SmNd isotopic and geochemical characterisation of the Paleoproterozoic Torngat orogen, Labrador, Canada

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Precambrian Research 81 (1997) 15-35

Sm-Nd isotopic and geochemical characterisation of the Paleoproterozoic Torngat orogen, Labrador, Canada R.J. Th6riault *, I. E r m a n o v i c s Continental Geoscience Division, Geological Survey of Canada, 601 Booth St., Ottawa, Ont. K1A-OE8, Canada

Received 12 June 1995; accepted 15 April 1996

Abstract

The Torngat orogen is a Paleoproterozoic, granulite-facies belt that welds the Archean components of the Nain and Rae provinces in eastern Labrador. Sm-Nd isotopic and supporting bulk rock major, trace and rare earth element data help elucidate the sources of the main Paleoproterozoic lithologies that comprise the orogen. In the foreland, Paleoproterozoic Ramah Group metasediments intercalated with reworked Nain Province gneiss show variable trace element and ENa(1.9 Ga) from - 2 . 9 to -12.0, ranging between Nain Province crust and Paleoproterozoic Mugford Group basalts compositions. The Ramah Group data suggest derivation of Ramah detritus from Archean Nain Province gneisses and from the Mugford Group. In the Lac Lomier complex (Rae Province hinterland), metasediments with variable trace element distribution patterns akin to Rae Province crest, and ENd(1.9 Ga) from -2.9 to -10.5 suggest derivation from Rae Province crust and from an unspecified juvenile component. In the core of the orogen, pelitic and psammitic protoliths of the Tasiuyak gneiss (ENd(1.9 Ga) from -1.9 to -2.6) were derived from crustal sources unknown in both Nain and Rae provinces, and may have been deposited in a trench environment starved of adjacent cratonic debris. Dioritic to granitic eastern and western metaplutonic suites intruding the Tasiuyak gneiss and the Lac Lomier complex display contrasting geochemical and Sm-Nd isotopic compositions. The eastern metaplutonic suite has a tendency toward lower SiO2, total alkalis, less fractionated REE and higher MgO, CaO and TiO2 relative to the western metaplutonic suite. The eastern suite yields end(1.9 Ga) from +3.8 to -6.7 and negative fsmma suggesting igneous precursors from Rae crust and a LREE-enriched juvenile component. The western suite yields eNd(l.9 Ga) from -1.5 to -8.5, suggesting an important contribution from Rae crustal sources in the petrogenesis of the igneous protoliths. The presence of Rae Province Nd isotopic compositions, and the absence of Nain Province signatures in these intrusions favors the eastern Rae Province as the active margin during Nain-Rae convergence. Keywords: Sm-Nd isotopes; geochemistry; Proterozoic; Laurentia

1. Introduction

The Torngat orogen is one of a number of Paleoproterozoic belts which weld Archean cratonic * Corresponding author. Fax: +1 613 995-9273. E-mail-address: rtheriault @gsc.nrcan.gc.ca

components of Laurentia. The high-pressure, hightemperature core of the Torngat orogen lies deeply exhumed near the Atlantic coast of Labrador (Fig. 1) where it overprinted a small remnant of the Archean Nain Province across a NNW-striking, > 4 0 0 km long, collisional zone, which joins the Rae and Nain Provinces. Recent geological investigations, based

0301-9268/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved. Pl1S0301-9268(96)00021-6

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R.J. Thdriault, L Ermanovics/Precambrian Research 81 (1997) 15-35 Ikl~in Dhifc~ni~

~'lite

"oup}Torngat orogen

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Fig. 1. Major lithological units of Labrador and location of the study area (after Ermanovics and Van Kranendonk, 1990). North River-Nutak map area is outlined in bold.

on detailed mapping of the southern segment of the Torngat, have focused largely on the structural, metamorphic and geochronological aspects of the orogen (Wardle, 1983; Korstggtrd et al., 1987; Mengel and Rivers, 1989; Ermanovics and Van Kranendonk, 1990; Van Kranendonk and Ermanovics, 1990;

Wardle et al., 1993; Ryan and Martineau, 1992; Van Kranendonk, 1992; Bertrand et al., 1993). Although a number of models of the precollisional configuration of eastern Laurentia in general (Hoffman, 1990; Van Kranendonk et al., 1993) and of the southern segment of the Tomgat orogen in particular have

R.J. Th&iault, I. Ermanovics / Precambrian Research 81 (1997) 15-35

been proposed (Van Kranendonk and Ermanovics, 1990; Bertrand et al., 1993; Rivers and Mengel, 1994; Scott, 1995b), limited information is available on the source of Paleoproterozoic rocks that might constrain the proposed models. In the southern segment of the orogen, the location of Paleoproterozoic intrusive rocks as well as structural and geochronological data prompted Van Kranendonk and Ermanovics (1990) and Bertrand et al. (1993) to suggest westward subduction of the Nain Province craton. However, geochronological and metamorphic arguments have led Rivers and Mengel (1994) to prefer eastward subduction of Rae crust. In the northern segment of the Torngat orogen (Burwell domain; Fig. 1), where the distinction between a considerably reworked Nain Province craton and the Rae Province is not readily made, subduction is postulated beneath both Nain and Rae Provinces, or beneath the Nain Province only, giving rise to continental arcs possibly in both provinces (Wardle et al., 1993; Campbell, 1994; Van Kranendonk and Wardle, 1994; Scott, 1995b; Scott and Machado, 1995). The present study is confined to an extensively mapped transect across the North River-Nutak map area in the southern segment of the orogen (Ermanovics et al., 1989; Ermanovics and Van Kranendonk, 1992) (Fig. 1). Here, Van Kranendonk (1992) and Bertrand et al. (1993) established and dated discrete tectonic events (D1 to D3 deformation) that include collision of Rae Province with Nain Province and crustal thickening (D1; 1860 Ma), strike-slip shear/transpression (D2; 1845-1822 Ma) and reactivation and uplift (D3; 1790-1780 Ma). Our study aims to: (1) elucidate the polarity of subduction by evaluating the crustal inheritance of the metaplutonic rocks, (2) constrain the sources and depositional setting of the Tasiuyak paragneiss protolith located in the core of the orogen and (3) evaluate the possible sources of Paleoproterozoic metasedimentary rocks located in Nain Province craton and the Torngat orogen. 2. Geology of the North River-Nutak area

Nain Province quartzofeldspathic gneisses, orthogneisses and supracrustal remnants at granulite and amphibolite grade record a protracted history in the range >3.6 to 2.55 Ga (Schictte et al., 1990, 1993; Ermanovics and Van Kranendonk, 1992; Van Kra-

17

nendonk, 1992; J.C. Roddick, GSC, pers. commun., 1994). Early Paleoproterozoic, brittle reactivation of this Nain crust is marked by local intrusion of small granite stocks at 2130 Ma (Emslie and Loveridge, 1992) and by wide-spread injection of the 1.9-2.2 Ga tholeiitic diabase dykes (Ermanovics and Ryan, 1990). These igneous events were succeeded by unconformable deposition of two cover sequences on Nain crust: the well-preserved, lower greenschistfacies Mugford Group metavolcanic rocks ( 1970 Ma, M.A. Hamilton, pers. commun., 1994) at the eastern margin of the Nain Province and the remnants of broadly coeval, amphibolite-facies Ramah Group metasediments in a reactivated foreland adjacent to the orogen in the western Nain Province. The granulite facies orogen is largely underlain by Archean (Ryan et al., 1991) to Paleoproterozoic rocks of the Lac Lomier complex (Scott, 1994) in the west, and by the Paleoproterozoic Tasiuyak gneiss in the east. The Tasiuyak gneiss constitutes an extensive belt, 12 to 50 km wide and 540 to possibly 740 km long, whose sedimentary protoliths are locally constrained to a 1940-1885 Ma depositional age (Scott and Machado, 1994). A zone of subvertical mylonite faults (D3) separates rocks of the Rae and Nain provinces. The latter is reworked in a 5 km-wide foreland east of the mylonite fault zone at amphibolite facies, which wanes to static subgreenschist facies 80 km eastward. Our study considers five, N-S-trending, Paleoproterozoic lithological assemblages from the North River-Nutak area, distributed from east to west as follows (Fig. 2): (i) rocks of the volcanic Mugford and metasedimentary Ramah Groups on the Nain craton, (ii) Tasiuyak gneiss in the core of the orogen adjacent to the Nain craton, (iii) Lac Lomier complex metasedimentary rocks in the Rae Province hinterland to the west, and an eastern (iv) and a western (v) suite of orthopyroxene metaplutonic rocks intruded into the core and hinterland, respectively. The Mudord Groupcomprises a succession of little altered, rift-related, tholeiitic plateau basalts and associated volcanogenic metasediments lying unconformably on a faulted block of amphibolite-facies Archean crust located on the easternmost margin of the Nain Province (Smyth, 1976; Hamilton, 1994) (Fig. 2). Basal sedimentary units of the group are correlated with basal members of the Ramah Group

18

R.J. Th~riault, 1. Errnanovics / Precambrian Research 81 (1997) 15-35

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R.J. Th~riault, L Ermanovics/ Precambrian Research 81 (1997) 15-35

