Yong Wng, Yong-hong Shi , Bi-lin Chen Ren-wen Tn , Yun Go , Jing-hui Shen

a Laboratory of Dynamic Digenesis and Metallogenesis, Institute of Geomechanics, Chinese Academy of Geological Sciences, Beijing 100081, China

b China University of Geoscience (Beijing), Beijing 100083, China

c China University of Geoscience, Wuhan 430074, China

1. Objective

The formation of East Gondwana and its combination with West Gondwana is a result of a series of complex orogenic events, known as Pan-African orogeny. The Northern Qinling Mountain is a microcontinent that formed on the ancient oceanic crust in the northern of Yangtze craton before Neoproterozoic. It is likely that the North Qinling,even the Yangtze craton and its surrounding oceanic crust belong to the ancient Gondwana tectonic domain and be a part of East Gondwana supercontinent (Chen Q, 2007). Therefore,it was believed that there were subduction collisions and convergences towards Gondwana continent (Australia) during the late Pan-African orogeny, and the records should be existed. In fact, the Qinling Orogenic Belt, even the Central Orogenic Belt are supposed to be superimposed composite orogenic systems, and most of tectonic and magma records of the Pan-African orogenic movement have been modified by the later tectonic movements, with very few preserved.Kroner A (1993) believed that the period of main convergence stage was 600-550 Ma. Gao JM et al. (2012) reported the age of the Liushuigou igneous complex is 548 Ma which indicate that there existed a “Pan-African orogeny ” tectonothermal event in the Central Orogenic System, however, it was little younger than main convergence stage. But the age data in Qinling Mountain is considered to be the main convergence stage of Pan-African orogeny is few, which heavily restricts the global comparative study of Pan-African events.

2. Methods

The study samples were collected in Qinling Group of North Qinling Mountain area, Fengxian County (Fig. 1a). The lithology was acid pyroclastic rocks with weak alteration and deformation. After zircon chosen from these samples, U-Pb age analysis were conducted by LA-ICP-MS at the State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry Chinese Academy of Sciences. The offline processing of the analytical data is completed by the software ICPMSDataCal (Liu YS et al., 2008, 2010).

3. Results

The U-Pb age analysis results of zircon are shown in Table 1.Most of the zircons were euhedral with grain sizes ranging from 30-80 μm and length/width ratios ranging from 1:1 to 3:1. Cathodeluminescence images showed that most zircons had internal oscillating bands (Fig. 1c). The Th/U value was 0.43-1.05, with an average value of 0.65, indicating the magma zircons characteristics. Nearly all the analysis points were located on the concordant line and no lead was lost. The206Pb/238U concordia age of the 15 points was (572 ± 3) Ma(MSWD = 0.36) (Fig. 1b), which represented the magmatic age.

Table 1. Zircon U-Pb age of acid pyroclastic rocks from Fengxian County in the West Qinling, China.

Fig. 1. Geotectonic position (a) and regional geological map of Fengxian area on the West Qinling (b); Cathodoluminescence images (c) and concordia diagram of some acid pyroclastic rocks (d).

4. Conclusions

(i) The formation age of acid pyroclastic rocks in the Qinling Group, Fengxian County was 572 Ma, geochemical characteristics showed that all zircons are magmatic origin which recorded the magmatic activities during the main convergence collision of the Pan-African orogenic movement in late Neoproterozoic.

(ii) The research provides a significant evidence for the existence of Pan-African orogenic movement in the Qinling Orogenic Belt, which also provides important information for the study of the tectonic evolution of the Central Orogenic Belt and the reconstruction of the ancient continent.

5. Acknowledgements

This work was financially supported by the Geological Survey Project of China Geological Survey (DD20190161 and DD20160053), Fundamental Research Funds of central welfare research institutes, China Academy of Geological Sciences (JYYWF20183702 and JYYWF20180602).