Excess radiogenic argon in pyroxenes and isotopic ages on minerals from Norwegian eclogites.

Potassium-argon measurements on eclogites enclosed in gneisses of the Nordfiord area, Norway, show that the pyroxenes contain excess radiogenic argon, indicating the presence of a high partial pressure of argon at one or more periods during the history of these rocks. Potassium-argon determinations on secondary hornblende suggest that the eclogites are as old as 1800 m. y. ; dates on phlogopite and hornblende in the range 950 to 1600 m.y. may be interpreted as indicating a metamorphism during this period. Micas from an altered eclogite and a country rock gneiss yield K-Ar ages of about 400 m.y., and Rb-Sr ages on two micas from altered eclogites also gi veĀ· dates of about 400 m. y., showing that the area was affected by the Caledonian orogeny.