In east-central Vermont and west-central New Hampshire, Siluro-Devonian mafic dikes intrude metasedimentary rocks of the Albee Formation. The dike swarms trend NE and crop out east of the Monroe Fault. The dikes are associated with the Comerford and Leighton Hill diorite-gabbro complexes. The igneous complexes and dikes exhibit mutual cross-cutting relationships and thus appear to represent the same magmatic episode. The Comerford complex has been dated by U-Pb systematics at 419.8 ± 2.6 Ma. Individual, 1 – 5m wide, sheeted dikes exhibit chilled margins against the country rock and against each other. The dikes have been metamorphosed to greenschist facies. Their mineralogy is dominated by albite, amphibole, chlorite, calcite, epidote and quartz. Despite extensive metamorphism, original igneous textures are preserved in some dikes.

The mafic dikes are tholeiitic basalts, based on their major and trace element compositions. Samples from different locations are all geochemically similar, and thus are assumed to be from the same magmatic suite. Rare earth element (REE) patterns are nearly flat to slightly light rare earth element enriched. Systematic trends in Mg-Ni, Ti-Zr, and Zr-Y, as well as a slight negative Eu anomaly in some samples, indicate that crystal fractionation of olivine, plagioclase and possibly pyroxene occurred during magma cooling. The source of the magma was probably asthenospheric mantle, similar to that which produces ocean ridge basalts. Tectonic discriminant diagrams, as well as the geological setting, suggest the magmas erupted in a continental extensional environment.

The dikes were intruded between the Taconic and Acadian orogenies during a time of crustal extension. The cause of the extension is not entirely clear; however, the dike geochemistry is consistent with derivation during back-arc extension behind a west-dipping subduction zone.