Crustal growth at ∼2.5 Ga in the North China Craton: evidence from whole-rock Nd and zircon Hf isotopes in the Huai'an gneiss terrane

The Huai'an gneiss terrane mainly consists of TTG gneisses and dioritic gneisses. Laser in situ U-Pb dating of magmatic zircon cores indicates that protolith of these gneisses was formed at similar to 2.5 Ga. The TTG gneisses have positive epsilon(Nd)(t) values of 2.7 to 4.3, and most of the magmatic zircons have positive epsilon(Hf)(t) values of 2.0 to 8.3. These positive epsilon(Nd)(t) and epsilon(Hf)(t)values are both similar to those of the contemporaneous depleted mantle at 2.5 Ga. Moreover, the young Hf model ages of 2.44 to 2.73 Ga for the magmatic zircons are close to the timing of the zircons growth. The whole-rock epsilon(Nd)(t) values are lower in the dioritic gneisses (0.8 to 1.7) than in the TTG gneisses due to the involvement of ancient crust in its source. However, many magmatic zircons from the dioritic gneisses have similar epsilon(Hf)(t) values (2.0 to 7.9) to that of the coeval depleted mantle; their Hf model ages of 2.49 to 2.75 Ga are close to the U-Pb ages of zircons. The highest epsilon(Hf)(t) values are close to the value of the depleted mantle, and the relatively high epsilon(Hf)(t) values corresponds to the relatively young Hf model age. These Nd and Hf isotope features suggest that these two types of gneisses of the Huai'an gneiss terrane originated from the juvenile crust at ca. 2.5 Ga.