Water-Saturated and -Undersaturated Melting of Metaluminous and Peraluminous Crustal Compositions at 10 kb: Evidence for the Origin of Silicic Magmas in the Taupo Volcanic Zone, New Zealand, and Other Occurrences

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Abstract
The melting relations of two proposed crustal source compositions for rhyolitic magmas of the Taupo Volcanic Zone (TVZ), New Zealand, have been studied in a piston-cylinder apparatus at 10 kb total pressure and a range of water activities generated by H2O-CO2 vapour. Starting materials were glasses of intermediate composition (65 wt.% Si02 representing a metaluminous ‘I-type’ dacite and a peraluminous ‘S-type’ greywacke. Crystallization experiments were carried out over the temperature range 675 to 975°C, with aH2O values of approximately 1·0, 0·75, 0·5, and 0·25. Talc-pyrex furnace assemblies imposed oxygen fugacities close to quartz-fayalite-magnetite buffer conditions. Assemblages in both compositions remain saturated with quartz and plagioclase through 675–700°C at high aH2O, 725–750°C at aH2O≈0·5, and 800–875°C at aH2O≈0·25, corresponding to aH2O. Biotite and hornblende are the only mafic phases present near the solidus in the dacite, compared with biotite, garnet, gedritic orthoamphibole, and tschermakitic clinoamphibole in the greywacke. Near-solidus melting reactions are of the type: biotite + quartz + plagioclase = amphibole ± garnet, potentially releasing H2O for dehydration melting in the greywacke, but producing larger amounts of hornblende and releasing little H2O in the dacite. At aH2O≈0·25 and temperatures ≥825–850°C, amphibole dehydration produces anhydrous mineral phases typical of granulite fades assemblages (clinopyroxene, orthopyroxene, plagioclase±quartz in the dacite; garnet, orthopyroxene, plagioclase±quartz in the greywacke) coexisting with melt proportions as low as 40%. Hornblendce-saturated liquids in the dacite are weakly peraluminous (0·3–1·6 wt.% normative C—within the range of peraluminous TVZ rhyolites), whereas, at aH2O≈0·25 and temperatures ≥925°C, metaluminous partial melt compositions (up to 1·8 wt.% normative Di) coexist with plagioclase, orthopyroxene, and clinopyroxene. At all water activities, partial melts of the greywacke are uniformly more peraluminous (1·5–2·6 wt.% normative C), reflecting their saturation in the components of more aluminous mafic minerals, particularly garnet and Al-rich orthopyroxene. A metaluminous source for the predominantly Di-normative TVZ rhyolites is therefore indicated. With decreasing aH2O the stability fields of plagioclase and quartz expand, whereas that of biotite contracts. These changes are reflected in the proportions of normative salic components in partial melts of both the dacite and greywacke. At high aH2O, partial melts are rich in An and Ab and poor in Or (trondhjemitic-tonalitic); with decreasing aH2O they become notably poorer in An and richer in Or (granodioritic-granitic). These systematic variations in salic components observed in experimental metaluminous to strongly peraluminous melts demonstrate that a wide variety of granitoid magmas may be produced from similar source rocks depending upon P-T-aH2O conditions attending partial melting. Some peraluminous granitoids, notably trondhjemitic leucosomes in migmatites, and sodic granodiorites and granites emplaced at deep crustal levels, have bulk compositions similar to near solidus melt compositions in both the dacite and greywacke, indicating possible derivation by anatexis without the involvement of a significant restite component.

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