Abstract
The Spences Bridge Group, a late Albian (100 Ma) age volcanic succession built on the western margin of Superterrane I, comprises two stratigraphic units with contrasting geochemistry. The lower unit, the Pimainus Formation, represents a set of stratovolcanoes with calcalkaline character. Lavas of the overlying Spins Formation, deposited as a shield volcano, are hybrids between Pimainus arc magma and "Spins-type" melts of intraplate affinity. Integration of regional geology suggests that the Pimainus Formation was produced by short-lived, easterly dipping subduction beneath Superterrane I. That convergence closed a narrow oceanic basin, now represented by the Methow-Tyaughton trough, bringing Superterrane II into collision with Superterrane 1. Accretion in the latest Albian resulted in cessation of Pimainus are volcanism, the eruption of Spins lavas, and easterly directed deformation of Methow-Tyaughton strata. Early to middle Cretaceous are volcanism of the Gambier Group on Superterrane II was produced by a separate, concurrently active subduction zone.
