Zircon U–Pb Dating of Tuff Layers in the Middle–Upper Triassic Strata of Successions and Sedimentation of Chang 7 Member in the Ordos Basin, China

During the Late Triassic, the southern margin of the Ordos Basin was influenced by the Indosinian Movement, in response to closure of the Paleo-Tethys Ocean. Thinly bedded tuff layers are exposed near the base of the Middle–Upper Triassic formations (Chang 8/7/6 members) in the Ordos Basin. Tuff samples from drilling core (well An-35) and outcrop (Motiangou section) samples in the Yanchang Formation were studied by the detailed field work. Igneous zircon crystals were picked and analyzed by the instruments of cathodoluminescence and laser ablation inductively coupled plasma mass spectrometery (LA-ICP-MS). The timing of volcanic activity was determined by means of high-precision zircon age. Combining tectonic setting and paleogeographic reconstruct, the ages of tuff layers indicate that volcanic activities occurred at three peaks (241.6 ± 1.1 Ma, 240.2 ± 1.1 Ma, and 233.5 ± 1.2 Ma) from the top of Chang 8 Member to the Chang 7 Member. The results implied that the volcanic activities lasted from Ladinian to Carnian, as well as define the sedimentary duration of the Chang 7 Member with a deposition rate of 1.34 cm/ka.