MAGMAS MOVE, MINGLE, AND MIX: A NEW INTERPRETATION OF THE CAMBRIAN COLD SPRINGS BRECCIA, WICHITA MTS., SW OKLAHOMA

The Southern Oklahoma Aulacogen is the result of Early Cambrian rifting that produced compositionally bimodal rocks exposed in the Wichita Mountains of southwest Oklahoma. Metaluminous A-type granites and rhyolites overlie hydrous gabbros and layered anorthosites. Evidence of co-existing mafic and felsic magmas come from interbedded mafic and felsic rocks found in drill cores and from identical U/Pb zircon ages for mafic and felsic igneous rocks. Evidence for Wichita bimodal igneous activity is nowhere more apparent than in the ~ 25 km² exposures of the Cold Springs Breccia (CSB) west of the Wichita National Wildlife Refuge. Lobate, fine-grained, mafic “pillows” ~30 cm across are suspended in intermediate plutonic rocks (tonalite, granodiorite, and quartz diorite) and commonly have crenulated margins. The CSB intrudes anorthosites of the Glen Mountain Layered Complex (GMLC). At the CSB/GMLC contact and commonly in sinuous channels within the intermediate rock is granite geochemically similar to the metaluminous A-type granites of the Wichita Granite Group. The mafic rocks of the CSB are fractionated within-plate tholeiitic diabase and diorite with Mg# = 51-58 and Ni = 40-60 ppm. Diabase mineralogy is dominated by sub- to euhedral Mg-hornblende and andesine-labradorite with sub-millimeter grain sizes. The intermediate rocks are hypidiomorphic and contain granular oligoclase-andesine, Mg-hornblende, orthoclase, and anhedral biotite in a quartz-rich matrix. The rocks of the CSB have trace element compositions that show some geochemical similarities, with some differences expected from partitioning by crystal fractionation. This can be seen in the variations in trace element ratios of CSB diabase, tonalite, and granite with mean Zr/Nb of 13, 16, 30, respectively as well as with rare earth elements with La/Yb ratios of 5.2, 7.6, and 13.1 respectively. We propose that CSB intermediate rocks are products of magma mixing formed when mafic magmas entered a chamber occupied by A-type granitic magma. Turbulence during injection of the hotter denser mafic magma through cooler less-dense felsic magma provided the opportunity for mixing the diabase and A-type granite to form intermediate igneous rocks.