A garnet-bearing leucogranite and two country rocks from the Transamazonian Tandilia belt of the Rio de la Plata craton were studied in detail. The leucogranite contains garnet with homogeneous composition of pyr6(gros+andr)2spes5alm87. In a garnet-biotite migmatite, the core and rim compositions of garnet are pyr1.7(gros+andr)5spes5.6alm87.7 and pyr1.2(gros+andr)5.5spes6.7alm86.6, respectively. These compositions in a sillimanite-garnet-muscovite migmatite are pyr4(gros+andr)2.7spes2.7alm90.6 and pyr2.7(gros+andr)4spes3.2alm90.1, respectively. We used this information to decipher the P-T evolution of the rocks applying P-T and T-H2O pseudosections with the PERPLE_X computer software package taking into consideration deformational microstructures. The leucogranite records an isothermal decompression from 5.3 to 3.8kbar at 665°C. The garnet-biotite migmatite was exhumed from 5.5kbar at 630°C to 4.3kbar at 615°C and the sillimanite-garnet-muscovite migmatite from supersolidus conditions of 670°C and 3.6kbar to 625°C at 2.4kbar. Late andalusite formed in this rock.Seventy four analyses of 28 monazite grains of the country rocks yielded three groups of U-Th-Pb ages which were related to a collisional event (I: ca. 2.13–2.14Ga.), a postcollisional thermal overprint (II: ca. 2.01Ga) and slow cooling of the orogen (III: 1.80–1.90Ga). Inherited ages of 2.28 and 2.25Ga could refer to an early accretionary stage of the orogen. An age of 2.41Ga indicates the presence of recycled Siderian continental crust. Synkinematic crystallization of melts and the subsolidus development of an S2-foliation, demonstrated by deformational microstructures, occurred during the exhumation of the studied area from depths of ~18km to ~8km in the time interval 2.01–1.90Ga.