Th–U–Pb isotopic systems and internal structures of complex zircons from an enderbite from the Pium Complex, Carajás Province, Brazil: evidence for the ages of granulite facies metamorphism and the protolith of the enderbite

SHRIMP U–Pb micro-analyses are reported for parts of complex zircons from a granulite facies enderbite from the Pium Complex in the Carajas Metallogenic Province of the Amazon Craton of Brazil. Zircons from the granulite consist of two types of grains — (a) those with oscillatory zoned cores surrounded by rims composed of two or more curved, overlapping, nebulously zoned domains distinguished by differences in cathodoluminescence intensity, and (b) grains which are composed entirely of such domains and have no obvious cores. The SHRIMP U–Pb age of 3002±14 Ma, determined for oscillatory-zoned zircon cores is interpreted as dating the parent rock of the enderbite. This age is resolved from the U–Pb age of 2859±9 Ma, determined on zircon rims, which is interpreted as a time point within the evolving granulite facies metamorphism. SHRIMP U–Pb ages of cores and rim-type zircon are generally concordant to slightly discordant where uranium contents are less than about 820 ppm, but once this value is exceeded the zircon becomes strongly discordant suggesting that a threshold value of radiation damage exists above which zircon stability with respect to Pb loss is sharply reduced. The results provide the first substantial age determination of the granulite metamorphism in the Carajas Province. The zircon U–Pb ages have not been influenced by the Transamazonian metamorphism dated at ca. 2.0–1.85 Ga from Ar–Ar and Rb–Sr mineral ages.