PETROGENESIS OF CENOZOIC BASALTS IN THE CENTENNIAL VALLEY AND VICINITY IN SW MONTANA: OUT-OF-SEQUENCE SNAKE RIVER PLAIN BASALT ERUPTIONS NORTH OF THE EASTERN SNAKE RIVER PLAIN?

Recent studies have identified <10 Ma mafic to intermediate magmatism, south of the Snake River Plain - Yellowstone (SRPY) volcanic province in Wyoming and Idaho, that predates voluminous felsic Snake River Plain eruptions. These mafic to intermediate rocks were interpreted as "hotspot out-of-sequence magmatism" because they lie within the Yellowstone hotspot seismic parabola but are out-of-sequence temporally (and geographically off the hotspot track) from the SRPY felsic eruptions. The Centennial Valley/Range study area is a potential northern analogue to these previously studied out-of-sequence igneous rocks, given its location north of the Snake River Plain. Miocene eruptive products in the area include <7 Ma unnamed basaltic lavas. Ongoing fieldwork has led to the discovery that at least one of these younger basalts erupted locally. Twenty-seven samples have been collected from the Centennial Range and Valley and from two areas mapped as Timber Hill basalt (one near Lima, MT and one at the ~6 Ma Timber Hill type section). Near vent facies shows that the Timber Hill basalt sample collected near Lima was locally erupted and not sourced from the SRPY. Eight samples thus far have been analyzed for whole-rock major and trace element geochemistry and compared to the four major compositional types of basalts found on the Snake River. The Timber Hill basalt from its type section most closely resembles the Craters of the Moon-style Snake River basalt, with high K₂O, FeO*, and Pb concentrations. This is consistent with a southern eruptive source (e.g., Heise volcanic field region). The other seven samples resemble the predominant type of SRPY basalt, Snake River olivine tholeiite, although they are slightly more potassic, with K₂O/TiO₂ ratios ranging from 0.34-0.54. When normalized to primitive mantle, all eight samples show similar depletions and enrichments to Snake River Plain olivine tholeiite. Characterizing these basalts allows for a better understanding of crustal interaction and/or mantle heterogeneity in the SRPY region, which is crucial to understanding the link between voluminous silicic caldera-forming eruptions and the basaltic magmas that drive them, and helps constrain the areal "footprint" of melting associated with the SRPY hotspot.