New radiometric ages and field relations within the Coastal Maine Magmatic Province suggest a bimodal distribution of igneous rocks in both space and time. The earlier magmatic event is represented by the development of large igneous complexes which occur along a magmatic axis extending from Calais to Penobscot Bay, Maine. These complexes are dominated by mafic magmatism ranging in composition from olivine norite through quartz diorite and granodiorite. Partially crystallized mafic magmas are often intruded by slightly peraluminous biotite ± hornblende granites as demonstrated by the occurrence of a wide range of enclave-host relationships. The igneous complexes are commonly cut by younger voluminous biotite ± hornblende granites and represent the development of a new tectonothermal perturbation of the crust. New radiometric data on igneous rocks from the Moosehorn Igneous Complex (MIC) near Calais, Maine indicate that the earlier magmatic event occurred during the interval of 440-420 m.y. This magmatic event in the MIC is marked by the emplacement of a large, complex assemblage of olivine norite, hypersthene gabbro, gabbro, biotite ± hornblende diorite, quartz diorite, and granodiorite. Compositional layering is present in many mafic bodies, and most commonly strikes northwest and dips gently to the southwest. The Moosehorn complex was intruded by the Baring granite before complete crystallization, resulting in the development of texturally diverse enclave swarms within the granite. / Master of Science