An appraisal on crustal growth and reworking in collisional high-temperature granites: The Neoproterozoic Serra dos Órgãos batholith, Central Ribeira belt, SE Brazil

Abstract

The Pan-African/Brasiliano orogeny, the result of the West-Gondwana assembly from the Neoproterozoic to the Early Paleozoic, culminated in a widespread, long-lived Ediacaran to Cambro-Ordovician granite production event at the Araçuaí-Ribeira Orogenic System. Collisional granitoids were emplaced along an interval wider than 100 m.y., following three subduction-accretion episodes. The Serra dos Órgãos batholith is the main granitoid intrusion emplaced during the syn-collisional episode at the Central Ribeira belt, southeastern Brazil. The integration of Nd–Sr–Hf isotopes and mineral chemistry data were employed to constrain the physical-chemical conditions concerning this intrusion and host rocks, along with potential sources. For this purpose, sensu stricto granites from Serra dos Órgãos batholith and the surrounding Sumidouro Leucogneiss were examined. Zircon and whole-rock isotopic signatures reveal distinct multi-component inheritance in both the intrusion and the country rocks. The Sumidouro Leucogneiss recorded diverse zircon populations of igneous origin, with juvenile to crustal Nd–Sr–Hf isotopic signatures in a large range of geochronological ages (200 m.y. interval), an evidence of zircon preservation in disequilibrium melting situations. The Sumidouro Leucogneiss must be the result of partial melting of multiple sources, behaving as a temporal archive of past events preceding the collision. Conversely, the Serra dos Órgãos batholith records a more mature, crustal signature, presenting a main zircon population interpreted as autocrystic (ca. 590 Ma), and minor inheritance given by an antecrystic population (ca. 610–620 Ma). The less spanned geochronological ages, more homogeneous Nd–Sr–Hf isotopic signatures, and the discrete preservation of an antecrystic zircon population are evidence of early zircon crystallization from a homogeneous source undergoing disequilibrium melting. An even older, ca. 1.9 Ga inherited/xenocrystic zircon population recorded in both the intrusion and the surrounding leucogneisses is also a piece of evidence of zircon preservation in disequilibrium melting situations. Zircon saturation and Ti-in-zircon temperatures strongly suggest thermal disequilibrium between relict zircon and magmas, consistent with the presence of antecrystic and inherited/xenocrystic zircon populations, previously distinguished by isotopic and UPb criteria. Ti-in-zircon thermometry suggests early crystallization temperatures for zircon precipitation, while biotite geothermometry yields near-solidus, late crystallization temperatures for the Serra dos Órgãos batholith. REE-in-zircon oxybarometry suggests fO₂ around FMQ conditions for the intrusion, contrasting with more oxidized fO₂ conditions for the country rocks. The evidence indicates that the studied sensu stricto granites were formed by water-undersaturated melting of crustal protoliths at temperatures up to 840 °C. Furthermore, the preserved zircon populations disclose a high-temperature, juvenile magma production more than 200 m.y. before the Serra dos Órgãos emplacement and the West Gondwana amalgamation.