Tracing the petrogenesis of Siletzia through geochemistry and petrography; the case for slab window-plume volcanism in the Crescent Formation

This paper presents new major, minor, and trace element analysis for rocks from the Black Hills in northern Washington while also presenting the single largest chemical dataset for Siletzia—an aggregate of decades of previous research. New samples from the Black Hills represent the subaerial extent of the Crescent Formation and support the interpretation of Siletzia as an accreted oceanic plateau responsible for clogging the North America Cordillera subduction zone. Geochemical data, geochronology, spatial trends, and tectonic reconstructions support that Siletzia drew from multiple distinct magma sources, including the Yellowstone Hotspot and Kula-Farallon spreading ridge. Northern Siletzia records a slab window generated by the subducting spreading ridge, while southern Siletzia displays more enrichment from plume-derived material. The Crescent Formation, therefore, records a unique transition from slab-window to ridge-plume-dominated volcanism. The quiescence between eruption of Siletzia and the Columbia River Basalt Group can be attributed to initiation of shallow subduction at the Cascadia trench, covering the mantle plume, which was finally punctuated by a slab tear preserved in the present-day Juan de Fuca Plate