Phase relationships of gabbro-tonalite-granite-water at 15 kbar with applications to differentiation and anatexis

Abstract

Phase relationships have been determined at 15 kbar with variable H_2O contents for
\nthree rocks-a gabbro, a tonalite, and a muscovite granite-representing the compositional
\ntrend of the calc-alkaline rock series. Experiments were conducted in 0.5-in. piston-cylinder
\napparatus using Ag-Pd or Pt capsules. The results obtained are acceptable representations
\nof the phase relationships for elucidation of petrological processes. Selected runs with 5% H_2O were completed using Au capsules to overcome the problem of iron loss, up to the
\ntemperature limit of Au, for reliable electron microprobe analyses of glass, garnet, clinopyroxene,
\namphibole, and plagioclase. No glasses could be analyzed in the gabbro. The
\ndata provide K_D values as a function of temperature for pairs of coexisting minerals and
\nliquid. The chemical variations are used to evaluate (1) the fractionation trends in hydrous
\nmagmas at 55 km in thickened continental crust or uppermost mantle and (2) the products
\nof anatexis of the thickened crust. At this depth, fractionation of hydrous basalt, or anatexis
\nof gabbro and tonalite or their metamorphosed equivalents, appears to produce liquids
\ndiverging from the calc-alkaline trend. Anatexis of granite produces a liquid less siliceous
\nthan itself, with some chemical characteristics of syenite.