Quantitative structural analyses and numerical modelling of ...

B10210ZÁVADA ET AL.: EXTREME DUCTILITY OF FELDSPAR AGGREGATESB10210Figure 1. Simplified geological map of the studied area around the Eger river valley, NE of Stráž nadOhří town [Hradecký, 2002] (scale 1:25000) and the position of the studied area in the Bohemian Massif[Franke, 2000] and European Variscides. AM, Armorican massif; MC, Massif Central; BM, Bohemianmassif.1987; Daines and Kohlstedt, 1997; Gleason et al., 1999;Rosenberg and Handy, 2000]. This phenomenon can beexplained either by dilation of granular aggregates due tomelt overpressure [Renner et al., 2000], when hydrostaticmelt pressure overcomes the confining pressure, or bypassive melt migration into the incipient voids during grainboundary sliding (GBS) [Čadek, 1988].[4] However, studies of natural microstructures showingevidence of large finite strains and interstitial quenchedmelt, which would be indicative for pronounced meltenhancedweakening and grain-scale melt migration arepractically absent. This is due to two reasons: (1) Naturalmelt-bearing microstructures equilibrated at high-grade conditionsare likely to be overprinted by subsequent static ordynamic recrystallization [Rosenberg and Riller, 2000;Rosenberg, 2001]. (2) High finite strains, slow creep ratesand high homologous temperatures necessary to introducehigh-temperature melt-enhanced deformation mechanisms(e.g., diffusion creep or dislocation climb accommodatedGBS [Boullier and Gueguen, 1975]) cannot be attainedusing conventional experimental facilities. There are threeapproaches that may help to understand the rheologicalbehavior of minerals deformed at high-grade conditions inthe presence of melt and their influence on rheology ofpolyphase rocks: (1) Investigation of high-grade rocks,where microstructural record was preserved due to rapidcooling rates, (2) comparison of high-grade rock microstructureswith results of various experiments on creepproperties of metals, alloys and steels, and (3) applicationof more efficient rock deformation laboratory facilities (e.g.,Paterson rig apparatus).[5] In this work, we characterize deformation microstructuresin a strongly deformed high-grade banded orthogneiss,where melting was concurrent with deformation. Meltbearing microstructures have been preserved due to rapidcooling that effectively increased quenching of melt instudied rocks [Zulauf et al., 2002]. Deformation mechanismsof constituent mineral phases are investigated usingquantitative microstructural analysis. The mechanism responsiblefor preferred distribution of interstitial melt insample with maximum strain intensity is critically evaluatedusing modeling of melt volume produced by a metamorphicreaction and molar volume change of the system. The effectof mobile melt phase on deformation mechanisms and bulkrock rheology is discussed. Finally, we consider the apparentstrength relationship between quartz and feldspars.2. Geological Setting[6] The rocks studied belong to a lower crustal hightemperaturemetamorphic unit that was rapidly exhumedalong the major Variscan tectonic boundary in the BohemianMassif called the North Bohemian Shear Zone (NBSZ ofZulauf et al. [2002]). The trend and spatial limits of theNBSZ are not defined in the field, because it is mostlycovered by Neogene volcanics and sedimentary sequences(Figure 1). The sampled unit forms the uppermost thrustsheet within a crustal nappe stack that displays typicalmetamorphic inversion and is called the ‘‘Upper CrystallineNappe’’ (UCN) [Konopásek and Schulmann, 2005]. TheUCN in the studied area consists of banded orthogneisses,high-pressure granulites and migmatites (Figure 1). PTcalculations of metamorphic conditions for the partiallymolten orthogneisses in this study reveal pressures of900 MPa and temperatures 700 ± 20°C. In contrast,the granulites exhibit peak metamorphic conditions of1600 MPa and 800°C [Kotková etal., 1996] indicating thatthe granulites and orthogneisses have been juxtaposed atmiddle crustal levels during differential exhumation. Furthermore,very fast cooling rates of 50°C +25°C/ 17°CMa 1 were deduced [Zulauf et al., 2002] from the U-Pbdating of formation of zircons and monazites (342 ± 1 Ma)and 40 Ar- 39 Ar cooling ages of muscovites and biotites(341 ± 4 Ma) from the orthogneisses.2of15280