Interplay between melt infiltration and deformation in the deep lithospheric mantle (External Liguride ophiolite, North Italy)

Mantle peridotites from the External Liguride Jurassic ophiolites (Northern Apennines, Italy) show diffuse occurrenceof pyroxenite bands, recording melt migration and crystallization at the lithosphere-asthenosphereboundary, during old, pre-Jurassic stages of their subcontinental lithospheric mantle evolution. We presentcoupled microstructural and geochemical study of profiles across various types of pyroxenite-peridotitelayering in these ophiolites, aiming to constrain the relative timing and potential interplay between melt infiltrationand mantle deformation. The mantle sequence is composed of lherzolite and harzburgite, occasionallyinterleaved with dunite, crosscut by centimeter- to decimeter-wide pyroxenite layers. The peridotites have aporphyroclastic texture and show a penetrative tectonic foliation subparallel to the pyroxenite layering.Peridotite-pyroxenite contacts are irregular at the grain scale. Olivine and pyroxenes in both peridotites andpyroxenites record moderate to strong crystallographic preferred orientations (CPO) with alignment of olivine[100] and pyroxenes [001] axes subparallel to the stretching lineation marked by olivine and pyroxenes elongation.This is compatible with coeval deformation of olivine and pyroxenes during a high-temperature, spinellherzolite-facies deformation event. The major and trace element compositions of peridotites record a metasomaticimprint that decreases with distance from the pyroxenite layers, whereas the strength of the olivine CPOdecreases from the country peridotites towards the pyroxenite layers. The parallelism between pyroxenitelayers and the peridotite foliation, their irregular contacts, as well as the spatial correlation between CPOand geochemical changes, are consistent with syn- to late-kinematic emplacement of the pyroxenites. Theseobservations point to a strong interplay between deformation and melt transport processes in the mantle,characterized by melt focusing in conduits parallel to the foliation and changes in the mantle deformation processesdue to the presence of melts. Exhumation of this mantle section in the Jurassic resulted in partial replacementof the spinel-facies assemblage by a plagioclase-bearing assemblage. Topotaxial relationshipbetween plagioclase and precursor spinel-facies minerals suggests that this exhumation was not associatedwith pervasive deformation of the peridotite, but rather accommodated by deformation localized in discreteshear zones, not sampled in the present study.