HYDROUS CORDIERITE IN GRANULITES AND CRUSTAL MAGMA PRODUCTION

Huge volumes of S-type granitic and volcanic rocks, as well as many migmatites, are the products of fluid-absent partial melting in metasedimentary protoliths at granulite-facies conditions. Thus, it is essential to know the controls on the fertility of metasedimentary rocks as magma sources. In the present experiments on fluid-absent partial melting of biotite in metasedimentary rocks, cordierite proportion is inversely correlated with melt fraction. At constant pressure (P) and temperature (T), this variation in melt proportion is a function of bulk-rock Mg number [100 Mg/(Mg + Fe)] and activity of alumina (a(Al2O3)). However, melt compositions are essentially unaffected. At 0.5 GPa, the calculated H2O contents of cordierite decrease as T increases from 850 to 1000 degrees C. In aluminous metasedimentary rocks, the cordierite stability field considerably overlaps with the conditions of fluid-absent melting in the middle to lower crust, severely restricting melt productivity. Consequently, metapelites may be much less fertile sources of granitic magma than metagraywackes.