Insight into lithospheric mantle beneath Devès volcanic field recorded by xenoliths from Mt. Coupet (Massif Central, France): preliminary data

Mafic volcanic rocks occurring in Devès volcanic field (3.5 – 0.5 Ma) are rich in peridotitic xenoliths sampling the southern domain of the French Massif Central (FMC). The origin of the relative fertile composition of the continental lithospheric mantle (CLM) beneath FMC is widely discussed in literature and two competing major mechanisms are proposed: 1) extraction of small amounts of partial melt(s), 2) refertilization by upwelling asthenospheric melts [1, 2]. Data from xenoliths at individual localities contribute to this discussion, providing insight into the local conditions of CLM and documenting its regional variability. In this study, we present major and trace elements chemistry of peridotites from Mt. Coupet xenolith suite.

Mt. Coupet peridotites (n = 24) are represented by porphyroclastic to fine–granular lherzolites, which are characterized by within-sample chemical homogeneity. Between samples, olivine Fo varies from 88.63 to 90.85. The Mg# and Al content in orthopyroxene (Opx) are: 0.90 – 0.92 and 0.12 – 0.20 a pfu, respectively. In clinopyroxene (Cpx), Mg# is 0.89 – 0.94 and Al content is 0.11 – 0.32 a pfu. In spinel, Cr# and Mg# are: 0.04 – 0.39 and 0.62 – 0.80, respectively. The REE patterns in Cpx vary, but are always above PM values (in 10 samples analyzed). Based on these patterns, three groups are recognized: (1) LREE–depleted, (2) LREE–enriched, and (3) spoon–shaped (i.e. LREE-enriched with inflection from La to Sm). Other trace elements contents, e.g. U, Th, Nb, Ta, Ti and Sr also differ between samples. Only 10 of the 24 samples are in major-element chemical equilibrium based on Opx–Cpx thermometry [3]. Thus, calculated temperatures are: 900 – 1070 ℃, suggesting sampling of a considerable depth interval if equilibrated to a conductive geotherm. No relationship between textures, geochemistry and thermometry were observed. By analogy to literature data [2, 4] we assume that the Mt. Coupet xenolith suite represents lithospheric mantle typical for the southern domain in FMC. Studies of REE and trace elements on greater number of samples from Mt. Coupet are, however, required to provide more profound interpretation which could contribute to the discussion on the structure and thermochemical evolution of CLM beneath FMC.

Funding. We gratefully acknowledge funding by the project of Polish National Centre of Research 2021/41/B/ST10/00900 to JP.

 

[1] Lenoir et al. (2000). EPSL 181, 359-375.

[2] Puziewicz et al. (2020). Lithos 362–363, 105467.

[3] Brey and Köhler (1990). JoP 31, 1353-1378.

[4] Uenver-Thiele et al. JoP 58, No. 3, 395–422.