Abstract: The plutonic complex in SE Jersey consists of a late Precambrian gabbro-diorite mass which has been intruded by several granites. The status of the diorites which, like the gabbros, often possess a layered structure, is not clear. New geochemical data, including rare earth and trace element analyses, suggest that acid-basic magma mixing was not responsible for the variety of intermediate rocks. Amphibole-controlled fractional crystallization of hydrous basic magma is also unlikely in view of the REE and HFS (high field strength) element distribution. The model which best fits all the available field, petrographic and geochemical evidence is one in which the chemical variation was initially produced by fractional crystallization of anhydrous basic magma and subsequently overprinted by granitic metasomatism. This superimposed calc-alkaline characteristics on the complex and produced hybrid diorites which, because they were above their solidus temperature, recrystallized with textures indicative of a magmatic origin.