NEW INSIGHTS INTO ADEL HILLS SHONKINITE FORMATION THROUGH MINERALOGY AND PETROLOGY

Shonkinites are a rare type of alkali-rich igneous rock found throughout the geological record. They occur across different tectonic settings, are often associated with light rare earth element (LREE) enrichments, and provide an opportunity to investigate alkali magma petrogenesis. This study focuses on the Upper Cretaceous shonkinites found in the Adel Hills Volcanic Field (AHVF) of central Montana and seeks to understand shonkinite petrogenesis within the context of the tectonomagmatic evolution of the Cordilleran fold and thrust belt. AHVF shonkinites are porphyritic and characterized by 25-30% clinopyroxene up to ~1 cm in maximum length set in a fine-grained, altered groundmass. Minor and trace minerals include plagioclase, K-feldspar, magnetite, apatite, zeolite and rarer amphibole. XRD analysis of bulk powders is consistent with mineralogy observed via PLM and thin sections subsequently studied via SEM-EDS. EPMA data collected on clinopyroxenes is consistent with diopside (n=100, Es_35-42, Fs_11-17, Wo_44-49) and confirms the presence of plagioclase (oligoclase, labradorite, bytownite, anorthite) and K-spar. Bulk powder wt. % SiO2 ranges between 47-49 wt. % with corresponding wt.% Na₂O + K₂O between 5.50-7.34. Bulk trace element analysis reveals a prominent Nb-Ta depletion accompanied by large-ion lithophile element (LILE) enrichment (e.g., Ba, Th), consistent with petrogenesis in an arc-related setting. Preliminary interpretations based primarily on the oscillatory zoning within the clinopyroxenes and the varied feldspar compositions infer the role of magma mixing during shonkinite petrogenesis. Future work will involve in situ trace element study of major and minor phases via LA-ICP-MS in addition to potential U-Pb dating of apatite to place alkali magmatism within the context of fold and thrust belt evolution.