Silurian granulite-facies metamorphism, and coeval magmatism and crustal growth in the Tongbai orogen, central China

Abstract

It is intriguing whether the formation of granulite in an orogenic belt is related to magmatism and crustal growth. The Tongbai orogen is one of the particular areas in the Qinling–Tongbai–Dabie–Sulu orogen that contains both granulites and associated arc magmatic rocks, and thus is an optimal place to address this issue. However, the formation age and genesis of the granulites and their relations to the magmatic rocks have not yet been well constrained. In situ LA (MC)-ICPMS U–Pb dating, and trace element and Lu–Hf isotope analyses were undertaken on zircon grains from gneiss, granulite, gabbro and granodiorite samples. Most zircons in three granulite samples and a gneiss sample have core–rim structures. The cores have oval or rounded shapes, oscillatory zones, high Th/U ratios and variable U–Pb ages, suggesting a detrital genesis. The youngest group of the detrital zircons yields a weighted mean 206Pb/238U age of 450±5Ma, which is interpreted as the maximum depositional age of their protoliths. A major age cluster is ca. 450–490Ma, corresponding to prolonged arc magmatism in the northern Tongbai and Qinling orogens. Another group of ages range from 660 to 950Ma, implying that the Qinling Group of the Tongbai orogen belongs to the South China Block (SCB). Metamorphic zircons in the granulites have weak or no zoning, low Th/U ratios and REE contents, negative Eu anomalies, and relatively flat HREE patterns, which indicate that they formed contemporaneously with garnet and plagioclase, and thus under granulite-facies metamorphic conditions. Their weighted mean age of 424±4Ma is taken as the best estimated age of the granulite-facies metamorphism. Zircons in a leucosome sample have significant negative Eu anomalies and relatively flat HREE patterns with a formation age of 428±4Ma, indicating partial melting coeval with the granulite-facies metamorphism. Metamorphic zircons in the gneiss show apparently negative Eu anomalies, and variable Th/U ratios and HREE contents. They yield a weighted mean 206Pb/238U age of 438±4Ma, which was suggested to record the timing of the prograde metamorphism. Zircon grains in a gabbro and a granodiorite yield U–Pb ages of 432±4 and 424±4Ma, respectively, which are interpreted as their formation ages and identical to the ages of the granulite-facies metamorphism. They have positive εHf (t) values of up to 5.1, showing clear evidence for crustal growth during their formation. The appearance of the Silurian granulite-facies metamorphism and magmatism in the Tongbai orogen provides a good paradigm of coeval granulite-facies metamorphism, magmatism and crustal growth in an arc-continent collision orogenic belt.