(Smyth and Knight, 1978) which is extensively exposed as a fold and thrust belt north of Saglek Fiord (Morgan, 1975; Knight and Morgan, 1981; Calon and Jamison, 1994) (Fig. 1). The Ramah Group records two contrasting depositional environments: west-facing shelf deposits, and overlying euxinic deposits recording basin subsidence, regarded by Hoffman (1987) as a west-migrating Paleoproterozoic foredeep deposit on the west margin of the Nain Province. In the study area, Ramah Group metapelite and quartzite are considered to be basal units of the group (Ermanovics et al., 1989). They are preserved as amphibolite-facies remnants reworked with Nain rocks in the foreland and mark the western limit of the Nain Province rocks truncated against the subvertical, D3 orogen-side-up, mylonite faults. The Tasiuyak gneiss, exposed in a 38 to 48 km wide domain immediately west of the mylonite faults, is a white, uniformly heterogeneous, granulite grade, garnet (5 to 25%)-quartz (25 to 80%)perthite/oligoclase-andesine gneiss which contains generally less than 12% sillimanite, biotite, graphite, sulphide, rutile, and zircon. Centimetre to decametre scale layers of mainly pelitic to quartzofeldspathic rock, interlayered with sparse amounts of feldspathic quartzite, calcsilicate and graphitic-sulphidic metapelites, attain up to 26% A1203 and 5% MgO. Protoliths of the gneiss are thus inferred to be mudstone, sandstone and wackes (Morgan, 1975; Wardle, 1983) preserved with remarkable uniformity in a belt of gneisses possibly up to 740 km long (Taylor, 1979). The eastern part of the gneiss is marked by subvertical protomylonite, mylonite and blastomylonite fabrics (Abloviak shear zone, D2 deformation) which wane westward preserving foliate, anatectic S-type, perthite/oligoclase granite derived from the gneiss. Peak granulite-facies conditions, P = 9.5 kbars, T = 950°C, occurred during D1 at 1860 Ma (Van Kranendonk, 1992; Bertrand et al., 1993). Modelled gravity and magnetic susceptibility data show that the Tasiuyak domain is a prismatic body 13 km deep in the east, feathering out over the Lac Lomier complex in the west (Feininger and Ermanovics, 1994). Supracrustal rocks constitute 15% of the Lac Lomier complex and include mainly remnants of granulite-facies semipelite, quartzite, marble, calcsilicate, quartzofelspathic gneiss and amphibolite (Ermanovics and Van Kranendonk, 1992), intercalated

19

with polycyclic Rae Province basement gneisses of Archean age (Rae Province gneisses west of the Nain Plutonic Suite yielded U-Pb zircon ages from 2572_+2o to 2807_+4~ Ma; Ryan et al., 1991). The supracrustal rocks are probably the high-grade equivalents of the Lake Harbour Group, more extensively exposed immediately north-northwest of the study area (Jackson and Taylor, 1972; Goulet and Ciesielski, 1990). According to these workers, the Lake Harbour Group metasediments represent steadily submerging Paleoproterozoic continental shelf and platform deposits sited on older Rae Province crust. Amphibolites of the group and some of its quartzofeldspathic members may be metamorphic equivalents of mafic to felsic volcanic rocks indicative of perhaps once extensive volcanic edifices (Ermanovics and Van Kranendonk, 1992). The orogen includes pre-D2 intrusions that are now represented by orthopyroxene-bearing metaplutonic rocks of granitic to tonalitic composition located with abrupt tectonic contacts with all rocks of the orogen (Fig. 2). Although they collectively portray calcalkaline affinities, the eastern metaplutonic suite, emplaced in the eastern part of the Tasiuyak gneiss, comprises a restricted suite of mainly dioritic and tonalitic rocks, whereas the western metaplutonic suite, in the western Tasiuyak gneiss domain and the Lac Lomier complex tends to include more potassic rocks. An extensively analysed sample from the western metaplutonic suite yielded a U-Pb zircon age of 1877 + 1 Ma and, in light of other geological and geochronological constraints, is regarded as a time of continental arc magmatism in Lac Lomier complex (Bertrand et al., 1993), which would pin the Tasiuyak gneiss to the complex at that time. A similar suite of intrusions in the Burwell domain are dated in the range >1859 Ma (Scott, 1995a,b) to 1910 + 2 Ma (Scott and Machado, 1995).

3. Whole rock geochemistry Fist-size or larger rock samples representing major lithologies from the study area (see Fig. 2 inset for sample location) were crushed and ground to 200-400 mesh size in a tungsten carbide ring mill. Major element data were obtained by XRF analysis on glass pellets made of fused LiBO2 and sample powder. Trace and REE data result from ICP-MS

20

R.J. Thgriault, L Ermanovics / Precambrian Research 81 (1997) 15-35

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samples of the North River-Nutak area. Shaded area represents compositionsof unmetamorphosedcommonigneousrocks (Shaw, 1968). analysis of samples fluxed with LiBO2, thus assuring digestion of refractory mineral phases. Analytical work was carded out at the geochemical laboratories of the Geological Survey of Canada in Ottawa. ICPMS data from some metaplutonic rocks and Tasiuyak gneiss samples were produced at C.N.R.S. in Nancy, France, and are the courtesy of J.M. Bertrand. Uncertainties for major and trace element concentrations are no greater than 5%. A representative subset of major, trace and rare earth element (REE) data are presented in Table 1. A larger data set, obtained by XRF analysis only, is available from the first author. Major element compositions graphically presented below encompass results from this larger data set.

3.1. Geochemical effects of metamorphism Because rocks of the Torngat orogen have undergone granulite grade metamorphism, the applicability of geochemical data to estimating protolith compositions must be considered. Effects of high grade metamorphism include removal or addition of certain elements in partial melts or CO2-rich fluids. Anatexis is ubiquitous, especially in the Tasiuyak gneiss where pods of anatectic granite occur (e.g. sample EES89-G1). K20/Rb ratios are considered as an approximation of extraction of large ion lithophile elements (LILE) in granulites relative to unmetamorphosed equivalents, as these two elements are

among the most mobile in high grade metamorphic conditions (e.g. Chacko et al., 1992; Rudnick et al., 1985). The shaded area in Fig. 3 represents the field encompassing 12 linear regressions for common unmetamorphosed igneous rocks ranging from basalts to granites (Shaw, 1968). Although deviation of lowK20 samples from the shaded field on Fig. 3 is apparent in our data set, most samples show K20/Rb ratios which are concordant with common unmetamorphosed rocks. Furthermore, the differences between metaplutonic suites are not only corroborated by variability in alkali elements, but also by differences in MgO, TiO2, Th and other less mobile elements. Although the present geochemical compositions are not considered exact reflections of the protoliths, this suggests that the bulk rock geochemical patterns in rock suites mostly reflect primary compositional differences between the suites, and effects of metamorphism have apparently not entirely erased many of the essential geochemical characteristics of the protoliths.

3.2. Metasediments The Ramah Group metasediments display a broad major (Fig. 4) and trace element (Fig. 5) compositional range. SiO2 (44-74%) and MgO (1.5-5.6%) suggest highly variable source compositions for the sedimentary protolith. The lower SiO2 and (NazO + K20), and higher MgO in some Ramah Group samples is consistent with contribution from an immature sedimentary component, whereas high silica and alkalis in other samples appear to reflect a geochemically mature source. Strong variations in Th/Nb (0.2-1.2), (La/Yb)y (3-48) and Zr/Y (1.431) are consistent with sedimentary provenance from variably fractionated sources. The higher (La/Yb)N and Zr/Y ratios in some Ramah Group samples are indicative of strong depletions in heavy rare earth elements (HREE) and Y (Fig. 5), which are consistent with Archean upper crustal sources (Condie, 1993). The most REE-fractionated Ramah Group samples (EEM89-NRP and EEM87-197E; Fig. 6) exhibit La/Yb, Gd/Yb and Eu/Eu* akin to those found in Mid Archean Nain Province gneisses (e.g. Schiette et al., 1993; Hamilton, 1993). The Lac Lomier metasediments also display compositional variability, as exemplified by SiO2 (52-

R.J. Thgriault, L Ermanovics / Precambrian Research 81 (1997) 15-35

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Table 1 Bulk-rock geochemical data for Paleoproterozoic rocks of the North River-Nutak area, southern Torngat orogen (oxides in wt%, metals in ppm) Tasiuyak gneiss

Ramah Group EEEM89- EEM89- EEC88- EEM87- EE88NRP SRP 079 197E 105C SiO2 TiO2

64.1 0.79 A1203 17.22 Fe203 a 9.87 MnO 0.03 MgO 2.4 CaO 0.33 Na20 0.58 K20 4.65 P205 0.04 K 38,593 Rb 146 Ba 467 Sr 71 Nb 20.4 Zr 321 Ti 4,723 Y 11 Th 13.62 La 51.55 Ce 106.99 Pr 11.67 Nd 39.88 Sm 6.03 Eu 0.73 Gd 3.89 Tb 0.54 Dy 2.43 Ho 0.4 Er 0.83 Tm 0.12 Yb 0.67 Lu 0.12

70.18 0.77 12.19 12.18 0.06 1.5 0.3 0.57 2.21 0.03 18,340 78 590 50 7.9 181 4,624 18 9.24 30.47 60.95 6.67 24.76 4 0.64 2.95 0.49 2.86 0.63 1.71 0.29 1.71 0.27

39.47 1.64 21.94 27.42 0.33 4.00 0.38 0.18 4.61 0.05 38,247 150 0 20 12.3 150 9,823 106 10.57 51.98 114.52 12.33 46.69 7.84 1.06 8.81 2.03 15.86 3.61 9.69 1.59 9.69 1.32

32.42 2.34 26.77 28.46 0.06 3.47 0.21 0.64 5.49 0.14 45,586 310 747 19 24.6 273 14,031 25 15.48 73.77 145.71 7.3 64.66 10.93 2 8.29 1.18 5.28 0.91 1.91 0.25 1.27 0.21

59.6 0.81 15.57 10.15 0.04 5.78 4.04 0.52 3.38 0.09 28,077 124 309 84 10.3 165 4,884 34 5.77 22.69 40.22 5.36 20.62 3.92 0.78 3.92 0.66 4.23 0.97 2.68 0.48 3.51 0.54

