Formation of diatexite migmatite and granite magma during anatexis of semi-pelitic metasedimentary rocks: An example from St. Malo, France

Petrological and geochemical variations are used to investigate the formati on of granite magma from diatexite migmatites derived fr om metasedimentary rocks of pelitic to greywacke composition at St. Malo, France. Anatexis oc curred at relatively low temperatures and pressures (<800<degrees>C, 4-7 kb ar), principally through muscovite dehydration melting. Biotite remained st able and serves as a tracer for the solid fraction during melt segregation. The degree of partial melting, calculated from modal mineralogy and reacti on stoichiometry, was <40 vol. %. There Is a continuous variation in textur e, mineralogy and chemical composition in the diatexite migmatites. Mesocra tic diatexite formed when metasedimentary rocks melted sufficiently to unde rgo bulk flow or magma flow, but did not experience significant melt-residu um separation. Mesocratic diatexite that underwent melt segregation during flow generated (I) melanocratic diatexites at the places where the melt fra ction was removed, leaving behind a biotite and plagioclase residuum (enric hed 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 plagioc lase (An(22)). Anatectic granite magma developed from the leucodiatexite, f irst by further melt-residuum separation, then through fractional crystalli zation. Most biotite in the anatectic granite is magmatic (mg-number 18-22) .