Background

Description

Hellancourt Formation (pPhe): Massive or Pillow Basalt, Some Mafic and Sedimentary Pyroclastic Rocks, Mudrock and Amphibolite

Hellancourt Formation 1 (pPhe1): Pillow Basalt

The pillow basalt unit of the Hellancourt Formation (pPhe1) is composed of homogeneous, aphanitic rock forming decimetric to metric pillows. The rock is generally massive, but includes schistose zones attributed either to small horizons containing sediments, small deformation corridors, or to more carbonated, chloritized or altered basalt. The unit contains an unestimated percentage of gabbro, generally interpreted as metric to decametric intrusions, conformable and contemporaneous to other Hellancourt units (pPhe, pPhe2 and pPhe5). The unit also contains multiple decimetric, 3 m thick horizons of schistose black mudrock (pPhe3), sometimes graphic or sulphidic.

Hellancourt Formation 2 (pPhe2): Massive Basalt

The massive basalt unit (pPhe2) shares several compositional and structural characteristics with the pillow basalt unit (pPhe1) above it. Part of this unit may correspond to the massive, schistose basalt of the undifferentiated unit (pPhe), but does not contain speckled basalt (or very little), a facies interpreted either as gabbro sills or coarse-grained lava. Massive basalt contains low-carbonate, friable black shale horizons and rare medium-grained horizons, interpreted as a microgabbro. Some of these intrusions could be ultramafic. The relationship between basalt and gabbro is often ambiguous, as contact is unclear and difficult to distinguish. These correspond to zones that are particularly rich in carbonates, as evidenced by the presence of small calcite clusters. Geochemical similarity also complicates the mapping of flows and sills.

The unit is usually composed of massive, homogeneous rock, fine to very fine-grained, even aphanitic, rarely marked by a deformation fabric. It is characterized by very low structural and compositional variability. The colour of the rock is similar to that of pillow basalt, which is beige-greenish in altered surface and greenish grey in fresh exposure. Distinctive components are in the presence of coarse plagioclase and amphibole crystals dominated by actinolite and, to a lesser extent, tremolite and hornblende. These minerals are generally sericitized and chloritized. A second generation of chlorite crystallized in parallel lath clusters marks the main schistosity, accentuating the lepidoblastic texture in places. Titanite is a minor phase, but it is characteristic of the Hellancourt Formation as a whole. Other matrices and secondary minerals include epidote, quartz and up to 2% disseminated sulphides, including framboidal pyrite. The rock contains very little carbonate, but dissolved cavities are frequently observed in outcrop. Another feature of this unit is the presence of centimetric chlorite porphyroblasts distributed heterogeneously in the rock. The latter indicate that the rock has been metamorphosed to greenschist facies, or even upper greenschist facies for areas containing hornblende. Garnet was not observed in this unit.

The schistose horizons conformable with stratification’s origin has not been further developed. Schistosity is local, not penetrative, and does not appear to be associated with specific petrographic variations, lithological changes, or shear corridors. Instead, deformed zones have anastomosed schistosity, rock discolouration, fracturing, silicification, or an abundance of quartz or carbonate veins and black chlorite veinlets. 

Hellancourt Formation 3 (pPhe3): Black Slate, Locally Pyritic, and Biotite Schist

Horizons of interstratified slate in the volcanic pile form sedimentary units of low extension and thickness ranging from <10 m to >30 m (Clark, 1980; Fournier, 1981; Wares and Goutier, 1989). The rock has thin, commonly wavy laminae. In places, current ripples and cross-bedding are observed (Clark, 1980). Slate is usually graphic and locally pyritic. It is black or dark grey in fresh exposure and rusty in altered surface due to pyrite oxidation, which is usually observed in thin laminas parallel to bedding and in cubic crystals (Clark, 1980; Fournier, 1981). Graphic black slate horizons are sedimentary or volcano-sedimentary (Clark, 1980).

