Synopsis

The Inzie Head gneisses of the NE Dalradian are syntectonic metapelitic schollen migmatites containing numerous sill-like bodies of granite and diorite. Leucosomes with clear evidence of melt-present crystallization exhibit a wide variety of segregation structures. Initial in situ melting occurred within, or at the margins of, fertile metasedimentary horizons and was strongly controlled by variations in bulk-composition. Intergranular melt-pockets coalesced into thin (<10 mm), discontinuous stromatic horizons which subsequently joined laterally and/or became interconnected in extensional shear-zones. A diktyonitic structure formed, enabling buoyancy aided upward migration of melt into low-strain sites such as boudin-necks and shear-zones. Metre-scale leucosome channels containing abundant schollen and having diffuse, unchilled contacts against the migmatites were supplied from this network and may represent the foci of H2O-rich fluid influx. Bodies of nebulitic granite contain abundant ‘ghost’ schollen in the final stages of assimilation. Diorite sills show extensive magma mingling and hybridization with granitic leucosome and were clearly contemporaneous with the crustal melting. A variably developed flattening fabric within the migmatites was in part due to inflation following intrusion of the diorites. Immediately beneath diorite intrusions, melt loss from the migmatites was pronounced indicating that pure-shear dominated deformation is a strong driving force for melt segregation. Garnetiferous aplites at higher crustal levels and contemporaneous with the peak of regional metamorphism represent melt frozen in channelways that potentially contributed to nearby, large-scale Grampian granitic bodies.

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