Stacking of Orogenic Phases: Empirical Evidence From the Grenville Province for a New Mode of Evolution of Large Hot Long-Duration Orogens
Numerical modelling predicts that large hot long-duration orogens (LHOs) develop a plateau in the hinterland beneath which some form of channel flow takes place in the mid and/or lower crust. Although the Grenville Province is an attractive candidate for an LHO, any realistic application to the Grenville Orogen must take into account the two-stage evolution involving high-grade Ottawan deformation and metamorphism in the orogenic hinterland between ca. 1090-1020 Ma and high-grade Rigolet deformation and metamorphism at the orogen margin from ca. 1005-980 Ma. The Ottawan and Rigolet orogenic phases were not only temporally and spatially distinct, they also developed under different geothermal gradients, resulted in different crustal-scale stacking sequences, and were separated by a period of widespread orogenic collapse that led to preservation of the Ottawan Orogenic Lid at the same level as Ottawan mid crust in the hinterland. It is argued that the Ottawan orogenic phase carries many of the hallmarks of an LHO, including long duration (ca. 70 My), high temperature of the orogenic mid crust (850-900 C at 1000 MPa = 30-35 km depth), evidence for major tectonic transport of mid and lower crust on the Allochthon Boundary Thrust, a candidate for the base of a mid-crustal channel, and termination of the plateau by orogenic collapse. After orogenic collapse at ca. 1020 Ma, penetrative deformation and metamorphism ceased for some 15-20 My before restarting at the orogen margin and thereby initiating the Rigolet orogenic phase, which in contrast exhibits many of the characteristics of a small cold short-duration orogen (SCO). The crustal-scale architecture of the Grenville Orogen results from stacking of the two orogenic phases, with the younger Rigolet SCO beneath the older Ottawan LHO. It is inferred this architecture was promoted by collapse of the plateau in the hinterland, which terminated the Ottawan orogenic phase and rendered the foreland a more favourable location for shortening. An interesting result of orogenic phase stacking is the juxtaposition of Ottawan and Rigolet high-pressure rocks on either side of the Allochthon Boundary Thrust, implying the latter was a material focal plane along which lower-crustal rocks from both the hinterland and foreland were exhumed.
The Adirondacks as the Grenville-Appalachian Link
The Adirondack (ADK) Highlands and Lowlands link the Grenville Province with the Mesoproterozoic cores of the Appalachians. The oldest linkage is the ca. 1.4-1.3 Ga calcalkaline Dysart-Mt. Holly arc suite rifted from the Laurentian margin at ca. 1.3 Ga and extending at least through the Hudson Highlands, NY, and Reading Prong, NJ. The rifted blocks form local basements for ca. 1.25-1.22 Ga Elzivirian arc magmatism. Details indicate that the ADK - NE Appalachian arc was distinct from the Composite Arc Belt (CAB) that docked against Laurentia at ca 1.2 Ga and whose eastern margin included the ADK Lowlands. At ca 1.21 Ga west-dipping subduction beneath the Lowlands produced 1.21-1.18 Ga calcalkaline magmatism followed by synkinematic ca 1.17 Ga granitic-tonalitic plutons as well as accretion of the Highlands- NE Appalachian arc from the east. These effects constitute the ca 1.2-1.16 Ga Shawinigan orogeny that extended throughout the CAB, Morin, ADK Highlands, ADK Lowlands, and Mekinac terranes. Locally, Shawinigan orogenesis transitioned into ca 1155 Ma AMCG magmatism. The Shawinigan orogen was two-sided with NW-thrusting at the western CAB margin and ENE-thrusting along the proto-Carthage-Colton Zone (CCZ)of the ADKs. The latter faults placed CAB lithic packages above the ADK Highlands and Morin terranes where they remained during the ca. 1080-1050 Ma Ottawan orogeny, thus escaping high-grade ca. 1050 Ma metamorphism. In the southern Appalachians, the Shawinigan orogeny is manifested by ca 1.2-1.15 Ga granitoid magmatism and is attributed to collision with Amazonia, which must also have been involved with ADK-CAB events. By ca 1.09 Ga continuing Amazonia-Laurentia collisions produced regional Ottawan metamorphism from Labrador to the southern Appalachians. Late Ottawan (ca 1.05 Ga) potassic-ferroan magmatism (including local alkalic anorthosite) occurred throughout. Within the ADK Highlands ca 1050-1040 Ma Lyon Mt Granite intruded during orogen collapse as the region evolved into a large, symmetric core complex. In the west, collapse occurred along the CCZ; whereas the eastern detachment appears more diffuse. Zircon and monazite dating in both regions establish the proper extensional shear senses. Along -strike southerly projection of the ca 1055 Ma, east-dipping Tawachiche Detachment Zone of the central Grenville Province places it in the Champlain Valley where it likely played a major role in collapse of the eastern ADKs. Thus, the proposed core complex includes both ADK and Morin terranes, and its effects were felt as far west as the Bancroft Shear Zone. It was during this late-Ottawan collapse that rocks of the tectonically sequestered CAB terranes, with their preserved high-grade Shawinigan assemblages, were dropped down to the west into their current positions. Finally, we interpret much of the dominant ADK fabric (eg, ribbon lineation) and late upright NNE and E-W folds to be products of extensional strains during orogen collapse. Despite post-1.2 Ga tectonic linkage between the ADKs and the southern and central Blue Ridge, Nd and Pb isotopic mapping strongly suggest that pre-1.2 Ga rocks of the latter terranes are exotic to Laurentia and were transferred from Amazonia during oblique collision. In contrast, isotopic mapping ties the Grenville Province, ADKs, and northeast Appalachian cores to the Mesoproterozoic Laurentian margin that passes through the St. Francois Mts., MO. and Llano uplift, TX.
