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

Abstract

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±3Ma 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 (1220–13323ppm) 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±3Ma, 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±3Ma, whereas only little overgrowth or recrystallization 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 415Ma, respectively. Therefore, the granulite-facies metamorphism would have lasted for more than 20Myr. The prolonged granulite-facies metamorphism might result from the continuous northward subduction of the Paleotethyan oceanic crust beneath the North Qinling terrane.