TRACKING SPATIAL AND TEMPORAL CHANGES IN MAGMATIC PROCESSES USING BENTONITES: THE CASE OF 40 MILLION YEARS OF MIGRATING MAGMATISM ACROSS THE IDAHO BATHOLITH THROUGH THICK AND THIN

Cretaceous strata preserved in Wyoming contain numerous thick bentonite beds formed from felsic ash, mainly derived from Idaho batholith magmatism. In a series of studies focused on bentonite geochemistry, bentonite zircon geochemistry and U-Pb geochronology, we have documented the wealth of information these particular beds contain for petrologic research. We used a multidisciplinary approach on clays and associated zircon -- including clay speciation via XRD, bulk Sr/Nd isotopic measurements, bulk geochemistry via XRF and ICP-OES, and on 697 grains from 44 bentonite beds, zircon Lu-Hf and trace element data, and U-Pb ages – to determine a 40 million year history of igneous processes, linked to the reconstruction of tectonic processes at the western Laurentian margin. In many our our zircon populations, xeno-, ante-, and autocrysts can be recognized. This history can be subdivided into three distinct phases of crust-magma interaction: (1) volcanism through thin Laurentia and exotic terrane crust during Aptian/Albian time; (2) development of MASH zone plutonism in thick, rigid Idaho crust during Cenomanian/Turonian time; and (3) forced magma migration into thin, Proterozoic-aged, sutured crust in western Montana where shallow subduction induced regional broadening of pluton emplacement during Campanian/Maastrichtian time. In addition, Ce, Ti, and U in zircon used to calculate f_O2 (Loucks et al., 2020) and other trace elements in zircon used as proxies for T and depth of magma evolution. In western Idaho and Montana, magmas had more direct conduits to the surface whereas subsurface processing via MASH occurred in central Idaho. Subduction-related magmas there assimilated anhydrous crust lowering T and f_O2 and increasing Eu fractionation. We encourage researchers to consider bentonites as a source of key plutonic data in regions where primary plutonic rocks have been lost or altered through geologic time (via metamorphism, replacement, and erosion). Our data demonstrate the utility of using both bulk bentonite clay and contained zircon 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.