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 three rocks-a gabbro, a tonalite, and a muscovite granite-representing the compositional trend of the calc-alkaline rock series. Experiments were conducted in 0.5-in. piston-cylinder apparatus using Ag-Pd or Pt capsules. The results obtained are acceptable representations of 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 temperature limit of Au, for reliable electron microprobe analyses of glass, garnet, clinopyroxene, amphibole, and plagioclase. No glasses could be analyzed in the gabbro. The data provide K_D values as a function of temperature for pairs of coexisting minerals and liquid. The chemical variations are used to evaluate (1) the fractionation trends in hydrous magmas at 55 km in thickened continental crust or uppermost mantle and (2) the products of anatexis of the thickened crust. At this depth, fractionation of hydrous basalt, or anatexis of gabbro and tonalite or their metamorphosed equivalents, appears to produce liquids diverging from the calc-alkaline trend. Anatexis of granite produces a liquid less siliceous than itself, with some chemical characteristics of syenite.