Browsing by Author "Powell, Benjamin N."
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Item Anorthosite-gabbro-granophyte relationships, Mount Sheridan area, Oklahoma(1975) Thornton, Edward Clifford; Powell, Benjamin N.Field work, phase analysis by microprobe, petrography, and oxygen isotope analysis have been undertaken in an effort to establish the relationships among the anorthositic, gabbroic, and granophyric rocks of the Mount Sheridan area in southwestern Oklahoma. The gabbro has been found to be somewhat finer-grained when in contact with the anorthosite. Petrographically, the gabbro is quite different from the anorthosite. The anorthosite is essentially a one-pyroxene rock (clinopyroxene) while the gabbro contains both augite and hypersthene. Quartz, micropegmatite, and biotite are present in the gabbro but essentially absent in the anorthosite. The anorthosite is a cumulate rock with well-developed igneous lamination whereas the gabbro is generally not laminated. The above information established that the anorthosite is the older unit and that the gabbro was subsequently intruded into it. The gabbro is transitional into the overlying granophyre through a zone of intermediate rock. The zone of transition has been found to extend into the gabbro itself as has been determined by phase analysis, in addition, diabasic inclusions are present in the granophyre-evidence of possible mixing of basaltic and granophyric magma. These observations suggest that gabbroic and granophyric magmas were emplaced simultaneously at Mount Sheridan and that the intermediate rock between the gabbro and granophyre may have been generated by mixing of the two magma types. The granophyre has a 618 value of + 4.7 +.7 (this study) while Johnson and Denison (1973) report that the initial value for the Sr87/Sr86 ratio is .77 + .1. The granophyre has a minor amount of ferroaugite (this study). Chemical and mineralogical data suggest that the granophyre may have been derived, in part, from a basaltic parent but volume relationships indicate that melting of crustal or upper mantle material was also important. A tectonic-magmatic model is presented to demonstrate the possible genetic relationships of the magma types.Item Petrology and geochemistry of intermediate rocks in gabbro-granite contact zones, Wichita Province, Oklahoma(1981) Kennedy, Jerry Wilson; Powell, Benjamin N.; Leeman, William P.; Baker, Donald R.The investigation of intermediate rocks in two granite-gabbro contact zones in the Wichita province of southwestern Oklahoma has established petrologic and geochemical constraints which suggest intrusive relationships exist between the major silicic and basic plutonic rocks. Field studies of the Poko Mountain area in the eastern portion of the province indicate the contact of the younger granite with the underlying gabbro is discordant and is characterized by the presence of gabbro xenoliths within the granite. Whole-rock and trace element chemistry disclose the intermediate rock found highest in the gabbro section at this exposure is more fractionated than the contact phase of the granite. Negative Eu anomalies demonstrate the granite has experienced strong plagioclase fractionation. The gabbro, however, shows no complimenatry positive Eu patterns. In addition, the plagioclases of the gabbro exhibit reverse cryptic variation with the most calcic plagioclase (An67) being found in the intermediate rock immediately below the granite contact. Petrogenesis of the intermediate rock at Poko Mountain is therefore attributed primarily to crystal fractionation of the gabbro and secondarily to alkali metasomatism from the overlying granite. Examination of the Twin Mountains outcrop in the western part of the province reveals the contact between granite and intermediate rock is knife-sharp. The intermediate rock, originally an anorthositic cumulate, is volumetrically composed of approximately 8% plagioclase but is now identified by a negative Eu anomaly. Contact thermal alteration and resorption of plagioclase within the intermediate rock are evident and indicate disequilibrium. The data suggest the development of the intermediate rock at this locale is due solely to hydrothermal contamination of the basic rock by alkali metasomatism from the adjacent granite. The evidence thus suggests that at neither outcrop are the granites a product of differentiation of the basic rocks. ' Most probably the granites originated by partial melting of the Precambrian granitic basement by upwelling basic magmas during the rifting associated with the evolution of the southern Oklahoma aulacogen.Item Phase chemistry of the layered series, Raggedy Mountain Gabbro Group, Oklahoma(1976) Phelps, David William; Powell, Benjamin N.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.Item The mineralogy and phase chemistry of silicic tephras erupted from Mount St. Helen's volcano, Washington(1980) Smith, Diane R.; Leeman, William P.; Powell, Benjamin N.; Schwarzer, Rudy R.The Recent eruptive history of Mount St. Helens volcano in Washington includes numerous explosive eruptions which produced voluminous and widespread silicic tephra layers. Many tephra sets have "been defined on the "basis of ferromagnesian phenocryst assemblages and carbon-l4 ages (Mullineaux et al., 1975) and provide an excellent record of explosive activity through time. They are useful time-stratigraphic markers as they are interbedded with other sedimentary and volcanic units in areas of the Pacific Northwest. Investigations of petrography, modal abundances of heavy minerals and phase compositions (determined by electron microprobe) were utilized in an effort to define parameters to unambiguously identify individual tephras. The characteristics which are most distinctive of different tephras include orthopyroxene compositions, iron-titanium oxide compositions, equilibration temperatures and oxygen fugacities obtained with magnetite-ilmenite geothermometry and geobarometry, and ferromagnesian silicate assemblages as defined by optical and x-ray diffraction techniques. No single characteristic is in itself distinctive and several characteristics should be used in identification of unknown tephras. The wide compositional ranges for silicate minerals, variable modal abundances of heavy minerals, and observed ranges of oxide equilibration temperatures and oxygen fugacities suggest that some of the magmas may have been chemically or physically heterogeneous. Iron-titanium oxide equilibration temperatures and oxygen fugacities correlate with mineral assemblage and composition. High temperatures and oxygen fugacities correspond with assemblages of hornblende + cummingtonite + orthopyroxene (relatively low in FeO) and lower temperatures and oxygen fugacities correspond with hornblende + orthopyroxene (relatively enriched in FeO). The Mount St. Helens cummingtonite-bearing tephras show higher temperatures and oxygen fugacities of equilibration of associated iron-titanium oxides compared to other cummingtonite-bearing silicic pumice deposits. Temporal trends of temperature and ferromagnesian silicate assemblages suggest that some of the tephras may be related, but limited geochemical data and biased sampling of only silicic tephras and not other products erupted during the same time intervals do not allow strong conclusions regarding petrogenetic relations.