TRACKING CRUST-MAGMA INTERACTION ACROSS THE IDAHO BATHOLITH USING ZIRCON U-PB AGES AND TRACE ELEMENT DATA FROM CRETACEOUS BENTONITES

Cretaceous strata preserved in Wyoming contain numerous large bentonite beds 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 magmatism as plutons migrated across the Laurentian margin. Using LA-ICP-MS, we analyzed the U-Pb ages and trace element compositions of nearly 700 zircon grains from 44 bentonite beds from the Bighorn Basin. Previous studies show that during the Middle to Late Cretaceous, plutons migrated eastwards across Idaho into Montana through three unique regions of crust-magma interaction: (1) thin Laurentia and exotic terrane crust during Aptian/Albian time; (2) thick, rigid Idaho crust inducing thick MASH zone stabilization during Cenomanian/Turonian time; and (3) thin Proterozoic-aged sutured crust in western Montana where shallow subduction induced regional broadening of pluton emplacement during Campanian/Mastrichtian time. In bentonite zircon, the Ce anomaly varies widely from equilibrium (Ce/Ce* = 1) to greater than 300, and increases linearly with an inverse garnet signature, Yb/Nd, indicating an increase in oxidation with a retention of heavy REE (less garnet fractionation), with the highest values from western Idaho and Montana. Regional data based on Ce/Ce* vs. Eu/Eu* anomalies further supports correlation between crustal thickness — enhanced magma differentiation — and magma oxidation. Idaho batholith magmatism, produced through MASH-zone processes, is more differentiated, recorded by lower Eu/Eu*, lower Yb/Nd, and is less oxidized, recorded by lower Ce/Ce* compared to regions characterized by complex suture zones. Our zircon data demonstrates a correlation between oxidation state and crustal thickness. This effect may be from the influence of fluids, seen when comparing Nb/Ta against total Hf, both indicators of enhanced aqueous activity. Bentonite zircons from western Idaho and Montana have a lower Nb/Ta signature in addition to higher Hf. The increased Hf, Ce/Ce*, and lower Nb/Ta are correlative with fluid migration and magma oxidation, likely enhanced due to changes in the crustal thickness across the region.