Introduction

The Adirondack Mts (Fig. 11.1) consist of a NNE-trending, elongate dome of high-grade Proterozoic gneisses that connect with the Grenville Province of Canada via the Frontenac Arch across the St Lawrence River. The region is divided into the Adirondack Highlands and Lowlands with the boundary between the two defined by a belt of high strain referred to as the Carthage–Colton mylonite zone (CCMZ, Fig. 11.1). This zone is believed to be continuous with the Chibougamau–Gatineau line that separates the Central Metasedimentary Belt from the Central Granulite Terrane of the Grenville Province (Geraghty et al., 1981). The Lowlands consist primarily of metasedimentary rocks, notably marbles, now at upper amphibolite grade; while the Highlands, which are dominated by orthogneisses with intervening synclinal keels, are predominantly at hornblende–granulite facies grade. Although metamorphic grade appears to be continuous across the CCMZ, continuity of lithic units across the zone remains uncertain and difficult to assess.

During recent years substantial progress has been made in recognizing and documenting the complex, polyphase deformation that characterizes both the Adirondack Highlands and Lowlands. These results are summarized by McLelland and Isachsen (1980,1985,1986) who emphasize the importance of large fold nappes in the structural framework of the region. Petrologic studies of high-grade metamorphism accompanying deformation have been summarized by Bohlen & Essene (1977) and Bohlen et al. (1985).