Geochronology, Potential Source and Regional Implications of Tuff Intervals in Chang-7 Member of Yanchang Formation, South of Ordos Basin

Key words: 
• Ordos Basin /
• Yanchang Formation /
• tuff /
• U-Pb zircon dating /
• Qinling orogenic belt

Abstract: Tuff intervals of Yangchang Formation are widespread in the south of Ordos basin, most of them are well documented at the bottom of Chang 7 segment. The geochronology and volcanic matters of tuff intervals have great significance on the formation of high quality source rock, the development time of hydrocarbon-rich sag and the occurrence of synchronous tectonic-magmatic events. This paper focuses on the tuff intervals, which are located at the bottom of Chang 7 segment in Hejiafang outcrop section, southern Ordos basin. Firstly, thin sections, scanning electron microscopy (SEM) and major and trace elements analysis methods were used to identify the petrographic and geochemical characteristics of the tuff. Then, zircon U-Pb dating method was used to study the geochronology of the tuff intervals as well as the sandstone at the same segment. The results indicate that the tuff has the characteristics of intermediate-acidic volcanic rocks with strong alteration. The weighted average ages of the dominant component in the two tuff samples are 226.5 ± 1.6 Ma (n = 24) and 229.7 ± 2.2 Ma (n = 21), respectively, while the youngest group ages of detrital zircons range from 235 ~ 243 Ma (n = 7, the weighted average age is 239.3 Ma). Combined with previous study results, it is concluded that the age of tuff intervals at the bottom of chang7 segment is around 230 Ma±, which can represent the peak period of Ordos basin's evolution and the beginning development of hydrocarbon-rich sag in Yanchang Formation. According to the characteristics of tuff intervals, the syntectonic events of Ordos Basin and Qinling Orogenic belt over this period, it can be deduced that the volcanic matters of tuff invtervals mainly derived from the Qinling area, related closely to the volcanic arc belt caused by the subduction of Mianlue oceanic crust to the south Qinling plate. The coupling process between the Qinling orogenic belt and the southern Ordos basin can be divided into three periods. The results have important indications to re-recognize the evolution of the Ordos Basin and the basin property at Chang 7 period.