Contrasting lithospheric mantle domains beneath the Massif Central (France) revealed by geochemistry of peridotite xenoliths

We report major and trace element analyses for 82 coarse-grained peridotite xenoliths from 25 Cenozoic volcanic centres throughout the Massif Central (France). These data cover a region of about 150x150 km, allowing an investigation of large scale compositional variations in the subcontinental lithospheric mantle (SCLM). In agreement with textural variations, geochemical data define two contrasting lithospheric domains, situated north and south of latitude 45 degrees 30'. Peridotites of the northern domain show protogranular textures, characterised by clustered pyroxene-spinel distributions. They are rather refractory and depleted in MREE relative to HREE, but pervasively enriched in LREE and other highly incompatible elements. The samples show mantle-normalised patterns with negative anomalies of Nb, Ta, Zr and Hf, similar to enriched mantle xenoliths ascribed to carbonatitic metasomatism. In contrast, the peridotites of the southern domain are devoid of pyroxene-spinel clusters and are therefore referred to as coarse-granular. They are distinguished from the northern suite by more fertile compositions and relatively flat MREE-HREE patterns. In addition, only the harzburgites and a few lherzolites are enriched in LREE. Most southern domain lherzolites are depleted in these elements and the average composition of the southern suite is comparable to that of depleted MORE-source mantle (DMM). The main compositional differences between the two domains cannot be accounted for by a secular evolution of the Massif Central SCLM caused by Cenozoic plume upwelling. Instead, these differences record the existence of distinct lithospheric blocks assembled during the Variscan orogeny. To some degree, the northern and southern domains are reminiscent of cratonic and circumcratonic SCLM domains. Being relatively refractory and pervasively enriched in LREE, the northern domain displays similarities with cratonic SCLM. It is interpreted as a relatively ancient (pre-Variscan) lithospheric block involved in the Variscan belt. Conversely, the fertile composition and the DMM signature of the southern domain evoke more juvenile lithospheric mantle, possibly accreted or rejuvenated during the Variscan orogeny. Geophysical data indicate that asthenospheric upwelling beneath Massif Central is focused beneath the southern domain and follows a NW-SE trend, roughly parallel to Variscan structures in the crust. Though poorly constrained in direction, the limit between the two SCLM domains recognised in this study is consistent with this trend. This may suggest a Link between the inherited architecture of the SCLM and channelling of asthenospheric upwelling. Secular variations in xenolith geochemistry, as well as correlations between trace element data and geophysical anomalies, suggest that the geochemical imprint of Cenozoic plume upwelling on SCLM xenoliths is limited to selective enrichments in U. Sr and Ph relative to Th and REE. (C) 2000 Elsevier Science B.V. All rights reserved.