Dehydration Melting and Water-Saturated Melting of Basaltic and Andesitic Greenstones and Amphibolites at 1, 3, and 6. 9 kb

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Abstract
The melting relations of five metamorphosed basalts and andesites (greenstones and amphibolites), collected from the late Jurassic Smartville arc complex of California, were investigated experimentally at 800–1000° C and 1, 3, and 6. 9 kb. Dehydration-melting (no water added) experiments contained only the water structurally bound in metamorphic minerals (largely amphiboles). They yielded mildly peraluminous to metaluminous granodioritic to trondhjemitic melts (Na/K is a function of starting composition) similar in major element composition to silicic rocks in modern oceanic arcs. The dehydration melts are water-undersaturated, with, and coexist with the anhydrous residual solid (restite) assemblage plagioclase + orthopyroxene + clinopyroxene + magnetite ± ilmen-ite, with plagioclase constituting ∼ 50% of the restite mode. In the dehydration-melting experiments at 3 kb the onset of melting occurred between 850 and 900 ° C, as amphibole and quartz broke down to yield pyroxenes plus melt. Total pressure is greater than in the dehydration-melting experiments and has little effect on melt composition or phase relations.

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