College of Earth Sciences��Jilin University��Changchun 130061��China

Mesozoic volcanic rocks in southeastern Mohe Basin include those from the Guanghua Formation and Ganhe Formation�� Zircon U- - Pb ages indicate that they were formed in 128 ��125 Ma and 126 ��120 Ma��re- spectively�� Guanghua Formation corresponding to rhyolite in chemical composition��is rich in silicon and alkalis�� poor in calcium��ferrum and magnesium��relatively significant fractionation of H��EE and L��EE �� w(La/Yb) N = 13. 01 ��17. 99��and middle Eu anoalies (δEu = 0. 49 �� 0. 67)��LILE (��b��K��Th��U) and depleted in HSFE (Nb��Ta��P��Ti); The ε Hf (t) values range from ��2. 5 to 3. 0��and the corresponding t DM2 vary from 989 Ma to 1 130 Ma�� The magma could be derived from partial melting of crustal material during late Mesoproterozoic�� The Ganhe Formation corresponding to trachyandesites and basaltic trachyandesites in chemical composition��is charac- terized by high potassium��rich alkali and high M values (Mg # ≈48)��enriched in L��EE �� w(La/Yb) N =12. 67 �� 23. 21��with weak Eu anoalies (δEu =0. 77 ��0. 91)��depleted in HFSE (Nb��Ta��P��Ti) and relatively rich in LILE (��b��Ba��K��Th��U); The ε Hf (t) values range from ��1. 2 to 4. 1�� The geochemical data suggest that the magma could be derived from partial melting of lithosphere mantle metasomatized by subduction fluids��and proba- bly experienced a degree of fractional crystallization��