Discovery of Variscan orogenic peridotites in the Pelvoux Massif (Western Alps, France)

Abstract

Small bodies of mantle-derived peridotites and other ultramafic rocks are commonly found in exhumed lower crustal units of collisional orogens. They provide a direct record of the complex evolution of the upper mantle before and during an orogeny, and are therefore key markers of the geodynamic evolution of an orogen. We report here the discovery of such mantle-derived peridotites in high-grade Variscan lower crust exposed in the Pelvoux massif (External Western Alps), which occur as fragmented enclaves in migmatites. A wide petrographic diversity has been observed, from very fertile, garnet-bearing lherzolites, to more depleted spinel/chromite-bearing harzburgites. Thermobarometric calculations on a garnet lherzolite indicate an initial stage at 3.0 ± 0.5 GPa and 973 ± 50°C, followed by exhumation to 0.8-1.3 GPa and 800-850°C, while the harzburgites do not show any evidence of equilibration in the garnet field and sample shallower mantle (<2.0 GPa). Petrological observations, whole-rock geochemistry and in-situ mineral compositions suggest the peridotites have undergone a complex history prior to their incorporation in the lower crust during the Variscan Orogeny. They derive from refractory mantle domains, which have experienced variable degrees of melt depletion, and have then been extensively refertilized. Cryptic metasomatism is observed in all samples. It is characterized by an enrichment in fluid-mobile incompatible elements relative to immobile ones (LILE vs HFSE), leading to the development of pronounced negative anomalies Nb and Ta, and is presumably related to percolation of hydrous fluids in the mantle. In addition, strong enrichment in incompatible LREE relative to HREE is observed in some samples, commonly associated with modification of the modal composition (crystallization of phlogopite and/or pargasite with accessory chromite and apatite). This modal metasomatism is attributed to percolation by a K2O-P2O5-Cr2O3-rich silicate melt, which might be at the source of syn-collisional ultrapotassic magmas (durbachites) emplaced in the crust during the middle-upper Carboniferous. These geochemical characteristics are in line with whole-rock Nd isotopic compositions, which indicate enrichment of the mantle by a continental crust component, presumably related to Variscan subductions. This evolution is consistent with that of other Variscan peridotites in the eastern Alps (Ulten) and the Bohemian massif, where multiple metasomatic episodes related to melts or fluids released in Variscan subduction zones have been documented.