Challenges in classifying I- and S-type granites: insights from the Sana and Itaoca plutons

Abstract The Sana (Sana suite) and Itaoca (Nova Friburgo suite) granites, emplaced in the Oriental Terrane of the Ribeira Belt during the Cambrian–Ordovician, provide key insights into post-collisional magmatism in a Gondwana segment marked by crustal recycling. We present new Sm–Nd and Rb–Sr isotope data for 26 samples from both plutons, combined with petrographic information, to evaluate magma sources and the applicability of I- and S-type classification. The Sana Granite is strongly peraluminous and dominantly S-type, derived mainly from metasedimentary sources of the São Fidélis Group with minor arc-derived input, supported by isotopic evidence and rare titanite (< 1%). In contrast, the Itaoca Granite is metaluminous, titanite-rich, and consistent with I-type, reflecting melting of mafic to high-K orthogneisses of the Rio Negro Magmatic Arc. Mixing models suggest ternary contributions from arc and sedimentary reservoirs but remain exploratory due to limited Sr data for sources. Both plutons share overlapping Nd and Sr isotope fields, with initial 87Sr/86Sr ratios of 0.7008–0.7078 and consistently negative εNd(t) (–11 to –13), highlighting isotopic convergence in the Oriental Terrane. These results show that, despite contrasting S- and I-type traits, both granites reflect hybridization and a shared tectonic framework, emphasizing the limitations of strict isotopic classification and the need for integrated petrographic approaches.