Tectonic Setting and Evolution of the Grenville Orogen: An Assessment of Progress Over the Last 40 Years

The Grenville Province is known for its high grade of metamorphism, complex ductile gneissic structure, and polyphase reworking, features indicative of residence in the deep crust (orogenic infrastructure) that hamper recognition of protoliths and original relationships and render tectonic interpretations especially challenging.  This paper charts the evolving understanding from the ‘Grenville Problem’ of the 1950s before the plate tectonic paradigm, through a speculative quasi-plate tectonic stage in the 1970s that effectively proved to be a dead end, and the first constrained plate tectonic models for pre-Grenvillian Laurentia in the 1980s, to the recent LHO (large hot orogen) and collapsed LHO models for the Grenville Orogen itself. The collapsed LHO model is based on the finding that significant amounts of the superstructure (upper orogenic crust) are preserved, and that the present crustal architecture can be explained by tectonic juxtaposition of infrastructure and superstructure in a late extensional event associated with crustal-scale collapse of a high-strain channel under an orogenic plateau. Conceptual breakthroughs and critical datasets assembled in the period 1980–2000 that were influential in guiding tectonic thinking are discussed and it is argued that present understanding was contingent on the results of 2-D numerical forward modelling of orogenesis, in particular the LHO experiments and the more recent models of orogenic collapse. As a result, for the first time a conceptual plate tectonic model for the convergence and collapse stages of the Grenville Orogen based on empirical field data (the inverse model) is broadly supported by numerical forward-modelling experiments constrained by physically plausible processes in a LHO – and both are available for future testing and refinement. Moreover, they may also have application to other enigmatic high-grade Proterozoic orogens that have resisted simple incorporation within the plate tectonic narrative.  SOMMAIRELa Province de Grenville est bien connue pour le métamorphisme élevés de ses roches, leur structure ductile gneissique complexe de ses roches et leur remaniement polyphasé, caractéristiques qui correspondent à un séjour dans la croûte profonde (infrastructure orogénique) ce qui gène la reconnaissance des roches d’origine et leurs relations, et rendent particulièrement difficile les interprétations tectoniques.  Le présent article  retrace l’évolution de la compréhension du « problème du Grenville », à partir des années 1950, avant l’avènement du paradigme de la tectonique de plaques, en passant par l’étape d’une interprétation quasi- tectonique de plaques des années 1970, laquelle s’est avérée une impasse, puis par les premiers modèles balisés de tectonique de plaques de la Laurentie pré-grenvillienne des années 1980, jusqu’aux modèles récents des grands orogènes chauds (LHO) et de LHO d’effondrement visant à expliquer l’orogène de Grenville lui-même.  Le modèle de LHO d’effondrement repose sur le fait que des portions importantes de la superstructure (croûte orogénique supérieure) sont préservées, et que l’actuelle architecture crustale peut s’expliquer par la juxtaposition tectonique de l’infrastructure et de la superstructure lors d’une phase d’extension tardive associée à un effondrement à l’échelle de la croûte d’un canal de fortes contraintes sous un plateau orogénique.  Nous présentons ici les percées conceptuelles ainsi que les bases de données essentielles constituées de 1980 à 2000 qui ont orienté la réflexion tectonique, et nous proposons que la compréhension actuelle découle des résultats de la modélisation prospective numérique 2-D de l’orogenèse, en particulier des expériences LHO et des modèles plus récents d’effondrement orogénique.  Et donc, pour la première fois, nous disposons d’un concept de modèle de tectonique de plaques permettant d’expliquer les phases de convergence et d’effondrement de l’orogène de Grenville qui découle de données empiriques de terrain (modèle inverse), et qui correspond largement aux résultats de modélisations prospectives numériques balisées conformes aux processus physiques d’un LHO, les deux étant disponibles pour essais et affinement.   En outre, ils peuvent aussi être appliqués à d’autres orogènes protérozoïques de nature semblable et qui n’ont pu s’expliquer par la logique de plaques tectoniques

Post-peak Evolution of the Muskoka Domain, Western Grenville Province: Ductile Detachment Zone in a Crustal-scale Metamorphic Core Complex

