Rhythmically layered anorthosite and gabbro are exposed in a 4–10-m thick interval at the base of the layered gabbro unit on North Arm Mountain, one of four massifs that compose the Bay of Islands ophiolite, Newfoundland. Within the rhythmically layered interval, up to 37 anorthosite layers 1–2 cm thick alternate with gabbroic layers 7–10 cm thick. Anorthosites are adcumulates (most contain <6ppm Zr) with 98–99% plagioclase (Plag) and 1–2% intergranular clinopyroxene (Cpx), whereas gabbros are adcumulates to mesocumulates (<6–20ppm Zr) with 35–55% Plag, and the balance olivine (Ol) + Cpx ± orthopyroxene (Opx). Average mineral compositions are: Ol mg-number [100 ×Mg/(Mg + Fe)]=84·9, NiO=0·13wt. % Plag An = 87·9; Cpx mg-number = 88·3, TiO₂=0·20 wt %; and Opx mg-number = 85·7. Rare earth element (REE) concentrations in clinopyroxene and plagioclase are low throughout the rhythmically layered interval (<5 times chondrites). The rhythmically layered interval is sandwiched between thick layers of adcumulate to orthocumulate uniform gabbro with average modal proportions of 54% Plag-39% Cpx-3% Ol-4% Opx. Average mineral compositions are: Ol mg-number = 75·5, NiO = 0·08 wt. %; Plag An=69%6; Cpx mg-number = 81·2, TiO₂ =0·53 wt. %, and Opx mg-number = 77·5. Clinopyroxene and plagioclase REE abundances are systematically higher in the uniform gabbro interval than in the rhythmically layered interval. Calculated fractional crystallization paths and correlated cryptic variation patterns suggest that uniform and rhythmically layered gabbros represent 20–30% in situ crystallization of two distinct magma batches, one more evolved and the other more primitive. When the more primitive magma entered the crystallization site of the NA300–301 gabbros, it is estimated to have been ∼40°C hotter than the resident evolved magma, and may have been chilled by contact with a magma chamber margin composed of uniform gabbro. In this model, chilling caused the liquid to become supercooled with respect to plagioclase nucleation temperatures, resulting in crystallization of gabbro deficient in plagioclase relative to equilibrium cotectic proportions. Subtraction of a plagioclase-poor melagabbro enriched the liquid in normative plagioclase, which in turn led to crystallization of an anorthosite layer. Alternating anorthosite and gabbro layers in the rhythmically layered interval built up by coupled and sustained variations in crystal nucleation and growth rates, and associated variations in liquid compositions at the crystallization front. Relatively stagnant magma-flow conditions may be required to accumulate substantial thicknesses of rhythmically layered cumulates by sustained oscillatory crystallization. The rarity of anorthosite-gabbro rhythmic phase layering on North Arm Mountain may indicate that convective magma currents in the Bay of Islands magma chamber were too vigorous for oscillatory crystallization to commonly occur.