THERMOCHRONOLOGY OF METAPELITES IN THE EASTERN ADIRONDACK MOUNTAINS, NEW YORK

Anatexis in metapelites, a widespread phenomenon throughout the Adirondack Mountains, has conventionally been associated with ca. 1020-1040 Ma Ottawan orogeny and high-grade metamorphism. Recent SHRIMP studies of zircon populations of migmatitic metapelites in the Adirondack Lowlands and western Highlands indicate that detrital zircons were derived exclusively from Elzevirian (ca. 1280-1340 Ma) sources, and, further, that leucosomes were formed about 1170 Ma, coeval with the Shawinigan orogeny. No zircons, or zircon overgrowths, of Ottawan age were found, indicating either that temperatures sufficient for zircon growth were not attained or that fluids necessary for zircon growth were not present. In contrast, SHRIMP studies of a new suite of migmatitic metapelites from the eastern Adirondacks reveal a similar suite of Elzevirian detrital zircons, but show zircon growth - either as overgrowths or discrete grains - during both Shawinigan (1140-1170 Ma) and Ottawan (1020-1050 Ma) orogenies. These results, which are supported by EMP dating of monazites, suggest that rocks exposed in the eastern Adirondacks were deeper, and thus at higher temperatures, during the Ottawan orogeny, or that fluids were more abundant in the eastern Adirondack region. Exposure of a deeper crustal level in the eastern Adirondacks implies either differential, east-side higher, tilting of the Adirondack dome, or uplift of the eastern portion of the Adirondacks along a major thrust fault. The thrust-fault hypothesis is consistent with the occurrence of a recently discovered, east-dipping, highly mylonitized quartzite-pegmatite zone in the southeastern Adirondacks. The Canada Lake fold nappe exposed west of Sacandaga Reservoir is repeated across this structure, indicating that it is a fault. When projected to the north (consistent with constraints due to folding), the fault appears to underlie much of the eastern Adirondack metapelitic sequence. In contrast to the rest of the Adirondack Highlands, this eastern region contains secondary muscovite indicating the availability of late fluids. We suggest that the proposed thrust served as a conduit for such fluids and, thus, for the increased incidence of late Ottawan anatexis.