Ultramafic and mafic xenoliths of magmatic origin, sampled in the Beaunit vent (northern French Massif Central), derive from the Permian (257 Ma) Beaunit layered complex (BLC) that was emplaced at the crust-mantle transition zone (∼1 GPa). These plutonic xenoliths are linked to a single fractional crystallisation process in four steps: peridotitic cumulates; websteritic cumulates; Al-rich mafic cumulates (plagioclase, pyroxenes, garnet, amphibole and spinel) and finally low-Al mafic cumulates. This sequence of cumulates can be related to the compositional evolution of hydrous Mg basaltic magma that evolved to high-Al basalt and finally to andesitic basalt. Sr and Nd isotopic compositions confirm the co-genetic character of the various magmatic xenoliths and argue for an enriched upper mantle source comparable to present mantle wedges above subduction zones. LILE, LREE and Pb enrichment are a common feature of all xenoliths and argue for an enriched sub-alkaline transitional parental magma. The existence of a Permian magma chamber at 30 km depth suggests that the low-velocity zone observed locally beneath the Moho probably does not represent an anomalous mantle but rather a sequence of mafic/ultramafic cumulates with densities close to those of mantle rocks.

The origin of magmatic layering is still hotly debated. To try to shed some light on this problem, two ultramafic–mafic layered xenoliths from Puy Beaunit (French Massif Central) were investigated in detail. The nodules belong to a stratiform intrusion emplaced in the deep crust during the Permian (257±6 Ma; Féménias, O., Coussaert, N., Bingen, B., Whitehouse, M., Mercier, J.-C., Demaiffe, D., 2003. A Permian underplating event in late- to post-orogenic tectonic setting. Evidence from the mafic–ultramafic layered xenoliths from Beaunit (French Massif Central). Chem. Geol. 199 293–315.). The 3 to 5 cm thick nodules have, in common, a central orthopyroxenite layer; the succession of layers is, respectively, norite–orthopyroxenite–norite (PBN 00-01) and norite–orthopyroxenite–harzburgite (PBN 00-03). The variations of both major (by electron microprobe) and trace, essentially the RE, elements (by LA-ICP-MS) were measured in major mineral phases (orthopyroxene, clinopyroxene, plagioclase, spinel) along cross-section perpendicular to the layering. Strong grain size, chemical and textural variations occur along these sections: they can be continuous or discontinuous, symmetrical or asymmetrical. Such complex variations cannot be solely related to a single magmatic history (fractional crystallisation, mineral sorting). Other processes such as element enrichment by residual liquid channelling along layer boundaries and/or sub-solidus recrystallisation and element redistribution must be invoked. It appears, in particular, that element distribution in the central orthopyroxenite layer could result from the injection of micro-sills of orthopyroxene-rich liquid between previously consolidated layers.

Mafic xenoliths, sometimes interlayered with magmatic peridotites, are abundant in the scoria cones of Puy Beaunit in the French Massif Central. These are mainly layered gabbronorites with some norites, pyroxenites and anorthosites; they probably derive from a Permian differentiated deep layered intrusion. Crystallisation conditions were estimated at about 1000 degrees C and I GPa. The rocks underwent sub-solidus re-equilibration at about 770 degrees C and I GPa (isobaric cooling) in the lowermost crust. Two distinct symplectitic textures (pyroxene-plagioclase-spinel intergrowths) have been observed; they result from the destabilization of magmatic garnet (750-800 degrees C, 0.55-0.8 GPa) and amphibole (990 degrees C, < 0.3 GPa). Melting of amphibole and destabilisation of orthopyroxene occurs during xenolith ascent. Garnet was destabilised either during tectonic uplift of the lower crust during Early Cenozoic or at the first step of xenolith ascent to the surface. Whole-rock REE concentrations show that some xenoliths are typically plagioclase-rich cumulates, others are pyroxene-rich cumulates. Few mafic xenoliths display HREE enrichment, (La/Yb)(N) ratios lower than 1, presumably indicating garnet cumulation. Major-element composition of the fine-grained plagioclase-orthopyroxene-spinel symplectites is indeed close to that of a pyrope-almandine garnet with significant grossular content. The presence of this magmatic garnet argues for a hydrated calc-alkaline high-alumina basaltic magma in which the various mafic cumulates crystallised at depth.

L’eon Archeen est marque par un phenomene majeur dans l’histoire de la terre : la stabilisation de du manteau lithospherique continental. Bien que le role reel de cette stabilisation en tant qu’acteur dans la geodynamique archeenne ne soit pas encore parfaitement etabli, elle a joue un role important, par la suite, dans le developpement et la preservation de la croute continentale stabilisee sous la forme de cratons. L’isolement de cette lithosphere par rapport a l’asthenosphere convective a du egalement avoir une incidence importante sur l’evolution du manteau et sur son regime thermique. Comprendre les processus qui ont forme et modifie le manteau lithospherique sub-cratonique a l’Archeen est donc essentiel pour apprehender l’evolution geodynamique du globe terrestre. L’origine et l’histoire de ce manteau lithospherique restent encore tres controversees car bien qu’il ait ete epargne depuis deux milliards d’annees de toute tectonique active, les roches qui le constituent ont subi des reequilibrages chimiques et/ou modaux suite a un ou plusieurs evenements metasomatiques. Ce travail s’inscrit en grande partie dans cette problematique avec une etude detaillee des equilibres existant entre les differentes phases mineralogiques des peridotites inclues dans les kimberlites de Thaba Putsoa et de Letseng-la-Terrae (Lesotho). L’etude de ces equilibres mineralogiques nous permet de determiner les phases qui resultent (ou ont ete modifiees) d’un phenomene metasomatique, et inversement d’en apprendre sur la nature de celui-ci. Le traitement des donnees petrographiques, geochimiques et thermodynamiques a etabli que la partie du manteau lithospherique archeen echantillonnee a Letseng et de Thaba Putsoa ne peut pas etre consideree comme homogene et a l’equilibre. On demontre qu’au moins deux episodes de metasomatisme aux caracteristiques chimiques distinctes ont affecte successivement ces roches. Bien que trois grandes familles de peridotites ont ete decrites, l’hypothese d’un protolithe commun de type harzburgite subcalcique a ete proposee pour l’ensemble de la lithosphere archeenne profonde. Ceci implique que tous les clinopyroxenes presents dans ces peridotites soient d'origine secondaire. La question de la validite des modeles thermodynamiques reposant sur le principe d’un equilibre du clinopyroxene avec les autres mineraux se pose des lors. Cela nous a conduits dans la seconde partie de la these a commencer le developpement d’un nouvel outil thermodynamique se basant sur une phase minerale plus stable comme l’orthopyroxene. Pour ce faire, un nouveau protocole d'inversion a ete propose permettant la determination d'un modele de solution fiable pour cette phase.

