Suprasubduction origin of the Neoproterozoic chromite-bearing serpentinites of the Candiotinha ophiolite, Porongos Complex, southern Brasiliano Orogen

The Candiotinha ophiolite, within the Neoproterozoic Porongos Complex of the southern Brasiliano Orogen, preserves depleted mantle-derived rocks that record key stages in the assembly of West Gondwana. We present the first integrated field, petrographic, and geochemical dataset for these ultramafic rocks, which occur as asymmetric lenses of variably altered massive and sheared serpentinites, with subordinate talc – carbonate and metachert. Cr-spinels occur disseminated or as aggregates within massive serpentinites. We define two compositional populations: Type I spinels (aggregates) display intermediate Cr# (0.53–0.55), moderate Mg# (0.52–0.65), and high Al# (~0.46), typical of forearc peridotites. Type II spinels (disseminated) exhibit higher Cr# (0.81–0.84) and lower Al# (~0.17), suggesting boninitic affinities; however, their compositions likely reflect partial post-magmatic modification. Whole-rock geochemistry indicates a refractory harzburgitic protolith formed by high-degree partial melting in a forearc mantle wedge, later modified by melt – harzburgite and fluid – rock interaction, as reflected in U-shaped REE patterns diagnostic of suprasubduction (SSZ) settings. These results support a multistage evolution involving depletion, SSZ melt interaction, and metamorphic overprint prior to tectonic incorporation into the continental margin during the Brasiliano – Pan-African Orogenic Cycle. The Candiotinha ophiolite thus provides key evidence for Neoproterozoic oceanic and collisional processes in SW Gondwana.