NATURE OF THE UPPER-MANTLE AND METASOMATI C PROCESSES ON EITHER SIDE OF THE SILLON-HOUILLER (FRENCH MASSIF-CENTRAL)

Peridotite xenoliths collected in two Mio-Pliocene volcanoes on either side of the Sillon Houiller (French Massif Central) have contrasting characteristics. East of the Sillon Houiller, the peridotites record a nomalously high temperatures (1,250 degrees C) and extensive reequilib ration with basaltic melts. To the west, equilibrium temperatures are much lower (950 degrees C) and the peridotites bear evidence for inter action with volatile-rich small melt fractions. In agreement with seis mic tomographic data, these results indicate that the Massif Central l ithospheric mantle is strongly affected by mantle plume upwelling to t he east of the Sillon Houiller, while this effect is moderate to the w est.