GRANOPHYRE AND THE WICHITA GRANITE GROUP, OKLAHOMA: A LOOK INTO AN EDGY TEXTURE

The Wichita Granite Group is a collection of Cambrian alkali-feldspar granite bodies emplaced as part of Southern Oklahoma Aulacogen magmatism. These granites are fine-grained seriate to coarse-grained subequigranular, a variation ascribed to emplacement depth, aH₂O, and cooling rates. Most of the fine-to-medium grained bodies exhibit variable granophyre, a texture characterized by skeletal intergrowths of Ksp and Qz. Although this microstructure is easily identifiable in thin section, and its origins tied to undercooling, its microstructural characteristics, and the specific conditions that determine its presence or absence in these granites, remains unexplored.

Expanding on prior work, we are using optical microscopy and SEM-EDS to characterize three different members of the WGG: the Rush Lake Granite, the Quanah Granite, and the Long Mountain Granite. The latter is characteristically strongly granophyric at all exposures, and the subject of prior study (Morgan and London, 2012, GSA Bull.). Element mapping of new samples confirms growth predominantly on Ksp phenocrysts and K enrichment with distance. Conversely, the Quanah Granite is free of granophyre (and approaching textural equilibrium), but shares an indistinguishable major-element composition with the porphyritic and granophyric western Rush Lake Granite. Microstructure analysis of this rock reveals that Ksp volumetrically increases with distance, and that a Ksp selvage bounds granophyric domains. Despite their presumed magmatic chemical similarity, the contrast between the Quanah and Rush Lake’s textures likely arose both as a consequence of the latter’s post-ponding decompression and shallower emplacement depth, conditions that promote greater undercooling and volatile loss during crystallization.