MIGRATING MAGMATISM ACROSS IDAHO: USING CRETACEOUS BENTONITES TO TRACE THE SHALLOWING OF FARALLON SUBDUCTION

During the Late Cretaceous, extensive magmatism occurred as a result of the subduction and protracted shallowing of the Farallon plate. Volcanism associated with this plutonic emplacement deposited ash across the Western Interior Seaway. These ash beds, or bentonites, are well preserved in the Bighorn Basin, preserving the geochemical signatures originating from the source and may provide a detailed record of geochemical changes and volcanic migration associated with the shallowing of the subduction angle. In this study, 87 bentonites were sampled and geochemically analyzed, spanning the entire Late Cretaceous. Using a combination of geochemical concentrations, ⁸⁷Sr composition, and ratio(s) of rare earth concentrations (e.g. Eu*, Zr/TiO2, La/Yb), we capture the geochemical evolution of volcanic material, likely tracking the history of plutonic emplacement in Idaho and Montana.

The radiogenic chemistry and geochemical signatures reveal 2 distinct phases (P1 and P2) of magmatic migration. These trends appear to indicate a transition in provenance through the Late Cretaceous, driven by the inboard migration of volcanism associated with shallowing subduction of the Farallon plate. Our data indicates that magmatism stalls in two localities—once, in central Idaho (P1), where the volcanic front migrated across the 0.706 isopleth (rapid transition of ⁸⁷Sr from 0.704 – 0.708) and the thick lithosphere results in a stabilized MASH zone differentiation with enhanced plagioclase fractionation (strongly negative Eu*). By the Campanian, the volcanic front stalled a second time in western Montana (P2), where the forced magmatic migration may have caused a reset of differentiation (weakly negative Eu*) and weakened assimilation (low ⁸⁷Sr), coinciding with the general composition of the Elkhorn Volcanic complex. As migration terminated, the Boulder batholith and surrounding plutonic complexes were buffered by high rates of assimilation (high ⁸⁷Sr), while continuing to produce less differentiated material. Additionally, the ratio of La/Yb, a REE signature that is highly sensitive to depth of partial melt generation, transitions from a high ratio in the Cenomanian (as high as 70), to a distinct decrease through P2 (10-20), providing additional evidence of magma influenced by the shallowing subduction zone.