Age assignment of the upper Carboniferous Arbasay Formation in Shichang Region, North Tianshan (NW China)

Abstract

Abstract The North Tianshan Orogenic Belt contains the youngest ophiolites in the Tianshan and provides some information on timing of the last closure of the Junggar-Balkhash Ocean. LA–ICP–MS zircon U–Pb dating was conducted to define the formation age of the Arbasay Formation in the Shichang Region of North Tianshan, which is exposed near the suture zone but its age remains debated. The Arbasay Formation is mainly composed of volcanic and volcaniclastic rocks with tuff interlayers. The zircons from the tuffs yield two age populations of 315 ± 3 Ma and 304 ± 2 Ma, constraining the commencement and demise timings of volcanism, respectively. Furthermore, zircon U–Pb age spectra of the tuffaceous sandstones display the youngest peak age at 308 Ma, indicating a < 308 Ma age for the depositional age of volcaniclastic rocks. The volcaniclastic rocks therefore were likely to deposit together with the syn-sedimentary volcanism during Late Carboniferous. This means that the Arbasay Formation in Shichang Region should be re-assigned to Late Carboniferous in age. Given that the Arbasay Formation was likely to be formed during the tectonic transition from compression to extension, the Junggar-Balkhash Ocean possibly closed during Late Carboniferous.

Keywords

• Xinjiang
• LA–ICP–MS zircon U–Pb dating
• Stratigraphic age
• Volcaniclastic rock
• Junggar-Balkhash Ocean