LA-ICP-MS monazite U-Pb age and trace element constraints on the granulite-facies metamorphism in the Tongbai orogen, central China

摘要

Granulite-facies metamorphic rocks in the Tongbai orogen have great implications for the evolution of the whole Qinling-Tongbai-Dabie orogenic belt. However, the age and tectonic setting of the granulite-facies metamorphism have not been well constrained yet. In this contribution, BSE imaging, trace element analysis and U-Pb dating of monazite were undertaken for three felsic granulites, a gneiss and a leucosome from the Tongbai orogen. Most monazite grains from the granulite and gneiss samples exhibit no zoning, relatively low Th/U ratios, significant depletion in HREEs and Y, and negative Eu anomalies, indicating their formation under granulite-facies metamorphic conditions. Their formation ages range from 423 ± 2 to 430 ± 3 Ma with a weighted mean of 426 ± 5, which is taken as the timing of the peak granulite-facies metamorphism. In the leucosome sample, the monazite shows a euhedral to subhedral shape, coupled with a core and rim structure. Besides being depleted in HREEs and Y, both of them have significantly negative Eu anomalies and variable U contents (1220-13323 ppm) and Th/U ratios (7.2-136.7). This suggests that their formation was coeval with the crystallization of feldspar, metamorphic zircon, and garnet. The monazites yield a weighted mean age of 426 ± 3 Ma, suggesting nearly coeval partial melting during granulite-facies metamorphism. The relationship between metamorphic monazite and zircon within the same metamorphic sample is complex as revealed by metamorphic monazite and zircon U-Pb ages for the gneiss, granulite and leucosome samples. In the other two granulite samples, the monazite yields U-Pb ages of 423 ± 2 and 425 ± 3 Ma, whereas only little overgrowth or recrys-tallization of metamorphic zircon. This indicates that monazite is more sensitive than zircon in recording of metamorphic age even at the granulite-facies conditions. Combined with previous metamorphic zircon U-Pb age results, the prograde, peak, and retrograde stages of the granulite-facies metamorphism in the Tongbai orogen can be constrained at ca. 440, 426, and 415 Ma, respectively. Therefore, the granulite-facies metamorphism would have lasted for more than 20 Myr. The prolonged granulite-facies metamorphism might result from the continuous northward subduction of the Paleotethyan oceanic crust beneath the North Qinling terrane.