An Indentor Tectonic Model for the Eastern Grenville Province and Implications for Rodinia Reconstruction
The easternmost Grenville Province is traditionally interpreted as projecting into the Atlantic Ocean and to correlate directly with the Sveconorwegian Province in Scandinavia. This concept requires modification. Geochronological, structural and geophysical data are used to demonstrate that, in contrast to frontal-thrust ramp tectonics in western and central Labrador (and farther west), Grenvillian orogenesis in eastern Labrador mostly involved a dextral strike-slip, lateral-ramp regime. It is also argued that allochthonous terranes in the western part of the EGP were displaced 150 km northwest, relative to their eastern counterparts during the late Grenvillian. Whereas central and western Labrador and adjacent Quebec can be successfully modeled in terms of frontal, collisional tectonism, utilizing channel-flow models that have been applied to the Himalayan-Tibetan tectonic system, eastern Labrador cannot. The point where frontal-thrust ramp tectonics gives way to a lateral-ramp regime is interpreted as an indentor corner. This model has important implications with regard to the reconstruction of Rodinia. Although both the Grenville and Sveconorwegian provinces experienced equivalent orogenesis, they did not belong to a single, continuous tectonic belt and must be modeled separately.
1.4 to 1.2 Ga crustal evolution of the central Grenville Province: new insights from the Manicouagan-Baie Commeau transect.
Integrated field relations, U-Pb geochronology and geochemistry in lithotectonic units exposed along the Manicouagan-Baie Comeau transect document two volcano-sedimentary sequences and related plutonic units formed within the 1.4 - 1.2 Ga time interval. While these sequences and units were metamorphosed under medium to high pressure granulite facies conditions during the subsequent Grenvillian orogeny, lithological associations and outcrop-scale to sample-scale features are consistent with the interpretations offered here. Key elements include the recognition of: (a) a 1.4 Ga supracrustal sequence, including layered mafic to intermediate-composition units, banded quartzo-feldspathic units, rusty aluminous gneiss, and minor calc- silicate and quartzite, intruded by 1.4 Ga granitoids (Canyon Domain); (b) 1.3 Ga granitoid plutons intrusive into Labradorian-age crust at the boundary between the Canyon Domain and structurally underlying units to the NNW (Lelukuau Terrane and Island Domain); and (c) 1.2 Ga intermediate composition and bimodal (mafic- felsic) supracrustal sequences (ex. Banded Complex) adjacent to the Canyon Domain to the ENE. The dominant lithological association of the Canyon Domain is correlative in terms of lithology and age with the mostly amphibolite facies Montauban Group - La Bostonnais complex of the Portneuf - St. Maurice Domain, 450 km to the SSW. The Montauban Group has been interpreted by others to represent remnants of an island arc, with the intrusion of La Bostonnais complex marking the younger age limit for its accretion to Laurentia. The Portneuf-St Maurice Domain is limited to the NNW by Grenvillian age batholiths, so its original extent is unknown. However, in light of the new data on Canyon Domain, it is likely that it represents the NNE continuation of the Portneuf-St Maurice Domain. Indeed, preliminary geochemical data on certain mafic dykes and layers in Canyon Domain are consistent with emplacement in an arc setting. The significance of the 1.3 Ga granitoids in Labradorian units at the NW boundary of Canyon Domain is ambiguous, as they may represent either: (a) "stitching" plutons, marking the accretion of the Canyon Domain to the Laurentian margin (in this case the accretion age would be younger than that inferred by the correlation between Canyon Domain and the Portneuf - St. Maurice Domain); or (b) the first stage in the development of a continental rift system (see below). The lithological association of the 1.2 Ga metavolcanic units of the Banded Complex, NNE of the Canyon Domain is consistent with rifting of continental crust and development of a back arc basin. The original location of the Banded Complex relative to the Canyon Domain is unknown. However the northern Canyon Domain is characterized by 1.2 Ga inherited monazite, and by mafic layers and dykes with the geochemical signature of within-plate basalts, consistent with a continental rift setting. We therefore suggest that the 1.2 Ga rifting was proximal to the 1.4 Ga crust of the Canyon Domain. This 1.2 Ga magmatic event is broadly coeval with the back arc magmatism in the Composite Arc belt (SW Grenville Province) and the Seal Lake group (foreland of the NE Grenville Province), which has been interpreted as a continental rift. However evidence for 1.2 Ga magmatism has been scarce elsewhere and the presence of an Elzevirian back arc system in the central Grenville Province is for the first time proposed here.
