Formation of diatexite migmatites and granite magma during anatexis of semi-pelitic metasediments: an example from St. Malo, France

Petrological and geochemical variations are used to investigate the formation of granite magma from diatexite migmatites derived from pelitic to greywacke metasediments at St. Malo. Anatexis occurred at relatively low temperatures and pressures (<800^oC, 4 to 7 kbars), 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 <40%.

There is a continuous variation in texture, mineralogy and chemical composition in the diatexite migmatites. The preserved mesocratic diatexite formed when metasediments 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; a) melanocratic diatexites at the places where the melt fraction was removed, leaving a biotite and plagioclase residuum behind, and, b) a complementary leucocratic diatexite where the melt-fraction accumulated. Leucocratic diatexite still contained 5 to 15 % residual biotite (magnesium number 40 to 44) and, 10 to 20 % 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 (magnesium number 18-22).

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