242-6 Insights into Arc-Related Magmatism During Petrogenesis of the Eocene Absaroka Supergroup, WY

Abstract:

Investigating the evolution of the lithosphere during arc-related magmatism contributes to our understanding of the Earth’s tectonomagmatic history. The Eocene-aged Absaroka Volcanic Supergroup (AVS, 53-43 Ma) located in NW Wyoming and southern Montana records a relatively recent cycle of tectonomagmatic activity associated with rejuvenated magmatism due to slab roll back, and subsequent extensional collapse. In the central region of the AVS, in eastern Yellowstone National Park, volcaniclastics and andesitic lava flows of the Trout Peak Trachyandesite are cross-cut by the intermediate intrusives of the Two Ocean and Langford Formation. In this study, the textural, mineralogical, and geochemical characteristics of these intrusives, along with their entrained xenolith populations, are investigated within the context of the AVS and evolution of the North American continental margin at the time. 

Sample collection was completed during the summer of 2022 in eastern Yellowstone National Park. From the study of thin sections via PLM and SEM (including BSE imaging and EDS mapping), host intrusives are porphyritic, 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. Entrained xenoliths are holocrystalline, 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 on the intrusives, major minerals are confirmed as amphibole (hornblende), plagioclase (andesine to labradorite), and quartz, with minor magnetite. Host intrusives are dioritic in composition (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 and 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 the amphibole populations throughout the sampled suite. Quantification of major and minor element compositions via EPMA will 1) determine amphibole type and 2) be applied to geothermobarometers to evaluate P-T-H₂O conditions during this stage of AVS magmatism. This dataset will be coupled with in-situ minor and trace element analysis via LA-ICP-MS to further investigate amphibole populations, their petrogenesis, and lithospheric evolution during the Eocene.

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