11-2 Investigating the Tectonomagmatic History of the Eocene Absaroka Supergroup, Wyoming, through a Petrological Investigation of Intrusives and their Entrained Xenoliths.

Session: Applications of Igneous Petrology, Geochemistry, and Geochronology to Understanding Earth and Other Planetary Systems (Posters)

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Abstract:

Earth’s tectonomagmatic record provides key insights into crustal evolution and differentiation, cycles of continental collision and break-up, and the composition of the lithosphere and asthenosphere. This is particularly true in the western United States, where multiple episodes of orogenesis and related collision, subduction, volcanic activity, and extension have occurred. In NW Wyoming and southern Montana, the Eocene-aged Absaroka Range records one of the more recent cycles of this tectonomagmatic activity, associated with rejuvenated magmatism and extensional collapse. Here, the Absaroka Volcanic Supergroup (AVS, 53-43 Ma) is characterized by volcaniclastics and andesitic lava flows and variably cross-cut by intermediate to evolved intrusives. This study focuses on investigating the textural, mineralogical, and geochemical characteristics of xenoliths hosted by the intrusive rocks (dikes) of the Two Ocean and Langford Formations, within the Trout Peak Trachyandesite of the AVS Sunlight Group. Collectively, these samples provide a unique opportunity to advance our understanding of magmatism following the cessation of contractional deformation. Samples were collected in summer 2022, in eastern Yellowstone National Park, in accordance with sampling permit YELL-2022-SCI-8238. From study of samples in thin section via microscopy (PLM and SEM-EDS), the host dike is porphyritic and characterized by ~40–50% euhedral to subhedral plagioclase with visible oscillatory zoning, and ~10–20% euhedral to subhedral amphibole in a microcrystalline, feldspathic groundmass with minor magnetite and ilmenite. The entrained xenoliths are holocrystalline and primarily characterized by ~40–80% subhedral to anhedral amphibole, ~10–30% subhedral to anhedral feldspar, and quartz, with minor apatite, biotite, ilmenite, and magnetite. From bulk powder XRD analysis of the host dike, major minerals are confirmed as amphibole (hornblende), plagioclase (andesine to labradorite), and quartz, with minor magnetite. The bulk composition of the host dike is andesitic/dioritic (wt. % SiO2 from 62.14 to 62.84 at wt. % Na₂O + K₂O from 6.49 to 6.56) and calc-alkaline based on the MgO-FeO-Na₂O+ K₂O ternary systematics. Bulk rock primitive mantle-normalized spidergrams exhibit a subduction-related signature as evidenced by LILE enrichment accompanied by Nb-Ta depletion with chondrite-normalized profiles exhibiting LREE enrichment (La_N/Lu_N: 34.74–39.10). Future work will focus on in-situ analysis of amphibole within the host dike and throughout the xenolith suite via EPMA to evaluate P-T conditions during this stage of the Absaroka Volcanic Supergroups’ tectonomagmatic history.

Geological Society of America Abstracts with Programs. Vol. 58, No. 1, 2026

doi: 10.1130/abs/2026SE-12675

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