Textural and compositional domains as tools for reconstructing retrograde UHT metamorphism: Insights from sapphirine-bearing granulites of the Salvador–Esplanada–Boquim Belt, São Francisco Craton, Brazil

This study addresses the scale of equilibrium in ultra-high temperature (UHT) rocks, investigating through thermodynamic modeling how local bulk compositional domains behave during texture development. In the southern Salvador-Esplanada-Boquim Belt (SEBB), ultramafic and sapphirine-bearing granulites occur as enclaves hosted within felsic granulites. The sapphirine-bearing granulite shows increasing grain size toward the host rock and a mineralogical zoning transition characterized by garnet clusters and orthopyroxene-bearing leucosome derived from the surrounding felsic granulite. Microscopically, the sapphirine-bearing granulite exhibits distinct compositional domains with symplectitic associations of sapphirine, spinel, and cordierite along garnet rims, in which orthopyroxene occurs varying locally the intergrowth phase. Phase equilibria modeling of the local bulk compositions of symplectites domains reconstructs the post-peak P–T path from 11.5 kbar/1030 °C followed by near-isothermal decompression associated to garnet breakdown, which generated the sequential symplectite formation: orthopyroxene + spinel (~11–9 kbar), orthopyroxene + sapphirine (~9 kbar/970 °C), followed by orthopyroxene + cordierite (~7.7 kbar/880 °C). The last stage at ~7 kbar/850 °C is associated with the crystallization of plagioclase close to the solidus. The felsic granulite recorded comparable clockwise P-T path from ~9.7 kbar/980 °C to ~7–5 kbar/880 °C. Ternary feldspar thermometry supports these constraints, yielding temperatures ranging from ~1000 °C to 850 °C. Field evidence, textural and compositional analyses suggest that the ultramafic granulite chemically interacted with the felsic granulite leucosome, the latter supplying SiO₂ to an originally SiO₂-undersaturated ultramafic protolith, leading to sapphirine-bearing granulite formation. Local bulk compositions derived from well-constrained metamorphic textures provided a robust approach, enabling reconstruction of the P-T paths of the UHT granulites of the SEBB. • Domain-scale equilibrium constrained by phase equilibria modeling in UHT granulites. • Sequential symplectites record garnet breakdown during decompression stages. • SiO₂-rich melt–ultramafic rock interaction produced sapphirine-bearing assemblages.