THE CAMBRIAN WICHITA GRANITE GROUP OF OKLAHOMA; REVIEWING THE MAFIC ASSEMBLAGES OF ITS “LESSER” LITHOSTRATIGRAPHY

The Cambrian Wichita Granite Group was defined by Myers et al. (1981, Ok. Geo. Notes) as ten formal lithodemic units as exposed within the Wichita Mountains. These are alkali-feldspar granites emplaced as shallow (1-4 km depth) plutons, and are the dominant exposed record of felsic magmatism in the Southern Oklahoma Aulacogen. Recent attention has focused on the larger bodies, the Mount Scott (212 km²) and the Quanah Granites (36 km²), the associated Medicine Park and Saddle Mountain Granites, as well as newer units, the Rush Lake and the French Lake Granites (collectively 11 km²). These have informed new models regarding the transport, emplacement, and crystallization of SOA felsic plutonism. With the exception of the Long Mountain Granite, the six other lithodemes have escaped additional fine-scale scrutiny. These are less exposed (68 km²) bodies partially obscured by Permian sedimentary cover. Presented here are results from a preliminary survey by XRD and SEM-EDS on the Cache, Cooperton, Long Mountain, Lugert, Reformatory, and Headquarters Granites.

Three are demonstrably heterogeneous: Myers et al. noted two facies in the Reformatory (A at Granite, OK and B at Flat Top Mtn.), and two in the Lugert (A, interior and B, reformatory contact); decameter-scale mapping (Murray, pers. comm.) reveals at least two distinct facies in the Headquarters (A, at Brown Mtn. and B, reformatory contact). Prior work defined the B facies as a contact unit or alternatively a forerunner intrusion of the lithodeme A facies.

With ~90% of the mode as Ksp and Qz, the mafic minerals are typically the best discriminators among lithodemes and their facies. All WGG contain Fe-dominated oxides; most surface samples contain Hem, arising from subsolidus replacement of Mag with Ilm as an exsolution phase. Long Mountain and Reformatory A exhibit potentially magmatic Ilm + Mag. Mafic silicates vary, and alteration to Chl is common. Headquarters B and Lugert A contain Hbl; Headquarters A, Lugert B, and Cooperton contain Bt. Reformatory A contains both, although the Bt appears to be a reaction texture in some samples. The Cache and many samples of the Long Mountain have none. The assemblage differences may be explained by variations in fO₂, aH₂O, or undercooling.