Discovery of metamorphic microdiamonds from the parautochthonous units of the Variscan French Massif Central

The Variscan French Massif Central (FMC) is classically described as a stack of nappes with increasing metamorphism from the bottom (parautochthonous unit) to the top (lower and upper gneiss units). Ultra-high pressure (UHP) metamorphism was already recognized in the uppermost units, with notably coesite-bearing rocks. We report the first finding of metamorphic microdiamonds in the parautochthonous unit, revealing that the UHP event affected the whole stack of nappes and also that the pressures reached are above what was previously expected, since the presence of diamond, according to the peak temperature estimates for this unit, indicates pressures of ca. 3 GPa (i.e. 100 km depth). At the scale of the FMC, this finding adds complexity to the established models. On a broader scale, this adds evidence of similarity between this part of the belt and other UHP Variscan terranes such as the Erzgebirge and the Bohemian Massif. (C) 2015 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.