MAGMATISM AS A GUIDE TO SILURO-DEVONIAN TECTONIC SETTINGS OF THE NASHOBA AND COMPOSITE AVALON TERRANES IN EASTERN MASSACHUSETTS

The Nashoba and Avalon (sensu stricto) terranes in eastern Massachusetts are peri-Gondwanan terranes with extensive, but contrasting, Siluro-Devonian magmatic histories. On the east, granitic magmatism of this age in the composite Avalon terrane is of classic A-type consisting of Late Ord. to Early Silurian alkalic (Cape Ann) and riebeckite-bearing peralkaline granitoids (Quincy, Blue Hills) and Late Silurian-Devonian alkalic granites (Peabody, Rattlesnake Hill, Wenham). In recent years, the bi-modal nature of this magmatism has been documented as a significant number of Siluro-Devonian alkalic dioritic to gabbroic plutons have been recognized, particularly along the western margin of the composite Avalon terrane.

In contrast to the alkalic magmatism of the Avalon terrane, the more highly metamorphosed and deformed Nashoba terrane was, at the same time, experiencing intrusion by a suite of calc-alkaline quartz diorites and widespread peraluminous granitoids. The diorites (Sharpners Pond, Straw Hollow, Assabet Quartz Diorite, dioritic phase of the Indian Head Hill pluton) range in age from Early Silurian to Mid-Devonian. The granites, largely included in the Andover Granite, consist of an older foliated phase that may be as old as Late Ord., and a younger, unfoliated, commonly pegmatitic phase of Early Devonian age.

Siluro-Devonian magmatism in these terranes occurred at the time of general consolidation of the terranes along the eastern side of the northern Appalachians, the Acadian Orogeny, and the development of major shear zones in eastern New England. Alkalic magmatism in the Avalon terrane in eastern Massachusetts is thought to have been generated in an extensional-transtensional setting within the overall compressional regime by intrusion of mafic magmas and the melting of the deep, dry crust. The calc-alkalic nature of the intermediate composition magmas in the Nashoba terrane indicates the presence of Siluro-Devonian subduction, likely oblique, during the closure of the Nashoba terrane with the Merrimack terrane to the west and/or the Avalon terrane. This interpretation is supported by the presence of calc-alkaline volcanic rocks in a fault sliver between the Nashoba and Avalon terranes in eastern MA and the generally high temperature lower pressure Silurian metamorphism in the Nashoba terrane.