TITANITE U-PB PETROCHRONOLOGY REVEALS ~26 MILLION YEARS OF DEFORMATION IN A MID-CRUSTAL HIGH-TEMPERATURE SHEAR ZONE

Dating recrystallized and neoblastic titanite associated with the fabric of deformed granitic plutons can test models of shear zone evolution through spatial-temporal patterns of strain localization. The western Idaho shear zone (WISZ) is a Cretaceous mid-crustal dextral transpressive shear zone developed within a continental magmatic arc. Previous studies have interpreted U-Pb zircon ages of ~90 Ma and ~88 Ma for weakly deformed cross-cutting dikes and plutons as minimum ages of deformation in the WISZ. Here we use titanite from variably deformed plutons to recover the full history of fabric development in the shear zone. Titanite is well-suited to dating high-temperature deformation due to its high closure temperature for the U-Pb system and its propensity to recrystallize or grow in response to changes in pressure, temperature, and differential stress.

We dated zircon and titanite using a sequential textural characterization, in situ analysis, microsampling, and isotope dilution petrochronology workflow for plutons within the WISZ and inboard of the arc-continent transition. Zircon ages, a proxy for magmatism, range from 105.23 ± 0.12 Ma to 82.50 ± 0.08 Ma. Ages for recrystallized and neoblastic titanite, a proxy for fabric development during deformation, range from 103.30 ± 0.26 Ma to 77.34 ± 0.65 Ma. Some peaks in the probability density function of titanite ages correlate with peaks in the probability density function of zircon ages, suggesting that titanite records localized zones of increased strain resulting from adjacent pluton intrusion. However, most titanite ages lag and/or are uncorrelated with zircon ages, suggesting that titanite also records ongoing strain accumulation in the shear zone. Overall, activity in the WISZ lasted ~26 million years, with multiple episodes of magmatism and changes in rates of deformation.

Our results reveal that deformation in the WISZ does not consistently migrate eastward with time, contrary to previous models. Rather, the locus of deformation moves back and forth across a 15 km wide shear zone, which is at least partially controlled by magmatism. This study illustrates how titanite and zircon petrochronology can parse the spatial-temporal patterns of fabric development within high-temperature shear zones.