DIVERSE PRIMITIVE MAGMAS IN THE CASCADE ARC, NORTHERN OREGON AND SOUTHERN WASHINGTON

Bulk-rock major- and trace-element composition, petrography and minera l compositions are presented for a diverse suite of 22 primitive mafic lavas in the Cascade Range of northern Oregon and southern Washington . With the exception of an early Western Cascade basalt, all the rocks are younger than 7 Ma. Intensive parameters [f(H2O),f(O-2), T, P] for the magmas have been inferred mostly from equilibrium olivine-liquid and plagioclase-liquid relations. Nearly anhydrous, MORE-like, low-K t holeiite was probably derived from relatively high degrees of decompre ssion-induced melting of shallow, depleted, relatively unmetasomatized lithospheric mantle during intra-are rifting. The degree of partial m elting decreases northward along the are, whereas the depth of average melt generation increases. OIB-like basalt represents deeper, wetter, smaller-degree melts of more enriched asthenospheric mantle, unaffect ed by subduction. Olivine analcimite resembles the silicate melt consi dered responsible for within-plate mantle metasomatism. Post-7-Ma subd uction-related basalt was derived by low degrees of partial melting of subduction-metasomatized garnet Iherzolite, similar to GIB-like basal t source-mantle before modification. The spectrum of subduction-relate d basalt from cooler and wetter (and slightly more oxidized) absarokit e to progressively hotter and drier high-g calc-alkaline basalt and ca lc-alkaline basalt seems to be due to Varying degrees of metasomatism of the deep mantle wedge by relatively cool, wet, LILE-rich absarokiti c magmas coming from near the subducted slab. Early Western Cascade ba salt is more typically are-like in its composition and mineralogy, and was probably generated under H2O-rich conditions when more vigorous s ubduction prevailed. Depleted basaltic andesite may have been generate d by low degrees of partial melting of residual harzburgite, possibly formed during the generation of early Western Cascade basalt.