Deformed and metamorphosed basaltic volcanic rocks (one pillow lava, one amphibolite) from the basement on Attu Island have yielded whole-rock K-Ar ages of about 30 m.y. The greenschist (or higher grade) assemblage of these rocks and their presumed correlation with late Eocene or older early-series rocks elsewhere in the Aleutian arc suggest that these dates indicate a time of metamorphism. Similar ages (≅30 m.y.) for two mafic intrusions into the basement may also signify a time of metamorphism. These four ages place an older limit on, and may coincide with, the mid-Tertiary tectonism that deformed rocks of the early series along the length of the Aleutian arc. One post-early-series intrusion on Agattu is 14 m.y. old, in agreement with previous results from Shemya on a similar rock and in accord with the observation of a relatively abrupt start of late Tertiary magmatism throughout the arc about 15 m.y. ago. The youngest recognized igneous activity in the Near Islands is represented by hornblende-bearing rocks (one andesite lava, one dacite dike) on Attu that have yielded ages of 6 to 8 m.y.

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