The generation of a diverse suite of Late Pleistocene and Holocene basaltic-andesite through dacite lavas from the northern…

This study highlights geochemical diversity in a spectrum of basaltic andesite through dacite lavas from Mount Baker, WA, and describes processes that are responsible for their generation. Petrographic observations, mineral chemistry, along with whole rock major oxide concentrations, and trace and REE data are provided for three Late Pleistocene and Holocene lava flows: the basaltic andesite of Sulphur Creek (SC) (52.5-55.8 wt.% Si02, Mg# 57-54), the andesite of Glacier Creek (GC) (58.3-58.7 wt.% Si02, Mg # 63-64), and the andesite and dacite of Boulder Glacier (BG) (60.2-64.2 wt.% Si02, Mg # 50-57). Major oxide concentrations for SC and BG display clear trends with increasing Si02. GC andesites are tightly clustered compositionally with elevated MgO and Ni compared to SC and BG for a given Si02. REE patterns are distinct for each unit but are not correlated with differentiation. The mafic lavas of SC have relatively elevated REE abundances with the lowest La/Yb (~4.5). The GC andesites have the lowest REE abundances and the largest La/Yb (~6.7). The BG lavas have intermediate REE abundances and La/Yb (~6.4). Phenocryst populations in all units display varying degrees of reaction textures and disequilibrium textures along with complicated zoning patterns indicative of magma mixing processes. None of units can be related to each other through crystal fractionation processes, nor can crystal fractionation explain the compositional diversity within each unit as suggested by several major and trace element models including MELTS (Ghiorso, 1993) and REE Rayleigh fractionation. However, magma mixing between the mafic SC lavas with compositions similar to the dacites of BG in the proportions 70 % SC with 30% BG can account for the chemical trends displayed by the SC lavas. Given that the BG dacite mixing end-member erupted at 80 ka, and was mixed with the SC lavas at 9.8 ka, the process that produced this felsic end-member has been active or periodically active for at least 70 ka. The BG mixing end-member is comparable to silicic mixing end-members at other Cascade volcanoes where crustal melting processes have been called upon to explain their origin and the SC mixing end-member is presumably mantle derived. GC andesites include a population of olivine that has been identified as xenocrystic based on size, composition and the observation of strong disequilibrium textures. The elevated MgO composition of the GC andesites is proposed as the result of the addition of ~4 wt% (by volume) of the xenocrystic GC olivine. The source of the xenocrystic olivine is unconstrained; however, it is most likely cumulate in origin or related to mafic plutonic roots. Compositions of GC andesites before the addition of 4% olivine lie on the straight line mixing trend formed by the SC basalts and BG dacites making them a possible product of magma mixing. All GC phenocrysts display varying degrees of reaction and disequilibrium textures along with complicated zoning patterns which are also supportive of magma mixing processes.