Metamorphic Minerals

Texture and Microstructure

Metamorphic Rocks

Chloritoid schist, Ile de Groix

Ile de Groix, situated about 10 km from the south coast of Brittany (Fig.1), represents the emerged part of NW-SE elongated metamorphic belt. This belt, covering an area of about 40 x 10 km, consists of rocks formed under high-pressure and low- or medium-temperature conditions (blueschist and eclogite facies). Geochronological investigations indicate that high-pressure metamorphism took place around 420-400Ma ago. Ile de Groix and Bois de Cené (Vendée, some 150-200 km SE) provide the only evidence for this type of metamorphism in the entire Variscan fold-belt of Western Europe.

Fig.1: Geological setting of the Ile de Groix in the frame of the Hercynian belt of southern Brittany. a) Simplified geological map of the metamorphic domain of southern Brittany with the location of the Ile de Groix. SASZ = South Armorican Shear Zone. b) Cross section AB (see location in a) showing the structural position of the Ile de Groix blueschists (modified after Philippon, M., 2009).

The Ile de Groix complex is made up of a series of metapelites intercalated with metabasic rocks. Petrological studies have shown that that glaucophane-schist facies metamorphism was followed by retrogression in the greenschist facies.
The P-T conditions (Fig.2-3) and the subdivision of the prograde metamorphic sequence are still controversial. On the basis of metabasic assemblages, Triboulet (1974) distinguished three zones (Fig.2):

I = eclogite facies zone (8.5 kbar, 530°C);
II = glaucophane-epidote-garnet facies zone (8kbar, 500°C);
III = greenschist facies zone (7.5kbar, 470°C);

Carpenter (1976) recognized two contrasting areas (Fig.2):

A ( P = 8 +/- 1.5 kbar, T = 400 +/- 50 °C) and B (P = 6.5-8 kbar, T < 400 °C).

The principal difference between these two areas is a lack of garnets in the metabasites of zone B. According to Quinquis (1980), Carpenter's isograd of garnet in metabasic rocks has a structural significance as well; the higher-grade zone A (eastern part of the island) seems to lie above the lower-grade zone B.

Fig.2: Sketch-map of Ile de Groix. I, II, III = metamorphic zones according to Triboulet (1974) (separated by solid line). A and B = metamorphic zones according to Carpenter (1976) and Quinquis (1980) (dash-dotted line). Modified after Dudek, K. (1989).

Fig.3: Metamorphic map of the Ile de Groix showing the location of the garnet isograd and the occurrence of lawsonite pseudomorphs. The lithological contours are only shown along the seashore. The PT paths for the western and eastern part of the island are shown in the insert (after Bosse et al., 2002). From Philippon, M., (2009).

The most common rocks of Ile de Groix series are mica schists. Intercalated with these are metabasic rocks, chiefly greenschists, amphibolites, blueschists and eclogites. The thickness of the metabasic intercalations varies from a few centimetres to several tens of metres.

Among the metabasic rocks of the eastern part of Ile de Groix (zone A according Carpenter and Quinquis) garnet glaucophanites (assemblage: glaucophane-garnet-pyroxene-epidote) and amphibolites (glaucophane-blue-green amphibole-epidote-garnet) are the most common. Scarce outcrops of eclogites, abundant in jadeitic pyroxenes, have been reported as well. Metapelites of this zone are represented by heterogeneous, quartzo-phengitic mica schists with chloritoid, chloritoid + garnet, or chloritoid + garnet + glaucophane. All these assemblages can contain such accessory minerals as quartz, phengite, chloritoid (metabasites), epidote, albite, tourmaline (mica schists) and biotite, stilpnomelane, apatite, sphene, rutile, and opaques (both groups of rocks).

West of the garnet isograd, the metabasites of zone B are represented by greenschists, amphibolites and prasinites. Except for lack of garnet, the abundance of the blue-green amphibole barroisite (more abundant than glaucophane) is of major importance. The typical assemblages are: (1) blue-green amphibole-glaucophane-epidote-chlorite and (2) albite-epidote-chloriteblue-green amphibole. The most common assemblage in the mica schist of this zone is: quartz-phengite-chloritoid-stilpnomelane-chlorite.

Sample done by Justine Francomme

Bibliography

• Philippon, M., Brun, J. P., & Gueydan, F. (2009). Kinematic records of subduction and exhumation in the Ile de Groix blueschists (Hercynian belt; Western France). Journal of Structural Geology, 31(11), 1308-1321.
• Dudek, K. (1989). Deerite from Ile de Groix, Brittany, France. Mineralogical Magazine, 53, 603-612.

