Late Oligocene OIB-like lavas in northern Idaho

The 26.3 to 25.3 Ma Potlatch volcanics in northern Idaho (Kuffman et al., 2006) consist of a suite of basalts, hawaiites, mugearites, benmoreites, trachytes and nepheline trachytes. The volcanic field was erupted on North American cratonic basement well to the northeast of the regional crustal suture with Phanerozoic terranes accreted during the Mesozoic, and predates Columbia River flood basalt activity in the area by 9 million years. The most primitive Potlatch lavas are porphyritic olivine basalts with 6 percent MgO and strongly OIB-like chemical affinities (La/Nb = 0.69 - 0.76, Th/Ta = 0.92 to 1.08, Pb/Ce = 0.029 to 0.033, 87Sr/86Sr = 0.70367 to 0.70476, 206Pb/204Pb = 19.254 to 19.504). Similarly, intermediate and felsic lavas and pyroclastics closely resemble differentiated members of typical sodic ocean island suites, but have additionally been affected by AFC involving small amounts of regional continental crust, which has acted to increase 87Sr/86Sr up to 0.70516. The Potlatch volcanics are geochemically unlike other regional Cenozoic volcanic suites including Eocene Challis rocks, basalts and rhyolites of the John Day Formation and other volcanic fields around the Blue Mountains to the south and southwest, and the later Columbia River basalts. Their occurrence represents a modification to the southward retreat pattern of early to mid-Cenozoic magmatism in northwestern North America. Kauffman, Bush, and Lewis (2006) ID Geol. Surv. Tech. Rep. 06-7, 11 pp.