GEOLOGY OF THE LITTLE MECATINA RIVER AREA (NTS 13D/NE ...
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83 83 12 47 41 59 44 44 26 49 79 43 43 74 33 17 54 73 73 50 39 25 26 63 50 67 37 36 25 40 44 53 52 75 68 82 82 71 31 45 71 58 37 49 66 73 72 74 21 77 5 61 81 66 79 65 67 66 80 68 76 72 71 63 73 66 58 70 27 39 60 69 58 70 59 65 67 21 0 49 57 67 85 57 83 48 52 37 45 71 79 90 46 68 55 90 60 78 54 65 15 40 54 18 20 42 34 30 38 34 55 20 38 32 30 34 28 54 36 35 28 28 34 47 42 40 57 28 58 34 54 38 48 48 80 37 35 54 60 50 43 36 20 44 52 52 75 70 37 52 48 50 76 45 78 27 58 46 70 73 80 52 75 54 77 57 50 35 52 80 46 76 74 75 70 78 75 78 35 48 37 48 21 55 75 62 53 64 38 52 80 50 52 68 52 80 68 55 70 52 57 57 50 35 54 54 50 75 75 70 62 48 40 38 27 46 30 36 33 47 31 40 32 37 34 45 44 44 48 42 scale: 1:100 000 1 2 3 4 5 0 kilometres Little Mecatina River Little Meactina River Little Meactina River Fig River Fig River Churchill River 52°30’ N 52°30’ N 53°00’ N 53°00’ N 63°00’ W 62°00’ W 62°30’W 52°45’ N 62°00’ W 62°30’ W 63°00’ W 52°45’ N NTS 13D/10 NTS 13D/15 NTS 13D/9 NTS 13D/16 No Exposure No Exposure No Exposure No Exposure a pproximate n ort hwest ern limit of hig hl y straine d ro c ks app roxim ate n o rth w es tern li mit of hi gh ly st rained ro cks P gbr P gbr P gbr P gbr P gbr P grn P grn P grn P grn P grn P ogn P msa P msa P dxt P dxt P dxt P msm P msm P msm P msm P msm P msm P gbr P gan P tan P tan P ggn P ggn P ggn P ggn P ggn P msd M grn M grn M grn M grn M grn M grn Hamilton River shear zone 50 67 45 21 76 12 57 52 Outcrop (massive or no structural fabrics measured) Grenvillian high-strain zone - boundary between the WLT and the MMT, teeth on structural hanging wall: assumed Grenvillian high-strain zone: assumed Contact: assumed Late- to post-Grenvillian fault (Neoproterozoic ?): assumed Foliation, vertical foliation Gneissosity, vertical gneissosity Gneissosity or foliation and contained mineral elongation lineation Planar fabric in highly strained rocks Dyke Igneous layering MESOPROTEROZOIC LEGEND PALEOPROTEROZOIC MEALY MOUNTAINS TERRANE (MMT) Orthogneiss: heterogeneous and unsubdivided unit consists of gneissic and foliated granitoid rocks including granite, granodiorite, quartz monzonite, local diorite and rare gabbro. Anorthosite: white- to grey weathering, recrystallized and foliated meta-anorthosite containing hornblende and biotite. Gabbro - gabbronorite: massive to foliated and recrystallized gabbro, biotite gabbro and gabbronorite. Gabbroic anorthosite: recrystallized and foliated gabbroic anorthosite, leucogabbro, gabbro and gneissic gabbro. Metasedimentary gneiss: pink- and grey-weathering, biotite + garnet + sillimanite migmatite, contains a high percentage of K-feldspar-bearing leucosome. Metasedimentary gneiss and related rocks: rusty- or pink-weathering, high-grade metasedimentary gneiss containing biotite + sillimanite and variable amounts of K-feldspar-bearing leucosome, also including rare occurrences of quartzite and calc-silicate schist. grey- and black-weathering, amphibolite- and upper amphibolite-facies metasedimentary gneiss containing biotite + sillimanite ± muscovite. pink-weathering, biotite + sillimanite-bearing diatexite containing abundant abundant inclusions of P msm metasedimentary gneiss. Gabbro and gabbronorite: variably deformed and recrystallized gabbro and gabbronorite. Grey orthogneiss: grey-weathering granitoid orthogneiss containing biotite + hornblende ± clinopyroxene. Granitoid rocks: consists mainly of recrystallized, foliated and locally mylonitic, biotite ± hornblende granite, granodiorite, monzonite, K-feldspar porphyritic granite and local clinopyroxene- bearing granitoid rocks. WILSON LAKE TERRANE (WLT) M grn Mealy Mountains Intrusive Suite (MMIS) P tan P gan P gbr P ggn P msd P gbr P grn P ogn P msa P msm P dxt Pre-MMIS rocks Granite: pink-weathering, mainly coarse- and medium-grained biotite granite, locally K-feldspar porphyritic granite; mainly isotropic, although contains a local, weak foliation (relict igneous foliation ?) Late- to post-Grenvillian intrusive rocks 23H 23J 24A 24H 24I 25A 23B 23I 23P 23A 23O 23G 24P 14M 13M 13L 13E 13D 13C 13B 13A 13H 13I 12P 13F 13J 13G 13O 13N 13K 14D 14C 14E 14F 14L 58° 58° 64° 64° 54° 54° 66° 66° 60° 60° 62° 62° 56° 56° 68° 68° 51° 52° 53° 54° 55° 56° 57° 58° 59° 60° 61° 150 km Atlantic Ocean NTS 13D/NE Figure 1. Index map of Labrador showing location of the NTS 13D/NE area. GRENVILLE FRONT Atlantic Ocean 100 km GRENVILLE FRONT MK GBT LMT HRT Pinware terrane Mealy Mountains terrane NP SLG NPS HLIS MI MB KSG WLT CFT LJT INTERIOR MAGMATIC BELT EXTERIOR THRUST BELT MLT GT SP SECP MAT WKT LRD Mecatina terrane Figure 2. Location of the Little Mecatina River area in relation to the tectonic and major lithotectonic units of northeastern Laurentia. Grenville Province: HRT - Hawke River terrane, LMT - Lake Melville terrane, GBT - Groswater Bay terrane, WLT - Wilson Lake terrane, CFT - Churchill Falls terrane, LJT - Lac Joseph terrane, MLT - Molson Lake terrane, GT - Gagnon terrane, MAT - Matamec terrane, WKT - Wakeham terrane, LRD - La Romaine domain. Archean divisions: SP - Superior Province, NP - Nain Province (Hopedale Block). Archean and Paleoproterozoic divisions: MK - Makkovik Province, SECP - Southeastern Churchill Province (core zone), KSG - Kaniapiskau Supergroup (2.25-1.86 Ga). Mesoproterozoic units: NPS - Nain Plutonic Suite, HLIS - Harp Lake intrusive suite, MB - Mistastin batholith, MI - Michikamau Intrusion, SLG - Seal Lake Group. Little Mecatina River area DATA POSITIONING On this map, structural and outcrop data are plotted in the position where the data were collected in the field (i.e., symbols representing tectonic structures or primary features are not offset from the outcrop location). The centre of the symbol representing a planar fabric element (e.g., foliation or gneissosity) or a planar primary feature (e.g., bedding or igneous layering) is plotted in the position where the data were collected in the field. The + symbol represents an outcrop that has a massive (i.e., isotropic) structure or one that does not contain a measurable fabric (e.g., frost heave or an extensively fractured outcrop). Data are plotted using a NAD 1927 projection, UTM Grid Zone 20. Recommended citation: James, D.T. and Nadeau, L. 2001. Geology of the Little Mecatina River area (NTS 13D/NE), Grenville Province, southern Labrador. Geological Survey of Newfoundland and Labrador, Department of Mines and Energy, Map 2001-18, Open File 013D/0029, 1:100 000-scale map with descriptive notes. Geology by D.T. James (Geological Survey of Newfoundland and Labrador) and L. Nadeau (Geological Survey of Canada, Québec), 2000. This map represents a preliminary interpretation of the geological field data and is subject to change without notice. Comments regarding revisions or errors are invited and should be directed to the Director, Geological Survey of Newfoundland and Labrador, P.O. Box 8700, St. John’s, Newfoundland, Canada, A1B 4J6. Corresponding author: D.T. James Geological Survey of Newfoundland and Labrador Department of Mines and Energy P.O. Box 8700, St. John’s, Newfoundland A1B 4J6 e-mail: [email protected] last modified: March 2001. DESCRIPTIVE NOTES INTRODUCTION REGIONAL GEOLOGY DESCRIPTION OF ROCK UNITS WILSON LAKE TERRANE (WLT) MEALYMOUNTAINS TERRANE (MMT) LATE- TO POST-GRENVILLIAN GRANITE (M grn) The northeastern part of NTS map area 13D (Figure 1), including sheets 13D/9, 13D/10, 13D/15 and 13D/16, were mapped at a scale of 1:100 000 in the 2000 field season (James and Nadeau, 2001). Mapping was accomplished by making helicopter landings on isolated outcrops. This work represents a continuation of 1:100 000-scale mapping in NTS map area 13C by James and Lawlor (1999) and James and Nadeau (2000a, b). NTS 13D was previously mapped by Stevenson (1969). Thomas (1994) mapped NTS 13D/NW. To the east, NTS 13C/NW was mapped by Wardle (1990, 2000). The rocks in the northern parts of NTS 13D/NE are well exposed. In contrast, the southern part of the area is extensively covered by bog, dense forest, or thick accumulations of Quaternary deposits, and there are few outcrops. Definitive contact relationships between units were not observed. To some extent, the locations of contacts and faults shown on the map were interpreted using regional aeromagnetic data. The study area is situated in the northeastern Grenville Province and straddles the boundary between the Exterior Thrust Belt and Interior Magmatic Belt ( Gower , 1991). It includes parts of the Wilson Lake and Mealy Mountains terranes (Figure 2). These structurally stacked terranes are Grenvillian tectonic divisions, although they consist primarily of late Paleoproterozoic crust. The Wilson Lake terrane (WLT) is dominated by