LIU Y, SHI Y S, SU J H, et al． Analysis of provenance and tectonic setting of the Middle Triassic series in the Tongchuan Area at the southern margin of the Ordos Basin [J]. Journal of Henan Polytechnic University( Natural Science) ， doi: 10.16186/j.cnki.1673-9787. 2025060009.

doi: 10.16186/j.cnki.1673-9787. 2025060009

Received： 2025-06-05

Revised： 2025-08-13

Online：2026-01-22

Analysis of provenance and tectonic setting of the Middle Triassic series in the Tongchuan Area at the southern margin of the Ordos Basin (Online)

Liu Yang1,2，Shi Yongsheng3，Su Jiahao1,2， Liu Hao1,2， Zhao Haibo1,2，Sun Jiaopeng4

1. Shenyang Research Institute Co., Ltd., China Coal Technology and Engineering Group , Shenyang-Fushun Demonstration Zone 113122，Liaoning，China; 2. No.107 Geological Team, Chongqing Bureau of Geological and Mineral Resource Exploration and Development , Chongqing 401120, China; 3. National Key Laboratory of Coal Mine Disaster Prevention and Control, Shenyang-Fushun Demonstration Zone 113122，Liaoning，China; 4. National Key Laboratory of Continental Evolution and Early Life, Department of Geology, Northwest University, Xi’an 710069, Shaanxi，China

Abstract: Objectives When was the Qinling Orogen uplifted during Triassic, a pivotal issue that was directly related to the changing in provenance and tectonic setting of the southern Ordos, remains controversial. During the Triassic, the Ordos Basin of the western North China Block bridged the Paleo-Asian Ocean to the north and the Paleo-Tethys Mianlue Ocean to the south (present coordinates). Thick accumulation of fluvial-lacustrine sediments within the Ordos Basin recorded detailed evolution clues of the ocean closure and resulted continental margin tectonic uplift. Methods We conducted field geological investigation in sedimentology on the Chang 9 member of the Yanchang Formation at the Qishui River section in Tongchuan, as well as sandstone petrology, zircon geochemistry and U-Pb geochronology. Results Field based sedimentological study revealed that the Chang 9 Member is composed of delta facies, with paleoflow direction mostly towards the south. Petrologic observations show that the Chang 9 Member sandstones display averaged Q/F/Lt ratio of 33/47/11, being clarified to as lithic arkose. Lithic fragments are predominantly sedimentary lithic fragments (averaging 8%, primarily composed of sandstone and carbonate rocks), followed by a few of metamorphic lithic fragments (averaging 1%, dominated by polycrystalline quartz) and magmatic lithic fragments (averaging 2%, mainly acidic volcanic rocks). Provenance discriminations based on the Dickinson discriminations indicate a mixed source involving detritus from magmatic arcs and ancient basement terrains. Detrital zircon ages are categized into 2 distinct groups, an older Precambrian constitution concentrated into two distinct groups of 1800~2000 Ma and 2300~2600 Ma as well as a younger Paleozoic one of 480~200 Ma, of which 2 age peaks at 430 Ma and 270 Ma can be defined, along with paucity of 1600~500 Ma age components. New data including paleocurrent, sandstone petrology, and detrital zircon ages refer to a sediment derivation from the Neimenggu continental arc to the north, rather than a southerly provenance from the Qinling. Zircon trace element data for Triassic detrital zircons further pinpoint that the Chang 9 Member was deposited in an arc associated tectonic setting. Crustal thickness reconstruction results indicate that the Neimenggu continental arc rapidly thickened and uplifted during Middle Triassic. Conclusions In combination with known geologic data, we propose that the eastern Qinling has not yet been intensely uplifted until ca. 240 Ma during the deposition of the Chang 9 Member, in contrast to the continental arc in the north that has been intensely uplifted and fed a large volume of detrital materials to the interior Ordos Basin.

Key words: Southern Ordos; Middle Triassic; Chang 9 Member; provenance study; uplift of the Neimenggu continental arc