EE88105D

EE88105

EEM88- EES89G1 194A

70.19 0.72 14 7.08 0.02 1.98 0.61 0.29 5.06 0.05 42,013 137 0 60 26.4 519 4,342 17 11.75 52.86 97.89 11.75 41.11 5.87 1.17 4.31 0.58 3.03 0.57 1.37 0.21 1.27 0.2

62.56 1.23 18.59 10.92 0.02 1.75 1.25 1.55 2.01 0.11 16,722 81 320 155 32.9 232 7,374 15 6.2 35.83 70.7 8.81 33.9 6 2.23 5.13 0.75 3.39 0.52 1.07 0.15 0.83 0.15

74.52 0.53 12.47 4.55 0.03 1.74 1.84 2.28 1.96 0.07 16,268 54 594 247 9.9 208 3,204 23 8.51 34.64 69.28 7.72 28.7 5.25 1.39 4.65 0.73 4.16 0,81 2.08 0.33 1.98 0.32

78%) and MgO (0.2-5.8%). Derivation of sedimentary protoliths from variably fractionated sources is suggested by the broad range of (La/Yb)N and Zr/Y (Fig. 5). Lac Lomier metasediments ((La/Yb)N 3-16) are generally less fractionated than Nain Province gneisses ((La/Yb)N commonly 50-100). However, the more REE-fractionated Lac Lomier samples do yield REE characteristics similar to many of the Rae Province gneisses investigated by Hamilton (1993). Also, two moderately light rare earth element (LREE)-enriched samples (EEF89-131A and EEM89-026C) with small or no Eu depletion also

69.28 0.54 13.7 6.85 0.04 0.75 1.48 2.54 4.61 0.22 38,257 235 470 88 15.7 196 3,226 29 23.48 45.01 97.85 11.74 45.01 8.61 0.83 6.75 0.98 5.09 0.97 2.54 0.43 2.54 0.37

EEG88- EEX88- EEC88- EEC88055 102 204 264 67.47 0.71 12.88 10.91 0.05 2.21 2.26 2.19 1.26 0.05 10,458 29 410 0 10.5 200 4,233 23 7.63 36.26 70.62 7.63 28.63 4.49 1.34 4.49 0.7 3.82 0.8 2,19 0,37 2,39 0,36

54.9 0.84 19,45 16.59 0.08 3.49 0.9 1.02 2.67 0.06 22,170 102 0 149 15.8 167 5,021 32 11.17 42.81 81.89 9.31 36.29 6.51 1.3 5.77 0.93 5.12 1.02 2.7 0.42 2,79 0,42

59.58 0.86 20.15 10.37 0.08 3.41 0.97 1.16 3.28 0.15 27,232 119 900 157 15 186 5,141 27 16.75 57.86 113.67 0 49.26 9.07 1.51 7.2 0 5.98 0 3.11 0 3.11 0.48

63.78 0.8 18.34 7.68 0.06 3.28 0.97 1.21 3.76 0.12 31,190 136 996 204 12.9 210 4,783 25 11.82 46.56 87.34 0 35.15 6.85 1.59 5.7 0 5.45 0 2.98 0 3.04 0.5

suggest input to the sedimentary protolith from relatively unfractionated sources. Although the Tasiuyak gneiss shows substantial geochemical variability (e.g. SiO2 58-75%), it is geochemically more uniform relative to the Ramah Group and Lac Lomier metasediments. A decrease in MgO, A1203 and (Na20 + I<20) with increasing SiO2 contents is generally observed in all metasediments of the study area, and may reflect varying degrees of chemical maturity of sedimentary protoliths. Peculiar to the Tasiuyak gneiss, a proportional increase of SiO2 with (Na20 + K20) may suggest

22

R.J. Th3riault, 1. Ermanovics / Precambrian Research 81 (1997) 15-35

Table 1 (continued) Tasiuyak gneiss

SiO2 TiO2 A1203 Fe203 a MnO MgO CaO Na20 K20 P205 K Rb Ba Sr Nb Zr Ti Y Th La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

EE88135A

EE88136B

70.08 0.55 14.27 4.47 0.04 0.78 1.64 2.77 5.12 0.26 42,541 253 512 108 16 180 3,300 26 30.03 45.4 96.01 0 43.24 8.73 0.91 6.57 0 5.61 0 3.11 0 3.09 0.45

70.81 0.55 14 4.2 0.03 0.71 1.62 2.77 5.05 0.26 41,901 240 505 104 12.6 183 3,295 23 19.49 44.68 95.71 0 42.28 8.66 0.88 6.33 0 5.1 0 2.56 0 2.55 0.38

Lac Lomier metasediments EEC88232A

60.07 1.13 17.53 9.06 0.08 2.62 4.24 3.04 1.43 0.79 11,885 36 385 286 15.4 336 6,782 26 3 54.64 121.45 0 61.27 13.32 1.96 10.01 0 6.81 0 2.35 0 1.81 0.25

EER8937A

90.56 0.31 2.99 3.93 0.04 1.13 0.15 0.1 0.76 0.03 6,292 33 349 16 4 180 1,853 8 3.29 9.28 19.95 1.99 7.58 1.4 0.25 1.4 0.25 1.5 0.28 0.68 0. I 1 0.67 0.1

EER8937B

61.98 1.07 12.81 15.58 0.09 1.65 0.77 0.95 4.97 0.12 41,289 104 0 104 23.7 0 6,430 77 24.68 199.33 351.2 35.12 161.36 22.78 4.46 18.03 2.66 14.24 2.94 8.16 1.23 7.88 1.23

EER8928E

74.43 0.57 12.64 5.62 0.03 1.58 1.53 1.6 1.94 0.05 16,072 75 411 110 11 221 3,428 20 14.04 43.13 87.27 10.03 38.12 5.82 0.99 4.31 0.66 3.51 0.7 1.91 0.28 1.91 0.29

Eastern metaplutonic suite EEM89026C

47.9 1.07 17.88 19.5 0.19 4.97 4.12 1.58 2.73 0.06 22,688 176 1.112 167 19.5 148 6,387 47 15.75 50.03 92.65 10.19 37.06 6.58 2.04 6.86 1.3 7.78 1.58 4.26 0.69 4.08 0.59

EEF89131A

50.41 0.66 12.14 25.95 0.23 3.22 6.32 0.54 0.35 0.19 2,889 6 205 170 11.6 125 3,958 43 1.87 16.95 41.94 5.18 23.2 5.44 1.34 6.87 1,16 7,23 1.52 3.93 0.65 4.1 0.61

EEF8995A

61.25 0.54 13.95 12.23 0.13 5.51 0.19 2.01 4.16 0.03 34,506 115 0 55 29.6 277 3,208 36 5.16 48.73 86.96 9.08 33.44 6.31 0.91 7.93 1.24 6.59 1.24 2.96 0.45 2.77 0.4

EEG88054

70.53 0.23 15.52 3.5 0,03 0.69 3,31 4.03 2.09 0.08 17,368 29 1.277 629 2.1 79 1,355 2 0.13 9.82 15.72 1.67 5.89 0.86 1.28 0.62 0.07 0.34 0.07 0.17 0.03 0.17 0.03

EEF88333A

59.51 0.78 18.81 6.41 0.06 2.13 5.34 4.31 2.31 0.34 19,214 44 1.323 503 6.7 244 4,685 9 0.8 42.36 77.72 0 32.41 4.96 1.82 3.6 0 1.73 0 0.74 0 0.61 0.11

EEF88- EEC88411A 214

42.51 1.51 1.51 24.81 0.18 6.43 11.33 1.32 0.29 0.12 2,406 13 54 202 8.1 88 9,056 26 0.39 6.59 18.44 2.63 13.17 3.78 1.23 4.83 0.77 4.39 0.97 2.37 0.39 2.46 0.37

54.24 0.79 17.8 10.39 0.11 4.91 5.79 4.12 1.36 0.5 11,279 16 577 1.138 6.3 134 4,731 13 1 37.74 76.65 0 36.15 5.99 1.81 4.4 0 2.81 0 1.16 0 0.9 0.13

p a r t i a l m e l t i n g as a r e s u l t o f u l t r a m e t a m o r p h i s m . Narrower ranges of Zr/Y (5-13), Th/Nb (0.7-1.1)

3.3. Metaplutonic rocks

and (La/Yb)N indicate a greater trace element homogeneity for the Tasiuyak gneiss compared with the

Although the eastern and western metaplutonic suites s h o w c o n s i d e r a b l e o v e r l a p i n w h o l e r o c k g e o c h e m i s t r y , t h e t w o suites d i s p l a y s o m e d i s t i n c t i v e m a j o r a n d t r a c e e l e m e n t f e a t u r e s . T h e eastern metaplutonic suite is r e s t r i c t e d to t h e q u a r t z diorite, q u a r t z m o n z o d i o r i t e , t o n a l i t e a n d g r a n o d i o r i t e fields o n the normative modal quartz-alkali feldspar-plagioclase d i a g r a m o f L e M a i t r e et al. ( 1 9 8 9 ) (Fig. 7). T h e e a s t e r n suite d i s p l a y s t h e l o w e s t SiO2 ( 4 8 - 7 2 % ) a n d ( N a 2 0 + K 2 0 ) (1.8--6.6%), a n d t h e h i g h e s t M g O (up to 9 . 4 % ) , C a O (up to 1 2 . 9 % ) a n d TiO2 ( u p

Ramah Group and Lac Lomier metasediments. REE features for the Tasiuyak gneiss are similar to those of post-Archean shales (cf. North American shale composite, Gromet et al., 1984). The trace element geochemistry of the Tasiuyak gneiss is consistent with derivation of the sedimentary protolith from crustal sources which are less REE-fractionated than the adjacent Nain and Rae Provinces.

