Chromian jadeite, phengite, pumpellyite and lawsonite in a highpressure metamorphosed gabbro from the French Alps

SUMMARY. Small ophiolitic bodies are enclosed in the calcschists ofthe Piemont zone (western Alps). They have been metamorphosed in the blueschist facies during the alpine orogeny. One of them, the Roche Noire massif, includes gabbro breecias. The magmatic mineralogy of the gabbro was plagioclase + clinopyroxene + minor chromite. There was no chemical homogenization during metamorphism because of the lack of penetrative deformation and on the site of previous chromites, chromium-rich high-pressure and low-temperature minerals (jadeite, phengite, pumpellyite, and lawsonite) were formed. The Al. ~ Cr substitution does not affect other P- and T-dependent substitutions. IN the western Alps the calcschists of the Piemont zone enclose numerous bodies of ophiolitic rocks. One of them, the Roche Noire massif (Lat. 44 ~ 38' N; Long. 6 ~ 53 ' E) is mainly composed of serpentin-ites. enclosing small masses of pillow-lavas and gabbro breccias (Lemoine, i97 0. The breccia is composed of big blocks of gabbro cross-cut by diabasic dykes, and smaller fragments of gabbro and minor diabase in a matrix made up of the same material but finely broken. It contains no fragment of the enclosing calcschists. This suggests that the breccia formed very early, in an oceanic environ-ment, probably along a fault scarp in the oceanic crust. Diabase breccias with a matrix of finely broken plagioclase and clinopyroxene crystals have been dredged, for example, in the Romanche frac-ture zone in the Atlantic ocean (personal observa-tions of rocks from cruise 73o 9 of the School of Marine and Atmospheric Sciences, Miami). Lemoine and Tricart (I979) suggest that the inclu-sion of the gabbros in the serpentinites might be the result of serpentinite diapirs in an oceanic fracture zone environment. During the alpine orogeny, the Roche Noire massif suffered ahigh-pressure and low-temperature metamorphism in the blueschist facies. The charac-Copyright he Mineralogical Society teristic assemblage in the basic rocks is jadeite