NEW SR ISOTOPE RESULTS FROM CAMBRIAN MAFIC THROUGH INTERMEDIATE WELL CUTTINGS FROM THE SOUTHERN OKLAHOMA AULACOGEN: TECTONOMAGMATIC IMPLICATIONS FOR MAGMA GENERATION

The failed arm of the Iapetus Ocean in the Southern Oklahoma Aulacogen (SOA) produced more than 200,000 km³ of early Cambrian mafic to silicic igneous rocks. In the Arbuckle Mountains, extraction-related well work documents 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 six others (Pan-Am Newberry, Pan-Am Jarman, Pan-Am Williams, Pan-Am Whyte, Blaik Oil Co. Mauldin, and Union Oil Co. Morton). X-ray fluorescence (XRF) analysis for whole rock major and trace elements has been performed at Franklin and Marshall College and Sr isotope analysis from a representative subset of the cuttings were obtained at the University of Kansas via TIMS. 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 clump with known EM1 OIB values but also suggest a role for open-system process in the generation of the evolved magmas. Applying a conservative age of 530 Ma for these rocks yields ⁸⁷Sr/⁸⁶Sr_i values from six samples that range from 0.7040 and 0.7064. ⁸⁷Sr/⁸⁶Sr_i increases with increasing wt. % SiO₂ 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 source. Nd isotope data from these samples are currently being evaluated to verify this hypothesis. Our results, coupled with existing isotope and trace element constraints from regionally exposed dikes and plutonic rocks that crop out in the Wichita Mts., will give better insight into understanding into what tectonic model (e.g., lower-mantle derived hotspot or upper-mantle extension-dominated) produced the SOA. Our results thus far appear to be more consistent with a lower-mantle origin for SOA mafic magmatism.