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

Kyanite and zoisite eclogites from the Limousin area in the Variscan French Massif Central (FMC) have been formed by deep subduction to 100km depth at peak pressure–temperature conditions around 660°C and 2.9GPa. They belong to a thin tectonic unit comprising ophiolitic bodies devoid of HP-UHP markers and forming a transported suture zone now separating the two main lithotectonic units of the FMC with contrasted tectono-metamorphic features (namely the lower and upper allochthons). Composition of kyanite–eclogites is similar to supra-subduction-zone basalts found in back-arc basins or island arcs (LREE depleted, negative Nb–Ta anomalies, εNd: +6 to +9). Zoisite eclogites have the major-element fingerprint of ultrabasic iron-rich plagioclase cumulates common in continental intrusions or forming Fe–Ti oceanic gabbros. The distribution of immobile trace-elements is however more comparable to alkaline Si-undersaturated rocks, but the Sr–Nd isotopic composition (87Sr/86Sr: 0.706; εNd: −3 to −5) suggests the intervention of an old radiogenic crustal component during their petrogenesis. The chemical composition of hydrous zoisite eclogites does not correspond closely to a common magmatic precursor. It is thus suggested that, by analogy with well exposed worldwide HP-UHP terranes, at least a part of the trace-element signature and the isotopic compositions are controlled by channelled fluid–rock interaction at UHP conditions, leading to localised segregation of zoisite that hosts nearly all Sr and LREE budgets of the bulk eclogite. Sr–Nd isotopic composition of these samples is thus controlled by zoisite and probably represents the isotopic composition of the fluid phase present during UHP recrystallisation. Zircons extracted from a zoisite eclogite were dated in-situ by LA-ICP-MS. Crystallisation of the magmatic precursor is bracketed between 489 and 475Ma and the UHP event is dated at 412±10Ma, in agreement with published ages for the Eo-Variscan HP stage in the FMC. A late resetting of the U–Pb isotopic system at 382±7Ma is related to an anatectic high pressure–medium pressure event, well known in the Limousin area. The oceanic eclogites dated in this study are tentatively linked to the Galicia–South Brittany Lower Paleozoic Ocean; they show evidence of the existence of an intra-oceanic subduction zone at the Lower Ordovician and of partial closure of this oceanic domain by ocean–continent subduction during Early Devonian.

The contact zone between two major allochthonous lithotectonic units in the French Massif Central (FMC) is characterized by the presence of corundum-bearing amphibolites associated with serpentinites, flaser-gabbros, eclogites and granulites. These unusual amphibolites are best preserved in the Western FMC, where they are found within the lower oceanic crust of the Limousin ophiolite. Mineralogical observations and thermodynamic modelling of the spinel–corundum–sapphirine–kyanite amphibolites in the CMASH system show that they were formed at peak P–T conditions around 800 °C/10 kbar in response to near isothermal burial followed by a retrogressive anticlockwise path. Metamorphic reactions are controlled both by modification of P–T conditions and by local chemical changes linked to fluid infiltration. Pargasite growth has been enhanced by infiltration of Ca- and Al-rich fluids whereas kyanite- and sapphirine-forming reactions are partly controlled by local inputs of MgO–SiO2 components, most probably during infiltration metasomatism. By analogy with worldwide ophiolites (Oman, Tethyan, Appalachian) and published numerical models, subduction of a still-hot oceanic ridge is proposed to form these Al-rich amphibolites from plagioclase-rich troctolites. The trace-element composition of high-Ti, fine-grained amphibolites (former fine-grained Fe–Ti gabbros) adjacent to the corundum-bearing ones, further indicates that the oceanic crust was initially created at a mid-ocean ridge (rather than within a back-arc basin), followed by the emplacement of supra-subduction zone-type magmas, probably due to intraoceanic subduction close to the ridge.

