-mantle origin for SOA mafic-intermediate magmatism, and indicate the potential for flood basalt volcanism.