A geochemical study of the Riddle Peaks gabbro, North Cascades: evidence for amphibole accumulation in the mid-crust of an arc

TL;DRAbstract

Mid-crustal arc rocks are not commonly exposed, hampering our understanding of magma differentiation processes and mineral crystallization in the mid-crust of arc systems. This thesis presents results of the study of one exposed mid-crustal arc pluton, which is a unique laboratory to understand the geochemical effects of crystallization in this type of system. I report on the major and trace element characteristics of amphibole, plagioclase, and apatite in hornblendite and hornblende gabbro cumulates from the ~44 km2 Riddle Peaks pluton (~77 Ma) in the North Cascades Crystalline Core (NCCC), Washington. Electron microprobe and laser ablation-induced mass spectrometry (LA-ICP-MS), coupled with whole rock major and trace element data, show that the Riddle Peaks contains low Mg# cumulates with 40.7-47.2 wt.% SiO2; Mg# 33-67, where Mg# is defined as 100*[(Mg/(Mg+Fe2+)]. The two rock types present in the pluton are a rhythmically layered gabbro, consisting of hornblendite and hornblende gab

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