The system tonalite-H_2O at 15 kbar and the genesis of calc-alkaline magmas

Abstract

Experimental phase relations and well-characterized mineral and melt compositions have been determined for a Sierra Nevada tonalite (andesite) with 2.5 to 10 wt% H_2O added at 15 kbar and 850 to 1100 °C. All results are from experiments conducted in Au capsules (minimal Fe loss to capsule) at oxygen fugacities measured to be near Ni-NiO. With increasing water content, the liquidus mineral changes from clinopyroxene at <3 wt% H20 to garnet, followed within 50 °C by clinopyroxene at 3 to 9 wt% H_2O, to hornblende at water contents greater than ~9 wt%. Microprobe analyses of glasses quenched from within the tonalite melting interval show an enrichment in Ca relative to (Mg + Fe), where Fe represents total Fe as Fe^(2+), with increasing SiO_2 instead of the typical calcalkaline trend of approximately constant Ca/(Mg + Fe) from basalt through rhyolite. The results document the importance of garnet and clinopyroxene in andesitic to dacitic compositions at deep-crustal to uppermost-mantle pressures, and they indicate that it is not possible to directly produce magmas of calc-alkaline composition at 15 kbar and the investigated range of water contents by either (1) partial melting of tonalitic to gabbroic (eclogitic) lower crust or (2) crystal fractionation from melts of andesitic to basaltic composition. Melting or fractionation at lower pressures or with lower water contents may produce calc-alkaline liquids as plagioclase replaces garnet as the major Al-bearing phase. Cale-alkaline rocks showing evidence of garnet-liquid equilibration in their REE patterns must also have undergone lower-pressure fractionation in order to explain their major-element compositions.