Assessing the Role of Silicate Partial-melts in the Textural Geochemical and Isotopic evolution of the South Bohemian Granulites

The project P12248 Geo was undertaken to assess the role of melting in the petrogenesis of the South Bohemian granulites. By means of geothermobarometric studies the peak P T conditions of granulite facies metamorphism could be constrained at 15-19 kb and 950-1050 C. Geochemical data indicate that most of the South Bohemian granulites had granitic protoliths. Although it is very likely that at such high P T conditions granitic lithologies would undergo dehydration melting as a consequence of biotite breakdown, a direct proof of a melt phase in the rocks and an estimation of melt amounts turned out difficult. An important evidence for melt presence has been provided by a magmatic growth zoning observed in zircon crystals. By means of SHRIMP U-Pb dating it could be clearly shown that these magmatic zircon domains grew during Variscan regional metamorphism. SHRIMP analyses were also made in zoned zircons from the Bittesch Gneiss, an amphibolite-facies metagranite located close to the granulite massifs. The work was undertaken to investigate whether prograde amphibolite-facies metamorphism has also caused a phase of zircon growth in the South Bohemian Massif. No Variscan zircon domains were recorded in this gneiss. However, an important by-product of this investigation was that in addition to magmatic zircons (formation age of the Bittesch Gneiss 5787 Ma), several inherited zircon generations with ages of c. 1.2, 1.5, 1.65-1.8, 2.0 and 2.5 Ga could be recognized. These data have turned out to be highly important for a terrane analysis of the eastern Variscan fold belt.