The Adirondack Mountains in upstate New York contain exposures of complex partially melted rocks, in addition to iron oxide-apatite (IOA) deposits with variable rare earth element (REE) concentrations. Previous workers have suggested that melting occurred during the ca. 1150 Ma Shawinigan and the ca. 1050 Ma Ottawan orogenies. However, there are challenges in determining the timing of melting and the number of partial melt events. Further, tectonic models must be developed to describe the petrogenesis of IOA and REE mineralization. Migmatites are present along Rt. 4/22 near Whitehall, NY. In chapter two, all layers of a single migmatitic rock were analyzed to describe the evolution of gneissic layering and the texture and timing of monazite and zircon developed during biotite dehydration melting. The gneissic layers are interpreted to reflect the differential preservation of reactants and products formed during the forward and reverse progress of the melting reaction. The heterogeneous layered gray gneiss provides robust constraints on the timing of melting (ca. 1050 Ma Ottawan orogenesis), melt crystallization, post-melting retrogression, and information about earlier metamorphic events. The IOA deposits are typically hosted in the Lyon Mountain Granite Gneiss (LMG) and are spatially correlated with extensive Na metasomatism. The results in chapter three demonstrate the utility of hand-held instruments (i.e., gamma ray and portable X-ray fluorescence spectrometers) for identifying cryptic alteration gradients in the LMG. Further, elevated Th and U values in the LMG proved to distinguish the high-REE deposits. The abundance of REEs is variable within the IOA deposits. The mineralogy, composition, and texture of high, medium, and low-REE deposits are compared in chapter four. High-REE deposits are characterized by clinopyroxene, apatite (14 wt% REEs), and magnetite (~4,000 ppm Ti) with accessory quartz (~12 ppm Ti). Medium-REE deposits are dominated by apatite (9 wt% REEs) and magnetite (~5,000 ppm Ti) with accessory quartz (~10 ppm Ti). The low-REE deposits consist of abundant quartz (~6 ppm Ti), magnetite (~60 ppm Ti), and chlorite. The three deposit types are interpreted to reflect varying temperatures of the immiscible liquids generated from a gabbroic magma and subsequently emplaced into dilatant zones coeval with the LMG.