Mbuji-Mayi (east Kasai province) and Kundelungu (Shaba province) are the two kimberlite fields known for a long time in the Democratic Republic of Congo (DRC). Mbuji-Mayi intrudes the Archean basement (Congo–Kasai Craton) and is diamond-rich, whereas Kundelungu cuts across Paleoproterozoic basement (Bangweulu block) and is diamond-poor. The megacryst suites (or discrete nodule suites) of both fields include garnet and clinopyroxene megacrysts. The pyrope-rich megacrysts can be subdivided in three groups on the basis of their Cr contents: low-Cr (0.00–1.79 wt.%Cr2O3; Mg#: 72.8–84.0); medium-Cr (1.93–5.16 wt.%Cr2O3; Mg#: 76.2–86.3) and high-Cr (5.42–7.10 wt.%Cr2O3; Mg#: 79.2–84.6). There are no significant geochemical differences between the garnets from Mbuji-Mayi and from Kundelungu. Polymineral inclusions composed of K-rich hydrated phases (phlogopite and amphibole), fresh glass and Cr-spinels are identified in garnets from all three groups, in both localities, which suggest a common origin. Two groups of diopside megacrysts from Mbuji-Mayi are distinguished on the basis of their Ca content: low-Ca (Ca#: 39.5–42.1; 0.61–0.92 wt.%Cr2O3) and medium-Ca (Ca#: 44.1–48.5; 0.41–1.09 wt.%Cr2O3); they differ from a third group of high-Cr diopsides (Ca#: 47.1–49.4; 1.31–2.77 wt.%Cr2O3). The major element compositions of DRC megacrysts are distinct from those of many other megacryst suites worldwide: the clinopyroxenes are lower in Fe and Ti and higher in Mg and the garnets contain more Cr and significantly less Ti, Fe and Al. These DRC megacryst compositions are intermediate between those of peridotite minerals and those of kimberlite megacrysts from other localities. Most garnets have “normal” REE profiles ((La/Yb)N=0.003–0.027), whereas clinopyroxenes display relative LREE enrichment ((La/Yb)N=5.1–43.2). The REE patterns of garnet and clinopyroxene megacrysts are similar to those from metasomatized South African mantle lherzolites. The differences in composition between DRC megacrysts and those from other kimberlites might reflect different modes of formation. Some megacryst suites are related by fractional crystallization processes, the DRC garnet and clinopyroxene megacrysts display geochemical similarities with peridotites and may originate by metasomatic transformation and recrystallization of mantle peridotites.

This study is a part of a wider investigation to evaluate how much can be learnt by using low-cost methods such as systematic moderate-resolution remote sensing to map and quantify geological structures at the regional scale on very large volcanic provinces only partly studied in the field. Volcanic-centre and cinder-cone distribution, faults and structural lineaments are mapped combining Shuttle Radar Topography Mission (SRTM), Digital Elevation Models (DEMs) and Landsat satellite images. As an example of the method, we present the interpretation of structural data and morphological features of three contrasted areas from the Cameroon Volcanic Line (Tombel graben, Upper Benue valley, and Ngaoundéré area) for which local field studies are available for comparison. At a local scale, this remote-sensing method of mapping displays good to excellent correlations with previously published data and, by itself, it allows one to constrain the structural setting of each area. Numerical treatment of vent and cinder-cone localisation can be related to tension fractures (T direction), whereas numerical treatment of the lineaments constrains the associated fault system to a single transtensional (strike-slip + extension) Riedel type fracture network. The first results on the Cameroon Volcanic Line are promising and could be used at a larger scale on numerous volcanic provinces for which field data are not yet available.

abSTRacT Gabbros from the Variscan Tisovita–Iuti ophiolite in Romania display the peculiar Cr-rich mica + calcite + quartz mineral association that is typical of listvenites. This metasomatic lithology appears as the end-product of a continuous petrographic series of transformed rocks from amphibolitized gabbros (resulting from an ocean-floor metamorphism) to listvenites. During these different degrees of modification, chromium is immobile and concentrated in the main silicate phases of each stage (amphibole, chlorite and white mica). This process could appear mineralogically and texturally continuous, but chlorite morphology and thermometry suggest two main processes. Listvenite formation characterizes a low-temperature (~300°C) metasomatic stage overprinted on a previously warmer (>450°C) more common process described as ocean-floor metamorphism.

The Motru Dyke Swarm intrudes the Precambrian Danubian basement of the Southern Carpathians (Romania). It is a marker of a sub-volcanic event that occurred during the early Palaeozoic (Cambrian to Ordovician). The geographical distribution of dykes on a ∼2,000km2 area is heterogeneous; several areas of high dyke density have been the subject of a detailed petrological and geochemical study. Taken altogether, the 150 samples define a single complete magmatic series, from basaltic andesite to rhyolite. Whole-rock major element variations show a medium- to high-K, calc-alkaline magmatic suite. The compositional variations and the general decrease of trace element contents (both compatible and incompatible, including REEs) from basaltic andesite to rhyolite are consistent with 1) the fractionation of the observed phenocryst assemblages, Ca-amphibole (Ti-pargasite to magnesiohornblende) followed by intermediate plagioclase, clinopyroxene and accessory biotite and quartz and 2) the absence of lower and/or upper crustal contamination. Trace elements diagrams display typical arc patterns (LILE, Pb and LREE enrichment and relative depletion in Nb-Ta, Zr-Hf and Ti). The Th/U, Nb/Ta and Zr/Hf ratios are constant and close to the mantle values throughout the whole series, which argues that the parental magma was generated from a single and homogeneous enriched lithospheric mantle source. The field regional evidence implies that melting occurred during a late- to post-orogenic period of lithospheric extension, and thus took place quite lately after the cessation of Pan-African subduction.

Ultramafic and mafic xenoliths of magmatic origin, sampled in the Beaunit vent (northern French Massif Central), derive from the Permian (257Ma) Beaunit layered complex (BLC) that was emplaced at the crust-mantle transition zone (∼1GPa). 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 30km 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.