|Author:||Bandyayera and Daoudene, 2018|
|Age:||Precambrian / Archean|
|Type area:||Boisrobert area (NTS sheet 32N11)|
|Geological province:||Superior Province|
|Geological subdivision:||La Grande Subprovince|
|Lithology:||Felsic intrusive rocks more or less foliated and migmatitized|
Table des matières
The Champion Complex is introduced to describe a large, polyphase, intrusive mass bounded by the Eastmain Group to the north, by the Nemiscau metasedimentary Subprovince to the west and south, and by the Lac des Montagnes Group to the east. It encompasses the Champion Lake plutonic terrain first assigned by Card and Ciesielski (1986) to the La Grande Subprovince, then reassigned to the Nemiscau Subprovince by Hocq (1994). New geophysical and geological data (D’Amours, 2011; Moukhsil, 2000) and results from the 2016 fieldwork (Bandyayera and Daoudene, 2018) show that the Champion Complex is the southern extension of the La Grande Subprovince.
The Champion Complex is a package of polyphase intrusive rocks hosted by tonalitic gneiss. It consists of six units: 1) a tonalitic gneiss unit (Achp1); 2) a unit of pyroxene-hornblende tonalite and granodiorite (Achp2); 3) a quartz diorite unit (Achp3); 4) a granodiorite unit (Achp4); 5) a porphyritic granodiorite unit (Achp5); and 6) a package of granodiorite and granite containing migmatite and tonalitic or granodioritic gneiss enclaves (Achp6).
Unit Achp1, outcropping northwest of Mezières Lake (sheet 33N06), is separated from the Lac des Montagnes Group by the Rivière Rupert Shear Zone (RRSZ). The tonalitic gneiss strips observed in the Nemiscau Subprovince near the Échancrure, Naquiperdu and Caumont lakes were also assigned to this unit. Tonalitic gneiss is medium-grained, light grey in altered surface and fresh exposure. It is heavily deformed, folded, locally banded, heterogeneous and low to highly magnetic. The gneissic texture is primarily marked by alternating millimetric to centimetric white and greyish ribbons. This banding is locally accentuated by centimetric intrusions of pink granite or granodiorite parallel to foliation planes and becomes even more pronounced near deformation zones. Fine to medium-grained tonalitic to dioritic ribbons are partially recrystallized. They contain 10 to 20% greenish hornblende and brownish to greenish-brown biotite in varying proportions. Biotite is chloritized and commonly contains zircon inclusions. Accessory minerals are allanite, sphene and apatite. South of the Mezières Lake area (sheet 32N06), the tonalitic gneiss that appears within the migmatites is highly magnetic and includes up to 10% magnetite. Chemically, tonalitic gneiss are metaluminous to locally peraluminous, type I, typically calcic and moderately enriched with potassium.
Granite gneiss outcrops mainly southwest of Indian Lake in the Champion Lake area (sheet 32N09). The rock is medium to coarse-grained, heterogeneous, strongly foliated to gneissic, pink in altered surface and fresh exposure. Unit Achp1 includes up to 20% enclaves or decametric strips of locally gneissic foliated magnetic diorite.
Unit Achp1 contains migmatites (metatexites and diatexites) following partial melting of tonalitic or dioritic gneiss; these metatexites and diatexites were assigned to subunits Achp1a and Achp1b respectively. These subunits are characterized by the presence of orthopyroxene (2-5%) and clinopyroxene (0-5%) in both neosome and paleoesome.
Champion Complex 1a (Achp1a): Metatexite derived from Tonalitic to Dioritic Rocks
Subunit Achp1a is located west and south of the Échancrure lake on the edge of the Champion Complex. The gneissic to foliated metatexite contains 20-50% mobilisate. It is derived from partial melting of a package of tonalitic to dioritic rocks. Migmatized tonalite contains up to 30% foliated and magnetic diorite green-grey enclaves. The highly deformed paleeosome, dioritic in composition, is rich in mafic minerals (50% hornblende, 10% biotite) and very magnetic. The mobilisate is a heterogranular and heterogeneous charnockite that typically contains hornblende schlierens. It appears in the form of clusters of leucosome or leucrocratic ribbons sub-parallel to foliation planes. In places, the mobilisate is also characterized by the presence of bluish quartz and orthopyroxene, indicating high temperature emplacement conditions.
Diorite rafts or enclaves are regularly observed in metatexite. It usually shows an agmatitic structure characterized by the presence of granitic mobilisate that includes sub-angular fragments with sharp or diffuse edges of diorite composition.
