Determining the Initiation of Shear Zone Deformation Using Titanite Petrochronology

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We present an integrative petrochronological approach to dating the initiation of shear zone deformation in granitic rocks, using the mineral titanite (CaTiSiO5). This method is suited to granitoid-hosted shear zones in continental arc settings, where the interplay between tectonics, magmatism, and deformation is actively debated. Microstructural observations including crystallographic misorientations, fabric context, and backscattered electron images were used to identify relict magmatic, partially to fully recrystallized, and neoblastic titanite crystals. Principal component analysis of trace element compositional variance in titanite was used to further distinguish and quantify the crystal chemical response to deformation. High-precision isotope dilution U-Pb geochronological measurements on relict magmatic, partially to fully recrystallized, and neoblastic titanite crystals was used to constrain the timing of shear zone initiation. For a sample of a porphyritic orthogneiss from the western Idaho shear zone of the northern U.S. Cordillera, U-Pb zircon geochronology dates emplacement of this unit to between ca. 105 and 103 Ma, whereas the age of partially recrystallized and neoblastic titanite grains indicate that the western Idaho shear zone initiated between ca. 98 and 96 Ma. The >5 Ma lag between pluton emplacement and the onset of deformation indicates that mid-Cretaceous deformation in the western Idaho shear zone is temporally distinct from Late Jurassic–Early Cretaceous accretionary tectonics. Our integrated petrochronological investigation documents that: 1) deformation induces textural, chemical, and isotopic changes in titanite; 2) the geochemical properties of primary magmatic titanite collapse into a discernible trend of subsolidus syn-deformational titanite compositions; and 3) the onset of deformation and progression of strain accumulation are preserved in the U-Pb ages of these titanite crystals.