No significant element transfer from the oceanic plate to the mantle wedge during subduction and exhumation of the Tethys lithosphere (Western Alps)

Abstract

In the Western Alps, Mesozoic ophiolites attest to the former existence of a slow-spreading rate ocean. Several units of this oceanic lithosphere, slightly to highly metamorphosed (up to the eclogite facies), are included in a nappe stacking formed during Tertiary subduction- and collision-related processes. Mantle and magmatic rock samples from five tectonic units, which have equilibrated at various metamorphic grades, were examined and analysed in order to estimate their prograde and retrograde metamorphic evolution and to evaluate the resulting chemical changes. Alpine metamorphism appears to be isochemical on the sample scale, or even on the mineral scale when the primary mineral is only pseudomorphosed at low-strain conditions. No significant loss or gain of trace elements, nor of Sr and Nd isotope exchange, appear to have occurred since alteration on the oceanic floor. As in the present oceans, the Sr isotopes recorded seawater contamination during sea floor spreading, whereas most trace element contents and the Nd isotopic signature of the magmatic and mantle protoliths remain unchanged. These results lead us to consider that no significant transfer of element occurred to the mantle wedge during subduction of the Tethyan units, even during the formation of eclogites at pressures up to 13 kbar. Serpentinized mantle peridotites retained the signature of a depleted mantle metasomatized by U-, Th- and LREE-enriched melts, similar to the European subcontinental mantle. As in the Ligurian ophiolites, gabbros and basalts cannot be related by fractional crystallization, and both originate from a same residual mantle source below the suboceanic mantle.