SYNTHESIS OF THE MESOPROTEROZOIC BASEMENT OF THE SOUTHEASTERN ADIRONDACK HIGHLANDS, ESSEX COUNTY, NEW YORK

The Adirondack Highlands are a southern extension of the Grenville Province in Canada. The 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. 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 of the Mesoproterozoic basement.

Granulite facies Mesoproterozoic paragneiss, marble, and amphibolite hosted the emplacement of anorthosite-mangerite-charnockite- granite (AMCG) suite rocks at ~1.18–1.15 Ga. The earliest of four phases of deformation (D1), expressed by a cryptic gneissosity and migmatization in the host rocks, pre-dated AMCG magmatism. D2 deformation and associated metamorphism produced a composite penetrative gneissosity, migmatite, and isoclinal F2 folds in AMCG rocks and older paragneiss. Near the end of D2, felsic magmatism, including the magnetite-bearing Lyon Mountain Granite Gneiss (LMG), intruded previously deformed rocks, and spread by penetrative migration as semi-concordant alkali feldspar granite sheets subparallel to S2. The LMG crystallized at ~1.15-1.14 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 occurred during D3 along a mylonitic extensional detachment, largely as an amagmatic 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. Deformation and metamorphism spanned the Shawinigan (1.2-1.14 Ga), Ottawan (1.09-1.03 Ga), and Rigolet (1.01-0.98 Ga) phases of the Grenville orogenic cycle.