DISTINCT PEGMATITE STYLES IN THE MT. SHERIDAN ROOSEVELT GABBRO, WICHITA MOUNTAINS, OKLAHOMA: IMPLICATIONS FOR RIGIDITY DOMAINS, MELT MIGRATION, AND MAGMA CHAMBER DYNAMICS

The vertically stratified Mt. Sheridan Roosevelt Gabbro is composed of layered gabbro (crystallized cumulate pile), homogenous quartz diorite, and felsic pegmatite. Pegmatites appear to be derived in situ and show evolution from a diffuse source to dikes over a meter in width. Five distinct pegmatite styles are observed in the field: 1) diffuse pegmatite 2) irregular pegmatite “pods,” 3) non-uniform (in thickness and strike) melanocratic dikes, 4) non-uniform leucocratic dikes, and 5) uniform (in thickness and strike) dikes (McEllen et al., 2006). Pegmatites typically contain Or, Qtz, Plag, Pyx, Amp, Ilm, Mt, Ap, Ttn, and Zrc + deuteric minerals. Textural and mineralogical similarities suggest pegmatite types preserve different stages of extraction and collection of evolved liquids (migrating residual melt). Each pegmatite style is interpreted to reflect different rigidity domains within the crystal framework at the time of formation. In low rigidity domains residual melt forms teardrop-shaped diapirs and may be preserved as pegmatite pods. With increasing rigidity, residual melt can form self propagating dikes and is preserved as non-uniform pegmatite dikes. Fractionation within the self propagating dikes allows separation of mafic minerals from the melt to form non-uniform melanocratic pegmatite dikes. The remaining felsic melt in the self propagating dikes migrates to form non-uniform leucocratic dikes. Uniform pegmatites are the result of brittle fracturing of the crystalline (very rigid) framework and filling by residual melt. Diffuse “protopegmatites” represent residual melt that had insufficient volume to overcome the strength of the crystal framework and migrate. A sufficient volume of residual melt must collect in order for melt to migrate as a diapir or as a self propagating dike. In order to reach a sufficient volume, crystals must develop a framework that traps sufficient residual melt to form pegmatites. Compaction of the framework (triggered through replenishments, seismic shaking, etc.) must take place in order to expel residual melt from interstitial spaces. Evidence for compaction triggers in the Mt. Sheridan gabbro includes scour channels, reverse zoning in plagioclase, and the repetition of olivine and quartz vertically within the pluton (Lasco et al., 2004).