The Ottawa River Gneiss Complex (ORGC) in the western Grenville Province of Ontario and Quebec is interpreted as the exhumed mid-crustal core of a large metamorphic core complex. This paper concerns the post-peak evolution of the Muskoka domain, the highest structural level in the southern ORGC that is largely composed of amphibolite-facies straight gneiss derived from retrogressed granulite-facies precursors. It is argued that retrogression and high strain occurred during orogenic collapse and that the Muskoka domain acted as the ductile detachment zone between two stronger crustal units, the underlying granulite-facies core known as the Algonquin domain and the overlying lower grade cover comprising the Composite Arc Belt. Formation of the metamorphic core complex followed Ottawan crustal thickening, peak metamorphism and possible channel flow, and took place in a regime of crustal thinning and gravitational collapse in which the cool brittle–ductile upper crust underwent megaboudinage and the underlying hot ductile mid crust flowed into the intervening megaboudin neck regions. Post-peak crustal thinning in the Muskoka domain began under suprasolidus conditions, was facilitated by widespread retrogression, and was heterogeneous, perhaps attaining ~90% locally. It was associated with a range of ductile, high-temperature extensional structures including multi-order boudinage and associated extensional bending folds, and a regional system of extension-dominated transtensional cross-folds. These ductile structures were followed by brittle–ductile fault propagation folding at higher crustal level after the gneiss complex was substantially exhumed and cooled. Collectively the data record ~60 m.y. of post-peak extension on the margin of an exceptionally large metamorphic core complex in which the ductile detachment zone has a true thickness of ~7 km. The large scale of the core complex is consistent with the deep level of erosion, and the long duration of extensional collapse is compatible with double thickness crust at the metamorphic peak, the presence of abundant leucosome in the mid crust and widespread fluid-fluxed retrogression, collectively pointing to the important role of core complexes in crustal cooling after the peak of the Grenvillian Orogeny.RÉSUMÉLe complexe gneissique de la rivière des Outaouais (ORGC) dans la portion ouest de la Province de Grenville au Québec et en Ontario est interprété comme le cœur d’un grand complexe métamorphique à coeur de noyau. Le présent article porte sur l’évolution post-pic du domaine de Muskoka, soit le niveau structural le plus élevé de l’ORGC composé en grande partie d’orthogneiss au faciès amphibolite dérivés de précurseurs au faciès granulite. Nous soutenons que la rétromorphose et les grandes déformations se sont produites durant l’effondrement orogénique et que le domaine de Muskoka en a été une zone de détachement ductile entre deux unités crustales plus résistantes, le cœur au faciès granulite sous-jacent étant le domaine Algonquin, et la chapeau sus-jacent à plus faible grade de métamorphisme comprenant le Ceinture d’Arc Composite. La formation du complexe métamorphique à coeur de noyau est survenue après l’épaississement crustale ottavien, le pic métamorphique et le possible flux en chenal, et s’est produit en régime d’amincissement crustal et d’effondrement gravitationnel au cours duquel la croûte supérieure refroidie a subit un mégaboudinage et où la croûte moyenne chaude et ductile sous-jacente a flué dans les régions entre les mégaboudins. L’amincissement crustale post-pic dans le domaine de Muskoka, qui a débuté en conditions suprasolidus, a été facilité par une rétromorphose généralisée, hétérogène, atteignant à peu près 90 % par endroits. Celle-ci a été associée avec une gamme de structures d’extension ductiles de haute température, incluant du boudinage de plusieurs ordres de grandeur et de plis de flexure d’extension, ainsi qu’un système régional de plis croisés d’origine transtensionnelle. À ces structures ductiles a succédé une phase de plissement de propagation de failles cassantes à ductiles à un plus haut niveau crustal, après que le complexe gneissique ait été exhumé et se soit refroidi. Prises ensemble, les données indiquent une extension post-pic sur la marge d’un complexe métamorphique à coeur de noyau exceptionnellement grand aux environs de 60 m.y. et dans laquelle la zone de détachement montre une épaisseur véritable d’environ 7 km. La grandeur de l’échelle du complexe métamorphique à coeur de noyau concorde avec le fort niveau d’érosion, et la grande durée de l’effondrement d’extension est compatible avec une croûte de double épaisseur au pic de métamorphisme, la présence de leucosomes abondants dans la croûte moyenne et d’une rétromorphose à flux fluidique généralisée, l’ensemble indiquant l’importance du rôle des complexes métamorphiques à coeur de noyau dans le refroidissement de la croûte après le pic de l’orogenèse grenvillienne

Tracking the evolution of the Grenvillian foreland basin: constraints from sedimentology and detrital zircon and rutile in the Sleat and Torridon groups, Scotland

