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Boulder complex
Stratigraphic label: [arch]blr
Map symbol: Ablr

First published: 21 October 2019
Last modified: 10 December 2020

 

 

 

  DISCLAIMER: This English version is translated from the original French. In case of any discrepancy, the French version shall prevail. 

Informal subdivision(s)
Numbering does not necessarily reflect the stratigraphic position.
 
Ablr2 Amphibolite
Ablr1 Tonalitic gneiss
 
Author: Gélinas, 1965
Age: Archean
Stratotype: Boulder Lake, located ~45 km west of Kuujjuaq, is the type locality
Type area: Kuujjuaq area (NTS sheet 24K)
Geological province: Churchill Province
Geological subdivision: Rachel-Laporte Lithotectonic Domain
Lithology: Tonalitic gneiss
Category: Lithodemic
Rank: Complex
Status: Formal
Use: Active
Related unit(s)

None

Background

Gélinas (1965) described gneiss in the Boulder Lake area by giving them the informal name of “Lac Boulder Complex”. Simard et al. (2013) formally named the unit Boulder complex. These gneiss are confined to a thrust sheet within the Rachel-Laporte Lithotectonic Domain and thus represent a structural complex.

 

Description

The Boulder Complex consists of two units: tonalitic gneiss (Ablr1) and amphibolite (Ablr2).

 

Boulder Complex 1 (Ablr1): Tonalitic Gneiss

Unit Ablr1 consists of grey tonalitic gneiss containing 15-40% millimetre to centimetre-wide bands of pink granite and, more locally, 10-20% whitish bands. Gneiss is locally migmatized to varying degrees. It is medium to fine grained and contains 5-10% ferromagnesian minerals (biotite ± hornblende ± epidote). Locally, the granitic phase dominates and contains remnants of tonalite.

 

Boulder Complex 2 (Ablr2): Amphibolite

Gneiss (Ablr1) contains several layers and enclaves of hornblende-biotite amphibolite, which are derived from basalt, gabbro or diorite, and which have been grouped into unit Ablr2. Amphibolite layers are dismembered or boudinaged, usually decimetre to metre-wide but locally reaching a few hundred metres, interstratified in gneiss sequences. Amphibolite contains >60% ferromagnesian minerals, mostly hornblende, and it is well foliated, locally mylonitic.

 

Thickness and distribution

The Boulder Complex is located within a thrust sheet ~30 km long by 5 km wide in the Rachel-Laporte Lithotectonic Domain (Lafrance and vanier, 2021). It is located in the axial part a large regional antiform structure oriented NW-SE and overturned towards the SW, in the south-central part of the Kuujjuaq area (sheet 24K). It covers an area of ~170 km2.

 

Dating

The Archean age around 2868 Ma is interpreted as that of the gneiss protolith. It is comparable to ages obtained in tonalites of the Douglas Harbour Domain (2879-2857 Ma) in the Superior Province, directly west of the Labrador Trough (Simard et al., 2008). The lithological assemblages and age of the complex are also similar to those observed in the different units of the Ungava Complex (ApPung).

Unit Sample Number Isotopic System Mineral Crystallization Age (Ma) (+) (-) Metamorphic Age (Ma) (+) (-) Reference(s)
Ablr1 Boulder dome U-Pb Zircon 2868 8 8 1783 11 11 Machado et al., 1989
Rutile       1740 5 5

Stratigraphic Relationship(s)

The Boulder Complex is interpreted as a thrust sheet (Moorhead and Hynes, 1990) in fault contact with Paleoproterozoic rocks of the Laporte Supersuite. The different thrust sheets of the Rachel-Laporte Lithotectonic Domain are interpreted to have overthrusted Paleoproterozoic rocks during the New Quebec Orogenesis (1.82-1.77 Ga; Machado et al., 1989). Unit Ablr1 rocks of the Boulder Complex are similar to unit ApPung3 of the Ungava Complex.

Paleontology

Does not apply.

References

Author(s) Title Year of Publication Hyperlink (EXAMINE or Other)
GÉLINAS, L. Géologie de la région de Fort Chimo et des lacs Gabriel et Thévenet, Nouveau-Québec. Thèse de doctorat, Université Laval, Québec, 212 pages. 1965
LAFRANCE, I. – VANIER, M.-A. Domaine lithotectonique de Rachel-Laporte, sud-est de la Province de Churchill, Québec, Canada : synthèse de la géologie. Ministère de l’Énergie et des Ressources naturelles. 2021 BG 2021-01
MACHADO, N. – GOULET, N. – GARIÉPY, C. U-Pb geochronology of reactivated Archean basement and of Hudsonian metamorphism in the northern Labrador Trough. Canadian Journal of Earth Sciences; volume 26, pages 1–15. 1989 Source
MOORHEAD, J. – HYNES, A. Nappes in the internal zone of the northern Labrador Trough: Evidence for major early, NW-vergent basement transport. Geoscience Canada; volume 17, pages 241–244. 1990
SIMARD, M. – LAFRANCE, I. – HAMMOUCHE, H. – LEGOUIX, C. Géologie de la région de Kuujjuaq et de la Baie d’Ungava (SNRC 24J et 24K). Ministère des Ressources naturelles, Québec; RG 2013-04, 60 pages. 2013 RG 2013-04

 

 

 

10 février 2020