high-grade metasedimentary gneisses having protolith depositional ages predating ca. 1720 Ma (i.e., pre-Labradorian Orogeny). The gneisses are derived primarily from pelitic and semi-pelitic sedimentary rocks, although very minor amounts of metasedimentary rocks derived from quartzite and siliceous carbonate also occur. The terrane also includes intrusions of late Paleoproterozoic gabbro and gabbronorite, and mafic gneisses possibly derived from supracrustal rocks. Intrusions of foliated granite and porphyritic granite, correlated on the basis of rock type with the ca. 1650 Ma Trans-Labrador batholith, and granitoid orthogneisses of uncertain age make up minor amounts of the WLT. Geochronological studies of monazite contained in sapphrine-bearing diatexite, occurring in the northern part of the WLT, and presumed to be derived from anatexis of the metasedimentary gneisses, indicate that high-grade metamorphism and attendant deformation in the WLT are Middle Labradorian (ca. 1640 Ma) (Corrigan , 1997). Corrigan ( ) have also shown, on the basis of U–Pb dating of monazite, that the terrane has been locally overprinted by Grenvillian (ca. 1000 Ma) metamorphism. Regionally persistent ductile high-strain zones separating the WLT from the Trans-Labrador batholith to the north, and the Mealy Mountains terrane to the south, are ca. 1010-990 Ma structures (Corrigan , 1997). The Mealy Mountains terrane (MMT) (Gower and Owen, 1984) consists primarily of late Paleoproterozoic (Labradorian-age) plutons of the Mealy Mountains intrusive suite (MMIS) ( Emslie, 1976; Emslie and Hunt, 1990; Krogh , 1996), minor amounts of Paleoproterozoic pre-Labradorian crust, and Pinwarian (1510 to 1450 Ma) and late- to post- Grenvillian intrusions (Gower, 1996). The MMIS consists mainly of an older group of anorthositic, leucogabbroic and leucotroctolitic rocks, and a younger group of pyroxene-bearing monzonite and quartz monzonite intrusions. Emplacement ages for units of MMIS monzodiorite orthogneiss, porphyritic quartz monzodiorite and pyroxene-bearing monzonite, determined by U–Pb geochronology of zircon and occurring in the Kenamu River area (NTS 13C/NE), are 1659 ± 5 Ma, 1650 ± 1 Ma, and 1643 ± 2 Ma, respectively (James , 2000). Rocks in the western part of the MMT are variably foliated and locally gneissic; in general they have northeast- to east-northeast-striking planar fabrics that are inferred to be Labradorian in age (James and Nadeau, 2000a). However, the local occurrence of deformed, Pinwarian age (ca. 1514 Ma) granite in the Kenamu River area (James , 2000) demonstrates the western part of the MMT has also been affected by Pinwarian and/or Grenvillian deformation. The MMT is intruded by plutons of unmetamorphosed, massive to weakly foliated granite and quartz monzonite. One of these plutons, occurring in the southwestern part of the MMT, is dated by U–Pb techniques on zircon to be 964 ± 3 Ma (James , 2001). This age is consistent with reported emplacement ages for widespread granitoid plutons in the northeastern Grenville Province ( Gower, 1996). In NTS 13D/NE, the WLT comprises approximately 60% metasedimentary gneiss and related rocks, including diatexite and granite. The most common metasedimentary rock type is a high-grade biotite and sillimanite-bearing migmatite (P msm) composed of a rusty or grey paleosome and variable amounts of pink- and white-weathering K-feldspar-bearing leucosome. Garnet occurs only rarely in P msm gneiss. The P msm gneisses are gradational, at all scales, into pink- and grey-weathering diatexite (P dxt). The diatexite is medium to coarse grained, variably foliated, has a varied composition from granite to quartz monzonite, and contains biotite and widespread sillimanite. Outcrops include less than 30% ‘screens’ of metasedimentary gneiss (P msm). Locally, outcrops are lacking in the metasedimentary gneiss ‘screens’ and they can be classified as sillimanite-bearing granite or sillimanite- bearing homogeneous diatexite. Gradational contacts with the surrounding high-grade metasedimentary gneiss unit, the widespread occurrence of metasedimentary gneiss ‘screens’ and the composition of the rocks suggest P dxt diatexite is derived from the nearly complete, melting of surrounding metasedimentary gneisses. The northeastern part of NTS 13D/16, north and east of the Churchill River, is mainly underlain by grey- and white- weathering or, locally, grey- and pink-weathering metasedimentary migmatite (P msa). The P msa rocks are inferred to be a somewhat lower grade version of the P msm gneisses. The P msa rocks commonly contain muscovite, and have lesser volumes of leucosome than P msm gneisses. The leucosome in P msa rocks is most commonly white weathering and K- feldspar-poor, whereas the high-grade P msm rocks contain abundant volumes of pink-weathering, K-feldspar-rich leucosome. The area underlain by P msa rocks is marked by a magnetic low; in sharp contrast, areas underlain by higher grade P msm rocks and diatexite (P dxt) are marked by a prominent, regional magnetic high. The composition of P msm and P msa metasedimentary gneisses suggests they are derived from aluminous, pelitic and semi-pelitic protoliths. Both units have a relatively consistent composition over large areas presumably reflecting a widespread and thick sedimentary precursor of consistent composition. However, there are a few local variations in the metasedimentary gneisses including rare occurrences of mafic gneiss having an uncertain protolith, quartzite, gneisses derived from siliceous carbonate, silicate-oxide iron formation and possible felsic volcanic rocks. Subdivisions of the different rock types in the metasedimentary gneiss units are not shown on the map because of their very limited extent. Metasedimentary gneisses in the northeastern part of NTS 13D/16 are intruded by granodiorite orthogneiss (P ogn). This orthogneiss is migmatitic containing thin (centimetre-scale), white- and pink-weathering layers of leucosome, and grey-weathering layers of paleosome containing biotite, hornblende, and possible clinopyroxene. The rocks locally contain K- feldspar augen that are presumed to be relicts of K-feldspar phenocrysts. The metasedimentary gneisses are intruded by bodies of variably deformed and recrystallized granite, K-feldspar porphyritic granite, granodiorite and monzonite. The different rock types that make up the P grn unit are not divided on the map. Medium to coarse grained, K-feldspar porphyritic, biotite- and hornblende-bearing granitic rocks are the most common type in the unit. Grey- to pink-weathering monzonitic rocks, locally porphyritic, and containing local clinopyroxene are a minor component of the unit. All rocks in the unit are foliated and some are gneissic. Highly strained and mylonitic rocks, forming a high-strain zone several kilometres wide, occur along the boundary with the MMT. Metasedimentary gneisses and granitic rocks in the WLT are intruded by bodies of metamorphosed and deformed gabbro, gabbronorite and lesser clinopyroxenite defined as unit P gbr. These rocks are mainly fine grained, have granoblastic textures, and are composed of varied proportions of plagioclase and pyroxenes. They may also contain minor amounts of biotite and quartz. The rocks are massive to foliated and locally gneissic. However, medium-grained rocks having a preserved igneous, intergranular texture also occur. Relict igneous layers occur in a few outcrops. The southwestern part of NTS 13D/10 contains several outcrops of pink-, grey- and rusty-weathering, upper amphibolite-facies metasedimentary gneiss (P msd). These rocks are very well layered, having a gneissosity defined by alternating biotite + sillimanite-rich, and K-feldspar-rich layers. P msd rocks contain abundant garnet, in contrast to the garnet-poor metasedimentary gneisses in the WLT. P msd metasedimentary gneiss is inferred to be derived from a pelitic sedimentary protolith. Occurrences of metasedimentary gneiss, similar to P msd gneiss, are relatively common in the eastern parts of the MMT (e.g., Gower and van Nostrand, 1996), although they are rare in the central and western parts of the terrane. The depositional age of the sedimentary protoliths is undetermined, but they must be older than the middle Labradorian rocks of the MMIS which intrude them. Gneissic and foliated, amphibolite-facies granitoid rocks, mainly including biotite ± hornblende ± clinopyroxene granite, granodiorite, quartz monzonite, and local diorite, are widespread in NTS 13D/NE. These rocks are very poorly exposed, and because they could not be reasonably subdivided in the field at the scale of mapping, they are collectively assigned to a single unit. P ggn rocks are somewhat similar to gneissic P mdq granitoid rocks, which comprise part of the