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利用锆石SHRIMP U-Pb定年方法,对甘肃靖远王家山地区南营儿群底部新发现的凝灰岩进行了锆石U-Pb定年分析,获得一组206Pb/238U加权平均年龄为(244.9±2.8)Ma。结合凝灰岩锆石阴极发光特征和锆石Th/U比值,认为该组年龄代表了凝灰岩的沉积年龄。这一高精度、高可置信度的研究成果充分说明南营儿群的底部存在中三叠世早期地层,建议将南营儿群底部的凝灰岩及其以下地层划归中三叠统丁家窑组。同时,也证实靖远王家山凝灰岩与宝积山凝灰岩为同一期火山喷发的沉积产物,该凝灰岩层可以作为区域地层划分和对比的标志层。上述研究成果对河西走廊地区中、上三叠统的划分与对比以及最终建立北方陆相三叠纪年代地层对比格架,都有一定的地层学意义。
U-Pb dating of newly discovered tuffs at the bottom of the Nanyingn group in the Wangjiashan area of Jingyuan, Gansu Province was carried out using zircon SHRIMP U-Pb dating. A set of 206Pb / 238U weighted average ages (244.9 ± 2.8) Ma. Combined with the cathodoluminescence characteristics of zircons and the Th / U ratio of zircons, the age of the tuffs is considered to represent the age of tuff deposition. This high-precision and high-confidence research fully shows that there are early Middle Triassic strata at the bottom of the Nanyang Clique. It is suggested that tuffs at the bottom of the Nanying Group and their lower strata should be assigned to the Middle Triassic Dingjia Kiln group. At the same time, it is also confirmed that the Jingyuan Wangjiashan Tuff and Bajianshan Tuff are the depositional products of the same volcanic eruption, which can be used as a marker layer for regional stratigraphic division and contrast. The above research results have certain stratigraphic significance for the division and contrast of middle and upper Triassic in the Hexi Corridor region and the eventual establishment of the continental shelf Triassic stratigraphic contrast framework.