- freely available
Geosciences 2013, 3(3), 448-465; doi:10.3390/geosciences3030448
Abstract: Over 100 new Nd isotope analyses for the central Grenville Province in the Parent-Clova region of Quebec help fill a major gap in understanding the crustal accretion history of the province. Nd model ages show that the Parent-Clova region consists of three crustal blocks: the Archean parautochthon in the north; a central block with mixed ages interpreted as an ensialic arc; and a southerly block forming an extension of the Mesoproterozoic Quebecia arc terrane. The Allochthon Boundary Thrust is believed to define the edge of the Archean parautochthon, which is bordered for a distance of 300 km by the ensialic arc block, within which model ages decrease consistently away from the craton. A similar negative correlation between Nd model age and distance from the craton is seen in published data for the Algonquin terrane in Ontario, but with a lower range of model ages. These comparisons show that in the Parent-Clova region, a Mesoproterozoic ensialic arc was established directly on the Archean margin, but further west, the Mesoproterozoic arc was built on a younger margin consisting of accreted Palaeoproterozoic arc crust. The use of large Nd data sets allows these distinct regional growth patterns to become clear and, hence, allows an understanding of Mesoproterozoic crustal evolution in the province as a whole.
Formed during the amalgamation of Rodinia, the Grenville Province represents a long-lived ancient orogenic belt, which comprises the southwestern margin of the Canadian Shield. New continental crust was intruded and sutured onto the Laurentian foreland for nearly a billion years until a terminal collision at 1.1 Ga halted subduction and crustal growth.
The continent-continent collision that formed the Grenville Province is analogous to that of the Himalayas  and resulted in considerable crustal shortening and thickening. The resulting high-grade metamorphism erased much of the evidence necessary to reconstruct the original growth history of the province and led large areas of the Grenville Province to be labeled as “seas of gneisses” . However, Nd isotope analysis has been successfully used to estimate crustal formation ages for high-grade gneisses in the Grenville Province  and, hence, to identify several large first-order accreted terranes that were amalgamated together on the Laurentian margin over the Paleo- and Mesoproterozoic (Figure 1).
Dickin  attributed much of the Eastern Grenville Province to the accretion of three very large terranes, formed around 1.9, 1.7 and 1.5 Ga, and termed Makkovikia, Labradoria and Quebecia, respectively (Figure 1). However, isotope mapping of the western part of the province has been complicated by the establishment of long-lived Andean-type arcs on the Laurentian margin [6,7], leading to more complex mixed isotope signatures in this area (Figure 1).
The clearest evidence for such mixing was seen in the Lac St Jean region , represented by the boxed area in Figure 1. At that time, the adjacent Parent-Clova area to the SW was not studied in detail, due to poor road access. However, recent upgrading of forest access roads has resulted in limited blasting of road-cuts, allowing Nd isotope mapping of this region to fill one of the last remaining gaps in the crustal formation age map of the Grenville Province. This reconnaissance Nd isotope mapping will hopefully reveal major terrane boundaries within the study area, and also allow a better understanding of the geological relationships between the eastern and western parts of the province and their differing history of crustal growth.
2. Regional Geological Background
The geological context of the present study area is shown in more detail on a terrane map of the SW Grenville Province in Figure 2. As noted above, the present study area (boxed) represents the last major sampling gap for Nd data in the western part of the province, but it also falls in a kind of conceptual gap between the geological complexity of the SW Grenville Province and the somewhat simpler structure of the Central Grenville Province.
Nd analysis in the Central Grenville Province by Martin and Dickin  showed it to consist of three major blocks or terranes. The western terrane consists of Tonalite-Trondhjemite-Granodiorite (TTG)-type Archean gneisses with homogeneous Nd isotope signatures that are interpreted as the lateral equivalents of Superior Province crust within the structural extent of the Grenville Province. The eastern terrane also has homogenous Nd isotope signatures, which gave rise to a 71 point Sm-Nd isochron age of 1.51 ± 0.05 Ga . This block also has TTG-type petrology and was identified by Dickin  as an accreted oceanic arc, termed Quebecia after Rondot . Between these large terranes is a narrow block with heterogeneous Nd isotope signatures and somewhat alkaline petrology, attributed by Martin and Dickin to a Mesoproterozoic ensialic arc established on the Archean margin. The Allochthon Boundary Thrust of Rivers et al. , separating largely in situ parautochthonous crust to the NW from far-traveled allochthonous crust to the SE, corresponds to the edge of the Archean terrane in this area, as originally recognised from Rb-Sr dating by Frith and Doig .
