Nd isotopic disequilibrium between minerals and Rb-Sr age of the secondaryphengite in eclogite from the Yangkou area, Qingdao, eastern China

There are two generations of white micas in retrograded coesite-bearing ecl ogite from the Yangkou area near Qingdao, eastern China. The secondary phen gite developed along the folliations in eclogite is the majority of the whi te micas. Nd and Sr isotopic disequilibriums between garnet and retrograded omphacite as well as secondary phengite have been observed. Consequently, the Rb-Sr ages ((193 +/- 4) Ma-(195 +/- 4) Ma) given by the tie lines of th e secondary phengite + garnet or whole rock may predate the formation time of the phengite. The Rb-Sr age of (183 +/- 4) Ma given by the secondary phe ngite + retrograde omphacite is much closer to the formation time of the ph engite indicating the retrograde age of eclogite instead of a cooling age o f eclogite at 500 degreesC.