Garnet crystal surrounded by chloritoid (green) and phengite (colorless) crystals. Metapelite from Ile de Groix (Brittany). PPL image, 2x (Field of view = 7mm)	Garnet crystal (isotropic) surrounded by chloritoid (low interference colors) and phengite (high interference colors) crystals. Metapelite from Ile de Groix (Brittany). XPL image, 2x (Field of view = 7mm)	Garnet crystal surrounded by chloritoid (green) and phengite (colorless) crystals. Metapelite from Ile de Groix (Brittany). PPL image, 2x (Field of view = 7mm)
Garnet crystal (isotropic) surrounded by chloritoid (low interference colors) and phengite (high interference colors) crystals. Metapelite from Ile de Groix (Brittany). XPL image, 2x (Field of view = 7mm)	Chloritoid (green) and phengite (colorless) crystals in the rock matrix. Metapelite from Ile de Groix (Brittany). PPL image, 2x (Field of view = 7mm)	Chloritoid (low interference colors) and phengite (high interference colors) crystals in the rock matrix. Metapelite from Ile de Groix (Brittany). XPL image, 2x (Field of view = 7mm)
Garnet crystal surrounded by chloritoid (green) and phengite (colorless) crystals. Metapelite from Ile de Groix (Brittany). PPL image, 2x (Field of view = 7mm)	Garnet crystal (isotropic) surrounded by chloritoid (low interference colors) and phengite (high interference colors) crystals. Metapelite from Ile de Groix (Brittany). XPL image, 2x (Field of view = 7mm)	Garnet crystal surrounded by chloritoid (green) and phengite (colorless) crystals. Metapelite from Ile de Groix (Brittany). PPL image, 2x (Field of view = 7mm)
Garnet crystal surrounded by chloritoid (green) and phengite (colorless) crystals. Metapelite from Ile de Groix (Brittany). PPL image, 2x (Field of view = 7mm)	Garnet crystal (isotropic) surrounded by chloritoid (low interference colors) and phengite (high interference colors) crystals. Metapelite from Ile de Groix (Brittany). XPL image, 2x (Field of view = 7mm)	Garnet crystal surrounded by chloritoid (green) and phengite (colorless) crystals. Metapelite from Ile de Groix (Brittany). PPL image, 2x (Field of view = 7mm)
Garnet crystal (isotropic) surrounded by chloritoid (low interference colors) and phengite (high interference colors) crystals. Metapelite from Ile de Groix (Brittany). XPL image, 2x (Field of view = 7mm)	Garnet crystal (isotropic) surrounded by chloritoid (low interference colors) and phengite (high interference colors) crystals. Metapelite from Ile de Groix (Brittany). XPL image, 2x (Field of view = 7mm)	Garnet crystal surrounded by chloritoid (green) and phengite (colorless) crystals. Metapelite from Ile de Groix (Brittany). PPL image, 2x (Field of view = 7mm)
Garnet crystal (isotropic) surrounded by chloritoid (low interference colors) and phengite (high interference colors) crystals. Metapelite from Ile de Groix (Brittany). XPL image, 2x (Field of view = 7mm)	br />Garnet crystal surrounded by chloritoid (green) and phengite (colorless) crystals. Metapelite from Ile de Groix (Brittany). PPL image, 2x (Field of view = 7mm)	br />Garnet crystal surrounded by chloritoid (green) and phengite (colorless) crystals. Metapelite from Ile de Groix (Brittany). PPL image, 2x (Field of view = 7mm)
br />Garnet crystal surrounded by chloritoid (green) and phengite (colorless) crystals. Metapelite from Ile de Groix (Brittany). PPL image, 2x (Field of view = 7mm)	Garnet crystal (isotropic) surrounded by chloritoid (low interference colors) and phengite (high interference colors) crystals. Metapelite from Ile de Groix (Brittany). XPL image, 2x (Field of view = 7mm)	br />Garnet crystal surrounded by chloritoid (green) and phengite (colorless) crystals. Metapelite from Ile de Groix (Brittany). PPL image, 2x (Field of view = 7mm)
Garnet crystal surrounded by chloritoid (green) and phengite (colorless) crystals. Metapelite from Ile de Groix (Brittany). PPL image, 2x (Field of view = 7mm)	Garnet crystal surrounded by chloritoid (green) and phengite (colorless) crystals. Metapelite from Ile de Groix (Brittany). PPL image, 2x (Field of view = 7mm)	Garnet crystal with chloritoid (green) inclusions. Metapelite from Ile de Groix (Brittany). PPL image, 2x (Field of view = 7mm)
