Sally Pehrsson

Collisional Snowbird tectonic zone resurrected: Growth of Laurentia during the 1.9 Ga accretionary phase of the Hudsonian orogeny

The ∼2800-km-long Snowbird tectonic zone is one of the most controversial tectonic features of the Canadian shield. Metamorphic and in situ geochronologic data reported here reveal that a 1.9 Ga medium- to high-pressure belt extends along most of this tectonic zone. In contrast to recent interpretations, a collisional origin is indicated by the length of this metamorphic belt, tectonic thickening documented in parts of it, geological contrasts across it, and subduction-type microdiamonds within it. This collisional event marks a pre–1.865 Ga phase of the Hudsonian orogeny involving microcontinent accretion that was fundamental to the growth of Laurentia.

Metallogeny and its link to orogenic style during the Nuna supercontinent cycle

Abstract The link between observed episodicity in ore deposit formation and preservation and the supercontinent cycle is well established, but this general framework has not, however, been able to explain a lack of deposits associated with some accretionary orogens during specific periods of Earth history. Here we show that there are intriguing correlations between styles of orogenesis and specific mineral deposit types, in the context of the Nuna supercontinent cycle. Using animated global reconstructions of Nunas assembly and initial breakup, and integrating extensive databases of mineral deposits, stratigraphy, geochronology and palaeomagnetism we are able to assess spatial patterns of deposit formation and preservation. We find that lode gold, volcanic-hosted-massive-sulphide and nickel–copper deposits peak during closure of Nunas interior ocean but decline during subsequent peripheral orogenesis, suggesting that accretionary style is also important. Deposits such as intrusion-related gold, carbonate-hosted lead-zinc and unconformity uranium deposits are associated with the post-assembly, peripheral orogenic phase. These observations imply that the use of plate reconstructions to assess orogenic style, although challenging for the Precambrian, can be a powerful tool for mineral exploration targeting. Supplementary material: Supplementary material including (1) tables (S1–S3) of Euler poles and palaeopoles used, summary of Nuna orogens; (2) a figure (S1) of modelled plate velocities; (3) mp4 files (S1 & S2) of the model with age data; ore deposits and VGPs; and (4) a zip file (S1) of the Gplates model is available at http://www.geolsoc.org.uk/SUP18822.

The Corner Brook Lake block in the Newfoundland Appalachians: A suspect terrane along the Laurentian margin and evidence for large-scale orogen-parallel motion

Available geochronological (U-Pb and Ar/Ar) and geological data indicate that the Corner Brook Lake block of the Humber zone in the western Newfoundland Appalachians has unique characteristics. Grenvillian ages (ca. 1.0 Ga), which are typical for the Laurentian Appalachian margin, are absent in the crystalline basement to the Corner Brook Lake block. This makes it unlikely that the block is underlain by true parautochthonous Humber margin basement. However, the lithological makeup of its late Neoproterozoic–Early Cambrian sedimentary cover and detrital zircon populations indicate a Laurentian-type provenance. In addition, there is no geochronological or other geological evidence for a Middle Ordovician tectono-thermal event in the Corner Brook Lake block, suggesting that it escaped the penetrative tectono-thermal effects of the Taconic collision present elsewhere in the Laurentian realm. Instead, the block underwent strong regional deformation and associated peak metamorphism during the Silurian Salinic orogeny. Combined evidence, including the absence of Grenvillian ages, the presence of late Neoproterozoic ca. 600 Ma granitoid plutons, the regionally distinct early Paleozoic tectonic history of the Corner Brook Lake block, and its fault-bounded nature, implies that the block represents an allochthonous terrane. Available data indicate that significant orogen-parallel movement of the block (possibly 400 km or more) could have taken place during the Appalachian orogeny. The possibility of large-scale strike-slip tectonics, in addition to the well-documented convergent motions, has significant implications for the tectonic interpretation of the early Paleozoic evolution of the Newfoundland Appalachians.

Deformation and extensional exhumation of 1.9 Ga high-pressure granulites along the Wholdaia Lake shear zone, south Rae craton, Northwest Territories, Canada

The origin of high-pressure granulites in the south Rae craton and Snowbird tectonic zone (STZ) is highly enigmatic. Current models for their formation and exhumation envisage continental collision at 2.55 Ga and intracratonic orogenesis at 1.9 Ga, or collision and exhumation at ca. 1.9 Ga. As an attempt to reconcile these disparate models, we conducted a regional and detailed mapping program along a geophysical discontinuity 100 km west of the STZ within the south Rae craton of the Northwest Territories, Canada. This work presents the discovery of a new crustal-scale shear zone, the Wholdaia Lake shear zone (WLsz), which deformed and transposed host rocks into a 20-km-wide and 300-km-long ductile high-strain zone. U-Pb zircon geochronology was utilized to establish host-rock crystallization ages, timing of deposition of metasedimentary rocks, and age constraints of metamorphism and ductile shearing. Hanging-wall metasedimentary rocks have a new depositional range of 1.98–1.93 Ga and contain abundant metamorphic zircon at 1.91 Ga. The protoliths of the footwall mafic granulite orthogneisses crystallized at 2.6 Ga and were metamorphosed at 1.9 Ga, which extends the known footprint of 1.9 Ga metamorphism 100 km west of the STZ. During and after 1.9 Ga metamorphism, the WLsz began progressively exhuming footwall rocks in three distinct stages, associated with (1) normal-oblique shearing at high-pressure granulite-facies conditions, (2) normal-oblique shearing accompanied by mylonitization at amphibolite-facies conditions, and (3) normal-oblique shearing with ultramylonite development at amphiboliteto greenschist-facies conditions. Ductile shearing was waning by 1.86 Ga, based on ages obtained from late synto postkinematic crosscutting dikes. Collectively, the WLsz in concert with other regional structures aided both extensional and thrust-sense exhumation of a large high-grade terrane at 1.9 Ga in the south Rae craton. LITHOSPHERE; v. 10; no. 5; p. 641–661; GSA Data Repository Item 2018268 | Published online 31 August 2018 https:// doi .org /10 .1130 /L704 .1