THE WICHITA GRANITE GROUP TEXTURE BY THE NUMBERS: IMAGE ANALYSIS OF QUARTZ MICROSTRUCTURE

The Cambrian Wichita Granite Group (WGG) includes roughly a dozen granite intrusive bodies. Each is a product of Southern Oklahoma Aulacogen magmatism, emplaced into the shallow crust. In spite of a gross similarity in bulk composition for much of the group, the WGG exhibits a range of microstructures among and within individual plutons and dike systems. The preponderance of granophyric quartz-feldspar intergrowth has long been noted, but the WGG includes textures that are hypdiomorphic to allotriomorphic granular. These may be subequigranular, seriate or porphyritic. Several are spherulitic, and some are recrystallized to a fine allotriomorphic granular or porphyritic texture.

To evaluate these textures numerically, we digitized examples of the WGG, processing each with ImageJ software. We digitized entire standard petrographic thin sections using a high-resolution flat-bed scanner, in both plane-polarized (PPL) and cross-polarized (XPL) light. These were encoded as RGB tiff files. In PPL, the quartz is transparent; all other minerals are colored (mafics and accessories) or turbid (feldspar). Thresholding a blue-channel split easily isolates quartz as particles; these are individual mineral grains in granular rocks and sections of skeletal crystals in granophyre. We also used the threshold to mask the XP image and reveal extinction ranges in quartz. Images were analyzed for quartz particle density (cm^-2), elongation ratios, formfactor (sphericity), and distribution (diameter ratio histogram) using ImageJ and the Particles 8 plugin (Landini, 2008).

Regardless of orientation, quartz particles in the WGG are not equant, and the averaged long axis for all WGG is elongated in random orientations by a factor of two. Particle formfactor is near uniformly 0.5 regardless of microstructure. The WGG shows a diversity of quartz particle density values: coarser grained granites contain only 280 to 300 cm^-2; the strongly granophyric texture typically has 600 cm^-2, but recrystallized granophyre is as high as 3500 cm^-2. Preliminary results indicate that the quartz in the WGG tend to three particle-size distributions: granular granites are broad and normal, these become log-normal with decreasing cm^-2, and the granophyric rocks exhibit narrow plateau distributions.