Instrumental neutron activation analysis (INAA) of rare earth element (REE) abundances for a northern portion of the main Adirondack (Marcy) anorthosite and the adjacent Tupper–Saranac mangerite indicates they are genetically unrelated. Marcy anorthosite characteristically has low REE abundances and a positive europium (Eu) anomaly and a thin discontinuous border facies with no Eu anomaly. Tupper–Saranac mangerite has a lower mafic-rich portion with a relatively low REE abundance and a positive Eu anomaly, and an upper quartz-rich portion with a higher REE abundance and a negative Eu anomaly. A thin border facies of basal mangerite has an intermediate REE abundance and no Eu anomaly. These data are best interpreted in terms of separate magmas and differentiation within the mangerite body. Rare earths were also determined for some Morin Complex rocks.The Sm/Eu ratio from our INAA data agrees well with that calculated from individual minerals for the early crystallized, lower mafic-rich portion of the Tupper–Saranac mangerite, but poorly for the later crystallized, upper quartz-rich portion. The poor agreement between calculated and experimental Sm/Eu ratios in the quartz mangerite is probably due to depletion of Eu relative to Sm during crystallization of the magma, and supports a differentiative relationship between the mafic and quartz mangerite units.