Subsolidus and Partial Melting Reactions in the Quartz-excess CaO+MgO+Al2O3+SiO2+H2O System under Water-excess and Water-deficient Conditions to 10 kb: Some Implications for the Origin of Peraluminous Melts from Mafic Rocks

Abstract

Experimental results up to 10 kb pressure are presented on the stability of amphibole in the quartz-excess CaO+MgO+Al2O3 (CMASH) system under H2O)-excess and H2O deficient conditions. Amphibole is stable above the solidus under H2O-excess conditions whereas under H2O-deficient conditions dehydration melting of amphibole-bearing assemblages defines the solidus. The successive appearance of amphibole, talc, and zoisite with increasing pressure considerably modifies the plagioclase-pyroxene-garnet-kyanite reactions documented experimentally in the CaO+MgO+Al2O3+SiO2 system for gabbro-granulite-eclogite transitions. Although both clino pyroxene and cordierite (with anorthite+orthopyroxene+quartz) may melt eutectically at one atmosphere to form diopside-normative and corundum-normative melts respectively, at higher pressures under H2O-excess conditions the peritectic melting of mafic rock compositions produces corundum-normative liquids together with either clinopyroxene or amphibole. Dehydration melting produces melts which are not corundum-normative. These data are used to discuss the origins and evolution of contrasting basalt-andesite-dacite-rhyolite volcanic suites and granitic plutons, many of whose silicic variants are corundum-normative in character, such as the Toba luff ignimbrites, Indonesia (Beddoc-Stephens et al., 1983) and I-type granite minimum melts (White & Chappell, 1977). In contrast, it is proposed that for the Cascades basalt-andesite-dacite-rhyolite suite the ortho pyroxene-plagioclase-quartz thermal divide was maintained up to rhyolite compositions, thereby prohibiting the derivation of corundum-normative rocks from diopside-normative parent magmas. The deduced reaction relations between pyroxenes, amphibole, plagioclase, quartz, and liquid are used to explain the absence or extreme scarcity of hydrous phases in some hydrous magmas. These phase relations can also explain the development of later plagioclase overgrowths on resorbed plagioclase cores in granitic intrusives, and the general absence of resorption and overgrowths in chemically equivalent extrusive rocks. A theoretical analysis of the partial melting of forsterite-bearing assemblages in the CaO+MgO+Al2O3+SiO2+H2O system shows that under H2O-excess conditions partial melting may generate corundum-normative (but low SiO2) melts from a peridotite source at shallow depths.