Xistos verdes do alto Araçuaí, Minas Gerais: vulcanismo básico do rifte neoproterozóico macaúbas

Abstract

Greenschists ( s.s. ) of the upper valley of the Aracuai River were grouped in four lithofacies, based on preserved macroscopic textures: i) schists with pillow structures and columnar joints, ii) banded schists, iii) breccias, and iv) schists with no primary structures. An interpretation of environments and processes involved in the formation of these lithofacies indicates interplay between submarine basaltic volcanism, volcaniclastic sedimentation and fire fountaining. The litho-structural framework indicates normal contact between the greenschists and the metasedimentary rocks of the Chapada Acaua Formation. U-Pb SHRIMP dating constrains the maximum age of the protoliths at ca. 1.1 Ga. The Sm-Nd model age of ca . 1.5 also corroborates that the greenschists are younger than the magmatism of the Espinhaco rift. The greenchists have tholeiitic protoliths, a within-plate basalt signature and inherited zircon crystal from older felsic rocks. However, a Nd(900 Ma) slightly positive (+ 0.23) and samples with oceanic signature suggest that the basic magma migrated trough a thinned continental crust, during the late rift stage. All data sets support the correlation between these rocks and the Rio Preto Member of the Chapada Acaua Formation (Macaubas Group).