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Formation of Diatexite Migmatite and Granite Magma during Anatexis of Semi-pelitic Metasedimentary Rocks: an Example from St. Malo, France 总被引:8,自引:2,他引:8
Petrological and geochemical variations are used to investigatethe formation of granite magma from diatexite migmatites derivedfrom metasedimentary rocks of pelitic to greywacke compositionat St. Malo, France. Anatexis occurred at relatively low temperaturesand pressures (<800°C, 47 kbar), principally throughmuscovite dehydration melting. Biotite remained stable and servesas a tracer for the solid fraction during melt segregation.The degree of partial melting, calculated from modal mineralogyand reaction stoichiometry, was <40 vol. %. There is a continuousvariation in texture, mineralogy and chemical composition inthe diatexite migmatites. Mesocratic diatexite formed when metasedimentaryrocks melted sufficiently to undergo bulk flow or magma flow,but did not experience significant meltresiduum separation.Mesocratic diatexite that underwent melt segregation duringflow generated (1) melanocratic diatexites at the places wherethe melt fraction was removed, leaving behind a biotite andplagioclase residuum (enriched in TiO2, FeOT, MgO, CaO, Sc,Ni, Cr, V, Zr, Hf, Th, U and REE), and (2) a complementary leucocraticdiatexite (enriched in SiO2, K2O and Rb) where the melt fractionaccumulated. Leucocratic diatexite still contained 515vol. % residual biotite (mg-number 4044) and 1020vol. % residual plagioclase (An22). Anatectic granite magmadeveloped from the leucodiatexite, first by further meltresiduumseparation, then through fractional crystallization. Most biotitein the anatectic granite is magmatic (mg-number 1822). KEY WORDS: anatexis; diatexite; granite magma; melt segregation; migmatite 相似文献
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