Zircon U-Pb dating of a tuff from the lower Taishanmiao Formation in the Nanzhao Basin,Northern Qinling and its geological significance
During the Late Triassic Carnian Period,there were significant global paleoclimate changes,in-cluding increased global precipitation and a rise in seawater temperature by about 6 to 8 ℃.Additionally,a large amount of terrigenous detrital material led to a crisis in carbonate production in the ocean.It is possible that this Carnian pluvial event was recorded in the Taishanmiao Formation of the Nanzhao Basin,Northern Qinling.Objectives To accurately determine the sedimentary age of the Taishanmiao Formation and highlight its geological significance,Methods laser ablation inductively coupled plasma mass spectrometry(LA-ICP-MS)was used for U-Pb isotopic dating analysis on zircons from a tuff bed located in the lower part of the Taishanmiao Formation.Results The results show that zircons from the tuff are granular and short columnar in shape,non-rounded,and exhibit an obvious oscillating ring structure in their cathodoluminescence images.Based on 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 removing zircons with high light rare earth element contents,15 zircons were selected for calculation,and their weighted average age was(241.4±1.3)Ma(1σ;MSWD=0.27),representing the crystallization age of the zircon in the tuff.According to previous studies,the average sedimentary rate of the lower Taishanmiao Formation was calculated to be 20.6 m/Ma,and it is estimated that the bottom of the Taishanmiao Formation has an age of about 246 Ma.Conclusions Therefore,it can be concluded that initial deposition of the Taishanmiao Formation occurred as early as Anisian Age(Middle Triassic),followed by successive sedimentation until end of Carnian Age(Late Triassic).Considering trace fossil evolution and plant pollen assemblage in the Nanzhao Basin during sedimentation of the Tai-shanmiao Formation,significant changes in paleoclimate from a dry and hot climate to a warm and humid one was observed.This study provides new insights into chronostratigraphic division within Triassic System in the Nanzhao Basin while also helping investigate basin's sedimentary response to Carnian Pluvial Episode.
国家科技期刊平台
NSTL
万方数据
