Age, Field, and Petrological Relationships of the Hyde School Gneiss, Adirondack Lowlands, New York: Criteria for an Intrusive Igneous Origin

Alaskitic and tonalitic rocks constituting Hyde School Gneiss (HSG) occur in 14 domical bodies in the Adirondack lowlands. Recent models have interpreted these bodies as metamorphosed rhyolitic and dacitic ash-flow deposits forming the basal member of a regional stratigraphic package. In contrast, this paper presents criteria and evidence for an intrusive origin for HSG. Field evidence includes intrusion breccias and complex crosscutting relationships involving mafic layers resembling synplutonic dikes. Petrological constraints supporting an intrusive origin include: (1) the common occurrence of quartz-mesoperthite hypersolvus assemblages; (2) magmatic features in tonalitic, trondhjemitic, and alaskitic facies; (3) local occurrences of orthopyroxene in all facies of HSG; and (4) the development of marginal garnet-sillimanite gneiss with corundum-spinel-garnet-sillimanite assemblages yielding paleotempera-tures of 780°-810°C and interpreted as restite remaining after anatexis of country rock metapelite by intrusions of hypersolvus granitoids. U-Pb zircon ages reported here suggest that the majority of the HSG was intruded at ca. 1230 Ma, contemporaneous with high-grade metamorphic activity in the Adirondacks and elsewhere in the SW sector of the Grenville Province. Geochronological evidence from a leucogranitic rock crosscutting metasediments on Welles-ley Island suggests that these metasediments are older than 1416 ± 5 Ma.