Morin Terrane Research Articles

New age constraints on magmatism and metamorphism in the Morin terrane (Grenville Province, Quebec)

The Morin terrane is an allochthonous crustal block in the southwestern Grenville Province with a relatively poorly constrained metamorphic history. In this part of the Grenville Province, some terranes were part of the ductile middle crust during the 1.09–1.02 Ga collision of Laurentia with the Amazon craton (the Ottawan phase of the Grenvillian orogeny), while other terranes were part of the orogen’s superstructure. New U–Pb geochronology suggests that the Morin terrane experienced granulite-facies metamorphism during the accretionary Shawinigan orogeny (1.19–1.14 Ga) and again during the Ottawan. Seven zircon samples from the 1.15 Ga Morin anorthosite suite were dated to confirm earlier age determinations, and Ottawan metamorphic rims (1.08–1.07 Ga) were observed in two samples. U–Pb dating of titanite in nine marble samples surrounding the Morin anorthosite suite yielded mixed ages spanning between the Shawinigan and Ottawan metamorphisms (n = 7), and predominantly Ottawan ages (n = 2). Our results show that Ottawan zircon growth and resetting of titanite ages is spatially heterogeneous in the Morin terrane. Ages with a predominantly Ottawan signature are recognized in the Morin shear zone, which deforms the eastern lobe of the anorthosite, in an overprinted skarn zone on the western side of the massif, and in the Labelle shear zone that marks its western boundary. In the rest of the Morin terrane, titanite with Shawinigan ages appear to have been only partially reset during the Ottawan. Further work is needed to better understand the relationship between the character of Ottawan metamorphism and resetting in different parts of the Morin terrane.

The Kilmar Magnesite Deposits: Evaporitic Metasediments in the Grenville Supergroup, Morin Terrane, Quebec

Mesoproterozoic magnesite deposits are found associated with dolomitic marble and intercalated with metasedimentary rocks of the Grenville Supergroup in the granulite facies Morin terrane (Grenville Province, Quebec). This study examines one of the remaining ore deposits exposed on the surface (at the Dobbie mine), and presents stable isotope and mineralogical data for a marine evaporitic origin. The magnesite ore zone has δ18O(Mag) = 25.5 ± 0.4‰ (VSMOW) and δ13C(Mag) = 1.7 ± 0.2‰ (VPDB; n = 7), while surrounding dolomitic marble has δ18O(Dol) = 24.2 ± 0.6‰ and δ13C(Dol) = −0.2 ± 0.7‰ (n = 11). These values are at the high end of the range for other Morin terrane marbles, and this and sharp transitions in stable isotope ratios between lithologies argue for preservation of evaporitic enrichment in δ18O and δ13C. Boron isotope ratios (δ11B = 15.5‰ to 22.7‰) are also consistent with a marine evaporite origin. Identifying evaporitic protoliths in metasedimentary rocks is important for determining pre-metamorphic depositional environments, and in this case links the sedimentary setting of the Morin terrane to the Adirondack Lowlands (New York, NY, USA). The identification of the Kilmar magnesite deposits as evaporitic also has implications for the formation of sedimentary exhalative base metal deposits in the Grenville Supergroup.

Detrital zircon constraints on Grenville sedimentation at the margin of Laurentia

The southern Grenville Province is made up of metaigneous and metasedimentary rocks that were formed and metamorphosed as part of the Mesoproterozoic active margin of Laurentia. Grenville Supergroup quartzites from the Adirondack Highlands (New York), the Morin terrane (Quebec), and New Jersey Highlands were analyzed to determine detrital and metamorphic zircon U-Pb ages and to help constrain Mesoproterozoic tectonics. These Grenville quartzites have broadly similar detrital age populations as each other, suggesting similar sedimentary source regions and depositional settings in back-arc basin environments at the Laurentian Margin during Geon 12. All samples contain detrital zircon with 1.45–1.25 Ga ages, consistent with proximal Grenville source regions. They also contain detrital ages that point to derivation from the Mazatzal (1.7–1.6 Ga), Yavapai (1.8–1.7 Ga), and Penokean (1.9–1.8Ga) orogens of the North American Midcontinent, and have a diverse population of Archean ages. These results and published data from Adirondack Lowlands metasediments (Chiarenzelli et al., 2015, 2017) show distinctly Laurentian sources, and the related and likely connected nature of the sedimentary histories of the Adirondack Highlands and Lowlands, Morin terrane and New Jersey Highlands. Metamorphic zircon formation is common in high-grade quartzites, and ages are variable between samples depending on the metamorphic history of each terrane and the melt productivity of different rocks (Peck et al., 2010a). Adirondack Highlands samples experienced metamorphic zircon grown during the Shawinigan orogeny and anorthosite-suite emplacement (1.19–1.14 Ga). Morin terrane samples are from the Morin Shear Zone, which was active and caused metamorphic zircon growth during the Ottawan phase of the Grenvillian orogeny (1.09–1.02 Ga). The New Jersey Highlands sample has metamorphic zircon formed during the Rigolet phase of the Grenvillian orogeny (1010–980 Ma).

