INSIGHTS INTO THE GROWTH OF SILICIC MAGMA SYSTEMS FROM THE GEOCHRONOLOGIC, ELEMENTAL, AND ISOTOPIC RECORD OF ZIRCON IN MIOCENE PLUTONS AND VOLCANIC ROCKS OF THE COLORADO RIVER EXTENSIONAL CORRIDOR, NEVADA

Recent geochronologic studies have shown overwhelmingly that zircons from small samples of plutons record intra- and intercrystalline age variability that make assigning a unique ‘crystallization age’ based on a zircon ‘date’ ambiguous. The story from mature silicic volcanoes is similar. What the age spread records is contentious but it typically exceeds the thermal lifetimes of large closed system magma bodies. Open system behavior is thus required to unite zircons grown at different times and sometimes in disparate environments, but the mechanisms that mix zircons and the specific sources of zircons remain poorly constrained. Elemental and isotopic studies of zircon offer additional valuable insights into where and how zircons are recycled in silicic magma systems, as demonstrated by our work from mid-shallow crustal Miocene plutons and coeval volcanic rocks in the northern Colorado River Extensional Corridor.

Zircons show appreciable inter- and intracrystalline age variability (several 10⁵ to 10⁶ years), and often pronounced changes in trace elements and ratios (e.g. Hf, Yb/Gd, Th/U, Eu/Eu*). These shifts record fluctuations in the chemical environments in which the zircons grew, and can often be correlated with temperature changes (Ti-in-zircon thermometry) and changes in whole rock geochemistry and/or saturation in new phases (e.g. sphene). Intercrystalline variation in Hf isotopes (typically 7 to 10 εHf units) and O isotopes (1-2‰ δ¹⁸O) from single samples also show that zircons grew in distinct melts, but a surprising result from the Hf isotope data is the remarkable lack of Precambrian zircons (n=4 from >400 analyses), even though whole rock isotope data require a Precambrian crustal component, and all plutons are intruded into Proterozoic crust. This suggests that initial anatectic melts were either undersaturated in zircon, and/or that subsequent replenishment (e.g. by a hotter, more mafic injection) resulted in strong undersaturation and eradication of zircon inherited from crustal sources. Altogether, the data show that the mid-shallow crustal magma chambers that coalesced to form the plutons and that were tapped during eruptions, were periodically dynamic environments in which much of the zircon growth occurred, and in which zircon was efficiently exchanged between magmatic pulses.