SAME SOURCE, DIFFERENT TEXTURES: THE CACHE AND QUANAH GRANITES, SOUTHWESTERN OKLAHOMA

The alkali feldspar granites of the Wichita Granite Group (WGG) of southwestern Oklahoma are products of Cambrian sheet plutons emplaced immediately below rhyolitic rift strata. The two prominent facies of the Quanah Granite and a facies of the Cache Granite are virtually indistinguishable with regard to major-element oxides and a number of trace elements. Thermodynamic modeling indicates liquidus crystallization of hypersolvus Ksp, followed by crystallization of Qz with decreasing T of 10-30 °C, and continued co-crystallization to the solidus. Their variable textures indicate additional individual complexity.

The Quanah’s typical facies is coarse and subequigranular, dominated by strongly exsolved perthite with overgrowths that form an intercumulus with glomerocrystic medium-grained equant Qz and smaller Arf + Fe-Ti oxides. Qz exhibits a low-intensity CL core mantled by two higher–intensity zones, with a ~60 micron, low-intensity rim that joins glomerocrystic grains. Including small inclusions, sections contain roughly 250 Qz particles per cm². The Quanah’s medium-grained Craterville facies is perthitic Ksp and equant glomerocrystic Qz with oxides ± Bt, surrounded by variable amounts of dense granophyre. Qz phenocryst cores have relatively elevated CL intensity, with ~60 micron rims that match the low CL intensity of the granophyric Qz. Sections contain 1050-1575 Qz particles per cm². In contrast, the type facies of the Cache Granite exhibits the strongly porphyritic microstructure more typical of the WGG. The medium-grained phenocrysts of perthitic Ksp and glomerocrystic equant quartz with oxides is surrounded by intense, very fine granophyre. These glomerocrysts display subtle CL zoning with lower intensity cores; their skeletal rim intensity is largely the same as the surrounding granophyre. Sections contain greater than 2300 Qz particles per cm².

Field relationships and high-precision dating show that the Quanah magma intruded the Cache Granite 110 to 650 Kyr after Cache crystallization. Their near-identical composition suggests separate pulses arising from the same source. Over this interval, emplacement conditions progressed from severe undercooling to those permitting the persistence of near-solidus temperatures.