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 Coastal New England (CNE) igneous province.

Plotting on a variety of chemical-tectonic discriminant diagrams suggests that the dike compositions are compatable with continental rifting. The more alkaline affinity of the composite swarm relative to ENA dolerites and CNE dolerites to the west and south may be a function of the swarm’s more central location on the horst adjacent to the Connecticut rift basin.