SHRIMP zircon U-Pb geochronological and geochemical data of the Early Carboniferous Dahalajunshan Formation: Evidence for the subduction processes in Western Tianshan

From east of Xinyuan County through Duku Highway to Yuximolegai area in the middle of the west Tianshan Mountains, the regional unconformity of Carboniferous Akeshake Group upon Lower Carboniferous Dahalajunshan Formation can be easily founded. This paper reports SHRIMP zircon U-Pb age of 359±2Ma from Dahalajunshan dacitic tuff near the Boerbosong River in Yining County, which constrains the age of the lower boundary of the west part of Dahalajunshan Formation. In combination with previously published results, the authors hold that Dahalajunshan Formation was formed at about 40Ma, with the characteristic that the early deposition appeared in the west and the later deposition occurred in the east. Taking into account all available chronological data published in the past ten years, petrological features, and major and trace elements data of these Carboniferous volcanic rocks, the authors consider that the basic lavas were most plausibly formed by mixing between enriched mantle sources and ocean floor sediments (melt) and modified by subduction zone fluids. Changes in volcanic rock composition, spatial distribution and temporal evolution of Dahalajunshan Formation correspond to features of the Tianshan oceanic subduction-collision process (from west to east), and therefore its disintegration is not necessary.