Provenance analysis and thermal evolution of Upper Triassic Yanchang Formation in the Southern Ordos Basin

The sedimentary sources and thermal evolution of Upper Triassic of Yanchang Formation in Xunyi area of Southern Ordos Basin were studied in this paper. With the method of LA-ICP-MS, the detrital zircons ages and compositions of sandstone samples from the study area were analyzed so as to investigate the sedimentary sources. The results show that there exist six age stages of the Chang 7 detrital zircons, i.e., 225-264 Ma, 276-408 Ma, 413-458 Ma, 727-851 Ma, 1249-1749 Ma and 1868-2469 Ma, corresponding to the tectonic movements of Indo-Chinese, Hercynin, late Caledonian, Rodinia break-up, early-middle Jinning movement and Paleoproterozoic periods. Sedimentary sources are complex in the study area, and they mainly came from the western section of northern Qinling, Qin-Qi orogenic belt, West Qinling, eastern section of Qilian orogenic belt, and Tianshan-Xingmeng orogenic belt. Alxa block and northern area of the basin might have been the sedimentary sources of the study area. Paleo-current statistics indicate that the source direction was close to the southern part of the study area. Apatite fission track analysis and thermal history reconstruction were carried out, and the results confirm that the study area experienced an essential tectonic thermal event at 100 Ma in the early Cretaceous period of Late Mesozoic, which was an important period of oil and gas formation. There existed four relatively evident thermal evolution periods after that:100 to 43 Ma was a slow uplift stage and experienced partially annealing with a cooling rate of around 0.44℃/Ma; 45 to 36 Ma was a rapid uplift stage and the cooling rate came to 4.3℃/Ma; 36 to 24 Ma was another slow uplift stage with the cooling rate about 0.52℃/Ma; 24 to 17 Ma was a rapid uplift stage and the cooling rate came to 1.5℃/Ma. However, the India-Asia collision has influenced the uplift and erosion in the study area since 43 Ma.