Current Research (2019) Newfoundland and Labrador Department of Natural Resources Geological Survey, Report 19-1, pages 147-156
GEOLOGICAL SETTING OF THE INGRID GROUP, LABRADOR
A. Hinchey and D. Corrigan1 Regional Geology Section 1Geological Survey of Canada, 401 Booth Street, Ottawa, ON, K1A 0E8
ABSTRACT
Research this past summer (2018) represented the completion of the field work component of the Geological Survey of Canada–Geological Survey of Newfoundland and Labrador–Nunatsiavut collaborative project aimed at upgrading the geo- scientific knowledge of, and stimulating mineral exploration in, the Hopedale region of Labrador. This project is supported by the Geomapping for Energy and Minerals (GEM) II program at Natural Resources Canada and the Geological Survey of Newfoundland and Labrador. The principal aims of GEM II are to assist in completing coverage of regional-scale geological maps across Canada’s north, and to enhance knowledge in regions to gain a better understanding of the geological evolution of key parts of the Canadian Shield. The overall GEM-II project addresses the latter objective by targeting specific areas along a transect from the Churchill Province to the Nain Province.
One such area is the Ingrid group, an isolated package of supracrustal rocks including volcanic and coarse-grained detri- tal rocks of presumed Paleoproterozoic age. The group is exposed as a fault-bound block between the Churchill Province to the west and the Archean Nain Province to the east. It is composed of subaerial lavas and upward-coarsening polymictic con- glomerate, and has been metamorphosed to the lower greenschist-facies conditions. The eastern part of the group is composed of coarse sandstone and mafic polymictic conglomerate that is in fault contact with Archean Nain Province. The western and southwestern part of the group is composed of mafic volcanic rocks, felsic sandstone and conglomerate, which are sheared and intercalated with protomylonite derived from metaplutonic gneisses of Churchill Province. The northern part of the group is truncated by felsic intrusive rocks of the Nain Plutonic Suite, although the contact is not exposed.
The age of the Ingrid group is not known. It is younger than the Kikkertavak dykes (2200 Ma) but older than the ca. 1.87–1.85 Ga Torngat orogeny. The Torngat orogeny is a zone of intense transpression and high-grade metamorphism that resulted from the amalgamation of the Core Zone and the Nain Province that deformed and metamorphosed the Ingrid group. This project aims to resolve many outstanding questions as to the nature, timing and tectonic history of the Ingrid group by studying the geochronology, metamorphic history, geochemistry of volcanic rocks, as well as the mineral potential of the group.
INTRODUCTION Atlantic Craton, which occurs in Greenland as well as north- west Scotland. The boundary between the Hopedale and A transect from the Core Zone of the Churchill Saglek blocks is assumed to be tectonic and of Neoarchean Province to the Hopedale Block of the Nain Province is the age; however, the exact location of the boundary has not focus of a joint Geological Survey of Canada–Geological been identified in the field (James et al., 2002). Part of the Survey of Newfoundland and Labrador–Nunatsiavut proj- current project included the acquisition of new aeromagnet- ect. This area is tectonically complex, located at the junction ic maps for the transect region, the results of which will be of four tectonic domains bound by Archean to published in the spring of 2019. Paleoproterozoic orogens (Figure 1). These are the 3.3‒2.8 Ga Hopedale Block, the 4.0‒3.2 Ga Saglek Block, the The Churchill Province comprises the Core Zone that is 2.8–2.3 Ga Core Zone, and the 1.88–1.74 Ga Makkovik separated from the North Atlantic Craton by the ca. 1.87– Province (James et al., 2002; Ketchum et al., 2002; Corrigan 1.85 Ga Torngat orogeny (Figure 1). The Torngat orogeny is et al., 2018). a zone of intense transpression and high-grade metamor- phism that mainly affected the Tasiuyak gneiss, the Lac The Hopedale and Saglek blocks jointly form the Nain Lomier complex, and the eastern edge of the Core Zone Province (Stockwell, 1963) and are part of the larger North (Wardle et al., 2002). The Core Zone and North Atlantic
147 CURRENT RESEARCH, REPORT 19-1
LEGEND 60o 06’
NEWQUÉBEC OROGEN o Torngat 1 47 34’ Orogen Rachel-Laporte zone 100 km Predominantly ma c volcanic rocks Labrador Trough North Kaniapiskau Supergroup Atlantic Reactivated Archean rocks (Superior crust?) Craton TORNGAT OROGEN Saglek Tasiuyak Gneiss Block Burwell Domain Falcoz River Mesoproterozoic Intrusions George Block Seal Lake Group River CORE ZONE Block Hutte Sauvage Group (Paleoproterozoic arkose) De Pas Batholith (1.84-1.81 Ga continental arc) Lac Lomier Complex (1.87-1.86 Ga continental arc) Mistinibi- Hopedale Quartzite, marble, metapelite (Lake Harbour Gp ?) Block Raude Archean to earliest-Paleoproterozoic rocks (mainly) Block Makkovik Tectonic Provinces Province of Labrador
Nain Province Churchill Province Makkovik Province Superior Superior Province Craton Grenville
o Grenville Province Province
69 37’ 52o 42’ Figure 1. Simplified geological map of the Precambrian Shield east of the Superior Craton in Québec and Labrador. Modified after James et al. (2003) and Corrigan et al. (2018).
