PB-PB ZIRCON AGES OF THE PORTO NACIONAL HIGH-GRADE METAMORPHIC TERRAIN, NORTHERN PORTION OF THE GOIÁS MASSIF, CENTRAL BRAZIL

Resumo

Single zircon Pb-evaporation ages were determined for a mafic granulite, two enderbites and a kinzigite of the Porto Nacional High-Grade Metamorphic Terrain (PNHGT) in the Goiás Massif. Zircons from mafic granulites and one of the enderbites yielded average ²⁰⁷Pb/²⁰⁶Pb ages of 2125 ± 3 Ma and 2153 ± 1 Ma, respectively, being interpreted as minimum ages of the igneous protoliths. The other enderbite, whose zircons present round terminations, yielded an average ²⁰⁷Pb/²⁰⁶Pb age of 2097 ± 2 Ma. Zircons from the kinzigite, displaying cores and rims overgrowth, presented ²⁰⁷Pb/²⁰⁶Pb ages ranging between 2072 and 2115 Ma. However, a very homogeneous zircon crystal without overgrowth yielded an age of 2100 ± 2 Ma, with a distinctive low Th/U value (0.02). This zircon is interpreted as a metamorphic crystal and its age probably sets the age of the granulite facies metamorphism, indicating that the PNHGT is a result of the Tranzamazonian thermotectonic event.