Searching study for a new indicator of partial melting of high-grade metamorphics

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The purpose of this study is to establish new indicators of partial melting of high-grade metamorphic rocks which formed at temperatures much higher than those of their minimum melting. First, we looked for textures and compositional features of rock-forming minerals that occur commonly in both igneous and metamorphic rocks, and have found the followings.1.The common occurrence of euhedral and calcic plagioclase inclusions in garnet porphyroblasts. Inclusion plagioclase is much more calcic compared to the matrix plagioclase.2.The chemical zoning of garnet and other coexisting cilicate minerals for phosphorus and heavy rare earth elements. Garnet porphyroblasts often consist of phosphorus-poor cores with euhedral and calcic plagioclase inclusions and phosphorus-rich rim with phosphorus mineral inclusions. In addition, garnet often preserves euhedral patterns for heavy rare earth element zonation while major elements zonation patterns are concentric and diffusion controlled. These clearly indicate that phosphorus minerals takes in part of the garnet-forming (partial melting) reaction.Thus, these features are well explained by the following continuous and incongruent melting reaction :sillimanite+biotite+sodic plagioclase+quartz+phosphorous minerals<plus-minus>H_2O=melt+garnet+calcic plagioclase+rutile (or ilmenite)The amount, morphology, internal structure and compositions of zircon in addition to phosphorus minerals such as apatite, monazite, xenotime and may be significant in geochemical modeling, as shown by the SHRIMP dating.Part of these features and insights have been reproduced and examined by high-temperature melting experiments with a HIP apparatus.We also have tried to precisely estimate the temperature, pressure and water content of large-volume felsic magmas erupted from the Aira volcano in Kyushu by experimentally producing phenocryst minerals in them.