Insights into the petrogenesis of alkalic, shonkinitic magmas from the Adel Hill Volcanic Field, Montana

Shonkinites are rare alkali-rich igneous rocks found in the geological record from the Precambrian to the Eocene. This study investigates the Upper Cretaceous shonkinites from the Adel Hills Volcanic Field (AHVF), central Montana. The AHVF shonkinites are porphyritic with large, euhedral to subhedral phenocrysts of diopside that exhibit sector zoning. Other major mineral phases include plagioclase, sanidine, and secondary zeolites. Minor and accessory phases identified with SEM-EDS include magnetite, apatite, rare ilmenite and pyrite, and secondary calcite. Bulk rock SiO 2 ranges from 47 to 49 wt% with Na 2 O + K 2 O varying from 5.50 to 7.34 wt% within that silica range. Normalized bulk trace elements indicate LILE enrichment and Nb Ta depletion, consistent with a volcanic arc setting. Bulk rock, diopside, and apatite chondrite-normalized REE signatures collectively exhibit a limited range of Eu anomalies (Eu/Eu* from 0.92 to 1.33). High-resolution elemental mapping of sector-zoned diopsides reveals that Si and Mg-enrichment is accompanied by Al, Ti, Na, and Cr-depletion within {−111} faces. In symmetrically equivalent prism sectors, Al, Ti, Na, and Cr enrichment with correlated depletions in Si and Mg is documented. This is interpreted to result from low degrees of undercooling during crystallization (ΔT = 13-25 °C). The lack of dendritic patterns at the Al sector boundaries further supports this. Apatite trace element chemistries are consistent with crystallization from mafic, alkali-rich melts while accompanying Ce/Ce* vs. Eu/Eu* systematics are consistent with formation in a hydrous, moderately oxidizing magmatic environment in which plagioclase crystallization was suppressed. Petrogenesis of the AHVF shonkinites is inferred to have occurred in a contractional tectonomagmatic setting in which variably metasomatized, heterogeneous mantle lithosphere experienced low degrees of partial melting. This occurred prior to the collapse of the Cordilleran fold and thrust belt and subsequent lithospheric extension in the middle Eocene-early Miocene. • The bulk geochemical signatures of clinopyroxene-phyric dikes are consistent with subduction-zone processes. • Diopsides exhibit sector and oscillatory zoning with Si, Mg-enrichment on {-111} and depletion on prism sectors. • Apatite chemistry indicates crystallization from a mafic melt from a hydrous, moderately oxidizing magmatic environment. • Globally, shonkinites are linked to metasomatized mantle lithosphere, variable partial melting, and convergent tectonics.