R.J. Th3riault, L Ermanovics / Precambrian Research 81 (1997) 15-35

23

Table 1 (continued) Western metaplutonic suite

Eastern metaplutonic suite

SiO2 TiO2 A1203 Fe203 a MnO MgO CaO Na20 I'(20 P205 K Rb Ba Sr Nb Zr Ti Y Th La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

EEC88083

EEC88255

EEF88339

EEC88116

EEM89008

EEM89019

EE89023

EEM89077A

EED89006B

EEM89050

EEG88071

66.15 0.41 15.85 6.32 0.06 1.39 5.03 4.11 0.56 0.12 4,631 13 157 303 7.9 137 2,464 14 0.15 9.49 21.53 2.54 10.76 2.25 0.81 2.35 0.37 2.06 0.43 1.17 0.2 1.27 0.21

56.28 1.42 13.73 17.37 0.16 2.19 3.75 3.84 0.94 0.32 7,824 7 247 293 20.1 92 8,503 17 0.17 20.13 42.09 5.03 21.05 4.21 1.28 4.3 0.58 3.02 0.59 1.46 0.24 1.46 0.24

52.51 0.88 14.24 10.5 0.16 9.46 8.01 2.45 1.45 0.34 12,061 52 522 665 6 103 5,287 17 0.3 19.58 43.36 0 23.98 5.17 1.46 3.74 0 3.03 0 1.49 0 1.48 0.22

55.44 0.95 15.11 13.99 0.1 3.6 5.85 3.24 1.49 0.22 12,390 44 0 399 8.3 204 5,724 19 0.27 20.39 43.57 5.1 21.32 3.99 1.3 4.36 0.58 3.24 0.65 1.67 0.24 1.67 0.24

60.57 0.63 16.05 8.96 0.09 2.11 4.79 3.82 2.77 0.22 22,988 76 744 616 11.7 225 3,755 25 2.25 46.97 107.64 12.72 49.9 8.9 1.57 6.75 0.92 4.6 0.88 2.35 0.36 2.15 0.34

74.31 0.18 13.45 1.7 0.01 0.33 1.26 2.53 6.2 0.03 51,449 152 647 162 7.9 162 1,091 7 68.75 78.86 151.66 15.17 52.57 7.38 0.99 4.45 0.5 1.82 0.25 0.43 0.07 0.35 0.07

65.99 0.68 12.75 10.63 0.08 0.84 2.43 2.8 3.66 0.14 30,398 85 1.159 164 21.3 357 4,055 32 0.16 24.15 48.31 6.09 26.09 5.6 2.03 5.8 0.9 5.12 1.06 3 0.49 3.09 0.5

75.63 0.17 12.69 2.78 -0.01 0.57 1.97 2.97 3.11 0.12 25,839 30 1.586 278 0.7 139 1,010 4 13.88 53.53 96.15 9.71 31.72 3.67 0.5 1.59 0.2 0.74 0.13 0.3 0.06 0.35 0.08

56.76 1.11 16.23 9.42 0.1 3.21 7.06 4.67 1.18 0.25 9,817 8 228 285 9.5 161 6,668 28 0.4 34.01 68.12 0 31.27 6.61 1.2 5.64 0 5.31 0 3.03 0 3.01 0.49

64.63 0.48 14.45 9.77 0.11 1.92 4.46 3.47 0.62 0.11 5,t17 2 231 241 11.6 318 2,888 25 1.54 26.01 54.9 6.45 24.08 4.82 1.44 4.53 0.71 4.05 0.87 2.41 0.41 2.7 0.43

70.31 0.65 13.08 5.23 0.06 0.97 2.44 2.76 4.35 0.14 36,129 154 856 107 14.9 275 3,917 23 0.2 19.89 41.41 0 23.62 5.53 1.56 4.77 0 4.72 0 2.71 0 2.96 0.46

a Total iron.

to 1.7%) contents of the two metaplutonic suites (Fig. 8). The eastern suite shows variably elevated Rb (RbN 10-100) and Ba (BaN 10-120), and moderately enriched Zr (ZrN 7-22) and La (LaN 10-50) relative to depleted mantle values. Nb values (NbN 3-40) are mostly depleted relative to other incompatible immobile trace elements. The eastern suite is LREE-enriched, moderately fractionated to unfractionated in HREE, and shows unfractionated to enriched Eu (Eu/Eu* 1--6) (Fig. 9). Western metaplutonic suite samples fall in the monzodiorite to granite fields on the normative QAP diagram (Fig. 7). The western suite shows tendencies toward higher SiO2

(57-76%) and (Na20 + K20) (4.2-8.6%), and lower MgO (up to 3.2.%), CaO (up to 7.1%) and TiOz (up to 1.1%) relative to the eastern suite. The western suite generally has higher Rb (Rbr~ < 200), Zr (ZrN 14-28) and La (LaN 28-111) relative to the eastern suite. Western suite samples show moderate to strong LREE-enrichment, small negative Eu anomalies (Eu/Eu* _>0.7), and weak HREE fractionation. 4. Sm-Nd isotope geochemistry Sm-Nd isotopic data for 34 whole rock samples are shown in Table 2, and represent the major map

24

R.J. Th~riault, I. Ermanovics / Precambrian Research 81 (1997) 15-35 30

•1

i"

r

"f

30 25 20

II<>Tasiwak gneiss D Ramah Group

[]|A Lac Lomler metasediments

D

25

ah Grouo iuyak gneiss o c Lomiermetasedirnen s~

151

o o

o%

20

,•LTaP.•

o

N 101

•

o

•

o

00<>oO

O N

o %

<>&

o •

lb

15

2'0

3'0

4b

La(N)/Yb(N) 10 6

i

5 ""

i

n

[]

D

4

i

•

oo &o

<> 0 ~ 0 0<> 0 0

O ~

Fig. 5. Zr/Y vs. [La/Yb]N diagram (after Condie, 1993) for the Ramah Group, Tasiuyak gneiss and Lac Lomier complex metasediments. Increase in Zr/Y and [La/Yb]N is generally associated with provenance from chemically evolved, highly fractionated sources.

[] []

O o<><><>

2 •

o

•

1 Ao~ 0

1

9

,

I

I

8

%0 0 0

~

7 ~A A 0

o

D

o

•

000

5

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[] <>~D <>o:> o

0 0

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÷

D

O []

2 1 1

I

I

I

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6O

70

80

Si02 (wt %) Fig. 4. Variation in AIeO3, MgO and (Na20 + K20) relative to SiO2 for Ramah Group, Tasiuyak gneiss and Lac Lomier complex metasediments. Note general decrease in A1, Mg and alkalis with SiO2 likely denoting increased sediment maturity. The increase in alkalis with SiO2 in the Tasiuyak gneiss may be a reflection of partial melting.

units (Ermanovics and Van Kranendonk, 1992). Locations of sampling sites are shown in Fig. 2. The sample set contains the rocks for which whole rock

geochemical data are shown in Table 1. The set includes seven samples of Ramah Group metapelites, five samples from the Tasiuyak gneiss, one anatectic granite derived from Tasiuyak gneiss, six samples of pelitic metasediments from the Lac Lomier complex, eight samples from the eastern metaplutonic suite, and seven samples from the western metaplutonic suite. The end values are calculated at 1.9 Ga for all rock types. Estimates of the isotopic compositions of the cratonic rocks welded by the Torngat orogen are critical to this study and are needed to identify the crustal reservoirs which contributed source or contaminant materials to the various Paleoproterozoic rocks. As an approximation of the Nd isotopic signature of the Rae Province craton west of the southern Torngat orogen, we consider data reported by Emslie and Thrriault (1991), Emslie et al. (1994), Hamilton (1993) and Hamilton and Shirey (1992) for Late Archean to Paleoproterozoic orthogneisses occurring west of the Mesoproterozoic Nain Plutonic Suite (Fig. 1). These Rae Province rocks yield Sm-Nd isotopic signatures that are similar to those of other regions of the Rae Province (McCulloch and Wasserburg, 1978; Patchett and Arndt, 1986; Stevenson et al., 1989; Dud~ts et al., 1991) suggesting crust formation in the 2.7 Ga to 3.0 Ga period. The Early Archean Uivak (Collerson and McCulloch, 1982) and Kiyuktok (Collerson et al., 1989) gneisses in the Saglek Fiord area, and granodioritic orthogneisses of probable Late to Mid Archean age occurring southeast of the study area near Okak Bay (Hamilton,

R.J. Thgriault, 1. Ermanovics / Precambrian Research 81 (1997) 15-35

25

1000

1O0

lO

I000

Tasiuyak gneiss 1O0

nJ u~ A

1000

l

l

t

l

i

~

t

l

l

l

l

l

J

l

r

i

l

l

F

l

l

l

,

r m

.~,Lac ~ m i e r

metas~imen~

100

10 V v

I

I

I

I

I

I

I

I

I

I

I

I

i

I

La CePr NdSmEu GdTb Dy Ho Er TmYb Lu Fig. 6. Chondrite-normalized REE profiles for Ramah Group, Tasiuyak gneiss and Lac Lomier complex metasediments. Note variation in degree of fractionation of Eu and HREE in the Rarnah Group, homogeneity of the Tasiuyak gneiss and heterogeneity of the Lac Lomier complex.

26

R.J. Thdriault, L Ermanovics / Precambrian Research 81 (1997) 15-35

Q 0 western metaplutonic suite • eastern metaplutonlc suite

2 3a

o 3b°t o~ , 9"

A

°'~0".~, P

Fig. 7. Modal quartz-alkali feldspar-plagioclasefeldsparplot for eastern and western metaplutonicsuites. Classification from LeMaitre et al. (1989). Relevantfields: 3b = granite;4 = granodiorite;5 = tonalite; 9* = quartz monzodiorite;10" = quartz diorite.

1993; SchiCtte et al., 1993) are assumed to represent the Nain Province east of the Torngat orogen.

ducible to 0.3%. eNd values were calculated assuming present-day CHUR 143Nd/144Nd = 0.512638 and 147Sm]144Nd = 0.1967.

