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.
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