Mantle domains in the lithosphere beneath the French Massif Central: trace element and isotopic evidence from mantle clinopyroxenes

Abstract

Lenoir et al. [Earth Planet. Sci. Lett. 181 (2000) 359] have suggested the existence of a lithospheric domain boundary at approximately 45°30′N within the French Massif Central because of differences in bulk rock geochemistry between mantle peridotite xenoliths from localities north and south of this latitude. New laser ablation ICP–MS analyses of trace element contents in clinopyroxenes from xenoliths from the northern domain, combined with Sr and Nd isotopic data for the same clinopyroxenes, clearly indicate significant differences between the northern and southern domains. LA–ICP–MS results show that the northern domain peridotites have experienced significantly more depletion than ones from the southern domain, having undergone almost twice as much fractional melting. Although they do not contain garnet at the present day, the northern domain peridotites appear to have contained garnet when they underwent melting. They can have extremely high Sm/Nd ratios (>0.5), indicating unusually strong depletion in Nd relative to Sm. However, they are also more enriched in LREE and Sr, and show significant fractionation of Zr from Hf, and of U from Th (Zr/Hf and Th/U≪chondritic values). This is due to metasomatism, possibly related to carbonitite infiltration. The Sr–Nd isotopic compositions of northern domain xenoliths are also unusual, in that many have high 143Nd/ 144Nd or high 87Sr/ 86Sr values which do not occur in samples from the southern domain. In contrast, the southern domain mantle shows low (MORB source) 87Sr/ 86Sr values not found in the northern domain. This confirms that significant differences exist between the two domains and offers strong support to the concept of the existence of lithospheric blocks with separate histories beneath the Massif Central. A remarkable similarity exists between the mantle beneath the northern domain of the Massif Central and that beneath the Rhön volcanic area (Germany) 1000 km to the northeast.