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Magmatic garnet-bearing mafic xenoliths (Puy Beaunit, French Massif Central)

¹ Section de Géologie Isotopique, Musée Royal de l'Afrique centrale, B-3080 Tervuren, Belgium
² Laboratoire de Géochimie Isotopique et Géodynamique Chimique, DSTE, Université Libre de Bruxelles (CP 160/02), 50 Avenue Roosevelt, B-1050 Brussels, Belgium
³ Unité de Géochimie et Minéralogie, Musée Royal de l'Afrique centrale, B-3080 Tervuren, Belgium

^* E-mail: julien.berger{at}africamuseum.be (J. Berger).

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, < 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.

Key-words: magmatic garnet, thermobarometry, mafic xenoliths, Puy Beaunit, pyrometamorphism, meta-cumulates.

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