Numerical analysis of the role of radiogenic basement on temperature distribution in the St. Lawrence Lowlands, Qu\xe9bec

Regions with low or medium surface heat flow in stable cratonic areas, such as in eastern Canada, have received little attention for geothermal energy. In the presence of a high heat-producing basement overlain by a sedimentary cover, however, such areas might be prospective. Their potential will depend on various parameters such as heat production within the basement, thermal conductivity of sedimentary formations, and structural context. In this study, we aim at quantifying the importance of these parameters on temperature distribution at depth for a model representative of the St. Lawrence Lowlands, Québec, where locally anomalously higher heat flow has been observed and a 3D model of sedimentary cover is available. Scenarios involving physical properties from neighbouring Grenvillian domains are considered: Portneuf–Mauricie domain with radiogenic heat production of 0.94–5.83 upmu hbox {W m}^{-3}, 0.02–4.13 upmu hbox {W m}^{-3} for the Morin Terrane, and 0.34–1.96 upmu hbox {W m}^{-3} for the Parc des Laurentides domain. The impact of radiogenic heating on temperature distribution at depth was simulated using the Underworld2 numerical modeling code. Results show that at 5 km depth, the range of temperature difference is 22 °C for all modeled scenarios. In addition, the benefit of the thermal blanket effect of the sedimentary cover can be significant, but depends strongly on the contrast in thermal conductivity between the basement and the cover, as well as on the structural context, and less on heat production in the basement. Finally, depth of the 120 °C isotherm varies by up to 1 km for the scenarios considered; carefully assessing the boundary conditions therefore, appears critical in an exploration context.

Geostatistical analysis on the spatial variation of radiogenic elements in the crystalline basement of Grenville Province in the southwestern Québec

Accurate assessment of deep geothermal resources remains a challenge from the practical point of view. Parameter uncertainties and partial knowledge of initial conditions limit the prediction of subsurface temperatures using a variety of thermal models strongly unreliable, and the temperature is highly dependent on the radiogenic heat production in the geological layers mainly affected by a number of factors including the concentrations of uranium, thorium and potassium, and rock density. In this paper, geostatistical methods were applied to investigate the spatial distribution of radiogenic elements (e.g., uranium, thorium, potassium) and their corresponding concentrations and radiogenic heat production. A representative region measuring 35 km × 80 km in the southwestern Quebec, and covering the domains of Portneuf-Mauricie, Morin Terrane and Parc des Laurentides in the Grenville Province was selected for this study because of its easy accessibility. Analysis results show that the concentrations of uranium, thorium and potassium for most rocks of the Grenville basement in the research region are in the range of 1–2 ppm, 3–10 ppm and 1–4%, respectively. Furthermore, 90% of the total samples analysed in this study show a uranium concentration of less than 3 ppm, 64% of the samples show a thorium concentration of less than 5 ppm, and 56% of the samples show a potassium concentration of less than 3%. This paper engaged both the ordinary kriging interpolation and sequential Gaussian simulation (SGS) methods to study the spatial distribution of radiogenic elements. Using density data for specific rocks, the distribution of radiogenic heat production in the study area of the southwestern Grenville Province was also simulated using the SGS method. Conclusively, results show that the difference between the minimum and the maximum value of radiogenic heat production is 30%, considering a significant proportion of heterogeneity in rock density.

