PETROCHRONOLOGY OF THE FISH CANYON TUFF

The petrologic complexity of the archtypical ‘monotonous intermediate’ Fish Canyon Tuff (FCT) has been previously established by a variety of mineralogical and geochemical proxies, and the unusual storage and eruptive dynamics of the FCT have been delineated by several geochronological studies. Zircon and titanite are equilibrium phases in the penultimate FCT magma, and their geochemical compositions may record an intracrystalline record of magma chamber dynamics. Titanite crystals were oriented and doubly-polished to yield characteristic wedge-shaped cross-sectional wafers approximately 300 µm in thickness. Back-scattered electron imaging guided LA-ICPMS analyses of a full suite of trace elements using a 25 µm beam diameter and crater depth on multiple locations across both sides of the wafer. Zircon crystals of a variety of shapes and sizes were ground to their centers and individual zones identified in cathodoluminescence imaging were analyzed using a variety of spot and shallow line geometries.

Most titanite crystals are characterized by large variations in trace elements, including at least two generations of REE-enriched, actinide-poor, low Sr, large Eu anomaly cores overgrown by REE-depleted, actinide-rich, high Sr domains with small Eu anomalies and distinctive concave-up middle to heavy REE patterns. Trace element contents and patterns correlate strongly with Eu anomaly; intermediate compositions are abundant and spatially correlated to reaction zones between core and rim domains.

Within the context of the batholithic rejuvenation model for the FCT magma, these trace element variations are interpreted to record the partial melting of a differentiated crystalline FCT precursor and its hybridization with a more ‘mafic’ flux. ID-TIMS dating of end-member titanites confirm older ages (ca 28.4 to 29.0 Ma) for cores and define a younger age for rejuvenation of ca 28.2 Ma, consistent with recent U-Pb zircon and ⁴⁰Ar/³⁹Ar studies. We will present a similar petrochronological tandem LA-ICPMS/ID-TIMS data set for zircon that tests the fidelity of these conclusions from titanite.