Geochemical Characteristics and Tectonic Implications of Wutongjin Granitic Gneisses in the Mazongshan Area, Gansu Province

The Wutongjin granitic gneisses is exposed in the Beishan area, Gansu Province. The Wutongjin granitic gneisses arem etalum in ous(Al₂O₃=11.45%～16.07%) and characterized by high SiO₂ (61.03%～75.28%),Na₂O+K₂O (6.74%～8.24%) and TFeO/MgO radios, and high REE(102.17×10^-6～372.17×10^-6), Y, Nb, and low CaO (0.13%～5.53%)contents. The characteristics are obviously different from those observed from I-and S-type granites, but quite similar to A2 type granites. The Wutongjin granitic gneisses has given a U-Pb isochron age of 558 Ma. Therefore, it is suggested that a crustal rifting eventmight have started in the Beishan a readuring Neoproterozoic.