LAUSR

Abstract Recent studies investigate the replacive formation of hybrid troctolites from mantle peridotites after multiple stages of melt-rock reaction. However, these studies are not conducted in a field-controlled geological setting displaying the clear evolution from the protolith to the end-product of the reactions. The Mt. Maggiore peridotitic body exposes a clear evolution from spinel lherzolite to plagioclase-bearing lithotypes (plagioclase peridotites, olivine-rich troctolites and troctolites) during two continuous episodes of melt-rock interaction. In the 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. The progressive evolution from spinel lherzolite to harzburgite to replacive dunite is accompanied by a change of olivine Crystallographic Preferred Orientation (CPO), from axial-[100] in the lherzolite to axial-[010] olivine CPO in the dunites, indicative of deformation in presence of melt. The initial percolating melt composition is consistent with single melt increments after 6% partial melting of a depleted mantle source. Reactive melt percolation leads to a progressive enrichment in the melt M-HREE absolute concentrations, while preserving its LREE depletion, consistent with the enriched analyzed HREE composition of olivine in the spinel dunite. In the shallower plagioclase facies, the melts modified by reactive melt percolation impregnate the spinel-facies lithotypes, leading to the dissolution of olivine and crystallization of plagioclase and orthopyroxene in the peridotites. This impregnation stage is also observed in the spinel dunites, forming hybrid olivine-rich troctolites and troctolites. The dissolution-precipitation reactions forming hybrid troctolites cause a progressive textural evolution of the olivine matrix, with the disruption of deformed coarse grains into undeformed small rounded grains. This textural evolution is not accompanied by clear changes in the olivine CPO, indicating low instantaneous melt/rock ratios during the impregnation process. Olivine, plagioclase and clinopyroxene REE compositions analyzed in troctolite fit a process of impregnation with a progressive closure of the porosity (at decreasing melt mass), leading to the crystallization of trapped melt and REE enrichments during the last crystallization increments.