Olivier Féménias’s research while affiliated with Université Libre de Bruxelles and other places

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.

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 (257Ma) 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 (F25%) 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 (F17%) produced high-Mg basalts with calc-alkaline trace element signature that gave rise to the Permian underplating episode recognised in western Europe.

The fabric in a dike is representative of the magmatic flow, considered as Newtonian. The anisotropy of magnetic susceptibility of the rocks gives a good representation of the shape-preferred orientation which, in turn, is a marker of the magmatic flow. Generally, a symmetrical pattern of the fabric across the dike is in agreement with a flow of magma within a channel: the flow direction is then reliable with this imbrication. An asymmetrical fabric is dependent on the flow and displacement of the wall. We present the case of both symmetrical and asymmetrical dike fabrics recording different emplacements. From a Pan-African calc–alkaline dike swarm (of basaltic–andesitic–dacitic–rhyolitic composition) of the Alpine Danubian window from South Carpathians (Romania), two populations of dikes have been described: thick (1–30 m) N–S-trending dikes and thin (<1 m) E–W dikes. The first display asymmetrical fabric and record the regional sinistral movement of the walls. In contrast, the thin dikes are symmetrical and frequently display an arteritic morphology that limits the dike length, with no cartographic extension. We propose to relate the two types of dikes to the same regional stress field in a continuum of emplacement during a regional brittle event.

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 (lherzolite, 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 calc-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 Permian 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.

The Puy Beaunit maar presents a large variety of mantle xenoliths (spinel peridotites, pyroxenites and layered rocks). A detailed study of the textures and mineral equilibria shows the unusual character of this occurrence and the local complexity of the upper mantle beneath the French Massif Central. Ultramafic nodules have a metamorphic, magmatic or pyrometamorphic origin; they display different stages of deformation, metasomatism, partial melting and fractional crystallisation. The upper mantle appears stratified (as in other regional occurrences of the area); it has been intruded by a differentiated magmatic complex.

A large diversity of xenoliths is present in the scoria cones of the quaternary Puy Beaunit (French Massif Central). Mafic xenoliths are abundant, they are mainly gabbronorites with minor pyroxenites and anorthositic gabbros. It has been shown recently that they derived from a deep layered complex emplaced at the crust-mantle boundary during Late Permian (257+/- 6Ma). These rocks still partly show magmatic structures. However, sub-solidus recrystallisation and pyrometamorphic overprinting (while the xenoliths were taken up by the lava) have partly blurred the magmatic primary textures. The xenoliths now have polygonal and coronitic textures, so intercumulus phases cannot be easily identified. The abundance of orthopyroxene (generally missing in alkaline series), the low content of REE and trace elements and the nearly flat spidergram profiles suggest that these xenoliths are of subalkaline affinity. They commonly show mm- to cm- scale layering indicative of a cumulate origin. Positive (in whole rock plagioclase-rich gabbros) and negative (in pyroxene-rich gabbros) Eu anomalies also argue for a cumulate origin. The trace element contents (REE, Zr, Hf, Rb, Sr, Nb, Ta, Ba) of the main silicate phases (plagioclase, orthopyroxene, clinopyroxene and the very fine-grained coronites) have been obtained by LA-ICP-MS. Two processes control the REE content of these mafic xenoliths: 1) the degree of differentiation, which is indicated by the Mg# of the cumulus orthopyroxene; 2) the proportion of trapped interstitial liquid which is indicated by the content of highly incompatible elements (U, Th, Zr and Rb) in whole rock analyses. Plagioclases and orthopyroxenes have relatively low REE contents compared to clinopyroxene and coronitic association suggesting that they are the cumulus phases. High REE and trace element contents of clinopyroxenes and of the very fine-grained coronites point to an intercumulus origin. Moreover, in situ REE patterns show that the very fine-grained symplectitic coronites could be derived from the breakdown of a former garnet corona.

The Puy Beaunit quaternary maar (Chaîne des Puys) is well known for its large diversity of xenoliths. Besides the typical crustal (granites and granulites) and mantle-derived (peridotites sensu lato) xenoliths, we have also identified magmatic mafic xenoliths, some of them displaying mm- to cm- scale layering. These mafic rocks are mainly gabbronorites with minor pyroxenites and anorthositic gabbros. It has been proposed that these mafic xenoliths are derived from a differentiated complex emplaced at the crust-mantle boundary during Permian times (257+/- 6Ma, zircon U-Pb SIMS data). These xenoliths have undergone a pyrometamorphic (HT-BP) event while they were brought up to the surface by the quaternary lava. Beside the classical polygonal texture, the xenoliths display four types of coronitic and symplectitic textures: 1) plag-opx-oxide symplectite between plagioclase and clinopyroxene; 2) brown-orange glass with skeletal and euhedral olivine around orthopyroxene; 3) poikilitic clinopyroxene with numerous inclusions of euhedral plagioclase and oxide (this texture is also observed around some clinopyroxene); 4) plag-opx-cpx-oxide symplectite around amphibole relics. Mafic samples with coronites are relatively poor in SiO2 (41 to 47wt% SiO2) when compared to polygonal mafic xenoliths (49 to 51wt% SiO2). It appears that the type of corona in a given sample depends on the composition of the primary (=magmatic) pyroxenes, plagioclase and ghost phase (amphibole?). Xenoliths with primary Fe-rich pyroxenes (opx: En59-67, cpx: En36-39) show poikilitic clinopyroxene textures. Xenoliths with primary An-poor plagioclase (An55) show coronas with olivine-bearing glass around orthopyroxene. Xenoliths with primary highly calcic plagioclase (An85-95) show plag-opx-oxide symplectite. Xenoliths with polygonal texture contain Mg-rich orthopyroxene (En70-80) and labrador plagioclase (An65-70). Thermobarometers based on the composition of amphibole relics give magmatic P-T crystallisation conditions of 7kb and 1000-1100°C respectively. Opx-cpx geothermometer gives sub-solidus recrystallisation temperature (700-900°C). Composition of pyroxenes in the coronas yields high T° (>1000°C at less than 5kb), interpreted, in a closed system, as the effect of the late pyrometamorphic event that occurred during the uptake of the xenoliths by the quaternary lava.