|Author:||Beauchamp and Massei, 2018|
|Type area:||Bohier Island region (NTS sheets 33A07, 33A08 and 23D05 and 23D12)|
|Geological province:||Superior Province|
|Geological subdivision:||Opatica Subprovince|
Table des matières
The Upper Eastmain Greenstone Belt (UEGB) was first mapped by the Geological Survey of Canada during geological survey work that covered a significant area of the Eeyou Istchee James Bay region. At that time, Eade (1966) identified a main klippe of green rocks north of the Otish Mountains, without really defining its extensions. The UEGB was better identified by Hocq (1985), Roy (1985, 1988), Couture (1986, 1987a, 1987b, 1987c, 1993) and Couture and Guha (1990). Couture (1986) was the first to divide the rocks of the UEGB into two lithostratigraphic packages, the Bohier Group consisting mainly of metasedimentary rocks, and the René Group, which comprises mostly metavolcanic rocks. Beauchamp and Massei (2018) added a new subdivision to the Bohier Group that includes paragneiss and biotite wackes (nAbh1), a unit that was observed in the eastern branch of the UEGB. They also ordered the units and tags of the Bohier Group to comply with the newly established stratigraphic order.
The Bohier Group refers to metasedimentary rocks belonging to the UEGB, which is located in the northeast part of the Opatica Subprovince. These rocks are metamorphosed to the amphibolites facies (Couture 1987a; Roy 1988; Talla Takam et al., in preparation). Compared to metasedimentary rocks of the Laguiche Complex (Opinaca subprovince), those of the Bohier Group were not or were not significantly affected by partial fusion.
The Bohier Group consists of three informal units, paragneiss and wacke biotite (nAbh1), nodular aluminosilicate paraschist (nAbh1a) and metaconglomerate (nAbh2).
The Bohier Group rock protolites are mainly wackes (Herron 1988). Their composition ranges from 60-70% SiO2, 12-17% Al2O3, 5-12% Fe2O3T, 1-6% MgO, 0.05-2.5% MnO, 1-5% CaO, 0.5-4% Na2O, 1-4% K2O and 0.3-1.5% TiO2. The rare earth spectra diagrams normalized to CI chondrite are fractionated and characterized by light rare earth enrichment relative to heavy rare earths. The nAbh1a subunit, which sometimes contains up to 25% garnet, is more enriched with heavy rare earth elements (La/YbCN = 7.6 and Gd/YbCN = 1.6) than the nAbh2 unit (La/YbCN= 13.2, Gd/YbCN = 2.1), which contains less than 2% garnet. The nAbh1a subunit is also distinguished by a negative anomaly in Eu stronger than in the nAbh1 unit. The Th/Sc vs. Zr/Sc diagram (McLennan et al., 1993) shows that Bohier Group sedimentary rocks were not affected by processes of recycling and concentration of heavy minerals. The tectonic environmental diagram (Bhatia and Crook, 1986) indicates that Bohier Group rocks would have developed in a context comparable to current continental arcs.
Unit nAbh1 represents a granoblastic wacke paragneiss and a stratified wacke containing 20 to 30% biotite. Primary bedding is locally visible, marked darker silt beds alternating with lighter sand beds. Sedimentary bedforms, such as grading, are preserved on the least recrystallized outcrops. Quartz and plagioclase grains are intertgrown and neoblastic. Foliation is poorly developed and characterized by a preferential orientation of biotite flakes. The typical assembage is made up of quartz, variously textured twinned plagioclase, biotite, K-feldspar, and traces of chlorite, epidote, apatite and zircon. These non-magnetic sedimentary rocks are cut by 2 to 10% millimetric to centimetric white quartz ± calcite veins. A massive, white to pinkish, coarse-grained pegmatic granite containing less than 5% muscovite and biotite intrudes metasedimentary rocks.
