HIGH-MG ANDESITES FROM THE NORTHERN CASCADE ARC: USING MINERAL CHEMISTRY TO DISTINGUISH BETWEEN HYPOTHESES FOR PETROGENESIS

High-Mg andesites and basaltic andesites (HMA and HMBA) are magmas that have an unusually high Mg# (molar Mg/(Mg+Fe)) relative to their SiO₂, and exhibit a steep rare earth element (REE) pattern with notable depletion in heavy REE and Ni and Cr enrichment. Proposals for their origin vary and provide numerous testable hypotheses, all of which include an important role for garnet since this steep REE pattern is acquired through separation of melt from garnet. The debate centers on where the garnet came from (subducting slab? mantle? basalt crystallization? thick lower crust?) and how the high-Mg signature is acquired. To better understand the generation of these magmas in the northern Cascade Arc, this study combined whole rock and mineral geochemical analyses of HMA from both Mount Baker and Glacier Peak, the two northernmost active volcanoes in Washington.

Lavas with multiple populations of clinopyroxene (cpx) and plagioclase, resorption textures in olivine and orthopyroxene (opx), and recrystallization and reverse zoning in cpx (Mg# increase in rims), indicate open system mechanisms such as mingling between basaltic and andesitic magmas. Olivine, cpx, and scarcity of chromite in a Mount Baker HMBA suggest fractionated basaltic parents, where sparse opx and plagioclase with thin sieved rims suggest the addition of a small andesitic component shortly before eruption. In contrast, the dominance and variety of plagioclase populations and plentiful opx in a Mount Baker HMA suggest contribution from an array of parent magmas. Sporadic, highly unstable olivines with large opx reaction rims suggest the addition of a small basaltic component to this HMA. At Glacier Peak, a compositionally zoned HMA-HMBA flow contains two distinct quenched inclusion types. Chromite compositions in the mafic endmember suggests derivation from a fractionated refractory peridotite source. The sharp decrease in olivine Fo from core to rim and meager, resorbed opx indicate disequilibrium and addition of a higher silica liquid, likely andesite.