Specimens from fourteen outcrops of bedrock in the Gulf of Maine were collected with the submersible Alvin and include one felsic aphanitic pyroclastic rock, two calc-alkalic granites, and eleven peralkalic granites. The peralkalic granites range from hypersolvus to subsolvus varieties and exhibit slightly different textures and mineralogical features that reflect minor differences in their local crystallization and subsolidus histories. Similar chemistry and zircon ages of 400 to 450 m.y. indicate that all of these peralkalic granites from the gulf are genetically related. On the basis of similar mineralogy, texture, rock chemistry, and primary radiometric ages, these granites are interpreted to have been intruded during the same igneous episode as the subaerial alkalic granites of eastern Massachusetts.
The Gulf of Maine lies within the Avalon Platform province. The local occurrences of Upper Ordovician–Lower Silurian alkalic granites may indicate an igneous-tectonic episode that accompanied intraplate epeirogenic doming following the major compressional phase of the Taconic orogeny which more severely deformed rocks farther west. Emplacement of alkalic granite probably was completed before renewed plate convergence and the final phase of Acadian plate collision during Devonian and throughout much of the rest of Paleozoic time.