CORRELATION OF CHEMICAL TRENDS IN CASCADE ARC GRANITOIDS WITH TERTIARY PLATE MOTIONS

The Cascade plutonic arc extends from Oregon to British Columbia and includes >70 intrusions that were emplaced in response to subduction of the Juan de Fuca plate over the past 37 Ma. These are among the youngest arc plutons exposed anywhere in the world, and as such provide an excellent opportunity to examine both temporal and spatial relationships between magma chemistry and subduction processes. The present study focuses on the Chilliwack batholith and seeks to determine whether temporal trends in pluton composition and magma production rate can be related to changes in: (1) the rate of subduction, and (2) the passage of fracture zones.

The lower crustal melting processes by which Chilliwack magmas form are strongly influenced by variations in the involvement of a slab-derived hydrous component (Tepper et al., 1993). Among intermediate composition plutons (quartz diorite - granodiorite) trace element evidence of this "subduction component" declines through time: Ba/Nb decreases from 90-110 (28-33 Ma plutons) to 40-60 (<12 Ma plutons); B/Be decreases from 9-11 to 6-7 over the same interval. Magma production rates (as inferred from pluton areas) also decrease during this time, falling from ~75 km²/Ma to <10 km²/Ma. Both trends may reflect the changing rate of subduction, which slowed by a factor of ~5 between 35 and 2 Ma (Verplanck and Duncan, 1987). Slower subduction should lead to progressively greater heating and devolatilization of the slab before it reaches magma source regions.

Because of oblique convergence, fracture zones (FZ) in the JDF plate migrate northward as they are subducted beneath the arc (Engebretson et al., 1985). Time intervals when fracture zones passed beneath the Chilliwack (Sila FZ at ~8 Ma; Aja FZ at ~24 Ma) coincide with emplacement of distinctive felsic plutons that based on their petrology are inferred to have originated at elevated water fugacity. These plutons display evidence for greater involvement of a hydrous "subduction component" (higher Ba/Nb and B/Be) and may record subduction of portions of the slab (i.e., FZ) that contained a higher than average volatile content. Ongoing work will establish whether similar hydrous plutons occur in other Cascade batholiths and whether their ages track the positions of FZ through time.