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The Antwerp-Rossie metaigneous suite (ARS) represents arc magmatism related to closure of the Trans-Adirondack backarc basin during Shawinigan collisional orogenesis (ca. 1200-1160 Ma). The ARS is of calc-alkaline character, bimodal, and lacks intermediate compositions. Primarily intruding marble and pelitic gneiss, the ARS is spatially restricted to the Adirondack Lowlands southeast of the Black Lake fault. On discrimination diagrams, the ARS samples plot primarily within the volcanic arc granite fi elds. Incompatible elements show an arc-like signature with negative Nb, Ta, P, and Zr and positive Cs, Pb, La, and Nd anomalies relative to primitive mantle. Neodymium model ages (T DM , depleted mantle model) range from 1288 to 1634 Ma; the oldest ages (1613-1634) and smallest epsilon Nd (ε Nd ) values are found in proximity to the Black Lake fault, delineating the extent of Laurentia prior to the Shawinigan orogeny. The epsilon Nd values at crystallization (1200 Ma) plot well below the depleted mantle curve. Geochemical and isotopic similarities to the Hermon granitic gneiss (HGG) (ca. 1182 Ma) and differences from the Hyde School Gneiss-Rockport Granite suites (1155-1180 Ma) suggest that arc plutonism rapidly transitioned into A-type AMCG (anorthosite-mangeritecharnockite-granite) plutonism. Given the short duration of Shawinigan subduction, apparently restricted extent of the ARS (Adirondack Lowlands), location outboard of the pre-Shawinigan Laurentian margin, intrusion into the Lowlands supracrustal sequence, bimodal composition, and recent discovery of enriched mantle rocks in the Lowlands, it is proposed the ARS formed as a consequence of subduction related to closure of a backarc basin that once extended between the Frontenac terrane and the Southern Adirondacks.

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Determining the relationship among crustal blocks within an orogen is a key factor in understanding the architecture and construction of that orogen. Within the large, mid-Proterozoic Grenville Province, the relationship between the Adirondack Lowlands and the adjacent Frontenac terrane is ambiguous. Review of previous work demonstrates that the Adirondack Lowlands have different plutonic suites, a lower grade of metamorphism, and a different geochemical signature. However, the timing and kinematics of deformation in the Lowlands, and their relation to major orogenic events, have not previously been well constrained, making comparisons with the Frontenac terrane diffi cult.On the northwestern edge of the Adirondack Lowlands, detailed structural analysis of upper-amphibolite grade migmatites and marbles reveals two penetrative deformation phases. Interference of F 1 and F 2 folds results in Type 3 fold interference patterns and is suffi cient to produce the regional map patterns. The Noname ductile shear zone, a 0.5-2-km-wide northeast-striking steep ductile shear zone with subvertical lineation, developed during D 2 . The steep geometry of the Noname ductile shear zone, paired with consistent sinistral kinematic indicators only found in subhorizontal surfaces, indicate that kinematics for D 2 was sinistral transpression.Sensitive high-resolution ion microprobereverse geometry (SHRIMP-RG) U-Pb zircon geochronology from three granitic samples that have well-defi ned relationships with D 1 and D 2 indicates that both deformation phases developed through continuous or progressive deformation during ca. 1185-1145 Ma. Zircon geochronology from a quartzite, and the presence of melt during all deformation phases, demonstrate that metamorphism was synchronous with deformation.This work reveals that the Shawinigan orogeny (1190-1140 Ma) developed the dominant structural features observed in the northwest Adirondack Lowlands. These structures are the result of the northward collision of a rifted slice of the Laurentian margin (Adirondis) into previously accreted terranes on the margin of Laurentia. Shawinigan deformation of the Adirondack Lowlands may have outlasted that of the Frontenac terrane across any potential terrane-bounding shear zone. While Frontenac terrane and Adirondack Lowlands geology are suffi ciently distinct to warrant separate terrane designation, evidence is lacking to indicate that a suture exists between them.

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