HR: 09:00h
AN: V51B-03    [Abstracts]
TI: Influence of Varied Extents of Slab Dehydration on Melting Conditions and Primary Mantle-derived Magmas, Northern Cascadia Subduction System
AU: * Green, N L
EM: ngreen@wgs.geo.ua.edu
AF: University of Alabama, Department of Geological Sciences 202 Bevill Building, Tuscaloosa, AL 35487-0338 United States
AU: Savov, I
EM: savovi@si.edu
AF: Smithsonian Institution, Department of Mineral Sciences National Museum of Natural History, Washington, DC 20560-0119 United States
AB: The northern Cascadia margin of North America represents the classic example of a "hot" subduction system. The downgoing Juan de Fuca plate ranges in age from about 10 m.y. along the coast of central Washington to less than 5 m.y. off central Vancouver Island. The Garibaldi volcanic belt (GVB) is a trench parallel, NNW-trending chain of small composite volcanoes, subglacial volcanic domes, and lava fields that lies 250 km east of the convergen margin. Beneath the GVB, the oceanic lithosphere attains an age that decreases northward from about 22 m.y to13 m.y. Primitive mafic lavas, which occur trenchward of the GVB volcanic front, range northward from high-alumina olivine tholeiites, magnesian andesites and LILE- and LREE-enriched calc-alkaline basalts at Glacier Peak, northern Washington, through transitional basalts in southern British Columbia to alkali olivine basalts and trachybasalts at Meager Mountain and Bridge River at the northern end of the volcanic belt. Concentrations of fluid-mobile-elements (FMEs) in the basalts suggest a significant interrelationship between forearc metamorphic dehydration/devolatization processes in the subducted plate and melting conditions within the mantle wedge beneath the GVB volcanic front. The forearc GVB mafic lavas generally possess an exceptionally low and more restricted range of boron abundances (0.9-5.7 ppm) compared to lavas of other subduction systems, including those of the High Cascades. The basalts also are characterized by low abundances of other FMEs (e.g., 1.5-7.9 ppm Pb, 5-11 ppm Rb, 0.10-0.36 ppm Cs, 0.7-1.7 ppm As, and 0.1-0.4 ppm Sb). Concentrations of B and FMEs in the lavas, as well as FME/LREE (e.g. Cs/La, Rb/La, and Pb/Ce) and ⁸⁷Sr/⁸⁶Sr ratios, correlate positively with the inferred age of the subducted slab immediately beneath the volcanic front. Such variations are consistent with increased metamorphic devolatilization of the Juan de Fuca lithosphere within the Cascadia seismogenic zone as the downgoing plate becomes younger and hotter northward along the convergent margin. Correlations between FME and major-element, REE and HFSE abundances suggest that more extensive dehydration of the slab beneath the northernmost basaltic centers resulted in lower degrees of wedge melting (5-10%) at higher P-T than conditions responsible for their more southerly counterparts (10-22%). The GVB mafic lavas therefore record a close relationship between the forearc metamorphic conditions and the degree of mantle melting beneath the volcanic front (i.e., the nature of primary arc magmas).
DE: 3640 Igneous petrology
DE: 8158 Plate motions--present and recent (3040)
DE: 8439 Physics and chemistry of magma bodies
DE: 9350 North America
DE: 9604 Cenozoic
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2005 Joint Assembly