GEOCHEMICAL CONSEQUENCES OF ARC TERMINATION: A PERSPECTIVE FROM THE NORTHERN CASCADE ARC

The northern termination of Cascadia is associated with magmas that are compositionally unusual for arc settings. The ~375 km long northern segment of the arc, known as the Garibaldi Volcanic Belt (GVB), includes 5 major volcanic centers and extends from northern Washington to southwestern British Columbia. High Cascade basalts are typically calc-alkaline or tholeiitic and record variably depleted mantle sources that are modified by slab components [1]. In contrast, GVB basalts display a gradual arc-parallel, northerly transition to alkalic compositions similar to oceanic intraplate basalts. To the north, Zr/Nb ratios of GVB basalts decrease and Nb-Ta anomalies gradually diminish, indicating a progressive shift to more enriched mantle sources and reduced slab input. The most highly alkalic basalts occur in the northernmost GVB at Salal Glacier and Bridge River, volcanic centers that lie above the inferred location of the Nootka fault, the boundary between the subducting Juan de Fuca plate and the near-stagnant Explorer plate [2]. Salal Glacier and Bridge River basalts record significantly hotter and deeper mantle segregation conditions (~1500°C, 3.4 GPa) than other Cascade basalts, consistent with the presence of asthenosphere upwelling through a slab gap at the Nootka fault. GVB basalts are isotopically distinct from other Cascadia basalts with lower ²⁰⁸Pb/²⁰⁴Pb and ²⁰⁸Pb*/²⁰⁶Pb*. In ε_Hf-ε_Nd space GVB basalts form an array extending from DM towards HIMU, with the most highly alkalic basalts plotting closest to HIMU. OIB and E-MORB from the NE Pacific basin, including the Explorer ridge [3] and the Cobb seamount chain [4], share the distinctive isotopic characteristics of the GVB. The presence of NE Pacific mantle on the North American side of the slab indicates that mantle is flowing through a slab gap at the Nootka fault. The arc-parallel geochemical gradients that are defined by GVB basalts are consistent with mixing between inflowing NE Pacific mantle and the depleted mantle that dominates the wedge farther south in Cascadia. The results of this study illustrate the far-reaching effects that slab edges and gaps can have on magma compositions in subduction settings.

[1] Mullen et al., this meeting; [2] Audet et al. (2008) Geology 36, 895-898; [3] B. Cousens (2007) unpub. data; [4] Chadwick et al. (2014) G3, accepted.