Garnet crystal with chloritoid (low interference colors) inclusions. Metapelite from Ile de Groix (Brittany). XPL image, 2x (Field of view = 7mm)	Garnet crystal surrounded by chloritoid (green) and phengite (colorless) crystals. Metapelite from Ile de Groix (Brittany). PPL image, 2x (Field of view = 7mm)	Garnet crystal (isotropic) surrounded by chloritoid (low interference colors) and phengite (high interference colors) crystals. Metapelite from Ile de Groix (Brittany). XPL image, 2x (Field of view = 7mm)
Garnet crystal surrounded by chloritoid (green) and phengite (colorless) crystals. Metapelite from Ile de Groix (Brittany). PPL image, 2x (Field of view = 7mm)	Garnet crystal (isotropic) surrounded by chloritoid (low interference colors) and phengite (high interference colors) crystals. Metapelite from Ile de Groix (Brittany). XPL image, 2x (Field of view = 7mm)	Garnet crystal surrounded by chloritoid (green) and phengite (colorless) crystals. Metapelite from Ile de Groix (Brittany). PPL image, 2x (Field of view = 7mm)
Garnet crystal (isotropic) surrounded by chloritoid (low interference colors) and phengite (high interference colors) crystals. Metapelite from Ile de Groix (Brittany). XPL image, 2x (Field of view = 7mm)	Chloritoid (green) and phengite (colorless) crystals in the rock matrix. Metapelite from Ile de Groix (Brittany). PPL image, 2x (Field of view = 7mm)	Chloritoid (low interference colors) and phengite (high interference colors) crystals in the rock matrix. Metapelite from Ile de Groix (Brittany). XPL image, 2x (Field of view = 7mm)
Chloritoid (green) and phengite (colorless) crystals in the rock matrix. Metapelite from Ile de Groix (Brittany). PPL image, 2x (Field of view = 7mm)	Chloritoid (green) and phengite (colorless) crystals in the rock matrix. Metapelite from Ile de Groix (Brittany). PPL image, 2x (Field of view = 7mm)	Chloritoid (low interference colors) and phengite (high interference colors) crystals in the rock matrix. Metapelite from Ile de Groix (Brittany). XPL image, 2x (Field of view = 7mm)
Chloritoid (green) and phengite (colorless) crystals in the rock matrix. Metapelite from Ile de Groix (Brittany). PPL image, 2x (Field of view = 7mm)	Chloritoid (low interference colors) and phengite (high interference colors) crystals in the rock matrix. Metapelite from Ile de Groix (Brittany). XPL image, 2x (Field of view = 7mm)	Chloritoid (green) and phengite (colorless) crystals in the rock matrix. Metapelite from Ile de Groix (Brittany). PPL image, 2x (Field of view = 7mm)
Chloritoid (low interference colors) and phengite (high interference colors) crystals in the rock matrix. Metapelite from Ile de Groix (Brittany). XPL image, 2x (Field of view = 7mm)	Chloritoid (green) and phengite (colorless) crystals in the rock matrix. Metapelite from Ile de Groix (Brittany). PPL image, 2x (Field of view = 7mm)	Chloritoid (low interference colors) and phengite (high interference colors) crystals in the rock matrix. Metapelite from Ile de Groix (Brittany). XPL image, 2x (Field of view = 7mm)
Chloritoid (green) crystals in the rock matrix. Metapelite from Ile de Groix (Brittany). PPL image, 2x (Field of view = 7mm)	Chloritoid (low interference colors) crystals in the rock matrix. Metapelite from Ile de Groix (Brittany). XPL image, 2x (Field of view = 7mm)	Chloritoid (green) crystals in the rock matrix. Metapelite from Ile de Groix (Brittany). PPL image, 2x (Field of view = 7mm)
Chloritoid (low interference colors) crystals in the rock matrix. Metapelite from Ile de Groix (Brittany). XPL image, 2x (Field of view = 7mm)	Chloritoid (green) crystals enclosed in a garnet crystal. Metapelite from Ile de Groix (Brittany). PPL image, 10x (Field of view = 2mm)	Chloritoid (green) and phengite (colorless) crystals in the rock matrix. Metapelite from Ile de Groix (Brittany). PPL image, 10x (Field of view = 2mm)