MMIS in the Kenamu River (NTS 13C/NE), Fourmont Lake (NTS 13C/SE) and Lac Arvert (NTS 13C/SW) areas ( James, 1999; James and Nadeau, 2000a). However, P ggn consists mainly of granite and granodiorite gneiss, whereas P mdq consists mainly of clinopyroxene-bearing monzonite or monzonite gneiss. P ggn rocks are interpreted to be equivalent to the Lower Brook metamorphic suite orthogneisses defined by Wardle (1990). Igneous emplacement ages of P ggn rocks are undetermined. Outcrops of P ggn rocks are commonly composite; they may contain xenoliths of grey-weathering hornblende + biotite diorite, and younger aplite dykes that are less deformed than the gneiss. In addition, P ggn gneisses are locally cut by deformed and metamorphosed mafic dykes. Weakly deformed and massive pegmatites form a minor part of the unit. The NTS 13D/10 area includes an elongate, east-west-trending intrusion of metamorphosed and deformed anorthosite. The rocks are white and grey on the fresh and weathered surfaces. They are variably recrystallized and foliated. Some rocks contain abundant relicts of coarse-grained, bluish-grey primary plagioclase grains that have fine-grained margins of recrystallized plagioclase. In contrast, other rocks consist entirely of white, fine- to medium-grained granoblastic plagioclase. Primary mafic minerals are replaced by a mixture of biotite and hornblende. Based on similarity of rock types, P tan anorthosite is provisionally correlated with Labradorian-age MMIS anorthosite occurring in the central (e.g., Gower, 1999) and northeastern (e.g., Nunn and van Nostrand, 1996) parts of the MMT. The P gbr unit consists mainly of a medium grey-weathering gabbro, commonly containing up to 10% medium- grained biotite. The biotite appears to be primary as opposed to metamorphic. The biotite gabbro has a distinctive porphyritic texture defined by biotite ‘phenocrysts’ up to 1 cm that consist of an aggregate of biotite grains. The rocks are generally massive to foliated and variably recrystallized, although subtle relicts of igneous layering, defined by composition and grain size, occur locally. In well-foliated and recrystallized rocks, the gabbro is converted to biotite amphibolite. The southwestern part of NTS 13D/9 includes several occurrences of metamorphosed gabbroic anorthosite, leucogabbro and mafic gneiss defined as unit P gan. The rocks have a fine-grained granoblastic texture and are strongly foliated. Locally, they contain relicts of coarse-grained clinopyroxene that appear to be mainly replaced by finer grained hornblende and biotite. The MMT is intruded by plutons of granite, quartz monzonite and, locally, K-feldspar porphyritic granite defined as unit M grn. These rocks are pink on the fresh and weathered surfaces, medium to coarse grained and contain less than 10% biotite. Generally they are massive, although weakly foliated rocks also occur. Of note, some plutons are marked by prominent circular or elliptical, magnetic highs, whereas others are marked by magnetic lows. M grn rocks are correlated, on the basis of composition and texture, with granites occurring in the NTS 13C/SE area, where one pluton has an emplacement age of 964 ± 3 Ma determined by U–Pb geochronology of zircon (James 2001). The MMT also contains widespread occurrences of massive, pink- or white-weathering pegmatites containing muscovite ± biotite. On the basis of their undeformed character, they are interpreted to be late- to post-Grenvillian intrusions. et al. et al. see et al. et al. et al. op cit et al. see et al. et al. et al. et al. see , ) insitu see et al. et al., Metasedimentary gneiss and related rocks (P msa, P msm P dxt Grey orthogneiss (P ogn) Granite, granodiorite and K-feldspar porphyritic granite (P grn) Gabbro and gabbronorite (P gbr) Metasedimentary gneiss (P msd) Orthogneiss (P ggn) Anorthosite (P tan) Gabbro, gabbronorite and gabbroic anorthosite (P gbr, P gan) quartz STRUCTURE AND METAMORPHISM WILSON LAKE TERRANE (WLT) MEALYMOUNTAINS TERRANE EXPLORATION POTENTIAL REFERENCES Foliation and gneissosity in metasedimentary migmatite, diatexite, orthogneiss and granitic rocks in the WLT are generally southwest-striking and moderately northwest dipping. Foliation is defined by alignment of the metamorphic minerals, or recrystallized mineral aggregates in the granitic rocks. In P msm metasedimentary migmatite, the metamorphic assemblage includes biotite + sillimanite + K-feldspar + granitic melt. This assemblage, and the fact that P msm migmatite is gradational into diatexite (P dxt), indicates upper-amphibolite to granulite-facies metamorphism. In NTS 13D/NE, metamorphic leucosome in P msm migmatite and diatexite have not been dated, although they are interpreted to be the product of ca. 1640 Ma Labradorian metamorphism. This interpretation is based on geochronology of monazite by Corrigan (1997), collected from diatexite occurring in the northwestern part of the WLT. In contrast to the P msm rocks, P msa metasedimentary migmatite, occurring in the northeastern part NTS 13D/16, commonly contains muscovite. Neither the age of the migmatization nor the muscovite growth is certain, but one possible scenario is that migmatization is Labradorian and the same age as high-grade metamorphism in P msm migmatite, whereas the muscovite is Grenvillian and represents the effects of a lower grade (amphibolite facies?) metamorphic overprint. This provisional model is consistent with data presented by Corrigan (1997) indicating that some rocks in the southeastern part of the WLT have been overprinted by ca. 1000 Ma metamorphism. In NTS 13D/NE, the tectonic boundary between the WLT and the MMT is not a single structure. Rather, it appears to consist of two geometrically discordant structures that separate MMT rocks from a structurally imbricated package of WLT rocks. Mylonitic rocks, derived from metasedimentary migmatite and granite, in the central and western parts of NTS 13D/NE, suggest this section of the boundary is a ductile high-strain zone. In these rocks, high-strain planar fabrics are southwest- to west-striking, moderately northwest to north dipping, and contain a northwest-trending mineral elongation lineation. The highly strained rocks do not appear to be retrogressed, suggesting that the high- strain event was synchronous with high-grade metamorphism. The interpretation of this part of the boundary as a mainly northwest-dipping structure is in general agreement with interpretations made by Thomas (1986) for areas west of NTS 13D/NE. In the northeastern part of the area, in NTS 13D/16, the boundary is very poorly exposed and its location is interpreted primarily on the basis of magnetic anomaly patterns. One outcrop, occurring near the inferred boundary location, consists of interlayered and strongly foliated mafic and metasedimentary gneisses that exhibit a very strong, moderately south southwest-dipping foliation and a southeast-plunging mineral elongation lineation. This single observation is consistent with mapping by Wardle (1990, 2000) in NTS 13C/NW which has shown the boundary between the WLT and the MMT to be a mainly southwest- to southeast-dipping high-strain zone in that area. Wardle (1990) defined this boundary as the Hamilton River Shear Zone (HRSZ). On the basis of U–Pb geochronological studies of mylonitized samples from the NTS 13C/NW area, Corrigan (1997) determined that thrusting of the MMT over the WLT, along the HRSZ, occurred at 1010 ± 4 Ma. A model involving early, northwest-directed Grenvillian transport of the MMT over the WLT on the HRSZ, followed by southeast- directed “back-thrusting” of WLT rocks over the MMT on different, northwest-dipping structures, could explain the apparent geometry of structures defining the terrane boundary. Foliation and gneissosity in metasedimentary gneiss (P msd), orthogneiss (P ggn) and rocks of the MMIS are highly varied in attitude, although in general, planar fabrics are southwest-striking. These fabrics, and the metamorphic minerals that define them, are tacitly assumed to be Labradorian. Geochronological studies are required to test this model. The metasedimentary and MMIS rocks indicate upper amphibolite-facies metamorphism was reached. The former are migmatitic rocks containing biotite + sillimanite + garnet + K-feldspar. The latter include gneissic and migmatitic granitoid rocks, and gabbroic rocks which have been converted to amphibolite. Primary mafic minerals in the anorthosite (P tan) have been replaced by hornblende and biotite. The Grenville Province of southern Labrador has some exploration potential ( Swinden , 1991; Gower, 1992; and Gower , 1995), although field work in 2000 did not result in any significant or promising discoveries of economic minerals. Mesoproterozoic and Paleoproterozoic anorthosite