Craton are bounded to the south by the Makkovik Orogen, a blocks is inferred to occur along a poorly defined, north– region of crustal reactivation and terrane accretion that northeast-trending high-strain zone and assumed to be formed during the Paleoproterozoic (Ketchum et al., 2002). Neoarchean (Connelly and Ryan, 1996; Wasteneys et al., 1996; James et al., 2002). The study area is also the focus of voluminous AMCG- type magmatism during the Mesoproterozoic, with intrusion The area has been the centre of voluminous AMCG-type of the Harp Lake anorthosite, the Mistastin pluton, the magmatism during the Mesoproterozoic (Emslie, 1978). This Flowers River intrusive complex, as well as the Nain includes: a) the ca. 1460 Ma Harp Lake Intrusive Suite, Plutonic Suite (Emslie, 1978). The focus of this report is the which stitches the boundary between the southeast Churchill Ingrid group, which is an isolated package of supracrustal and Nain provinces and the Torngat orogen (Emslie, 1980); rocks exposed as a faulted block preserved between the b) rocks of the long-lived, ca. 1351– 1292 Ma Nain Plutonic Churchill Province to the west and the Archean Nain Suite (NPS; Hill, 1982; Ryan et al., 1991; Thomas and Province to the east (Figure 2). Morrison, 1991), which intrudes along the boundary between the Saglek and Hopedale blocks; and c) the ca. 1293–1271 GEOLOGICAL SETTING Ma Flowers River intrusive complex (Hill, 1982; Thomas and Morrison, 1991) which masks the Hopedale‒Saglek In Labrador, the North Atlantic Craton is referred to as boundary. All rocks of the region, with the apparent excep- the Nain Province and is composed of two major Archean tion of the Flowers River intrusive complex, are cut by the crustal fragments, the Saglek and Hopedale blocks, which northeast-trending, tholeiitic Harp dykes (Hill, 1982) dated at are inferred to have been juxtaposed in the late Archean ca. 1273 Ma (Cadman et al., 1993); however, the newly (Connelly and Ryan, 1996; Wasteneys et al., 1996; James et acquired geophysical survey data indicate that the Flowers al., 2002). The boundary between the Hopedale and Saglek River intrusive complex is cut by the Harp dykes.
148 A. HINCHEY AND D. CORRIGAN
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54o 30’ o o 54 30’ 62 00’ 59o 00’ LEGEND PROTEROZOIC Harp Lake and Nain Plutonic Suites Reworked Archean Gneiss ARCHEAN Ingrid Group Gneiss and granitoids of Hopedale Block Moran Lake Group Hunt River and Florence Lake Groups Island Harbour Plutonic Suite
Figure 2. Location of the Ingrid group relative to the major tectonic blocks and younger igneous suites. Red box shows the location of the Ingrid group.
