We request one year of funding to complete a study of the relationships between granites and the mafic rocks (diorites and gabbros) commonly accompany them in continental arcs. In many arcs granites bear a strong isotopic resemblance to coeval mafic rocks, suggesting that the two suites are linked by a differentiation process such as fractional crystallization or partial melting. Remelting of mafic lower crust is one way (but not the only way) to generate granitoid magmas with mantle-like isotopic characteristics. We have undertaken a study of the dehydration melting of hornblende gabbros and diorites with variable K2O contents to investigate the origin of granitoid magmas of the Sierra Nevada batholith and elsewhere. Melting experiments at 8 kilobar pressure on Sierran mafic rocks have successfully produced silicic to intermediate liquids similar in composition (including K2O and Na2O) to common I-type granite and granodiorite. We request funding to complete this study. Work to be performed will include further electron microprobe analyses of synthesized mineral phases and glasses, ion microprobe analysis of experimental glasses, and geochemical modeling to test the hypothesis that the granites could be derived by melting of mafic lower crust.
