Subduction or sagduction? Ambiguity in constraining the origin of ultramafic–mafic bodies in the Archean crust of NW Scotland

摘要: Abstract The Lewisian Complex of NW Scotland is a fragment the North Atlantic Craton. It comprises mostly Archean tonalite–trondhjemite–granodiorite (TTG) orthogneisses that were variably metamorphosed and reworked in late Neoarchean to Paleoproterozoic. Within granulite facies central region mainland Complex, discontinuous belts composed ultramafic–mafic rocks structurally overlying garnet–biotite gneiss (brown gneiss) are spatially associated with steeply-inclined amphibolite shear zones have been interpreted as terrane boundaries. Interpretation primary chemical composition these complicated by partial melting melt loss during metamorphism, contamination melts derived from adjacent migmatitic TTG host rocks. Notwithstanding, layered suggestive protolith formed differentiation tholeiitic magma, where ultramafic portions bodies represent cumulates mafic fractionated liquids. Although brown does not clearly discriminate protolith, it most likely represents sedimentary or volcano-sedimentary sequence. For rocks, particularly those facies, geochemical characteristics typically used for discrimination paleotectonic environments neither strictly appropriate nor diagnostic. Many ‘arc-like’ trace element signatures. These signatures reflect derivation hydrated enriched mantle and, case gneisses, source containing garnet Ti-rich phase, probably rutile. However, becoming increasingly recognised such may be unique subduction environment but relate processes delamination dripping. Consequently, unclear whether gneisses products plate margin intraplate magmatism. subduction-related origin possible, we propose an equally plausible. If second alternative correct, remnants intracratonic greenstone sank into deep crust due their density contrast underlying partially molten low viscosity orthogneisses.