The SW Grenville Province is more complex and is divided into several major lithotectonic terranes in Figure 2, which will be summarized briefly in order of decreasing age. In this area, Archean crust reaches as far as 150 km SE of the Grenville Front, but in some areas, it has been strongly reworked by later plutonism, and in other areas, it is covered by metasedimentary rocks that sampled material with mixed sources . In some areas, Archean crust is over-ridden by the allochthonous belt, which forms large nappe lobes, but elsewhere, Archean crust is sutured against an accreted Palaeoproterozoic arc terrane with homogeneous Nd isotope signatures that gave rise to a 53 point Sm-Nd isochron age of 1.75 ± 0.05 Ga .
In contrast to this terrane with homogenous Nd signatures, the over-riding Allochthonous Belt, represented in this area by the Algonquin and Lac Dumoine terranes, has a heterogeneous Nd isotope signature with depleted mantle (TDM) model ages ranging from ca. 1.4 to 1.8 Ga . The Algonquin terrane was, in turn, overthrust by the Muskoka and Parry Sound terranes with somewhat younger Nd model ages (ca. 1.4–1.7 Ga). Further to the SE, still younger model ages are also seen in the Mont Laurier Terrane, Central Granulite Terrane and the Adirondacks (ca. 1.33–1.55 Ga), attributed to crustal formation in an Elzevirian continental margin arc that gradually stepped off the earlier Mesoproterozoic margin to create a strip of juvenile crust along the edge of the continent [7,8,12].
At the peak of development of this continental margin arc, back arc spreading led to the formation of an Elzevirian back-arc rift system , sub-divided into an ensimatic rift zone in Ontario and an ensialic rift zone in Quebec. These zones are equivalent in extent to the so-called Composite Arc Belt of Carr et al. , but are attributed to rift-related magmatism rather than arc magmatism, consistent with the geochemistry of mafic units in this area .
3. Geology of the Study Area
The field area encompasses ca. 40,000 km2 of south-central Quebec, extending from ca. 47° to 48°30' N and from ca. 73° to 75° W (Figure 2). The lack of major roads throughout this region has hindered mapping efforts, which have thus been neglected to this point. Due to the large size of the area, reconnaissance scale mapping of crustal formation ages will allow initial identification of first-order crustal terranes and investigation of the possible continuation of the ensialic arc block of Martin and Dickin  between Archean and Mesoproterozoic terranes to the north and south.
Previous reconnaissance scale mapping in this area was conducted by the Ministère des Ressources naturelles de la Faune et des Parcs, Gouvernement du Québec (MRNFP), leading to a series of 1:250,000 scale regional geological maps that have, in turn, been used to generate a compilation map of the whole Grenville Province .
Much of the field area is dominated by basement lithologies at high metamorphic grades, including orthogneisses and minor paragneisses, whose precursors have been subjected to upper amphibolite to granulite grade metamorphism. The extent of granulite-facies metamorphism (charnockite and mangerite) is clearly indicated on maps of the area, but such variations in grade may actually be a late feature that has little significance for the earlier geological evolution of the region. Finally, a large portion has been characterized as undifferentiated or mixed gneisses, attesting to the complex geology and relative lack of geologic investigation of the region.
