To understand the petrogenesis of peraluminous granites syntectonic to the Dorsal de Canguçu Transcurrent Shear Zone in the Sul-rio-grandense Shield, Brazil, melting experiments were performed on one of the potential protoliths, a cordierite-bearing semi-pelitic metasedimentary gneiss (PE-1). Experiments were conducted at pressures of 5, 10 and 15 kbar, at temperatures of 700–900°C, and under fluid-absent and 5% H₂O-present conditions. The experiments show that fluid-absent melting begins at near-solidus conditions, around 700°C, promoted by participation of retrogressive phengitic muscovite in the reaction Mus + Kf ± Qz = melt ± Fe–Ti oxide ± Als, producing a very small amount of melt (800°C) produced S-type granitic melts promoted by participation of biotite or cordierite in the reactions Bio + Pl + Crd + Qz = Px + Fe–Ti oxide + melt at 5 kbar, and Bio + Pl + Crd ± Qz = Grt + Als ± Kf + melt at 10 and 15 kbar, both producing a high amount of melt (10–63% by volume). The melt compositions obtained at 900°C and 15 kbar under fluid-absent conditions, promoted by biotite or cordierite breakdown, are similar to the syntectonic granites. However, it is unlikely that the granites were formed at this pressure (corresponding to a depth of melting of ∼54 km).

The Cerro Olivo Complex is one of the few occurrences of the basement rocks in the Dom Feliciano Belt. It contains migmatitic paragneisses and orthogneisses that host granites of ca. 600–540 Ma Aiguá Batholith. The main orthogneisses are rich in orthopyroxene + Ca-plagioclase (Cerro Bori unit), but K-feldspar augen gneisses are also common (Centinela-Punta del Este unit). The paragneisses (Chafalote unit) are semi-pelitic migmatites that contain restites of metapelites, quartzites, amphibolites, and calc-silicate rocks. A clockwise pressure–temperature–time (P–T-t) path and two deformational events affected the Cerro Olivo Complex rocks. Granulitic high-pressure (HP)–high-temperature (HT) peak conditions were followed by low pressure (LP)–HT decompression. The first deformation (K₁) developed an E–W gneissic foliation and westward-stretching lineations, whereas the second (K₂) produced NS to NE–SW low-temperature mylonitic foliation and southward-stretching lineations. New SHRIMP U–Pb data from zircon cores in magmatic textural domains yield an intrusive age of 782 ± 7 million years for the Cerro Bori unit. The zircon rims have an age of 657 ± 7 million years, reflecting a younger partial melting event. Inherited ages in zircon xenocrysts span from 2655 to 768 million years, but are mostly ca. 1.0–1.2 thousand million years old. Bulk-rock geochemistry indicates a magmatic arc setting for the source rocks. The Cerro Bori unit represents calk-alkaline tonalitic and granodioritic rocks mixed with minor gabbros; in contrast, the Centinela unit consists of post-orogenic granites. A continental magmatic arc developed between ca. 800 and 770 Ma attending convergence of the Kalahari and Rio de la Plata palaeocontinents, but prior to their collision.