Background

Description

Hutte Complex 1 (Ahue1): Tonalitic Gneiss, Locally Granitic; Granodiorite in Some Areas

The tonalitic to locally granitic gneisses are pale grey on the altered surface and greyish on the fresh surface. The grain size is generally medium, but can be locally coarse or, more rarely, fine. These gneisses are usually homogeneous, but show some heterogeneity in places. This heterogeneity is due to several causes, including variations in grain size that can change rapidly from fine to coarse, and the presence of diorite enclaves and pegmatitic granite injections without preferential direction. The Ahue1 unit is weakly to strongly magnetic and shows foliated, folded and banded structures. The gneissic structure is defined essentially by alternating grayish millimetric to centimetric bands rich in biotite and hornblende (~15%) with whitish bands richer in plagioclase (>50%). The banding is locally accentuated by centimetric injections of pink granite or granodiorite arranged parallel to the foliation planes. This banding becomes even more pronounced in the vicinity of deformation zones. The fine- to medium-grained tonalitic ribbons are partially to completely recrystallized. The most common accessory minerals are biotite (10-15%), hornblende (1-5%) and magnetite (trace-3%). The magnetite content is more important in the gneisses east of Glas Lake. In thin section, the biotite is locally chloritized and contains zircon inclusions. There are also traces of apatite, sphene and allanite. The gneiss typically contains 1 to 20% decimetric to metric enclaves or decametric strips of foliated, locally gneissic, magnetic diorite. Granitic gneiss outcrops mainly SW of Indien Lake in the Champion Lake area (sheet 32N09). The rock is medium to coarse grained, heterogranular, highly foliated to gneissic, and pink both on altered and fresh surfaces.

Hutte Complex 2 (Ahue2): Foliated Biotite ± Hornblende Tonalite

The biotite ±hornblende tonalite is intensely foliated, locally gneissic. The rock is homogeneous, medium grained, light gray on altered surface and dark gray on fresh surface. The rock typically contains 15% biotite, 5% hornblende and traces of magnetite. The Ahue2 unit contains 2-5% enclaves of homogeneous, granoblastic diorite that is dark gray on both altered and fresh surfaces. These enclaves are strongly stretched parallel to the foliation planes and include 40-50% ferromagnesian minerals (biotite and hornblende). The rock is cut by several multiphase injections of pegmatitic and massive pink granite, locally showing a lining of biotite, that can be confused with a structure commonly associated with partial melting.

Hutte Complex 3 (Ahue3): Foliated Granite

The granite is medium to coarse grained, locally pegmatitic. It is slightly to moderately magnetic. It is medium light pink on a fresh surface and light pink on an altered surface. The rock is foliated to locally gneissic. It is homogeneous and contains 3-10% biotite and 1-3% magnetite.

Thickness and Distribution

Most of the rocks in the Hutte Complex form domes 3 to 12 km long and 0.5 to 5 km wide. They are mainly located in sheets 32N09, 32O12, 32O14 and 32O15. They are elongated along a NE-SW axis and spatially associated with the Lac des Montagnes Belt. In the Lac de la Marée region (sheet 32O16), the orientation of the Hutte Complex units, as well as the Lac des Montagnes Belt that encloses them, changes from NE-SW to E-W (Bandyayera and Caron-Côté, 2022).

Dating

Two ages were obtained for the tonalitic gneisses of the Hutte Complex. The sample from outcrop 2017-DT-5144 yielded a metamorphic age of 2804 ±5.1 Ma, interpreted as that of the gneiss, and an age of 2952.2 ±3.8 Ma, interpreted as that of the protolith of the gneissic tonalite. Bayesian modelling of the same geochronological data from sample 2017-DT-5144A suggests two distinct ages at 2792.0 ±2.7 Ma and 2834.8 ±4.1 Ma, which is an alternative interpretation of the tonalitic gneiss age. The sample from outcrop 2018-CS-4142 yielded an age of 2790.4 ±5.4 Ma, interpreted as the protolith of the tonalitic gneiss.

These ages support the interpretation that the gneissic unit (Ahue1) corresponds to a Mesoarchean basement on which supracrustal rock units such as the Lac des Montagnes Group and the Voirdye Formation were deposited.

Stratigraphic Relationship(s)

Although the contact between the Hutte Complex and supracrustal rocks could not be observed at any outcrop, the age of the Ahue1 unit (David, 2020a-b) and the piling of volcano-sedimentary units suggest that the Hutte Complex forms the basement on which the Lac des Montagnes Group and Voirdye Formation were deposited. In addition, subhorizontal foliation observed in the Hutte Complex supports the interpretation that the unit consists of domes exposed in windows in the axial part of anticlines. The Hutte Complex is cut by Proterozoic diabase dykes belonging to the Mistassini and, more locally, Senneterre dyke swarms.

Paleontology

Does not apply.

References

Publications Available through Sigéom Examine

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