4.1. Analytical methods 4.2. Ramah Group metasediments The methods employed are a variant of those described by Richard et al. (1976). Whole rock powders were spiked with a mixed 148Nd-149Sm solution and dissolved in an HF-HNO3 mixture. Separation of bulk rare earth elements followed standard cation exchange chromatography. Purification of Sm and Nd was done by HDEHP-Teflon powder chromatography. Total procedure blanks were approximately 200 pg for Nd and less than 100 pg for Sin. Mass analysis was carried out on a MAT-261 solid source mass spectrometer in static mode. Nd isotopic compositions were normalized to 146Nd/144Nd = 0.7219 and corrected to LaJolla mNd/144Nd = 0.511860. Repeated analysis of an Ames Nd metal solution yielded 143Nd/144Nd ----- 0.512120 4- 10 (external 2or error) (equivalent to La Jolla 143Nd/144Nd --0.511830). The external reproducibility of individual end values is within 4-0.5 ENd unit. On the basis of replicate analyses, 1478m]144Nd are generally repro-

The Ramah Group metasediments display crustal fsmmd ranging from -0.41 to -0.54. end(1.9 Ga) values vary from - 2 . 9 to -12.0. The Ramah Group data infsmmd vs ENd(1.9 Ga) space (Fig. 10a) define a linear array that spans from the field of Nain Province orthogneisses near Okak Bay (Hamilton, 1993; Schi0tte et al., 1993) to a less evolved Nd isotopic composition (c.f. sample EE88-105E; ENd(I.9 Ga) = -2.9). The linear arrangement of the data is interpreted to represent mixing of sediments shed from a crustal source such as the Nain Province gneisses studied by both Hamilton (1993) and SchiCtte et al. (1993), and a source with a more juvenile character. The more juvenile component may be Mugford Group basaltic crest, with ENd(1.9 Ga) of +0.8 to - 6 . 3 and fsmma of approximately - 0 . 3 to - 0 . 4 (Hamilton, 1994), which overlaps with the most juvenile compositions obtained in the Ramah Group.

R.J. Th(riault, I. Ermanovics / Precambrian Research 81 (1997) 15-35 I

.

to -10.5 and -0.24 to -0.54, respectively. Except for one sample (EEF89-131A; ENd(1.9 Ga) = -2.9, fsmmd = -0.24), the Lac Lomier complex data are similar to data for Rae Province gneisses in Labrador (Emslie and Thrriault, 1991; Hamilton and Shirey, 1992; Hamilton, 1993; Emslie et al., 1994) (Fig. 10a). Furthermore, the Lac Lomier complex data display no affinity for the more negative Eya(1.9 Ga) and fsm~a values that generally characterize the Nain Province.

I

eastern metap(utonicsuite

/o w e ~ e m metaplutonic suite o

o

0

•

% o

o oe °

e o

OoO

o

0

co

O

I

4.4. Tasiuyak gneiss

t

12

Five samples of Tasiuyak gneiss and one sample of anatectic granite derived from fusion of Tasiuyak metasediments yield a tight group of ENd(1.9 Ga) values (from -1.9 to -2.6) and Jsn~a (from -0.41 to -0.49). The consistency in fSmmd values suggests that the Sm-Nd system was not seriously disturbed during metamorphism. The isotopic homogeneity displayed by the Tasiuyak gneiss does not require provenance of precursor detritus from a single, homogeneous source, as homogenization of detritus from variable sources could have occurred following source erosion. The ENd(1.9 Ga) values suggest that the source material (or at least the dominant component of the source materials) was provided by crust younger than the known Nain Province or Rae Province crustal reservoirs. A single analysis of Tasiuyak gneiss from the Burwell domain yielded a ENd(1.9 Ga) of -3.8 (Campbell, 1994), suggesting derivation from older crustal sources than those of the Tasiuyak gneiss in the North River-Nutak area.

10

.-,,

=<

8 o

6

e*

o"

0 (.3

e••

•

o

o

4 o °

oo

o o

0

I

I

10 9

,-.

ae •.-*

0 o

o

8

0

~

o

7

o

•

o +

6 •

•

o

°o

~o

s

o

4

Z

3

N

o

o Q

4.5. Eastern metaplutonic suite

2 1 0 40

27

I

50

r

I

60

70

80

Si02 (wt %) Fig. 8. Variation in MgO, CaO and (Na20 + KzO) relative to SiO2 for eastern and western metaplutonic suites. Despite considerable compositional overlap, the eastern suite tends to more primitive compositions.

4.3. Lac Lomier complex metasediments

The six samples of Lac Lomier complex metasediments range in end(1.9 Ga) and fsmma from -2.9

The eight samples of eastern metaplutonic suite show considerable variation in Sm-Nd compositions with ENa(1.9 Ga) spanning from -6.7 to +3.8 and fsmmd ranging from -0.54 to -0.06 (Fig. 10b). With the exception of two samples (EEG88-054 and EEC88-255) the eastern suite has more positive Er~d(1.9 Ga) values than the Nain or Rae crust into which their igneous precursors were presumably emplaced. The eastern suite rocks show no evidence for the presence of a crustal component older than Rae Province crust, though this is permissible if mixing between an older crustal component and a juvenile

28

R.J. Th~riault, 1. Errnanovics / Precambrian Research 81 (1997) 15-35

1000

.

.

.

.

.

.

.

.

.

.

.

East. metaplutonic suite 100

10

'C.

1

cO c-" L) (L) O.

I

I

I

La CePr

E looo

i

I

I

I

I

I

I

I

I

I

t

I

N d S m E u GdTo Dy Ho Er T m Y b Lu

~

r

,

,

i

l

,

,

r

,

west. metaplutonic suite 10C

10

I

I

La C e R

I

I

I

t

I

I

I

r

I

I

I

I

NdSmEu GdTb Dy Ho Er TmYb Lu

Fig. 9. Chondrite-normalizedREE profilesfor eastern and western metaplutonic suites. Note lack of negative Eu anomalies and in some cases almost fiat patterns in the eastern suite. Western suite features negativeEu anomalies, and generally stronger LREE-enrichment.

source has occurred. The scatter of data on Fig. 10b may imply derivation of the melts from multiple sources which were variably evolved in terms of Nd isotopic compositions, and variably fractionated in terms Sm and Nd. The Nd isotopic composition of the eastern metaplutonic suite are similar to those of the 1.9 Ga tonalitic to dioritic orthogneisses in the eastern part of the Burwell domain, for which Campbell (1994) reports ~Nd(1.9 Ga) values of - 0 . 8 to +3.1

4.6. Western metaplutonic suite

The seven samples from the western metaplutonic suite show ~Nd(1.9 Ga) values which range from - 1 . 5 to - 8 . 5 and fSmmd values which vary from - 0 . 3 3 to -0.55. The ENd(1.9 Ga) values are generally lower than those of the eastern suite. Excluding samples EE89-023 (ENO(1.9 Ga) = - 1.5) and EEG88-071 (eNd(1.9 Ga) = -2.0), these end(1.9 Ga) values overlap with Rae Province crust values (Fig. 10b). The close resemblance to Rae Province

R.J. ThLriault, L Ermanovics / Precambrian Research 81 (1997) 15-35

29

Table 2 Sm-Nd isotopic data for Paleoproterozoic rocks of the North River-Nutak map area, southern Torngat orogen 147Sm/144Nd

fSm/Nd

143Nd/144Nd(20")

ENd (1.9 Ga)

63.67 43.01 22.83 97.58 19.85 45.20 37.61

0.1019 0.0898 0.0959 0.1005 0.1167 0.0909 0.1129

-0.48 -0.54 -0.51 -0.49 -0.41 -0.54 -0.43

0.511023(10) 0.510690(5) 0.510884(7) 0.511037(6) 0.511262(9) 0.510820(9) 0.511443(9)

-8.4 -12 -9.7 -7.8 -7.4 -9.7 -2.9

0.85 4.90 4.98 2.33 4.73 4.64 6.78 5.06

5.71 23.64 33.16 10.58 23.99 15.24 40.76 25.32

0.0901 0.1253 0.0908 0.1331 0.1192 0.1843 0.1006 0.1208

-0.54 -0.36 -0.54 -0.32 -0.39 -0.06 -0.49 -0.39

0.510962(6) 0.511513(6) 0.511296(7) 0.512038(8) 0.511697(9) 0.512625(3) 0.511361(9) 0.511678(7)

-6.7 -4.6 -0.4 +3.8 0.5 +2.8 -1.5 -0.2

EEM89-019 EE89-023 EEM89-008 EEM89-077A EEM89-006B EEM89-050 EEG88-071

8.10 6.78 9.58 3.69 6.50 4.97 5.80

54.70 30.86 52.85 32.81 32.52 25.93 25.83

0.0895 0.1328 0.1096 0.0679 0.1208 0.1158 0.1357

-0.55 -0.33 -0.44 -0.66 -0.39 -0.41 -0.31

0.510950(8) 0.511765(5) 0.511193(3) 0.510593(10) 0.511426(6) 0.511314(10) 0.511774(8)

-6.8 - 1.5 -7 -8.5 -5.2 -6.1 -2.0

EEG88-055 EEX88-102 EEC88-264 EEM89-194A EEC88-204

5.18 7.82 6.87 5.42 10.61

31.03 43.31 36.82 30.43 60.28

0.1008 0.1091 0.1128 0.1079 0.1064

-0.49 -0.45 -0.43 -0.45 -0.46

0.511321(6) 0.511447(8) 0.511466(7) 0.511396(10) 0.511398(11)

-2.3 - 1.9 -2.4 -2.6 -2.2

47.44 158.5 26.96 8.42 44.74 37.57 38.21

0.1153 0.0966 0.1488 0.1119 0.1061 0.1179 0.1188

-0.41 -0.51 -0.24 -0.43 -0.46 -0.40 -0.40

0.511532(9) 0.511022(6) 0.511890(12) 0.511181(9) 0.511202(3) 0.511417(10) 0.511129(7)

