Sedimentary and geochronological studies on tuff levels from Triassic strata in the Nanzhao Basin, North Qinling Orogenic Belt, and their geological significance

摘要

The Ordos Basin is the second largest oil- and gas-bearing sedimentary basin in China. This basin formed during the Triassic. At present, the location of the basin margins is still highly debated in the literature, especially its eastern and southern margins. The Triassic Yanchang Formation extensively outcrops in the Ordos Basin, and its distribution can provide a significant constraint on the basin extension. In this study, we focus on the measurement and correlation of the Middle-Late Triassic sedimentary strata from Nanzhao Basin, which is located to the south-east of the present-day Ordos Basin. The results of this study indicate that the sedimentary sequence of Triassic strata from Taizishan Formation and Taishanmiao Formation in the Nanzhao Basin is comparable with that of the Yanchang Formation in the Ordos Basin. Tuff intercalations in clastic rocks from Taizishan Formation in the Nanzhao Basin were studied with zircon chronology and Hf isotope analysis. The results show that the tuff sample has concord U-Pb age of 234.6 +/- 1.5 Ma, which is considered as the boundary age between Middle and Late Triassic with a similar age in respect to that of the Yanchang Formation. The T(DM2)values for the studied sample range from 1.0 to 1.4 Ga with the peak at 1.2 Ga, which is also similar to T(DM2)values for tuffs encountered in the Yanchang Formation. On the other hand, the corresponding epsilon(Hf)(t) values of tuff in Nanzhao Basin range from -1.1 to +4.2 with an average of 1.2 and peak at 2, which are different with respect to those of the Yanchang Formation. According to the geochronological data, the tuffs in Nanzhao Basin might be mainly derived from a depleted-mantle source from Late Meso-Proterozoic period, and contaminated by a continental Early-Middle Meso-Proterozoic crust. Therefore, we suggest that in Middle-Late Triassic, the southeastern margin of the Ordos Basin may have extended to Nanzhao area and the different epsilon(Hf)(t) values between tuffs in Nanzhao Basin and present Ordos Basin may be the result of scissor-type collision of North China Block and South China Block during the Triassic.