CONTINUED EVALUATION OF GRANOPHYRE IN THE LONG MOUNTAIN GRANITE, WICHITA MOUNTAINS, SOUTHWESTERN OKLAHOMA

In the Wichita Mountains, many of the shallow-emplaced Cambrian plutons of the Wichita Granite Group (WGG) contain granophyre, a microstructure characterized by intergrowth of quartz and alkali feldspar. The texture constitutes at least half of the Long Mountain Granite. The remainder is comprised of 2-3 mm quartz and 2-5 mm perthite phenocrysts, with iron-titanium oxides, trace zircon, and few mafic silicates. The type locality at Long Mountain served as the model for Morgan and London's (GSA Bull., 2012) seminal investigation of the texture, which constrained granophyre formation to undercooling of roughly 70-150 °C.

The Long Mountain Granite is exposed over 12.8 km² in the southern-most central Wichitas, as a 20 km north-arcing, east-west elongated crescent. In most fresh expressions, the rock is pink-red and medium grained; extensive quarrying at the type locality has exposed a gray-green colored (unaltered) facies (Hamilton et al., GSA Bull., 2016).

To further characterize the granophyre within the Long Mountain, we examined four widely distributed samples. In many regards the samples exhibit strong similarities in phenocryst size and composition and distribution of granophyre. From the east to west, the Long Mountain samples are LM61017, easternmost exposure; LM60717, central exposure; LM60817, type locality; and LM121720, westernmost exposure. Whole-rock data, including new analyses from the two marginal exposures, indicate the Long Mountain to be largely among the low Ca, high Ba, intermediate Sr (Reformatory) class of WGG. LM121720 is marked by a large concentration of vapor cavities with an elevated LOI value (1.0 wt.%). Both features indicate a water-rich crystalizing environment, atypical of other locations (and the bulk of the WGG). This western sample also appears more intensely oxidized, contains biotite and fluorite, and has granophyre that coarsens into quartz and a relatively homogenous binary alkali feldspar. We additionally noted a patchy zoning in the granophyre feldspar of this and the eastern margin sample. In contrast, the type locality demonstrated an even distribution of Na- and K-elevated feldspar.