1. Institute of Geology and Geophysics, Chinese Academy of Sciences, No. 19, Beitucheng Western Road, Chaoyang District, Beijing 100029, China.
2. Corresponding author.
3. College of Earth Sciences, University of Chinese Academy of Sciences, No. 19A, Yuquan Road, Beijing 100049, China.
4. Shenhua Geological Exploration Co. Ltd, No. 22, West Riverside Road, Dongcheng District, Beijing 100011, China.