Abstract
Synopsis Whole-rock Nd-isotopic data from the Ness anorthosite are consistent with an early-Proterozoic age for this body rather than the late-Archaean (Scourian) age inferred from geological relationships. Sm–Nd isotope systematics are interpreted as the result of interaction of an early-Proterozoic ( c. 2200 Ma) mantle-derived anorthositic magma with late-Archaean Lewisian gneisses. An early-Proterozoic protolith age suggests a possible correlation with the South Harris anorthosite. A Pb/Pb isochron age of 1860 ± 240 Ma is thought to represent the same early-Laxfordian high-grade mctamorphism that occurred in the South Harris complex. The similarity of Nd-isotope systematics, probable metamorphic history, and tectonic setting are considered to be strong evidence that the Ness and South Harris anorthosite bodies are coeval. However, a single early-Proterozoic anorthosite protolith body would need to be regionally extensive and is considered unlikely.
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