Age and tectonic setting of neoproterozoic granitoid rocks, antigonish highlands, Nova Scotia, Canada: Implications for avalonia in the northern appalachian orogen

Abstract

The oldest rocks in the Avalonian Antigonish Highlands of northern mainland Nova Scotia, Canada, are late Neoproterozoic (>618 Ma) volcanic and sedimentary rocks of the Georgeville Group intruded by gabbroic/dioritic to granitic plutons. New U-Pb zircon ages presented here for five samples from plutons intruded into the James River and (or) Keppoch formations of the Georgeville Group have ages ranging from ca. 615 to 604 Ma. They have petrological characteristics of expanded calc-alkalic “Andean-type” suites but are compositionally biased toward evolved high-silica compositions and are interpreted as evolved I-type granites. They were emplaced at shallow depths and some were likely comagmatic with felsic volcanic components of their host rocks. These plutons are younger and show less varied Sm-Nd isotopic compositions than most plutonic rocks formed in the early Ediacaran “main arc phase” elsewhere in Avalonian terranes in the northern Appalachian orogen, although they are similar in age to plutons in southeastern New England and in the Bass River and Jeffers blocks of the Cobequid Highlands, Nova Scotia. The Jeffers block of the Cobequid Highlands appears to be most similar to the Antigonish Highlands but both areas record a Neoproterozoic history less protracted than in other parts of Avalonia.