DIGITAL IMAGE ANALYSIS OF VARIABLY GRANOPHYRIC TEXTURE IN THE MOUNT SCOTT GRANITE, WICHITA MOUNTAINS, SOUTHERN OKLAHOMA

Granophyre is a feature most commonly found in shallowly-emplaced A-type granites, attributable to an undercooling range of 50-150 °C (Morgan and London, 2012, GSA Bull., 124, p1251). The Mount Scott Granite is a chemically-homogenous, alkali-feldspar granite associated with Eocambrian rift magmatism. This uniformly porphyritic rock contains a matrix that varies from granophyric to largely allotriomorphic granular texture. If properly characterized, the distribution of this textural variation might be useful for assessing internal pluton structure and cooling history.

To quantify and characterize the variation through digital image processing, thin sections were digitized into an RGB tiff file at 2400 dpi, in plane (PP) and crossed-polarized light (XP). The PP image was cropped and split into RGB channels. Thresholding the upper grayscale (GS) values isolated the quartz, resulting in a binary image with white (GS=255) quartz on black (GS=0). Quartz modes and granophyric fractions were measured. A GIF transparency of this binary was overlain on the XP image, split for the blue channel, and thresholded for the bright (first-order white) quartz. The resulting binary, a black on white map of the individual bright quartz particles, revealed single non-granophyric grains and optically continuous granophyre domains. This image was analyzed for particle geometry and ellipse parameters.

The 5mm, rounded alkali feldspar phenocrysts serve as the dominant nucleation site for granophyre. Smaller phenocrystic quartz grains and glomerocrysts may be mantled by continuous quartz or granophyre. The quartz mode drops slightly from 25% from a sample with no discernible granophyre to 20% in examples where granophyric texture exceeds 90% of the total quartz. Likewise, individual particle size increases with decreasing granophyric content. All samples show similar log-normal particle size distributions and random ellipse primary axis orientations.