LIU C，LIANG T，YANG W T,et al. Zircon U-Pb dating of a volcanic tuff from the lower Taishanmiao Formation in the Nanzhao Basin, North Qinling and its geological significance[J].Journal of Henan Polytechnic University( Natural Science), doi:10.16186/j.cnki. 1673-9787. 2023110026.

Received：2023-11-13

Revised：2024-02-04

Online：2024-04-11

Zircon U-Pb dating of a volcanic tuff from the lower Taishanmiao Formation in the Nanzhao Basin, North Qinling and its geological（Online）

LIU Chao¹，LIANG Tian¹，YANG Wentao¹，CHEN Hao¹，LIU Xiaoxiao¹，AN Haihua¹，LI Xin¹，MA Qiangfen²

（1.School of Resource and Environment，Henan Polytechnic University，Jiaozuo 454000，Henan，China；2.College of Earth Sciences，Guilin University of Technology，Guilin 541004，Guangxi，China）

Abstract: Objectives During the Late Triassic Carnian Period, the global paleoclimate changed significantly, i.e. global precipitation increased, seawater temperature rose by about 6 to 8 °C, and a large amount of terrigenous detrital material led to a crisis of carbonate production in the ocean. This Carnian pluvial event might have been recorded in the Taishanmiao Formation of the Nanzhao Basin, North Qinling. In order to accurately determine the depositional age of the Taishanmiao Formation and highlight its geological significance, Methods laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) was used to conduct U-Pb isotopic dating on zircons from a tuff bed in the lower part of the Taishanmiao Formation. Results The results show that zircons from the tuff are granular and short columnar in shape, without rounding, and their cathodoluminescence images exhibit obvious oscillating ring structure. According to the CL image and Th/U ratios, it is considered that the 18 zircons from the tuff in the lower part of the Taishanmiao Formation are of volcanic origin. After the zircons with high light rare earth element contents were removed, 15 zircons were selected for calculation, and the weighted average age was 241.4 ± 1.3 Ma (1σ; MSWD = 0.27), representing the crystallization age of the zircon in the tuff. Combined with previous studies, the average depositional rate of the lower Taishanmiao Formation was calculated to be 20.6 m/Ma, and the age of the bottom of the Taishanmiao Formation is about 246 Ma. Conclusions Therefore, the initial deposition of the Taishanmiao Formation occurred as early as the Anisian Age (Middle Triassic), and this formation then had a successive sedimentation till the end of the Carnian Age (Late Triassic). According to the evolution of trace fossil and plant pollen assemblage in the Nanzhao Basin, we conclude that the paleoclimate during the deposition of the Taishanmiao Formation in the Nanzhao Basin has undergone a significant change, from a dry and hot climate to a warm and humid climate. This study sheds new light on the chronostratigraphic division of the Triassic System in the Nanzhao Basin and is helpful to investigate the sedimentary response of the basin to the Carnian Pluvial Episode.

Key words：Carnian Pluvial Episode；U-Pb isotopic dating；Triassic；Nanzhao Basin；Taishanmiao Formation；terrestrial deposits；volcanic tuff

CLC: P534.51