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 gabbro layered with anorthite to plagioclase-rich gabbro, and a massive hornblende gabbro. The layered gabbro has higher Mg# amphibole (60-70, with the majority 66-70) than massive gabbro (60-63) and more anorthitic plagioclase (layered gabbro = An81-85; massive gabbro = An71-77), suggesting that it was formed by a more primitive liquid. This is supported by modeling that shows that equilibrium liquids from the massive gabbros could have been produced by 40% crystallization of a hornblende gabbro lithology from the parent, calculated liquids in equilibrium with the layered gabbros. Equilibrium liquid calculations also allow for calculation of new apatite partition coefficients for 16 trace elements and REE in a mid-crustal, basaltic andesite system. This study finds that cumulate amphiboles crystallized from a basaltic andesite parent are responsible for increasing La/Yb ratios in derivative melts, such as arc magmas, continental crust and NCCC magmas (NCCC magmas approximated by liquid compositions from the Cardinal Peak and Tenpeak plutons). Amphibole crystallization decreases Dy/Yb in derivative melts; these results are in accordance with predictions from observed arc magmas. Other observed ratios in arc and crustal magmas, such as high Sr/Y (16-20), low Nb/Ta (10-17) and Ti/Zr (30) relative to primitive mantle/chondritic values (Sr/Y = 4.6; Nb/Ta = 18-20; Ti/Zr = 115) are not explained by amphibole crystallization. It has been suggested that amphibole-rich plutons could fractionate certain incompatible trace element pairs to explain the differing ratios in arc magmas and continental crust versus primitive mantle values. With the exception of REE, the Riddle Peaks pluton does not fractionate these ratios sufficiently to explain the differing ratios. If mineral fractionation is occurring, another mineral partitions these elements; or, another process occurs
