Lac au Bout Intrusion
Stratigraphic label: [narc]but
Map symbol: nAbut

First published: 15 May 2018
Last modified:











Translation of original French

Informal subdivision(s)
Numbering does not necessarily reflect the stratigraphic position.
Author: Bandyayera and Daoudene, 2017
Age: Neoarchean
Reference section: None
Type area: Lake au Bout area, in the Ouasouagami River area (NTS sheet 32N03)
Geological province: Superior Province
Geological subdivision: Opatica Subprovince
Lithology: Diorite and monzodiorite
Type: Lithodemic
Rank: Lithodeme
Status: Formal
Use: Active


The Lac au Bout Intrusion is a unit introduced by Bandyayera and Daoudene (2017) to describe an intrusive package of intermediate rocks in the Rodayer Lake area, emplaced between migmatitic paragneiss of the Nemiscau Subprovince to the north and the Colomb-Chaboullié Group volcano-sedimentary assemblage to the south.


The Lac au Bout Intrusion consists of highly foliated, granoblastic, medium-grained, generally magnetic and hematitized diorite and monzodiorite. In altered surface, the rock is whitish grey and often shows reddish spots in the centre of plagioclase crystals. In fresh exposure, the rock is rather reddish to greenish. The reddish colour is due to pronounced hematitization of plagioclase, while the greenish colour reflects amphibole which makes up 30 to 50% of the rock. In addition to hornblende and feldspars, the rock contains biotite, chlorite and epidote. In places, epidote occurs as veinlets.

Diorite is homogeneous and even grained, while monzodiorite is characterized by the presence of locally zoned centimetric K-feldspar phenocrystals (10%). At the edge of the intrusion, the rock is heavily deformed, porphyroclastic and shows plagioclase fragments reoriented in the mylonitic foliation. In porphyraceous monzodiorite, K-feldspar phenocrystals are deformed, stretched or flattened.

Thickness and Distribution

The Lac au Bout Intrusion is over 25 km long and 5 km wide. It is primarily located between the Broadback River and the Lake au Bout.



Stratigraphic Relationship(s)

The Lac au Bout Intrusion cuts volcanic rocks of the Colomb-Chaboulliée Group dated 2760.3 ±6.4 Ma (David, personal communication, 2017). At the regional level, it shows several petrographic similarities with the Bezier Suite (2712.3 ±5.8 Ma) identified by Goutier et al. (2001) in the La Grande Subprovince,which is further north. 


Does not apply.


Author(s) Title Year of Publication Hyperlink (EXAMINE or Other)
BANDYAYERA, D. – DAOUDENE, Y. Géologie de la région du lac Rodayer (SNRC 32K13-32K14-32N03 et 32N04-SE). Ministère de l’Énergie et des Ressources naturelles, Québec; RG 2017-01, 60 pages. 2017 RG 2017-01
GOUTIER, J. – DION, C. – OUELLET, M.-C. Géologie de la région de la colline Bezier (33G/12) et du lac de la Montagne du Pin (33G/13). Ministère des Ressources naturelles, Québec; RG 2001-13, 53 pages. 2001 RG 2001-13
12 septembre 2019