STRUCTURAL EVOLUTION OF THE EASTERN ADIRONDACK HIGHLANDS, SOUTHERN GRENVILLE PROVINCE

The Adirondack Highlands expose a large region of uplifted Grenville-age rocks that formed and evolved from ca. 1.13 – 0.95 Ga. Recent 1:24,000 scale mapping of the Eagle Lake quadrangle yields new insight into the Mesoproterozoic structural evolution of the region. The oldest structural fabric (D₁) is only recognized in supracrustal rocks (pelitic and amphibolitic gneisses) of the Grenville Supergroup (thought to be older than about 1.2 Ga, based on previous detrital zircon age data). It is commonly defined by migmatitic layering in the gneisses, both of which contain either garnet and orthopyroxene as a peritectic phase. Given the restriction of D₁ to rocks within the Grenville Supergroup, we interpret that deformation phase to be the result of Shawinigan orogenesis at ca. 1.17 Ga, prior to plutonism of the anorthosite-mangerite-charnockite-granite (AMCG) suite (ca. 1.17 – 1.15 Ga). The second phase of deformation involved isoclinal folding of S₁, magmatic fabric, and sills of ca. 1170 Ma megacrystic granitoids associated with mesoperthite pegmatite. The F₂ fold axes plunge moderately to the SE. Associated with F₂ is a penetrative axial planar fabric (S₂), which is present within the post-Shawinigan AMCG suite (ca. 1.15 Ga). S₂ within AMCG rock types is associated with the development of a metamorphic assemblage containing hornblende + garnet ± clinopyroxene, indicative of granulite facies metamorphic conditions. The third phase of deformation (D₃) involves large amplitude, east-west trending, folds (calculated axis 110°, 12°) and involved folding of preexisting granulite facies metamorphic assemblages and fabrics. The ca. 1.06 Ga Lyon Mountain granite gneiss (LMG) is variably deformed by F₃ folds, and commonly forms plutons within F₃ hinge regions. D₃ folding, and the generation of LMG leucogranite, are interpreted to be syn-kinematic. The LMG crosscuts and contains xenoliths of rocks that contain S₂, suggesting that D₂ deformation must predate emplacement of LMG. D₂ remains the prime candidate as representing the Ottawan orogeny in the region, but better geochronologic constraints and better links between metamorphism, deformation, and absolute time (studies currently in progress) will improve understanding of this important phase of Adirondack tectonism.