The Allochthon Boundary Thrust (ABT), which has been traced along much of the Grenville Province , is believed to cross the northern portion of the study area (heavy line in Figure 2). South of the ABT lies a band of early Mesoproterozoic mafic gneisses and amphibolites with a few identified granite intrusions generally occurring along regional fault structures. The eastern portion of the study area is dominated by a wide range of lithologies, including mixed, mafic and undifferentiated gneisses. Some anorthosite-mangerite-charnockite-granite (AMCG) suite complexes also trend along regional fault structures to the east . These variable lithologies are consistent with the existence of an ensialic arc in this area, as identified to the NE by Martin and Dickin .
In the south, the western extent of Quebecia has been currently limited to the Tawachiche Shear Zone (TSZ in Figure 2) by both U-Pb  and Sm-Nd dating . Further sampling to the northwest of the TSZ will be used to bridge the remaining gap in Quebecia’s southwestern limit, and identify any crustal mixing  that may have resulted from the younger (<1.45 Ga) Elzevirian arc established on the continental margin .
4. Sampling and Analytical Techniques
Since the objective of this study is to characterize the protolith age of the crust as an estimate of its regional crustal formation age, the strategy adopted was to limit sampling to granitoid orthogneisses that are believed to form by anatexis of primitive arc crust. Previous studies have shown that granitoids of this type have Nd isotope signatures that are consistent and predictable, allowing reliable estimates to be determined of the formation age of the crust using the depleted mantle model of DePaolo . Mafic gneisses were excluded as far as possible, because of the increased likelihood of a younger mantle-derived component in these rock-types. Metasedimentary gneisses were also excluded, because of their uncertain sedimentary provenance.
On average, 1 kg of rock was crushed, after the removal of any weathered, veined or migmatized material; and careful attention was given to obtain a fine powder that was representative of the whole rock. Major element analyses were performed by Activation Laboratories, Ancaster, Ontario, using Li-borate fusion inductively couple plasma (ICP) analysis. The accuracy of their data was ensured by the inclusion of international standards as part of the analytical protocol.
Sm-Nd analysis followed our established procedures . After a four-day dissolution at 125 °C in sealed digestion vessels using HF and HNO3, samples were converted to the chloride form before splitting and spiking. Standard cation and reverse phase column separation methods were used. Nd isotope analyses were performed on a VG Isomass 354 mass spectrometer (VG instruments, Winsford, UK) at McMaster University using double filaments and a 4 collector peak switching algorithm, and were normalised to a 146Nd/144Nd ratio of 0.7219. Average within-run precision on the samples was ±0.000013 (2σ), and an average value of 0.51185 ± 2 (2σ population) was determined for the La Jolla standard during this work. The reproducibility of 147Sm/144Nd and 143Nd/144Nd is estimated at 0.1% and 0.002% (1σ), respectively, leading to an analytical uncertainty on each model age of ca. 20 Myr (2σ). Duplicate dissolutions in Table 1 support this estimate, except for two samples with anomalously old ages (#100 and #101) that may contain an inherited refractory sedimentary component.
|Map||Field||UTM N||UTM E||ABT||Nd||Sm||147Sm||143Nd||E Nd||TDM||Q||P||Met|
|#||#||NAD 83||NAD 83||km||ppm||ppm||144Nd||144Nd||1.45Ga||Ga||grade|
|Ensialic West, young|
|Quebecia West, old|
5. Nd Isotope Results
Over 100 new Nd model ages for granitoid orthogneisses from the study area are presented in Table 1. Localities are based on Universal Transverse Mercator (UTM) grid references using the 1983 North American Datum. They are plotted in Figure 3 using colours to indicate approximate ranges of model ages. Dark blue = Archean (>2.4 Ga); red, yellow and pale blue = Palaeoproterozoic (ca. 1.64–2.4 Ga); green and pink = largely Mesoproterozoic (1.37–1.64 Ga). Based on these colour schemes, it can be seen that samples with Archean TDM ages are restricted to the area NW of the ABT, while the line across the middle of the map separates most samples with model ages above and below 1.64 Ga. Hence, this line is argued to be a continuation of the boundary between the Eastern and Central blocks of Martin and Dickin , corresponding to the Quebecia and ensialic arc terranes.
