K-Ar and 40Ar-39Ar incremental-release ages have been determined for biotite and hornblende from Grenville basement gneiss units of the northeastern Reading Prong west of the Hudson River. Biotite ages range from 768 ± 15 to 819 ± 18 m.y. (average of 790 m.y.), and those for unaltered hornblende range from 869 ± 20 to 949 ± 24 m.y. (average of 900 m.y.). Individual biotite and hornblende release spectra show negligible variations in 40Ar/39Ar ratios, and total-gas and incremental-release ages are similar.

These ages are discordantly younger than zircon ages of about 1,060 m.y. from this area; however, the consistent correlation of K-Ar, total-gas, and incremental-release ages argues against the discordancy resulting from partial argon loss during a distinct post-Grenville thermal event. The correlation suggests that the ages date times during post-Grenville metamorphic cooling when temperatures dropped below those required for argon retention in the minerals.

The ages place constraints on post-metamorphic cooling history and allow development of a model for the late Proterozoic uplift of this segment of the Appalachian Grenville terrane.

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