-1.8 -7.2 -2.9 -7.8 -6 -4.6 -10.5

Number in Fig. 2

Sample

Sm (ppm)

Ramah Group 1 2 3 4 5 6 7

EEC88-079 EEM89-NRP EEM89-SRP EEM87-197E EE88-I05C EE88-105D EE88-105E

10.74 6.39 3.62 16.23 3.83 6.80 7.02

Nd (ppm)

Eastern metaplutonic suite 8 9 10 11 12 13 14 15

EEG88-054 EEC88-255 EEF88-333A EEC88-083 EEC88-116 EEF88-411B EEC88-214 EEF88-339

Western metaplutonic suite 16 17

18 19 20 21 22

Tasiuyak gneiss 23 24 25 26 27

Anatectic granite Lac Lomier metasediments 28 29 30 31 32 33 34

EES89-G1 EER89-37B EEF89-131A EER89-37A EEM89-26C EEF89-95A EER89-28E

9.05 25.35 6.64 1.56 7.86 7.32 7.51

signatures is consistent with a Rae Province crustal protolith, given the close spatial association between the western suite and Rae Province basement gneisses of the Lac Lomier complex. The involvement of more juvenile source materials may be manifested by the less evolved Sm-Nd isotopic signatures of samples EE89-023 and EEG88-071. In the western part of the Burwell domain, a sample of 1895 Ma

charnockitic orthogneiss yielded a ENd(1.9 Ga) value of -6.7 (Campbell, 1994), falling within the range of ENd(1.9 Ga) values of the western metaplutonic suite. 5. Discussion The Sm-Nd isotopic and geochemical data indicate possible sources for the Paleoproterozoic meta-

30

R.J. Thdriault, I. Ermanovics / Precambrian Research 81 (1997) 15-35

0.4

o

-E

Ramah Group

oTasiuyak gneiss • Lac Lomier metasediments

0.2 0

Mugfctd Gp:'......... ,,

-0.2.

/'

t Ee.~yArchean ....... " ......... ~

_rl A / ~" ~

"O

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....... ~ Z .---:'?:~'"

,;

8

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0.4 •eastem

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o western rnetaplutonic suite

0 -0.2 -0.4

Ec~t Arc ~ N ~ n ProCI, ce j"

.......

..................... ,i / ......

../o

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./:. . . . . )<

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1~Nd(1.9 Ga) Fig. 10. fSm/Nd VS. ~Nd(1.9 Ga) for (a) Ramah Group, Tasiuyak gneiss and Lac Lornier metasedirnents and (b) eastern and western orthogneiss suites. Nain Province crust is represented by Early Archean Uivak (Collerson and McCulloch, 1982) and Kiyuktok (Collerson et al., 1989) gneisses near Saglek Fiord, and Mid Archean gneisses exposed south of Okak Bay (Hamilton, 1993; Schiette et al., 1993). Rae Province crust is represented by Late Archean gneisses exposed west of the Mesoproterozoic Nain Plutonic Suite (Emslie and Thdriault, 1991; Hamilton and Shirey, 1992; Hamilton, 1993; Emslie et al., 1994). Field for Mugford Group volcanic rocks after Hamilton (1994).

sedimentary and metaplutonic rocks of the study area. The bulk rock geochemistry and Sm-Nd isotope systematics of metasediments from the Ramah Group, Lac Lomier complex and Tasiuyak gneiss support distinct provenances for the metasediments. Specifically, some Ramah Group metasediments demonstrate REE geochemical and Sm-Nd isotopic affinities for Late to Mid Archean Nain Province crust such as exposed in the Okak Bay area (Hamilton, 1993; Schi0tte et al., 1993). Paleocurrent data

in the Ramah Group suggest a general southeastern derivation of detritus (Knight and Morgan, 1981), and support the argument that gneisses in the Okak Bay area are potential source rocks for Ramah Group sediments. Although the Ramah Group Sm-Nd isotopic data partly overlap those of the Rae Province, the latter is not considered a likely source for Ramah Group sediments, given that the Ramah Group is deposited on Nain Province, the southeastern paleocurrent directions in Ramah Group strata, and that Ramah deposition occurred prior to Nain-Rae docking. As noted earlier, the linear array defined by the Ramah Group data in fsmmd vs. eNd(1.9 Ga) space (Fig. 10a) projects from Nain Province compositions to Mugford Group compositions, and may suggest that Ramah Group protoliths were derived from Nain Province and Mugford Group sources. The presence of material derived from the Mugford Group is supported by: (i) stratigraphic correlations between the Mugford Group and lower parts of the Ramah Group (Smyth and Knight, 1978), (ii) paleocurrent directions in the lower parts of the Ramah Group suggesting southeasterly-derived detritus (Knight and Morgan, 1981), (iii) relatively unfractionated REE patterns and comparatively more primitive bulk compositions for some Ramah Group samples, (iv) immobile trace element ratio variations with Nd isotopic compositions showing Ramah Group data points distributed in an array bridging fields occupied by data from Mugford Group and Nain Province gneisses south of Okak Bay (Fig. 11). Relationships between Nd isotopic and major element compositions for the Mugford and Ramah groups, and Nain Province are not as well defined as the trace element--ENd relationships shown on Fig. 11, but do support contributions from both high MgO-low SiO2-high end and low MgO-high SiO2-1ow ENd sources. A striking feature of the data set for the Tasiuyak gneiss is the REE and Sm-Nd isotopic homogeneity (eNd(1.9 Ga) from -1.9 to -2.6; fsmmd from -0.41 to -0.49) exhibited by this voluminous metasedimentary assemblage. Such compositional homogeneity suggests that REE fractionation did not occur in the Tasiuyak gneiss in spite of granulite facies conditions. The robust nature of the REE is further demonstrated by a sample of anatectic granite (EES89-G1) derived from melting of Tasi-

R.J. Thgriault, I. Ermanovics / Precambrian Research 81 (1997) 15-35

100

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SNO[1.90a] Fig. 11. La/Nb and La/Yb vs. ~Nd(1.9 Ga) plots for Ramah Group metasediments, Mugford Group basalts and Nain Province gneisses near Okak Bay. The data set supports derivation of Ramah sedimentary protolith from Mugford and Nain sources. Data from Mugford Group and Nain Province from Hamilton (1994).

uyak metasediments (Bertrand et al., 1993) which yields a Sm-Nd signature (ENd(1.9 Ga) = -1.8; fsmma = -0.41) and REE pattern that do not deviate significantly from the uniformity exhibited by the metasedimentary gneiss. The REE patterns and ENd(1.9 Ga) values of the Tasiuyak gneiss in the study area clearly indicate sedimentary sources which are different from Nain and Rae provinces. The post-Archean shalelike REE patterns (LREE-enrichment, strong negative Eu anomaly, flat HREE patterns) are consistent with derivation from crustal sources which are less fractionated than either Rae or Nain crusts. The ENd(1.9 Ga) values of approximately - 2 imply that, at least in the study area, provenance of the sed-

31

imentary protolith was from more juvenile crustal sources than Rae or Nain crusts. Distal (up to 1000 km; e.g. southern Chile, Schweller and Kulm, 1978) provenances for sediments in a forearc or trench setting are a possible explanation for the exotic REE and Nd isotopic compositions of the Tasiuyak gneiss. Axial transport along a trench system may account for distal sedimentary sources. The Lac Lomier complex metasediments yield Sm-Nd isotopic signatures which are generally similar to those of Rae Province gneisses (Emslie and Thrriault, 1991; Hamilton and Shirey, 1992; Hamilton, 1993; Emslie et al., 1994). This is consistent with derivation of the Lac Lomier complex metasediments from older Rae Province sources. The broad range of end(1.9 Ga) values (-10.5 to -2.9) and variable REE patterns and abundances reflect heterogeneity in age and composition of source materials. In end(1.9 Ga) vs fsmmo space, the Sm-Nd data of the Lac Lomier metasediments produce a linear array trending from Rae Province compositions toward more LREE-depleted and isotopically juvenile compositions. This suggests that Lac Lomier complex sedimentary protoliths also received detritus from other more primitive, undefined sources. An important implication of our data is the recognition of two distinct suites of metaplutonic rocks, defined by spatial distribution, geochemistry and Nd isotopic signature. The eastern suite covers the compositional range of the western suite, yet the eastern suite has a tendency toward more primitive geochemical compositions and more juvenile Nd isotopic compositions. The compositional affinities of the eastern suite are highlighted by lower SiO2, (K20 + Na20), Rb and Th, greater Mg and Ti, and weakly LREE-enriched to unfractionated REE distribution patterns. Initial end values ranging from --6.7 to +3.8 are indicative of heterogeneous melt compositions incorporating juvenile and older crustal components. The nature of the juvenile component remains ambiguous, although positive end(1.9 Ga) and negative fsrnmd values are consistent with melting of recently LREE-enriched mantle or young LREEenriched crust. The old crustal component in the eastern orthogneiss suite may include Rae Province crust and Tasiuyak gneiss because the ENd(1.9 Ga) versus fSmmd values for the eastern metaplutonic suite overlap Rae and Tasiuyak values (Fig. 10).