Tectonic reactivation of transverse basement structures in the Grenville orogen of SW Quebec, Canada: Insights from gravity and aeromagnetic data

NW-SE trending, transverse lineaments, including the lithospheric-scale Mont-Laurier lineament, are interpreted from regional Bouguer gravity of the Grenville orogen of SW Quebec and adjacent Superior Craton in southeastern Canada. These lineaments, transverse to the ENE trending Grenville orogen, are inferred to correspond to Palaeoproterozoic structures in Archaean basement that have played an important role: (i) in the development of volcano-sedimentary back-arc basins along this segment of the Laurentian margin; (ii) on the geometry of thrust sheets and folds formed during thrusting in the ca. 1.23–1.2Ga Elzevirian orogeny and incorporation of the basins within the orogen; (iii) on reorientation of early-formed structures in the Central Metasedimentary Belt of Quebec (CMB-Q) during ca. 1.19–1.17Ga post-Elzevirian orogenic collapse; and (iv) for development of syn-plutonic deformation corridors and shear zones at the onset of the emplacement of the Morin anorthosite-mangerite-charnockite-granite (AMCG) suite. In the CMB-Q, a 100km wide megakink zone developed during ca. 1.19–1.17Ga differential, post-Elzevirian orogenic collapse in the upper-most nappe above transverse sinistral shear corridors 10–20km wide located in an underlying thrust sheet or “lower-deck”. Emplacement of 1.17Ga Chevreuil intrusive suite preferentially occurred within the megakink zone, starting late in the post-Elzevirian collapse and culminating during a switch to local shortening early in (and in part as a consequence of) the emplacement of the voluminous Morin anorthosite and associated AMCG-suite plutons. The Labelle deformation zone separating the CMB-Q and Morin terrane is interpreted as a post-Shawinigan, reverse shear zone that truncates folded lithological layering in the eastern CMB-Q and western Morin terrane that is either subsequently folded above the Mont-Laurier lineament during its further reactivation, or developed as a curved shear zone stepping across the Mont-Laurier lineament. The Grenville Province of SW Quebec therefore provides an example of strain partitioning and distinct deformation responses at different crustal levels during reactivation of basement structures.

Reconnaissance geochronology and geochemistry of the Mont-Tremblant gneiss of the Morin terrane, Grenville Province, Québec

The Mont-Tremblant gneiss is a granulite-facies quartzofeldspathic orthogneiss unit in Morin terrane of the southwestern Grenville Province. These gneisses are the oldest rocks in the Morin terrane and country rocks to intrusions of the 1.15 Ga anorthosite-mangerite-charnockite-granite (AMCG) suite. The Mont-Tremblant gneiss may also be basement to the Grenville Supergroup, but until now it has not been directly dated. Sensitive high-resolution ion microprobe–reverse geometry (SHRIMP-RG) geochronology of zircon from two penetratively deformed samples of Mont-Tremblant gneiss yields igneous ages of 1324 ± 38 Ma and 1333 ± 32 Ma (2σ), and one sample shows metamorphic zircon growth during the Shawinigan orogeny at 1159.4 ± 15.6 Ma. Geochemically, Mont-Tremblant gneiss samples are calc-alkaline, granitic, and show hallmarks of an evolved arc environment. These characteristics are consistent with the Mont-Tremblant gneiss being correlative with the Geon 14 Lacoste and Bondy suites in the Central Metasedimentary Belt of Quebec to the west, but contrast with the more juvenile tonalitic Geon 14 calc-alkaline suites in the Adirondack Highlands and other Grenville outliers in the Appalachian Mountains. This transition may reflect differences in the lithosphere along strike of the Laurentian margin during Geon 14 subduction.

