Quantitative structural analyses and numerical modelling of ...

B10406SCHULMANN ET AL.: RHEOLOGY OF PARTIALLY MOLTEN GNEISSESB10406Figure 2. Macroscopic samples of deformed metagranite divided in three types according to thedeformation intensity and the macroscopic appearance. X, Y, and Z refer to the axes of the finite strainellipsoid. (a) Sample S1 is a weakly deformed metagranite with large centimeter-sized grains of quartzand feldspar representing Type I rock. (b, c) Sample M1 is an augen orthogneiss corresponding to Type IIrocks and intermediate strain intensity (equivalent to samples T1, T2, M2, and R4), (d, e) Sample V1 is abanded mylonite corresponding to Type III rock and the highest intensity of deformation (equivalent toother highly strained samples R5, R3, V2, and H1).[Treagus, 2002]. This allows constraining the degree ofstrain partitioning in rocks and the viscosity ratios betweenthe individual mineral phases.[8] The shape analysis of the feldspars and the quartzpolycrystalline aggregates was carried out on 17 sectionscut both perpendicular to the foliation and parallel to thestretching lineation and perpendicular to both the foliationand the lineation, i.e., parallel to the XZ and YZ sections ofthe finite strain ellipsoid (Figure 2). The K-feldspar cannotbe distinguished from plagioclase in highly deformed macroscopicsamples and therefore both minerals were groupedtogether for finite strain measurements. All studied samplesare composed on average of 60–70% feldspars, 35–25%quartz and up to 10% of biotite and muscovite. The almostconstant mineral composition for highly variable strainintensities and constant bulk rock chemistry shows the lackof chemical variations with strain (Table 1).[9] In our study we classified three major types ofdeformed orthogneiss according to the deformation intensitiesat the macroscopic scale: Type I is represented byweakly deformed metagranite (Figure 2a); Type II correspondsto augen orthogneiss with quartz porphyroclast(Figures 2b and 2c); Type III is a banded mylonite orthogneiss(Figures 2d and 2e).[10] Mineral shape data are plotted into a Flinn diagram[Flinn, 1965] (Figure 3). The analyses of feldspar polycrystallineaggregates and quartz of the individual samples areconnected by tie lines with the vertical ellipse representingthe bulk strain value of the whole rock. An importantfeature of all the studied samples is that feldspars showhigher strain intensities than quartz for any bulk strain(Figures 3a and 3b). In several samples of the so-calledDoubravčany pencil gneiss, the strain intensities cannot beidentified because the stretching of feldspar and quartzlayers exceeds the length of the samples (Figure 2d). Thestrain symmetry, represented by the K values of Flinn[1965], varies from prolate to oblate shapes (K = 2.7 to0.3). For quartz with prolate shapes, the correspondingfeldspar strain symmetry is close to plane strain, while foroblate quartz, the feldspars show the same shape or slightlymore oblate shapes (Figure 3b). Strain intensities areexpressed using Ramsay’s D value, which is an alternativeexpression of viscosity ratio values of Gay [1968a, 1968b] alsoused by Schulmann et al. [1996]. The inspection of the diagramin Figure 3b shows that the highest strain ratios are achievedbetween D fel /D qtz for Type III orthogneiss and some Type IIorthogneiss samples marked by high bulk strain intensities. Therelatively small ratio between D fel /D qtz suggests similar yield-3of20297