32

R.J. Thdriault, L Ermanovics / Precambrian Research 81 (1997) 15-35

The dominantly felsic composition of the western metaplutonic suite is consistent with its strongly negative eNd(1.9 Ga) and fsmma values. Considering the field setting of the western suite and the similarity between the Nd isotopic composition of this suite with Rae Province crust, derivation of the suite's plutonic precursors involved pervasive assimilation of Rae crust in primary melts of more juvenile composition, or intracrustal melting of Rae Province crust. Minor input from a more juvenile component is suggested by ENd(1.9 Ga) of - 1.5 and -2.0 and fsmmd values of -0.3 for samples EE89-023 and EEG88071. Similar to the eastern metaplutonic suite, the western suite shows no evidence for Nain Province crustal sources. The absence of Nain Province StaNd isotopic signatures does not preclude the presence of a Nain Province crustal component in the petrogenesis of the eastern or western suites, yet favors the involvement of Rae Province crust as the crustal component intermixed to variable extents with a juvenile component. Recycling of uniquely Rae Province crust in Torngat orogen magmatism implies that, at least at the latitude of the study area, magmatism related to the Nain-Rae collision may have developed on the Rae Province margin. If continental arc magmatism is represented largely by parts of the western metaplutonic suite, it follows that subduction of oceanic lithosphere occurred beneath the active Rae Province margin. Considering that the eastern metaplutonic suite is located strictly within the Tasiuyak gneiss, the eastern suite may represent forearc magmatism. Phanerozoic examples of forearc magmatism include the Cretaceous tonalite-trondhjemite plutonic suite of southern Alaska which displays many petrological similarities with the eastern metaplutonic suite (e.g. lithological types and major element geochemistry) (Pavlis et al., 1988). The end(1.9 Ga) values of -0.8 to +3.1 for 1.91 Ga tonalitic to dioritic metaplutonic bodies (DTG suite of Van Kranendonk et al., 1993) in the eastern part of the northern Torngat orogen (Burwell domain), and the ENa(I.9 Ga) of -6.7 for a charnockitic body (Campbell, 1994) in the western part of the Burwell domain are similar to the spatial distribution of Nd isotopic compositions and general lithologies of the eastern and western metaplutonic suites of this study (more juvenile in the east, more evolved

in the west). However, Scott (1995a,b) suggests that Paleoproterozoic plutonism in the Burwell domain resulted from eastward subduction beneath the Nain Province, and that the DTG suite represents continental arc plutonism in the Nain Province. Furthermore, Scott (1995b) proposes that the entire Torngat orogen evolved under such a tectonic regime, and argues that differential uplift or Rae Province crust in the southern part of the orogen resulted in exhumation and erosion of Paleoproterozoic plutonic rocks emplaced in the western Nain Province at the latitude of this study area. According to Scott (1995b), the western metaplutonic suite would represent the roots of syn- and post-collision (syn- and post-D1) plutonic bodies which would have developed by melting of thickened Rae Province crust following collision and tectonic stacking. Regardless of the orientation of subduction, the similarities in lithology, Nd isotopic composition and east-west disposition of dioritic to granitic Paleoproterozoic metaplutonic rocks in this study area and the Burwell domain are striking. Such similarities may indicate common sources and processes in the petrogenesis of Paleoproterozoic plutonic rocks along the entire length of the Torngat orogen, thus suggesting a common tectonic framework for the entire orogen. 6. Conclusions Our Sm-Nd isotopic and accompanying geochemical data reveal the following aspects of the Torngat orogen as represented in the North RiverNutak transect area. (1) The metasediments of the Ramah Group were likely derived from detritus shed from Late to Mid Archean Nain Province crust and Mugford Group mafic volcanic rocks. (2) The sources of the Tasiuyak gneiss do not lie in the Archean rocks of the Rae or Nain Provinces, but in a potentially distal crustal source with more juvenile Nd isotopic compositions and less fractionated REE than Rae or Nain crust. The protoliths of the Tasiuyak gneiss were likely Paleoproterozoic foredeep shales starved of detritus from adjacent cratons. (3) The Lac Lomier metasediments were mostly derived from older Rae Province crust with a minor input of a more juvenile, yet undefined component.

R.J. Thgriault, 1. Ermanovics /Precambrian Research 81 (1997) 15-35

(4) The eastern metaplutonic suite may represent forearc plutonism in the eastern part of the Tasiuyak gneiss. Generation of the igneous protoliths likely involved melting and mixing of LREE-enriched mantle or juvenile (oceanic?) crust, Rae crust and possibly Tasiuyak gneiss. (5) The western metaplutonic suite may represent continental arc and post-collision (pre-, synand post-Dr) plutonic rocks emplaced along a Rae Province active margin. Petrogenesis involved pervasive assimilation or melting of Rae Province crust. (6) The Nd isotopic compositions of the eastern and western suites resemble Nd isotopic compositions of tonalitic-dioritic and charnockitic orthogneisses in the eastern and western Burwell domain, respectively. The similarity in lithologies, Nd isotopic compositions and spatial distribution of Paleoproterozoic metaplutonic rocks in the southern and northern Torngat orogen may be viewed as evidence that the entire Torngat orogen developed under a single tectonic regime.

Acknowledgements We are grateful to Bill Davis, Simon Hanmer, Randy Parrish, Dave Scott and Tom Skulski for stimulating discussions and reviews. Journal reviewers K. Condie and A. Goodwin are acknowledged for their constructive comments. J.M. Bertrand is thanked for providing REE data on Tasiuyak gneiss and metaplutonic gneiss samples. This is Geological Survey of Canada contribution No. 16995.

References Bertrand, J.-M., Roddick, J.C., Van Kranendonk, M.J. and Ermanovics, I., 1993. U-Pb geochronology of deformation and metamorphism across a central transect of the Early Proterozoic Torngat orogen, North River map area, Labrador. Can. J. Earth Sci., 30: 1470-1489. Calon, T. and Jamison, W.R., 1994. Structural evolution of the eastern borderland of the Torngat orogen, Lake Kiki transect, Saglek Fiord area, northern Labrador. In: Eastern Canadian Shield Onshore-Offshore Transect (ECSOOT), transect meeting (December 10-11, 1993), Universit6 du QuEbec ~ Montreal, Lithoprobe Report, 36: 89-99. Campbell, L.M., 1994. Preliminary results from an Sm-Nd isotopic study of Proterozoic crustal formation and accretion in the Torngat orogen, northern Labrador. In: Eastern Canadian Shield Onshore-Offshore Transect (ECSOOT), transect

33

meeting (December 10-11, 1993), Universit6 du QuEbec MontrEal, Lithoprobe Report, 36: 100-107. Chacko, T., Kumar, G.R.V., Meen, J.K. and Rogers, J.J.W., 1992. Geochemistry of high-grade supracrustal rocks from the Kerala Khondalite Belt and adjacent massif charnockites, South India. Precambrian Res., 55: 469-489. Collerson, K.D. and McCulloch, M.T., 1982. The origin and evolution of Archaean crust as inferred from Nd, Sr and Pb isotopic studies in Labrador. 5th International Conference on Geochronology, Cosmochronology and Isotope Geology, Extended Abstracts, pp. 61-62. Collerson, K.D., McCulloch, M.T. and Nutman, A.R, 1989. Sr and Nd isotope systematics of polymetamorphic Archean gneisses from southern West Greenland and Northern Labrador. Can J. Earth Sci., 26: 446466. Condie, K.C., 1993. Chemical composition and evolution of the upper continental crust: contrasting results from surface samples and shales. Chem. Geol., 104: 1-37. Dud~is, EO, LeCheminant, A.N. and Sullivan, R.W., 1991. Reconnaissance Nd isotopic study of granitoid rocks from the Baker Lake region, District of Keewatin, N.W.T., and observations on analytical procedures in Radiogenic Age and Isotopic Studies: Report 4, Geol. Surv. Can., Pap., 90-2:101-112. Emslie, R.E and Loveridge, W.D., 1992. Fluorite-bearing early and middle Proterozoic granites, Okak Bay area, Labrador: geochronology, geochemistry and petrology. Lithos, 28: 87109. Emslie, R.E and Tberiault, R.J., 1991. Sm-Nd and Rb-Sr isotopic characteristics of ferrodiorites and related rocks associated with anorthositic complexes, central Labrador. GAC/MAC Annual Meeting, Progr. Abstr., 16: A34. Emslie, R.E, Hamilton, M.A. and ThEriault, R.J., 1994. Petrogenesis of a Mid-Proterozoic anorthosite-mangeritecharnockite-granite (AMCG) complex: isotopic and chemical evidence from the Nain Plutonic Suite. J. Geol., 102: 539-558. Ermanovics, I. and Ryan, B., 1990. Early Proterozoic orogenic activity adjacent to the Hopedale block of southern Nain Province. Geosci. Can., 17: 293-296. Ermanovics, I. and Van Kranendonk, M.J., 1990. The Torngat Orogen in the North River-Nutak transect area of Nain and Churchill provinces. Geosci. Can., 17: 279-283. Ermanovics, I. and Van Kranendonk, M.J., 1992. Geology, North River-Nutak map area, Labrador. Geological Survey of Canada, Map, scale 1:100,000, Open File 2443. Ermanovics, I., Van Kranendonk, M., Corriveau, L., Mengel, F., Bridgewater. D. and Sherlock, R., 1989. The boundary zone of the Nain--Churchill provinces in the North River-Nutak areas, Labrador. Geol. Surv. Can., Pap., 89-1C: 385-394. Feininger, T. and Ermanovics, I., 1994. Geophysical interpretation of the Torngat orogen along the North River- Nutak transect, Labrador. Can. J. Earth Sci., 31: 722-727. Goulet, N. and Ciesielski, A., 1990. The Abloviak shear zone and the NW Torngat orogen, eastern Ungava Bay, QuEbec. Geosci. Can., 17: 269-272. Gromet, L.R, Dymek, R.E, Haskin, L.A. and Korotev, R.L., 1984. The 'North American Shale Composite': its compilation, major and trace element characteristics. Geochim. Cos-