Champion Complex 1b (Achp1b): Diatexite derived from Tonalitic to Dioritic Rocks
Subunit Achp1b was recognized east of Naquiperdu Lake. It consists of diatexite following partial melting of the gneissic unit Achp1. In this unit, the dioritic paleosome is significantly more abundant than that of tonalitic composition. Mobilisate amounts 50 to 90% of the diatexite and is similar to the metatexite mobilisate of unit Achp1a. It is composed of in situ or segregated leucosome, heterogenranular and with hornblende schlierens. It typically contains 10% centimetric mafic fragments rich in hornblende (5-10%) and pyroxene (orthopyroxene and clinopyroxene; 5%). The mobilisate contains up to 30% partially digested enclaves and restites containing up to 40% hornblende, 10% clinopyroxene and some biotite.
In places, subrounded fragments of foliated diorite or enclaves of banded amphibolite derived from diorite are floating in the hornblende-rich mobilisate, defining a raft structure. The agmatitic structure, which is widespread within unit Achp1b, is due to the presence of fragments of banded diorite floating in the heterogeneous hornblende-rich leucosome. The nebulitic structure, also very common, is characterized by the presence of relics of melted diorite enclaves cut by diffuse leucosome, which are included in the heterogranular mobilisate.
Unit Achp2 consists of a package of tonalite and granodiorite. North of the Rivière Rupert Shear Zone (RRSZ), tonalite dominates over granodiorite. Tonalite contains biotite (10%), hornblende (5%), magnetite (3%), and, in smaller proportions, epidote, sphene and apatite. The medium-grained rock is locally hematitized. It is homogeneous or weakly foliated in appearance and has a granoblastic and equigranular texture. It is pale grey or pinkish white in altered surface and beige in fresh exposure. Unit Achp2 also contains bands of granodiorite that are difficult to distinguish from tonalite in outcrops. It also contains up to 10% medium-grained and granoblastic diorite enclaves characterized by a dark-grey alteration patina. These enclaves are flattened and elongated parallel to foliation planes.
South of the RRSZ, rocks assigned to unit Achp2 show mainly a granodioritic to locally granitic composition. They are highly magnetic and are medium to coarse-grained. Deformation is moderate to severe. Granodiorite is characterized by the presence of hornblende fragments or porphyroclasts (up to 10%) closely associated with biotite, as well as the pervasiveness of pyroxenes (orthopyroxene and clinopyroxene). Partial melting affects this rock in places. The neosome resembles the diatexite of subunit Achp1b. This migmatite is distinguished by the presence of many amphibole-rich enclaves digested by granodiorite likely representing restites following partial melting of an unknown diorite or amphibolite. The granodioritic mobilisate shows hornblende, biotite and magnetite schlierens aligned with foliation. It is also characterized by the pervasiveness of orthopyroxene in very variable quantities (1-5%). In this area, the tonalite-granodiorite assemblage contains 5-10% amphibolite or epidotitized diorite enclaves. These centimetric to decimetric enclaves are highly magnetic and generally elongated parallel to foliation.
Unit Achp3 is best exposed to the northeast of the Lake Boisrobert area (sheet 32N11), close to the contact of the Champion Complex with the Anatacau-pivert Formation. It consists of biotite-rich intermediate rocks that vary in composition from diorite to quartz diorite. These medium-grained and homogeneous diorites are weakly to strongly foliated and have a granoblastic texture. They are light grey in altered surface and beige in fresh exposure. The rock typically contains 40-60% of mafic minerals (hornblende and biotite) and 2% of magnetite. Chemically, diorites and quartz diorites of unit Achp3 are metaluminous, of calc-alkaline affinity, and characterized by the absence of europium anomalies.
Champion Complex 3a (Achp3a): Monzodiorite and Quartz Monzodiorite
Subunit Achp3a is better exposed to the northeast of Amikap Lake (sheet 32N11), following unit Achp3. It was created to describe a group of monzodiorite and quartz monzonite, generally foliated, locally massive, homogeneous, medium to coarse-grained. These intermediate rocks are characterized by the presence of 20% disseminated euhedral to subhedral phenocrystals of plagioclase or K-feldspar, 0.5 to 1 cm on the longest axis. Rocks are white to slightly pinkish in altered surface and beige to pinkish grey in fresh exposure. The pinkish colour is due to hematization. The rock is weakly to strongly hematitized and contains chloritized hornblende (10-30%), biotite (5-15%), chlorite (1-10%), disseminated or in veinlets epidote (10%), and accessory apatite and allanite. Monzodiorites of unit Achp3a are also characterized by the presence of centimetric to metric diorite enclaves of unit Achp3, rich in mafic minerals (>55%), fine-grained, stretched parallel to foliation planes and partially digested.
