|Author:||Gigon et al., in preparation|
|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|
Table des matières
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.
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).
The gneiss has been identified in three NTS sheets: 33F01, 33F02 and 33G04. It covers an area of approximately 66 km².
A U-Pb isotopic dating sample was taken from outcrop 2016-JG-2856 located 1.4 km west of Marjoulet Lake (Goutier, 2018).
|Isotopic System||Mineral||Crystallization Age (Ma)||(+)||(-)||Reference(s)|
|U-Pb||Zircon||2689||4||4||David, personal communication, 2018|
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.
Does not apply.