The northern islands of the Galapagos Archipelago (Wolf, Darwin, Pinta, Marchena, and Genovesa) are located between 50-100 km south of the Galapagos Spreading Center (GSC) and constitute a distinct geologic province from the main island chain. One of the most notable petrologic characteristics shared by the northern islands is the predominance of basaltic flows containing up to 60% high-An plagioclase feldspar megacrysts (An 80-95; up to 3 cm long), a feature not observed in the rest of the archipelago. Lavas from Wolf, Darwin, and Genovesa Islands were examined using XRF, ICP-MS, and electron microprobe to characterize the basalts and determine the magmatic history recorded by the large plagioclase crystals. Megacrysts of high-An plagioclase, accompanied by minor clinopyroxene ( < 5%) and/or olivine and orthopyroxene phenocrysts, dominate 30-60% of the volume of the plagioclase ultraphyric lavas, with clinopyroxene, plagioclase, and Fe-Ti oxides comprising the groundmass. The plagioclase crystals exhibit complex oscillatory zoning, glomerocrystic textures, quench rims of lower An content than the cores, abundant melt inclusions, and inclusions of Cr-Spinel (absent on Genovesa). Thermodynamic calculations (by MELTS) suggest that the megacrysts are not in equilibrium with the groundmass, which corroborates textural evidence for a xenocrystic origin for the megacrysts. According to statistical analyses, the plagioclase megacryst compositions from each island are not significantly different from each other. Nevertheless, preliminary electron microprobe analyses of rehomogenized plagioclase-hosted melt inclusions indicate that island-specific evolutionary trends are preserved in the crystals from each volcano. Our results suggest that a common feature of the northern islands is that they all formed from high temperature Al₂O₃ rich basalts, conducive to the formation of significant volumes of An-rich plagioclase. The composition of the mantle source, however, differs subtly from island to island, varying from nearly pristine depleted upper mantle (Genovesa) to mixtures of enriched plume and upper mantle (Wolf, Darwin). On the basis of these observations, each island must be supplied by a relatively long-lived magma reservoir (at least 30,000 years) to produce the large megacrysts. Such conditions may be a manifestation of the proximity of the Galapagos Plume to the GSC, where regional stresses from plume-ridge interaction initiate tensional volcanism. The locally homogeneous melts supplying each volcano reflect regional heterogeneities caused by variable plume contamination.