Trace element and isotopic evidence for two types of crustal melting beneath a high cascade volcanic center, Mt. Jefferson, Oregon

Mt. Jefferson is an andesite-dacite composite volcano in the Cascade Range, the locus of andesite and dacite-dominated volcanism for at least 1 million years. A large trace element data set for Mt. Jefferson and its surrounding mafic volcanic platform effectively rules out any fractionation based model (FC or AFC) for the generation of Mt. Jefferson andesites. Several incompatible element (Zr, Nb, Y) concentrations decrease in the range from basalt to andesite, and then increase in the range from andesite to rhyodacite. Others (Ba, Rb, La, Th) remain constant or show a slight increase in the basalt to andesite range, with modest increases from andesite to rhyodacite. Systematic variations in highly incompatible element ratios such as Ba/La and Rb/Th suggest magma mixing dominates the trace element signatures. Rhyodacites are isotopically uniform (⁸⁷Sr/⁸⁶Sr = 0.70325-0.70343;²⁰⁶Pb/²⁰⁴Pb = 18.75-18.85; ∂¹⁸O = 6.3 ± 0.3), whereas andesite and dacite are more variable (⁸⁷Sr/⁸⁶Sr = 0.70291-0.70353;²⁰⁶Pb/²⁰⁴Pb = 18.59-18.86;∂¹⁸O = 6.0±0.6). Typical basaltic andesite has⁸⁷Sr/⁸⁶Sr = 0.70326-0.70358, ²⁰⁶Pb/²⁰⁴Pb = 18.78-18.85, and ∂¹⁸O = 5.9±0.4. Sr-rich (> 1, 000 ppm) basaltic andesite is more variable (⁸⁷Sr/⁸⁶Sr = 0.70300-0.70360;²⁰⁶Pb/²⁰⁴Pb = 18.70-18.89; ∂¹⁸O = 5.9±0.4). The data define mixing arrays with one end member at⁸⁷Sr/⁸⁶Sr = 0.7029;²⁰⁶Pb/²⁰⁴Pb = 18.59, another at rhyodacite, and a third at⁸⁷Sr/⁸⁶Sr = 0.7036;²⁰⁶Pb/²⁰⁴Pb = 18.89. The first end member is defined by Sr-rich (800-1, 200 ppm) andesite with high Al₂O₃, and low K₂O, Ba, and Rb/Th; the third one by K₂O- and very Sr-rich (> 2, 000 ppm) shoshonite. Isotopic data for basalts in northern Oregon preclude any fractionation relationship between basalt and either rhyodacite or Sr-rich andesite (e.g., the minimum²⁰⁶Pb/²⁰⁴Pb ratio in basalt is 18.83). Considered in light of geophysical models for the Cascades, these data suggest two types of crustal melting beneath the arc. Rhyodacite may be generated at 25-30 km depth by partial melting of arc basalt-like amphibolite at 850-900 °C. Sr-rich andesite may be formed by partial melting of depleted MORB-like mafic granulite at 35-45 km depth at 1, 000-1, 100 °C. Experimental and REE evidence supports these interpretations as does the restriction of Sr-rich andesite in the Cascades to the area south of the 100 mW/m² heat flow contour between Mt. Jefferson and Mt. Hood. Thick crust and high heat flow are necessary to produce such andesite.