THE SHAWINIGAN OROGEN IN THE ADIRONDACKS AND LINKAGE TO AMCG PLUTONISM

The Shawinigan Orogeny was defined as an accretionary event in south central Grenville Province of Quebec. In the Adirondacks, the orogen spanned ~60 million years (ca. 1200-1140 Ma) culminating in the intrusion of the voluminous Anorthosite-Mangerite-Charnockite-Granite (AMCG) suite. Here the orogen records the closure of a back-arc basin, ophiolite obduction, and collision with an outboard (1350-1300 Ma) arc fragment. Metasedimentary rocks of the Grenville Supergroup track the rift-drift phase of the basin, development of a foredeep and eventual basin restriction, compression, and deposition of sedimentary exhalatives and evaporates. Calc-alkaline magmatic suites (ca. 1200-1180 Ma, Antwerp-Rossie, Herman Granite, and Piseco Lake) are recognized throughout the Highlands, and are abundant in the Central Adirondacks occurring largely along the Piseco Lake shear zone (PLsz). Subduction and oblique collision was followed by strike-slip motion along the PLsz (> 30 km wide) that imparted the structural fabric of the Central Adirondack region; E-W-trending L and L-S mylonites. The PLsz separates ca. 1350 Ma arc rocks of the Southern Adirondack terrane from the Central Adirondacks. Widespread melting and deformation of pelitic gneisses throughout the Adirondacks (ca. 1180-1160 Ma) followed intrusion of arc granitoids. Anatexis partially overlapped intrusion of the AMCG intrusive suite (ca. 1165-1150 Ma), with high-T crustal melts preceding the composite anorthosite massifs. The AMCG suite was emplaced largely to the north of the PLsz, where the rise of the asthenospheric sourced magma was channeled by slab breakoff as subduction transitioned to collisional processes. Left-lateral transpressional shearing, continued during intrusion of the AMCG suite as ductile lithologies folded and warped about rigid anorthosite massifs. Given the relatively short time (60 my), near continuous magmatism, and high-grade metamorphism and deformation, the intrusion of the AMCG suite accompanied Shawinigan orogenesis in the Adirondacks. Remaining problems include differentiation of Shawinigan and Ottawan metamorphism and deformation, evaluating the possibility of Shawinigan orogenic collapse, and the origin of N-S trending structures in the eastern Adirondacks and lithologic similarities with the Lowlands.