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A new stratigraphic and structural synthesis is presented for Precambrian rocks of the Adirondack Mountains, New York, an amphibolite-granulite facies terrane in the 1.1-b.y.-old Grenville province exposed in a dome on the North American craton. The geology of the Adirondacks appears to be explicable in terms of a stratigraphic sequence that has been subjected to multiple folding, metamorphism, and intrusive activity. This stratigraphic sequence is correlated across the entire width of the Adirondacks and westward into Ontario. Recognition of the widespread nature of this stratigraphic sequence has resulted in a coherent structural framework for the Adirondacks, consisting of two stages of nappe formation followed by three stages of upright to overturned folding. Both widespread intrusive activity and subsequent mylonitization of intrusive rocks occurred mostly during the second phase of nappe formation.

The stratigraphy of the Adirondacks is interpreted to consist of an older granitic basement, referred to as the Piseco Group, overlain unconformably by a metamorphosed clastic/carbonate sequence, referred to as the Oswegatchie Group in the northwest and the Lake George Group in the east. The oldest recognized formation in the Piseco Group is the Pharaoh Mountain Gneiss, consisting of charnockitic and granitic gneiss. This unit is overlain in many places by the Alexandria Bay Gneiss, consisting of pink leucogranitic gneiss. The Alexandria Bay Gneiss is equivalent to the Brant Lake Gneiss in the eastern Adirondacks.

The basal formation of the metasedimentary rocks of the Oswegatchie Group is the Baldface Hill Gneiss. This thin and discontinuous unit consists of garnet-sillimanite gneiss and quartzite. The overlying Poplar Hill Gneiss consists of biotite-quartz-plagioclase gneiss that contains granitic portions. Rocks of the Baldface Hill and Poplar Hill may represent metamorphosed basal quartz sand and conglomerate, shale, shaly arkose, and possibly reworked Fe- and Al-rich regolith that was formed by weathering of the basement prior to deposition of the cover rocks. The Baldface Hill and Poplar Hill are equivalent to the Eagle Lake Gneiss of the Lake George Group.

Overlying these thin basal clastic deposits of the Oswegatchie Group is the Gouverneur Marble that consists of five members, two of which contain three subdivisions within them. Member A at the base consists of thick, calcitic, dolomitic, and siliceous marbles. Member B is a thin, pyritic biotite schist. Member C consists of interbedded siliceous marbles, quartzites, and calc-silicate rocks. Member D consists of well-layered calcareous gneiss, and Member E, only locally present, is a quartz-feldspar granulite. To the east, the Gouverneur Marble correlates with carbonate rocks of the Cedar River, Blue Mountain Lake, and Paradox Lake Formations, and correlates via facies changes to metamorphosed calcareous clastics of the Cranberry Lake, Sacandaga, Tomany Mountain, and Springhill Pond Formations. The previously defined upper and lower marble may be stratigraphically equivalent in the Northwest Lowlands and possibly in the Adirondack Highlands.

The Pleasant Lake Gneiss overlies the Gouverneur Marble and consists largely of migmatitic gneiss equivalent to the Treadway Mountain Formation of the Lake George Group. K-feldspar megacrystic granitic gneisses overlie the Pleasant Lake Gneiss. These rocks are equivalent to the Lake Durant Formation in the Lake George Group and probably represent intrusive sheets.

Anorthosite, charnockite, hornblende granite, and gabbro successively intruded the metamorphosed sedimentary rocks and themselves were later metamorphosed and deformed. Mangerite-charnockite suites that mantle anorthosite contain xenoliths of anorthosite and are thought to be produced by partial melting of Pharaoh Mountain Gneiss by heat from the anorthosite. Megacrystic hornblende granitic gneisses intrude various formations of the Lowlands and Highlands but show gross structural concordance.

Five phases of folding affected all stratigraphic units, but only the last four phases affected the intrusive rocks. The first phase of folding resulted in northwest-directed nappes and formation of regional foliation and lineation. A second phase of isoclinal folding folded the regional foliation and lineation. Intrusion of anorthosite, charnockite, hornblende granite, and gabbro accompanied second-phase folding, as well as local mylonitization of charnockite and local thrusting. The third-phase folds are upright to overturned and responsible for the “grain” of the Adirondacks. The axial traces of these folds form an arc convex to the north that swings continuously from N70°W to east-west to N45°E from south to northwest. Peak 1.1 to 1.02-b.y.-old granulite facies metamorphism outlasted third-phase folding in the Adirondack Highlands and second-phase folding in the Lowlands. Fourth-phase, northwest-trending folds are open and best developed in the northwestern Adirondacks, where they are associated with retrograde metamorphism and possibly with intrusion of diabase dikes at mid-amphibolite facies. Fifth-phase, north-northeast-trending folds are open and best developed in the Adirondack Highlands, where they are associated with retrograde metamorphism and with 930-m.y.-old pegmatite dikes. The fourth- and fifth-phase folds interfere with third-phase folds to produce dome and basin map patterns. Hook and heart and anchor map patterns result from interference of the later folds with first- or second-phase isoclinal folds.

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