Roche Noire west slope, Ceillac, Briançon, Hautes-Alpes, Provence-Alpes-Côte d'Azur, Francei

Regional Level Types
Roche Noire west slope	Mountain Area
Ceillac	Commune
Briançon	Arrondissement
Hautes-Alpes	Department
Provence-Alpes-Côte d'Azur	Region
France	Country

Metagabbros occur in a small ophiolitic body enclosed in calcschists on the west slope of the Roche Noire (Cristillan valley). The ophiolitic body is composed of serpentinites enclosing small masses of pillow-lavas and gabbro breccias. The breccia is composed of big blocks of gabbro crosscut by basalt dykes, and smaller fragments of gabbro and minor basalt in a matrix made up of the same material but finely broken. It contains no fragment of the enclosing calcschists.

In the blueschist facies metamorphosed gabbro the magmatic assemblage was plagioclase + clinopyroxene + minor chromite. Relict clinopyroxene and chromite are still present. Because of the lackof deformation, the metamorphic phases crystallising are highly dependent on the nature of the magmatic minerals that they replace. Clinopyroxene is replaced by glaucophane + jadeite, plagioclase by lawsonite + phengite ± jadeite. Green patches, up to 5 mm in size, are scattered in the gabbro. They are dark green in hand specimen and are seen in thin section to consist of jadeite, lawsonite, phengite, and members of pumpellyite group, characterized by very high chromium contents. Sometimes a chromite grain is still present in the centre of the patch. Jadeite, phengite, and lawsonite occur in the rest of the rock as non-chromian varieties and attest the equilibrium bluesehist paragenesis (Mevel & Kienast, 1980).

Phengite is widespread in the gabbro. In the green patches it is replaced by a Cr-rich variety (mariposite) distinctly green in thin section despite the tiny size of the crystals (< l0 μm). Chromian jadeite occurs as small crystals (< 75 μm), strongly pleochroic from bright grass-green to yellow. Lawsonite is not abundant in the green patches, where it occurs only at the their periphery, and it contains Cr also. It forms elongated prisms, up to 0.2 mm in size, slightly pinkish. Chromian "pumpellyite" occurs as small crystals (< 75 μm), with low birefringence and high dispersion, scattered in the phengite. Unlike the green phengite and jadeite, chromian "pumpellyite" is distinctly pink. The Cr₂O₃ content reaches 26.48 wt.% but varies widely from one crystal to the other. Also Al content strongly varies. In the case of the Cr-richest and Al-poorest "pumpellyite", as the water content is not known, the empirical formula calculated on the basis of eight cations, gives: (Ca_1.57Mg_0.14Fe_0.12Mn_0.05)_Σ1.88(Cr_0.75Mg_0.25)_Σ1.00(Cr_1.07Al_0.93)_Σ2.00Si_3.00O_10+x(OH)_3+y - interpretable as aluminian shuiskite-(Cr).


The greenschist facies metamorphosed gabbroic rocks from Roche Noire (Cristillan valley) are mainly hornblende gabbros. They generally consist of hornblende and plagioclase with minor epidote-clinozoisite, tremolite, chlorite and titanite. The hornblende is usually replaced by tremolite or chlorite. The plagioclase (An_35-45) is partly replaced by epidote-clinozoisite and kaoline. It is cracked, the cracks are filled by titanite, epidote-clinozoisite and chlorite (Abdel-Karim & Puskás, 1989).

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References