PETROGRAPHIC ANALYSIS OF THE CRYSTAL RICH HARMONY HILLS TUFF, CONDOR CANYON, LINCOLN COUNTY, NEVADA

The stratigraphy of the southern Great Basin Ignimbrite Province is very well exposed in Condor Canyon of eastern Nevada. These ignimbrites represent large-scale silicic volcanic activity that spanned the Oligocene and early Miocene. The source calderas have been identified by Best et al. (2013) to be along the Utah-Nevada border, near intersections of major tectonic fabrics.In Condor Canyon, the Miocene Harmony Hills Tuff, in hand sample, a light grey, phenocryst-rich tuff with conspicuous euhedral biotite crystals and subhedral feldspar crystals, looks remarkably similar to “salt and pepper” granite. Given the increase in viscosity with increasing phenocryst content, this texture begs the question “Why did the Harmony Hills Tuff erupt?” In addition, the Harmony Hills Tuff provides an opportunity to gain insight into the nature of magma chambers that are close to transitioning from the volcanic to the plutonic phase in their life span. We report here preliminary petrographic description of the Harmony Hills Tuff in an effort to constrain answers to these two fundamental questions. The dominant phenocryst phase is euhedral to anhedral plagioclase which is seriate in texture, the largest several mm’s in size. Plagioclase phenocysts can exhibit sharp crystal faces, embayed edges, and form rare fragments(?). Biotite phenocrysts are also euhedral to anhedral. Well-formed crystals several mm’s in size are a diagnostic feature of the tuff. Amphibole crystals are typically subhedral, smaller than biotite, zoned, and appear to have a thin reaction rim. Large zoned amphibole phenocrysts (several mm’s in size) are rare. Pyroxene crystals are typically anhedral and small (mm’s). These phenocrysts locally have touching grain boundaries, but are typically separated by a small amount of matrix. Textural evidence for extensive magma-mingling (e.g., ocelli, extensively resorbed crystals) has yet to be observed as expected if the eruption was triggered by mafic magma recharge. Amphibole rims may record volatile degassing, but remarkably, crystals are typically euhedral rather than broken as might be expected for a violent gas-charged eruption of a crystal-rich tuff. The Harmony Hills Tuff appears to have been well on the way to forming a granitoid and the reason for the eruption interruption from a petrographic stand-point remains an enigma.