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 millio n years. A large trace element data set for Mt. Jefferson and its surroundi ng mafic volcanic platform effectively rules out any fractionation based mo del (FC or AFC) for the generation of Mt. Jefferson andesites. Several inco mpatible element (Zr, Nb, Y) concentrations decrease in the range from basa lt 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 b asalt 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. Rhyo dacites are isotopically uniform (Sr-87/Sr-86 = 0.70325-0.70343; Pb-206/Pb- 204 = 18.75-18.85; partial derivative O-18 = 6.3 +/- 0.3), whereas andesite and dacite are more variable (Sr-87/Sr-86 = 0.70291-0.70353; Pb-206/Pb-204 = 18.59-18.86;partial derivative O-18 = 6.0 +/- 0.6). Typical basaltic and esite has Sr-87/Sr-86 = 0.70326-0.70358, Pb-206/Pb-204 = 18.78-18.85, and p artial derivative O-18 = 5.9 +/- 0.4. Sr-rich ( >1,000 ppm) basaltic andesi te is more variable (Sr-87/Sr-86 = 0.70300-0.70360; Pb-206/Pb-204 = 18.70-1 8.89: partial derivative O-18 = 5.9 +/- 0.4). The data define mixing arrays with one end member at Sr-87/Sr-86 = 0.7029; Pb-206/Pb-204 = 18.59, anothe r at rhyodacite, and a third at Sr-87/Sr-86 = 0.7036; Pb-206/Pb-204 = 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-r ich (>2,000 ppm) shoshonite. Isotopic data for basalts in northern Oregon p reclude any fractionation relationship between basalt and either rhyodacite or Sr-rich andesite (e.g., the minimum Pb-206/Pb-204 ratio in basalt is 18 .83). Considered in light of geophysical models for the Cascades, these dat a suggest two types of crustal melting beneath the arc. Rhyodacite may be g enerated at 25-30 km depth by partial melting of arc basalt-like amphibolit e at 850-900 degreesC. 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 degree sC. Experimental and REE evidence supports these interpretations as does th e restriction of Sr-rich andesite in the Cascades to the area south of the 100 mW/m(2) heat flow contour between Mt. Jefferson and Mt. Hood. Thick cru st and high heat flow are necessary to produce such andesite.