High-pressure partial melting and melt loss in felsic granulites in the Kutná Hora complex, Bohemian Massif (Czech Republic)

Abstract

Felsic granulites from the Kutná Hora complex in the Moldanubian zone of central Europe preserve mineral assemblage that records transition from early eclogite to granulite facies conditions, and exhibits leucocratic banding, which is interpreted as an evidence for melt loss during the decompression path. The granulites are layered and consist of variable proportions of quartz, ternary feldspar, garnet, biotite, kyanite, and rutile. In the mesocratic layers, garnet grains show relatively high Ca contents corresponding to 28–41mol% grossular end member. They have remarkably flat compositional profiles in their cores but their rims exhibit an increase in pyrope and a decrease in grossular and almandine components. In contrast, garnets from the leucocratic layers have relatively low Ca contents (15–26mol% grossular) that further decrease towards the rims. In addition to modeling of pressure–temperature pseudosections, compositions of garnet core composition, garnet rim-ternary feldspar–kyanite–quartz equilibrium, ternary feldspar composition, and the garnet–biotite equilibrium provide five constraints that were used to reconstruct the pressure–temperature path from eclogite through the granulite and amphibolite facies. In both layers, garnet cores grew during omphacite breakdown and phengite dehydration melting at 940°C and 2.6GPa. Subsequent decompression heating to 1020°C and 2.1GPa produced Ca- and Fe-poor garnet rims due to the formation of Ca-bearing ternary feldspar and partial melt. In both the mesocratic and leucocratic layer, the maximum melt productivity was 26 and 18vol.%, respectively, at peak temperature constrained by the maximum whole-rock H2O budget, ~1.05–0.75wt.%, prior to the melting. The preservation of prograde garnet-rich assemblages required nearly complete melt loss (15–25vol.%), interpreted to have occurred at 1000–1020°C and 2.2–2.4GPa by garnet mode isopleths, followed by crystallization of small amounts of residual melt at 760°C and 1.0GPa. Phase formation and melt productivity were independently determined by experiments in the piston-cylinder apparatus at 850–1100°C and 1.7–2.1GPa. Both the thermodynamic calculations and phase equilibrium experiments suggest that the partial melt was produced by the dehydration melting: muscovite+quartz=melt+K-feldspar+kyanite. The presence of partial melt facilitated attainment of mineral equilibria at peak temperature thus eliminating any potential relics of early high-pressure phases such as phengite or omphacite. By contrast, adjacent mafic granulites and eclogites, which apparently share the same metamorphic path but have not undergone partial melting commonly preserve relics or inclusions of eclogite-facies mineral assemblages.