Abstract

INTRODUCTION

Massif-type anorthosite complexes, typified by the Adirondack Mardy massif (Fig. 1) consist predominantly of large masses of pure or nearly pure plagioclase rocks (>90% An40–65), with minor facies containing larger proportions of mafic silicates, Fe-Ti–oxide minerals, and apatite (Buddington, 1939, 1969). Within the massifs, purer anorthosite is generally restricted to the central regions, and the percentages of mafic and oxide minerals gradually increase toward the margins. The border zones consist predominantly of leucogabbro and leuconorite (65% to 90% plagioclase), with local segregations of more mafic- and oxide-rich rocks. Mafic-rich dikes are found sporadically throughout the massifs.

Spatially associated with most, if not all, massif-type anorthosite bodies is a suite of orthopyroxene-bearing silicic commonly referred to as the “mangerite-charnockite series” (Fig. 1). Contacts between the anorthosite and mangerite suites may appear sharp, with intimate interlayering (Crosby, 1969), or gradational, with a complete lithologic transition (de Waard and Romey, 1969b; Letteney, 1969; Philpotts, 1966). Mangeritic rocks are in places intrusive into rocks of the anorthosite suite, forming dikes (Seifert, 1978) or extensive intrusive breccias (Buddington, 1953; de Waard, 1970). Locally mangeritic bodies are separated from anorthosite by screens of metasedimentary rocks, or the silicic series may be absent altogether (Buddington, 1969; Davis, 1971). In some localities mangerite-charnockite complexes are spatially removed from anorthosite (Hargraves, 1969).

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