The Ticunzal Formation in central Brazil: Record of Rhyacian sedimentation and metamorphism in the western border of the São Francisco Craton

Abstract

The Ticunzal Formation is exposed in the northern portion basement of the external zone of the Brasília Fold Belt and corresponds to a sequence of biotite-garnet paragneisses and mica-graphite schists with mineral assemblages indicating retrograde metamorphism under greenschist facies conditions. The formation displays local mylonitic structures and is intruded syn-to post-tectonically by 2.11–2.16 Ga peraluminous plutons of the Aurumina Suite. Detrital zircon U-Pb ages, Nd model ages between 2.23 and 2.88 Ga and geochemical data reflect a provenance probably represented by the 2.20 and 2.46 Ga tonalitic/granodioritic rocks of the Almas-Dianópolis terrane, exposed northwards of the study area. Zircon U-Pb geochronology and field observations suggest an age interval for the deposition of the precursor sediments of the Ticunzal Formation between 2.16 and 2.19 Ga. Raman spectroscopy analyses on graphite indicate high-crystallinity internal structures consistent with peak metamorphic temperatures between 620 and 630 °C. Trace element geochemical data of Ticunzal Formation samples suggest that precursor sediments were deposited in a continental arc-related basin, defining a setting consistent with an active mobile belt at the western margin of the São Francisco Craton during Rhyacian times, as inferred also from the characteristics of rocks from the Almas-Dianópolis terrane and the Aurumina Suite.