PISECO LAKE GNEISSES RECORD SUBDUCTION-RELATED MAGMATISM AND OBLIQUE COLLISION PRIOR TO AMCG PLUTONISM

The Piseco Lake Gneisses represent an intensively deformed batholithic belt located between AMCG (Anorthosite-Mangerite-Charnockite-Granite) suite rocks of the Central Adirondack Highlands (ca. 1165-1155 Ma) and older tonalitic gneisses of the southern Adirondacks (ca. 1350-1300 Ma). Structurally the rocks form a belt of east-west trending, sinistral L-S tectonites, with a shallowly plunging, stretching lineation that parallels foliation. Spatially associated with foliation-defined, transpressional domes, 1-2 km wide zones of L-tectonite form within the vertical western limb of the Piseco Antiform known as the Piseco Lake Shear Zone. Geochemically the rocks are predominantly megacrystic granites of limited SiO₂ range (70.1±5.7 wt %), consistent Nd systematics (εNd 2.18-3.95; T_DMNd = 1366-1544 Ma), and calc-alkaline arc/subduction zone affinity (anomalies Pb↑; Nd, Ta, Sr, and Ti ↓. The rocks contain abundant high-U zircon with a complicated Pb-loss history. U-Pb analyses by LA-MC-ICPMS have documented concordant ages for four samples. The samples include a megacrystic granite form the Ohio Gorge of the West Canada Creek (1187±8.4; MSWD = 1.5), two samples of megacrystic L-tectonite from Piseco Lake (1182±13; MSWD = 0.42) and (1189±3.5; MSWD = 1.2), and a granitic mylonite where the PLsz abruptly bends to the south at Spier Falls (1179±7.2; MSWD = 0.79). The data indicate the PLsz rocks were emplaced: 1) prior to intrusion of the voluminous Adirondack AMCG suite (ca. 1165-1155 Ma); 2) just prior to widespread anatexis in spatially associated pelitic to psammitic gneisses of the Grenville Supergroup throughout the Lowlands and Highlands (1180-1160 Ma); and 3) coincident with arc-rocks of the Adirondack Lowlands (ca. 1200-1180 Ma). Zircons show abundant growth of Shawinigan rims (ca. 1160) but lack Ottawan (ca. 1050 Ma) zircon growth. In most respects the Piseco Lake Gneisses resemble rocks of the Hermon Granitic Gneiss and Antwerp Rossie suite intruded into the rocks of the Grenville Supergroup in the Adirondack Lowlands at ca. 1200-1180 Ma. They are believed have been emplaced during northward subduction of oceanic crust prior to the Shawinigan Orogeny and subsequently deformed during oblique collision and accretion of the Southern Adirondack Terrane prior to the intrusion of the AMCG suite.