Crustal anatexis in the Tongbai orogen: Evidence from LA-ICP-MS zircon U-Pb age of migmatite

The Tongbai orogen is a key region to investigate the evolution of the Qingling-Tongbai-Dabie-Sulu metamorphic belt. However, the age of the crustal anatexis in the Tongbai high-grade metamorphic complex has not been well constrained, and thus hinders our understanding of the tectonic evolution of this area. Three migmatite samples from the Tongbai complex, including one melanosome and two leucosomes, were selected for LA-ICPMS zircon U-Pb dating. Zircon from the melanosome defines upper and lower intercept U-Pb ages of 859 +/- 73Ma and 135 +/- 250Ma, respectively. Six spot analyses near the upper intercept yield a weighted mean (206)Pb/(238)U age of 828 +/- 7Ma (MSWD = 0. 57). This age consists with the upper intercept age, interpreting as the age of the protolith crystallization. The protolith formation age might correspond to magmatism during the Mid-Neoproterozoic in the northern margin of the South China Block. On the other hand, new growth zircons in the leucosome samples show planar or weakly oscillatory zoning, low Th/U ratios. The morphological and internal structure features of zircons suggest that their growths were associated with partial melting. They yielded (206)Pb/(238)U weighted mean ages of 135Ma and 131Ma, which were interpreted to register the ages of migmatization. The timing of the anatexis in the Tongbai high-grade metamorphic complex is coeval with the widespread post-collision magmatism in the Tongbai-Dabie orogen. It suggests that the onset of extensional tectonism in the Tongbai orogen is no later than 135Ma.