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 Vojte ˇch Janous ˇek, Fritz Finger, Malcolm Roberts, Jir ˇı ´ Fry ´da, Christian Pin and David Dolejs ˇ ABSTRACT: The prominent felsic granulites in the southern part of the Bohemian Massif (Gfo ¨hl 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. Contrary to previous studies, the bulk of felsic Moldanubian granulites do not appear to represent separated, syn-metamorphic Variscan HP–HT melts. Instead, they are interpreted as metamor- phosed (partly anatectic) equivalents of older, probably high-level granites subducted to continental roots during the Variscan collision. Protolith formation may have occurred within an Early Palaeozoic rift setting, which is documented throughout the Variscan Zone in Europe. KEY WORDS: Austria, Czech Republic, high-grade metamorphism, Nd isotopes, partial melting, Sr isotopes, Variscan orogeny. Felsic garnet kyanite-bearing high-pressure and high- temperature (HP–HT) granulites with broadly granitic com- positions are prominent constituents in the central part of the European Variscides (e.g. Pin & Vielzeuf 1983; O’Brien & Ro ¨ tzler 2003). These light-coloured, SiO 2 –rich rocks are known from many places of the Bohemian Massif, namely from the Moldanubian (Lower Austria and southern Czech Republic; Fig. 1) and the Saxothuringian zones (the Saxonian Granulite Massif and Erzgebirge; Romer & Ro ¨ tzler 2001; Ro ¨ tzler & Romer 2001; O’Brien & Ro ¨ tzler 2003). Since the Moldanubian granulites may provide insights into processes related to deep subduction and subsequent exhumation of the continental crust, research over the past 40 years has focused mainly on constraining their precise pressure–temperature– time (P–T–t) paths (for reviews, see O’Brien 2000; O’Brien & Ro ¨ tzler 2003). However, their pre-metamorphic history has been largely neglected. Concerning the protolith, the felsic granulites were con- sidered as former rhyolites or granites which acquired their high-grade metamorphic character during the course of the Variscan collision (Fiala et al. 1987a, b; Vellmer 1992). Based on existing P–T estimates and fluid-absent melting models for crustal rocks, Roberts & Finger (1997) proposed that they probably experienced a certain degree of partial melting and melt persisted during much of their metamorphic history. Other authors (Vra ´na & Jakes ˇ 1982; Vra ´na 1989; Jakes ˇ 1997; Kotkova ´ & Harley 1999) went further and suggested that the granulites actually represent high-pressure granitic magmas which formed during the Variscan metamorphic cycle. Clearly, the problem of granulite genesis cannot be resolved without involving additional aspects of granite petrology and geochemistry, constraining the protolith formation and metamorphic reworking. The present paper characterises the compositional variation of the felsic Moldanubian granulites and provides their petrogenetic interpretation. The authors demonstrate that most, if not all, of their whole-rock geochemical signature may be related to a pre-existing Early Palaeozoic igneous-rock suite, whereas the Variscan high- pressure metamorphism could have been nearly isochemical. 1. Geological setting and petrology 1.1. Moldanubian Zone The Moldanubian Zone represents remnants of the crystalline core of the Variscan orogen in central Europe (e.g. Dallmeyer et al. 1995). In the southern part of the Bohemian Massif, shared by the Czech Republic and Austria, the Moldanubian Zone consists of several crustal segments with contrasting ages and complex polyphase deformational histories, intruded by numerous, mainly post-tectonic granitic masses (Dallmeyer et al. 1995; O’Brien 2000 and references therein). It has been Transactions of the Royal Society of Edinburgh: Earth Sciences, 95, 141–159, 2004