Melting Relations in Part of the System CaO-MgO-Al2O3-SiO2-Na2O-H2O under 5kb Pressure

In the system CaO-MgO-Al2O3-SiO2-Na2O-H2O under 5 kb pressure the invariant equilibrium forsterite-orthopyroxene-Ca-rich clinopyroxene-amphibole-plagioclase-liquid-vapour has been identified at 960±12 °C. A similar invariant assemblage with spinel replacing Ca-rich clinopyroxene exists at 950±8 °C. The liquid in the former equilibrium contains 16.5 per cent (wt.) normative quartz and 3 per cent Na2O; the plagioclase is more calcic than An87; the pyroxenes contain about 3 per cent Al2O3 and the amphibole is hypersthene-normative. Two anhydrous thermal maxima, the olivine-Ca-rich clinopyroxene-plagioclase and the orthopyroxene-Ca-rich clinopyroxene-plagioclase divide zones are not encountered in this system, and nepheline-normative liquids may crystallize amphibole±olivine±Ca-rich clinopyroxene to produce quartz-normative residual liquids of andesite-type composition. A thermal maximum involving amphibole-olivine-Ca-rich clinopyroxene-liquid-vapour exists for liquids containing approximately 11 per cent normative nepheline and liquids more undersaturated than this will crystallize these phases to produce extremely nephelinitic liquids. Phase diagrams are presented which facilitate the prediction of crystallization sequences and liquid evolution paths for any basic or intermediate composition under the conditions employed here.