- Inheritance from pre-existing mantle domains and fluid and melt contributions from active subduction together produce the geochemical signatures of mantle-derived arc basalts. In this context, this work evaluates the evolution of Cascadia mantle sources by documenting the isotopic and compositional characteristics of primitive basalts along a transect across the Eocene-Oligocene Proto-Cascadia (EOPC) arc at
~44.5-45.5° N. Primitive EOPC flows, dikes, and sills are exposed across a ~300 km transect that includes the Oregon Coast Range in the Cascadia forearc, the Western Cascades, flanking the modern arc, and the John Day and Eastern Clarno formations east of the Cascades. Like the modern arc, EOPC was built upon accreted terranes of western North America and within the Columbia embayment, which is lithosphere of oceanic affinity that crops out as the Siletzia terrane in the forearc and extends beneath the arc to the backarc. Potential mantle source reservoirs for EOPC magmas include contributions from mantle domains related to pre-existing underlying terranes, distinct North America lithosphere, and depleted Pacific-like upper mantle. In addition, the geochemical characteristics of EOPC magmas have likely been overprinted by subduction processes.
Major, trace element, and isotopic data from the EOPC reveal a heterogeneous mantle source that was variably influenced by subduction processes. In the forearc, the high field strength (HFSE) enriched basalts of the Oregon Coast Range represent low degree partial melts of a relatively enriched mantle source. Despite this enriched character, there is little evidence in the trace element or isotopic data to suggest that
the OCR (Oregon Coast Range) samples have been strongly influenced by either a crustal or subduction component. Their distinctive ²⁰⁶Pb/²⁰⁴Pb enrichment (as compared to ²⁰⁷Pb/²⁰⁴Pb) distinguishes the forearc magmas from the arc and backarc magmas and also from a hypothetical Cascadia subduction component. Forearc EOPC magmas share a mantle source with the accreted Siletzia terrane, as evidenced by their shared ²⁰⁶Pb/²⁰⁴Pb enrichment. At the apparent arc axis, the Western Cascades produced a diversity of primitive magmas that are, for the most part, interpreted to result from higher degrees of partial melting of a less enriched source. Fluid fluxing appears to have facilitated mantle melting beneath the Western Cascades. Additionally, the mantle beneath the arc may be slightly influenced by the ²⁰⁶Pb/²⁰⁴Pb enriched source underlying the
forearc. Though this effect is difficult to resolve, the Western Cascades samples appear to be slightly more variable in ²⁰⁶Pb/²⁰⁴Pb (and generally higher in ²⁰⁶Pb/²⁰⁴Pb at a given ²⁰⁷Pb/²⁰⁴Pb) than their backarc counterparts, suggesting such an influence. Both trace element and isotopic data suggest a significant subduction contribution to EOPC backarc primitive magmas. The EOPC backarc magmas appear to have originated from a heterogeneous mantle and are variably influenced by a
subduction component. Though this subduction influence could be an inherited feature, the affinity between John Day and Eastern Clarno magmas and a modern Cascadia sediment source suggest that this is at least a relatively young feature. Taken together, these results demonstrate the heterogeneity of source and process across the
Eocene-Oligocene arc. Apparent subduction contribution increases from the
geographic forearc to backarc, perhaps indicating a wider arc than is typically envisioned at the time. The distinct ²⁰⁶Pb/²⁰⁴Pb enrichment associated with the Siletzia mantle and apparent in the forearc appears to wane in influence to the east, having at most a minor influence on the Western Cascade arc magmas. As with the Siletzia mantle domain in EOPC arc and backarc, mantle domains associated with the Western Cascades and John Day/Eastern Clarno magmas do not persist in the High Cascades or High Lava Plains. Beneath and behind the arc, mantle reservoirs appear to have been largely replaced since the Eocene-Oligocene. However, the High Lava Plains basalts appear to carry a paleo-enrichment signature that may be an inherited feature from advected mantle. If this enriched mantle was
advected from beneath North America, it may explain the observation that High Lava Plains basalts are more enriched to the east.