Hornblende-pyroxene granitic gneisses, quartz syenitic gneisses, and anorthosite, cropping out in the centers of structural domes and in the cores of nappe structures in the eastern and south-central Adirondacks, may represent igneous intrusions into the Grenville metasediments or remnants of an older basement upon which the Grenville sediments were deposited. Whole-rock Rb/Sr age analyses of eight samples of the granitic gneiss from the eastern Adirondacks yield excellent agreement at 1035 ± 20 million years (m.y.) with initial Sr87/Sr86 equal to 0.7098. Whole-rock Rb/Sr analyses of gneisses from the mantling sequence yield 1100 ± 125 m.y. and 1040 ± 40 m.y., and minerals from a pegmatite in the mantling gneiss are 1060 ± 75 m.y. old. These ages all lie within the time of the Grenville metamorphism (K/Ar and Rb/Sr on biotite) reported by others. These data are consistent with the hypothesis that the granites intruded the metasedimentary rocks or with the hypothesis that the granites formed from a pre-Grenville basement by metasomatism during the Grenville orogeny. It is unlikely that granitic gneisses could have been of granitic composition before 1035 ± 20 m.y.ago.

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