Tracking 40 Million Years of Migrating Magmatism across the Idaho Batholith Using Zircon U-Pb Ages and Hf Isotopes from Cretaceous Bentonites

Abstract

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 has been 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 pulses 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 that follow plutonic emplacement across the Laurentian suture zone in western Idaho and into western Montana with the onset of Farallon slab shallowing. Our data demonstrate the utility of using zircons in preserved tephra 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.

Combined Zircon/Apatite U-Pb and Fission-Track Dating by LA-ICP-MS and Its Geological Applications: An Example from the Egyptian Younger Granites

“…The U-Pb system is extremely resistant to thermal overprinting (>900 • C; [22]) and is, therefore, unaffected by thermal events when the most suitable mineral, zircon, is analyzed. These advantages and improvements in the technology of LA-ICP-MS made it a rapid, precise and accurate method for U-Pb geochronology which has recently been extended [23][24][25][26][27][28][29][30][31][32]. The combination of U-Pb and FT methods could provide insights on history of the provenance that has undergone metamorphism.…”

Timing of the Transition From Sevier‐ to Laramide‐Style Tectonism in Southwestern Montana Based on the Provenance of the Frontier Formation, North American Cordillera

“…This potential high‐flux magmatic episode of the Idaho batholith is based on inheritance of 100–85 Ma ages in the 83–67 Ma Atlanta peraluminous suite of the Idaho batholith (Gaschnig, Vervoort, et al., 2017), the documentation of voluminous magmatism and high‐grade metamorphism in central Idaho (Ma et al., 2021), and DZ age spectra from marine strata in the Bighorn Basin, Wyoming (May et al., 2013); the age of this proposed magmatic flux is strikingly similar to the apparent magmatic flux curve for the Sierra Nevada batholith (Ducea, 2001). More recently, work primarily in the Bighorn Basin, which focused on U‐Pb zircon geochronology of marine bentonite beds, has also documented magmatism between 100 and 85 Ma (Hannon et al., 2020, 2021). These recent studies are consistent with—and refine—previous interpretations based on DZ age spectra (May et al., 2013) because the primary source for zircon grains in bentonites of the Bighorn Basin was interpreted as ash‐fall derived from the Idaho batholith based on age and trace element geochemistry (Hannon et al., 2020, 2021).…”

“…More recently, work primarily in the Bighorn Basin, which focused on U‐Pb zircon geochronology of marine bentonite beds, has also documented magmatism between 100 and 85 Ma (Hannon et al., 2020, 2021). These recent studies are consistent with—and refine—previous interpretations based on DZ age spectra (May et al., 2013) because the primary source for zircon grains in bentonites of the Bighorn Basin was interpreted as ash‐fall derived from the Idaho batholith based on age and trace element geochemistry (Hannon et al., 2020, 2021).…”

“…The ∼15 m.y. spread of ages within the <100 Ma fraction may be explained by the presence of xenocrystic zircons (Gaschnig, Vervoort, et al., 2017; Hannon et al., 2021), or by incorporation of older portions of the volcanic stratigraphy during explosive eruptions (Schwartz et al., 2021). We interpret that during eruption, the older volcanic cover would have been incorporated into the eruptive material and settled as ash‐fall in the nonmarine Idaho‐Montana sector of the foreland basin system.…”