CHEMICAL AND PETROGRAPHIC ANALYSIS INDICATES DIFFERENCES IN THE ORIGIN FOR GRANITIC GNEISSES WITHIN THE POPPLE HILL GNEISS IN THE ADIRONDACK LOWLANDS, NEW YORK

The Popple Hill Gneiss (PHG), a locally migmatitic, biotite-quartz-two feldspar gneiss, is volumetrically the largest non-carbonate lithology in the Adirondack Lowland, lying chronostratigaphically between the Lower Marble and the Upper Marble. However, regionally concordant, discontinuous layers and lenticular bodies, ranging from a few meters to 100’s of meters in width and up to kilometers in length, of chemically and petrographically diverse granitic gneisses are intimately interwoven throughout the PHG, possibly occupying as much as 35-50% of the exposed lithostratigraphic unit. Some of the more definitively identifiable bodies have been designated as Hermon Granite (HG), a microcline megacrystic, well-foliated, calc-alkaline metaintrusive emplaced within the PHG and subsequently deformed during the Shawinigan orogeny (1180-1150 Ma). Relict igneous textures are pervasive, perthite and myrmekite are common, and strain is particularly evident in bent twin lamellae, lenticulation, and tension gashes in feldspars. Chemically, HG granitoids are fairly homogeneous within bodies, but range from 60-78 wt. % SiO₂. Geochemical results from this study match well those published by Peck, et al. (2013), supporting their hypothesis that the HG may have been derived from melting of metasomatized mantle during the collision of the Adirondack Lowlands and Frontenac terrane. However, the majority of the granitic gneiss components of the PHG are more heterogeneous in their color, texture, and composition for major and trace elements. They range from white to pink to darker in color, almost all are inequigranular, fine- to coarse-grained, but lack megacrysts, and many are only weakly foliated. Microcline, plagioclase, and quartz are ubiquitous and biotite is generally a significant accessory. Chemically they constitute a calc-alkaline suite that is largely indistinguishable from the PHG, displaying similar trends on a variety of chemical and tectonic discrimination diagrams. This, combined with the intimate interwoven association between these large scale granitic gneisses and the PHG, leads to the conclusion that they were derived by syntectonic anatexis of lower PHG that was substantially thickened by the Shawinigan collisional event and represent intrusions that were transposed during emplacement.