Decouple of monazite U-Pb and Th-Pb ages of the Paleo-proterozoic UHT pelitic granulite in Sulu orogen, eastern China

Monazite is a common accessory mineral in metapelites. High-Th monazite has previously been recorded only in granitic-pegmatitic rocks. In-situ studies of metamorphic monazite in the Weihai UHT granulite (Sulu orogen) show them to be strongly zoned in Th content, Th/U ratio and U-Th-Pb age. The pelitic granulites occur as lenses within the orthogneiss. The pelitic granulites have a peak mineral assemblage of garnet + sillimanite + ternary-feldspar + plagioclase (antiperthite) + quartz + rutile ± biotite. Zircons from the granulite have low Th/U ratios (<0.1 mostly), flat HREE patterns with negative Eu anomalies, suggesting a typical of granulite-facies metamorphic zircon. Ti-in-zircon thermometers give very high temperatures ranging from 911 to 954 ℃ with a weighted mean of 928 ± 10 ℃, suggesting an ultrahigh temperature (UHT) metamorphism. These metamorphic zircons yield a concordant ²⁰⁶Pb/²³⁸U age of 1843 ± 17 Ma (MSWD=12, n=26) and an upper intercept age of 1842 ± 15 Ma (MSWD=1.7), suggesting a Paleo-proterozoic UHT granulite-facies metamorphism. The monazites in the granulite generally have zoned structure: gray core and bright rim. Compared with the gray cores (Th = 45429 - 165236 ppm; U = 2227 - 6044 ppm; and Th/U = 18 - 28), the bright rims have high Th (194616 - 285224 ppm) but low U (1145 - 3209 ppm) contents, resulting in very high Th/U ratios (69 - 217). Both the monazite cores and rims have same ²⁰⁶Pb/²³⁸U ages. They yield a concordant ²⁰⁶Pb/²³⁸U age of 1837 ± 9 Ma (MSWD=3.5, n=27) and an upper intercept age of 1846 ± 27 Ma (MSWD=3.0), which is similar to zircon ²⁰⁶Pb/²³⁸U ages. However, the high-Th rims generate anomalously old ²⁰⁸Pb/²³²Th ages ranging from 2280 to 2534 Ma. We suggested that the decouple of the monazite U-Th-Pb age system in the bright rims might be caused by fractionation of Th and U (reflected by an increased in Th/U ratios in rims relative to cores). The high-Th rims could be generated from incongruent breakdown of biotite to produce a melt metasomatism under UHT granulite-facies conditions. Simultaneously, this study provides a possible explanation for some inauthentic age information recorded in the high-grade metamorphic rocks.