Quantitative structural analyses and numerical modelling of ...

96 O. LEXA ET AL.subsolidus conditions. Such a reconstructed granitemineral composition calculated at 650 °C and 9 kbar is8 mol.% muscovite, 6.5 mol.% biotite, 39 mol.%quartz, 30 mol.% plagioclase and 16 mol.% K-feldspar(Fig. 12b). On heating, the melt proportion forthe reconstructed granite at 16 kbar and 860 °C is8 mol.%, and on isothermal decompression to 8 kbarand 860 °C it increases to 13 mol.%. The calculationspredict 14 mol.% of melt at 16 kbar and 1000 °C thatincreases to 37 mol.% at 8 kbar and 1000 °C, confirmingthe earlier estimates by Roberts & Finger(1997) and Janousˇek et al.(2004b).The modelling demonstrates that heating of graniticcrust with a restricted amount of muscovite and biotitecan generate a garnet-bearing residue and up to7 mol.% of melt at temperatures 900 °C. This temperaturewas used as a threshold for melt loss relatedremoval of radioactive elements and the switching offof the heat production in lower crustal rocks in thenumerical experiments.The detailed microstructural studies of Franěk et al.(2006, 2011b) showed that the origin of the granulitemicrostructure is related to significant deformation,dynamic recrystallization and viscous flow during thevertical extrusion stage. Before the vertical extrusionstage, the granulite facies rocks acquired a metamorphicfabric in a rather static environment. The deformationoccurred via diffusion-assisted grain boundarysliding, which is an efficient mechanism to extract meltfrom a continuously deforming source by a combinationof dynamic dilation and compaction (Za´vadaet al., 2007). The mechanism of dynamically developedporosity and melt extraction was discussed by Hasalova´et al. (2008a), who showed that a small amount ofmelt may be efficiently extracted from the source farbelow the rheologically critical melt percentagethreshold (>20–25 vol.% melt; Vigneresse et al.,1996). Therefore, we consider 900 °C to be a reasonabletemperature when melt can be extracted from theparent rock via the grain boundary sliding mechanismleaving behind a garnet-bearing mylonitic rock – theMoldanubian granulite. However, this processrequired significant deformation related to the verticalextrusion event to extract the melt. It is the pervasiveearly vertical fabric which is related to the efficient lossof melt (even at very small melt fraction), resulting insignificant depletion of the parental rocks in U, Th, Cs,Li ± Rb, but leaving the rest of the geochemical signatureunaffected.A consequence of melting and melt extraction is theremoval of a significant part of the radioactive elementbudget from the system. This is particularly true for Uand Th hosted by accessories such as zircon or monazite,which would resist fluid loss in course of theprogressive heating. On the other hand, they shoulddissolve readily even at low degrees of melting, as thepartial melt was likely to be rather corrosive, being hotand rich in alkalis with fluorine (Finger & Cooke,2004; Janousˇek et al., 2007). In granitic rocks, therelevant accessories are mostly enclosed in biotite (Bea,1996) that was undergoing melting. Moreover, as arguedby Watson et al. (1989) on theoretical grounds,the larger accessories are likely to be progressivelyconcentrated at grain boundaries in the course of highgrademetamorphism. Finally, there is a directevidence for the presence of low-degree, high-T, traceelement-richmelt in the felsic HP granulites, as thenewly grown metamorphic zircon and rutile are rich inU, Th and LREE or Zr and Nb, respectively (Finger &Cooke, 2004). Thus, the accessories hosting U and Thin the protolith to the HP Moldanubian granulitesseem to have been largely accessible to, and dissolvedin, the low-degree HP melt.The 1D thermal modelling has demonstrated that,for radioactive heat production of 4 lW m )3 , thetemperature threshold of 900 °C would be reached indeeply buried crust after 7 Myr (at 70 km) to 15 Myr(at 60 km). A radioactive heat production of4 lW m )3 is similar to the average radioactive heatproduction at c. 340 Ma calculated for the Fichtelgebirgemetaigneous crust, which is considered tocorrespond to the most appropriate protolith of felsicgranulites (Janousˇek & Holub, 2007). Moreover, themodelling shows that heat necessary for crustal meltingindeed could have been produced internally, within thetime frame allowed by the available geochronologicaldata (5–15 Myr).The heat source located at lower crustal depthswould also lead eventually to partial melting of theunderlying metasomatized and hydrated subcrustalmantle lithosphere. Melting of such an anomalous andfertile mantle source could have produced, soon afterthe HP metamorphic peak, ultrapotassic rocks withmixed crustal–mantle signatures. The effective removalof U and Th from the partially molten felsic metaigneousrocks (future felsic granulites) by melt extractionwould have grave consequences for the thermal evolutionof the whole system. First, the depletion ofradioactive elements from the granulite means that themelting process must have rapidly switched off. Second,the extracted melts could have partly mixed withenriched mantle-derived ultrapotassic magmas, whichinvaded the overlying partially molten crust, contributingto their further enrichment by U and Th.However, this mixing was probably volumetricallyrather insignificant and essentially different fromanother, large-scale hybridization event with S-typeleucogranitic magmas assumed for the durbachiteseries in Bohemia (Holub, 1997) and the Rastenbergsuite in Lower Austria (Gerdes et al., 2000). Thehybrid magmas finally intruded syn-tectonically orpost-tectonically at mid- to high-crustal levels in closespatial and temporal association with the HP granuliteand orthogneiss complexes so typical of the Variscanorogenic crust in the Moldanubian domain of theBohemian Massif (Schulmann et al., 2005; Zˇa´k et al.,2005; Tajcˇmanova´ et al., 2006; Janousˇek & Holub,2007).Ó 2010 Blackwell Publishing Ltd198