TRACKING 40 MILLION YEARS OF MIGRATING MAGMATISM ACROSS THE IDAHO BATHOLITH USING ZIRCON U-PB AGES AND HF ISOTOPES FROM CRETACEOUS BENTONITES

Cretaceous strata preserved in Wyoming contain numerous large bentonite deposits formed from the felsic ash of volcanic eruptions, mainly derived from Idaho batholith magmatism. These bentonites preserve a near-continuous 40 m.y. chronology of volcanism and their whole-rock and mineral chemistry can be used to document igneous processes and reconstruct the history of Idaho magmatism as emplacement migrated across the Laurentian margin. Using LA-ICP-MS, we analyzed the U-Pb ages and Hf isotopic compositions of nearly 700 zircon grains from 44 bentonite beds from the Bighorn Basin, Wyoming. Zircon populations contain magmatic autocrysts and antecrysts which can be linked to the main pluses of the Idaho batholith, and xenocrysts ranging from approx. 250 Ma to 1.84 Ga from country rocks and basement source terranes. Initial εHf compositions of Phanerozoic zircons are diverse, with compositions ranging from -26 to nearly +12. Based on temporal trends in zircon ages and geochemistry, four distinct periods of plutonic emplacement are recognized during the Mid to Late Cretaceous: (1) Aptian/Albian: magma intruded primarily into the accreted terranes of the Salmon River Suture Zone; (2) Cenomanian/Turonian: magmatism shifted to central Idaho, generating the Northern and Southern Atlanta lobes of the Idaho batholith during an approx. 10 million year phase of stationary magmatism; (3) Campanian: coinciding with a decrease of the Farallon subduction angle, spreading the active magmatic zone across Idaho and into western Montana, and initiating Boulder batholith magmatism and the extrusive Elkhorn sequence; and (4) Maastrichtian: volcanism diminished in Idaho while forming extensive plutonic bodies in western Montana, including continued Boulder batholith magmatism and the related Pioneer and Tobacco Root plutons. Calculated Hf depleted mantle model ages of assimilated basement crust supports this spatial migration of volcanism through time. Our data demonstrate the utility of using zircon in preserved tephra to not only reconstruct the chronology of ancient igneous activity, but to track the regional-scale evolution of convergent margins related to terrane accretion and the spatial migration of magmatism related to changes in subduction dynamics.