In the Roberts Synclinal area, the unit consists of a few very fine-grained slate schist beds. This rock is black in fresh exposure and shows a completely altered and friable rust-coloured surface. Schist is composed of quartz (33-67%), biotite (20-54%), muscovite (0-12%), chlorite (0-16%), epidote (0-8%) and plagioclase (1-25%). Accessory minerals are hornblende, microcline, garnet, opaque minerals, calcite, apatite and tourmaline (Hardy, 1976). 

Hellancourt Formation 4 (pPhe4): Mafic Pyroclastics

Mafic pyroclastics (pPhe4) interstratified in the basalt sequence are scarce in the Hellancourt Formation. They include mafic pyroclastic rocks, flow top breccias, pillow breccias and rarely mafic tuff and chert. Volcanic breccias form horizons ranging in thickness from 0.3 to 1 m (Fournier, 1981) and up to 5 m (Sauvé and Bergeron, 1965). These brecciated horizons are interbedded with pillow basalt flows or form the summit of pillow or massive flows (Clark, 1979, 1980; Fournier, 1981). They are composed of fragmented basaltic material with structures that resemble small pillows (Clark, 1980). The fragments, angular, subrounded or irregular in shape, have a diameter ranging from <1 cm to about 30 cm and a tight arrangement. The matrix is composed of carbonate, quartz and chlorite (Sauvé and Bergeron, 1965; Clark, 1979, 1980; Fournier, 1981). Basaltic tuff is present in horizons of a few metres thick (Clark, 1980).

A 3 m thick flow breccia of hyaloclastic material was observed by Goulet (1986) at Baie-aux-Feuilles (sheet 24K13). Goulet (1995) also reported the presence of channelized conglomerates interbedded between pillow basalt flows. Conglomerate beds are less than 3 m thick. They consist of vesicular lava, pyritic gabbro, mudrock and pyrite fragments. Pebbles are rounded to subrounded and rarely exceed 10 cm in diameter. The matrix, characterized by a very dark colour, is composed of reworked tuff (Goulet, 1995).

Hellancourt Formation 5 (pPhe5): Plagioclase Porphyritic (Spotted) Basalt

Unit pPhe5 groups outcrops where the speckled metagabbro and porphyritic basalt predominate over other basaltic units of the Hellancourt Formation. In the Roberts Synclinal area, this unit is predominantly present in the massive to schistose basalt at the volcanic sequence’s base (pPhe). The rock is characterized by homogeneity, fine to medium grain size, and a unique speckled (spotted) texture. It can be interpreted as microgabbro sills or coarse-grained lava, their composition and geochemical signature being similar to the fine-grained basalt hostrock. Contacts between gabbro and basalt are generally not marked, progressive or non-visible, which accentuates the similarities between these lithologies. Sill thickness is estimated to be a few tens of metres.

A variety of gabbro sills interpreted as late to post-volcanic intrusions are distinguished by sharp contacts and particularly schistose and altered basalt walls. However, their geochemical and structural affinities with the other gabbros favour their integration into the Hellancourt Formation. Other gabbros have a subophitic texture, a characteristic that is more representative of the Montagnais Sills (pPmon). However, field relationships and the nature of the contacts with the basalts support their belonging to the Hellancourt Formation.

The rock is generally mesococratic, with a brownish alteration patina that can be confused with that of basalt. It is greenish-grey to dark greenish brown in fresh exposure, giving it a locally melanocratic appearance. The main components are acicular amphiboles and plagioclase. The unit is distinguished from other Hellancourt facies by the presence of damouritized plagioclase or chloritized amphibole phenocrystals, the best criterion for their identification in the field. Few samples appear to contain some hornblende. Secondary components are chlorite, epidote, carbonate and titanite, the lalter being ubiquitous in all the facies of the Hellancourt Formation. Epidote can make up 20% of the rock, especially in small euhedral crystals of metamorphic origin, but also in veinlets or associated with altered plagioclase. Sulphides account for up to 2-3% of the rock and are concentrated in microfractures.

Rocks are commonly cut by veinlets, veins or clusters up to 5 cm wide and composed of soft blackish material interpreted as chlorite. The latter is also a constituent of quartz veins that abound in the schistosity plane, as well as carbonate, as evidenced by the dissolved cavities in the veins and surrounding rock.

