Pyroxenite layers in the orogenic spinel lherzolite massifs of Ariège are porphyroclastic textured and range in composition from spinel websterite to garnet clinopyroxenite. Each pyroxenite type forms individual layers or occurs as part of composite layers in which the Opx/Cpx and Sp/Gt ratios decrease from margins to core. They are interpreted as crystalline segregations separated by flow crystallization from continental tholeiites en route to the surface. The primary magmatic phases consist of Al-rich pyroxenes, together with a minor amount of spinel, which started to crystallize at 1400°C and 20–22 kb pressure; the pyroxenes have locally survived plastic strains and subsolidus rccrystallizations and now occur in the form of clinopyroxene and orthopyroxene megacrysts displaying unmixing features. Although the differentiated silicate liquid was fully expelled during the flow crystallization process, the layered pyroxenites have concentrated the highly incompatible elements S and Cu and locally display significant chalcophile platinum-group element enrichment (Pd, Pt). Cu and S behave coherently over the whole range of pyroxenite composition; their highest concentrations are found in the thinnest websterite layers or at the margin of composite layers. Microscopic investigation of 214 polished thin sections shows these elements to be present as accessory Cu-Fe-Ni sulfides interstitial among the silicate phase or forming discrete bodies included in the relic pyroxene megacrysts. All these features indicate the presence of a sulfide liquid, immiscible with the silicate magma, during the crystallization of the layered pyroxenites. Sulfide liquid immiscibility probably occurred in response to thermal contrast between the pyroxenites and the cooler surrounding peridotites. It is proposed that the megacryst-hosted sulfide inclusions were trapped as linear arrays arranged on host megacryst growth planes. Due to the slow cooling and complex unmixing history of the megacrysts, these arrays have been transformed into coarse, isolated sulfide inclusions by subsolidus migration and spheroidization processes. They started to crystallize at T = 1200°C as monosulfide solid solution (MSS), probably coexisting with a minor amount of Ni- and Cu-rich sulfide liquid down to r=900°C. The reconstruction of the bulk chemistry of each individual inclusion reveals significant between-inclusion variations of Cu/Ni+ Fe and Ni/Fe ratios, which would result from strain-induced immobilization of these liquids. On cooling, the high-temperature MSS has decomposed below 230°C into Ni-rich pyrrhotite, nickeliferous pentlandite, chalcopyrite and minor pyrite. The post-magmatic history of the interstitial sulfide grains was not unlike that of the inclusions, except at near-surface temperatures where the primary sulfides resulting from unmixing of MSS have been partly altered into secondary sulfides by serpentinizing aqueous fluids. In spite of these post-magmatic alterations, the inclusions and the interstitial sulfide phases are remarkably homogeneous as regards their bulk Ni/Cu ratio, which is close to 3. This value is characteristic of sulfide separated from primary rather than partially differentiated tholeiitic melts. It is thus concluded that the continental tholeiite parent to the layered pyroxenites was saturated with sulfides when it left its mantle source regioa In this aspect, it would not be different from MORBs which contain similar sulfide compositions. In both cases, sulfide fractionation cannot be ignored in models for chalcophile trace element fractionation during initial evolution of these magmas.
The Oligocene basanites from Montferrier near Montpellier contain various ultramafic xenoliths, predominantly spinel-lherzolites locally rich in amphibole, and a unique garnet-clinopyroxenite. The sulfide content of spinel-lherzolites is unusually high for xenoliths and approaches that of MORB mantle sources. The garnet-clinopyroxenite, now consisting of recrystallized cpx + garnet + opx + sp assemblage, results from subsolidus evolution of a primary subcalcic clinopyroxene (plus minor spinel) compositionally similar to those experimentally obtained during high-pressure crystallization of MORB; such an origin is also attested by the Ni/Cu ratio of the coexisting sulfide component which is close to 1. The spinel lherzolites display generally fine-grained porphyroclastic textures with varying proportions of porphyroclasts and neoblasts, and more rarely granuloblastic textures. Within each single sample, the cores of the pyroxene and spinel porphyroclasts have nearly constant compositions. Significantly inhomogeneous chemical changes appear at the extreme edges of some large crystals, as well as in the fine-grained pyroxenes and spinel, whereas olivine composition is invariable. Such compositional variations result from the superimposed effects of two episodes of deformation and recrystallization. Geothermometric determinations show that all the spinel lherzolites and the garnet-pyroxenite attained an equilibrium state around 950°C. From a discussion on the origin of relict opx–cpx–sp clusters and relatively high sulfide contents in peridotites, on the reconstructed primary paragenesis of the garnet-pyroxenite, and from a comparison with the North Pyrenean ultramafic associations, it is inferred that this equilibration occurred at the end of an episode involving partial melting, crystallization, and subsolidus recrystallization consequent on plastic deformation. Sulfides were probably retained as sulfide melt early in this episode while metasomatism responsible for the crystallization of amphibole occurred late. This event is ascribed to a diapiric uplift up to a 45–50 km depth in relation with Oligocene rifting which started 35–40 m.y. ago in the Languedoc area. The compositional disequilibrium observed in a number of lherzolites is related to a second episode of shearing deformation responsible for generating porphyroclastic textures, grading locally into mylonites. As a consequence of a concomitant cooling, Al and Cr contents decrease in both pyroxenes, and Na content also decreases in clinopyroxene. The main chemical change of spinel consists of a systematic substitution of MgAl2O4 by FeCr2O4, linked to the development of a porphyroclastic texture, i.e. to the degree of shearing deformations imposed upon the peridotites. The temperature decrease down to 750°C resulted from the ascent of previously equilibrated mantle blocks into colder parts near the Moho. This event occurred a few millions years before the collection of xenoliths by the Montferrier basanites, probably as the Cevennes fault zone below Oligocene grabens was reactived at depth.