intrusions are potential exploration targets for Ti (Fe) oxide and magmatic Ni-Cu sulphide deposits. The QIT Fer et Titane Inc. Ti-oxide deposits in Mesoproterozoic anorthosite near Harve-St-Pierre, Québec, provide the best local example of economic Ti-oxide mineralization in Grenville Province anorthosite (Perreault and Jacob, 1999). Of note, the Mesoproterozoic (ca. 1130 Ma) Atikonak anorthosite, occurring to the west, in NTS 13D/SW, is approximately the same age as the intrusion which hosts the QIT deposits. The anorthosite (P tan), discovered in the 2000 field season and inferred to be Paleoproterozoic in age, offers a new exploration target in the region for magmatic Ni-Cu sulphide deposits. In addition, Paleoproterozoic gabbro and gabbronorite intrusions in the study area, and elsewhere in the region, also have some potential for hosting Ni-Cu and PGE mineralization. Several outcrops in the study area contain a few percent pyrite, although no significant sulphide occurrences were discovered. Granitic rocks may be of some exploration interest. Late- to post-Grenvillian granite plutons in southwestern Norway are hosts to Mo mineralization and are locally anomalous in U and F. Also associated with these intrusions are fault-controlled polymetallic (Au–Ag–Cu–Pb–Zn) veins ( Gower, 1992). There is no known Mo mineralization in the late- to post-Grenvillian granite plutons in Labrador, although typically these intrusions are very poorly exposed and have not been mapped in any detail. An outcrop of gossanous P ggn orthogneiss (Sample: NK-00-8060, UTM 522150m E, 5826410m N) contains several percent sulphide mineralization and 2788 ppm copper. In addition, an outcrop of granite and associated pegmatite, inferred to be late- to post-Grenvillian, contains several percent pyrite and 215 ppm copper (Sample: DJ-00-9026, UTM 546365m E, 5828086m N). Emslie, R.F. 1976: Mealy Mountains complex, Grenville Province, southern Labrador. Report of Activities, Part A. Geological Survey of Canada, Paper 76-1A, pages 165-170. Emslie, R.F. and Hunt, P.A. 1990: Ages and petrogenetic significance of igneous-charnockite suites associated with massif anorthosites, Grenville Province. Journal of Geology, Volume 98, pages 213-231. Corrigan, D., Rivers, T. and Dunning, G.R. 1997: Preliminary report on the evolution of the allochthon boundary thrust in eastern Labrador: Mechin River to Goose Bay. Eastern Canadian Shield Onshore-Offshore Transect (ECSOOT), Transect Meeting, April 1997. R.J. Wardle and J. Hall. The University of British Columbia, LITHOPROBE Secretariat, Report 61, pages 45-56. Gower, C.F. 1992: The relevance of Baltic Shield metallogeny to mineral exploration in Labrador. Current Research. Newfoundland Department of Mines and Energy, Geological Survey, Report 92-1, pages 331-366. 1996: The evolution of the Grenville Province in eastern Labrador. Precambrian Crustal Evolution in the North Atlantic Region. T.S. Brewer. Geological Society Special Publication No. 112, pages 197- 218. 1999: Geology of the Crooks Lake map region, Grenville Province, eastern Labrador. Current Research. Newfoundland Department of Mines and Energy, Geological Survey, Report 99-1, pages 41-58. Gower, C.F., Heaman, L.M., Loveridge, W.D., Schärer, U. and Tucker, R.D. 1991: Grenvillian granitoid plutonism in the eastern Grenville Province, Canada. Precambrian Research, Volume 51, pages 315-336. Gower, C.F., James, D.T., Nunn, G.A.G. and Wardle, R.J. 1995: The Eastern Grenville Province. The Geology and Mineral Deposits of Labrador: A Guide for the Exploration Geologist. R.J. Wardle. Workshop Handout, Geological Survey, Department of Natural Resources, pages 73-101. Gower, C.F. and Owen, V. 1984: Pre-Grenvillian and Grenvillian lithotectonic regions in eastern Labrador - correlations with the Sveconorwegian Orogenic Belt in Sweden. Canadian Journal of Earth Sciences, Volume 21, pages 678-693. Gower, C.F. and van Nostrand, T. 1996: Geology of the southeast Mealy Mountains region, Grenville Province, southeast Labarador. Current Research. Newfoundland Department of Natural Resources, Geological Survey, Report 96-1, pages 55-71. James, D.T. 1999: Geology of the Kenamu River area (NTS 13C/NE), Grenville Province, southern Labrador. Geological Survey of Newfoundland and Labrador, Department of Mines and Energy, Open File 013C/0040, 1:100 000-scale map with descriptive notes. James, D.T., Kamo, S. and Krogh, T.E. 2000: Preliminary U–Pb geochronological data from the Mealy Mountains terrane, Grenville Province, southern Labrador. Current Research Newfoundland Department of Mines and Energy, Geological Survey, Report 2000-1, pages 169-178. James, D.T., Kamo, S., Krogh, T.E. and Nadeau, L. 2001: Preliminary U–Pb geochronological data from the Mealy Mountains and Mecatina terranes, Grenville Province, southern Labrador. Current Research. Newfoundland Department of Mines and Energy, Geological Survey, Report 2001-1. James, D.T. and Lawlor, B. 1999: Geology of the Grenville Province, Kenamu River area (NTS 13C/NE), southern Labrador: Preliminary observations of Labradorian and Pre-Labradorian(?) intrusions. Current Research. Newfoundland Department of Mines and Energy, Geological Survey, Report 99-1, pages 59-70. James, D.T. and Nadeau, L. 2000a: Geology of the Minipi Lake area (NTS 13C/South): New data from the southern Mealy Mountains terrane, Grenville Province, Labrador. Current Research. Newfoundland Department of Mines and Energy, Geological Survey, Report 2000-1, pages 179-196. 2000b: Geology of the Minipi Lake area (NTS 13C/South), Grenville Province, southern Labrador. Geological Survey of Newfoundland and Labrador, Department of Mines and Energy, Open File 013C/0041, 1:100 000scale map with descriptive notes. 2001: Geology of The Little Mecatina River (NTS 13D/NE) and Lac Arvert (NTS 13C/SW) areas, Grenville Province, southern Labrador. Current Research. Newfoundland Department of Mines and Energy, Geological Survey, Report 2001-1, pages 55-73. Krogh, T.E., Gower, C.F. and Wardle, R.J. 1996: Pre-Labradorian crust and later Labradorian, Pinwarian and Grenvillian metamorphism in the Mealy Mountains terrane, Grenville Province, eastern Labrador. Proterozoic Evolution in the North Atlantic Realm. C.F. Gower. COPENA-ECSOOT-IBTA conference, Goose Bay, Labrador, Program and Abstracts, pages 106-107. Nunn, G.A.G. and van Nostrand, T. 1996: Geology of the Kenemich River map area (NTS 13G/SW), Labrador. Current Research. Newfoundland Department of Natural Resources, Geological Survey, Report 96-1, pages 73-83. Perreault, S. and Jacob, H-L. 1999: Les minéraux industriels en Minganie: une ressources exploitée depuis 75 ans. Explorer au Québec: Le défi de la connaissance. Programme et résumes, 1999, page 62. Stevenson, I.M. 1969: Lac Brule and Winokapau Lake mapareas, Newfoundland and Quebec (NTS 13D, 13E). Geological Survey of Canada, Paper 6769, 15 pages, and includes Map 261967, scale: 1:253 440. Swinden, H.S., Wardle, R.J., Davenport, P.H., Gower, C.F., Kerr, A., Meyer, J.R., Miller, R.R., Nolan, L., Ryan, A.B. and Wilton, D.H.C. 1991: Mineral exploration opportunities in Labrador: A perspective for the 1990's. Current Research. Newfoundland Department of Mines and Energy, Geological Survey, Report 91-1, pages 349- 390. Thomas, A., Nunn, G.A.G. and Krogh, T.E. 1986: The Labradorian orogeny: evidence for a newly identified 1600 to 1700 Ma orogenic event in Grenville Province crystalline rocks from central Labrador. The Grenville Province. J.M. Moore, A. Davidson, and A.J. Baer. Geological Association of Canada, Special Paper 31, pages 175-189. Thomas, A., Culshaw, N.G. and Currie, K.L. 1994: Geology of the Lac Ghyvelde Lac Long area, Labrador. Scale: 1:100 000. Geology of the Lac Ghyvelde Lac Long area, Labrador and Quebec. Geological Survey of Canada, Bulletin 448, 37 pages. Wardle, R.J., Crisby-Whittle, L.V.J. and Blomberg, P. 1990: Geology of the Minipi River area (NTS 13C/NW). Current Research. Newfoundland Department of Mines and Energy, Geological Survey, Report 90-1, pages 267- 276. 2000: Bedrock geology map of the Minipi River area (13C/NW), Labrador. Map 200013, Scale: 1:100 000. Newfoundland and Labrador Department of Mines and Energy, Geological Survey, Open File 013C/0042. et al. et al. et al. et al. et al. et al. see et al. et al. see In In Edited by In In Edited by In In Compiled by In In In In In In In Compiled by In In In Edited by In In Boundary between the WLT and the MMT GOVERNMENT OF NEWFOUNDLAND AND LABRADOR Department of Mines and Energy Geological Survey Joint project of the Geological Survey of Newfoundland and Labrador and the Geological Survey of Canada. OPEN FILE 013D/0029 MAP 2001-18 BEDROCK GEOLOGY GEOLOGY OF THE LITTLE MECATINA RIVER AREA (NTS 13D/NE), GRENVILLE PROVINCE, SOUTHERN LABRADOR.
