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

Abstract

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 ∼2.5 Ga. The TTG gneisses have positive eNd(t) values of 2.7 to 4.3, and most of the magmatic zircons have positive eHf(t) values of 2.0 to 8.3. These positive eNd(t) and eHf(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 eNd(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 eHf(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 eHf(t) values are close to the value of the depleted mantle, and the relatively high eHf(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.