Late Proterozoic igneous rocks in the Boston-Avalon zone of eastern Massachusetts are dominated by voluminous calc-alkalic intrusives and extrusives that yield radiometric ages between 600 and 650 Ma. This “main phase” Avalonian volcano-plutonic series, which includes the Dedham Granite and Lynn Volcanics, was emplaced over a time span that was both preceded and followed by separate episodes of mafic volcanism. The Middlesex Fells Volcanic Complex temporally preceded the emplacement of the Dedham and Lynn, whereas the Brighton Volcanics, part of the stratified section of the Boston Basin, clearly postdate them.

Geochemical study of the mafic members of the Middlesex Fells Volcanic Complex, which were regionally and thermally metamorphosed to the greenschist facies, reveals that these alkaiic and transitional basalts have a geochemical signature characteristic of modern continental rift zones. In contrast, flows, pillows, and pyroclastics of the basalts, basaltic andesites, and andesites of the Brighton Volcanics are high-alumina basalts and andesites of calc-alkaline character resembling those from modern subduction-related magmatic regimes.

Magmatism thus provides evidence of the changing tectonic regime during the late Proterozoic in the Avalon terrane of southeastern New England. An initial period of continental rifting preceded the extensive calc-alkaline magmatism associated with the Avalonian orogeny. Subsequently, the terrane experienced a brief period of continental subduction-zone-related magmatism before becoming a stable shelf in the early Paleozoic.