BEDROCK GEOLOGIC MAPPING IN THE ADIRONDACK HIGHLANDS, NEW YORK

The Adirondack Highlands are a southern extension of the Grenville Province of Québec. These complexly deformed Mesoproterozoic crystalline rocks present significant challenges for geologic mapping. 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. Advanced new detailed bedrock geologic mapping in Essex County, New York and adjacent Vermont integrates lidar, geophysics, geochemistry, petrology, geochronology, and economic geology to improve our understanding of the framework geology.

Granulite facies Mesoproterozoic paragneiss, marble, and amphibolite hosted the emplacement of charnockite-mangerite-granite (MCG) suite rocks at approximately 1.18–1.15 Ga. The earliest of four phases of deformation (D1) pre-dated MCG magmatism and is represented by gneissosity and migmatite in the paragneiss host rocks. D2 deformation produced a composite penetrative gneissosity, migmatite, and isoclinal F2 folds in MCG rocks and older paragneiss. Near the end of D2, felsic magmatism, including the magnetite-bearing granites of the Lyon Mountain Granite Gneiss (LMG), spread by penetrative migration as semi-concordant alkali feldspar granite sheets subparallel to S2 into previously deformed lithologies. The LMG crystallized at approximately 1.15 Ga and displays late syn-kinematic F2 folds, schlieren, abundant screens and xenoliths, and previously unmapped complex internal F2 and F3 folds. Exhumation of the Marcy anorthosite massif, and regionally the entire Adirondack Highlands, occurred during D3 along a mylonitic extensional detachment, as a type of core complex, and was accompanied by large scale boudinage. Protracted D3 produced regional domes and basins, reactivation of the S2 foliation, partial melting, metamorphism, metasomatism, ore mobilization, and intrusion of magnetite-bearing pegmatite both as layer-parallel sills and crosscutting dikes. Finally, D4 created NE- and NW-trending boudinage, continued local high-grade ductile shear zones, and crosscutting granitic pegmatite dikes at around 1.03-1.0 Ga. Protracted deformation and metamorphism spanned the Shawinigan (1.2-1.14 Ga), Ottawan (1.09-1.03), and Rigolet (1.01-0.98 Ga) phases of the Grenville orogenic cycle.