Background

Description

Montauban Group 1 (mPmt1): Pyritic Paragneiss, Quartzite, Amphibolite, Pegmatite

Montauban Group 2 (mPmt2): Mesocratic Paragneiss, Locally Migmatitic

This unit consists of fine to medium-grained, grey mesocratic paragneiss, which locally has a banded to migmatitic texture. It also forms horizons in other units of the Montauban Group. Paragneiss is locally banded and contains 10 to 25% mafic minerals such as hornblende and biotite. The rock is fine grained and is characterized by alternating laminas rich in green hornblende, biotite, quartz (undulatory extinction) or plagioclase. Cordierite crystals are observed in several locations. The latter represents a typical assemblage of the high-grade facies, but forms less than 1% of the rock and does not exceed 0.5 mm in size. In addition to sulphides (pyrite), paragneiss also contains sillimanite, zircon, allanite, epidote, titanite and K-feldspar (partially sericitized). 

Montauban Group 3 (mPmt3): Metapelite, Amphibolite and Calcosilicate Rock Boudins

This unit consists of garnet porphyroblastic metapetite, fine to medium-grained, light grey and locally banded (centimetre scale). It contains 10 to 25% biotite and garnet with some sillimanite. The rock is foliated and generally migmatitic. Garnet porphyroblasts make up 15% of the rock and are euhedral, reddish and purplish in the presence of sillimanite.

Montauban Group 4 (mPmt4): Quartzite

This unit is less represented in the Borgia Lake area. It consists of quartzite, as well as horizons of amphibolite and tuff belonging to unit mPmt5. Quartzite is impure, light grey and has a granoblastic to holohyaline texture. It contains garnet porphyroblasts (1-2 mm), plagioclase (3 mm) and trace biotite.

Montauban Group 5 (mPmt5): Amphibolite, Lapilli Tuff

This unit consists of volcanic (basalt) granoblastic amphibolite and lapilli tuff. It also forms boudins in other units of the Montauban Group. Amphibolite has a rust-coloured patina associated with traces of iron sulphide. The rock is fine grained, generally homogeneous and locally has strong centimetric banding. The latter is marked by a gradual decrease in the order of 20% of mafic minerals in metric amphibolite horizons. It has a granoblastic texture and contains amphibole, biotite, plagioclase in small polygonal crystals, twinned clinopyroxene (aegerine), trace epidote and, locally, garnet porphyroblasts. Amphibolite is also caraterized by hornblende, hornblende-garnet porphyroblastic and hornblende-clinopyroxene facies. Pillow metabasalts were identified 1 km west of the old Montauban Mine (Pyke, 1967; Bernier, 1992).

Lapilli tuff is intermediate in composition (andesite) and contains calcic plagioclase and clinopyroxene. Some plagioclases are altered and others have rounded edges. Plagioclase is commonly rimmed by biotite and clinopyroxene (<2 mm), forming scattered nodules in the rock. These nodules are separated by small grains of polygonal quartz with undulatory extinction. Several plagioclase crystals contain greyish micro-inclusions, possibly quartz. It could be a filling of microvesicles formed in a volcanoclastic rock. Variable amounts of biotite paragneiss, quartzofeldspathic gneiss and garnetite are associated with this unit. Quartz-plagioclase-biotite felsic to intermediate horizons are also observed. Small biotite crystals are associated with quartz and plagioclase, which are concentrated in millimetric horizons. The whole is oriented parallel to the unit’s planar fabric.

From a geochemical perspective, rocks sampled during the 2017 mapping campaign (Moukhsil and Côté, 2018) have SiO₂ contents ranging from 53.92% to 71.05%. The Nesbitt geochemical diagram (2003) suggests that the paragneiss source of units 1, 2 and 5 is granitic to tonalitic. In the same diagram, a few samples mainly located in paragneiss units 1 and 2 approach the muscovite and illite pole, which could reflect an early alteration event. For volcanic rocks, spider diagrams normalized to the primitive mantle and discrimination diagrams of tectonic environment for basalts indicate emplacement in a MORB environment. Rocks sampled in the summer of 2017 correspond to the same tectonic setting environment as those of the Wabash Complex; however, they differ from those sampled in the same unit by MacLean et al. (1982).

Thickness and Distribution

Units of the Montauban Group are best known for their assemblage of metavolcanics, including amphibolitized and pillow basalts of tholeiitic affinity, which have a greater thickness north of the city of Trois-Rivières. To the north of this region, however, these assemblages are more difficult to identify because they are thinner and more dismembered in the surrounding rocks.

Dating

 A lapilli tuff was dated 1.45 Ga (Nadeau and van Breemen, 1994).

Stratigraphic Relationship(s)

The Montauban Group is cut by rocks of the La Bostonnais Complex. It also forms dismembered boudins in it and in surrounding intrusive rocks.

Paleontology

Does not apply.

References