149 CURRENT RESEARCH, REPORT 19-1
A detailed map of the Ingrid group was published by imentary rocks, such as bedded greywacke and black- Ermanovics (1981; Figure 3). The lithological units remain laminated siltstone, are a minor but widespread compo- unchanged from this mapping and the descriptions herein nent indicating a proximal sedimentary source rock that are based largely on the earlier work. This project will apply predates deposition of the conglomerate (Ermanovics, geochronology, geochemistry and isotopic geochemistry to 1981, 1993). correlate this belt with other prospective supracrustal belts in the region (e.g., Moran Lake Group, Aillik Group). 2) Mafic volcanic conglomerate; minor felsic polymictic conglomerate, grit, sandstone, purple siltstone, and INGRID GROUP minor lavas; thickness 200 to 800 m (Alcm). This unit of coarse-grained, generally massive, mafic polymictic An isolated area (2.4 by 0.8 km) of altered, schistose, fragmental rocks (Plate 1B) has a mafic matrix and vol- andesitic rocks mapped by Taylor (1977, 1979) and inter- canic-derived gravels and boulder-sized clasts. Clasts preted as Proterozoic in age, was mapped in more detail and also include metaplutonic rocks and rare siltstone, sand- named the Ingrid group by Ermanovics (1981). These stone, greywacke, and felsic volcanic rocks supracrustal rocks are exposed over an approximate 3 by 6 (Ermanovics, 1981). Due to the angularity of the clasts, km area and are composed of subaerial lavas and upward- the volcanic material is interpreted to be locally derived coarsening polymictic conglomerate that have been meta- and further supported by the presence of porphyritic morphosed to the lower greenschist-facies conditions basalt boulders in the conglomerates that are in contact (Ermanovics, 1981), likely during the Torngat orogeny as with porphyritic basalt. This unit also contains rare mineral fabrics are more or less parallel to the latter. intercalated sandstone beds that preserve overturned younging directions. The eastern part of the group is composed of coarse sandstone and mafic polymictic conglomerate that extends 3) Felsic sandstone and minor purple siltstone and silty to the base of a fault juxtaposing the group against Archean mudstone; minor dacite; minor polymictic conglomer- gneiss of the Hopedale Block. The western and southwest- ate having a maximum thickness of 100 m (Als). This ern parts of the group are composed of mafic volcanic rocks unit is composed of a heterogeneous mixture of arkosic and felsic greywacke sandstone and conglomerate. These greywacke, sandstone, purple siltstone, mudstone, rocks are sheared and intercalated with proto-mylonite polymictic conglomerate, and minor amounts of felsic derived from metaplutonic gneisses of the Churchill (possibly volcanic) rocks (Plate 1C; Ermanovics, 1981). Province. The northern part of the group is truncated by Primary features are locally preserved and include grad- intrusive rocks of the NPS. Although the contact with the ed bedding, crossbedding, ball and flame structures, NPS is not exposed, Ermanovics (1981, 1993), interpreted rip-up clasts and evidence of soft-sediment slumping. weak effects of contact metamorphism in rocks in the north- The unit is in gradational contact with felsic conglom- ernmost outcrops and in float in the intervening river valley, erate (not map scale). suggesting an intrusive contact. 4) Porphyritic basalt, minor mafic to intermediate lavas Based on the field observations, Ermanovics (1981, and mafic volcanic conglomerate (Alpv). This unit has a 1993) divided the Ingrid group into six informal units as fol- maximum thickness of 200 m, and consists of alkaline‒ lows (Figure 3): subalkaline volcanic rocks composed of moderately to highly fractionated porphyritic basalt (Ermanovics, 1) Felsic polymictic conglomerate; minor mafic volcanic 1981). Porphyritic basalt contains white, sericitized conglomerate, sandstone, and purple siltstone; includes andesine phenocrysts and andesine + quartz aggregates cobbles and boulders of felsic metaplutonic rocks. This (up to 1 by 3 cm) within a dark-green, nonmagnetic unit has a thickness between 100 and 400 m (AIcf), and aphanitic matrix (Ermanovics, 1981). is confined to the southern part of the map area. Contacts with the surrounding volcanic rocks are not 5) Mafic to intermediate lavas, minor porphyritic basalt preserved, and are interpreted as fault contacts and mafic volcanic conglomerate and rare pillowed (Ermanovics, 1981). Bedding is 10–70 cm thick and lavas (Almv). This unit has maximum thickness of 500 graded bedding is locally preserved. The matrix is lith- m. Massive basalts are medium grained to aphanitic, ic (silt to sand) and the unit is poorly sorted (Plate 1A). pale to dark green, and are locally finely schistose (Plate Clasts include foliated granite, orthogneiss, massive 1D). The rocks are weakly to strongly magnetic and con- gabbro, foliated granodiorite, shale, siltstone, quartzite, tain scattered veins (up to 5 mm thick) of pyrite and cal- chert and limestone. Rare graded bedding indicates cite. Ermanovics (1981, 1983) reported three localities overturning to the east and west. Angular clasts of sed- where pillow lavas were identified, with individual pil-
150 A. HINCHEY AND D. CORRIGAN
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Figure 3. Detailed geological map of the Ingrid group (after Ermanovics, 1993).
