INCOMPATIBLE TRACE ELEMENT CONSTRAINTS ON MANTLE SOURCE CHANGES OF EOCENE-OLIGOCENE BASALTS FROM THE DILLON VOLCANIC FIELD, SOUTHWEST MONTANA (USA)

The northern Rockies/Pacific Northwest (USA) was affected by Eocene Farallon plate subduction, partially resulting in ~54-44 Ma Challis-Absaroka (CA) magmatism, followed by collision of Siletzia. Coeval and adjacent to CA eruptions was volcanism in the Dillon volcanic field (DVF), Montana. After a pause, Dillon magmatism resumed from ~33-16 Ma and was spatially coincident with local uplift and extensional basin formation. This may have occurred due to lithospheric drip or extension resulting from Farallon slab steepening and/or slab window growth. Middle (33-27 Ma) and upper (27-16 Ma) DVF mafic-intermediate rocks are the focus of this study, with the goal of better understanding the tectonic driver(s) of magmatism. The DVF erupted mafic-intermediate magmas including <33 Ma alkali basalts in the Gravelly range (MT). The alkali basalts resemble asthenosphere-derived alkali basalts from the modern Basin and Range province (NV). All DVF rocks are strongly potassic. Lower DVF rocks range from basalt to rhyolite, with alkaline to subalkaline affinities; they are enriched in light earth elements (LREE) and large-ion lithophile elements (LILE) and depleted in high-field strength elements (HFSE). Middle and upper DVF rocks show slightly less positive LILE enrichments and less negative HFSE depletions than the lower DVF. DVF basalt trace element ratios clarify mantle sources. Lower DVF La/Nb = ~1.65 to 2.04 while Nb/Zr = ~0.06 to ~1.6, which are MORB-like. Middle DVF La/Nb = ~0.60 to ~1.16 with Nb/Zr = ~0.07 to ~0.26, which overlap with MORB and OIB values. Lower Dillon basalt Ta/Th = ~0.17 to ~0.24, while middle DVF basalt Ta/Th = ~0.15 to 1 (e.g., Ta/Th < ~0.2 associated with lithospheric mantle; Ta/Th > ~0.6 suggest asthenosphere sources). The lower DVF trace element ratios suggest a lithospheric mantle source while the middle DVF has intermediate Ta/Th, consistent with their more OIB-like La/Nb and Nb/Zr values. We interpret these trace element variations to reflect a change from a lithospheric mantle source (lower DVF) to melt production dominated by asthenosphere upwelling and melting (e.g., OIB-like Ta/Th ratios in middle DVF basalts). These differences are most consistent with a tectonomagmatic change due to slab rollback and/or extension, from ~54-27 Ma. Detailed work on upper DVF basalts is in progress and will help clarify the initial interpretations presented here.