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leucosome A coarse-grained, quartzofeldspathic vein, varying in thickness from a few centimetres to a metre or two, and found as a high-grade metamorphic product (see METAMORPHISM) in rocks of pelitic (see PELITE) to psammitic (see PSAMMITE) composition. The finer material between the leucosomes is enriched in mafic and aluminous minerals. Leucosomes may represent a low-melting-point liquid segregated from the metasediment during high-grade metamorphism to give rise to what is, in effect, a migmatite fabric.