Bohier Group 1a (nAbh1a): Nodular Aluminosilicate Paraschist and Conglomerate Horizons
Unit nAbh1a represents a package of aluminous metasedimentary rocks of pelitic origin. The paraschist consists of a biotite-muscovite-quartz-plagioclase-sillimanite (or fibrolite)-garnet-cordierite assemblage. Traces of staurotide, tourmaline, amphibole, chlorite, epidote, apatite, zircon, sphene, rutile and opaque minerals were observed in thin section. These rocks underwent metamorphism at the upper amphibolites facies. Primary bedding is well preserved within this metasedimentary unit. It is recognizable by variations in metamorphic assemblages. The clay beds, which are more aluminous, are lined with aluminosilicates, mainly sillimanite. Sand beds contain fewer aluminosilicates and contain garnet. Aluminosilicate paraschist outcrops are heavily deformed and affected by tight isoclinal folding. Under the microscope, the samples show the presence of at least two schistosities, and the angular relationship between primary bedding and the main regional fabric. Sillimanite porphyroblasts are very common in paraschists. They form nodules or whitish fibres in positive relief on the surface of the outcrops. The sillimanite nodules are millimetric, usually centimetric (0.5 to 5 cm). They appear to replace andalusite pseudomorphs and are partially retrograded to muscovite. Some of the outcrops in the heart of Bohier Island are covered with sillimanite, which can account for up to 50% of the rock. Garnet, milimetric to centimetric, is euhedral and poeciloblastic. It contains several inclusions of quartz, biotite and muscovite. An angular relationship between the orientation of the garnet inclusions and the trace of primary foliation is noted in some thin sections. Garnet is isolated or in clusters. A quartz rim forming a depletion halo is present around the garnet. Locally twinned plagioclase is partially to 100% damouritized. Biotite forms flakes whose preferential orientation highlights the main schistosity. It is often intergrown with muscovite. Two generations of muscovite are present, the first form of thin flakes aligned with the main foliation, the second forms large prisms that overprint foliation. Cordierite, when present, forms highly pinitized poikilitic areas. Some paraschist outcrops on Bohier Island have undergone extensive penetrative alteration in garnet-quartz ± tourmaline ± amphibole.
Unit nAbh2 represents a package dominated by polygenic metaconglomerates. The percentage of fragments varies between 50 and 85%. The proportion of clasts is locally difficult to assess. Because of recrystallization, fragments with the same composition as the matrix are indistinguishable. The coarse fraction of the metaconglomerate is made up of blocks and stones rounded to subrounded of granitoids (tonalite, granodiorite, quartz diorite and diorite) and gabbro. Stones and cobbles of felsic and intermediate intrusive rocks, metavolcanic rocks (felsic, mafic and ultramafic), metatuffs and metasedimentary rocks are also present in the metacongerate. Fragments are millimetric to decametric, and their morphology shows that they have been deformed more in flattening than constriction. The largest intrusive rock clasts are up to 70 cm wide, indicating a dynamic system of emplacement.
Some highly deformed metaconglomerate outcrops have a banded appearance caused by a strong local flattening of the clasts (outcrop 17-FM-2063).
The composition of the metaconglomerate matrix varies laterally. The metaconglomerate in the western branch (Dolent Lake area) is distinguished by its dark green matrix composed of amphibole (50-75%), plagioclase (15-25%), quartz (5%), epidote and traces of sphene, rutile and opaque minerals.
The metaconglomerate in the Bohier Island area exhibits a pale grey matrix. The matrix has a similar composition to the nAbh1a paraschist. The matrix contains muscovite, biotite, quartz, plagioclase, garnet, and lesser amounts of sillimanite, staurotide and cordierite.
On Bohier Island, two types of metaconglomerate were observed. The most common is the joint-fragmented metaconglomerate, supported by felsic clasts. The latter is in sharp contact with a monogenic metaconglomerate with floating fragments (outcrop 17-AB-1071). The monogenic metaconglomerate with floating fragments contains between 5 and 10% quartz pebbles, as well as felsic pebbles (metavolcanics and tonalite). The matrix of the paraschist-type monogenic metaconglomerate is rich in quartz, muscovite, feldspath, biotite with minor amounts of sillimanite and millimetric garnet (trace-1%).
