LATE PROTEROZOIC STRATIGRAPHY AND STRUCTURE IN THE AVALONIAN MAGMATICARC, SOUTHWEST OF BOSTON, MASSACHUSETTS

The Avalon zone in southeastern New England contains calc-alkaline gra nitoids and volcanic rocks formed during Late Proterozoic arc magmatis m. Southwest of Boston, Massachusetts, arc-related units include the D edham Granite, rhyolitic ash-flow tuffs of the Mattapan Volcanic Compl ex, and the Westwood Granite. Roxbury Conglomerate and interbedded Bri ghton Volcanics overlying these ''basement'' units have previously bee n interpreted as distinct rift-related deposits formed during the tect onic transition from Avalonian magmatic arc to stable Cambrian platfor m. The present study shows that several east-northeast-trending faults control thickness changes and clast assemblage variations in the cong lomerate, thus strengthening the case for Late Proterozoic rifting. he calc-alkaline Brighton Volcanics, however, are here expanded beyond t raditionally accepted interbeds near the base of the Roxbury Conglomer ate to include geochemically similar andesites and basaltic anaesites on the west side of the map area formerly assigned to the Mattapan Vol canic Complex. The expanded Brighton Volcanics with minor sedimentary interbeds measure at least 2100 m on the west and interfinger eastward with the Roxbury Conglomerate to form a volcanic-sedimentary complex that is locally conformable on the Mattapan Volcanic Complex. These re lationships indicate continuing arc magmatism throughout Roxbury depos ition. In this context, the Roxbury/Brighton Complex (proposed name) i s interpreted as graben fill in depressions reflecting intra-arc exten sion as found in modern and ancient arc systems. The Avalonian arc/pla tform transition should be sought stratigraphically above the Roxbury/ Brighton Complex.