Les péridotites et pyroxénolites à grenat du Bois des feuilles (Monts du lyonnais) (France)

The author describes a new occurrence of garnet peridotite and garnet pyroxenite interlayered in the biotite-sillimanite-garnet gneisses at the top of the granulitic serie of the Monts du Lyonnais (Massif Central français). Its dimensions are rather significant for a crustal gisement (500×100 m). It is only composed of forsterite, enstatite, chromiferous diopside, pyrope and spinel peridotites with their products of retrograde transformations as kelyphites, amphiboles, chlorites, lizardite, ores, etc. The petrographic studies show the heterogeneity of the massif and the anteriority of the red spinel upon the garnet which always forms a corona around the spinel. The peridotites are intermingled with numerous streched and dislocated layers of garnet websterites with rare centimetric levels. These pyroxenites would be derived of particular magmatic processes (partial anatectic melting followed by cristallisation) developped from an upper mantle level in a primary pyrolitic lherzolitic (s. l) or garnet peridotitic material. The garnet peridotite of “Le Bois des Feuilles” would be, in fact, a “secondary garnet lherzolite” derived: - either from a spinel lherzolite intermingled with garnet websterite layers and their “dunitic” remnants, to form a “pseudo-garnet lherzolite” like this of Beni Bouchera described by Kornprobst; - or from a spinel lherzolite associated with garnet websterites and submitted temporarily, at the time of its diapiric rising movement from the mantle towards the crust to the conditions of the spinel garnet lherzolite facies. The plastic deformations and intense laminations form blastomylonites of mixed rocks recristallised ultimately under granulitic facies conditions. These rocks are, pro parte, not very different from the other crustal garnet peridotites, in spite of the frequency of the spinel inclusions in garnet. In corollary, it seems that numerous crustal garnet peridotites would have the same origin.