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

Zircons in Zhungashun granodiorites are all euhedral to subhedral in shape with homogeneous oscillatory growth bands and high Th/U ratio (0. 67 ��1. 26)��which suggests their magmatic origin�� LA--ICP--MS zir- con U--Pb dating shows the weighted age of (433 ±2) Ma with MSDW =0. 44��indicating the intrusion formed in Early Silurian�� The rocks have a SiO 2 content variety from 62. 74% to 64. 50%��high CaO (4. 47% ��6. 44%) and Al 2 O 3 (16. 51% ��17. 22%)��low K 2 O (1. 51% ��2. 02%) and Na 2 O (2. 21% ��3. 54%)��belonging to calc-alkaline series�� The rocks are characterized by enrichment in LILE (such as K����b��Ba and Sr) and rela- tively depleting in HFSE (such as Nb��Ta��Hf��Zr and Ti)��and the chondrite-normalized ��EE patterns exhibit a right-dipping type�� Low Mg # values (39 ��43 ��45) and compatible elements like Cr (3. 12 × 10 ^��6 �� 24. 6 × 10^��6 ) and Ni (5. 57 ×10 ^��6 ��11. 6 ×10 ^��6 ) suggest that the primary magma was derived from lower crust�� Combined with the data of igneous rocks of the same age��the authors conclude that the formation of Zhungashun grano- diorites were related to the subduction of the Paleo-Asian oceanic plate��