Th-U-Pb isotopic systems and internal structures of complex zircons from an enderbite from the Plum Complex, Carajas 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 Plum Complex in the Carajas Metallogen ic Province of the Amazon Craton of Brazil. Zircons from the granulite cons ist of two types of grains - (a) those with oscillatory zoned cores surroun ded by rims composed of two or more curved, overlapping, nebulously zoned d omains distinguished by differences in cathodoluminescence intensity, and ( b) grains which are composed entirely of such domains and have no obvious c ores. The SHRIMP U-Pb age of 3002 +/- 14 Ma, determined for oscillatory-zon ed 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 zirc on 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 gen erally concordant to slightly discordant where uranium contents are less th an about 820 ppm, but once this value is exceeded the zircon becomes strong ly discordant suggesting that a threshold value of radiation damage exists above which zircon stability with respect to Pb loss is sharply reduced. Th e results provide the first substantial age determination of the granulite metamorphism in the Carajas Province. The zircon U-Pb ages have not been in fluenced by the Transamazonian metamorphism dated at ca. 2.0-1.85 Ga from A r-Ar and Rb-Sr mineral ages. (C) 2000 Elsevier Science B.V. All rights rese rved.