Discovery of the Permian volcanic rocks in the Bingdaban-Houxia, Tianshan Mountains, and its geological significance

For many years there has been much controversy over the stratigraphic division of the Paleozoic in the Bingdaban-Houxia area, North Tianshan. Through the 1∶50000 regional geological mapping, a suite of volcanic rocks was discovered in the original Upper Carboniferous Qirgustaw Formation. Intensive study of the rock association, isotope ages, basal conglomerate and sedimentary environment indicates that this suite of volcanic rocks belong to the Middle Permian Arbasay Formation, being an association of continental purplish red intermediate-basic-acid volcanic rocks. It consists predominantly of andesite, dacite and rhyolite and breccia, and in addition, breccia, brecciated lava, volcanic agglomerate and volcanic breccia are often seen. It has an unconformable contact with both the underlying Houxia Formation and the overlying Triassic Xiaquangou Group. The single-zircon SHRIMP U-Pb dating of the volcanic rocks yielded an age of 272±4 Ma, indicating a Middle Permian age. The volcanic rocks formed in a tectonic environment of deep thermal collapse eruption in the Early-Mid Permian late-orogenic phase. This study offers an important basis for the correlation of the stratigraphic units and tectonic division of the East and West Tianshan.