Anorthosite-gabbro-granophyte relationships, Mount Sheridan area, Oklahoma
Thornton, Edward Clifford
Powell, Benjamin N.
Master of Arts
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.