Abstract: The volcanics of the Upper-Silurian Bayinbuluke Formation in the northern part of the Southern Tianshan are mostly intermidiate-basic in composition, are associated with gabbro and contain abundant pyroclastic rocks. The Zr/Nb values (26~47, averaged 37), Hf/Ta values (11.9~19.9, averaged 14.8) of the volcanic-magmatic complexes indicate that, similar to the N-MORB, they have a depleted mantle derivation. Their (La/Yb)_N values range from 2.92 to 5.21) and Nb/La values from 0.22 to 0.33, averaged 0.29, showing that they are enriched in LREEs and LILEs and depleted in HFSEs(especially with apparent negative Nb and Ta abnormalies), features similar to those of the island arc magmatic rocks and an apparent indication of formation in island arc setting. In space, the Bayinbuluke Formation is near an ophiolitic mélange belt on the southern margin of the Central Tianshan; in time, these strata almost have similar age as the subducting age of the Southern Tianshan Ocean. Thus, it stands a good chance that the formation of these volcanicmagmatic complexes owes to late-Silurian subduction of the Southern Tianshan Ocean.

Key words: volcanic-magmatic complex, tectonic setting, Silurian, Bayinbuluke, Southern Tian-shan