THE CRAIG HILL DIKE, A UNIQUELY COMMON EXPRESSION OF WICHITA IGNEOUS PROVINCE DIABASE, OKLAHOMA

A largely subhorizontal expression of Cambrian diabase dike occurs along the south wall of a cut for the eastbound lane of Highway 49 at Medicine Park, Oklahoma. The dike cross-cuts the 530.45 ± 0.14 Ma Mount Scott Granite. The host granite expresses minor thermal alteration and a network of fractures. The dike orientation differs from common subvertical diabase intrusions that cut through the products of the Wichita Igneous Province (WIP), a region of intense magmatism associated with rifting of the Laurentian margin. This current study evaluates the mineralogy, microstructure, and geochemistry of this outcrop and compares it to other nearby expressions.

Hand-transit measurements coupled with high-accuracy GPS readings defined orientation. The dike is subhorizontal from its exposure at its west end for 109 m to the east. We observed its westernmost orientation as N48°W 06°S, with an average mapped trend of N69°W. The dike gently rolls to an upward inclination for a minimum of 19 m. Measurements on this portion were N25°W 40° and N46°W 43°, with an average mapped trend of N58°W.

Apart from a unique orientation, the dike is a typical, although weathered, expression of WIP diabase. Qualitative XRF analysis reveals 44.4 wt.% SiO₂, 18.7 wt.% Fe₂O₃, 7.1 wt.% MgO, and 7.62 wt.% CaO. The tholeiitic values are comparable to other WIP diabase. Despite recent excavation, the dike is heavily weathered by extensive groundwater incursion. XRD analysis indicates an abundance of montmorillonite, in addition to the primary mineralogy of plagioclase and augite. Plagioclase laths are roughly 100 x 30 μm in subophitic augite that has substantially altered to chlorite and clay minerals. Laths exhibit preferential alignment in three directions, a behavior within the range of plagioclase orientation distributions we measured in other WIP diabase. Alignment of the plagioclase within four other dikes varies from unidirectional to random.