The Strathy Complex of the Scottish Caledonides is a bimodal association of amphibolites and siliceous grey gneisses that structurally underlies adjacent metasediments of the Moine Supergroup. Both rock units record a common polyphase Caledonian tectonometamorphic history. New elemental and radiogenic isotope data indicate that both end-members of the Strathy suite were derived from a depleted mantle source, that they are cogenetic and that they may have been related by crystal fractionation. δ18O values and their correlations with major and trace elements suggest that the protoliths were hydrothermally altered at temperatures below 200 °C. Tectonomagmatic discrimination based on relatively immobile elements and isotope systems, plus comparison with geochemically similar bimodal supracrustal associations elsewhere, strongly support the conclusion that the igneous protoliths of the Strathy Complex formed in an oceanic destructive margin setting. If TDM model ages of c. 1000 Ma approximate protolith crystallization, the Strathy Complex may have formed as juvenile crust in the peri-Rodinian ocean broadly contemporaneous with the Grenville orogenic cycle.