Abstract

High pressure trondhjemitic rocks interbedded with eclogites and high pressure amphibolites have been recognized in the leptyno-amphibolitic series of the Rouergue and Marvejols metamorphic terranes. These rocks appear to have been derived by partial melting of the surrounding amphibolites (or amphibole-gabbro) at graphic (<7 × 105 kPa). The initial mineral assemblage of these layers (Q, Pl, Ky, Ga, Zo, etc.) is suggested by microstructural evidence to have crystallized directly under high pressure granulite facies conditions (12.5 kbar <PT <20 kbar (12.5 × 105 kPa < PT <20 × 105 kPa); 750 °C<T<840 °C) from the trondhjemitic liquid. The partial melting has taken place before the well-known Barrovian metamorphism that has affected the whole crystalline basement of the French Massif Central. This older metamorphic event is considered to be equivalent to the high pressure granulite facies metamorphism recognized in the Variscan of Europe. This acid–basic high pressure bimodal association argues for a close similarity between Archean greenstone belts and the leptyno-amphibolitic series.

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