Olivier Femenias

A Permian underplating event in late- to post-orogenic tectonic setting. Evidence from the mafic-ultramafic layered xenoliths from Beaunit (French Massif Central)

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.

Calcic amphibole growth and compositions in calc-alkaline magmas: Evidence from the Motru Dike Swarm (Southern Carpathians, Romania)

Abstract A Late Pan-African calc-alkaline dike swarm (basalt-andesite-dacite-rhyolite) has been investigated in a region of over 2000 km2 in the Alpine Danubian window, South Carpathians (Romania). Amphibole phenocrysts and microphenocrysts have been investigated by wavelength-dispersive microprobe analysis and BSE imaging. The Ca-amphibole population, represented in all the lithologies, displays a large compositional range, interpreted as the result of two processes: (1) magmatic evolution (kaersutite → Ti-pargasite → pargasite → Ti-magnesiohastingsite → magnesiohastingsite → edenite → tschermakite → magnesiohornblende) linked to magmatic differentiation from andesitic basalt to rhyolite; and (2) deuteritic alteration of the primary amphibole related to late-emplacement hydrothermal activity (yielding numerous varieties comprising those cited above). In all rock types, amphibole phenocrysts equilibrated at a nearly constant pressure of about 0.6 ± 0.1 GPa, but their temperatures of crystallization ranged from 1000.900 °C for basaltic andesites to 700.600 °C for dacites. In rhyolites, edenite to magnesiohornblende crystals reflect a continuous range of P-T conditions from 700 °C/0.6 GPa to 600 °C/0.1 GPa, in agreement with their change of habit from euhedral to subhedral. Complex zonations in pargasite-magnesiohastingsite (including resorption) are interpreted in term of self-organization of oscillatory zoning without significant heating and/or magma mixing.

A Variscan slow-spreading ridge (MOR-LHOT) in Limousin (French Massif Central): magmatic evolution and tectonic setting inferred from mineral chemistry

Abstract The Limousin ophiolite (French Massif Central) occurs as elongate bodies forming a (nearly) continuous suture zone between two major lithotectonic units of the French Variscan belt. The mantle section of the ophiolite is made of diopside-bearing harzburgite, harzburgite and dunite characteristic of a lherzolite-harzburgite ophiolite type (LHOT). The plutonic section is essentially composed of troctolites, wehrlites and gabbros locally intruded by ilmenite-rich mafic dykes. All the rocks were strongly affected by an ocean-floor hydrothermal metamorphism. The composition and evolution of primary magmatic phases (olivine, clinopyroxene, plagioclase and spinel) throughout the lowermost magmatic sequence correspond to those described in oceanic cumulates (ODP data). The Limousin ophiolite is thus of MOR type instead of SSZ type. The whole lithological section, the mineral chemistry, the extensive hydrothermal oceanic alteration and the relatively thin crustal section are typical of a slow-spreading ridge ocean (i.e. Mid-Atlantic ridge). Comparison of the Limousin ophiolite with other ophiolites from European Variscides suggests that the oceanic domain was actively spreading during the Late Palaeozoic and extended from the Armorican massif to the Polish Sudetes.

Magmatic garnet-bearing mafic xenoliths (Puy Beaunit, French Massif Central): P-T path from crystallisation to exhumation

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°C and 1 GPa. The rocks underwent sub-solidus re-equilibration at about 770°C and 1 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°C, 0.55-0.8 GPa) and amphibole (990°C,

Pétrologie des xénolites ultramafiques du puy Beaunit (Massif central francais): Un gisement atypique du manteau sous-continental

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.

Deformation-driven Differentiation during in situ Crystallization of the 2·7 Ga Iguilid Mafic Intrusion (West African Craton, Mauritania)

The 2 7 Ga Iguilid mafic intrusion is a post-metamorphic plutonic body made of cumulate gabbronorite and norite characterized by limited variations in modal proportions but significant disparities in trace element contents. The cumulates display microtextural evidence for syn-magmatic deformation, leading to reorientation of cumulus minerals, indented contacts at plagioclase grain boundaries, local bending and fracturing of euhedral cumulus plagioclase and interstitial melt migration/ segregation structures. All these features suggest that the Iguilid intrusion formed after in situ crystallization and syn-magmatic deformation of an unconsolidated mush within solidification fronts. Quantitative investigations of syn-magmatic fabrics together with geochemical estimations of the trapped melt fraction reveal a strong inverse correlation between the anisotropy degree P and the volume of trapped liquid retained in the cumulates. We interpret the dominantly vertical planar and linear fabrics in the Iguilid rocks as a result of horizontal shortening of the cumulates induced by filter pressing of a partly solidified mush against the vertical walls of the intrusion. Flattening of the cumulates occurred in response either to a syn-magmatic rotation of the Iguilid body relative to the tectonic main stress or to stages of overand under-pressurization owing to cycles of magma recharge or discharge. Deformation of the unconsolidated cumulates led to migration of the silicic interstitial melt and controlled their bulk content in incompatible elements. This migrating liquid had similar trace element and isotopic compositions to synchronous 2 7 Ga dioritic to granodioritic plutons, forming a bimodal, post-metamorphic, magmatic suite with the Iguilid body. The interstitial melt extracted from the Iguilid mafic intrusion could have fed synchronous metaluminous intermediate to felsic plutons.