Background

Description

Joubert Suite (nAjbt): Hornblende-Biotite Tonalite or Granodiorite

Joubert Suite 1 (nAjbt1): Foliated, Banded or Gneissis Biotite-Magnetite Tonalite and Granodiorite

Unit nAjbt1 is the main unit of the Joubert Suite. It consists of a package of foliated, banded or gneissic biotite-magnetite tonalite and granodiorite. Fresh and altered surfaces are grey. The rock is foliated, granoblastic, and fine to medium grained. As the deformation intensity increases, the texture changes to gneissic and banded. Tonalite and granodiorite are composed of biotite (5-40%) and magnetite (<3%) as well as local epidote (<1%), sphene (<1%), hornblende (<8%), pyrite and pyrrhotite (<1%). The relative abundance of plagioclase is confirmed by stains that indicate a tonalitic composition close to a granodiorite.

Joubert Suite 2 (nAjbt2): Biotite-Hornblende Tonalitic and Dioritic Gneiss

Unit nAjbt2 unit is observed in the area of sheets 33H07, 33H08, 33H09 and 33H10. It consists of very deformed biotite-hornblende tonalitic and dioritic gneiss. The intense foliation or banding is folded at the outcrop scale. The rock consists of alternating decimetric tonalitic gneiss and dioritic gneiss. Both lithologies contain biotite, hornblende and, sparsely, sphene (1-2%), diopside, pyrite (1%), magnetite (1%), epidote (1%) and chlorite. They are distinguished primarily by the abundance of ferromagnesian minerals: in tonalitic gneiss, biotite varies between 1 and 20% and hornblende is scarce, whereas in dioritic gneiss, hornblende dominates (10-50%) and biotite accounts for only 10% of the total. This difference in composition influences the rock colour in fresh and altered surface, which is beige grey for tonalite and greenish grey for diorite. Gneiss are fine to medium grained and may have biotite schlierens or granite and diorite boudins, which are evidence of significant deformation. Dioritic gneiss is granoblastic. Unit nAjbt2 is also cut by numerous conformable and boudinaged centimetric quartz veins and by millimetric to decimetric granite intrusions. Granitic intrusions parallel to foliation accentuate the banded appearance of gneiss. Hornblendite or amphibolite enclaves or centimetric lenses were observed.

From a geochemical perspective, rocks of units nAjbt1 and nAjbt2 are calc-alkaline, peraluminous to metaluminous, and derived from a type-I magma that has evolved in a tectonic context comparable to volcanic arcs (Guemache et al., 2017).

Thickness and Distribution

This unit has only been recognized in the western portion of the Grande Rivière Domain in the La Grande Subprovince (sheets 33H and 23E). The southern part of this suite has the peculiarity of being emplaced in rocks belonging to both the La Grande and Opinaca subprovinces. To the north (e.g., sheets 23E13 and 23L04), more detailed mapping would be required to determine the Joubert Suite extent.

Dating

Two samples were collected in 2015 from the main Joubert Suite unit (nAjbt1) in sheets 23E03 and 23E04 (outcrops 2015-SB-3144 and 2015-PB-1039). The two samples come from a foliated biotite-magnetite tonalite. One of the outcrops contained enclaves of foliated diorite and biotite-magnetite paragneiss. LA-ICP MS zircon isotopic analyses yielded ages of 2679.4 ±9.9 Ma and 2831.4 ±6.2 Ma for one sample, and 2681.7 ±5.6 Ma, 2729.8 ±7.8 Ma and 2820 ±23 Ma for the other (David, 2018). The ages of 2679.4 ±9.9 Ma and 2681.7 ±5.6 Ma are interpreted as tonalite crystallization ages, while the other ages would be associated with inherited zircons.

Stratigraphic Relationship(s)

The presence in the Joubert Suite of diorite and paragneiss enclaves showing varying degrees of migmatitization, cross-cutting relationships and the two new isotopic datings indicate that this suite is younger than paragneiss of the Laguiche Complex and Rivière Salomon Formation. Lamothe et al. (2000) suggest that the Joubert Suite is a lateral equivalent of the Marquiset Suite tonalitic unit (Amaq), but the age difference between these units (~2680 Ma compared to 2704 ±2 Ma) indicates that this is not the case (Gigon and Goutier, 2017). 

Paleontology

Does not apply.

References

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