FILLING THE CRACKS: MAGMA INJECTIONS AT THE EDGE OF THE QUANAH PLUTON, OKLAHOMA

The granites of the Wichita Mountains of southwestern Oklahoma are a product of the Eocambrian Southern Oklahoma Aulacogen. The Quanah Granite pluton is one of roughly a dozen A-type sheet-like granitic intrusions that comprise much of the Wichita Granite Group (WGG). These plutons intruded the upper crust, finding the horizon between the Glen Mountains Layered Complex (GMLC) and Carlton Rhyolite Group (CRG). The northern margin of the Quanah Pluton contains its typical, relatively-coarse grained rock, and two finer grained facies. Adjacent to the north contact are a series of granitic dikes that cross-cut the GMLC. The dikes are exposed over three square kilometers, ranging in width from a few centimeters to almost two meters. The longest is continuously exposed for a third of a kilometer.

We sampled the typical Quanah Granite, the finer-grained facies, and nine dike exposures. The typical Quanah is exsolved alkali-feldspar, quartz, and arfvedsonite. The sodic amphibole arfvedsonite uniquely appears as a general phase, and is not seen in the bulk of the other WGG plutons. The finer facies occur as dike-like bodies: a fine-grained rock with 2mm alkali-feldspar and quartz with smaller biotite grains, and a porphyritic rock with 5mm alkali-feldspar and biotite ± calcic amphibole in a sub-millimeter matrix.

The fine- to very coarse-grained granitic dikes north of the boundary are aplitic, seriate, granophyric, and pegmatitic (including vugs); the textures vary among and within the dikes. The dikes are dominated by perthitic alkali feldspars and, in one exposure, amazonite. One of the sampled dikes contains arfvedsonite while others contain biotite ± calcic amphibole. Biotite occurs as acicular grains, or small books, or similar to smaller crystals disseminated with other mafic grains. And one dike exposure exhibits mafic schlieren.

The differing mafic mineral contents in the pluton’s margins suggest that the fine and porphyritic units arose from a magma distinct from that of the typical Quanah Granite. The dike-like geometries of the fine-grained and porphyritic facies indicate that these cross-cut relatively cold, brittle granite. The mafic mineral contents in the adjacent dikes suggest two or more generations; at least one from the typical Quanah-forming magma and one from the magma that formed the later fine-grained and porphyritic units.