Serpentinites from the central portion of the Rio Piranhas-Seridó Domain (RPSD), in the northeast portion of the Borborema Province (BP), occur in association with the Caicó Complex gneisses, paragnaisses/schists of the Seridó Group. Serpentinites, which were sampled in two localities near the city of São Tomé-RN, are melanocratic rocks, fine-grained, greenish in color and foliation consistent with the regional trend. The mineralogical composition is primarily composed of serpentine, which occurs as a replacement for the iron and magnesium minerals of the protolith, with chlorite, muscovite, tremolite, calcite-dolomite, quartz, and opaque minerals as accessory phases. Chemical analyzes show SiO2 values ranging from 35.72 to 43.48%, MgO> 28.88%, Fe2O3 between 7.08 and 11.29%, Al2O3 < 3.42% and very low TiO2, CaO, K2O and Na2O, suggesting an ultrabasic composition. Graphs obtained with the major elements ratios indicate olivine as the primary mineral, and despite the influence of metamorphic processes, the rock still retains the chemistry predominant in the protolith. The high magnesium content points to a komatiitic signature, implying in high temperature, high pressure and high degree of partial melting in the protolith genesis. Data acquired in this study corroborate the bibliography and suggest a compatible scenario for the studied serpentinites to be part of a greenstone belt.
