Magmatic inheritance vs. UHT metamorphism: Zircon petrochronology of granulites and petrogenesis of charnockitic leucosomes of the Socorro–Guaxupé nappe, SE Brazil

We present a detailed study of zircon petrochronology of UHT granulites combined with charnockite petrogenesis, using geochemical and SrNd isotopic constraints for the Socorro-Guaxupé Nappe (SGN) in southeast Brazil. This multi-technique approach is used to refine the timescales of pre-collisional arc magmatism and syn-collisional UHT metamorphism and to constrain the P-T-t evolution of granulites. The SGN features the transition from a subduction-related to a continental-collision setting, involving the formation of in-situ orthopyroxene- and hornblende-bearing leucosome. The application of the REE array plot approach enabled a clear distinction between pre-collisional (prior to garnet growth) and syn-collisional (during or after garnet growth) zircon crystallization events in the SGN. A period of pre-collisional arc magmatism at ca. 730–640 Ma is identified. Al-in-orthopyroxene thermometry yields minimum temperatures of ca. 900 °C, 12 kbar in the garnet-rich granulite, consistent with UHT metamorphism, followed by cooling with re-equilibration to ca. 770 °C at 10 kbar. Garnet growth near peak conditions prior to zircon, and small-scale equilibrium between later garnet cores and ca. 615 Ma zircon rims are revealed by the array plot approach. Zircon growth during melt crystallization is documented in well-developed sector- and oscillatory-zoned overgrowths and “soccer ball” zircon at ca. 613 Ma and 608 Ma. Additionally, the origin and sources of both granulites and leucosomes are investigated. Orthopyroxene- and hornblende-bearing leucosomes are present both as fractionated melts and cumulates. Leucosomes and their granulite residues belong to the shoshonitic series, characterized by high K2O contents, and they display negative ƐNd(t) values (−11.7 to −5.1) and high initial 87Sr/86Sr ratios (0.7055–0.7080). The combination of geochemical and isotopic evidence, in addition to zircon inheritance patterns, point to enriched-mantle sources in their origin rather than crustal recycling involving either Archean or Paleoproterozoic older crust. The enrichment of subcontinental lithospheric mantle is interpreted to be related to the subduction stage prior to collision.