Arc-related and high-grade dry metamorphic conditions of the Búzios Orogeny: A case study of. ∼500 Ma orthogranulites from Ilha Grande, Rio de Janeiro – Brazil

In situ U-Pb and Lu-Hf analyses performed by laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) on zircons and cathodoluminescence (CL) images were performed to isotopically characterize the granulitic metamorphism of the Ribeira Belt in Ilha Grande (Rio de Janeiro State, SE Brazil). Our study provides the first detailed and accurate U-Pb ages of the subduction stage of this magmatic arc with respective mantle signatures, as demonstrated by the Hf isotopic results. Zircon grains in charnockitic rocks have a heterogeneous internal texture or cores preserved with overgrowth, and Th/U ratios generally <0.1. However, a group of zircons was identified that present the typical zoning of magmatic origin and high Th/U ratios generally >0.4. The preserved grains and with core provide U/Pb ages between 540-530 Ma and 177 Hf/ 176 Hf values between 0.2323 and 0.2324 and 177 Lu/ 176 Hf values between 0.2322 and 0.2321. Most metamorphic zircons grew in a dry environment, as indicated by their high 176 Lu/ 177 Hf ratios resulting in newly formed metamorphic domains with Th/U values (between 2.5 and 0.5) similar to magmatic domains, suggesting their formation by solid-state recrystallization without a presence of aqueous fluid. This event was defined by the metamorphic U/Pb age of 510-496 Ma in zircon grains, consistent with the granulation age of the Búzios Orogeny. The results indicate that the protolith of the charnockites is type-I granitic rocks crystallized between 550-510 Ma. These rocks are the product of oceanic crust followed by oceanic crust subduction and crustal duplication during the collision of the São Francisco and Congo cratons during the amalgamation of the Gondwana supercontinent.