Diatexite and metatexite from the Higo metamorphic rocks, west-central Kyushu, Japan

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A variety of anatectic migmatites occur in high-grade zones of a Mesozoic high-T/P typemetamorphic belt — the Higo metamorphic belt, Kyushu, Japan. This paper deals with two major lithotypes of the migmatites — metatexite and diatexite — and attempts to characterize them in terms of field occurrence, texture, mineralogy and whole-rock chemistry. The metatexite is a layered migmatite that is composed of leucosome and melanosome with metamorphic fabrics, while diatexite is a massive and more homogeneous type and has plutonic igneous textures. Both types of migmatite occur in a high grade garnet-cordierite zone (D zone) and an even higher grade garnet-hypersthene zone (E zone) of the Higo metamorphic belt, and they are both considered to represent the products of partial melting of pelitic and psammitic gneisses. The diatexite occurs in masses, sporadically, in the matrices of metatexite both in the D zone and E zone, and it does not appear to increase in abundance with the increase of metamorphic grade. Another important finding is that no systematic compositional difference has been detected between the metatexite and the diatexite despite their textural and structural differences, if compared within the same D zone. Treating the metatexite and diatexite together, it was shown that the E-zone migmatites are chemically and mineralogically more depleted than the D-zone migmatites, which may imply that the former represents the refractory residues of partial melting. Some leucogranites, which form veins, dikes and pods in metamorphic rocks, have compositions of anatectic melts and are considered to be compositionally complementary to the residual E-zone migmatites. Although the D-zone rocks must also have been partially molten at the peak of metamorphism, melt segregation probably did not take place effectively there, and, as a whole, they largely retain their original metasedimentary composition. The factor controlling the structural types of the migmatite is also discussed.
日本鉱物科学会の論文
2005-02-01

Diatexite and metatexite from the Higo metamorphic rocks, west-central Kyushu, Japan

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