High-pressure partial melting and melt loss in felsic granulites in the Kutna Hora complex, Bohemian Massif (Czech Republic)
Abstrakt
Felsic granulites from the Kutna Flora complex in the Moldanubian zone of central Europe preserve mineral assemblage that records transition from early eclogite to granulite fades 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 28-41 mol% 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 (15-26 mol% 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 degrees C and 2.6 GPa. Subsequent decompression heating to 1020 degrees C and 2.1 GPa 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 18 vol.%, respectively, at peak temperature constrained by the maximum whole-rock H(2)O budget, similar to 1.05-0.75 wt.%, prior to the melting.