OXYGEN ISOTOPES IN THE GRENVILLE AND NAIN AMCG SUITES: REGIONAL ASPECTS OF THE CRUSTAL COMPONENT IN MASSIF ANORTHOSITES

Oxygen isotope ratios in anorthosites are sensitive to supracrustal input into magmas, and shed light on the origin of the parent magmas of anorthosite and of contemporaneous granitic rocks. Oxygen isotope ratios of anorthosites range from 4 to 11‰ and indicate variable supracrustal input into their parent magmas that correlates with boundaries between crustal blocks. Oxygen and neodymium isotope ratios of 1.3 Ga Nain Plutonic Suite (NPS) anorthosites reflect the different ages and composition of the lower crust of the Archean Nain Province and the Late Archean to the Paleoproterozoic Churchill Province, which are country rocks to the anorthosites. These data are consistent with derivation from a mantle-derived magma contaminated by continental rocks, the anorthosites in the Churchill Province showing high oxygen isotope ratios suggestive of involvement of the regional Tasiuyak paragneiss at depth. Oxygen isotope ratios of NPS granitoids are similar across the boundary between the Nain and Churchill provinces, which implies that anorthosites and granitoids in the NPS are not derived from the same materials in the lower crust. In the Grenville Allochthonous Polycyclic Belt, oxygen isotope ratios in anorthosites of 1.6–1.0 Ga are consistent with mantle derivation and limited contamination by crust [δ 18 O(whole rock) ≈ 7‰]. High magmatic δ 18 O(whole rock) values (8–11‰) are only observed in the Adirondack Highlands and Morin terrane of the Grenville Allochthonous Monocyclic Belt, indicating the presence of supracrustal materials at depth such as hydrothermally altered ocean crust. This can be explained by tectonic models where the Adirondack Highlands and Morin terrane are built on arcs amalgamated to Laurentia at ca . 1.3 Ga. Taken together, Grenville and Nain anorthosites show that parent magmas of anorthosites can contain a significant crustal component, and compositional variations can be used for basement mapping. In some suites, contemporaneous granitic rocks do not match oxygen isotope ratios in anorthosites, arguing against comagmatic models for anorthosite – mangerite – charnockite – granite suites.

Polymetamorphism of marbles in the Morin terrane, Grenville Province, Quebec

The Morin terrane (Grenville Province, Quebec) is dominated by the 1.15 Ga Morin Anorthosite Massif and related granitic intrusions, all of which exhibit granulite-facies mineral assemblages. Anorthosite-suite rocks are deformed both in shear zones and in the interior of the terrane and show intrusive contact relations with marble along road cuts near the village of St. Jovite. Intrusive rocks exposed in these road cuts have well-developed skarns, which were deformed with the intrusions after emplacement. Skarn minerals are consumed by garnet-forming reactions (e.g., An + Wo = Gr + Qtz) that preserve granulite-facies temperatures and pressures. Calcite–graphite thermometry of Morin terrane marbles records temperatures of 755 ± 38 °C (n = 21), independent of proximity to anorthosite-suite plutons. Preserved metamorphic conditions and the retrograde pressure–temperature (P–T) path in the Morin terrane are very similar to conditions during the 1.07 Ga Ottawan orogeny in the Adirondack Highlands. Metamorphism and deformation of anorthosite-suite rocks and marbles of the Morin terrane are consistent with anorthosite intrusion followed by a distinct granulite-facies overprint.

Geochronological constraints on the last stages of terrane assembly in the central part of the Grenville Province

New U–Pb zircon and monazite ages for syn- and post-kinematic pegmatites that cut major shear zones within the western Quebec portion of the Grenville Province constrain the timing of deformation and final terrane assembly within this region. From the Grenville Front SE along the present section, the Grenville Province consists of a reworked Archean promontory partly covered by Proterozoic allochthons and two Proterozoic metasedimentary and metaigneous terranes, which are probably allochthonous. The age of the Grenville Front is still poorly constrained in this area as pegmatites intersecting high-grade rocks exhumed some 30 km south of the Front have upper intercept U–Pb ages of 2642+6/−5 Ma. Field and geochronological data in this paper, however, lend support to the possibility of late-Archean exhumation of high-grade terranes along the Front. South of the Front, two types of ductile shear zone are responsible for the geometry of terranes in this part of the Grenville Province. The first type corresponds to the boundary of allochthons transported onto the Archean parautochthon. The corresponding structures are not directly dated, but reheating of the parautochthon at about 1020 Ma (U–Pb ages on monazite) sets a minimum age for thrusting in this area. Late movement along an east-dipping shear zone located within the parautochthon, the Cabonga thrust (CT; 998+9/−5 Ma), is rather unique in that it involves rocks of the parautochthon and is characterized by a westerly directed transport. The shear zones of the second type are subvertical, NE-trending, generally sinistral and intersect the parautochthon, the allochthon or both. From the Front towards the SE, these shear zones are: the Cadgecrib shear zone (998+16/−6 Ma) intersecting the parautochthon, the Renzy shear zone (1003+4/−5 Ma) involving allochthonous metasediments, the Baskatong shear zone (1020+2/−1 Ma) which marks the NW limit of Proterozoic rocks of the Mont-Laurier terrane, the Labelle shear zone (1078±6 Ma) separating the Mont-Laurier and the Morin terrane, and the Taureau shear zone (1074±4 Ma) bounding the Morin terrane to the east. All the above timing constraints are from U–Pb ages on syn- to post-kinematic pegmatites emplaced within the respective shear zones. This age distribution suggests a propagation of transcurrent deformation towards the NW, which is responsible for the final configuration of terranes.

