IMPLICATIONS OF CASCADES FRONTAL ARC (FA) MAGMATISM - BORING VOLCANIC FIELD

The Boring Volcanic Field represents largely Quaternary (³⁹Ar-⁴⁰Ar dates: 3.1 – 0.05 Ma) FA magmatism associated with the Cascades subduction zone, but extending at least 90 km west of the modern volcanic front. Mapping defines >100 eruptive units/lava packages of at least four distinct magma types based on geochemical criteria (e.g., Ba/Nb, Sr/Y, K/Ti, etc.; >1300 analyses). Low-K tholeiites (LKT) dominate early activity, initially from vents in the westernmost High Cascades, then from Portland basin itself (~2.6-1.1 Ma). Also, OIB-like, calcalkalic (CAB), and absarokitic (ABS) basalt types and derivative lavas (47-60% SiO₂) erupted in part coevally across the FA region from ~2.6 Ma to 58 ka. Many of the basaltic lavas are quite primitive (MgO > 8 wt%). Such diversity is unusual for the Cascades and for volcanic arcs in general.

The most primitive LKTs and OIBs seem to have little or no subduction contribution whereas the CAB/ABS types do, based on their systematic enrichments in Ba and Sr relative to Nb. These data preclude cogenetic relations between the groups, and implicate formation of the respective magmas by melting of multiple distinct mantle sources. As do higher Mg#s and olivine Fo content in primitive CAB and ABS lavas, that suggest they are derived from more refractory mantle sources compared to LKT or OIB magmas.

In more evolved samples, compositional distinctions are less clear. Textures and mineral compositions indicate that magma mixing was common, and chemical variations in some units clearly reflect open system behavior. Thus, the nature of magma sources at depth cannot be reliably inferred from such samples.

Mapping and geochronology suggest a general NW migration of vent localities over time, but no systematic pattern is seen in the distribution of magma types. Magmatism derives from heterogeneous mantle domains present beneath the forearc region, and segregation depth thermobarometry of primitive samples implies that the respective domains may be stratified (top to bottom) as follows: ABS/CAB, OIB, LKT. We propose that melting initiated by decompression of the LKT source domain, followed by heating of variably modified lithospheric mantle (CAB/ABS), possibly augmented by asthenospheric upwellings (OIB) through tears in the slab.

* This is a tribute to Russ Evarts – Deceased 11 July 2017