VARIABLE MANTLE AND CRUSTAL INFLUENCES ON MAGMATISM IN THE CASCADE ARC

The Cascade arc is associated with slow (~4.5 cm/yr) subduction of young ocean lithosphere (<10 ma at the trench) and as such represents an endmember “hot” subduction environment. The ~1300 km arc is divided into five segments due to structural changes in the subduction system. The northern segment has the narrowest arc width, is more deeply dissected exposing plutonic roots of the ancestral Cascade arc, and has a crustal thickness of ~45 km. The middle and southern segments are wider, with distributed peripheral vents surrounding stratovolcanoes. The entire arc is built on crust of juvenile accreted terranes that do not provide a strong contrast in basement composition.

Large volumes of mafic magmas have erupted in the middle and southern Cascades, but their volume dramatically decreases northward. In the entire arc however, compositional variability of mafic magmas is attributed to three distinct mantle reservoirs. Compositional types that characterize these reservoirs include hydrous calcalkaline basalt (CAB), dry decompression-related low-K olivine tholeiite (LKOT), and alkalic ocean island basalt (OIB). Primitive high-Mg basaltic andesites are also present in the southernmost and northernmost (Mt. Baker) segments of the arc. Calcalkaline lavas extend the length of the arc, LKOTs (with variable trace element chemistry) decrease in abundance northwards but are present as far north as Mt. Baker, and OIBs are not found north of Mt. Rainier. In all arc segments, felsic magmas are focused primarily in the stratovolcanoes, and mafic magmas in the surrounding vents.

In the northernmost segment (Garibaldi belt), slab age decreases northwards, and as a result, degree of partial melting in the mantle wedge decreases north of Mt. Baker. Glacier Peak (dacitic) and Mt. Baker (andesitic) have average compositions that are distinct, but they have strikingly similar mafic and felsic endmembers. In particular, the most felsic lavas erupted at each volcanic center, regardless of volume, are characteristically depleted in middle and heavy REE compared to the mafic magmas. As such, they are clear geochemical markers of magma mixing, as are abundant disequilibrium textures. Mixing of these characteristic, probably crustally-derived, felsic endmembers with diverse mafic magmas occurs over large spatial and temporal scales.