IT’S COMPLICATED: FINE AND PORPHYRITIC GRANITES ASSOCIATED WITH THE COARSE-GRAINED QUANAH GRANITE, SOUTHWESTERN OKLAHOMA, USA
The pluton’s northern margin includes typical Quanah Granite, quartz-rich hydrothermal products, some with feldspar ± biotite, and outcrops of the finer-grained rock. We determined the finer-grained exposures to hold two textures with distinct mafic assemblages, one a medium-grained porphyritic granophyre with hornblende and biotite, and the other a fine-grained granular felsite with biotite. These are grossly similar to other granite units in the Wichita Mountains. Their arfvedsonite-free composition and dike-like structures suggest these to be intrusions of separate, later magma(s), which we designate as the French Lake Granite. The crosscutting relationships make it one of the younger felsic products of SOA magmatism. We consequently redefine the Quanah Granite as the more voluminous, coarse-grained arfvedsonite granite.
We are now evaluating the pluton’s interior. As noted by prior work, the typical Quanah Granite contains hydrothermal pods and dikes, as well as dikes of very-coarse granite, pegmatite, and aplite, each a likely product of late-stage residual Quanah magma. But we have also noted granite outcrops with alkali-feldspar phenocrysts in a variably granophyre matrix. These contain hematite as 300 micron or smaller grains and lack mafic silicates. Some contain mm-sized miarolitic cavities with appreciable secondary hematite. Outcrop geometries seem more complex than the dike-like structures on the northern margin. These preliminary results indicate that the interior of the pluton either included sub-kilometer-sized xenoliths of older granites or was intruded by additional later magmas. Further work will assess these bodies and their relationship to the Quanah Granite Pluton.