FIELD RELATIONSHIPS AND ZIRCON TEXTURES CONSTRAIN RELATIVE TIMING OF PERALUMINOUS MAGMATISM AND METAMORPHISM IN THE RUBY MOUNTAINS, NEVADA

The Ruby Mountain–East Humboldt Range metamorphic core complex (northeast Nevada) experienced multiple periods of protracted peraluminous magmatism between 160 and 29 Ma. Zircons from many of the granitic rocks have discordant ages leading to conflicting models for magma generation and the magnitude and duration of partial melting events. A detailed map of a complex polylithological outcrop with clearly defined crosscutting relationships located near Liberty Pass was prepared to better constrain the relative timing relationships between different magmatic events. Accessory minerals from a pegmatitic muscovite granite gneiss were characterized using back scatter electron (BSE) and cathodoluminescence (CL) imaging. The pegmatitic gneiss represents the geochronologically most complex lithology in the core complex and is an important key to establishing the nature, duration and timing of magmatism between ~90 and ~40 Ma. The complexity is reflected in zircon separated from the gneiss. Based on morphology, there are at least three populations: euhedral prismatic zircon characteristic of igneous crystallization; elongate bladed crystals that may represent a short-lived high-temperature event; and, a rounded morphology that is characteristic of a metamorphic zircon population. The textural relationships, morphologies and zoning patterns establish relative timing relationships between the populations, and suggest that each population grew during a distinct tectonic and/or thermal event. The data favor a model for zircon growth in distinct and separate geologic events, rather than a period (40-50Ma) of prolonged partial melting and magmatism.