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

The existence of excess Ar in phengites from coesite-bearing ultrahigh-pressure eclogites of the Dabie Mountains and Su-Lu terrain, China, has been confirmed by dating of minerals using Sm-Nd, Rb-Sr and 40Ar/39Ar methods. 40Ar/39Ar ages of phengites for two samples studied are 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 and phengite 40Ar/ 39 Ar age of 877.5 +/- 15.9 Ma; the corresponding ages 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 (40Ar/39Ar, phengite). An age of approximately 225 Ma based on the Sm-Nd and Rb-Sr mineral isochron ages is consistent with the time inferred from paleomagnetic and geological data for collision of the North China and Yangtse Blocks and formation of these eclogites. We call for greater caution in 40Ar/39Ar dating of phengites from other occurrences of ultrahigh-pressure environments. The existence of excess Ar in phengites from coesite-bearing ultrahigh-pressure eclogites of the Dabie Mountains and Su-Lu terrain, China, has been confirmed by dating of minerals using Sm-Nd, Rb-Sr and 40Ar/39Ar methods. 40Ar/39Ar ages of phengites for two samples studied are 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 and phengite 40Ar/ 39 Ar age of 877.5 +/- 15.9 Ma; the corresponding ages 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 (40Ar/39Ar, phengite). An age of approximately 225 Ma based on the Sm-Nd and Rb-Sr mineral isochron ages is consistent with the time inferred from paleomagnetic and geological data for collision of the North China and Yangtse Blocks and formation of these eclogites. We call for greater caution in 40Ar/39Ar dating of phengites from other occurrences of ultrahigh-pressure environments.