The Grenville Orogen, although occupying a key position in the Rodinia supercontinent, lacks a clear foreland basin in its type area in eastern Canada. Early Neoproterozoic siliciclastic rocks in northern Scotland, however, are now interpreted as remnants of a proximal Grenvillian foreland basin. Analysis of the sedimentology and detrital zircon and rutile of the Torridon and underlying Sleat groups provide new constraints on the evolution of this basin. Youngest U-Pb detrital zircon grains yield ages of 1070–990 Ma in both groups, consistent with a Grenvillian source. The proportions of older age components vary throughout the stratigraphy. The lower Sleat Group shows a dominant ca. 1750 Ma peak, likely derived from local Rhinnian rocks in Scotland and Ireland uplifted within the Grenville Orogen. In the upper Sleat Group and Torridon Group, detrital zircon peaks at ca. 1650 Ma and ca. 1500–1100 Ma become increasingly important. These latter peaks correspond with Labradorian Pinwarian, Elzevirian and early Grenvillian protolith ages within the eastern Grenville Province in Canada, and reflect exhumation and erosion of different mid-crustal complexes within that sector of the orogen. There is no difference in detrital zircon ages across the low-angle Sleat/Torridon unconformity. Detrital rutile in the Torridon Group yields a significant ca. 1070 Ma, Grenville-age peak, but older grains (1700–1200 Ma) also occur, suggesting derivation from the cool (T < 600 °C) upper orogenic crust. The detrital mineral data and sedimentology suggest the following evolution of the Grenvillian foreland basin in Scotland: i) early deposition in a narrow marine foreland basin (lower Sleat Group), sourced from the Irish-Scottish sector of the Grenville Orogen, with orogen-normal fill; ii) within the narrow Sleat Group basin a gradual switch to more distal sources in the Canadian sector of the Grenville Orogen, via axial transport; iii) an abrupt switch in basin dynamics (but not in source) across the Sleat-Torridon boundary to fluvial braidplain deposition in a much wider, Torridon-Morar basin; iv) followed by a gradual retrogradation of that basin. The Torridon-Morar groups represent a major denudational event of the Grenville Orogen that we infer was linked to more distal deposits in East Greenland, Svalbard and Norway

Trace element partitioning between coexisting biotite and muscovite from metamorphic rocks, Western Labrador: structural, compositional and thermal controls

Coexisting biotite and muscovite in ten metapelitic and quartzofeldspathic rocks from western Labrador have been analyzed by electron microprobe for major and minor elements and by a laser ablation microprobe coupled to ICP-MS (LAM-ICP-MS) for selected trace elements—Li, Sc, V, Cr, Mn, Co, Ni, Cu, Zn, Rb, Sr, Y, Zr, Nb, Cs, Ba, REE, Hf and Ta. The samples have experienced a single prograde Grenvillian metamorphism ranging from 490 to 680°C and from 7 to 12 kbar. The trace element compositions of coexisting micas in the metamorphic rocks are used to assess the effects of crystal structure, major element composition and temperature on the partitioning of each element between biotite and muscovite. Overall, trace element distributions are systematic across the range of metamorphic grade and bulk composition, suggesting that chemical equilibrium was approached. Most distribution coefficients (biotite/muscovite) show good agreement with published data. However, distribution coefficients for Co and Sr are significantly different from previous determinations, probably because of contamination associated with older data obtained by bulk analysis techniques. The sequence of distribution coefficients is governed mainly by the ionic radii and charges of substituting cations compared to the optimum ionic radius of each crystallographic site in the micas. In particular, distribution coefficients exhibit the sequence Cr3+ (0.615 Å) > V3+ (0.64 Å) > Sc3+ (0.745 Å) in VI-sites, and Ba2+ (1.61 Å) > Sr2+ (1.44 Å) and Cs+ (1.88 Å) > K+ (1.64 Å) > Rb+ (1.72 Å) > Na+ (1.39 Å) in XII-sites. The distributions of Li, Sc, Sr and Ba appear to be thermally sensitive but are also controlled by major element compositions of micas. V and Zr partitioning is dependent on T and may be used to cross-check thermometry calculations where the latter suffer from retrograde re-equilibration and/or high concentrations of Fe3+. The ranges and dependence of distribution coefficients on major element compositions provide important constraints on the values that can be used in geochemical modeling

The synmetamorphic P-T-t path of granulite-facies gneisses from Torngat Orogen, and its bearing on their tectonic history

Granulite-facies gneisses in the Abloviak shear zone in the internides of the Torngat Orogen are characterized by subvertical shear fabrics and associated subhorizontal stretching lineations. P-T vectors, derived from individual samples of these gneisses by conventional geothermobarometry of equilibrium core, rim and replacement symplectite assemblages, yield evidence of over 3 kbars decompression associated with cooling of approximately 150°C. When the sample population is considered together, a P-T-t path involving over 5 kbars decompression and 250°C cooling is defined. Such paths are compatible with theoretical models of synmetamorphic uplift following doubling of crustal thickness during thrusting, and imply that transcurrent motion took place in tectonically thickened crust, and was coeval with uplift. In a regional context, the Torngat Orogen preserves evidence of the oblique collision of two Archean cratonic blocks, the Nain and Rae provinces, during the Early Proterozoic and their amalgamation with Laurentia