Partial melting of crustal rocks

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Shales and graywackes were first metamorphosed at 650°C and then partially melted at 700 and 750°C at 2, 4, 6, and 8 kilobars in the presence of 0.75 m NaCl−0.45 m KCl and 0.225 m CaCl 2−0.750 m NaCl solutions. In experiments with shales, KK+Na ratio in the decreases with increasing pressure at 650 and 700°C; however, at 750°C this ratio is equal to 0.5 at all pressures investigated. This suggests that melts at 700°C and at 2 to 8 kilobars pressure may be affected metasomatically whereas melts at 700°C and in the same pressure range will not. Melt composition produced in the shale-KCl NaCl experiments is granite at 2, 4 and 6 kilobars pressure, whereas the melt compositions in the shale-CaCl 2 NaCl experiments range from quartz monzonite (2–5 kilobars) to granodiorite (above 5 kilobars). Experiments with graywacke-KCl NaCl produced melts of trondhjemite composition at 2, 2.5, 4, 6, and 7.5 kilobars. These results indicate that partial melting of crustal rocks such as metamorphosed shales and graywackes in the deeper parts of the crust can produce large volumes of granitic magmas ranging in composition from true granite to trondhjemite to quartz monzonite and granodiorite.