The Lovozero alkaline massif (Kola Peninsula, Russia) is composed of three major units. The central unit (80% of the volume) comprises numerous well developed layers composed, from bottom to roof, of an urtite–juvite–foyaite–lujavrite continuous lithological sequence (ijolite–foid-bearing alkali feldspar syenite in IUGS nomenclature). The mode of emplacement of the massif and the mechanism of formation of the layering are still under debate. Petrological, mineralogical (two stages of crystallisation) and structural evidence from the detailed analysis of one of these layers (unit II-7) is interpreted in terms of both mechanical (magmatic to sub-solidus, non-coaxial deformation) and thermal differentiation operating on a crystal-laden (alkali feldspar, high T nepheline, aegirine-augite) material of foyaitic composition. Textural and mineralogical data suggest that a sheet of foiditic magma intruded into solidified earlier units of the Lovozero layered sequence and acquired a sill-like structure on cooling.

Mantle xenoliths from Puy Beaunit (French Massif Central) are compositionally varied, ranging from relatively fertile spinel lherzolites to refractory spinel dunites. Fertile peridotites have registered a modal (amphibole-bearing lherzolites) and cryptic metasomatic event that took place before the last Permian (257 Ma) melting episode. Depletion processes have been constrained by chemical modelling: the depletion is related to different degrees of partial melting, but two major melt extraction episodes are needed to explain the range of major element composition. The second event was responsible for the local large-scale dunitification of former residues. The first melting event (F≈25%) and metasomatic enrichment are attributed to an ancient fluid and/or liquid infiltration that could be related to a pre-Variscan regional subduction (located to the north of the Beaunit area). Texture acquisition and major deformation of the mantle xenoliths were sub-contemporaneous of the subduction and would result from lithospheric delamination. The second melting event (F≈17%) produced high-Mg basalts with calc-alkaline trace element signature that gave rise to the Permian underplating episode recognised in western Europe.

of the 7th International Kimberlite Conference, Cape Town, 934-936. Van Achterbergh, E., Griffin, W. L., Stiefenhofer, J., 2001. Metasomatism in mantle xenoliths from the Letlhakane kimberlites: estimation of element fluxes. Contributions to Mineralogy and Petrology 141, 397-414. Yaxley, G. M., Green, D. H., Kamenetsky, V., 1998. Carbonatite metasomatism in the southeastern Australian lithosphere. Journal of Petrology 39, 1917-1930. Contact: N Coussaert, Musee Royal d’Afrique Centrale, 13 Leuvensteenweg, B-3080, Tervuren, Belgium. E-Mail : ncoussae@africamuseum.be

The Puy Beaunit volcano vent, French Massif Central, displays a population of plutonic mafic to ultramafic xenoliths, commonly showing asymmetric, millimetre to centimetre thick, layering. Layers are pyroxenitic to gabbroic, and less commonly peridotitic (Iherzolite, dunite, websterite) and anorthositic. These xenoliths are interpreted as samples of a layered intrusion, located at the crust-mantle boundary. Primary cumulate phases are olivine and orthopyroxene, followed by clinopyroxene and plagioclase; rare intercumulus accessory phases (apatite, rutile and zircon) are observed in the most differentiated layers. Homogeneous xenoliths, interpreted as single cumulate layers, have a cale-alkaline geochemistry with LREE and large ion lithophile elements (LILE) enrichments relative to Nb, Ta and Ti. The negative Eu anomaly of pyroxenite can be related to earlier plagioclase fractionation, as observed in the gabbroic layers. Trace element laser ablation inductively coupled plasma emission mass spectrometry (LA-ICP-MS) and secondary ion mass spectrometry (SIMS) analyses of plagioclase, orthopyroxene and zircon from layered rocks suggest equilibrium and cogenetic relations between the silicate phases. U-Pb SIMS dating of a 1.5 mm zircon crystal gives a magmatic or sub-solidus equilibration age of 257 +/- 6 Ma.The Beaunit layered intrusion belongs to the large Pennian within-plate magmatic episode commonly of calc-alkaline geochemical signature observed over Europe and North Africa. It probably corresponds to a mafic underplating event spatially controlled by post-Variscan trans-tensional to trans-pressional basin tectonics in an intracontinental setting. The subduction-related geochemical signature of the magmatic suite is interpreted as resulting from the passive remobilisation of a mantle source, which was previously metasomatised during the Variscan subduction. (C) 2003 Elsevier Science B.V. All rights reserved.