Paleoproterozoic ultrahigh-temperature granulite-facies metamorphism in the Sulu orogen, eastern China: Evidence from zircon and monazite in the pelitic granulite

摘要

Because of their essential information for constraining the timescales of high-temperature processes, zircon and monazite are the two most important minerals for understanding crustal processes. Zircon and monazite are common accessory minerals in pelitic granulites (Sulu orogen, eastern China). The pelitic granulites have a peak mineral assemblage of garnet + sillimanite + antiperthite + plagioclase + 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 temperature estimates ranging from 911 to 954 degrees C with a weighted mean of 928 +/- 10 degrees C, suggesting a ultrahigh temperature (UHT) metamorphism. These metamorphic zircons yield a concordant Pb-206/U-238 age of 1843 +/- 17 Ma (n = 26) and an upper intercept age of 1842 +/- 15 Ma (n = 28), suggesting a Paleoproterozoic UHT metamorphism. The monazites in the pelitic granulite record the same Paleoproterozoic age (ca. 1840 Ma) as the zircons. The monazites generally exhibit a zoned structure in BSE images: gray core and bright rim. Both the cores and rims display significant depletion in HREEs and Y, and negative Eu anomalies, indicating their formation under granulite-facies metamorphic conditions. Notably, the bright rims are more depleted in HREEs and Y, and more negative Eu anomalies than the gray cores. Compared with the gray cores (Th = 45,429-165,236 ppm; U = 2227-6044 ppm; and Th/U = 18-28), the bright rims have high Th (194,616-285,224 ppm) but low U (1145-3209 ppm) contents, resulting in very high Th/U ratios (69-217). These results establish zircon-monazitegarnet- feldspar REE equilibrium and their U-Th-Pb system under UHT metamorphic processes. Paleoproterozoic UHT granulite-facies metamorphism in the Sulu orogen provides a crutial clue to understand its early geodynamic setting and tectonic evolution.