Challis volcanics comprise Eocene age mafic, intermediate, and silicic lavas and ash flow tuffs erupted from fissure vents and calderas in central and southwestern Idaho. We have analyzed a suite of mafic to intermediate composition lava flows from central Idaho, and ash flow tuffs and dikes from southwestern Idaho for major elements, trace elements, and Pb isotopic compositions. These data provide constraints on the petrogenesis of the Challis volcanics and on the tectonic setting of the northwestern United States during the Eocene. The mafic to intermediate composition lavas have low Al2O3 and high MgO contents, K2O generally≥Na2O, and phenocryst assemblages suggesting an absarokite-shoshonite affinity. Compatible element (e.g., Mg. Ni, and Cr) abundances in these lavas are comparable to those in oceanic basalts, consistent with a mantle source.

The high concentrations of some incompatible trace elements (e.g., Rb, Th, U, and Pb) and the values of key trace element ratios (Nb/U, Ce/Pb, and Sr/Nd) in these lavas, however, are more consistent with a crustal component. Pb isotopic compositions of these lavas (206Pb/204Pb=17.9--18.5, 207Pb/204Pb =15.5--15.6, and 208Pb/204Pb=38.4--39.0) are independent of fractionation indicators, suggesting the magmas escaped significant assimilation during crystallization. Therefore the mixture of mantle and crystal components must have occurred prior to the crystallization history recorded by these lavas, either in the source or during the early stages of magma generation. Although Challis volcanism is obviously related to an evolving convergent mergin setting, the mafic to intermediate Challis lavas do not appear to be direct products of a subduction-related magma system. We have found no evidence for an asthenospheric mantle contribution to even the most mafic Challis basalts; rather, the trace element and Pb isotopic data suggest a more direct connection with the continental lithosphere. At this time, we favor a model for the generation of the Challis volcanics by melting entirely within the lithospheric mantle and lower crust, possibly in response to the tectonic thickening and subsequent extension experienced by the region following late Mesozoic and early Cenozoic compressional deformation. ¿ American Geophysical Union 1991