PRELIMINARY FINDINGS ON THE CACHE GRANITE, WICHITA MOUNTAINS SOUTHWESTERN OKLAHOMA

The Southern Oklahoma Aulacogen is an expression of early Cambrian magmatic activity, and the Wichita Mountains contain exposures of its shallow-intrusive and extrusive products. The Cache Granite is one of the oldest plutons in the Wichita Granite Group (WGG), among the first to intrude an unconformity between the preceding rhyolite activity and an underlying layered mafic complex. Although the constraints that promoted this style of plutonism are known, the nature of the transition between the early volcanic and the later plutonic events remains poorly understood.

The Cache is distinct from most WGG units because of its heterogeneous composition. Prior work shows the Hill 1545 locality on the pluton's western margin (Gilbert, OGS GB21, 1982) to have 76.8 wt.% SiO₂, with 92 and 7 ppm Ba and Sr, whereas the outcrop adjacent to the northwest contact has 73.5 wt.% SiO₂ and 800 and 80 ppm Ba and Sr. Current fieldwork is expanding mapping and sampling; this study analyzed six samples through XRD, SEM, and optical petrography.

Three samples were within the pluton. A sample from Hill 1545 and one from the pluton's eastern margin are nearly identical; both contain abundant granophyre on medium grain size phenocrysts of perthitic feldspar and quartz. Biotite, chlorite, fluorite, and zircon are present only in accessory quantities. The northwest contact-adjacent rock is similar, but uniquely contains rapakivi feldspar (plagioclase rimmed perthite). The texture is also noted in the neighboring Mount Scott Granite, attributable to crystallization at a ponding depth, followed by decompression resorption due to ascent into warm crust, and resumed growth after emplacement. In contrast to Mount Scott, rapakivi feldspar in the Cache Granite are angular, not rounded, implying a lack of resorption prior to emplacement growth.

Three potentially related samples were collected from areas adjacent to the pluton. Located 0.5 km within the Quanah Granite, a thin, inclined body contains rock identical to the Hill 1454 sample, and likely represents included material. The two others are distinct and non-granophyric. An anomalously-textured rock from Quanah Mountain was weakly porphyritic and fine granular. A sample of an exposure south of Hill 1545 was coarse granular. We consider both to be likely products of the Quanah pluton, not the Cache.