DISTRIBUTION OF MARGINAL TEXTURES IN THE QUANAH GRANITE PLUTON, WICHITA MOUNTAINS, OKLAHOMA

The Wichita Mountains are largely the exposed magmatic products of the Southern Oklahoma Aulacogen, an Eocambrian rift. The Quanah Granite pluton is one of roughly a dozen individual granite bodies. Previous workers had noted that the pluton is dominated by a coarse-grained seriate texture that gives way to finer-grained facies on its northern margin. These are exposed in the Central Lowlands adjacent to Burford, Fish and French Lakes. We report here the preliminary results of a project focused on mapping the distribution of facies.

Field reconnaissance and mapping was augmented by sampling. Mapping used a handheld GPS-WAAS unit and topographic base map. We marked locations of outcrops and lithologic contacts, as well as excavation sites and irregular features. And we collected several hand samples at key localities. Upon return to the lab, we compiled observations using DeLorme’s Topo North America 10 digital mapping software on USGS DOQQ 1m resolution areal imagery.

This study has thus far documented three different facies based on rock textures: coarse-grained, fine-grained, and porphyritic. The coarse-grained facies has 6mm kspar with slightly smaller quartz. The fine-grained facies has 2mm kspar and quartz. The porphyritic facies contains 5mm kspar phenocrysts with a sub-millimeter matrix of kspar and quartz. These three distinct textural facies exhibit limited, continuous geographical distribution. The coarse-grained facies is dominant in the bulk of the pluton to the south, but it is found adjacent to contacts around Burford Lake. The fine-grained facies is much of the margin from Burford Lake to the east of French Lake. The porphyritic facies dominates the pluton margin at French Lake. For both porphyritic and fine-grained facies, the contacts with the coarse facies are typically linear, but somewhat diffuse. Contacts between the fine and porphyritic facies remain more enigmatic. Their distribution may result from cooling variations along the pluton margin or small variations in composition.

We also noted at least one marginal pegmatite pod; a square meter dominated by polycrystalline quartz grading to intergrown quartz and feldspar at its margins. We also documented a dike or screen of Carlton Rhyolite. Additionally, our map reveals a pluton contact with an irregular geometry in places.