However, these relatively broad age divisions conceal more subtle internal age variation within these terranes. Therefore, to clarify these internal variations, TDM model ages are plotted against distance from the ABT in Figure 4, in a similar manner to Martin and Dickin .
This plot reveals that the central block in the present study area has exactly the same age structure as the central block of Martin and Dickin, with correlated trends of the Nd model age against distance that are completely overlapping (red circles and vertical crosses in Figure 4). Similarly, the young (southerly) terrane in the present study area (green circles) overlaps strongly with Quebecia (black diamonds). Hence, this terrane is here referred to as Quebecia West (Figure 4).
One difference is that Western Quebecia has a larger number of samples with Nd model ages below 1.5 Ga, whereas in the main Quebecia terrane, only four such samples were seen amongst 71 with model ages in the range 1.5–1.65 Ga . This is attributed to the effects of the younger Elzevirian arc, which is believed to have straddled the edge of the continent in this area, reworking the southern edge of the Quebecia terrane and generating new crust offshore, which now forms the Adirondacks .
In addition to the 88 samples in Table 1 that define these regular patterns, there are 14 samples that show anomalous behavior. For example, within the Western Quebecia terrane, there are five samples that show abnormally old ages. However, because rocks with these old ages are not contiguous, they are unlikely to represent specific structural features, such as tectonic windows, to an underlying thrust sheet. Instead, they are attributed to contamination of the granitoid source by inclusion of small pockets of older sediment into the melting zone.
There are also a few samples from the ensialic and Archean terranes that yield abnormally young model ages. These are attributed to younger plutons that introduced new mantle-derived material into the crust. Although these samples contribute to the geological noise, they can also yield important information. For example, the three abnormally young samples (pale blue) within the Archean terrane show the effects of mixing Archean crustal Nd with (probable) juvenile Mesoproterozoic Nd. Hence, we infer that similar mixing process gave rise to the older (ca. 2 Ga) model ages within the ensialic block. The fraction of old crustal Nd involved in mixing apparently decreased steadily southwards, as proposed by Martin and Dickin .
Six samples (coloured pink) that yield abnormally young model ages within the ensialic block are attributed to two stages of Nd mixing. The first of these stages yielded the relatively coherent signature of the Nd mixing line described above. Superimposed on this older mixing event was a more scattered younger mixing event, which introduced small amounts of new mantle-derived material into the older mixed crust of the ensialic arc block. This latter event probably represents distal plutonism related to the Elzevirian continental margin arc to the south .
Isotope signatures analogous to this younger mixing event were generated in crust southeast of the Algonquin Ramp in Ontario (diagonal crosses in Figure 4). This area of Ontario is part of the Algonquin Terrane that overlies the main ramp of the ABT  and was attributed by Dickin and McNutt  to Mesoproterozoic ensialic arc magmatism established on an older Palaeoproterozoic margin.
The overlapping distributions of pink circles and diagonal crosses in Figure 4 are both attributed to mixing of ensialic arc magma with an older crustal component having Nd model ages around 1.8–2 Ga. However, the origins of the older components are different in the two regions. In Ontario, the older component is attributed to an accreted Palaeoproterozoic arc with geographically homogeneous Nd signatures that yield a 1.75 Ga Sm-Nd isochron , as described in the introduction. In contrast, the older component in the present study area was itself the product of earlier mixing, demonstrated by its highly variable model age structure.
The calculation of model ages for mixed components involves extrapolation from the time of mixing to the apparent intersection with the mantle growth curve . Hence, this can cause error magnification and, therefore, increased data scatter. To overcome this problem, an alternative data presentation involves plotting epsilon Nd values calculated at the estimated time of mixing. It is not necessary to know the precise time of mixing, because the narrow range of Sm/Nd ratios in granitoid rocks causes most Nd isotope growth lines to be sub-parallel. Hence, the epsilon model is quite insensitive to the exact time (t) chosen, provided this is the same for all samples. The result (Figure 5) effectively reproduces the effects seen in Figure 4, but with less scatter about the mixing lines. The result is strong support for the distance-modulated crustal mixing model.
