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, which occur as fragmented enclaves in migmatites of the high-grade Variscan lower crust exposed in the Pelvoux Massif (external Western Alps). 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 3.0–4.0 GPa and 970–1140 °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. 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 a refractory mantle, which has experienced variable degrees of melt depletion, and has then been extensively refertilized. Cryptic metasomatism is observed in all samples. It is characterized by an enrichment in large-ion lithophile elements (LILE, in particular Cs, Rb, U and Pb) relative to high field strength elements (HFSE), in particular Nb and Ta. This cryptic metasomatism is presumably related to percolation of subduction-related fluids or melts in the mantle. In addition, modal metasomatism occurred in some samples, where crystallization of phlogopite, pargasite, chromite and apatite has been observed. This modal metasomatism resulted in significant enrichment in K2O, P2O5 and Cr2O3 of the bulk rock, together with a strong enrichment in incompatible LREE relative to HREE. These geochemical characteristics are strikingly similar to that of syn-collisional, Mg–Cr–LILE rich mantle-derived (ultra)-potassic magmas such as durbachites and vaugnerites, which are ubiquitous in the Variscan metamorphic allochthons of Massif Central, external Alps, Vosges and Bohemian Massif. We therefore suggest that this metasomatism results from dynamic percolation of the peridotites by K2O–P2O5–Cr2O3-rich melts from which the durbachites and vaugnerites are primarily derived. 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.
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