LA-ICP-MS zircon U-Pb age and tectonic background of the Dahalajunshan Formation volcanic rocks in Wusunshan area, West Tianshan Mountains

Volcanic rocks in the Dahalajunshan Formation of Wusunshan area in West Tianshan Mountains are composed of basalt, andesite, dacite, rhyolite and relevant volcaniclastic rocks, with andesite and rhyolite being dominant. The LA-ICP-MS zircon dating results show that adesite and andestic tuff lava in the Dahalajunshan Formation from Wusunshan area yield concordant 206Pb/238U ages of 353.9±6.5Ma and 356.3±4.4Ma, implying that they were erupted at the early stage of Early Carboniferous. Regional correlation shows that the volcanic magmatism gradually became younger from the north and the south edge of Yili-Central Tianshan plate to Yili basin, and the eruption time span is small (about 40Ma). Therefore, the age of Dahalajunshan Formation at the south edge of Yili block should be confined between 363Ma and 326Ma, namely Late Devonian to Early Carboniferous and mainly in Early Carboniferous Tournaisian period-Visean period. The geochemical characteristics show that the volcanic rocks belong to the calc-alkaline series, enriched in LREE and depleted in HREE relatively. The primitive mantle -normalized trace element patterns show that intermediate volcanic rocks are slightly enriched in large-ion-lithophile elements(LILE) such as Cs, Rb, Th and U and relatively depleted in HFSE and have obvious negative Nb, Ta, Ti anomalies, and that acid rocks are characterized by Rb, Th, U, Ta enrichment and Ba, Sr, P, Eu, Ti depletion. In combination with the south edge of Central Tianshan tectonic evolution, the characteristics of volcanic rocks in Dahalajunshan Formation suggest that they were probably formed in an active continental margin environment.