THE TRACE ELEMENT GEOCHEMISTRY OF BENTONITE ZIRCONS RECORDS IGNEOUS PROVENANCE AND MAGMATIC PROCESSES: AN EXAMPLE USING CRETACEOUS BENTONITES FROM THE BIGHORN BASIN, WYOMING

Zircons separated from bentonites from the Appalachian foreland have been dated, adding high precision timelines to the regional stratigraphy, but recent studies using detailed bentonite zircon geochemistry to test tectonic models are few (e.g., Herrmann et al., 2021 Lithos 398-399). In addition to their relevance to tectonics, bentonite zircon geochemistry records igneous processes and conditions. We have dated nearly 700 zircons from 44 Cretaceous bentonite beds in the Bighorn Basin, Wyoming and have also analyzed them for Nd isotopes and trace elements to determine a 40-million-year history of igneous processes, linked to the reconstruction of tectonic processes at the western Laurentian margin. Zircon populations contain magmatic autocrysts and antecrysts which are linked to the main pulses of the Idaho batholith as well as xenocrysts from country rocks and basement source terranes. Initial εHf compositions are used to track changes in magmatic source rocks. Analyzed trace elements, including Ce, Ta, Nb, La, and Yb, record magmatic processes, in finer detail than bentonite whole-rock major element chemistry. Taken together, our age and geochemical data record the migration of Cretaceous magmatism across the western Laurentian margin from accreted terranes, eastwards into thickened Laurentian crust, and then with shallowing of Farallon subduction, pushing farther eastwards into thinner crust. More reducing, mantle-fed MASH magmatism was established in thick crust during late Cenomanian-early Turonian time. During Campanian time, magmatism was more oxidizing magmatism with faster pathways to eruption in thinner crust. The analysis of large numbers of zircons from single bentonite beds reveals details in both magmatic source rocks and their mineralogy, and igneous conditions, and in the Bighorn Basin, significant changes in magmatism through time is recorded by the tremendous succession of bentonite beds and their zircons. Details of tectonic models of the Appalachian orogen could be tested by our approach.