PROTOLITH DETERMINATION OF THE HYDE SCHOOL GARNET-SILLIMANITE MARGINAL GNEISSES, ADIRONDACK LOWLANDS, NY

Elemental and carbon isotope analysis was performed on the distinctive garnet-sillimanite gneisses, which envelope the Hyde School Gneiss (HSG), located in the NW Adirondack Lowlands. The findings bolster the intrusive model for this Mesoproterozoic suite, opposed to a metavolcanic origin. The trends observed from variation diagrams coupled with REE plots suggest the garnet-sillimanite marginal mylonite is genetically linked to the alkali-granitic lithology of the HSG through magmatic differentiation. The garnetiferous margins are LREE-enriched and some have negative europium anomalies. The garnet-sillimanite gneisses do not show the HREE-enrichment, which is expected if there had been voluminous melt extraction from these garnet-rich rocks. Also, significant variability and low values of carbon isotope ratios of graphite from the HSG margin rocks challenges the importance of marble assimilation during the formation of the garnet-sillimanite gneisses. The enrichment of Fe and Al within the marginal mylonite is most likely due to hydrothermal alteration. Determining the protolith of the Hyde School Marginal Gneisses aids in the reconstruction of the crustal conditions along the Black Lake Shear Zone, which is theorized to have played an important role in the assemble of the Adirondack Highlands and Lowlands.