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