MINERALOGY OF THE LUNA VISTA SILL, BREWSTER COUNTRY, TEXAS

A textural, mineralogical, and geochemical study was conducted on five samples, top to bottom, of the Luna Vista Sill (LVS). The LVS is a hypabyssal peralkalic quartz trachyte located in southern Brewster County, Texas and is part of the Trans Pecos Magmatic Province (TPMP). Samples were collected at a roadside outcrop of the LVS at distances of 8, 26, 38, 52, and 62 meters from the northernmost edge (top) of the outcrop. Thin sections of the samples were studied under petrographic microscope, and further analyzed by Energy Dispersive Spectroscopy (EDS) using a Scanning Electron Microscope (SEM). Mineralogical observations were compared with geochemical data collected in EDS. Minerals found include alkali feldspar as phenocrysts and in the groundmass, arfvedsonite, eckermannite, ferro-eckermannite, aegirine augite, apatite with rare earth elements (REEs), zircon, ilmenite, hematite, calcite, and quartz. The alkali feldspars found in the groundmass, here referred to as microperthite, all exhibited trachytic texture and similar chemistry with the exception of the base (62 meters). The base was finer grained, different in color, and contained a higher amount of albite in the microperthite than the other four sections, which possessed more anorthoclase and sodic sanidine. The abundance of sodic feldspars, sodic amphiboles and aegirine-augite indicate high amounts of sodium in the LVS and classify the intrusion as peralkalic. This project began as a McNair Scholars Program undergraduate research project and will continue, using the mentioned methods along with x-ray fluorescence (XRF), to examine the LVS in further detail. It is part of a growing database of detailed chemistry and mineralogy of TPMP rocks being compiled at Sul Ross State University.