Multi-stage evolution of Continental Lithospheric Mantle beneath Devès volcanic field (Massif Central, France): an example from Allègre xenolith suite

Cenozoic volcanic rocks occurring in Devès volcanic field (3.5 – 0.5 Ma) in the southern mantle domain of the French Massif Central (FMC) carry abundant peridotite xenoliths sampling Continental Lithospheric Mantle (CLM) [1, 2]. CLM in this area is fertile and might have formed due to 1) extraction of small amounts of partial melt(s) from mantle, and/or 2) refertilization of depleted mantle by asthenosphere-derived melts [1, 2]. We present mineral data for peridotites from several xenolith localities at Devès, in order to shed new light on the problem and document regional-scale CLM variability. Here, we complement the existing set of mineral chemical data from Allègre [2], Mt. Coupet [3] and Mt. Briançon [4] with a new data set on peridotitic xenoliths from Allègre volcano.

Peridotite xenoliths from Allègre (n = 16) are represented mostly by fine- to medium-granular lherzolites. Forsterite in olivine varies from 89.34 to 91.42%. The Mg# and Al content in orthopyroxene are: 0.89 – 0.92 and 0.06 – 0.22 apfu, respectively. In clinopyroxene, Mg# is 0.88 – 0.93 and Al content is 0.03 – 0.32 apfu. In spinel, Cr# and Mg# are: 0.09 – 0.50 and 0.63 – 0.76, respectively. Three major groups are recognized based on clinopyroxene REE patterns: (A) LREE-depleted, (B) LREE-enriched and (C) moderately LREE-enriched (spoon-shaped). However, two samples are characterized by significantly higher Cr# (0.38 – 0.50), lower Mg# (0.63 – 0.68) in spinel and lower Al in Opx and Cpx (0.06 – 0.13 and 0.03 – 0.20 apfu, respectively), along with strong LREE-enrichment and were classified as group D.

The mineral major element compositions for peridotite xenoliths from Allègre resemble those from other xenolith suites at Devès [2, 3, 4]. The only difference is recognized in the composition of spinel, which in peridotites from Allègre has higher Cr# (higher by up to ~0.20) and lower Mg# (~0.05) than that from other Devès localities (including previous data from Allègre [2]). Moreover, the trace element composition of pyroxenes is very similar in all three localities. Therefore, we assume that Allègre peridotites share an evolution with peridotites from other Devès localities. They record multi-stage metasomatism, including reaction with MORB-like melt (group A) and overprint by percolating alkaline melts (group B), additionally documented by transitional lithologies (group C).  On the other hand, the chemical composition of group D peridotites, which are more refractory but more strongly incompatible element-enriched, suggests the existence of mantle domains, which were not affected by MORB-like metasomatism observed in group A. Thus, despite the generally fertile composition of peridotites typical for the southern FMC mantle domain [1, 2], isolated relic pockets of more refractory material persist in the CLM, offering the rare opportunity to unravel regional CLM evolution prior to pervasive refertilization.

 

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] Mazurek et al. (2024). Abstract EGU24-8658

[4] Ziobro-Mikrut et al. (2024). Lithos 482-483, 107670.