Abstract:

 The “Lagged arc volcanic rocks” distributed in Urho area in the south of Hala’alat mountain of West Junggar. It is composed of Urho dyke swarms and Baiyanghe basalt, which constitute a complete continental volcanic apparatus and is angular unconformably overlying on the Jiamuhe molasse formation of Lower Permian system. The dyke swarms are composed of diabase, diabase porphyrite and gabbrodiorite. The LA ICP MS zircon U Pb ages of them are 283.6±2 Ma (MSWD=0.57, 95% conf.), 286.3±4.0 Ma (MSWD=0.32, 95% conf.) and 286.5±5.6 Ma (MSWD=0.3, 95% conf.). The LA ICP MS zircon U Pb age of Baiyanghe basalt is 283.7±3.1 Ma (MSWD=0.45, 95% conf.). All of these are in the late stage of the Early Permian. Urho dyke swarms and Baiyanghe basalt both have similar geochemical properties which are related to subduction of island arc, showing the features of middlelow K calcalkaline basalt to andesite series and enrichment of LREE. The anomaly of δEu is not obvious. All samples are characterized by the enrichment of LILE (K, Ba, Sr), the depletion of HFSE (Nb, Ta, Zr, Hf), positive εNd(t) values and relatively low initial Sr isotopic ratios, indicating that the source of the volcanic rocks is the enriched mantle. “Lagged arc volcanic rocks” occurred after the closure of ocean basin and the disappearance of trench arc basin system in surface, and later than postorogenic sedimentary record, suggesting that the final completion of the subduction in Western Junggar was in the late Early Permian. The outcrops of “lagged arc volcanic rocks” distributed along a belt, which may be the joint position of Siberia plate and TarimTianshan plate.

Key words: lagged arc volcanic rocks, dyke swarms, basalt, Early Permian, West Junggar