TITANITE PETROCHRONOLOGY DATES SHEAR ZONE INITIATION TO BETWEEN 99 AND 96 MA IN THE WESTERN IDAHO SHEAR ZONE

Constraining the timing of deformation is critical for reconstructing plate tectonic processes in continental arc settings. Titanite, a mineral commonly found in granitoids that recrystallizes and grows in response to changes in temperature, pressure, and differential stress, is useful as a U-Pb geochronometer for assessing the timing of deformation. However, titanite can form in magmatic to subsolidus conditions and thus can record a protracted history of pluton emplacement and subsequent deformation. We developed a method of integrating titanite microstructures, geochemistry, and high precision geochronology to distinguish the onset of deformation in granitoid-hosted shear zones.

We demonstrate this method on a sample of porphyritic orthogneiss from the western Idaho shear zone (WISZ), a crustal-scale transpressional shear zone. From a single hand sample, we document a span of titanite ages from 105.0 ± 0.1 to 82.6 ± 2.9 Ma. We use backscattered electron images, electron backscattered diffraction analysis, and trace element geochemistry to characterize titanite crystals and crystal domains as magmatic, partially to fully recrystallized, or neoblastic. Specifically, we use the dimensional reduction of principal component analysis to identify key trace element parameters (total rare earth elements (REE), U, Nb/Th) that track titanite recrystallization associated with subsolidus fabric development, and we relate geochemical trends to intragrain misorientation patterns and fabric context. Through interactive plots of total REE vs. U vs. Nb/Th coded by temperature and age, we show how the expulsion of REE and high field strength elements during partial to full recrystallization marks the onset of deformation in this sample.

Our integration of microstructures, geochemistry, and geochronology reveals that the WISZ shearing initiated between 99 and 96 Ma, consistent with a ca. 100 Ma event in western North America. These data, combined with a U-Pb zircon pluton emplacement age of 103 Ma, show that deformation lagged pluton emplacement by 4 to 7 Ma. This implies that the WISZ shearing event was distinct from Jurassic to Early Cretaceous accretionary tectonics.