On the relationship between subducted slab age and arc basalt petrogenesis, Cascadia subduction system, North America

Olivine-normalized less than or equal to 2.0 Ma magnesian basalts erupted c lose to the volcanic axis of the Cascadia subduction system exhibit are-par allel compositional variations compatible with a northward decrease in slab -derived components in the underlying mantle wedge. Inferred decreases in s lab input correlate strongly with a systematic decrease in age of the ocean ic crust along the convergent margin. Geochemical trends are most pronounce d in southwestern British Columbia and northern Washington basalts, where t he Garibaldi belt strikes obliquely to the trend of isochrons on the subduc ted plate. From southern Washington to northern California, High Cascades a re segments trend nearly parallel to subducted plate isochrons, and basalts show more subdued are-parallel variations. These observations support a mo del in which temperature and depth of melting in the mantle wedge beneath t he us are influenced by the age of the subducted plate. Where the us is und erlain by relatively young and hot oceanic crust, only a minor amount of he ating during subduction is required before dehydration reactions begin in t he subducted plate. The slab loses much of its volatiles trenchward of the are, and the volatile budget in the mantle wedge beneath the are is relativ ely low. In the Garibaldi belt, which overlies very young oceanic crust, th is produces progressively more alkalic basalts to the north. The High Casca des overlies older oceanic Lithosphere which was cooler at the time of subd uction. As a result, dehydration reactions are delayed and a greater amount of volatiles are released beneath the are. This results in lower melt temp eratures and higher degrees of melting, producing predominantly low-K thole iite and LILE-enriched HFSE-depleted calc-alkaline basalt magmas. (C) 1999 Elsevier Science B.V. All rights reserved.