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Casterne Pluton
Stratigraphic label: [narc]cst
Map symbol: nAcst

First published: 14 June 2019
Last modified:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Translation of original French

 

 

Informal subdivision(s)
Numbering does not necessarily reflect the stratigraphic position.

None

 

Author: Côté-Roberge and Goutier, 2019
Age: Neoarchean
Reference section: Representative outcrops (e.g., isotopic dating site 2018-MY-6315) are located in the NE corner of NTS sheet 33G03
Type area: Casterne Lake area (NTS sheets 33G03 and 33G04)
Geological province: Superior Province
Geological subdivision: Opinaca Subprovince
Lithology: Quartz monzodiorite
Type: Lithodemic
Rank: Lithodeme
Status: Formal
Use: Active

 

 

Background

The Casterne Pluton is a formal unit of the Féron Suite established by Côté-Roberge and Goutier (2019). The name refers to Casterne Lake, located less than 5 km from the type outcrop used for isotopic dating (2018-MY-6315).

 

Description

The composition of the Casterne Pluton is intermediate to felsic. The most commonly observed lithology is a porphyritic quartz monzodiorite. There are also smaller amounts of monzonite, quartz monzonite, monzodiorite, quartz diorite and granodiorite. These lithologies are grey in fresh exposure and commonly intruded with coarse-grained pink granite.

The intensity of deformation varies from low to medium. A south-dipping E-W to NE-SW foliation can be measured on most outcrops. In places, rocks show a L/S tectonite fabric with secondary and tectonometamorphic mineral lineations defined by the alignment of mafic minerals shallowly dipping towards the ENE to NE.

K-feldspar phenocrystals make up 1-10% of the rock and are 1-2 cm long, five times coarser than the medium-grained matrix. Petrographic observations indicate that these phenocrystals are orthose, although microcline is abundant in the matrix. The pluton’s mineralogy is characterized by hornblende (5-15%), biotite (3-15%) and trace magnetite. Sphene and epidote are observed in a few places.

 

Thickness and Distribution

The Casterne Pluton is ovoid in shape and shallowly dips eastward. It is 20 km long, parallel to the main foliation, by 10 km wide. The Casterne Pluton is distinguished from adjacent units by its distinct magnetic signature. Several elongated magnetic ridges are arranged concentrically with a closure in the west that is better developed than in the east, where the pluton is cut by the Vieux Comptoir Granitic Suite. It covers an area of 76 km2 in the NW part of sheet 33G03, 17 km2 in the NE of sheet 33G04, and less than 1 km2 in sheet 33G06.

Dating

Isotopic dating was performed on an orthose porphyritic hornblende-biotite-magnetite-epidote monzodiorite sampled in 2018 on outcrop 2018-MY-6315. The age of 2701.5 ±2.4 Ma obtained is considered to be that of pluton intrusion and is also a minimum age for deposition of Laguiche Complex paragneiss in the Casterne Lake area.

Unit Isotopic System Mineral Crystallization Age (Ma) (+) (-) Reference(s)
nAcst U-Pb Zircon 2701.5 2.4 2.4 David, personal communication, 28 February 2019

Stratigraphic Relationship(s)

The Casterne Pluton cuts paragneiss and migmatites of the Laguiche Complex, as well as a biotite-magnetite tonalite-granodiorite intrusion in sheet 33G04. In sheets 33G03 and 33G06, it is completely isolated from paragneiss by a granite of the Vieux Comptoir Granitic Suite. It is also cut by a dyke belonging to the Mistassini Dyke Swarm.

Paleontology

Does not apply.

References

Publications available through SIGÉOM Examine

CÔTÉ-ROBERGE, M., GOUTIER, J. 2019. Géologie de la région du lac Casterne, sous-provinces de La Grande et d’Opinaca, Eeyou Istchee Baie-James, Québec, Canada. MERN. BG 2019-05, 2 plans.

GOLDAK AIRBORNE SURVEYS. 2008. LEVE AEROMAGNETIQUE SUR LE TERRITOIRE DE LA BAIE-JAMES – OPINACA, SUD DE LG-3 ET SUD DE LG-4. DP 2008-01, 50 pages and 58 plans.

 

11 novembre 2019