Deciphering the petrogenesis of deeply buried granites: whole-rock geochemical constraints on the origin of largely undepleted felsic granulites from the Moldanubian Zone of the Bohemian Massif (original) (raw)

2004, Earth and Environmental Science Transactions of the Royal Society of Edinburgh

The prominent felsic granulites in the southern part of the Bohemian Massif (Gföhl Unit, Moldanubian Zone), with the Variscan (w340 Ma) high-pressure and high-temperature assemblage garnet+quartz+hypersolvus feldspar kyanite, correspond geochemically to slightly peraluminous, fractionated granitic rocks. Compared to the average upper crust and most granites, the U, Th and Cs concentrations are strongly depleted, probably because of the fluid and/or slight melt loss during the high-grade metamorphism (900-1050(C, 1·5-2·0 GPa). However, the rest of the trace-element contents and variation trends, such as decreasing Sr, Ba, Eu, LREE and Zr with increasing SiO 2 and Rb, can be explained by fractional crystallisation of a granitic magma. Low Zr and LREE contents yield w750(C zircon and monazite saturation temperatures and suggest relatively low-temperature crystallisation. The granulites contain radiogenic Sr ( 87 Sr/ 86 Sr 340 = 0·7106-0·7706) and unradiogenic Nd ( 340 Nd = 4·2 to 7·5), indicating derivation from an old crustal source. The whole-rock Rb-Sr isotopic system preserves the memory of an earlier, probably Ordovician, isotopic equilibrium.