Transcript of GEOLOGY OF THE LITTLE MECATINA RIVER AREA (NTS 13D/NE ...
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Little Mecatina River
Little Meactina River
Little Meactina River
Fig
River
Fig River
Churchill River
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53°00’ N 53°00’ N
63°00’ W 62°00’ W62°30’W
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62°00’ W62°30’ W63°00’ W
52°45’ N
NTS 13D/10
NTS 13D/15
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NTS 13D/16
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Outcrop (massive or no structural fabricsmeasured)
Grenvillian high-strain zone - boundarybetween the WLT and the MMT, teeth onstructural hanging wall: assumed
Grenvillian high-strain zone: assumed
Contact: assumed
Late- to post-Grenvillian fault(Neoproterozoic ?): assumed
Foliation, vertical foliation
Gneissosity, vertical gneissosity
Gneissosity or foliation and containedmineral elongation lineation
Planar fabric in highly strained rocks
Dyke
Igneous layering
MESOPROTEROZOIC
LEGEND
PALEOPROTEROZOIC
MEALY MOUNTAINS TERRANE (MMT)
Orthogneiss: heterogeneous and unsubdivided unit consists of gneissic and foliated granitoid rocksincluding granite, granodiorite, quartz monzonite, local diorite and rare gabbro.
Anorthosite: white- to grey weathering, recrystallized and foliated meta-anorthosite containinghornblende and biotite.
Gabbro - gabbronorite: massive to foliated and recrystallized gabbro, biotite gabbro andgabbronorite.
Gabbroic anorthosite: recrystallized and foliated gabbroic anorthosite, leucogabbro, gabbro andgneissic gabbro.
Metasedimentary gneiss: pink- and grey-weathering, biotite + garnet + sillimanite migmatite,contains a high percentage of K-feldspar-bearing leucosome.
Metasedimentary gneiss and related rocks:
rusty- or pink-weathering, high-grade metasedimentary gneiss containing biotite + sillimanite andvariable amounts of K-feldspar-bearing leucosome, also including rare occurrences of quartzite andcalc-silicate schist.
grey- and black-weathering, amphibolite- and upper amphibolite-facies metasedimentary gneisscontaining biotite + sillimanite ± muscovite.
pink-weathering, biotite + sillimanite-bearing diatexite containing abundant abundant inclusions ofP msm metasedimentary gneiss.��
Gabbro and gabbronorite: variably deformed and recrystallized gabbro and gabbronorite.
Grey orthogneiss: grey-weathering granitoid orthogneiss containing biotite + hornblende ±clinopyroxene.
Granitoid rocks: consists mainly of recrystallized, foliated and locally mylonitic, biotite ±hornblende granite, granodiorite, monzonite, K-feldspar porphyritic granite and local clinopyroxene-bearing granitoid rocks.
WILSON LAKE TERRANE (WLT)
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Mealy Mountains Intrusive Suite (MMIS)
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Pre-MMIS rocks
Granite: pink-weathering, mainly coarse- and medium-grained biotite granite, locally K-feldsparporphyritic granite; mainly isotropic, although contains a local, weak foliation (relict igneousfoliation ?)
Late- to post-Grenvillian intrusive rocks
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Figure 1. Index map of Labrador showing location of the NTS 13D/NE area.
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Figure 2. Location of the Little Mecatina River area in relation to the tectonic and major lithotectonic units of northeastern Laurentia.
Grenville Province: HRT - Hawke River terrane, LMT - Lake Melville terrane, GBT - Groswater Bay terrane, WLT - Wilson Lake
terrane, CFT - Churchill Falls terrane, LJT - Lac Joseph terrane, MLT - Molson Lake terrane, GT - Gagnon terrane, MAT - Matamec
terrane, WKT - Wakeham terrane, LRD - La Romaine domain. Archean divisions: SP - Superior Province, NP - Nain Province
(Hopedale Block). Archean and Paleoproterozoic divisions: MK - Makkovik Province, SECP - Southeastern Churchill Province (core
zone), KSG - Kaniapiskau Supergroup (2.25-1.86 Ga). Mesoproterozoic units: NPS - Nain Plutonic Suite, HLIS - Harp Lake intrusive
suite, MB - Mistastin batholith, MI - Michikamau Intrusion, SLG - Seal Lake Group.
Little MecatinaRiver area
DATA POSITIONING
On this map, structural and outcrop data are plotted in the position where the data were collected in the
field (i.e., symbols representing tectonic structures or primary features are not offset from the outcrop location).
The centre of the symbol representing a planar fabric element (e.g., foliation or gneissosity) or a planar primary
feature (e.g., bedding or igneous layering) is plotted in the position where the data were collected in the field. The
+ symbol represents an outcrop that has a massive (i.e., isotropic) structure or one that does not contain a
measurable fabric (e.g., frost heave or an extensively fractured outcrop).
Data are plotted using a NAD 1927 projection, UTM Grid Zone 20.
Recommended citation:James, D.T. and Nadeau, L. 2001. Geology of the Little Mecatina River area (NTS 13D/NE), GrenvilleProvince, southern Labrador. Geological Survey of Newfoundland and Labrador, Department of Mines andEnergy, Map 2001-18, Open File 013D/0029, 1:100 000-scale map with descriptive notes.
Geology by D.T. James (Geological Survey of Newfoundland and Labrador)and L. Nadeau (Geological Survey of Canada, Québec), 2000.
This map represents a preliminary interpretation of the geological field data and is subject to change withoutnotice. Comments regarding revisions or errors are invited and should be directed to the Director, GeologicalSurvey of Newfoundland and Labrador, P.O. Box 8700, St. John’s, Newfoundland, Canada, A1B 4J6.
Corresponding author:D.T. JamesGeological Survey of Newfoundland and LabradorDepartment of Mines and EnergyP.O. Box 8700, St. John’s, Newfoundland A1B 4J6
e-mail: [email protected]
last modified: March 2001.
DESCRIPTIVE NOTES
INTRODUCTION
REGIONAL GEOLOGY
DESCRIPTION OF ROCK UNITS
WILSON LAKE TERRANE (WLT)
MEALY MOUNTAINS TERRANE (MMT)
LATE- TO POST-GRENVILLIAN GRANITE (M grn)
The northeastern part of NTS map area 13D (Figure 1), including sheets 13D/9, 13D/10, 13D/15 and 13D/16, weremapped at a scale of 1:100 000 in the 2000 field season (James and Nadeau, 2001). Mapping was accomplished by makinghelicopter landings on isolated outcrops. This work represents a continuation of 1:100 000-scale mapping in NTS map area13C by James and Lawlor (1999) and James and Nadeau (2000a, b). NTS 13D was previously mapped by Stevenson (1969).Thomas (1994) mapped NTS 13D/NW. To the east, NTS 13C/NW was mapped by Wardle (1990, 2000).
The rocks in the northern parts of NTS 13D/NE are well exposed. In contrast, the southern part of the area isextensively covered by bog, dense forest, or thick accumulations of Quaternary deposits, and there are few outcrops. Definitivecontact relationships between units were not observed. To some extent, the locations of contacts and faults shown on the mapwere interpreted using regional aeromagnetic data.
The study area is situated in the northeastern Grenville Province and straddles the boundary between the ExteriorThrust Belt and Interior Magmatic Belt ( Gower , 1991). It includes parts of the Wilson Lake and Mealy Mountainsterranes (Figure 2). These structurally stacked terranes are Grenvillian tectonic divisions, although they consist primarily oflate Paleoproterozoic crust.
