STYLE AND TIMING OF FOLDING IN THE WHITEHALL QUADRANGLE, EASTERN ADIRONDACKS: IMPLICATIONS FOR THE TECTONIC HISTORY OF THE ADIRONDACK HIGHLANDS

Large NW-oriented folds and later E-plunging folds have been recognized in the Eastern Adirondack Mountains, but the nature, timing and P-T conditions of folding events are not yet constrained. At least four tectono-metamorphic events are recognized in the region: the 1.2-1.16 Ga Shawinigan Orogeny, 1.16-1.14 Ga emplacement of the AMCG igneous suite, the 1.09-1.05 Ga Ottawan Orogeny and post-Ottawan tectonism. Field mapping and structural/petrographic analysis has been carried out in the western portion of the Whitehall quadrangle, where a kilometer-scale southeast-plunging synform has been hypothesized. Major rock units include: quartzofeldspathic gneisses with varying amounts of pyroxene, hornblende and/or mica, massive to weakly foliated gabbro, and local khondalite (Grt-Sil-Qtz-Fspar, interpreted to be restite), especially near gabbro. It is unclear whether the sequence contains multiple gabbro units or one unit repeated by isoclinal folding. The region can be separated into a western domain that displays a dominant NNE-striking foliation and continuous layering, and an eastern structural domain characterized by complex folding on several scales. Outcrop-scale folds are isoclinal and recumbent with gently East-plunging axes (10°→105°). The presence of folded gabbro layers that cut an earlier gneiss foliation, folded inclusion trails in garnet that is wrapped by a strong foliation, and recumbent style isoclinal folding of khondalite, indicate that the region underwent at least two periods of deformation. Structures and fabrics in both domains suggest that the second deformation event involved intense east-west-directed shearing. Current work involves detailed microstructural analysis, petrologic analysis, geothermobarometry and timing of folding and metamorphism using monazite geochronology.