Pétrologie des pyroxénites litées dans les complexes ultramafiques de l'Ariège (France) et autres gisements de lherzolite à spinelle. I. Compositions minéralogiques et chimiques, évolution des conditions d'équilibre des pyroxénites

Abstract

The layered pyroxenites in the main lherzolitic bodies from Ariege recrystallized from primary paragenesis composed of Al₂O₃-rich bronzites and Al₂O₃ and MgSiO₃-rich augites in variable proportions, commonly associated with small amounts of spinel (pleonaste to picotite). Pyrope-rich garnet or olivine are present in a few types. The relationships between pyroxenites and adjacent peridotites, as well as the mineralogical and chemical zonation of the layers, indicate that the primary paragenesis represent segregates of the phases which crystallized successively from melts ranging from picritic tholeiitic to normative nepheline basalt compositions. Olivine being the liquidus phase in the primary melt, its extraction leads to orthopyroxene as the primary phase in the residual liquid. As this segregation process goes on, bronzite is replaced by clinopyroxene and by clinopyroxene plus garnet in the last residual melts preserved in the ultramafic body. This crystallization sequence and the initial chemical composition of the pyroxenes are compatible with P-T conditions starting at 1600° C between 15 kb and 20 kb and ending under isobaric or quasi isobaric conditions at about 1200° C. The primary paragenesis recrystallized during two major episodes, the first one before the plastic flow, the second one during the deformations. Observed subsolidus reactions and the chemical compositions of the recrystallized minerals indicate that the last major episode of recrystallization occurred at 800°-900° C between 12 kb and 15 kb, i. e. near crystallization pressures. From a review of published data it is concluded that the pyroxenites associated with spinel lherzolites everywhere in the world, either in complexes or among xenoliths in alkali basalts, originate from the same segregation process. They seem to have crystallized and recrystallized under physico-chemical conditions close to those defined in the Ariege district, i. e. in the uppermost parts of the upper mantle, between about 35 to 70 km deep. Temperatures and pressures under which pyroxenites crystallized imply that spinel lherzolite bodies have been built up either from a crystal mush, mainly of olivine and orthopyroxene, or from a liquid of picritic composition. Examination of this alternative can be reduced to the discussion of two hypothesis : in the first one, the layered pyroxenites crystallize from melts originated by one or several partial fusion processes occurring as ultramafic bodies move towards the surface ; in the second one, periodites as well as pyroxenites are of primary magmatic origin. Both hypothesis are discussed in the second part of this paper.