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

First published: 21 October 2019
Last modified:

 

 

 

  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”.

 

Description

The Boulder Complex consists of Archean rocks that are circumscribed within a thrust sheet from the Archean basement. These rocks were believed to have been emplaced within Paleoproterozoic volcano-sedimentary rocks of the Rachel-Laporte Lithotectonic Domain during the New Quebec Orogenesis (1840-1775 Ma). The complex consists of two units: tonalitic gneiss (Ablr1) and amphibolite (Ablr2).

 

Boulder Complex 1 (Ablr1): Tonalitic Gneiss

Unit Ablr1 of the Boulder Complex is similar to unit ApPung3 of the Ungava Complex. It 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

Boulder Complex 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. 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).

 

Dating

The Archean age 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

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, within the Rachel-Laporte Lithotectonic Domain.

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
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