The Wilson Lake terrane (WLT) is dominated by high-grade metasedimentary gneisses having protolithdepositional ages predating ca. 1720 Ma (i.e., pre-Labradorian Orogeny). The gneisses are derived primarily from pelitic andsemi-pelitic sedimentary rocks, although very minor amounts of metasedimentary rocks derived from quartzite and siliceouscarbonate also occur. The terrane also includes intrusions of late Paleoproterozoic gabbro and gabbronorite, and mafic gneissespossibly derived from supracrustal rocks. Intrusions of foliated granite and porphyritic granite, correlated on the basis of rocktype with the ca. 1650 Ma Trans-Labrador batholith, and granitoid orthogneisses of uncertain age make up minor amounts ofthe WLT.
Geochronological studies of monazite contained in sapphrine-bearing diatexite, occurring in the northern part ofthe WLT, and presumed to be derived from anatexis of the metasedimentary gneisses, indicate that high-grade metamorphismand attendant deformation in the WLT are Middle Labradorian (ca. 1640 Ma) (Corrigan , 1997). Corrigan ( )have also shown, on the basis of U–Pb dating of monazite, that the terrane has been locally overprinted by Grenvillian (ca. 1000Ma) metamorphism. Regionally persistent ductile high-strain zones separating the WLT from the Trans-Labrador batholith tothe north, and the Mealy Mountains terrane to the south, are ca. 1010-990 Ma structures (Corrigan , 1997).
The Mealy Mountains terrane (MMT) (Gower and Owen, 1984) consists primarily of late Paleoproterozoic(Labradorian-age) plutons of the Mealy Mountains intrusive suite (MMIS) ( Emslie, 1976; Emslie and Hunt, 1990; Krogh
, 1996), minor amounts of Paleoproterozoic pre-Labradorian crust, and Pinwarian (1510 to 1450 Ma) and late- to post-Grenvillian intrusions (Gower, 1996). The MMIS consists mainly of an older group of anorthositic, leucogabbroic andleucotroctolitic rocks, and a younger group of pyroxene-bearing monzonite and quartz monzonite intrusions. Emplacementages for units of MMIS monzodiorite orthogneiss, porphyritic quartz monzodiorite and pyroxene-bearing monzonite,determined by U–Pb geochronology of zircon and occurring in the Kenamu River area (NTS 13C/NE), are 1659 ± 5 Ma, 1650± 1 Ma, and 1643 ± 2 Ma, respectively (James , 2000).
Rocks in the western part of the MMT are variably foliated and locally gneissic; in general they have northeast- toeast-northeast-striking planar fabrics that are inferred to be Labradorian in age (James and Nadeau, 2000a). However, the localoccurrence of deformed, Pinwarian age (ca. 1514 Ma) granite in the Kenamu River area (James , 2000) demonstrates thewestern part of the MMT has also been affected by Pinwarian and/or Grenvillian deformation.
The MMT is intruded by plutons of unmetamorphosed, massive to weakly foliated granite and quartz monzonite.One of these plutons, occurring in the southwestern part of the MMT, is dated by U–Pb techniques on zircon to be 964 ± 3 Ma(James , 2001). This age is consistent with reported emplacement ages for widespread granitoid plutons in thenortheastern Grenville Province ( Gower, 1996).
In NTS 13D/NE, the WLT comprises approximately 60% metasedimentary gneiss and related rocks, includingdiatexite and granite. The most common metasedimentary rock type is a high-grade biotite and sillimanite-bearing migmatite(P msm) composed of a rusty or grey paleosome and variable amounts of pink- and white-weathering K-feldspar-bearing
leucosome. Garnet occurs only rarely in P msm gneiss.
The P msm gneisses are gradational, at all scales, into pink- and grey-weathering diatexite (P dxt). The diatexite
is medium to coarse grained, variably foliated, has a varied composition from granite to quartz monzonite, and contains biotiteand widespread sillimanite. Outcrops include less than 30% ‘screens’ of metasedimentary gneiss (P msm). Locally, outcrops
are lacking in the metasedimentary gneiss ‘screens’ and they can be classified as sillimanite-bearing granite or sillimanite-bearing homogeneous diatexite. Gradational contacts with the surrounding high-grade metasedimentary gneiss unit, thewidespread occurrence of metasedimentary gneiss ‘screens’ and the composition of the rocks suggest P dxt diatexite is
derived from the nearly complete, melting of surrounding metasedimentary gneisses.
The northeastern part of NTS 13D/16, north and east of the Churchill River, is mainly underlain by grey- and white-weathering or, locally, grey- and pink-weathering metasedimentary migmatite (P msa). The P msa rocks are inferred to be a
somewhat lower grade version of the P msm gneisses. The P msa rocks commonly contain muscovite, and have lesser
volumes of leucosome than P msm gneisses. The leucosome in P msa rocks is most commonly white weathering and K-
feldspar-poor, whereas the high-grade P msm rocks contain abundant volumes of pink-weathering, K-feldspar-rich
leucosome. The area underlain by P msa rocks is marked by a magnetic low; in sharp contrast, areas underlain by higher
grade P msm rocks and diatexite (P dxt) are marked by a prominent, regional magnetic high.
The composition of P msm and P msa metasedimentary gneisses suggests they are derived from aluminous,
pelitic and semi-pelitic protoliths. Both units have a relatively consistent composition over large areas presumably reflecting awidespread and thick sedimentary precursor of consistent composition. However, there are a few local variations in themetasedimentary gneisses including rare occurrences of mafic gneiss having an uncertain protolith, quartzite, gneisses derivedfrom siliceous carbonate, silicate-oxide iron formation and possible felsic volcanic rocks. Subdivisions of the different rocktypes in the metasedimentary gneiss units are not shown on the map because of their very limited extent.
Metasedimentary gneisses in the northeastern part of NTS 13D/16 are intruded by granodiorite orthogneiss (P
ogn). This orthogneiss is migmatitic containing thin (centimetre-scale), white- and pink-weathering layers of leucosome, andgrey-weathering layers of paleosome containing biotite, hornblende, and possible clinopyroxene. The rocks locally contain K-feldspar augen that are presumed to be relicts of K-feldspar phenocrysts.
The metasedimentary gneisses are intruded by bodies of variably deformed and recrystallized granite, K-feldsparporphyritic granite, granodiorite and monzonite. The different rock types that make up the P grn unit are not divided on the
map. Medium to coarse grained, K-feldspar porphyritic, biotite- and hornblende-bearing granitic rocks are the most commontype in the unit. Grey- to pink-weathering monzonitic rocks, locally porphyritic, and containing local clinopyroxene are aminor component of the unit. All rocks in the unit are foliated and some are gneissic. Highly strained and mylonitic rocks,forming a high-strain zone several kilometres wide, occur along the boundary with the MMT.
Metasedimentary gneisses and granitic rocks in the WLT are intruded by bodies of metamorphosed and deformedgabbro, gabbronorite and lesser clinopyroxenite defined as unit P gbr. These rocks are mainly fine grained, have granoblastic
textures, and are composed of varied proportions of plagioclase and pyroxenes. They may also contain minor amounts ofbiotite and quartz. The rocks are massive to foliated and locally gneissic. However, medium-grained rocks having a preservedigneous, intergranular texture also occur. Relict igneous layers occur in a few outcrops.
The southwestern part of NTS 13D/10 contains several outcrops of pink-, grey- and rusty-weathering, upperamphibolite-facies metasedimentary gneiss (P msd). These rocks are very well layered, having a gneissosity defined by
alternating biotite + sillimanite-rich, and K-feldspar-rich layers. P msd rocks contain abundant garnet, in contrast to the
garnet-poor metasedimentary gneisses in the WLT. P msd metasedimentary gneiss is inferred to be derived from a pelitic
sedimentary protolith. Occurrences of metasedimentary gneiss, similar to P msd gneiss, are relatively common in the eastern
parts of the MMT (e.g., Gower and van Nostrand, 1996), although they are rare in the central and western parts of the terrane.The depositional age of the sedimentary protoliths is undetermined, but they must be older than the middle Labradorian rocksof the MMIS which intrude them.
Gneissic and foliated, amphibolite-facies granitoid rocks, mainly including biotite ± hornblende ± clinopyroxenegranite, granodiorite, quartz monzonite, and local diorite, are widespread in NTS 13D/NE. These rocks are very poorlyexposed, and because they could not be reasonably subdivided in the field at the scale of mapping, they are collectivelyassigned to a single unit. P ggn rocks are somewhat similar to gneissic P mdq granitoid rocks, which comprise part of the
MMIS in the Kenamu River (NTS 13C/NE), Fourmont Lake (NTS 13C/SE) and Lac Arvert (NTS 13C/SW) areas ( James,1999; James and Nadeau, 2000a). However, P ggn consists mainly of granite and granodiorite gneiss, whereas P mdq
consists mainly of clinopyroxene-bearing monzonite or monzonite gneiss. P ggn rocks are interpreted to be
equivalent to the Lower Brook metamorphic suite orthogneisses defined by Wardle (1990). Igneous emplacement ages ofP ggn rocks are undetermined.
