The Paleoproterozoic volcano-sedimentary rock sequences of the Lake Harbour Suite were described by Davison in 1959 on southern Baffin Island. These sequences were extended into NE Québec and Labrador (« Dorset fold belt ») and introduced as a group by Jackson and Taylor (1972). Taylor (1979) took up the term, but reduced the unit to the rank of formation in order, in his view, to comply with the North American Stratigraphic Code (NACSN, 1983, 2005). Hoffman (1988) did, however, use the informal term “Koroc River sediments” while acknowledging its similarity to the Lake Harbour Group. For priority reasons, Verpaelst et al. (2000) and Simard et al. (2013) maintained the term Lake Harbour Group in the Koroc River and Kuujjuaq areas. Although the correlation with Baffin Island units remains uncertain, Lafrance et al. (2015, 2016) and Mathieu et al. (2018) also assigned volcano-sedimentary rock sequences located in the Falcoz Lithotectonic Domain to the Lake Harbour Group to provide some continuity with the units recognized further north.

Given the degree of metamorphism and loss of primary structures, the use of the term group is inappropriate. Hence, the unit was renamed « Lake Harbour Suite » by Lafrance and Vanier (2022) as part of a synthesis of the southeastern Churchill Province (SEPC; Lafrance et al., 2018) to follow the nomenclature of the North American Stratigraphic Code (NACSN, 1983, 2005). Lafrance and Vanier (2022) also reassigned the metasedimentary rock units located east of the Blumath Shear Zone (ZCblm) and previously included in the Sukaliuk Complex to the Lake Harbour Suite. These changes in nomenclature are detailed in the table below.

Description

Lake Harbour Suite 1 (pPhb1): Paragneiss

The paragneiss is the dominant unit of the Lake Harbour Suite. It usually has a banded or stromatic texture materialized by the presence of 5 to 25% millimetric to centimetric white leucosome bands. The rock composition varies depending on the percentage of K-feldspar (1-30%), quartz (5-30%) and ferromagnesian minerals (biotite and garnet, 10-25%). Its colour varies from grey to dark grey in fresh surface with a distinctive rusty brown or brownish patina. Paragneiss is fine to medium grained, well foliated and has a well-developed granoblastic texture. Biotite and garnet are locally associated with sillimanite. Garnet occurs as small disseminated grains or as centimetric poikiloblasts containing inclusions of other minerals observed in the paragneiss. Sillimanite occurs in several forms: 1) small disseminated prisms; 2) fibrous, associated with mica (muscovite and biotite); 3) millimetric to centimetric nodules; and 4) flexuous clusters surrounding feldspar phenocrysts. Muscovite, cordierite, zircon, apatite, epidote, graphite, allanite, tourmaline, magnetite, rutile and sulphides are the most common secondary minerals.

Lake Harbour Suite 2 (pPhb2): Quartzite

Impure quartzite layers contain up to 10% K-feldspar or plagioclase, as well as laminae rich in muscovite, sillimanite or graphite. Sillimanite occurs as flexuous clusters of fine needles. Biotite (<12%), brown to green and with zircon inclusions, garnet (<2%) and tourmaline are also commonly observed. Accessory minerals are apatite, clinopyroxene, zircon, graphite, magnetite and sulphides. In several areas, particularly near ultramafic sills of the Nuvulialuk Suite (pPnuv), quartzite contains pyrite and graphite mineralization (5-25%), disseminated and as laminae.

Lake Harbour Suite 2a (pPhb2a): Impure Quartzite

Unit pPhb2a consists of quartzite outcrops located east of the ZCblm. The quartzite is generally impure and forms klippes <1 km wide. It contains >75% quartz, feldspars, biotite, and muscovite. Accessory minerals observed are apatite, tourmaline, zircon, and graphite. Quartzite may also contain sulphides and magnetite. Quartz grains may be indented, lenticular or in bands. Micas in places give a lepidoblastic texture to the quartzite, especially evident in thin section, but which can give the rock a bedded appearance. Quartzite is commonly cut by quartz veins and veinlets.

Lake Harbour Suite 3 (pPhb3): Calc-silicate Rocks and Marble

Calc-silicate rocks and marbles are commonly located in contact between Archean gneiss and paragneiss. Since many of these contacts are tectonic, it is possible that these lithologies acted as ductile materials that facilitated movement on shear zones. They may also represent alteration zones near NE-SW fault zones. Moreover, these rocks contain millimetric quartz bands, indicating a protomylonitic fabric, and generally form centimetre- to metre-long boudins in the different units of the Lake Harbour Suite. In this case, reaction rims are observed around the boudins.

