Rhyacian evolution of high-grade metamorphic rocks of the Porto Nacional Granulite Complex, based on geocronological data U-Pb-Hf in zircon and U- Pb in monazite

Abstract

The Porto Nacional Complex in the northern portion of the Goiás Massif, in north-central Brazil, represents a Paleoproterozoic/Archean microcontinent reworked in the Neoproterozoic during the Brasiliano Cycle. This NE-SW-trending belt of high-grade metamorphic rocks consists, predominantly, of orthogranulites in addition to aluminous paragneisses that reached high-grade metamorphic conditions in the granulite facies (∼860 °C and 9 kbar). The zircon U–Pb geochronological results obtained on orthogranulite (enderbite) crystal cores yielded an age of 2.16 Ga, however, the crystal borders present lower values (2.09 Ga). These results represent, respectively, the age of igneous crystals of tonalitic protoliths, and high-grade metamorphism. The zircon crystals of another enderbite sample yielded lower values of about 2.09 Ga, reinforcing the interpretation of metamorphic zircons. The U–Pb geochronological results between 2.09 Ga and 2.10 Ga obtained on monazite from the aluminous paragneisses, coincide with the values obtained for enderbite metamorphic zircon crystals, allowing us to define the high-grade metamorphism age and the formation of igneous protoliths in the Rhyacian. This small age difference demonstrates the metamorphism role contemporary to the magmatism of the tonalitic protoliths of the Porto Nacional Complex, related to the Late-Transamazonian orogeny. The Hf-TDMC model ages suggest two crust formation episodes that gave rise to the following rocks: one from the Siderian (2.40–2.48 Ga) and another from the Neo-Mesoarchean (2.52–3.01 Ga). The positive petrogenetic parameters ƐHf(t) (+3.9 to +5.2) evidence a mantle source derivation for the Siderian material, while the negative and positive petrogenetic parameters ƐHf(t) (−4.6 to +3.3) indicate a mixture between crustal and juvenile material for the Neo-Mesoarchean material. These results allow us to disregard the proposed correlation between this high-grade metamorphic terrain with the Porangatu-Talismã Granulitic Belt, which occurs to the south of this region, whose thermo-tectonic evolution is related to the late Neoproterozoic. On the other hand, there are records of several other Rhyacian similar granulitic terrains, such as the granulites of the Bacajá and Tartarugal Grande in the Amazonian Craton, the Itabuna-Salvador-Curaçá Granulitic Belt in the São Francisco Craton, as well as in the Bakhuis Belt, in Suriname, and Limpopo Belt in the West African Craton.