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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 and, as such, are related to Atlantic rifting. Unlike the eastern North American tholeiites (ENA) of Weigand and Ragland (1970), the Boston-terrane Mesozoic composite swarm also includes alkaline dolerites, transitional alkalic dolerites, and olivine-normative dolerites (latter absent in ENA dikes north of Maryland). Boston-terrane dikes also have much higher average TiO2 and K2O contents and lower average SiO2 and MgO contents (for quartz- and olivine-normative dikes, respectively). These differences indicate the Mesozoic Bostonterrane dolerites were derived from different magma sources than the ENA dikes and are probably a southern extension of the Mesozoic Coastal New England igneous province of McHone and Butler (1984).

Unlike the rest of New England, the majority of the Boston-terrane dikes trend northwest rather than northeast. This, along with field evidence and 7 K-Ar ages, suggests most of the NW-trending dikes are Paleozoic, with some possibly late Proterozoic. A number of dikes (andesites, diorites, hybridized dolerites, and a few dolerites) appear to have intruded into mobile granitic magmas and are undoubtedly the same age as the late Proterozoic and Silurian or Ordovician granites they intruded. Other dike segments and irregular bodies of diorite and hybridized dolerite occur as xenoliths in the Cape Ann Granite and themselves contain very coarse to pegmatitic anorthosite xenoliths. A NW-trending swarm of felsic dikes in the Cape Ann zone is probably associated with the Cape Ann Granite. Dacite and andesitic dikes in the Danvers-Dedham zone appear to be late Proterozoic to early Paleozoic in age. Most of the remaining NW-trending (and E-W–trending) dolerite dikes are believed to be middle to late Paleozoic.

The dominant northwest trend is consistent with intrusion into fissures opened during oblique NE-SW extension associated with Paleozoic (and Proterozoic?) pulses of right-lateral movements within the NE-trending Nashoba thrust belt (and bounding Bloody Bluff fault) immediately west of the Boston terrane. Alternatively, the NW-trending swarms may have been intruded during Paleozoic (and Proterozoic?) episodes of crustal rifting (or aborted rifting) along northwest trends.

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