EXCESS ARGON IN PHENGITE FROM ECLOGITE - EVIDENCE FROM DATING OF ECLOGITE MINERALS BY SM-ND, RB-SR AND AR-40 AR-39 METHODS/

The existence of excess Ar in phengites from coesite-bearing ultrahigh -pressure eclogites of the Dabie Mountains and Su-Lu terrain, China, h as been confirmed by dating of minerals using Sm-Nd, Rb-Sr and Ar-40/A r-39 methods. Ar-40/Ar-39 ages of phengites for two samples studied ar e much older than the Sm-Nd and Rb-Sr mineral isochron ages: sample QL -1 has Rb-Sr age of 219.5 +/- 0.5 Ma, Sm-Nd age of 226.3 +/- 4.5 Ma an d phengite Ar-40/ 39 Ar age of 877.5 +/- 15.9 Ma; the corresponding ag es for sample ZB-4 are: 223.9 +/- 0.9 Ma (Rb-Sr), 228.4 +/- 6.0 Ma (Sm -Nd) and 943.3 +/- 12.4 Ma (Ar-40/Ar-39, phengite). An age of approxim ately 225 Ma based on the Sm-Nd and Rb-Sr mineral isochron ages is con sistent with the time inferred from paleomagnetic and geological data for collision of the North China and Yangtse Blocks and formation of t hese eclogites. We call for greater caution in Ar-40/Ar-39 dating of p hengites from other occurrences of ultrahigh-pressure environments.