Quantitative structural analyses and numerical modelling of ...

B10210ZÁVADA ET AL.: EXTREME DUCTILITY OF FELDSPAR AGGREGATESB10210Figure 11. Stereographic projection of CPO data and fracture poles of 100 fractured grains. X and Zdesignate direction of lineation and pole to the foliation, respectively. Symbols in the center projectiondesignate the misorientation angle of (001) planes with respect to the corresponding fracture pole (right).Lower hemisphere, equal-area projection, contoured at multiples of uniform distribution.[24] The rocks studied are represented by banded orthogneissesconsisting of K-feldspar (Kfs) (39%), plagioclase(Pl) (25%), quartz (Qtz) (26%), biotite (Bt), and muscovite(Ms) (9%) and minor garnet (Grt) (1%). P-T pseudosectionfor this rock was calculated with the Perple_X software set[Connolly, 1990; Connolly and Petrini, 2002] using phaseend-members thermodynamic data by Holland and Powell[1998]. Mixing properties of phases used for the calculationwere taken from Berman [1990] for garnet, Newton et al.[1980] for plagioclase, Thompson and Hovis [1979] for K-feldspar, Powell and Holland [1999] for biotite and clinopyroxene,and White et al. [2001] for melt.[25] The obtained pseudosection calculated inNCKFMASH system with composition taken from thewhole rock analysis of a typical banded orthogneiss (sampleB13) is shown in Figure 13a. Stable mineral assemblageKfs-Pl-Qtz-Ms-Bt-Grt-melt, and composition of coexistingFe-Mg phases suggest that the highest metamorphic temperaturesassociated with melting are 700°C at a pressure of9.5 kbar. Calculated isopleths of the melt mode indicatethat for the given melt model and estimated P-T conditions,the rock contained 2–4 vol % of the melt phase(Figure 13a) at its metamorphic peak. This is, however, aminimum estimate, because the chosen system compositiondoes not take into account the amount of water releasedduring melt crystallization. The model of White et al. [2001]anticipates that the melt does not contain any CaO componentat the estimated PT conditions, which is in goodagreement with albitic composition of plagioclase crystallizedfrom the interstitial melt in the rock. Generalizedreaction leading to melt production in the stability field ofassemblage Kfs-Pl-Qtz-Ms-Bt-Grt-melt can be derived fromthe changes in modal proportions of phases in the stabilityfield of interest (Figure 13b). Such changes suggest that theincrease in melt content is associated with the crystallizationof garnet as a result of the reaction: Bt + Ms + Pl + Qtz = Grt +Kfs + melt.[26] The inferred melt-producing reaction correspondswell with the distribution of mineral phases in the rockmicrostructure. Small garnet grains (50 mm) form clusters onboundaries between plagioclase bands and mica aggregatesin the vicinity of quartz grains (Figure 14). Agreement ofmodeled melt composition with that encountered in intergranularvoids of K-feldspar indicates that the melt migratedonly in between adjacent feldspar aggregates. Therefore wecan exclude melt loss or melt infiltration from externalsources, which would produce different reaction texturesand could result in crystallization of interstitial plagioclasewith different composition.7. Deformation Mechanisms[27] Microstructural features in quartz observed in samplesof all strain intensities reveal migrated grain boundariesand large grain size that increases even more during thedevelopment of type 3 microstructure. Microstructures andCPO patterns of quartz in type 2 and 3 samples show clearactivity of grain boundary migration (GBM) accommodateddislocation creep mechanism [Jessell, 1987; Hirth andTullis, 1992] and CPO patterns correspond to the shapesof deformation inferred from the strain analysis.[28] Microstructural data show isometric shapes of polygonalfeldspar grains, absence of shape preferred orientationand weak or no CPO for both feldspars in type 2sample. These microstructural features and high finite strainintensities attained are indicative for grain boundary diffusionaccommodated grain boundary sliding (D gb -GBS) inboth feldspars [Poirier, 1985; Wadsworth et al., 1999].Although large grain size (K-feldspar 300 mm and plagioclase200 mm) is not characteristic for such deformationmechanism at experimental strain rates [Tullis and Yund,Figure 12. Statistics of angles between poles to (001)planes and corresponding fractures crosscutting the grainsrepresented by frequency histogram and a doughnutdiagram. Colors in the doughnut diagram represent thethree groups of grains as depicted in the histogram.9of15287