Abstract
The age and stratigraphic position of the basaltic greenschists of the Rio Preto valley, located in the western part of the Aracuai Belt ( ca . 60 km north of Diamantina, Minas Gerais) were a matter of controversy in the geologic literature, because several authors correlated them to the Neoproterozoic Macaubas Group but others to the lower Espinhaco Supergroup (ca. 1.7 Ga). However, detailed studies demonstrate that these greenschists represent an interplay of submarine basaltic volcanism, volcanoclastic sedimentation and fire fountaining, and that they belong to the Chapada Acaua Formation of the Macaubas Group (Gradim et al ., 2005). Geochemical studies indicate that the greenschist protoliths evolved in a continental intraplate environment. Zircon crystals were separated from a greenschist sample and analyzed by the U-Pb SHRIMP method. A Sm-Nd whole-rock isotopic analysis was obtained from the same sample. The Sm- Nd model age of ca . 1.52 Ga suggests that the greenschist protoliths are younger than the magmatism of the Espinhaco rift. Most analyzed zircon crystals show features of detrital grains. The older ages indicate zircon grains inherited from the Archean- Paleoproterozoic basement and from magmatic rocks of the Espinhaco rift. The younger U-Pb values constrain the maximum age of the greenschist protoliths at ca. 1,16 Ga.
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