Abstract

We report the Sm-Nd systematics in seven whole-rock anorthosites, in plagioclase and garnet mineral separates from one anorthosite, and in five mangerite-norite-charnockite rocks from the Snowy Mountain dome in the south-central Adirondacks. The plagioclase, two garnet separates, and the whole-rock anorthosite define a four-point internal isochron of age 1,098 ± 7 (2σ) m.y. The six whole-rock anorthosites and the anorthositic gabbro in conjunction with the above internal isochron define a similar age of 1,095 ± 7 (2σ) m.y. and an initial 143Nd/144Nd ratio of 0.51135 ± 1 (2σ). This age of 1,100 m.y. is considered to be the time of crystallization of the anorthositic rocks. The initial Nd isotope ratio of the combined whole-rock and mineral isochron indicates an ϵNd value of +2.5, which suggests a light rare-earth-element-depleted source for the anorthositic parent magma in the upper mantle. The trace-element and isotopic data also support the view that gabbroic anorthosite is the parent magma for the anorthosites. Five whole-rock samples of norite-mangerite-charnockite, found in close association with the anorthosites, do not define a precise isochron. The initial ϵNd values of these rocks at 1,100 m.y. show a range from +2.7 to +5.1, which demonstrates that the anorthositic rocks and the mangerite-charnockite are not cogenetic. The isotopic data further demonstrate that the mangerite-charnockite rocks are mantle-derived meta-igneous rocks, and not of sedimentary derivation.

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