VARIATIONS IN CASCADE ARC MAGMATISM THROUGH SPACE AND TIME: A COMPARISON OF GEOCHEMICAL AND GEOGRAPHIC TRENDS IN VOLCANIC AND PLUTONIC ROCKS FROM THE CHILLIWACK-MT. BAKER AND FIFES PEAKS-BUMPING LAKE AREAS, WASHINGTON

Volcanic and plutonic rocks of the Cascade arc extend from northern California to British Colombia and preserve a record of subduction-related magmatism over the past ~44 Ma. During this interval the arc has experienced variations in several parameters that have the potential to influence magmatism including the rate of subduction, the age and volatile content of the subducting plate, and the stress regime, thickness and composition of the overlying continental crust. This study compares spatial and temporal patterns of magmatic activity in two areas of the Washington Cascades with the goals of: (1) identifying trends that may be related to changing subduction parameters, and (2) contrasting the records of plutonic versus volcanic rocks. The two areas under study are the Chilliwack-Mt. Baker area (CMB), which includes >40 plutons of the Chilliwack batholith (35 Ma to <3 Ma), and the Fifes Peaks-Bumping Lake area (FPBL), which contains over 46 dated volcanic and plutonic units (32 Ma to Recent). In both areas the interval 27-22 Ma was marked by voluminous activity that included emplacement of distinctive leucocratic granitoids indicative of hydrous melting. Both areas then experienced a hiatus (~22–12 Ma) after which magmatism resumed but was different in character. In the CMB magmatism since 12 Ma is confined to a NE-trending belt within which there is a younger-to-the-SW progression. Plutons in this belt generally have lower Ba/Nb, lower B/Be, higher incompatible element concentrations (e.g., Nb, Th), and smaller areas than the older plutons. These differences may reflect declining fluid flux from the slab (and decreased melting) as the rate of subduction slowed. In the FPBL the hiatus was followed (11-2 Ma) by emplacement of hornblende dacites that have adakite traits (e.g., Sr/Y > 40, Y < 15 ppm) and define a younger-to-the-SE progression. The presence of adakites in the FPBL but not the CMB may reflect a difference between the two areas in the age of the subducting slab. Preliminary results of this study suggest that many of the temporal and spatial trends observed in CMB and FPBL are strongly influenced by local variations in subduction parameters including slab age and geometry.