Previous studies of strontium isotopes in minerals from mantled gneiss domes, made before recognition of the importance of strontium redistribution, were interpreted as indicating that radiogenic strontium was retained almost completely by K-feldspar in spite of a subsequent orogeny. An isochron plot of new whole-rock samples from gneiss domes near Baltimore, Maryland, indicates an age of 1050 m.y., in agreement with previous ages reported on K-feldspar and zircon. Primary Sr87/Sr8G = 0.705. However, measurements on separated minerals show that the values obtained earlier on K-feldspar probably were obtained only because this mineral had nearly the same Rb/Sr ratio as the whole rock. Measurements on plagioclase and K-feldspar, as well as on biotite, now show that extensive strontium redistribution took place in the Baltimore Gneiss as a consequence of the Appalachian orogeny.

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