AN EXAMPLE OF INTRUSION RELATIONSHIPS ALONG THE NORTHERN MARGIN OF THE QUANAH GRANITE, WICHITA LIP, SOUTHWESTERN OKLAHOMA

The Wichita Mountains in southwestern Oklahoma expose approximately a dozen granite plutons, all part of A-type silicic magmatism within the Southern Oklahoma Aulacogen or Wichita Large Igneous Province. Contact relationships indicate it to be the youngest of the plutons. Exposures include 35 km² of the Quanah Granite pluton, including its northern margin contact with an older gabbroic layered complex. At this margin, we noted 3 facies. These are 1.) the typical seriate hypidiomorphic coarse-granular facies (CF), 2.) fine-grained allotriomorphic facies (FF), and 3.) fine-grained hypidiomorphic porphyritic facies (PF), which is weakly granophyric. All are alkali feldspar granites dominated by perthitic feldspar and quartz. CF has 6mm alkali-feldspar and smaller quartz, FF has 2mm alkali-feldspar and quartz, and PF has 5mm alkali-feldspar phenocrysts with a sub-millimeter matrix. The CF grain size is slightly diminished at the margins compared to exposures in the interior of the pluton. Addditionally, the margin contains several pegmatite pods and dikes, and inclusions of rhyolite and granite. Granitoid dikes are found adjacent to the northern contact.

Mapping the FF and PF revealed these to have dike-like geometries that cut the CF, suggesting that both are late intrusions. To resolve relationships, we assessed the three facies through petrography, X-Ray Diffractometry (XRD), and Electron Probe Microanalysis (EPMA). Prior researchers noted CF’s sodic amphiboles (confirmed as arfvedsonite) that make the Quanah unique among the Wichita granite plutons. PF contains biotite and/or calcic amphibole, both occuring as anhedral interstitial crystals. In our samples, FF contains only biotite. The pegmatitic bodies are quartz ± orthoclase ± biotite or arfvedsonite.

The textures and mafic mineral assemblages of the different facies imply that the voluminous CF magma first intruded this area, followed by the magma(s) that gave rise to the FF and PF. The difference in texture suggests a change in the local thermal profile over the time interval. As such, the FF and PF facies may record one of the last gasps of felsic magmatism in the province.