INSIGHTS FROM MAFIC MINERAL VARIATIONS AMONG AN A-TYPE LIP GRANITE SERIES FROM THE SOUTHERN OKLAHOMA AULACOGEN

The Wichita Granite Group (WGG) is the expression of shallow (emplacement ≤ 2 km) plutonic felsic magmatism, part of the Eocambrian Southern Oklahoma Aulacogen or Wichita Large Igneous Province. The WGG currently includes 13 lithodemic units as exposed over 325 km² in the Wichita Mountains, but a much larger area of material underlies younger strata along a trend that extends from Dallas into the Texas Panhandle and beyond. Recent work reexamines and characterizes their mafic mineral assemblages in order to constrain chemical relationships. Furthermore, these data permit comparison of surface exposures with rotary-drill cuttings from basement-penetrating wells.

A-type granites typically are ferroran and calc-alkalic to alkalic (wt% Na₂O + K₂O - CaO), with elevated Nb and F and depressed H₂O. The WGG is alkali feldspar granite and similar granitoids that embody these characteristics. The range in alkalis is sufficient to vary mafic-mineral assemblages. The bulk of the granites contain biotite ± hornblende (hastingsite and ferro edenite, ΣFM=15, 23O basis). Afvedsonite and katophorite are found in one pluton each, but sodic amphiboles dominate the volumetrically diminutive pegmatitic granitoids. For all, 0 < ^BΣM²⁺ < 12, increasing with ^BCa. Clinopyroxene is rare, but increasingly present in samples that preserve mafic-felsic interactions. Alteration of the primary mafic silicates to chlorite-group minerals is pervasive, particularly in subsurface samples. Initial assemblages of magnetite (± ilmenite) are altered to hematite; the rocks contain extensive hematitic staining in and along grains. Hematite alteration is diminished in deep quarry cuts and holes in exposed granites, but is curiously near-universally present in subsurface granites (as deep as 3 km).

The findings indicate that despite a broad similarity in chemical composition and emplacement history, the diversity in mafic silicate assemblages records the complications endemic to voluminous magmatism constrained by a narrow space (e.g a rift valley). Subsolidus alteration is surprisingly prevalent given low magmatic H₂O.