Metasomatic origin of large parts of the Adirondack Phacoliths

A metasomatic origin seems established for large parts of the granite phacoliths in the northwest Adirondack Mountains, New York. This conclusion is based upon the discovery and detailed mapping of a blurred but widespread stratigraphic sequence in the phacoliths. Highly complicated patterns of relict beds are defined by alternations of granitic gneiss, amphibolite, oligoclase-quartz gneiss, and a predominant alaskitic granite. Regional reconstructions indicate these relict beds comprise a major basal formation, probably of arkosic and calcareous quartzites, in the exposed Grenville metasedimentary rocks. The existing amphibolite interlayers formed early in the metasomatic epoch, probably replacing the more calcareous quartzite beds. This mafic metasomatism was overlapped and followed by pervasive granitization of the arkosic and highly quartzose members. Pink alaskitic granite is the final metasomacic product. It tends to replace all pre-existing rock types, especially in the cores of the antiforms. This mafic and granitic metasomatism occurred during the evolution of the major folds in the metasedimentary sequence. As the folds evolved, rising domes in the quartzite probed upward into successively higher horizons in the overlying marbles. At least three quartzite antiforms (California, Clark Pond, and South Edwards phacoliths) punctured the overlying marble, invading the basal layers of the Adirondack paragneiss. The crests of the evolving folds in the basal quartzite, capped by carbonate-rich marble, acted as traps for aqueous and carbonated, alkali-bearing fluids, and perhaps some associated anatectic granitic magma generated in the deeper, hotter basement to the Grenville.