LATE OTTAWAN (CA. 1035 MA) HYDROTHERMAL SIGNATURES IN THE SOUTHEASTERN ADIRONDACK LOWLANDS: NEW GEOCHRONOLOGICAL, STABLE ISOTOPE AND FLUID INCLUSION RESULTS

The southeastern portion of the Adirondack Lowlands province formed the upper plate of a major detachment structure during the tectonic collapse of the Grenville Orogen following the compressional Ottawan Orogeny. Previous work demonstrates that extensional motion on the Carthage-Colton Shear Zone (CCSZ) was contemporaneous with intrusion of Lyon Mountain Granite on the southwest side of the CCSZ at ca. 1035 Ma (Selleck et al. 2005). Within the Adirondack Lowlands immediately to the northwest of the CCSZ, a variety of hydrothermal features are present, and are related to brittle structures in the upper plate. In the vicinity of Harrisville, NY, high-grade ductile shear zones and pegmatites containing ca. 1035 Ma titanite and zircon (Chappell et al. 2006) are associated with hydrothermally-weakened zones of mylonite and ultramylonite. Veins associated with mylonite contain calcite have δ¹⁸O= 11.4 to 22.6‰ SMOW, similar to typical Adirondack marbles. Carbon isotopes are also similar (-8.4 to 1.1‰ PDB), with some samples showing an organic carbon signature. Later epidote-bearing veins cut earlier ductile structures, and contain calcite with meteoric oxygen isotopes (3.2 to 6.5 ‰) and low-salinity fluid inclusions.

Cl-amphibole+scapolite veins and scapolite+tourmaline pegmatites are a common feature within the Adirondack Lowlands immediately northwest of the CCSZ. Zircons from scapolite+tourmaline pegmatites near Hermon and Edwards, NY, have well-preserved xenocrystic cores with U-Pb SHRIMP ages ca. 1140 Ma; ages on igneous rims are discordant but may indicate late Ottawan zircon growth. Near Russell, NY, quartz-scapolite-tourmaline pegmatites with Cl-amphibole halos cut earlier ductile fabrics; quartz-feldspar-calcite-chlorite veins contain ca. 1035 Ma titanite; calcite contains halite-saturated fluid inclusions. Collectively, these features document hydrothermal infiltration into the cooler Adirondack Lowlands during late Ottawan tectonic collapse, with fluids evolving from early CO₂-rich, high-salinity compositions to low-salinity meteoric waters.