NEW BEDROCK GEOLOGIC MAPPING IN THE ADIRONDACK HIGHLANDS, NORTHERN NEW YORK

The Adirondack Highlands are underlain by complexly deformed Mesoproterozoic crystalline rocks that present significant challenges for geologic mapping and assessment of natural and environmental resources. The vast ~30,000 km² area has the lowest density of peer-reviewed 1:24,000-scale bedrock geologic maps in the northeastern United States. New detailed bedrock geologic mapping in southern Essex County, New York, integrates lidar, geophysics, geochemistry, petrology, geochronology, and economic geology to improve our understanding of the framework geology.

Granulite facies Mesoproterozoic paragneiss, marble, and amphibolite were intruded by anorthosite-mangerite-charnockite-granite (AMCG) plutonic suite rocks at about 1.18-1.15 Ga during the Shawinigan orogeny. The earliest of four phases of ductile deformation (D1) is characterized by gneissosity, rarely preserved F1 isoclinal folds, and migmatite in both AMCG and host rocks. Subsequent D2 deformation produced a composite penetrative gneissosity, migmatite, and large isoclinal F2 folds. Syn- to late-D2 felsic magmatism, that included the regionally extensive Lyon Mountain Granite Gneiss (LMG), intruded as semi-concordant alkali feldspar granite sheets emplaced by pervasive migration, sub-parallel to S2. Contrary to previous interpretations, the LMG is recently shown to have crystallized at about 1.15 Ga marking the end of AMCG magmatism. The LMG locally displays syn-kinematic F2 folds that constrain the time of D2 deformation. Exhumation of the crust during the Ottawan orogeny produced D3 regional domes and basins, local re-activation of the S2 foliation, partial melting, granulite to upper amphibolite facies metamorphism, metasomatism, iron ore re-mobilization, and intrusion of magnetite-bearing pegmatite both as layer-parallel sills and cross-cutting dikes. D4 created NE- and NW-trending boudins, granulite to upper amphibolite facies ductile shear zones, and cross-cutting granitic pegmatite dikes. Kilometer-scale lineaments readily seen in lidar data are both Ediacaran mafic dikes that are truncated by the Great Unconformity and Phanerozoic brittle faults. This complex geologic framework provides insight for ongoing investigations of historically significant iron and graphite deposits in the Adirondacks.