The northeast-trending composite swarm of Mesozoic mafic dikes in eastern Massachusetts consists of tholeiitic olivine dolerites, transitional-alkalic dolerites, alkaline dolerites, and alkaline lamprophyres. Chemical subtypes thus far identified among the dolerites include dikes relatively high (>9.00 wt%) or low in MgO as well as dikes having relatively low TiO₂(<3.00 wt%) and low P₂O₅ (<1.00 wt%) contents. The low-MgO dolerites also include high-TiO₂ and high-P₂O₅ varieties.

The major and trace element compositions indicate that the Eastern Massachusetts dolerites differ markedly from Eastern North America (ENA) dikes and are a part of the...