Five mafic and four intermediate to felsic swarms have been recognized among the dikes of the Avalon Boston terrane on the basis of their field, petrographic, and petrochemical characteristics and a limited number of K-Ar ages. The Boston terrane in Massachusetts is divided into three dike zones from north to south as follows: the Cape Ann, Danvers-Dedham, and Boston Basin zones. A new nomenclature is introduced for classifying dike systems into sets, simple swarms, swarms, compound swarms, and dike complexes.

Cross-cutting relations and three K-Ar ages suggest most of the NE-trending dolerites and lamprophyres of the Boston terrane are Mesozoic...