Origin of amphibole in evolving Variscan subcontinental lithospheric mantle: the Marais de Limagne xenoliths, Massif Central, France
摘要
Amphibole in peridotite xenoliths in Cenozoic volcanic rocks of the Massif Central, France, is generally sparse but widely distributed. Here, we discuss its origin in the xenolith suite of Marais de Limagne (Devès volcanic field) and its bearing on evolution of lithospheric mantle in the southern part of the Massif Central. The xenoliths are lherzolites containing (1) no amphibole, (2) 2–5 vol. % of dispersed amphibole or (3) 12–22 vol. % of amphibole forming aggregates. The amphibole-free lherzolites are similar to LREE-depleted lherzolites typical for lithospheric mantle underlying southern part of the Massif Central. Increasing amphibole volume is correlated with a decrease of Al contents in both pyroxenes (opx: 0.19 → 0.14, cpx 0.30 → 0.21 Al atoms per formula unit, a pfu), with an increase of spinel Cr/(Al + Cr) from 0.09 to 0.17, and with a transition from LREE-depleted to LREE enriched clinopyroxene REE patterns. By comparison, the lherzolites with amphibole aggregates contain pyroxenes that are more Al-depleted (opx ~ 0.10, cpx ~ 0.13 Al a pfu) and Cr-enriched spinel (Cr/(Cr + Al) 0.21–0.39). The amphibole is pargasitic and locally was affected by incipient melting producing secondary clinopyroxene, olivine and glass. In-situ determination of 87Sr/86Sr ratios in clinopyroxene and amphibole, which are largely in isotopic equilibrium, yields values from 0.70319 to 0.70371, i.e. moderately higher than the Depleted Mantle. We suggest that heterogeneous percolation of a highly mobile metasomatic agent that contained water, Ba, Sr, K, Na and was LREE-enriched, resulted in variable addition of amphibole to anhydrous lherzolitic protoliths. Amphibole crystallized at the expense of spinel and clinopyroxene, which were the source of Al. Spinel was passively enriched in Cr as amphibole volume increased. Amphibole occurring in low volumes inherited the chemical characteristics of protolith minerals, whereby 87Sr/86Sr ratios up to 0.7037 may reflect an earlier, Variscan subduction imprint. In contrast, the composition of amphibole occurring in large volumes reflects the composition of the metasomatic medium. The low and rather homogeneous amphibole 87Sr/86Sr (0.7033–0.7034) in amphibole-rich samples, combined with published elevated hydrogen isotope ratios for amphibole in other xenoliths from the southern Massif Central, suggest relatively recent derivation of the metasomatic agent from an asthenospheric mantle source that was polluted by young subducted oceanic crust, with limited influence of sedimentary components.