DISCLAIMER: This English version is translated from the original French. In case of any discrepancy, the French version shall prevail.
|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|
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.
The Boulder Complex consists of two units: tonalitic gneiss (Ablr1) and amphibolite (Ablr2).
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.
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.
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.
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).
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.
Does not apply.