PETROLOGY OF THE IGNEOUS ROCKS OF THE CENTENNIAL & HENRY'S MOUNTAINS, IDAHO, U.S.A

The Centennial and Henry’s Mountains sit near the Continental Divide on the Idaho-Montana border and contain ~40 km³ of Eocene shoshonite aa lava flows (max. thickness >= 350 m) that overlie local Paleozoic strata and are cut by feeder dikes. These rocks are exposed in three discontinuous areas each roughly 5 km (E-W) wide in a 10 x 30 km region. The youngest flow from the westernmost area has a ⁴⁰Ar/³⁹Ar age of 49.94 ± 0.038 Ma. Because these rocks are located between the Absaroka (to the east) and Challis (to the west) volcanic fields their petrogenesis may yield useful insights into the Eocene petrotectonic history of western North America. The volcanic features, petrography, and composition of the Sawtell Peak shoshonites are remarkably uniform. The aa flows are 1-10 m thick and are composed of a thin lower rubble zone, a dense interior, and a thick upper rubble zone. Where slopes are steep the cross section of individual flow lobes are exposed. No pyroclastic rocks or lahars were observed. In a very few locations the flows are separated by small, thin (<3 m) sedimentary deposits. The lava flows are cut by linear, 1-2 m thick feeder dikes. Some dikes have chilled margins and others are surrounded by zones of hydrothermal alteration. Surface exposures of dikes are 10-100 m long. The flows are sometimes vesicular or amygdaloidal, are crystal rich, and contain euhedral cpx phenocrysts, commonly-altered ol phenocrysts, and microlites of plag and Fe-Ti oxides. Modal proportions are: 61-69% matrix, 22-34% cpx, and 5-12% ol. Two units (one flow and one dike) were observed to contain biotite. The rocks are uniformly shoshonitic (K₂O 2-5%), silica-oversaturated, magnesian, and calc-alkalic. Major (in wt. %) and trace element (in ppm) variations are relatively small, e.g., SiO₂ varies from 51.6-54.5, MgO from 5–9, TiO₂ from 0.7–0.8, Cr from 274-417, Ni from 54–106, Rb from 54–199, Sr from 412-799, Ba from 1389–1774, and Zr from 72-129. Major and trace element compositions suggest that these rocks erupted during (at least) three eruptive episodes; are compositionally similar to the mafic rocks of the Absaroka (rather than the Challis) volcanic field; and include a significant subduction zone compositional component—likely from the subduction zone that existed beneath western North America during the Eocene.