34

R.J. Thdriault, L Ermanovics / Precambrian Research 81 (1997) 15-35

mochim. Acta, 48: 2469-2482. Hamilton, M.A., 1993. Contamination of Massif Anorthosite and Precambrian Crustal Evolution in Central Labrador: A Combined Trace Element and Sr, Nd and Pb Isotopic Study. Unpubl. Ph.D. dissertation, Univ. Massachusetts, 195 pp. Hamilton, M.A., 1994. Tholeiitic and weakly alkalic basaltic volcanism of the Mugford Group, northern Labrador: preliminary geochemical results. Geol. Surv. Can., Current Res., 1994-C: 333-342. Hamilton, M.A. and Shirey, S,B., 1992. Nd and Sr isotopic variations in anorthositic rocks of the Nain Plutonic Suite, Labrador. Eos, Trans. AGU, Spring Meeting 1992, 73 Suppl., p. 355. Hoffman, RE, 1987. Early Proterozoic foredeeps, foredeep magmatism, and Superior-type iron-formations of the Canadian Shield. In: A. Krrner (Editor), Proterozoic Lithospheric Evolution. Am. Geophys. Union, Geodyn. Ser., 17: 85-98. Hoffman, P.F., 1990. Dynamics of the tectonic assembly of northeast Laurentia in geon 18 (1.9-1.8 Ga). Geosci. Can., 17: 222-226. Jackson, G.D. and Taylor, EC., 1972. Correlation of major Aphebian rock units in the northeastern Canadian Shield. Can. J. Earth Sci., 9: 1650--1669. Knight, I. and Morgan, W.C., 1981. The Aphebian Ramah Group, northern Labrador. In: EH.A. Campbell (Editor), Proterozoic Basins of Canada. Geol. Surv. Can., Pap., 81-10: 313-330. Korstgftrd, J., Ryan, B. and Wardle, R., 1987. The boundary between Proterozoic and Archean crustal blocks in central West Greenland and northern Labrador. In: R.G. Park and J. Tarney (Editors), Evolution of the Lewisian and Comparable Precambrian High Grade Terrains. Geol. Soc. London, Spec. Publ., 27: 247-259. LeMaitre, R.W., Bateman, R, Dudek, A., Keller, J., Lameyre Le Bas, M.J., Sabine, P.A., Schmid, R., Sorensen, H., Streckeisen, A., Woolley, A.R. and Zanettin, B., 1989. A Classification of Igneous Rocks and Glossary of Terms. Blackwell, Oxford. McCulloch, M.T. and Wasserburg, GJ., 1978. Sm-Nd and RbSr chronology of continental crust formation. Science, 200: 1003-1011. Mengel, E and Rivers, rE, 1989. Thermotectonic evolution of the Proterozoic and reworked terranes along the Nain/Churchill boundary in the Saglek area, northern Labrador. In: J.S. Daly, R.A. Cliff and B.W.D. Yardley (Editors), Evolution of Metamorphic Belts. Geol. Soc. London, Spec. PUN., 43: 319-324. Morgan, W.C., 1975. Geology of the Precambrian Ramah Group and basement rocks in the Nachvak Fiord-Saglek Fiord area, north Labrador. Geol. Surv. Can., Pap., 74-54, 42 pp. Patchett, P.J. and Arndt, N.T., 1986. Nd isotopes and tectonics of 1.9-1.7 Ga crustal genesis. Earth Planet. Sci. Lett., 78: 329-338. Pavlis, T.L., Monteverde, D.H., Bowmann, J.R., Rubenstone, J.L. and Reason, M.D., 1988. Early-Cretaceous near-trench plutonism in southern Alaska: a tonalite-trondhjemite intrusive complex injected during ductile thrusting along the boarder ranges fault system. Tectonics, 7:1179-1199. Richard, P., Shimuzu, N. and All~gre, C.J., 1976. t43Nd/146Nd, a

natural tracer: an application to oceanic basalts. Earth Planet. Sci. Lett., 31: 269-278. Rivers, T. and Mengel, F., 1994. A cross-section of the Abloviak shear zone at Saglek Fiord, and a preliminary tectonic model for Torngat orogen. In: Eastern Canadian Shield OnshoreOffshore Transect (ECSOOT), transect meeting (December 10--11, 1993), Universit6 du Qufbec a Montrral, Lithoprobe Report, 36: 171-184. Rudnick, R.L., McLennan, S.M. and Taylor, S.R., 1985. Large ion lithophile elements in rocks from high-pressure granulite facies terrains. Geochim. Cosmochim. Acta, 49: 1645-1655. Ryan, B., and Martineau, Y., 1992. Geology of the Saglek FiordHebron Fiord area, Labrador (NTS 14/2,3,6,7), 1:100,000 scale. Newfoundland Department of Mines and Energy, Geological Survey Branch, Open File Map 92-18 and Geological Survey of Canada, Open File 2466. Ryan, B., Krogh, T.E., Heaman, L., SchS_rer, U., Philipe, S. and Oliver, G., 1991. On recent geochronological studies in the Nain Province, Churchill Province and Nain Plutonic Suite, north-central Labrador. In: Current Research, Newfoundland Dept. Mines and Energy, Report, 91-1: 257-261. Schi0tte, L., Noble, S. and Bridgwater, 1990. U-Pb mineral ages from northern Labrador: possible evidence for interlayering of early and middle Archean tectonic slices. Geosci. Can., 17: 227-231. Schi0tte, L., Hansen, B.T., Shirey, S.B. and Bridgwater, D., 1993. Petrological and whole rock isotopic characteristics of tectonically juxtaposed Archaean gneisses in the Okak area of the Nain Province, Labrador: relevance for terrane models. Precambrian Res, 63: 293-323. Schweller, W.J. and Kulm, L.D., 1978. Depositional patterns and channelized sedimentation in active Pacific trenches. In: D.J. Stanley and G., Kelling (Editors), Sedimentation in Submarine Canyons, Fans and Trenches. Dowden, Hutchinson and Ross, Stroudsburg, Penn., pp. 311-324. Scott, D.J., 1994. U-Pb geochronology of the northeastern Rae Province, Part 1. A preliminary report on the Lac Lomier complex, northern Labrador. In: Eastern Canadian Shield OnshoreOffshore Transect (ECSOOT), transect meeting (December 10-11, 1993), Universit6 du Qurbec 5 Montrral, Lithoprobe Report, 36: 90--109. Scott, D.J., 1995a. U-Pb geochronology of the Nain craton on the eastern margin of the Torngat Orogen, Labrador. Can. J. Earth Sci., 32: 1859-1869. Scott, D.J., 1995b. U-Pb geochronology of a Paleoproterozoic continental magmatic arc on the western margin of the Archean Nain craton, northern Labrador, Canada. Can. J. Earth Sci., 32: 1870-1882. Scott, D.J. and Machado, N., 1994. U-Pb geochronology of the northern Torngat Orogen; results from work in 1993 in 1994. In: Eastern Canadian Shield Onshore--Offshore Transect (ECSOOT), transect meeting (December 10-11, 1993), Universit6 du Qurbec ~ Montrral, Lithoprobe Report, 36: 141-155. Scott, D.J. and Machado, N., 1995. U-Pb geochronology of the northern Torngat Orogen, Labrador Canada: a record of Paleoproterozoic magmatism and deformation. Precambrian Res., 70: 169-190.

R.J. Thgriault, L Ermanovics/Precambrian Research 81 (1997) 15-35

Shaw, D.M., 1968. A review of K-Rb fractionation trends by covariance analysis. Geochim. Cosmochim. Acta, 32: 573601. Smyth, W.R., 1976. Geology of the Mugford Group, northern Labrador. Newfoundland Department of Mines and Energy, Mineral Development Division, Report, 77-1: 72-79. Smyth, W.R. and Knight, I., 1978. Correlation of the Aphebian supracrustal sequences, Nain Province, northern Labrador. Newfoundland Department of Mines and Energy, Mineral Development Division, Report, 78-1: 59--64. Stevenson, R.K., Patchett, P.J. and Martin, R.E, 1989. Sm-Nd isochron from a granodiorite-granite complex in the Portman Lake region, Northwest Territories. Can. J. Earth Sci., 26: 2724-2729. Taylor, EC., 1979. Reconnaissance geology of a part of the Precambrian Shield, northeastern Qu6bec, northern Labrador and Northwest Territories. Geol. Surv. Can., Mem., 393. Van Kranendonk, M.J., 1992. Geological Evolution of the Archean Nain Province and the Early Proterozoic Torngat Orogen As Seen Along a Transect in the North River-Nutak Map Area, Northern Labrador, Canada. Unpubl. PhD thesis, Department of Geological Sciences, Queen's University, Kingston, Ontario, 477 pp.

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Van Kranendonk, M.J. and Ermanovics, I., 1990. Structural evolution of the Hudsonian Torngat Orogen in the North River map area, Labrador: evidence for east-west transpressive collision of Nain and Rae continental blocks. Geosci. Can., 17: 283-288. Van Kranendonk, M.J. and Wardle, R.J., 1994. Geological synthesis and musings on possible subduction-accretion models in the formation of the northern Torngat Orogen. In: Eastern Canadian Shield Onshore-Offshore Transect (ECSOOT), transect meeting (December 10-11, 1993), Universit6 du Qutbec Montrtal, Lithoprobe Report, 36: 62-80. Van Kranendonk, MJ., St-Onge, M.R. and Henderson, J.R., 1993. Paleoproterozoic tectonic assembly of Northeast Laurentia through multiple indentations. Precambrian Res., 63: 325-347. Wardle, R.J., 1983. Nain-Churchill Province cross-section, Nachvak Fjord, northern Labrador. In: Current Research, Newfoundland Dept. Mines and Energy, Report, 83-1: 68-89. Wardle, R.J., Van Kranendonk, M.J., Mengel, E, Scott, D., Schwarz, S., Ryan, B. and Bridgwater, D., 1993. Geological mapping in the Torngat Orogen, northwestern Labrador: Report 2. In: Current Research, Newfoundland Dept. Mines and Energy, Report, 93-1: 77-89.