PEGMATITES OF THE MOUNT SHERIDAN GABBRO, WICHITA MOUNTAINS, OKLAHOMA

Felsic pegmatite in the compositionally stratified Mt. Sheridan Roosevelt Gabbro, Wichita Mountains, southwestern Oklahoma, appear to be derived in situ and show evolution from a diffuse source to dikes over a meter in width. Pegmatite is common to locally abundant, but may occur in greater abundance in the lower portion of the intrusion. Pegmatite occurs in several distinct forms that are readily identified in the field –from the most mafic to the most felsic: 1) diffuse areas of felsic enriched gabbro “protopegmatite,” 2) areas of irregular, isolated pegmatite “pods,” 3) irregular pods and stringers with a common planar alignment, 4) dikes of pegmatite which contain amphibole and pyroxene throughout the dike, and 5) dikes of pegmatite which contain amphibole and pyroxene, primarily as combs at the gabbro-pegmatite boundary with leucocratic aplitic or graphic cores. Pegmatite typically contains orthoclase, quartz, plagioclase, pyroxene, amphibole, FeTi oxides, apatite, titanite, and zircon. Pervasive deuteric alteration includes epidote, sericite, chlorite, and amphibole. Pods generally contain more pyroxene and amphibole than the average pegmatite. Evolved dikes are zoned from pyroxene rich margins to almost pure quartz and orthoclase centers. Quartz and orthoclase typically exhibit micrographic texture (<3mm) or occur as very fine grained crystals. Plagioclase forms small, typically saussruitized, (<5mm) euhedral crystals. Pyroxene occurs either as small tabular crystals (0.5-1 cm) or as elongate hollow crystals (up to at least 8cm long) and are typically rimmed by amphibole. Cores of these crystals can be unaltered and are rarely completely replaced. Texturally, the pegmatite's share several distinguishing features; graphic texture, hollow crystals of pyroxene commonly twined and rimed by amphibole, and saussruitization of plagioclase crystals. Smooth transitions between different pegmatite types have been observed. This, along with textural and mineralogical similarities, suggests pegmatite types preserve different stages of extraction and collection of evolved liquids from filter pressing of the cumulate pile during crystallization of the Mount Sheridan magma chamber.