Hellancourt Formation 6 (pPhe6): Amphibolized Basalt

Amphibolite of the Hellancourt Formation (pPhe6) is interpreted as a higher metamorphic grade equivalent of the other Hellancourt units. The complete loss of primary structures associated with metamorphism does not make it possible to know with certainty which of the original units the rock belongs. In the field, rocks of the pPhe6 unit may be confused with some dark-coloured metasediments. Amphibolite is generally grey to dark green to blackish.

The rock is fine to very fine-grained, granoblastic, equigranular and generally mesocratic and banded. Amphibolite is composed on average of 60% amphibole and 40% plagioclase. Amphibole bands are largely dominated by stocky recrystallized hornblende, with rare actinolite. Non-twinned plagioclase crystals combine with quartz to form alternating leucocratic bands.

South of the Arnaud River (sheets 24N12 and N13), amphibolite is composed of approximately 5% coronitic garnet, resulting from a higher metamorphic pressure than that which affected rocks of the Roberts Synclinal. Garnet is medium-grained, poikilitic with quartz or plagioclase inclusions, homogeneously distributed and surrounded by quartz.

Thickness and Distribution

The Hellancourt Formation is part of the Labrador Trough’s second volcano-sedimentary cycle. It outcrops in the north-central (southwest of Kuujjuaq) to the far north of the Trough (north of Kangirsuk). In the Kuujjuaq region, unit thickness was estimated to be between 1000 and 1500 m on a regional fold’s western flank, reduced in thickness on the eastern flank due to a shear zone (Sauvé and Bergeron, 1965). In the Kangirsuk region, the Hellancourt Formation is the thickest of Paleoproterozoic units. In the Roberts Synclinal, the unit’s thickness also varies significantly due to the presence of numerous local folds and imbrication of the sequence along thrust fronts east of the synclinal. Its thickness is roughly estimated at a minimum of 1500 m on the east side and up to 8000 m on the west side.

Dating

None.

Stratigraphic Relationship(s)

A glomerophyric gabbro dyke intruding the Hellancourt Formation’s summital strata was analyzed to constrain the age of the end of volcanism in the Baie-aux-Feuilles region (Machado et al., 1997). The average results show an age of 1874 ±3 Ma that was correlated with an age of 1870 ±4 Ma obtained for a rhyodacite in the Douay Formation (Machado et al., 1997). The authors conclude that the Hellancourt Formation’s age is 10 to 14 Ma younger than that of the Sokoman Formation‘s iron formations.

The Hellancourt Formation overlies turbidites of the Menihek Formation with interlocking contact over a transition zone of at least 1 km. Cartographic contact between the two units occurs when the proportion of basalt at the top of the Menihek Formation exceeds 50% in outcrop. The unit stratigraphically overlying the Hellancourt Formation is completely eroded in the Roberts Synclinal area.

The normal polarity of the Hellancourt units is reflected in a basal undifferenciated sequence (pPhe) cut by abundant speckled gabbro sills (pPhe5), some of which are possibly progressive contact coarse-grained lava, overlain by massive flows (pPhe2), and then pillow flows (pPhe1) at the top. Some of the summital units are intruded with gabbro (pPhe5) and others contain thin horizons of black slate (pPhe3) and sulphide clusters. All contacts are sharp and interlocking. There are a few exceptions where massive flows change laterally to pillow lava (Sauvé and Bergeron, 1965). Gabbro sills (or coarse-grained flows) of the pPhe5 unit were observed alternating with mudrocks and quartz arenites of the Menihek Formation (pPme), underlying the Hellancourt Formation. The Hellancourt Formation’s base can be followed continuously in contact with the Baby Formation’s ferriferous upper member in the Kuujjuaq region, without the presence of the Menihek Formation (Sauvé and Bergeron, 1965). Finally, there does not appear to be any apparent relationship between the Hellancourt Formation and other mafic units in the region, be it the Montagnais Sills or the Baie Kyak Intrusion.

Paleontology

Does not apply.

References

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