262 / 2019-07-01 00:55:24

Sedimentary and Geochronological studies on tuff from Triassic strata in Nanzhao Basin, North Qinling orogen belt, and its indication for prototype basin study of Ordos Basin in Middle-Late Triassic

Geological study, Geochemistry, Zircon U-Pb age, Hf isotopic chronology, tuff, Taishanmiao Formation, Taizishan Formation, Nanzhao Basin, Ordos Basin

Ordos Basin is the second largest oil and gas bearing sedimentary basin in China, formed at Triassic. But where margin of the basin located, especially eastern and southern margin, is still uncertain during this time. Triassic Yanchang Formation extensively outcrop in the Ordos Basin, and its distribution can give the significant constraint of the basin range. In this study, we focus on the measurement and correlation of the Middle-Late Triassic sedimentary strata from Nanzhao Basin, which located on the southeastern part of present Ordos Basin. The results indicate that the sedimentary sequence of Triassic strata Taizishan Formation and Taishanmiao Formation in Nanzhao Basin is comparable with Yanchang Formation. Then,
we sample the tuff intercalation in clastic rocks from Taizishan Formation in Nanzhao basin and carry out zircon chronology and Hf isotope analysis. The results show that the rhyolitic tuff has concord U-Pb age of 234.6±1.5 Ma, which is the boundary age of Middle Triassic and Late Triassic，and has the same deposition age with Yanchang Formation. The TDM2 values for the samples range from 1.0 to 1.4 Ga with the peak at 1.2 Ga, that is also similar with tuffs in Yanchang Formation, but the corresponding εHf(t) values of tuff in Nanzhao Basin, that ranging from -1.1 to +4.2 with average of 1.2 and peak at 2 are different with Yanchang Formation. According to our geochemical data, the tuffs in Nanzhao Basin might be mainly derived from depleted-mantle source in Late Meso-proterozoic and contaminated by continental crust in Early-Middle Meso-proterozoic. Therefore we regard that in Middle-Late Triassic, the southeastern margin of Ordos Basin may had extended to Nanzhao area and the slightly difference of εHf(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 in Triassic.