RELATION OF CONVERGENT MARGIN VOLCANISM TO SLAB AGE AND THERMAL STRUCTURE: POSSIBLE INFLUENCES ON ANDESITIC MAGMATISM OF THE NORTHERN CASCADIA SUBDUCTION SYSTEM

The Cascadia subduction system of western North America represents the type example of a continental margin underthrust by extremely young (<25 m.y.) and presumably hot oceanic lithosphere. Although the High Cascades volcanic arc of the southern Cascadia system overlies a subducted Juan de Fuca plate of nearly uniform age (15-17 m.y.), more northerly Garibaldi belt andesitic to rhyodacitic volcanoes developed above oceanic crust that apparently decreases in age from about 22 m.y. beneath Glacier Peak to roughly 14 m.y. below the Bridge River Cones. Individual intermediate to silicic eruptive suites of the 15-km-wide Garibaldi belt exhibit sub-parallel to overlapping trends of decreasing TiO₂, FeO_t, CaO, MgO, Ni, V, and Cr, and increasing K₂O, Rb, and Ba with increasing silica content. Within most of these suites, Al₂O₃, Na₂O, P₂O₅, and to a lesser extent, Sr initially increase as SiO₂ increases to about 58 wt. %, then steadily decline with increasing silica contents.

Major- and trace-element compositions of Cascadia basaltic lavas have been shown to exhibit significant correlations with slab age, suggesting magma generation was strongly influenced by along-strike variations in the thermal state of the underlying subducted plate. Garibaldi belt mafic andesites also appear to exhibit relationships to the age of the subducted oceanic crust, but correlations are not as strong and some relationships are opposite to those observed between associated basalts and slab age. Arc-parallel variations in compositions of these medium-K andesitic magmas typically include: 1) slight increases in Al₂O₃, Na₂O, K₂O, P₂O₅, K/Rb, Ba/Ta, Ba/Th, Ba/U, Sm/Yb, and Sr/Nd; 2) near constant CaO, FeO_t, Nb/Ta (~17), Ce/Ce* (~1), and Ba/Sr; 3) wide but similar ranges in Rb, Sr, La, Zr, Hf, La/Yb (<25), Zr/Hf (>35), Th/Rb, Ba/La, Ba/Zr, Ba/Nb, Eu/Eu*, K/Sr, La/Nb, (La/Sm)_N, and Sr/Y (20-110): 4) modest decreases in TiO₂, MgO, Th, V, Y, Yb, Th, Rb/Ba, Th/Nb, Zr/Nb, and Mg# (0.7-0.5); and 5) decreases in maximum Cr, Ni, Nb, Nb/Zr, and (Sr/P)_N. Compositions of more evolved Garibaldi belt magmas (siliceous andesite-to-rhyodacite) tend to lack similar correlations with slab age. These variations may place constraints on processes operating within and above the Cascadia subduction zone and on any genetic connections between mafic and evolved magmas.