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DOI

10.19111.bulletinofmre.1764813

Abstract

The Bracco chromitites are hosted in the Mesozoic Ligurian Ophiolites (Italy) and provide key insights into the magmatic and post-magmatic (i.e. metamorphic and hydrothermal) evolution of gabbro-hosted chromitites in an oceanic mantle. Petrographic and mineralogical analyses reveal that the Bracco chromitites comprise cumulitic, massive to disseminated, layered chromitites overprinted by multi-stage alteration within altered olivine–clinopyroxene–anorthite cumulates. Detailed Cr–Al– Fe³+ systematics indicates that primary Cr- to Al-rich chromite, affected by metamorphic-hydrothermal processes under sub-greenschist facies conditions, locally escaped recrystallization and metasomatic modification. Consequently, chromite cores preserve their primary magmatic compositions consistent with crystallization from aluminous melts produced by low-degree partial mantle melting at a midocean ridge (MOR) setting. Metamorphic-hydrothermal alteration is marked by multi-stage ferrian chromite rims, whereas based on their Mg content the associated chlorite is classified as clinochlore. Chlorite geothermometry indicates alteration temperatures in the range of ~100-300 °C, consistent with oceanic serpentinization under prehnite-pumpellyite facies conditions. The hydrothermal fluids were oxidizing, enriched in SiO2 and MnO, and circulated through fracture networks in the shallow oceanic lithosphere. Elevated MnO amounts in alteration rims suggest widespread Mn-enrichment in these fluids, potentially linking them to seafloor Mn deposits in the Ligurian Ophiolites. Together, these findings indicate that the Bracco chromitites, their gabbroic hosts, and associated lherzolitic mantle rocks were at least partially exposed at the Tethyan seafloor prior to their final emplacement during the Alpine orogenetic phase, where serpentinization promoted complex chromite alteration.

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