ZIRCON AND TITANITE PETROCHRONOLOGY OF DEFORMED AND NON-DEFORMED PAYETTE RIVER TONALITE NORTH OF MCCALL, ID: IMPLICATIONS FOR THE TIMING OF HIGH TEMPERATURE DEFORMATION IN THE WESTERN IDAHO SHEAR ZONE

North of McCall, Idaho, the Payette River tonalite (PRT) straddles the eastern margin of the dextral western Idaho shear zone (WISZ), a lithospheric-scale feature that accommodated significant transpression during the Cretaceous. PRT emplacement has been hypothesized to be contemporaneous with shearing. We present new zircon and titanite (sphene) ages, temperatures, and trace element data from deformed and non-deformed samples of the PRT to determine the timing and conditions of emplacement and deformation. Additionally, we will present a new method to measure pressure using paired zircon thermometry and titanite thermobarometry. This technique will allow us to quantify the amount of transpression in the WISZ by comparing emplacement and deformation depths.

Magmatic zircon gives an emplacement age of 90.39±0.28 Ma (isotope dilution thermal ionization mass spectrometry (ID-TIMS) weighted mean age, 2σ error, n=6) for the PRT, consistent with earlier work in the area. Titanite from deformed PRT is 83.3±3.3 Ma (laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS), 2σ error, n=82, MSWD=1.7) and suggests that high temperature WISZ shearing post-dates emplacement by ~7 Ma. The age of the nominally non-deformed PRT, 83.1±1.8 Ma (LA-ICPMS, 2σ error, n=114, MSWD=1.6), is the same as the strongly foliated PRT. These ages are curiously young but do fall between the zircon crystallization age and previously reported Ar-Ar cooling ages (Giorgis et al., 2008). Titanite directly dates high-temperature solid-state deformation because it readily recrystallizes, has a high Pb closure temperature (>700°C), and participates in fabric-forming reactions. We will present the results of titanite ID-TIMS geochronology for these samples to provide further insight into the tempo of deformation. Positive Eu anomalies in PRT titanite indicate growth during submagmatic to subsolidus anorthite breakdown. Zr-in-titanite temperatures range from ~600° to ~750°C, suggesting magmatic, submagmatic, and solid-state growth.