1. Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94709, USA.
2. Yichang Institute of Geology and Mineral Resources, Post Office Box 502, Yichang 443003, People's Republic of China
3. Department of Geology and Geophysics, University of California, Berkeley, CA 94720, USA.
4. Department of Earth and Environmental Sciences, University of Rochester, Rochester, NY 14627, USA.