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Petrological and geochemical variations are used to investigate the formation of granite magma from diatexite migmatites derived from metasedimentary rocks of pelitic to greywacke composition at St. Malo, France. Anatexis occurred at relatively low temperatures and pressures (&amp;lt;800°C, 4–7 kbar), principally through muscovite dehydration melting. Biotite remained stable and serves as a tracer for the solid fraction during melt segregation. The degree of partial melting, calculated from modal mineralogy and reaction stoichiometry, was &amp;lt;40 vol. %. There is a continuous variation in texture, mineralogy and chemical composition in the diatexite migmatites. Mesocratic diatexite formed when metasedimentary rocks melted sufficiently to undergo bulk flow or magma flow, but did not experience significant melt–residuum separation. Mesocratic diatexite that underwent melt segregation during flow generated (1) melanocratic diatexites at the places where the melt fraction was removed, leaving behind a biotite and plagioclase residuum (enriched in TiO2, FeOT, MgO, CaO, Sc, Ni, Cr, V, Zr, Hf, Th, U and REE), and (2) a complementary leucocratic diatexite (enriched in SiO2, K2O and Rb) where the melt fraction accumulated. Leucocratic diatexite still contained 5–15 vol. % residual biotite (mg-number 40–44) and 10–20 vol. % residual plagioclase (An22). Anatectic granite magma developed from the leucodiatexite, first by further melt–residuum separation, then through fractional crystallization. Most biotite in the anatectic granite is magmatic (mg-number 18–22).

Formation of Diatexite Migmatite and Granite Magma during Anatexis of Semi-pelitic Metasedimentary Rocks: an Example from St. Malo, France