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.

EXPLOR_STEP=$WICRI_ROOT/Wicri/Terre/explor/NickelMaghrebV1/Data/France/Analysis

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{{Explor lien
   |wiki=    Wicri/Terre
   |area=    NickelMaghrebV1
   |flux=    France
   |étape=   Analysis
   |type=    RBID
   |clé=     ISTEX:04ABEB846D4AA558C2D67D2B6B67A26CA91D1B63
   |texte=   Petrology and Thermal History of Highly Deformed Mantle Xenoliths from the Montferrier Basanites, Languedoc, Southern France: A Comparison with Ultramafic Complexes from the North Pyrenean Zone
}}