Recent discussions on partial melting in high-grade metamorphic rocks are briefly reviewed. Garnets in high-grade pelitic rocks commonly contain euhedral calcic plagioclase inclusions. In addition, they display compositional growth zoning for phosphorus as well as major elements (Fe, Mn, Mg and Ca) and other minor elements (e.g., Y), though modified to various degrees by volume diffusion. The phosphorus zonation is defined by a low-P core with a distinct high-P rim, and correlates with the presence or absence of euhedral calcic plagioclase inclusions. Apatite, xenotime and other phosphate minerals as well as K-feldspar are present in the rock matrix and as inclusions in the garnet rim, whereas these minerals are usually absent in the garnet core. This suggests phosphate undersaturation during the growth of the core. All these features are satisfactorily explained by the following partial melting reaction.
Biotite + sillimanite + sodic plagioclase + K-feldspar + quartz + phosphates (+H2O)
=granitic melt +garnet core+calcic plagioclase+rutile (or ilmenite).
The garnet rim and the rock matrix may have formed from the granitic melt upon cooling.
These data would be a new indicator for previous partial melting of pelitic gneisses and granulites.