METALLOGENIC CONSTRAINTS ON NORTH AMERICAN CENTRAL CORDILLERA EOCENE PALEO-TOMOGRAPHY: THE MAGMATIC AFFINITY OF THE ABSAROKA AND CENTRAL IDAHO CHALLIS VOLCANIC PROVINCES

Igneous rock geochemical-isotope data, coupled with geophysical constraints, are often used to reconstruct paleo-tomography. However, these datasets can be at odds, especially further back in time and in areas affected by post-volcanic reshuffling via strike-slip faults, like the Cordilleran margin, making slab reconstructions ambiguous. We use geochemical data to calculate bulk rock Cu-fertility geochemical indices and integrate those results with geological data. We show that a combination of differences in porphyry ore systems (e.g., Cu- vs. Mo-dominated), combined with volcano types and magma Cu-fertility from broadly coeval igneous provinces, can be integrated to better decipher magmatic affinity. A major part of the central North American Cordillera contains a discontinuous north-northwestern aligned zone of Eocene volcanic fields (e.g., the Challis-Kamloops volcanic belt), spanning >2000 km from Idaho and Wyoming (USA) to British Columbia, Canada. The focus of this study is the southern end of the belt, the Absaroka and central Idaho Challis volcanic provinces. Comparative Sr/Y versus wt% SiO₂ and (Eu/Eu*)/Yb data (e.g., mean Sr/Y = 77.1), signifying Cu-fertile magma chemistries, coupled with Cu porphyry deposits associated with an ~230-km-long belt of stratovolcanoes, supports an arc origin for the Absaroka province. The central Idaho Challis province has chemical indices not conducive to Cu porphyry mineralization (e.g., Cu-infertile chemistries; mean Sr/Y = 24.0), which, coupled with the presence of Mo-porphyry mineralization, caldera and fissural vents, and a lack of stratovolcanoes, supports an extensional tectonomagmatic origin. We integrate these datasets and create an updated tectonic model for the Absaroka and central Idaho Challis provinces that involves shallow slab subduction and interaction with the Wyoming Craton, slab tearing, and eventual foundering of the Farallon plate between 55 – 45 Ma. The combination of regional differences in porphyry systems, magma chemistry as a proxy for Cu porphyry deposits, and geological characteristics of volcanic fields, can be used to identify volcanic arcs that form via subduction (e.g., Cu-fertile magmas with Cu porphyry deposits) in other locations and can clarify the tectonism in specific locales (e.g., subduction vs. extension).