A MULTI-MINERAL PETROCHRONOLOGIC APPROACH FOR UNDERSTANDING CRYSTAL-RICH RHYOLITIC MAGMA SYSTEMS: AN EXAMPLE FROM THE MIOCENE APACHE LEAP TUFF, CENTRAL AZ

Zircon petrochronology, a technique that places temporal constraints on thermochemical magma evolution, is an increasingly common tool for understanding rhyolite petrogenesis. Investigations into large-volume, crystal-rich rhyolites using this technique have captured the complexity of crystal-rich rhyolitic systems, including apparent prolonged magma residence and mineral disequilibria. Zircon crystals record extended magmatic histories due to their high saturation temperature (~850°C) and propensity to recycle through multiple stages of magmatism. We posit that studies of pre-eruptive magmatic processes are improved with combined zircon-titanite petrochronology because the zircon record alone may smear the complexity of rhyolite petrogenesis. Titanite saturates at relatively low temperature in rhyolitic magmas (~760°C) and has strong, coarse compositional zoning, which provides a high-fidelity record of near-solidus magmatism. Thus, employing tandem zircon-titanite petrochronology is critical for understanding the continuous and episodic histories of rhyolitic magmatic systems, particularly when investigating near-solidus and pre-eruptive magma dynamics.

To demonstrate, we use zircon-titanite petrochronology to investigate magma dynamics leading to eruption of the Apache Leap tuff, an ~800 km³ crystal-rich (~40 vol%) rhyolite in the Superstition Volcanic Field, central Arizona. Zircon in the Apache Leap tuff record >200 ka of magmatism and have Ti-in-zircon temperatures ~850 to 700°C. Thermometry from zircon crystal faces analyzed via LA-ICPMS rasters suggests final zircon growth at <750°C. Zr-in-titanite thermometry suggests titanite growth occurred from ~760 to 700°C, and core-to-rim analyses of euhedral titanite crystals reveals both up- and down-temperature growth. Our high-precision ID-TIMS U-Pb zircon and titanite dates are accompanied by a new eruption age obtained via ⁴⁰Ar/³⁹Ar incremental heating experiments on single sanidines. Zircon-titanite petrochronology results are combined with constraints from rhyolite-MELTS modeling to provide further insights into pre-eruptive magma dynamics of large, crystal-rich rhyolitic magma reservoirs.