CENOZOIC BASALTIC VOLCANISM IN THE CENTENNIAL VALLEY AND VICINITY: IMPLICATIONS FOR THE AREAL EXTENT OF SNAKE RIVER OLIVINE THOLEIITES ASSOCIATED WITH THE YELLOWSTONE HOTSPOT

The Centennial Valley and surrounding areas in southwestern Montana, located north of the eastern Snake River Plain (ESRP), host small volume basalts that range in age between 6.75 and 2 Ma. The area is notable for the Centennial fault, an E-W trending normal fault that dips away from the ESRP and is analogous to the Teton fault. Displacement along the Centennial fault began ~2.1 Ma and is associated with the passage of the North American plate over the Yellowstone hotspot. We present here new field, temporal, geochemical, and Sr-Nd-Pb isotope data from basalts in the Centennial Valley and link them to “typical” basalts that characterize the Snake River Plain. Centennial basalts can be classified into two chemotypes: Type 1, which best resembles Snake River Olivine Tholeiites (SROT), and type 2, a more alkaline basalt that also resembles some ESRP basalts. Type 1 basalts are more depleted in Rb, Sr, and Ba and have higher values of MgO, FeO*, and V compared to type 2. Nd, Sr, and Pb isotope data further corroborates the similarity of type 1 to SROT (i.e. slightly negative eNd). Type 2 basalts are more K-rich (1.50 – 3.07 wt. %) and have isotope ratios consistent with crustal contamination. New Ar-Ar ages fill in gaps of previous K-Ar dating of these basalts and range from 6.20 – 6.75 Ma, most of which span the time between the emplacement of the Blacktail Creek and Walcott tuffs that erupted from the Heise volcanic field. Though some of the outcrops of these basalts are inferred to have originated from the Heise volcanic field and flowed north through paleovalleys, the presence of scoriaceous lapilli and bombs (e.g., near vent facies) provides evidence for local eruptions of some the basalts. Our results suggest that the areal footprint of melting associated with the Yellowstone hotspot is larger than previously understood and extends north of the ESRP into Montana and the Centennial valley. These eruptions are likely analogous to similar off-hotspot track basaltic volcanism, south of the ESRP (e.g., Blackfoot volcanic field, ID).