PRIMITIVE MAGMAS AT 5 CASCADE VOLCANIC FIELDS - MELTS FROM HOT, HETEROGENEOUS SUB-ARC MANTLE

Major and trace element concentrations, including REE by isotope dilut ion, and Sr, Nd, Pb, and O isotope ratios have been determined for 38 mafic lavas from the Mount Adams, Crater Lake, Mount Shasta, Medicine Lake, and Lassen Volcanic fields, in the Cascade are, northwestern par t of the United States. Many of the samples have a high Mg# [100Mg/(Mg + Fe-T) > 60] and Ni content (>140 ppm) such that we consider them to be primitive. We recognize three end-member primitive magma groups in the Cascades, characterized mainly by their trace-element and alkali- metal abundances: (1) High-alumina olivine tholeiite (HAOT) has trace element abundances similar to N-MORB, except for slightly elevated LIL E, and has Eu/Eu > 1. (2) Are basalt and basaltic andesite have notab ly higher LILE contents, generally have higher SiO2 contents, are more oxidized, and have higher Cr for a given Ni abundance than HAOT. Thes e lavas show relative depletion in HFSE, have lower HREE and higher LR EE than HAOT, and have smaller Eu/Eu (0.94-1.06). (3) Alkali basalt f rom the Simcoe volcanic field east of Mount Adams represents the third end-member, which contributes an intraplate geochemical signature to magma compositions. Notable geochemical features among the volcanic fi elds are: (1) Mount Adams rocks are richest in Fe and most incompatibl e elements including HFSE; (2) the most incompatible-element depleted lavas occur at Medicine Lake; (3) all centers have relatively primitiv e lavas with high LILE/HFSE ratios but only the Mount Adams, Lassen, a nd Medicine Lake volcanic fields also have relatively primitive rocks with an intraplate geochemical signature; (4) there is a tendency for increasing Sr-87/Sr-86, Pb-207/Pb-204, and delta(18)O and decreasing P b-206/Pb-204 and Nd-143/Nd-144 from north to south. The three end-memb er Cascade magma types reflect contributions from three mantle compone nts: depleted sub-are mantle modestly enriched in LILE during ancient subduction; a modern, hydrous subduction component; and GIB-source-lik e domains. Lavas with an: and intraplate (GIB) geochemical signatures were erupted close to HAOT, and many lavas are blends of two or more m agma types. Pre-eruptive H2O contents of HAOT, coupled with phase-equi librium studies, suggest that these magmas were relatively dry and las t equilibrated in the mantle wedge at temperatures of similar to 1300 degrees C and depths of similar to 40 km, virtually at the base of the crust. Are basalt and basaltic andesite represent greater extents of melting than HAOT, presumably in the same general thermal regime but a t somewhat lower mantle separation temperatures, of domains of sub-are mantle that have been enriched by a hydrous subduction component deri ved from the young, relatively hot Juan de Fuca plate. The primitive m agmas originated by partial melting in response to adiabatic upwelling within the mantle wedge. Tectonic extension in this part of the Casca de are, one characterized by slow oblique convergence, contributes to mantle upwelling and facilitates eruption of primitive magmas.