Angular unconformity at the bottom of the Lower Carboniferous Dahalajunshan Formation: constraints on the Palaeozoic ocean-continent transition in the Tianshan Mountains

The volcanic-sedimentary rocks from the Lower Carboniferous Dahalajunshan Formation are widely exposed in the western Tianshan Mountains. There is a wide regional angular unconformity between the Dahalajunshan Formation and its underlying strata, i. e., the Precambrian crystalline basement or preCarboniferous folded basement. The regional angular unconformity represents a strong fold and uplift orogenic event judged from the angular unconformities and the alluvial-proluvial clastic rocks or continental volcanic rocks at the bottom of the Lower Carboniferous Dahalajunshan Formation. The stratagraphic sequences of initial deposits overlying the unconformity surface represent the onset of a new round of sedimentary cycles in the Carboniferous post-collision rift basin. The interbedded andesite samples collected from the bottom of the Lower Carboniferous Dahalajunshan Formation gives a LA-ICP-MS zircon U-Pb age of 359±2.3 Ma, which not only defines the age of the above-mentioneol regional angular unconformity, but also represents an initial age of the Tianshan post-collision rift basin. Therefore, we can draw the conclusion that the time duration of the Palaeozoic ocean-continent transition should be limited at the boundary of the Late Devonian to the Early Carboniferous, followed by the post-collision rift evolutionary stage of the Tianshan orogenic belt.