MSA DANA MEDAL LECTURE: ORIGINS OF CASCADE MAGMAS

The Cascade magmatic arc stands out in its accessibility and the youth of its subducting plate. Easy access and high populations stimulated research on geothermal resources, hazards, and igneous and volcanic processes. Proximity of the Juan de Fuca ridge leads to predictions that the downgoing oceanic plate is one of the hottest presently subducting. Suggestive of widespread slab melting is that volcanic rocks with trace element signatures ascribable to residual garnet (e.g. Sr/Y >50) erupt along the entire arc, although they are a minority of its Quaternary products (~15%), are most abundant and strongly developed at its southern end, and are nearly all silicic andesites and dacites. Origin of the garnet signature at mantle depths is supported by (1) a general correlation with low Nb concentrations and low Nb/Zr attributable to residual rutile, requiring ≥1.5 GPa to be stable in mafic-intermediate sources, and (2) a tendency for the higher Sr/Y rocks to have the least radiogenic Sr and Pb of their volcano. Most Quaternary volcanic rocks of the arc lack pronounced residual-garnet signatures and instead form a continuum from basalts to rhyolites. Cascade basalts were long shown to define three principal types: (1) low-K, (2) within plate, (3) and calc-alkaline (with typical arc trace element features). The usual processes of differentiation, mixing, and assimilation can account for the basalt through rhyolite suite, although zircon ages and other results make it increasingly clear that petrogenesis is polycyclic: Crustal roots heat and cool repeatedly over the 1-5 ×10⁵ year activity of individual volcanoes, so later magmatic replenishments mainly encounter and pass through antecedent intrusions from the same magmatic system. Mixing with residual liquids and low-degree partial melts from antecedent intrusions is a major differentiation process in low-flux systems, whereas higher-flux systems can undergo more ordinary progressive crystallization-differentiation. Petrogenesis thus appears to be two-fold: dominantly complex differentiation from basaltic parents, accompanied by subordinate slab melts that pass through the mantle wedge reacting insufficiently to become basalts, or that consistently differentiate to silicic andesite and dacite, perhaps driven by degassing of high H₂O contents.