Volcanism in the central High Cascade Range has been dominated since late Miocene time by the construction of a mafic platform of coalescent shield volcanoes within an intra-arc graben that developed in response to extensional plate tectonics. Graben development along the Cascade crest was possibly activated by failure of crust that had become thermally weakened during previous episodes of calc-alkaline magmatism. Lithospheric extension along the central Cascade arc is attributed to a decrease in the convergence rate of the Farallon-North American plate system since early Tertiary time. Geochemical variations, compiled from new and existing data, are used to delineate four major categories of basalts (SiO2=47--53 wt %) and basaltic andesites (SiO2=53--60 wt %): (1) early High Cascade high-alumina olivine tholeiitic (HAOT) basalts, (2) normal High Cascade HAOT basalts, (3) Mount Washington (MW) type basaltic andesites, and (4) North Sister (NS) type basaltic andesites. Compositional variations among the four types possibly reflect source differences although much of the variation, especially in basaltic andesites, might be related to open-system evolution of primary magmas.

Early High Cascade basalts, erupted during late Miocene to early Pliocene time, display depleted light rare earth element (REE) and other large ion lithophile element (LILE) abundances similar to those observed in mid-ocean ridge basalts (MORB). Later basalts, erupted since the Pliocene, have variably enriched LILE and a wide range of REE fractionation (e.g., La/Yb ratios), and chemically resemble ocean island basalts (OIB). Pliocene to Holocene basaltic andesites exhibit ranges in total REE, Zr, Hf, and Sc abundances that are similar to those in normal basalts, yet the abundances of these elements in NS types are overall lower than the average basalt contents. Fractionation among the relative abundances of REE, Zr, Hf, and Sc between NS and MW types are not appreciable. Compared to normal basalts, both types of basaltic andesite exhibit uniformally elevated abundances of mobile LILE (Cs, Rb, K, Sr, Ba, and Th) and marginally depleted abundances of high field strength elements (HFSE) (Ta and Nb) typical of magmas erupted in magmatic arcs. Chemical signatures in central High Cascade basalts imply that the mantle beneath the central Cascades has been subjected to variable influences of MORB- and OIB-like components.

The implication of oceanic mantle beneath the central Cascades is consistent with the existence of the Columbia embayment into which a segment of oceanic lithosphere was compressed and thickened. Relatively high mobile LILE and somewhat depleted HFSE in the basaltic andesites, considered to be secondary effects to OIB/MORB mantle evolution, are believed to result from interaction of aqueous subduction-derived fluids withevolving magmas or their sources in lower crustal or upper mantle regions. The preponderance of HAOT basalts that exhibit within-plate chemical signatures argues for less input, relative to typical calc-alkaline magmatic arcs, of aqueous fluids due to subduction processes. The petrologic significance of a subduction zone beneath the Cascade arc may be restricted to the tectonic overprinting of an extensional system, caused by a reduction of convergence rate, on a waning of calc-alkaline magmatism. Physical models of the High Cascade subduction system are presented to explain the apparent chemical decoupling between calc-alkaline and tholeiitic magmatism. ¿ American Geophysical Union 1990