Outcrops of P ggn rocks are commonly composite; they may contain xenoliths of grey-weathering hornblende +
biotite diorite, and younger aplite dykes that are less deformed than the gneiss. In addition, P ggn gneisses are locally cut by
deformed and metamorphosed mafic dykes. Weakly deformed and massive pegmatites form a minor part of the unit.
The NTS 13D/10 area includes an elongate, east-west-trending intrusion of metamorphosed and deformedanorthosite. The rocks are white and grey on the fresh and weathered surfaces. They are variably recrystallized and foliated.Some rocks contain abundant relicts of coarse-grained, bluish-grey primary plagioclase grains that have fine-grained marginsof recrystallized plagioclase. In contrast, other rocks consist entirely of white, fine- to medium-grained granoblasticplagioclase. Primary mafic minerals are replaced by a mixture of biotite and hornblende. Based on similarity of rock types, P
tan anorthosite is provisionally correlated with Labradorian-age MMIS anorthosite occurring in the central (e.g., Gower, 1999)and northeastern (e.g., Nunn and van Nostrand, 1996) parts of the MMT.
The P gbr unit consists mainly of a medium grey-weathering gabbro, commonly containing up to 10% medium-
grained biotite. The biotite appears to be primary as opposed to metamorphic. The biotite gabbro has a distinctive porphyritictexture defined by biotite ‘phenocrysts’ up to 1 cm that consist of an aggregate of biotite grains. The rocks are generallymassive to foliated and variably recrystallized, although subtle relicts of igneous layering, defined by composition and grainsize, occur locally. In well-foliated and recrystallized rocks, the gabbro is converted to biotite amphibolite.
The southwestern part of NTS 13D/9 includes several occurrences of metamorphosed gabbroic anorthosite,leucogabbro and mafic gneiss defined as unit P gan. The rocks have a fine-grained granoblastic texture and are strongly
foliated. Locally, they contain relicts of coarse-grained clinopyroxene that appear to be mainly replaced by finer grainedhornblende and biotite.
The MMT is intruded by plutons of granite, quartz monzonite and, locally, K-feldspar porphyritic granite definedas unit M grn. These rocks are pink on the fresh and weathered surfaces, medium to coarse grained and contain less than 10%
biotite. Generally they are massive, although weakly foliated rocks also occur. Of note, some plutons are marked by prominentcircular or elliptical, magnetic highs, whereas others are marked by magnetic lows. M grn rocks are correlated, on the basis of
composition and texture, with granites occurring in the NTS 13C/SE area, where one pluton has an emplacement age of 964 ±3 Ma determined by U–Pb geochronology of zircon (James 2001).
The MMT also contains widespread occurrences of massive, pink- or white-weathering pegmatites containingmuscovite ± biotite. On the basis of their undeformed character, they are interpreted to be late- to post-Grenvillian intrusions.
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Metasedimentary gneiss and related rocks (P msa, P msm P dxt
Grey orthogneiss (P ogn)
Granite, granodiorite and K-feldspar porphyritic granite (P grn)
Gabbro and gabbronorite (P gbr)
Metasedimentary gneiss (P msd)
Orthogneiss (P ggn)
Anorthosite (P tan)
Gabbro, gabbronorite and gabbroic anorthosite (P gbr, P gan)
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STRUCTURE AND METAMORPHISM
WILSON LAKE TERRANE (WLT)
MEALY MOUNTAINS TERRANE
EXPLORATION POTENTIAL
REFERENCES
Foliation and gneissosity in metasedimentary migmatite, diatexite, orthogneiss and granitic rocks in theWLT are generally southwest-striking and moderately northwest dipping. Foliation is defined by alignment of themetamorphic minerals, or recrystallized mineral aggregates in the granitic rocks. In P msm metasedimentary
migmatite, the metamorphic assemblage includes biotite + sillimanite + K-feldspar + granitic melt. Thisassemblage, and the fact that P msm migmatite is gradational into diatexite (P dxt), indicates upper-amphibolite
to granulite-facies metamorphism. In NTS 13D/NE, metamorphic leucosome in P msm migmatite and diatexite
have not been dated, although they are interpreted to be the product of ca. 1640 Ma Labradorian metamorphism.This interpretation is based on geochronology of monazite by Corrigan (1997), collected from diatexiteoccurring in the northwestern part of the WLT.
In contrast to the P msm rocks, P msa metasedimentary migmatite, occurring in the northeastern part
NTS 13D/16, commonly contains muscovite. Neither the age of the migmatization nor the muscovite growth iscertain, but one possible scenario is that migmatization is Labradorian and the same age as high-grademetamorphism in P msm migmatite, whereas the muscovite is Grenvillian and represents the effects of a lower
grade (amphibolite facies?) metamorphic overprint. This provisional model is consistent with data presented byCorrigan (1997) indicating that some rocks in the southeastern part of the WLT have been overprinted by ca.1000 Ma metamorphism.
In NTS 13D/NE, the tectonic boundary between the WLT and the MMT is not a single structure. Rather,it appears to consist of two geometrically discordant structures that separate MMT rocks from a structurallyimbricated package of WLT rocks.
Mylonitic rocks, derived from metasedimentary migmatite and granite, in the central and western partsof NTS 13D/NE, suggest this section of the boundary is a ductile high-strain zone. In these rocks, high-strain planarfabrics are southwest- to west-striking, moderately northwest to north dipping, and contain a northwest-trendingmineral elongation lineation. The highly strained rocks do not appear to be retrogressed, suggesting that the high-strain event was synchronous with high-grade metamorphism. The interpretation of this part of the boundary as amainly northwest-dipping structure is in general agreement with interpretations made by Thomas (1986) forareas west of NTS 13D/NE.
In the northeastern part of the area, in NTS 13D/16, the boundary is very poorly exposed and its locationis interpreted primarily on the basis of magnetic anomaly patterns. One outcrop, occurring near the inferredboundary location, consists of interlayered and strongly foliated mafic and metasedimentary gneisses that exhibit avery strong, moderately south southwest-dipping foliation and a southeast-plunging mineral elongation lineation.This single observation is consistent with mapping by Wardle (1990, 2000) in NTS 13C/NW which has shownthe boundary between the WLT and the MMT to be a mainly southwest- to southeast-dipping high-strain zone inthat area. Wardle (1990) defined this boundary as the Hamilton River Shear Zone (HRSZ). On the basis ofU–Pb geochronological studies of mylonitized samples from the NTS 13C/NW area, Corrigan (1997)determined that thrusting of the MMT over the WLT, along the HRSZ, occurred at 1010 ± 4 Ma. A model involvingearly, northwest-directed Grenvillian transport of the MMT over the WLT on the HRSZ, followed by southeast-directed “back-thrusting” of WLT rocks over the MMT on different, northwest-dipping structures, could explainthe apparent geometry of structures defining the terrane boundary.
Foliation and gneissosity in metasedimentary gneiss (P msd), orthogneiss (P ggn) and rocks of the
MMIS are highly varied in attitude, although in general, planar fabrics are southwest-striking. These fabrics, and themetamorphic minerals that define them, are tacitly assumed to be Labradorian. Geochronological studies arerequired to test this model.
The metasedimentary and MMIS rocks indicate upper amphibolite-facies metamorphism was reached.The former are migmatitic rocks containing biotite + sillimanite + garnet + K-feldspar. The latter include gneissicand migmatitic granitoid rocks, and gabbroic rocks which have been converted to amphibolite. Primary maficminerals in the anorthosite (P tan) have been replaced by hornblende and biotite.
The Grenville Province of southern Labrador has some exploration potential ( Swinden , 1991;Gower, 1992; and Gower , 1995), although field work in 2000 did not result in any significant or promisingdiscoveries of economic minerals.
Mesoproterozoic and Paleoproterozoic anorthosite intrusions are potential exploration targets for Ti(Fe) oxide and magmatic Ni-Cu sulphide deposits. The QIT Fer et Titane Inc. Ti-oxide deposits in Mesoproterozoicanorthosite near Harve-St-Pierre, Québec, provide the best local example of economic Ti-oxide mineralization inGrenville Province anorthosite (Perreault and Jacob, 1999). Of note, the Mesoproterozoic (ca. 1130 Ma) Atikonakanorthosite, occurring to the west, in NTS 13D/SW, is approximately the same age as the intrusion which hosts theQIT deposits. The anorthosite (P tan), discovered in the 2000 field season and inferred to be Paleoproterozoic in
age, offers a new exploration target in the region for magmatic Ni-Cu sulphide deposits. In addition,Paleoproterozoic gabbro and gabbronorite intrusions in the study area, and elsewhere in the region, also have somepotential for hosting Ni-Cu and PGE mineralization. Several outcrops in the study area contain a few percent pyrite,although no significant sulphide occurrences were discovered.
