Petrology and geochemistry of coeval andesite flows at Mount Baker, North Cascades

At the Mt. Baker volcanic field in the northern Cascade arc, the coeval Coleman Pinnacle (CP) hornblende andesite and the Table Mountain (TM) two-pyroxene andesite have overlapping eruption ages (305±5 ka) and field relations show flow interlayering (Hildreth et al., 2003). Although primary vents can no longer be recognized, the distribution of dikes and outcrop patterns are consistent with eruption from vents close to the southern rim of Kulshan Caldera. Although both flow sets have similar ranges in SiO₂, they have markedly different phenocryst assemblages, initial water contents, oxidation states, K₂O contents, isotopic ratios and trace element abundances. In the more silicic CP andesite samples, plagioclase and hornblende are the dominant phenocrysts with microphenocrysts of titanomagnetite, ilmenite and apatite set in a fine-grained, crystalline groundmass. The most mafic CP andesite flow also contains orthopyroxene and clinopyroxene and inclusions of two-pyroxene andesite rich in apatite, Fe-Ti oxides (including pseudobrookite) and abundant quenched interstitial rhyolitic glass. In Table Mountain andesites, phenocrysts are plagioclase, orthopyroxene, clinopyroxene, titanomagnetite and ilmenite with minor resorbed olivine and late-stage amphibole in the lowermost flow. Coleman Pinnacle andesite is richer in K₂O (2.7-3.2 wt.%), H₂O (5-6 wt.%) and is more oxidized (f_O2: NNO+1 to +1.7) than Table Mountain andesite (K₂O: 1.6-2.4 wt.%, H₂O: ~3 wt.%, f_O2: NNO to NNO+0.7). Breakdown rims on CP amphibole are absent or very narrow while TM amphibole rims range from 25-30 μm, indicating integrated ascent rates of ~0.003 m/sec. Mg-rich rims on most CP amphiboles provide evidence for late magma mixing. High Fe³⁺ contents of amphiboles indicate dehydrogenation during the eruptive event. Phase equilibria for CP and TM flows are consistent with crystallization at P_H2O of ~2kb but both flow sequences define distinct fractionation trends on [Pl] Ol-Wo-Qz projections. In Pb-Pb and Sr-Nd isotope plots, CP and TM andesites plot along a mixing line between Mt. Baker basalts and more-radiogenic Kulshan caldera rhyodacites, indicating crustal assimilation by basalt and/or mixing of crustal melts with basalt. The pseudobrookite-bearing CP flow lies off the mixing lines and clearly requires an additional component, probably subducted sediment. The TM andesite has an Os isotope ratio (γOs = 65) similar to those of the most primitive Mt. Baker basalts, indicating that crustal input had a minimal effect on γOs. Trace element abundances of both TM and CP are consistent with AFC modeling in which parent basalts similar to those at Tarn Plateau and Park Butte assimilate silicic crust and fractionate plagioclase and amphibole. Reference: Hildreth et al. (2003) GSA Bull 115, 729-764.