INTRUSIVE SILL MORPHOLOGY OF THE EARLY CAMBRIAN COLD SPRINGS BRECCIA, WICHITA MTNS., SW OKLAHOMA

The Wichita Mountains in southwest Oklahoma preserve excellent exposures of bimodal igneous rocks produced as a result of Early Cambrian rifting that formed the Southern Oklahoma Aulacogen. Layered anorthosites of the Glen Mountain Layered Complex (GMLC) and hydrous gabbro are overlain by metaluminous A-type granites of the Wichita Granite Group (WGG) and Carlton rhyolites. The ≈25 km² Cold Springs Breccia (CSB) located west of the Wichita National Wildlife Refuge demonstrates that mafic and felsic magmas co-existed in time and space and give clues to their interactions. The CSB is not a breccia but reflects co-mingling and incomplete mixing of magmas. Three lithologies comprise the CSB: A-type granite that is geochemically similar to the WGG, within-plate tholeiitic diabase and diorite with Mg# = 51-58 and Ni = 40-60 ppm, and a hypidiomorphic intermediate rock containing oligoclase-andesine, Mg-hornblende, orthoclase, and anhedral biotite in a quartz-rich matrix. The granite is always in contact with GMLC, commonly as a fine-grained upper horizon separating CSB and GMLC. The diabase forms lobate pillows of varying size (3-100 cm in largest dimension) with crenulated oxide-dominated margins enclosed in sinuous channels of intermediate rock between mafic pillows. In previous presentations we proposed that a granitic magma intrusion within the GMLC was followed by an energetic pulse of diabase magma and proposed that the intermediate rock is a product of magma mixing of these two end-members. Field observations of limited exposures, many of which are on private land, reveal a complex network of sills ranging in thickness from 5 cm to 20 m, although the lack of exposed bottom contacts suggests that some units could be thicker. The CSB is highly variable with thicker units displaying some stratigraphic zonation. In addition, a new georeferenced geologic map of the CSB has been produced.