The authors are grateful to Prof. R. Brousse for his helpful
discussions. This petrographic and geochemical study was carried
out at the Tohoku University, and the manuscript completed
at the Universit€ de Paris-Sud during the senior author's tenure
of a Fellowship from Ministere des Universites, France. A part
of expenses of this study was defrayed with a Grant-in-Aid for
Special Project Research (Nos. 420901 and 520102) from the
Ministry of Education, Science and Culture of Japan.