GEOCHEMICAL EVIDENCE FOR SLAB-TEAR RELATED MAGMA GENERATION IN THE CASCADES COLUMBIA TRANSECT

The Cascades Columbia Transect (CCT) of southern Washington and northern Oregon is unusual among volcanic arcs in that Quaternary magmas vented across ~175 km perpendicular to the arc axis from the rear-arc Simcoe volcanic field and into the forearc accretionary complex near Beaverton, OR. No crustal structures distinguish the CCT forearc from its amagmatic northern and southern counterparts, and the shallow estimated depths to the Juan de Fuca (JdF) slab surface beneath the western CCT (45–50 km) combined with that area’s ~40 km crustal thickness seemingly allow for little to no mantle wedge melting. These raise the question of how magmas form across such an expanse and so close to the trench. CCT magmatic products are compositionally diverse, with andesite and dacites primarily focused at two stratovolcanoes (Mts. Adams and St. Helens). Basalts and basaltic andesites predominate and span between low-K tholeiite (LKT), intra-plate basalt (IPB), and calc-alkaline basalt (CAB) endmembers. Endmember primitive magmas erupt within km of each other but retain their distinct characteristics, implying short residence times in the crust and somewhat isolated ascent pathways.

Each endmember primitive magma type has a consistent trace element or isotopic composition across the CCT and can be modeled with a widespread “uniform” LKT mantle source and variable contributions from sediment and igneous ocean crust melts but limited inputs from fluids. This implies that the CAB arc signature primarily results from trace phases residual upon slab melting. Uniform endmember compositions across the CCT also indicate near-uniform conditions of melting across the corridor, seemingly conflicting with the large difference in mantle wedge thickness and proximity to the trench. Lateral transport of magmas from the arc axis into the nose of the mantle wedge is unlikely to preserve the diversity of magma types observed. Additionally, rear-arc magmas are IPB with lower ¹⁷⁶Hf/¹⁷⁷Hf and higher ²⁰⁶Pb/²⁰⁴Pb isotopic compositions than the rest of the CCT and cannot have the same source. A tear or gap in the subducting slab and influx of sub-slab asthenosphere likely provides the thermal source needed for melting across the CCT, supported by tomographic and shear wave splitting studies that observe segmentation of the JdF slab just south of the CCT.