Unit Achp4 outcrops on the southern edge of the Champion Complex, between Nemiscau Lake and the Jolliet River (sheets 32N06 and 32N07). It is also observed in the Champion Lake area (32N09), north of the Nemiscau and Caumont lakes. Unit Achp4 is separated from the Lac des Montagnes Group by the RRSZ. On aeromagnetic maps, it is distinguished from the surrounding rocks by low magnetic susceptibility. This unit consists of medium-grained, homogeneous, weakly to strongly foliated granodiorite. Petrographic and chemical analyses show that locally, unit Achp4 consists of tonalite. However, to the naked eye, these two rocks are very similar. The rock is light grey in altered surface, locally pinkish, and light grey in fresh exposure. It generally contains 10% biotite, 2-10% hornblende and 1-2% magnetite. The foliated rocks are also the most rich in mafic minerals (hornblende and biotite) with contents of up to 20%. Unit Achp4 contains up to 10% centimetric to metric amphibolite and quartz diorite enclaves that contain up to 50% hornblende. Chemically, these rocks are the type I, bordering metaluminous and pareluminous, of calc-alkaline affinity and poorly to moderately potassium enriched.
Unit Achp5, located in the heart of the Champion Complex, consists of granodiorite characterized by a porphyraceous texture linked to the presence of 1-2 cm euhedral to subrounded K-feldspar phenocrystals. The proportion of phenocrystals varies from 2 to 20%, but is on average around 10%. In places, K-feldspar (orthose) phenocrystals have white plagioclase rims, giving the rock a rapakivi texture. Massive to weakly-foliated granodiorite is a homogeneous, medium-grained lithology, usually magnetic. It is pinkish grey in fresh exposure and altered surface. Weak foliation is highlighted in places by the alignment of K-feldspar phenocrystals, which likely represents magmatic foliation associated with intrusion of the granodiorite. The rock contains less than 15% mafic minerals including partially chloritized biotite (10-15%) and hornblende (1-10%). Sphene, allanite and magnetite are the most common accessory minerals.
In places, outcrops assigned to unit Achp5 contain up to 30% of granite in diffuse or transitional contact with granodiorite. Unit Achp5 also contains metric to decametric enclaves of tonalite, amphibolite or diorite.
Unit Achp6 outcrops northwest of the Lake Boisrobert region (sheet 32N11), in contact with the Northern Anatacau-pivert Formation, and the Rupert Group in the west and south. It consists of a package of granodiorite and granite characterized by a significant amount of migmatite and tonalitic or granodioritic gneiss enclaves. These enclaves generally show a very high degree of assimilation and form ribbons that resemble schlierens.
Granodiorite and granite are very heterogeneous lithologies, with a heterogranular, foliated and locally gneissic texture. These magnetic and hematitized rocks are brownish beige in altered surface and light grey in fresh exposure. They contain strongly chloritized biotite schlierens, hornblende and magnetite. Migmatite enclaves include migmatitized paragneiss, metatexites and diatexites, folded and stretched in foliation planes. The northern part of this unit, located in contact with the Anatacau-pivert Formation, is composed mainly of a homogeneous, massive to weakly foliated, pink granodiorite. This area is an area of high magnetic susceptibility.
The Champion Complex is the southern extension of the La Grande Subprovince. New geophysical and geological data show that this unit extends to NE for 65 km in width and 185 km in length. In the west end of the Rupert River, granitic and granodioritic rocks account for 80% of the Champion Complex. The oldest rocks in the Champion Complex are tonalitic gneiss, about 8 km wide and 20 km long.
The Champion Complex is one of the oldest units in the Rupert River area. It forms the host unit of the majority of intrusive units in the region and is located in enclaves in these same units. Gneissic rocks of the Champion Complex are also observed as domes within the highly magnetic granulitic domain of the Rupert Group. In the south, the Champion Complex is separated from the Lac des Montagnes Group by the Rivière Rupert Shear Zone (RRSZ), a major deformation zone of regional importance with a sinistral movement. The various units of the Champion Complex are cut by pink granite and granitic pegmatite intrusions assigned to the Kaupanaukau Intrusive Suite.
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