MICROANALYTICAL CHARACTERIZATION OF SUBSURFACE SAMPLES OF THE WICHITA GRANITE GROUP, SOUTHERN OKLAHOMA AULACOGEN

The Southern Oklahoma Aulacogen (SOA) is an Eocambrian rift that extends from the Dallas-Fort Worth area northwestward into the Texas Panhandle and beyond. The rift contains a number of largely bimodal igneous systems that hold further clues on the construction of the SOA in its distribution of individual magmatic events. The aulacogen rocks are exposed in the Arbuckle and Wichita Mountains, but the bulk is covered under Paleozoic sediments. Fortunately, the buried SOA is extensively sampled by oil and gas exploration wells; many of which produced numerous small (~5 mm) cuttings from rotary drilling.

We prepared cuttings from 12 Wichita Granite Group samples from wells in the western Wichita Mountains. The wells were selected because of their proximity to outcrop, documented penetration of the basement, and sample availability. We characterized the chemical differences of mafic grains using laser induced breakdown spectroscopy (LIBS) and running the acquired spectra through principal component analysis (PCA).

We also compared these Wichita Mountain cuttings to previously acquired Arbuckle Mountain samples, and found a general compositional similarity. PCA revealed substantial variance in the Ca, Na, Mg, and Fe peaks. Electron microprobe analysis revealed that samples with strong variance are those with amphibole altered to chlorite group minerals. Further LIBS work on the chlorites document the significant variance in Ca and Mg peaks that may be used for future discrimination. In amphibole-bearing Arbuckle samples, those from the west exhibit variance in the first principal component that differ from those in the east, driven largely by the Na peaks. This difference is confirmed by electron microprobe analysis. All of the granite samples are calcium amphiboles, but the amphiboles in the eastern samples are hastingsite, similar to those from members of the exposed Wichita Granite Group, whereas the western example is ferro-actinolite.

Curiously, we found one granitoid with a distinctive high color index. The Mobil Mauldin well encountered a dark rock at 6,100 feet (1,860 m) that contains a disequilibrium mixture of clinopyroxene, plagioclase, amphibole, and alkali feldspar. Amphibole compositions vary within cuttings, but include a kataphorite-like sodium-calcium chemistry. This rock bears some similarity to an exposed igneous breccia in the Wichita Mountains and further indicates the interactions of mafic and felsic liquids in the aulacogen.