The epsilon versus distance plot can also be used to test another feature of the sample suite, namely the effect of metamorphic grade. The study area displays variable metamorphic grade from amphibolite-facies to granulite-facies (A and G, respectively, in Table 1). It is possible that the variable grade of metamorphism could have affected the closed-system assumption inherent in Nd isotope mapping. However, granulite-facies samples (shown in darker red and green) show no consistent difference from amphibolite-facies samples. Hence, it is argued that metamorphic grade has not affected the Sm-Nd closed-system assumption in the large whole-rock samples analysed.
6. Petrochemical Data
In order to compare the petrology of the analysed samples with those of the surrounding region, major element analysis was performed on selected samples. These data were used to determine the quartz (Q) and plagioclase (P) indices of Debon and LeFort , which are intended to classify granitoid rocks following Streckeisen , but using whole-rock chemical data. The new data (Table 1) are plotted in Figure 6, along with data from Martin and Dickin .
The results in Figure 6 show that the terranes identified by Martin and Dickin  continue to the west with similar petrology. Thus, the Archean samples in both areas are restricted to the quartz diorite and tonalite fields, whereas the ensialic arc in both areas trends across the middle of the diagram from diorite to granite, similar to the Blanco Batholith in Peru, which provides a modern analogue of continental arc magmatism . However, whereas Quebecia samples are largely restricted to the tonalite-granodiorite-monzogranite fields, the western extension of Quebecia is more variable, with scattered samples in the lower part of the diagram. This is consistent with the reworking of the western extension of Quebecia by the Elzevirian arc, which yielded several monzonitic to syenitic rocks in the Mont Laurier area to the SW .
New Nd data from the Parent-Clova region of south-central Quebec help to fill in a major gap in the crustal formation age map of the Grenville Province. They show clearly that the ensialic arc block of Martin and Dickin  continues a further 100 km to the SW, forming an almost parallel-sided strip between Archean crust and the Quebecia terrane along a distance of over 300 km (Figure 7).
The positive correlation between epsilon (1450 Ma) values and the distance from the ABT in the ensialic arc block provides strong evidence that this is an isotopic mixing line, and because the mixing line projects towards the compositions of scattered Proterozoic plutons within the Archean terrane (Figure 5), this suggests strongly that the crustal end-member of the mixing line was Archean. Hence, the mixing line suggests that Mesoproterozoic crustal growth occurred directly on the Archean margin in the present study area, in the absence of the accreted Palaeoproterozoic arc crust.
However, around 76° west, there is an important change in the nature of the Laurentian margin. From this point westwards, Archean crust was fringed by an accreted Palaeoproterozoic arc named Barilia by Dickin . This transition has been emphasized in Figure 7 by leaving the Archean crust west of 76° longitude un-coloured. South of the Palaeoproterozoic margin in Ontario, the Algonquin terrane represents an ensialic arc analogous to the central block of Martin and Dickin , forming a similar mixing line on the epsilon Nd versus distance plot. The samples that gave rise to this mixing line came from the area shaded dark pink in Figure 7. However, the Algonquin mixing line projects towards a Palaeoproterozoic rather than an Archean end-member (Figure 4, Figure 5). Hence, we can see that a Mesoproterozoic ensialic arc was established along this whole margin, but it crossed from Archean crust in the east to Palaeoproterozoic crust in the west.
It was suggested by Martin and Dickin  that the ensialic arc magmatism that gave rise to the Central Block occurred due to a subduction flip after the accretion of Quebecia. However, the new data presented here show that the reworking of Quebecia in the Elzevirian arc gave rise to a much more scattered mixing signature than the more regular mixing line seen in the Central Block. Therefore, we now suggest that an early Mesoproterozoic ensialic arc was established on the Laurentian margin as part of the Pinwarian magmatic event , before the accretion of Quebecia (Figure 8).
