PETROGRAPHIC COMPARISON OF SUBSURFACE ALKALI-FELDSPAR GRANITES TO THE EXPOSED PLUTONS OF THE SOUTHERN OKLAHOMA AULACOGEN, U.S.A

Felsic magmatism in the Southern Oklahoma Aulacogen (SOA) is expressed as granite plutons and rhyolite flows. The latter is exposed in the Wichita Mountains and the Arbuckle Mountains, while surface exposures of the former, collectively mapped as the Wichita Granite Group (WGG), are unseen at the surface in the Arbuckles. Petroleum exploration within the Arbuckles penetrated into hundreds of meters to 1.3 km of granitic materials. We examined samples from four holes: three of the penetrations fall along a 25 km line striking S57°E, parallel and adjacent to the controlling reverse fault that superimposes SOA rocks on lower Paleozoic sediments; a fourth is 10 km to SW on the upthrown block. The granite fragments pulled from these holes are strikingly similar to examples of the WGG.

In the Wichita Mountains, WGG members are A-type, alkali-feldspar granites. Individual plutons are distinguished by location coupled with subtle changes in texture and mineral assemblage, as well as chemical composition. Most have brick- to orange-red weathered surfaces; excavations in two plutons exhibit pink to gray colors with significant depth below the surface. Individual plutons are either fine-to-medium grained porphyry with granophyric to hypidiomorphic-granular texture, or less typically coarser-grained seriate. Alkali feldspars are invariably perthitic. All contain titaniferous oxides ± biotite ± amphibole; zircon ± fluorite ± titanites are common accessory minerals. The mafic and accessory minerals are typically glomerocrystic, also occurring as enclaves in some plutons.

We characterized small (0.1 - 1.0 cm) granite sample chips and thin sections of chips collected from several depths during drilling. Generally, these are orange- and brick-red alkali feldspar granites, with lighter color in some of the deeper samples. Each hole contained chips with granophyre nucleated on medium-grained phenocrysts of perthitic alkali feldspar. Some samples also contained chips exhibiting hypidiomorphic-granular texture. Most chips contain a small amount of glomerocrysts of substantially altered biotite, hornblende, and hematite. Accessory minerals include zircon, titanite, and fluorite. The textures, mineralogy, and alterations seen in these chips are wholly consistent with those seen in plutons from the Wichita Mountains.