Nd Isotope Mapping of Manicouagan, Quebec, in the Grenville Province
The Grenville Province is a 1Ga orogenic belt, composed of much older terranes whose age and extent require mapping in order to gain a better understanding of its geological evolution. Manicouagan, Quebec, in the Grenville Province, has experienced both large scale orogenic events, as well as being the site of a 215 Ma hypervelocity impact event, which has further complicated its geological structure. By analyzing surface samples from this area, as well as drill core of the country rocks involved in the impact, the original age of crustal formation for the various terranes can be calculated, thus revealing the evolutionary history of the Manicouagan area. Nd isotopes are resistant to metamorphic disturbances, therefore permitting accurate calculations of original crustal formation ages. Through the use of Nd-isotopic analysis of granitoid orthogneisses, three major crustal formation age groups have been identified in the area. These are: Mesoproterozoic (1.58 Ga), Paleoproterozoic (1.86 Ga), and Archean (2.8 Ga). These depleted mantle model ages (TDM) correspond well with Sm-Nd isochron reference lines, supporting their validity as actual geologic events. Our results indicate that the Archean basement extends throughout the western side of the Manicouagan reservoir, whereas the eastern side is dominated by Proterozoic crust. In contrast the Manicouagan Imbricate Zone (MIZ) has a larger scatter of data attributed to mixing between Archean and Proterozoic sources. The orientation of the model ages suggests that the MIZ is sandwiched between the Archean basement and a Proterozoic terrane, which was thrust over it from the southeast. The location of the Allochthon Boundary Thrust (ABT), between the Archean and Proterozoic terranes, is largely agreed upon to the east and west of the impact, however, it becomes highly ambiguous near the impact itself. It is likely that the boundary crosses onto the island on the NW side of the reservoir, and exits to the left of the previously established Cryptic Shear Zone (CSZ) at the southern end of the impact. Model ages to the southeast indicate a Mesoproterozoic crust, which may indicate the presence of an additional thrust boundary running sub parallel to the ABT in this region.
A Multi-Disciplinary Approach to Mapping the Grenville Province: Using Neodymium Isotopic Analysis Accompanied by Spatial Information to Construct a More Accurate Geological Map
Geochronological study of the Grenville Province has been an ongoing project at McMaster for the past 20 years. Over 1,500 orthogneiss samples have been collected from a 700,000 square kilometer region, and geochemically analyzed for Neodymium model ages. This vast area has been subjected to multiple accretion and ductile deformation episodes, creating great geological complexity. The two main belts of the Grenville Province are the Allochthonous Belt, consisting of terranes which were laterally transported by the Grenville orogeny, and the underlying Parautochthonous Belt. The boundary separating these two major belts is termed the Allochthon Boundary Thrust (ABT) and represents the northwesterly limit of major crustal movement during the Grenville orogeny. Due to the high degree of exhumation of mid to lower crustal levels, identification of the ABT is difficult within Southwestern Ontario. As a result, lithological maps, such as the Geological Survey of Canada (GSC) map of the Grenville Province created by Davidson (1998), do not correctly identify sections of the ABT. In addition, important geological features such as structural outliers (klippen) of the Allochthonous belt are not correctly identified. Along much of its length the ABT defines the boundary between Archean- Paleoproterozoic (> 1.8 Ga) and the Paleo-Mesoproterozoic (<1.8 Ga) crust. Therefore, by measuring the isotopic composition of either side of the ABT the boundary can be clearly defined. Previously collected Nd isotope data were plotted on ESRI's Geographical Information System (GIS) and digitally represented. Interpolation based on sample age distribution was conducted to assess completeness of the data set. Interpolation techniques included tessellations, krigging and inverse distance weighting, which were used to obtain global and localized trends. This approach highlighted discrepancies between geological and geospatial interpretation, therefore highlighting areas which needed additional sampling. Nd analysis of orthogneiss from four areas collected within Northeastern Ontario and Southwestern Quebec have provided additional information to this interpolation process. Through the addition of these samples the boundaries of the ABT and klippen have become much more precise. Digital mapping of the terranes of the Grenville Province through integration of the aforementioned techniques has provided an improved understanding to the geological history of the Grenville Orogeny.