[1] Russian Acad Sci, Siberian Div, Joint Inst Geol Geophys & Mineral, Novosibirsk 630090, Russia
[2] Free Univ Brussels, Lab Geochronol, B-1050 Brussels, Belgium
[3] Royal Mus Cent Africa, Dept Geol & Mineral, B-3080 Tervuren, Belgium
[4] Russian Acad Sci, Siberian Div, Inst Earths Crust, Irkutsk 664033, Russia