In view of the isotopic homogeneity and TTG-type petrology of the Quebecia terrane, Dickin  proposed that this was an accreted oceanic arc. An alternative possibility proposed by Rivers and Corrigan  is that the ensialic arc that gave rise to the Central Block continued to step further away from the continent, so that it generated juvenile ensimatic crust off-shore. However, in the Manicouagan area, homogeneous Mesoproterozoic arc crust abuts directly against Palaeoproterozoic crust , suggesting that in that area, Quebecia was a discrete accreted terrane. In that case, a possible scenario that could explain crustal evolution in the present study area would be two subduction zones, as shown in Figure 8.
Another reason for believing that Quebecia is a discrete accreted terrane is that no such crustal unit is seen in Ontario. Here, it seems that the (Pinwarian) ensialic arc that gave rise to the Algonquin Terrane was followed by continued ensialic arc magmatism on the continental margin, forming the Muskoka Terrane [6,20,27].
Slagstad et al.  suggested that a 1.55 Ga TDM boundary line, identified in the U.S. Central Plains by van Schmus et al. , could be traced through the Muskoka terrane of the Grenville Province. However, the data presented here, building on the study of Dickin et al. , show that no meaningful 1.55 Ga TDM boundary can be traced through Ontario, because the crustal context in Ontario and western Quebec is different from the Central Plains.
Van Schmus et al.  interpreted the 1.55 Ga TDM line as “a fundamental crustal feature representing the southeastern limit of Palaeoproterozoic crust in Laurentia”. This older Laurentian crust is relatively homogeneous in the Central Plains region, with an average TDM age of 1.7 Ga over large areas. However, the crust to the south of the line is even more homogeneous, with an average TDM age of 1.50 Ga that is barely older than the U-Pb ages of these rocks. Hence, Van Schmus et al.  suggested that this younger crust “could consist of one or more juvenile terranes accreted to the southeastern margin of early Mesoproterozoic Laurentia”. The 1.55 Ga TDM boundary, therefore, approximates a crustal suture line.
A similar scenario to this is seen in eastern Quebec (Figure 1), where the homogeneous juvenile Quebecia terrane (TDM 1.55 Ga) is juxtaposed against the Palaeoproterozoic Labradoria terrane (TDM 1.75 Ga) along a sharp boundary line . However, the situation in Ontario is quite different, because “primary Palaeoproterozoic crust” (with an average TDM age of 1.9 Ga) is here fringed by the Algonquin ensialic arc mixing zone, which was itself reworked by Elzevirian continental arc magmatism. Therefore, in Ontario, the edge of primary Palaeoproterozoic crust is best approximated by a 1.8 Ga demarcation line, which also corresponds to the location of the ABT . In contrast, a 1.55 Ga age cut-off runs through the middle of the mixing line in the Algonquin ramp, as indicated by the black horizontal bar in Figure 4.
It is not surprising that suture boundaries of the type identified by van Schmus et al.  cannot be traced across a whole continent. Most arc systems have a finite geographical extent, so that long-lived active continental margins are expected to periodically break down into separate segments, some of which are characterized by the accretion of large arc terranes, while others are characterized by long-lived ensialic arc subduction zones. Both types are here demonstrated on the Laurentian margin.
New Nd data for the Parent-Clova region of Quebec fill a major gap in understating between the geological evolution of the eastern and western Grenville Province. The Parent-Clova area contains a 60 km-wide zone of ensialic arc crust, located between the Archean craton to the north and an accreted Mesoproterozoic arc to the south. This ensialic arc extended NE–SW for a distance of 300 km along the Archean margin, before crossing onto accreted Palaeoproterozoic crust in western Quebec and Ontario. These inferences have been made possible by the analysis of large Nd data sets, which reveal distinct Nd isotope mixing zones on the edge of the Archean and Palaeoproterozoic margins, along with other areas of homogeneous crust attributed to accreted oceanic arcs.
We appreciate the invitation to participate in the Continental Evolution special issue and thank the journal reviewers for their constructive comments. Financial support is acknowledged from McMaster University and the Natural Science and Engineering Research Council of Canada.
Conflict of Interest
The authors declare no conflict of interest.
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