Graham B. Baird

Introduction: New developments in Grenville geology: In honor of James McLelland

The Grenville Province exposes the interior of an ancient mountain belt whose scale rivals the modern Himalayan-Alpine Orogen ([Rivers, 2008][1]; [McLelland et al., 2010][2]). The geology of this vast tectonic collage holds a record of mid-Proterozoic continental margin dynamics, including the

Shawinigan arc magmatism in the Adirondack Lowlands as a consequence of closure of the Trans-Adirondack backarc basin

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 (TDM, depleted mantle model) range from 1288 to 1634 Ma; the oldest ages (1613–1634) and smallest epsilon Nd (eNd) 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.

Intrusive age and geochemistry of the Kebne Dyke Complex in the Seve Nappe Complex, Kebnekaise Massif, arctic Sweden Caledonides

The Seve Nappe Complex of the Scandinavian Caledonides is predominately metamorphosed rift-related igneous and sedimentary rocks formed during Rodinia breakup in the Neoproterozoic. The Kebne Dyke Complex of the Kebnekaise Massif, arctic Sweden, is one such unit within the Seve Nappe Complex and is mostly composed of dolerite dykes metamorphosed and thrust onto Baltica during the Caledonian orogeny. Structurally adjacent to the dyke complex are geochemically similar amphibolites that have a common origin as the metadolerite dykes. Collectively, these rocks have transitional mid-ocean ridge basalt geochemistry similar to correlative Seve Nappe Complex rocks to the north (Indre Troms dykes) and south (Sarek Dyke Swarm). U–Pb single-crystal chemical abrasion–thermal ionization mass spectrometry of zircon from a metagabbro and a metagranitoid, showing co-mingling magmatic textures with the metadolerite, produced 11 concordant analyses with 206Pb/238U ages ranging from 608 to 596 Ma, which includes the magmatic age of the dyke complex. These results support the idea that the Kebne Dyke Complex, Sarek Dyke Swarm and Indre Troms dykes constitute the tholeiitic continent–ocean transition subdivision within the Seve Nappe Complex. However, the Kebne Dyke Complex differs from the Sarek Dyke Swarm and Indre Troms dykes as it is the least enriched among these, nearly lacks rift-related metasedimentary rocks, and may be younger than the Sarek Dyke Swarm by up to 14 Myr.

Timing and kinematics of deformation in the northwest Adirondack Lowlands, New York State: Implications for terrane relationships in the southern Grenville Province

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 difficult. 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 sufficient 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 microprobe–reverse geometry (SHRIMP-RG) U-Pb zircon geochronology from three granitic samples that have well-defined 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 sufficiently distinct to warrant separate terrane designation, evidence is lacking to indicate that a suture exists between them.