Granitic rocks may be of some exploration interest. Late- to post-Grenvillian granite plutons insouthwestern Norway are hosts to Mo mineralization and are locally anomalous in U and F. Also associated withthese intrusions are fault-controlled polymetallic (Au–Ag–Cu–Pb–Zn) veins ( Gower, 1992). There is no knownMo mineralization in the late- to post-Grenvillian granite plutons in Labrador, although typically these intrusionsare very poorly exposed and have not been mapped in any detail.
An outcrop of gossanous P ggn orthogneiss (Sample: NK-00-8060, UTM 522150m E, 5826410m N)
contains several percent sulphide mineralization and 2788 ppm copper. In addition, an outcrop of granite andassociated pegmatite, inferred to be late- to post-Grenvillian, contains several percent pyrite and 215 ppm copper(Sample: DJ-00-9026, UTM 546365m E, 5828086m N).
Emslie, R.F. 1976: Mealy Mountains complex, Grenville Province, southern Labrador. Report of Activities, PartA. Geological Survey of Canada, Paper 76-1A, pages 165-170.
Emslie, R.F. and Hunt, P.A. 1990: Ages and petrogenetic significance of igneous-charnockite suites associatedwith massif anorthosites, Grenville Province. Journal of Geology, Volume 98, pages 213-231.
Corrigan, D., Rivers, T. and Dunning, G.R. 1997: Preliminary report on the evolution of the allochthon boundarythrust in eastern Labrador: Mechin River to Goose Bay. Eastern Canadian Shield Onshore-Offshore Transect(ECSOOT), Transect Meeting, April 1997. R.J. Wardle and J. Hall. The University of British Columbia,LITHOPROBE Secretariat, Report 61, pages 45-56.
Gower, C.F. 1992: The relevance of Baltic Shield metallogeny to mineral exploration in Labrador. CurrentResearch. Newfoundland Department of Mines and Energy, Geological Survey, Report 92-1, pages 331-366.
1996: The evolution of the Grenville Province in eastern Labrador. Precambrian Crustal Evolution in theNorth Atlantic Region. T.S. Brewer. Geological Society Special Publication No. 112, pages 197-218.
1999: Geology of the Crooks Lake map region, Grenville Province, eastern Labrador. Current Research.Newfoundland Department of Mines and Energy, Geological Survey, Report 99-1, pages 41-58.
Gower, C.F., Heaman, L.M., Loveridge, W.D., Schärer, U. and Tucker, R.D. 1991: Grenvillian granitoid plutonismin the eastern Grenville Province, Canada. Precambrian Research, Volume 51, pages 315-336.
Gower, C.F., James, D.T., Nunn, G.A.G. and Wardle, R.J. 1995: The Eastern Grenville Province. The Geologyand Mineral Deposits of Labrador: A Guide for the Exploration Geologist. R.J. Wardle. WorkshopHandout, Geological Survey, Department of Natural Resources, pages 73-101.
Gower, C.F. and Owen, V. 1984: Pre-Grenvillian and Grenvillian lithotectonic regions in eastern Labrador -correlations with the Sveconorwegian Orogenic Belt in Sweden. Canadian Journal of Earth Sciences, Volume 21,pages 678-693.
Gower, C.F. and van Nostrand, T. 1996: Geology of the southeast Mealy Mountains region, Grenville Province,southeast Labarador. Current Research. Newfoundland Department of Natural Resources, Geological Survey,Report 96-1, pages 55-71.
James, D.T. 1999: Geology of the Kenamu River area (NTS 13C/NE), Grenville Province, southern Labrador.Geological Survey of Newfoundland and Labrador, Department of Mines and Energy, Open File 013C/0040, 1:100000-scale map with descriptive notes.
James, D.T., Kamo, S. and Krogh, T.E. 2000: Preliminary U–Pb geochronological data from the Mealy Mountainsterrane, Grenville Province, southern Labrador. Current Research Newfoundland Department of Mines andEnergy, Geological Survey, Report 2000-1, pages 169-178.
James, D.T., Kamo, S., Krogh, T.E. and Nadeau, L. 2001: Preliminary U–Pb geochronological data from the MealyMountains and Mecatina terranes, Grenville Province, southern Labrador. Current Research. NewfoundlandDepartment of Mines and Energy, Geological Survey, Report 2001-1.
James, D.T. and Lawlor, B. 1999: Geology of the Grenville Province, Kenamu River area (NTS 13C/NE), southernLabrador: Preliminary observations of Labradorian and Pre-Labradorian(?) intrusions. Current Research.Newfoundland Department of Mines and Energy, Geological Survey, Report 99-1, pages 59-70.
James, D.T. and Nadeau, L. 2000a: Geology of the Minipi Lake area (NTS 13C/South): New data from the southernMealy Mountains terrane, Grenville Province, Labrador. Current Research. Newfoundland Department of Minesand Energy, Geological Survey, Report 2000-1, pages 179-196.
2000b: Geology of the Minipi Lake area (NTS 13C/South), Grenville Province, southern Labrador.Geological Survey of Newfoundland and Labrador, Department of Mines and Energy, Open File 013C/0041,1:100 000scale map with descriptive notes.
2001: Geology of The Little Mecatina River (NTS 13D/NE) and Lac Arvert (NTS 13C/SW) areas, GrenvilleProvince, southern Labrador. Current Research. Newfoundland Department of Mines and Energy,Geological Survey, Report 2001-1, pages 55-73.
Krogh, T.E., Gower, C.F. and Wardle, R.J. 1996: Pre-Labradorian crust and later Labradorian, Pinwarian andGrenvillian metamorphism in the Mealy Mountains terrane, Grenville Province, eastern Labrador. ProterozoicEvolution in the North Atlantic Realm. C.F. Gower. COPENA-ECSOOT-IBTA conference, GooseBay, Labrador, Program and Abstracts, pages 106-107.
Nunn, G.A.G. and van Nostrand, T. 1996: Geology of the Kenemich River map area (NTS 13G/SW), Labrador.Current Research. Newfoundland Department of Natural Resources, Geological Survey, Report 96-1, pages 73-83.
Perreault, S. and Jacob, H-L. 1999: Les minéraux industriels en Minganie: une ressources exploitée depuis 75 ans.Explorer au Québec: Le défi de la connaissance. Programme et résumes, 1999, page 62.
Stevenson, I.M. 1969: Lac Brule and Winokapau Lake mapareas, Newfoundland and Quebec (NTS 13D, 13E).Geological Survey of Canada, Paper 6769, 15 pages, and includes Map 261967, scale: 1:253 440.
Swinden, H.S., Wardle, R.J., Davenport, P.H., Gower, C.F., Kerr, A., Meyer, J.R., Miller, R.R., Nolan, L., Ryan,A.B. and Wilton, D.H.C. 1991: Mineral exploration opportunities in Labrador: A perspective for the 1990's.Current Research. Newfoundland Department of Mines and Energy, Geological Survey, Report 91-1, pages 349-390.
Thomas, A., Nunn, G.A.G. and Krogh, T.E. 1986: The Labradorian orogeny: evidence for a newly identified 1600to 1700 Ma orogenic event in Grenville Province crystalline rocks from central Labrador. The GrenvilleProvince. J.M. Moore, A. Davidson, and A.J. Baer. Geological Association of Canada, Special Paper 31,pages 175-189.
Thomas, A., Culshaw, N.G. and Currie, K.L. 1994: Geology of the Lac Ghyvelde Lac Long area, Labrador. Scale:1:100 000. Geology of the Lac Ghyvelde Lac Long area, Labrador and Quebec. Geological Survey of Canada,Bulletin 448, 37 pages.
Wardle, R.J., Crisby-Whittle, L.V.J. and Blomberg, P. 1990: Geology of the Minipi River area (NTS 13C/NW).Current Research. Newfoundland Department of Mines and Energy, Geological Survey, Report 90-1, pages 267-276.
2000: Bedrock geology map of the Minipi River area (13C/NW), Labrador. Map 200013, Scale: 1:100 000.Newfoundland and Labrador Department of Mines and Energy, Geological Survey, Open File 013C/0042.
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Boundary between the WLT and the MMT
GOVERNMENT OFNEWFOUNDLAND AND LABRADOR
Department of Mines and Energy
Geological Survey
Joint project of the Geological Survey of Newfoundland and Labradorand the Geological Survey of Canada.
OPEN FILE 013D/0029
MAP 2001-18
BEDROCK GEOLOGY
GEOLOGY OF THE LITTLE MECATINA RIVER AREA
(NTS 13D/NE), GRENVILLE PROVINCE, SOUTHERN LABRADOR.