151 CURRENT RESEARCH, REPORT 19-1
Plate 1. A) Poorly sorted, metaconglomerate with sub- rounded to angular clasts of felsic and mafic volcanic rocks and minor clasts of foliated granodiorite. The foliat- ed granodiorite was sampled for geochronological study; B) Mafic conglomerate; C) Cherty, pink to mauve sand- stone. Bedding is 2 to 10 cm thick; D) Weakly, foliated, fine-grained, greenschist-facies metabasalt. Foliation in the metabasalt is north‒south and steeply dipping.
lows ranging from 10 by 30 cm wide and up to 1 m long. ately east of the Ingrid group (Figure 3; Ermanovics, Porphyritic basalt (Alpv) is also locally observed. 1981, 1993). The contact between the eastern grey- wacke is not exposed. 6) Mafic outliers are mapped as a separate unit (Al) as the relationship with the rest of the sequence is ambiguous. METAMORPHISM Several outliers of fine-grained, laminated amphibolite and layered orthogneiss occur within sheared quart- Ermanovics (1993) reported metamorphic assemblages zofeldspathic orthogneiss west of the Ingrid group preserved in the Ingrid group as representative of lower (Figure 3). In addition, a thin veneer of mafic greenschist-facies conditions. Metamorphic assemblages greywacke outcrops within the Archean rocks immedi- are best preserved in mafic units and in fine-grained sedi-
152 A. HINCHEY AND D. CORRIGAN mentary rocks. Metamorphism is accompanied by the devel- grained sediment, typical of unstable depositional environ- opment of a weak foliation. The western contact of the ment (Plate 2B). Ingrid group contains clasts that are stretched and deformed in parallelism with mylonitized Churchill Province gneisses. Formational contacts, bedding, tectonic alignment of This metamorphism and deformation is interpreted as result- clasts, and metamorphic foliations, all strike north-north- ing from the Torngat orogeny (Ermanovics, 1981). west and parallel to the Nain‒Churchill boundary and Churchill Province tectonic grain. Metamorphic foliation is TECTONIC EVOLUTION generally parallel to bedding and may represent axial-planar foliations caused by folding (Ermanovics, 1981). Indicators Although not exposed, a well-sorted succession of epi- of younging directions suggest an overturned succession clastic sediments is inferred based upon coarse clasts that (Plate 2C). Ermanovics (1993) suggested that the group is are preserved in the conglomerates. This inference suggests part of the nose of a recumbent fold, with downward-facing deposition in a high-energy aqueous environment decollement folds on the lower limb, which is mylonitized (Ermanovics, 1993). Rapid emergence of this succession and intercalated with mylonitic gneisses of the Churchill would have given rise to intraformational rip-ups, soft-sedi- Province. Metamorphism and deformation would have ment slumping and large isolated clasts in finer grained beds occurred as part of the distal effects of the Torngat orogeny. that are observed in these units. Rapid deposition of coarse detritus, derived from Archean metaplutonic rocks, basalts AGE AND CORRELATION and epiclastic sediments is interpreted to have occurred fol- lowing emergence (Plate 2A; Ermanovics, 1993). The The age of the Ingrid group is not well established. It is coarse, angular detritus occurs within unsorted, poorly younger than the Kikkertavak dykes (2.2 Ga) but older than defined beds of sandstone and minor argillaceous fine- the ca. 1.87–1.85 Ga Torngat orogeny. The Torngat orogeny
Plate 2. A) A foliated subrounded Archean metaplutonic clast in the polymictic mafic conglomerate; B) Polymictic conglomerate to the east with abundant felsic and mafic volcanic clasts. The angular nature of the clasts indicates a proximal source. Younging is interpreted to be to the west, based on preserved foreset beds climbing to the north; C) Cherty, pink sandstone interbedded and polymic- tic conglomerate with abundant felsic volcanic and purple sandstone clasts. Younging to the west, with foreset beds that are climbing to the north. The source of the clasts is interpreted to be proximal, based upon their very frag- mented and angular nature.
