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 ve· dates of about 400 m. y., showing that the area was affected by the Caledonian orogeny.
Excess radiogenic argon in pyroxenes and isotopic ages on minerals from Norwegian eclogites.