A Suture–related Accretionary Wedge Formed in the Ediacaran Neoproterozoic Brazil

A suture–related accretionary wedge formed in the Neoproterozoic Araçuaí orogen (SE Brazil) during Western Gondwanaland assembly

Authors:

Peixoto et al

Abstract:

The Araçuaí orogen represents a branch of the Brasiliano orogenic system developed between the São Francisco and Congo cratons in Neoproterozoic time. We conducted detailed studies on a complex schist belt located to the west of the Rio Doce magmatic arc, along the assumed suture zone of the Araçuaí orogen. This 30 km–wide and 100 km–long, NS–trending belt includes pelitic schists with intercalations of quartzites, metaultramafic schists and diopsidites, intruded by collisional granites. U–Pb ages from detrital zircon grains point to distinct provenances for different portions of the schist belt. The lower succession shows an age spectra and maximum depositional age (819 Ma) similar to passive margin deposits of the precursor basin. The upper succession yielded a maximum deposition age around 600 Ma, pointing toward the Rio Doce arc as the main sediment source. Thrust to the west onto the Guanhães basement and separated on the east from the Ediacaran Rio Doce magmatic arc by reverse–dextral faults, the schist belt exhibits the architecture of an asymmetric flower structure developed in transpressional regime. The distribution of metamorphic assemblages across the schist belt characterizes a collisional, Barrovian–type metamorphic zoning, in which the sillimanite, kyanite, staurolite and garnet zones are locally duplicated by thrusts. P–T conditions range from 700 °C at 7.5 kbar, at the western base of the pile, to 550 °C at 5.5 kbar, at the eastern top of the package. Zircon U–Pb ages record metamorphic overprinting on the sheared top of the basement at 560 ± 20 Ma and crystallization of collisional granites at 544 ± 10 Ma. Our results allow us to interpret the schist belt as a suture-related accretionary wedge and suggest that basin closure during the assembly of West Gondwnaland lasted to the Ediacaran-Cambrian boundary in the Araçuaí orogen.