SR, ND, AND PB ISOTOPE CONSTRAINTS ON THE PETROGENESIS OF MT. ADAMS, A CASCADE ARC VOLCANO

Extensive Quaternary lava flows in the Mt. Adams region of the Cascade arc allow for a detailed isotopic investigation of the petrogenesis of a young continental arc volcanic field constructed on young crustal material accreted to the western edge of the North American plate.  We present Sr, Nd, and Pb isotope compositions for 12 basalts, 14 andesites, and 4 dacites from the Mt. Adams stratovolcano and its basaltic periphery in southern Washington, USA.  Mt. Adams lavas have ⁸⁷Sr/⁸⁶Sr ratios that are slightly higher (0.7029 to 0.7039), e_Nd values that are lower (+5.3 to +7.4), and ²⁰⁶Pb/²⁰⁴Pb ratios that are higher (18.85 to 19.00; ²⁰⁷Pb/²⁰⁴Pb = 15.55 to 15.60; ²⁰⁸Pb/²⁰⁴Pb = 38.40 to 38.71).  Mt. Adams Sr and Nd isotope compositions lie on the high side of the range defined by Cascade arc rock, and isotope compositions also lie on a mixing line between Juan de Fuca-MORB and pre-Tertiary rocks (Borg and Clynne, 1998), but generally lie closer to MORB values, suggesting that interaction with pre-Tertiary rocks is minimal.  Mt. Adams Pb isotope compositions lie on a mixing line between Pacific MORB and NE Pacific sediments.  The data plot on or just above the NHRL in terms of ²⁰⁸Pb, and have generally lower ²⁰⁷Pb/²⁰⁴Pb ratios for a given ²⁰⁶Pb/²⁰⁴Pb ratio compared to other Cascade arc rocks.  Isotopic compositions of the volcanic rocks do not correlate with degree of evolution (SiO₂ wt.%), and basaltic rocks define the upper and lower limits in the ranges of isotopic values.  These variations in Sr, Nd, and Pb isotope compositions suggest that volcanic rocks at Mt. Adams derive their isotopic compositions primarily from the mantle and from deep-crustal mafic material young enough not to have developed measurable changes in Sr, Nd, and Pb isotope compositions, a conclusion supported by previous Os isotope analyses.