Rb‐Sr isotope measurements from the basement Houghton Inlier near Adelaide suggest that they were metamorphosed to upper amphibolite facies grade about 870 m.y. ago. One microcline mineral separate indicates that the minerals are grossly affected by the later Kanmantoo metamorphism (biotite grade in this region), but the total rock systems appear to have remained closed. The age of most of the Adelaide Supergroup is limited by this new result.