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

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. In the water-saturated (water added, so that experiments, melts formed at 3 kb and above are strongly peraluminous, rich in Ca and poor in Fe, Mg, Ti, and K. Their compositions are unlike those of most silicic igneous rocks. These melts coexist with the amphibole-rich, plagioclase-poor restite assemblage amphibole + magnetite ± clinopyroxene ± plagioclase ± ilmenite. The highly aluminous nature of the melts and the plagioclase-poor nature of the restite both reflect the substantial contribution of plagioclase (along with quartz) to melts in high-pressure water-saturated systems. Water pressure equals Ptoul in the water-saturated experiments and has a profound effect on both melt composition and phase relations. At 1 kb, the water-saturated experiments yielded melt and mineral products with some characteristics of the dehydration-melting experiments (no amphibole at high T), and some characteristics of the 3-kb, water-saturated experiments (amphibole plus melt coexisting at lower T, elevated Al, lowered Fe). As pressure is increased from 3 to 6. 9 kb, the stability fields of both plagioclase and clinopyroxene decrease relative to amphibole and the Al contents of the melts increase. These experiments have important implications for the petrogenesis of low-K silicic rocks in arcs. First, dehydration melting is a viable mechanism for the formation of these rocks; water-saturated melting is not. Second, because of the influence of rock composition on melt composition, low-grade metamorphic and hydrothermal processes that alter the alkali contents and Na/ K in arc basement terranes may have a direct impact on the petrogenesis of silicic magmas in arcs, particularly the formation of extremely low-K trondhjemites. Third, the experiments predict that anhydrous, pyroxene- and plagioclase-rich ‘granulitic’ restite assemblages should develop as a result of partial melting in arc terranes. Such assemblages occur in at least two deeply eroded arc complexes.