Using texture and geochemical data to assess the stratigraphic relationships between the Marcus Wash and Rojo Grande units, Picacho State Recreation Area, SE California
Advisor Dr. Gary Girty
The Picacho State Recreation Area (PSRA), located in the lower Colorado River region SE California, contains a thick sequence of Tertiary volcanics. Based on published work, Miocene volcanic rocks have been subdivided, from lowest to highest, into the following 6 mappable units: Quechan volcanics, Marcus Wash rhyolite, bedded pyroclastic and epiclastic unit, White Wash rhyolite, Walker volcanics, and the ignimbrite of Ferguson Wash. Earlier workers also recognized, in and around Rojo Grande, a unit of rhyolite that they named the Rojo Grande rhyolite. This unit was thought to occupy the same stratigraphic position as the Marcus Wash rhyolite. In the field, the so called Rojo Grande rhyolite has prominent light and dark flow banding, abundant ~1-6 millimeter-size plagioclase phenocrysts, and an aphanitic matrix. The purpose of this study was to investigate the degree that alteration may have affected the chemical composition of the Rojo Grande rhyolite, and to address whether or not this unit is chemically similar to the Marcus Wash rhyolite.
Thin section study revealed that all 18 samples collected from the Rojo Grande rhyolite had some degree of hydrothermal alteration. Alteration included local replacement of feldspar by calcite and white mica (sericite) and the replacement of amphibole by opaques. Also, many samples contain veinlets of calcite and quartz. Because of ubiquitous alteration, samples evaluated during this study cannot be classified utilizing the TAS (total alkali-silica) scheme of the IUGS. Instead, the conventional immobile trace-element magma-series discrimination diagram was used.
The results of this study suggest that alteration does not correlate with proximity to structures, and is therefore likely the result of a regional hydrothermal alteration event or events. On the immobile trace-element magma-series discrimination diagram, samples from this study cluster in the trachyte field. Hence, the rhyolitic character commented on by previous authors may be due to the introduction of silica during hydrothermal alteration. Though published results from the Marcus Wash rhyolite overlap the field defined by samples analyzed during this study, they tend to cluster below the main grouping of Rojo Grande samples. I tentatively recommend that the name Rojo Grande trachyte be retained until additional geochronological and rare earth element analysis are completed from both the Rojo Grande and Marcus Wash units.