Master of Science, Miami University, 2025, Geology and Environmental Earth Science

Shonkinites are rare alkali-rich igneous rocks found in various tectonic settings. This study investigates the Upper Cretaceous shonkinites from the Adel Hills Volcanic Field (AHVF) in central Montana. The AHVF shonkinites exhibit porphyritic textures, featuring large pyroxenes with a mineralogical composition defined by clinopyroxene (diopside), plagioclase, K-feldspar, and secondary zeolite. SEM-EDS aided in identifying magnetite, rare ilmenite, rare pyrite, and calcite. Bulk rock wt. % SiO2 ranges from 47 to 49 at wt. % Na2O + K2O from 5.50 and 7.34 wt. %. Normalized bulk trace elements indicate LILE enrichment and Nb-Ta depletion, consistent with a volcanic arc setting. Bulk rock and mineral chondrite-normalized REE signatures lack an Eu-anomaly. For diopside, Si- and Mg-enrichment is accompanied by Al, Ti, Na, and Cr-depletion on {-111}. This is interpreted as resulting from low degrees of undercooling during crystallization (ΔT = 13-25 degrees C). Apatite Ce/Ce* vs. Eu/Eu* systematics are consistent with a hydrous, oxidizing magmatic environment in which plagioclase crystallization was suppressed. The AHVF shonkinites are interpreted to have formed in a contractional environment where a metasomatized arc mantle wedge underwent low-degree partial melting. Future research should employ bulk rock and in situ isotopic analysis to assess contributions from crustal and mantle sources.

Committee: Claire McLeod (Advisor) Subjects: Geology; Petrology