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Marjoulet Gneiss
Stratigraphic label: [narc]gma
Map symbol: nAgma

First published: 15 March 2018
Last modified: 7 May 2018

 

 

 

 

 

Translation of original French

 

 

Informal subdivision(s)
Numbering does not necessarily reflect the stratigraphic position.
None
 
Author:Gigon et al., in preparation
Age:Neoarchean
Reference section: 
Type area:Representative outcrops for this unit are located just west of Marjoulet Lake (NTS sheet 33F01). This lake is located 4 km south of Laguiche Lake and 23 km east of the eastern end of Sakami Lake.
Geological province:Superior Province
Geological subdivision:Opinaca Subprovince
Lithology:Biotite ± hornblende ± magnetite tonalitic gneiss, banded and foliated
Type:Lithodemic
Rank:Lithodeme
Status:Formal
Use:Active

 

 

Background

The Marjoulet Gneiss was first described following the 2015 fieldwork in sheet 33F01 (Gigon et al., in preparation). The name refers to Marjoulet Lake located in this area.

Description

A banded to foliated biotite ± hornblende tonalitic gneiss was observed in the central portion of sheet 33F01. This gneiss appears as a 30 km-long band with an apparent thickness between 400 m to almost 3 km. This unit is hosted in metasedimentary rocks of the Laguiche Complex and follows the structural grain and large folds. Furthermore, it is distinguished by a higher magnetic susceptibility.

In the field, the gneissic structure has not always been listed since these rocks have sometimes been associated with ultrametamorphic granite, banded granite or a granite-intruded foliated tonalite to explain compositional banding. Furthermore, the heterogeneity of the rock and the presence of biotite schlierens make mineral percentages highly variable and difficult to assess. The amount of biotite generally ranges from 2% to 10% and reaches up to 25% in tonalite. Magnetite (<1%) and hornblende (<8%) are also present. Grain size varies from fine to medium. Foliation is highlighted by schlierens of biotite. Banding is formed by more felsic centimetric bands, more ferrromagnesian-rich bands and dioritic horizons, which are usually boudinaged. Enclaves and centimetric to metric horizons of amphibolite, paragneiss or ultramafic rock are also observed. A large volume of granitic intrusions is also present in this unit (Gigon et al., in preparation). 

Thickness and Distribution

The gneiss has been identified in three NTS sheets: 33F01, 33F02 and 33G04. It covers an area of approximately 66 km². 

Dating

A U-Pb isotopic dating sample was taken from outcrop 2016-JG-2856 located 1.4 km west of Marjoulet Lake (Goutier, 2018). 

Isotopic SystemMineralCrystallization Age (Ma)(+)(-)Reference(s)
U-PbZircon268944David, personal communication, 2018

Stratigraphic Relationship(s)

The Marjoulet Gneiss is hosted in units nAlgi2a, nAlgi3a and nAlgi4a of the Laguiche Complex. It is highly deformed and metamorphosed to the amphibolite facies, as are host rocks. Dioritic intrusions found in the gneiss are younger than the tonalite, but they have also been heavily deformed. The Marjoulet Gneiss is cut by dykes and pegmatitic granite intrusions associated with the Vieux Comptoir Granitic Suite. It is also cut by diabases of the Mistassini Dyke Swarm and the Lac Esprit Dykes. 

Paleontology

Does not apply.

References

Author(s)TitleYear of PublicationHyperlink (EXAMINE or Other)
GIGON, J. – GOUTIER, J. – CÔTÉ-ROBERGE, M.

Géologie de la région du lac de Villaret, Municipalité d’Eeyou Istchee Baie-James. Ministère de l’Énergie et des Ressources naturelles, Québec.

 

In preparation
GOUTIER, J.

 

Géologie de la région du lac Ewart, sous-provinces de La Grande et d’Opinaca, à l’est de Radisson, municipalité d’Eeyou Istchee Baie-James, Québec, Canada. Ministère de l’Énergie et des Ressources naturelles, Québec.

2018

 

Geological Bulletin

 

 

 

8 novembre 2018