Geocronologia U-Pb-Hf em zircão e U-Pb em monazita das rochas de alto grau metamórfico do Complexo Porto Nacional, Norte do Maciço de Goiás

Abstract

The Porto Nacional Complex is inserted in the Tocantins Province’s central-north portion, as part of the northern Goiás Massif, which represents a Paleoproterozoic microcontinent remnant of tectonics Brasilian Cycle, which formed the orogens Araguaia, Brasília, including Goiás magmatic arc, and Paraguai. The CPN constitutes a belt of high-grade metamorphic rocks in the NE-SW direction, affected by the Porto Nacional Shear Zone, part of the Transbrasiliano Lineament, and predominantly gathers orthogranulites which composition vary widely (mafic granulites, enderbites, charnoenderbites and charnockites) with minor occurrences of aluminous paragneisses, more or less migmatized tonalitic orthogneisses and Stype granitoids. Petrographic analyzes identified paragenesis defined the following metamorphic paragenesis: Opx + Cpx + Pl ± Hb (mafic granulites); Opx + Cpx + Pl + Qtz ± Mc ± Bt ± Hb (enderbites); Pl ± Mc + Qtz + Bt + Grt ± Sil ± Ky ± Grf (kyanite-sillimanite-garnet gneisses). These data reveal high-grade metamorphism’s performance, under conditions of granulite facies with a metamorphic peak reaching temperature and pressure above 850 ºC and 8 kbar, respectively. For geochronological studies were selected two samples of enderbite and two sillimanite-garnet gneiss where U-Pb-Hf methods in zircon and U-Pb in monazite were applied, respectively. Scanning electron microscopy images by cathodoluminescence of enderbite’s zircon shows two types: (I) long prismatic crystals with irregular contours, homogeneous and without evident zoning; (II) short prismatic crystals with a well-defined inner core, surrounded by an external zone with a lighter tonality, suggesting metamorphic overgrowth. Back-Scattered Electron of paragneisse’s monazite showed homogeneous crystals rounded to rounded, without internal structure. U-Pb geochronological in zircon’s core indicated ages of 2.16 Ga, and the crystal’s rims showed lower values (2.09 Ga), interpreted as zircon crystallization age of tonalitic protolites, and high-grade metamorphism, respectively. In another sample, zircon ages also indicated values in the order of 2.09 Ga, reinforcing the interpretation of being metamorphic zircons. Th/U ratio zircon obtained for zircon crystals vary 0.04 – 0.95 and 0.01 – 1.15, respectively, suggesting metamorphic and igneous origin. Hf-TDM C model ages in zircon crystals analyzed suggest two continental crust formation episodes: a source from Siderian (2.40 to 2.48 Ga) and another from Neo-Mesoarquean (2.52 to 3.01 Ga). Positive (+3.9 to +5.2) ƐHf(t) petrogenetic parameters shows mantle source derivation for Siderian rocks, and negative and positive (4.6 to +3.3) ƐHf(t) petrogenetic parameters shows mixture between crustal and juvenile material derivation for Neo-Mesoarquean rocks. U-Pb geochronological results in paragneisse’s monazite indicates ages of 2.09 Ga and 2.10 Ga, slightly below the age of metamorphic zircons and are interpreted as high-grade metamorphism age that affected the Porto Nacional Complex in x the Palaeoproterozoic. This demonstrates the paticipation of granulitic metamorphism almost contemporary to the magmatic crystallization of protoliths. This chronology is similar to other high-grade metamorphic terrains in Brazil and other countries, such as West African Craton granulites (Limpopo Belt); São Francisco Craton, in Bahia, represented by Itabuna-Salvador- Curaçá Block granulitic belt and in Minas Gerais in the basement of Araçuaí Belt; Bacajá Domains, Amazonian Craton and Amapá Block granulitic strips; and Guiana Shield. The analysis of the geochronological data demonstrates that the formation of the magmatic protoliths and metamorphism happened close chronologically in the Ryacian.