CRYSTALLIZATION DYNAMICS OF THE MT. SHERIDAN ROOSEVELT GABBRO, WICHITA MOUNTAINS, OKLAHOMA

Three texturally distinct rock types in the Mt. Sheridan Roosevelt Gabbro suggest distinct modes of crystallization. The vertically stratified pluton is composed of layered gabbro in the lower elevations (the growing cumulate pile) which is overlain by homogenous quartz gabbro (the active magma chamber). Felsic pegmatite is present, and locally abundant, throughout the pluton (see McEllen et al., this session). These three rock types, though texturally distinct, are genetically linked during crystallization of the magma chamber. Reverse zoning in plagioclase and the repetition of olivine-bearing and quartz-bearing rocks vertically within the pluton indicates the magma chamber received multiple mafic replenishments during crystallization (Lasco et al. 2004). Isolation and extreme fractionation of the mafic magma could produce a felsic melt that, if kept isolated from the active chamber, would be preserved as pegmatite. Compaction of the cumulate pile by dense mafic replenishments or seismic shaking would allow intercumulus melt to migrate and collect. A critical amount of this melt must collect in order to form dikes that can self propagate through the cumulate pile. This amount varies as the cumulate pile rigidity and melt properties (i.e., viscosity) vary. Intercumulus melts with low viscosities or in regions with low rigidity will be more efficiently extracted from the cumulate pile than intercumulus melts with high viscosities or in regions with high rigidity. Field evidence suggests that felsic melts were moving through the cumulate pile (e.g., boudinaged pegmatite dikes) and may have reached the active magma chamber allowing batches of highly evolved felsic melt to be incorporated back into the remaining magma. Felsic melts which did not reach the active portion of the magma chamber were preserved as pegmatites within the cumulate pile. The type of pegmatite produced (diffuse, pod, or dike) depended upon the amount of melt that collected. We suggest that incorporation of a significant amount of this melt into the active magma chamber could drastically change the composition of the magma, resulting in the crystallization of a more evolved homogenous quartz gabbro.