Nicolas Coussaert

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.

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,