According to Verpaelst et al. (2000), marble forms thin sequences, usually at the base of the Lake Harbour Suite. Most of it is calcitic, although dolomitic marble is also found. It is generally interbedded between layers of paragneiss or more or less pure quartzite. Marble is medium to coarse grained, heavily deformed and granoblastic. It can be equigranular or heterogranular and it is generally well foliated. During a low temperature deformation episode, mylonitization caused some marbles to reduce their grain size. Minerals observed are carbonates, phlogopite, tremolite, olivine, diopside and scapolite. Olivine is variably altered to serpentine and iddingsite; some crystals are partly replaced by chondrodite. Marble also contains a wide variety of accessory minerals such as sulphides, apatite, graphite, sphene, garnet, epidote, zircon, tourmaline, quartz and feldspar.

Calc-silicate rocks are generally associated with marbles and have roughly the same pattern of occurrence in the field. Their mineralogy is varied and displays alternating composition in bands or millimetric to decimetric clusters. They are fine to medium grained with a light greenish colour. In addition to carbonates, diopside, tremolite, epidote, scapolite, plagioclase, quartz and sphene are the common minerals of these rocks. Hornblende, apatite, olivine, phlogopite, garnet, muscovite, spinelchlorite, zircon and allanite may also be present locally. Olivine is coronitic (clinopyroxene crown) and usually serpentinized.

Lake Harbour Suite 4 (pPhb4): Amphibolite

Amphibolite is interstratified with other units of the Lake Harbour Suite, primarily paragneiss and quartzite. Unit pPhb4 is fine grained or aphanitic, granoblastic, banded and dark grey-green to black. In the Koroc River area (Verpaelst et al., 2000), basalt-derived amphibolite forms massive, pillow or breccia flows. However, it is too deformed for polarities to be determined. Its actual thickness is also difficult to assess due to significant deformation. Amphibolite contains 45 to 65% ferromagnesian minerals, consisting of green hornblende (40-50%), clinopyroxene (2-20%) and brown biotite (2-10%). Quartz (<3%), sphene, sulphides, magnetite, chlorite, apatite, garnet and actinolite are the main accessory minerals. Hornblende is biotitized locally. Epidote appears as a plagioclase and ferromagnesian mineral alteration mineral. Quartz is interstitial or occurs as globules in hornblende. Locally, garnet can account for up to 35% of amphibolite.

Thickness and Distribution

The Lake Harbour Suite covers an area of ~4703 km² in the Falcoz Lithotectonic Domain. The largest sequences occur in the Koroc River area (sheet 24I). The paragneiss of the pPhb1 unit (~2461 km²) forms sequences a few kilometres to nearly 30 km wide east of the ZCblm. Unit pPhn1b (~1492 km²) is the second largest unit and forms sequences up to 10-15 km wide by 30-70 km long, as well as elliptical-shaped bodies 3-4 km wide by 6-10 km long. Quartzite in unit pPhb2 (~457 km²) may also represent sequences >10 km wide. The most prominent calc-silicate rock and marble layers of unit pPhb3 (~114 km²) are in the Keglo Bay (sheet 24P04) and Abrat River (sheet 24P03) areas, where they reach a few kilometres in apparent width. The pPhb1a (~52 km²), pPhb2a (~67 km²) and pPhb4 (~60 km²) units and subunits cover smaller areas.

Dating

On Baffin Island, dating obtained from detrital zircons of the Lake Harbour Suite impose a sediment deposition age younger than 1934 ±2 Ma (Scott and Gauthier, 1996). According to the work of Charette (2016) and Charette et al. (2021), about 10 fractured zircon cores gave heritage ages between 2597 Ma and 2019 Ma (sample taken from outcrop 2013-IL-3008). Finally, analysis of sulphur isotopes from two paragneiss samples also indicates that they are derived from a Proterozoic protolith (Wing et al., 2012).

East of the ZCblm, detrital ages obtained from two samples (outcrops 2013-BC-6150 and 2013-DB-1098) range from 2824 Ma to 1884 Ma (Charette et al., 2021), indicating a slightly younger maximum age of deposition than on Baffin Island. Paleoproterozoic ages of zircons and monazites allow us to place the age of leucosome crystallization between 1825 Ma and 1812 Ma. Lu-Hf ages on garnets by Charette et al. (2021), however, indicate that melting of the metasedimentary rocks began earlier, around 1896 Ma.

Stratigraphic Relationship(s)

Metamorphosed volcano-sedimentary rocks of the Lake Harbour Suite unconformably overlie gneissic Archean basement of the Falcoz Lithotectonic Domain. However, the contacts observed today are tectonic in nature and can be obliterated by the partial melting that affected all these rocks in the Paleoproterozoic as well as by the presence of late granite and pegmatite intrusions. Paragneiss and quartzite of the Lake Harbour Suite are cut by mafic and ultramafic sills and dykes of the Nuvulialuk Suite (pPnuv).

Paleontology

Does not apply.

References

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