Phase chemistry of the layered series, Raggedy Mountain Gabbro Group, Oklahoma
Phelps, David William
Powell, Benjamin N.
Master of Arts
Gabbroic igneous rocks of lower Cambrian age outcrop in southwestern Oklahoma in the Wichita Mountains and are here referred to as the Raggedy Mountain Gabbro Group. The Raggedy Mountain Gabbro Group consists of small gabbroic to dioritic sills and dikes and a large layered intrusion (4 km by 8 km in the subsurface) referred to as the layered series. The layered series has only approximately 2 m of section exposed at the surface which has been divided into three units (K, L, and M zones) on the basis of lithology. Approximately 1 samples were collected from different stratigraphic elevations from three locations in the layered series. All handsamples were thinsectioned, examined optically and selected samples chosen for microprobe analysis. Petrographically the rocks of the layered series are adcumulates. Plagioclase is a cumulus phase and the most abundant mineral in all thin sections. Clinopyroxene occurs in large oikocrysts with two distinct textures distinguished by the size of plagioclase chadacrysts. Olivine is cumulus; most grains are surrounded by symplectic coronas of orthopyroxene and magnetite. Orthopyroxene also occurs as an intercumulus phase. Microprobe analyses were performed on olivine, clinopyroxene, orthopyroxene and plagioclase. Oscillatory zoning observed in some plagioclase crystals is tentatively attributed to circulation of the crystals by convection currents. Highly calcic partial rims on plagioclase crystals may be a result of increasing PH during crystallization of trapped pore liquid. There is evidence that the symplectic reaction rims around olivine crystals are a result of increasing oxygen fugacity in the crystallizing trapped pore liquid. The intercumulus orthopyroxene crystallizing from pore liquid in communication with overlying magma, the low Al2O3 and TiO2 values in clinopyroxenes, and the presence of low calcium pyroxene lamellae in clinopyroxene indicate crystallization from a tholeiitic series. Compositions of clinopyroxenes vary from Fs. Cumulus clinopyroxenes symplectic coronas range from Fs magma. Increase in iron in the cumulus ferromagnesian minerals and the increase in sodium in the plagioclase upwards in the section suggests that crystal settling out of a differentiating basaltic magma is responsible for the formation of the layered series. The present level of exposure may be approximately midway between the top and bottom of the intrusion.