153 CURRENT RESEARCH, REPORT 19-1 is a zone of intense transpression and high-grade metamor- Establishing the age, as well as the geochemical and phism that separates the Core Zone and the North Atlantic isotopic signature of the Ingrid group will allow for a robust Craton (Figure 1). This orogenic event would have been comparison and potential correlation with the responsible for the deformation and metamorphism pre- Paleoproterozoic supracrustal sequences preserved in the served in the Ingrid group. The metamorphic grade increas- region. In turn, this will allow for potential mineralizing es to the west from the greenschist Ingrid group through to environments to be targeted based on the similarities the Tasiuyak and Lac Lomier complex where upper gran- between the belts. ulite facies conditions are preserved. Basement rocks beneath and surrounding the Ingrid group do not appear to CONCLUSIONS have acquired the same greenschist-facies metamorphic overprint, suggesting that the Ingrid group was emplaced as The Ingrid group is an isolated package of supracrustal a nappe in thin-skin tectonic event. rocks that include volcanic and coarse-grained detrital rocks. The group is exposed as a faulted block preserved POTENTIAL CORRELATIVES between the Churchill Province to the west and the Archean Nain Province to the east. The group is composed of sub- There are several Paleoproterozoic supracrustal aerial lavas and upward-coarsening polymictic conglomer- sequences preserved in the study area. The Moran Lake and ate, and has been metamorphosed to the lower greenschist Post Hill (not shown on Figure 2 map) groups are comprised facies. The age of the Ingrid group is not well known. It is of siliciclastic sedimentary and mafic volcanic rocks, and younger than the Kikkertavak dykes (2.2 Ga) but older than are correlated on the basis of their similar stratigraphy and the ca. 1.87–1.85 Ga Torngat orogeny. The Torngat orogeny rock types (Marten, 1977; Wardle and Bailey, 1981). The a zone of intense transpression and high-grade metamor- Post Hill group contains a mafic metavolcanic unit in its phism that affected the Ingrid group. Future studies will lower tectonostratigraphy that was deposited ca. 2178 Ma, explore the formation and mineral potential of the Ingrid but units higher in the sequence were deposited after ca. group based on more abundant and precise U–Pb zircon and 2013 Ma (Ketchum et al., 2002). The Moran Lake Group monazite ages, as well as new ground observations support- remains undated, but is unconformably overlain by ca. 1850 ed by high-resolution aeromagnetic data. Ma siliciclastic sedimentary rocks (Sparkes et al., 2016), and is in thrust contact with the Nain Craton. ACKNOWLEDGMENTS
Younger supracrustal sequences are dominated by shal- Heather Campbell, Deanne Van Rooyen, Nicole low-water to terrestrial sedimentary rocks and subaerial fel- Rayner, Hamish Sandeman and Étienne Girard are warmly sic volcanic rocks of the Aillik Group dated between 1883 thanked for their contribution in the field, and to an overall and 1856 Ma (Schärer et al., 1988; Hinchey and Rayner, understanding of the geological history. Neil Stapleton is 2008). The Bruce River Group sits unconformably upon the thanked for cartographic support. Wayne Tuttle helped Moran Lake Group. The highest exposed stratigraphy with- immensely with logistics and safety. Universal Helicopter in the Bruce River Group contains a felsic volcanic tuff that Services provided excellent rotary-wing support. The manu- was dated ca. 1650 Ma (Schärer et al., 1988; Sparkes et al., script was improved by the helpful review by John Hinchey. 2016). However, a felsic tuff layer within the basal units of the sequence of the Bruce River Group produced an age of REFERENCES ca. 1850 Ma (Sparkes et al., 2016), indicating the occur- rence of an unrecognized fault or unconformity. Cadman, A.C., Heaman, L., Tarney, J., Wardle, R. and Krogh, T.E. The Post Hill group, Aillik Group, the Bruce River 1993: U-Pb geochronology and geochemical variation Group, and Moran Lake Group are associated with econom- within two Proterozoic mafic dyke swarms, Labrador. ic mineral occurrences, generally consisting of uranium Canadian Journal of Earth Sciences, Volume 30, pages mineralization. The Aillik, Bruce River and Post Hill groups 1490-1504. contain several uranium deposits (e.g., Kitts, Michelin, Jacques Lake, Moran Lake Lower C deposits). In addition to Connelly, J.N. and Ryan, B. uranium occurrences (e.g., Armstrong deposits), the Moran 1996: Late Archean evolution of the Nain Province, Lake Group also contains an iron-rich breccia that has ele- Nain, Labrador: imprint of a collision. Canadian vated values of gold and copper (Moran Lake Upper C Zone Journal of Earth Sciences, Volume 33, pages 1325- deposit; Sparkes et al., 2016). 1342.
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