The mineralogy and petrology of compositionally zoned ash flow tuffs, and related silicic volcanic rocks, from the McDermitt Caldera Complex, Nevada‐Oregon

Abstract

The McDermitt caldera complex, Nevada‐Oregon, is a composite collapse structure formed following eruption of three ash flow tuffs from a single compositionalIy zoned magma chamber. The major and early erupted portions of the tuffs are mildly peralkaline high‐silica comendite. Later erupted portions of two ash flow sheets are metaluminous low‐silica rhyolite. Associated with the caldera complex are precaldera dacite to rhyolite lava flows and postcaldera rhyolite domes and intrusives. Systematic variations in mineral compositions and whole‐rock chemistry throughout the entire silicic volcanic suite record the petrogenesis of highly silicic comendite through fractional crystallization of dacitic or rhyodacitic parental magma. Fractionation from daciterhyodacite to metaluminous rhyolite is characterized by initially decreasing light and middle rare earth elements; little change in Zr and Hf; and increasing U, Th, K, Rb, and Ba; controlled by removal of plagioclase, low and high‐Ca pyroxenes and significant (5%) amounts of apatite, magnetite, and ilmenite. Fractionation from rhyodacite‐low‐silica rhyolite to high‐silica comendite is characterized by increasing rare earth elements, U, Th, K, Rb, Zr, and Hf, with drastically decreasing Eu and Ba, controlled by removal of ternary‐alkali feldspar, quartz, ferroaugite‐ferrohedenbergite, fayalite and lesser ilmenite, magnetite, and apatite. Heavy rare earth element enrichment (HREE) from dacite to rhyolite is pronounced in ash flow tuffs and postcaldera volcanic rocks but not in precaldera lavas; this is possibly due to the variable presence of F as a HREE complexing agent during crystal fractionation in the caldera magma chamber. Fe‐Ti oxide compositions and phase equilibria indicate progressively decreasing magmatic temperature and oxygen fugacity: 865–940°C in dacite‐low‐silica rhyolite, with fo2 below the fayalite‐magnetite‐quartz buffer, and approximately 800°C in high‐silica comendite, with fo2 slightly above the wustite‐magnetite buffer. In the ash flow tuffs, observed patterns of vertical compositional zonation and phenocryst distributions reveal preeruptive processes important for the genesis of mildly peralkaline silicic magmas. Influx of heat is required to maintain crystallization of relatively mafic, high‐temperature mineral assemblages, notably with ternary feldspar, and prohibits further crystallization of derivative melts. Heat transfer occurred throughout a density‐stratified magma chamber via convection within compositionally discreet zones of low‐ and high‐silica rhyolite.