Quartz microstructures and c-axis preferred orientations in high-grade gneisses and mylonites around the Morin anorthosite (Grenville Province)

Quartz in deformed rocks from two large, high-grade shear zones around the Morin anorthosite (Morin terrane, Grenville Province) displays distinctive microstructures, as well as c-axis preferred orientations. In the west-dipping Morin shear zone, east of the Morin anorthosite, four distinct quartz microstructures (types 1–4) are identified, based on deformation features and grain size. The c-axis orientations are characterized by a single maximum near the stretching lineation and two maxima in type 1 microstructure, and by an asymmetrical, single girdle in type 2 microstructure. Quartz c axes show crossed-girdle pattern in type 3 microstructure. Both quartz microstructures and c-axis preferred orientations suggest that crystal–plastic slip and dynamic recrystallization are the dominant deformation mechanisms. The asymmetry of c-axis orientations with respect to the mylonitic foliation, as well as the substructures developed in quartz, indicates a dextral sense of shear in the Morin shear zone. Type 4 microstructure, which developed in some gneisses and granulites, is interpreted to record influence of postdeformation annealing by which quartz c-axis orientations were partially modified. In the north-northeast-trending, subvertical Labelle shear zone that separates the Morin terrane from the Mont-Laurier terrane, metamorphic assemblages and structural elements suggest that an early, sinistral strike-slip deformation occurred under granulite-facies conditions. This was overprinted by a late downdip movement of the Mont-Laurier terrane under retrogressive conditions. Quartz in felsic gneisses from this zone shows two types of microstructures: one is similar to type 4 from the Morin shear zone, the other is named type 5. Quartz c-axis orientations are complex and less systematic, due to overprinting by two episodes of deformation and possible annealing. These complexities limit the utility of quartz microstructures and c-axis data in the structural analysis of the Labelle shear zone.

Metamorphic constraints on the tectonic evolution of the allochthonous monocyclic belt of the Grenville Province, western Quebec

The tectono-metamorphic evolution of Grenvillian monocyclic terranes in western Quebec has been constrained along a northwest–southeast transect from Réservoir Cabonga to the Morin anorthosite by the study of garnet zoning and the application of geothermobarometric techniques to appropriate mineral assemblages. In all three terrenes examined (Réservoir Cabonga, Mont-Laurier, and Morin terranes), the results are consistent with Grenvillian metamorphism induced by thrusting. Additional magmatic heat may have been provided by the cooling of igneous rocks related to the Morin anorthosite complex.After the peak of the Grenvillian metamorphism, the monocyclic rocks were sliced, and three Grenvillian crustal levels, sharing some of their lithologic elements but having contrasting Grenvillian peak-metamorphic conditions, were subsequently juxtaposed. The deepest crustal level exposed in the transect (level A) comprises the Réservoir Cabonga terrene and the margins of the Mont-Laurier terrene. This level consists of metasediments of the "Grenville Supergroup," garnet amphibolites, and synmetamorphic plutonites whose emplacement predated the thrusting. Metamorphic conditions in level A are estimated at 725–875 MPa and 725–800 °C. Level C, which is exclusively in the central part of the Mont-Laurier terrene, comprises metasediments of the Grenville Supergroup that were metamorphosed at 425–600 MPa and 625–725 °C. Synmetamorphic igneous rocks of the Morin plutonic complex in the Morin terrane were metamorphosed at 600–800 MPa and 650–775 °C, probably in an intermediate crustal level B, which is missing in the Mont-Laurier terrane. It is thus suggested that juxtaposition of the Mont-Laurier terrane and the Morin terrane along the Labelle shear belt occurred after the peak of metamorphism.A comparison of the metamorphic evolution of these monocyclic terranes with that of their adjacent polycyclic counterparts suggests that (i) the Réservoir Cabonga terrane was thrust over the parautochthonous Réservoir Dozois terrane after the peak of metamorphism and (ii) level A rocks of the Mont-Laurier terrane and the Réservoir Baskatong terrane were juxtaposed during the peak of metamorphism.