The change in matrix composition and clast variety tend to indicate a gradual erosion of proximal volcanic rocks during sedimentation. The metaconglomerate having a dark green matrix and located at the heart of the western branch of the UEGB (Dolent Lake area) is believed to have been the result of erosion of the underlying volcanic mafic sequence. The metaconglomerate of Bohier Island, which has a pale grey matrix and contains mostly felsic fragments, would have been formed following the erosion of a pluton and an altered felsic edifice nearby.
The Bohier Group belongs to the UEGB, located in the northeast part of the Opatica Subprovince. The metaconglomerate (nAbh2) occupies the central part of the western branch of the UEGB and western Bohier Island. The paragneiss and wacke unit (nAbh1) occupies the central part of the east branch. The nAbh1 and nAbh2 units occupy the axial part of synclinals, bordered on both sides by mafic metavolcanics of the René Group. The aluminosilicate paraschist unit (nAbh1a) is located in the south branch of the UEGB and in the Bohier Island area. On Bohier Island, there is a gradual passage from unit nAbh2 to the nAbh1a subunit going south and east. Metaconglomerate beds (nAbh2) are interbedded with paraschist beds (nAbh1a) (outcrops 17-JM-6080 and 17-FM-2057). Metasedimentary rocks of the Bohier Group have a lateral extension of 50 km in a NE-SW direction, and a thickness of 0.3 to 3 km.
Two samples of Bohier Group metaconglomerate (nAbh2) were collected during the summer of 2017 for U-Pb isotopic dating of detrital zircons (Beauchamp and Massei, 2018). The former has a pale grey matrix (outcrop 17-JM-6027) while the latter has a dark green matrix (outcrop 17-JF-5071).
The pale matrix conglomerate is located in the heart of Bohier Island and consists mainly of tonalite clasts and felsic volcanic rocks. Only one zircon population has been identified. The age of detrital zircons indicates that this conglomerate is mainly derived from erosion of felsic rocks dated 2763 ±3 Ma. This conglomerate comes from the erosion of volcanic rocks of the Roman Formation (2770 ±3 Ma, Davis, personal communication; 2774 ±6 Ma, Talla Takam et al., in preparation) and the Île Bohier synvolcanic Pluton (2771 ±6 Ma Davis, personal communication).
The dark-green-matrix conglomerate outcrop is located in the western branch of the UEGB; it is located 1.5 km south of Dolent Lake. It shows an amphibole-rich matrix and contains tonalitic felsic clasts, basalt or gabbro mafic clasts, and a minor fraction of intermediate clasts. The majority of ages from zircons of the dark matrix metaconglomerate are distributed around two ages, 2712 ±6 Ma and 2836 ±10 Ma (Davis, personal communication). This conglomerate would come from the erosion of a Mesoarchean basement and Neoarchean rocks. The youngest zircon population dated 2712 ±6 Ma (Davis, personal communication) represents the maximum sedimentation age of Bohier Group rocks.
The small variation in age suggested that the source of the eroded material would be limited and that the distance of transport would be restricted.
Metasedimentary rocks of the Bohier Group (nAbh) differ from the René Group metavolcanic units (Beauchamp and Massei, 2018). This stratigraphic interpretation differs from that proposed by Hocq (1985), Couture (1987) and Roy (1988), which placed the Bohier Group sedimentary rocks at the base of the sequence under the René Group units. It is closer to that of Boldy et al. (1984) who proposed placing the René Group metavolcanic rocks at the base of stratigraphy.
Units nAbh1 and nAbh2 are cut by a coarse-grained to pergmatitic granite containing muscovite and minor amounts of biotite and tourmaline (outcrop 17-AB-1071) similar to the Wahemen Granite. Quartz veins and veinlets, and folded and boudinaged granite intrusions are present in metasedimentary rocks of the Bohier Group. The Cadieux Pluton cuts the southern part of the nAbh1 unit in the eastern and western branches of the UEGB. The Bart Thrust Fault marks a sharp contact between the aluminosilicate paraschists in this belt and the migmatised biotite wacke paragneiss (± grenat ± cordierite) in the Opinaca SubProvince.
Does not apply.