Résumé
The Alpine ophiolites represent sub-continental and oceanic lithosphere remnants of the narrow Jurassic Ligurian Tethys. They allow the study of the lithospheric evolution during adiabatical melting and melt percolation history in slow- spreading environments. The Mt. Maggiore ultramafic unit (Corsica, France) displays field associations that testify of a complex history of melt reactive percolation from deep levels (spinel facies) to shallower depths (plagioclase facies). We present a field-based microstructural and geochemical investigation of depleted peridotites and associated pyroxenite layers and olivine-rich troctolites. The Mt. Maggiore peridotites display a clear evolution from spinel lherzolite to plagioclase harzburgite during two continuous episodes of melt-rock interaction. At spinel facies, the reactive percolation of a LREE-depleted melt leads to the dissolution of mantle pyroxenes and the growth of olivine crystals, forming replacive spinel dunites. In the shallower plagioclase facies, the melts modified by the previous reactive melt percolation impregnate the spinel-facies lithotypes, leading to the dissolution of olivine and crystallization of plagioclase and orthopyroxene in the peridotites. The impregnation stage is also observed in the spinel dunites, leading to the formation of replacive olivine-rich troctolites. Additionally, field and petrographic evidence indicate that pyroxenite layers formed prior to the melt-rock interaction history. Moreover, both the parental melts of pyroxenites and the melts involved in the subsequent percolation were characterized by similar Na2O-poor, LREE- depleted compositions. This implies that they represent the continuous evolution of depleted melts leading to different processes (pyroxenite segregation and later melt-rock interaction) during their upward migration. To support the genetic relation and the continuity between the formation of pyroxenites and the subsequent melt- rock interaction history, we modeled all the documented processes in sequence, i.e.: i) formation of depleted melts after 6% mantle decompressional fractional melting; ii) high- pressure segregation of pyroxenites; iii) spinel- facies reactive porous flow, iv) plagioclase-facies melt impregnation. These field-based chemical models allow to constrain all melt percolation features observed at the Mt. Maggiore as resulting from the percolation of a single parental melt in a thick oceanic lithosphere from deep levels to shallow depths.