Background

Description

Druillon Suite 1 (nAdru1) : Monzodiorite and Diorite

According to Gosselin et al. (2002), unit nAdru1 consists of coarse-grained antiperthitic feldspar monzodiorite, monzonite and diorite. Thériault and Chevé (2001) noted that monzonitic and monzodioritic rocks are generally composed of plagioclase (50-60%), K-feldspar (20-35%), quartz (10-15%), hornblende (5-15%), clinopyroxene (5-10%) and biotite (5%). The rock has a characteristic satin appearance, although its colour varies widely; the most common colours being bluish grey, beige and pink (Gosselin et al., 2002). In addition, it is distinguished by its coarse grain size and antiperthitic feldspar porphyraceous texture visible in staining. This unit is massive or has a slight foliation marked by tabular minerals, mainly feldspar and amphibole.

In thin sections, monzodiorite and diorite are composed mainly of feldspar with fine antiperthitic to perthitic exsolution textures that indicate crystallization at high temperature (Gosselin et al., 2002). Thériault and Chevé (2001) noted that plagioclase forms subhedral crystals and phenocrystals that are slightly recrystallized and moderately sericitized. In addition, K-feldspar crystals occur in two different habits, namely, as anhedral grains interstitial to plagioclase and quartz, or forming centimetric phenocrystals. Quartz is typically interstitial and partially granoblastic. Ferromagnesian minerals (5-35%) form compact, interstitial grains to coarse feldspars (Gosselin et al., 2002). According to Thériault and Chevé (2001): “poikilitic (or poikiloblastic) crystals of clinopyroxene highly retromorphosed to bluish green hornblende constitute the two main ferromagnesian phases, while brownish to olive-green biotite is partially altered to chlorite.” The most abundant ferromagnesian minerals are hornblende and biotite, while clinopyroxene and orthopyroxene occur in lower amounts. Hornblende forms the core of ferromagnesian mineral clusters. Biotite occurs as inclusions or rims around other ferromagnesian minerals. Magnetite, ilmenite, titanite, apatite and sulphides are also associated with these clusters (Gosselin et al., 2002).

Druillon Suite 2 (nAdru2) : Amphibole Granite

Unit nAdru2 forms kilometric amphibole granite intrusions emplaced and confined within the Brésolles Suite (Gosselin et al., 2002). The old Ossant Suite is recognized by its pink and grey colour, which gives it a spotted appearance. Granite is homogeneous, massive to very slightly foliated and porphyraceous. The rock is composed mainly of perthitic feldspar phenocrystals in a quartz-plagioclase matrix.

According to Gosselin et al. (2002), this unit contains 5-10% ferromagnesian minerals, mostly sodic amphibole (5-8%) in clusters. Biotite and chlorite rim or replace amphibole. Clusters also include epidote, allanite, apatite, titanite and iron oxides.

Stratigraphic Relationship(s)

According to Simard (2008): “The Druillon Suite was not dated. Gosselin et al. (2002) considered intrusions of the Turbar and Ossant suites to be relatively young due to their massive, homogeneous and minimally deformed appearance. Thériault and Chevé (2001) mentioned that Druillon Suite intrusions could be associated with the same magmatic event responsible for the formation of the Maurel Suite (2707-2685 Ma). Simard et al. (2008) associate an age between 2705 Ma and 2680 Ma to the Druillon Suite.”

Paleontology

Does not apply.

References

Publications available through SIGÉOM Examine

Suggested Citation

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