Quantitative structural analyses and numerical modelling of ...

B10210ZÁVADA ET AL.: EXTREME DUCTILITY OF FELDSPAR AGGREGATESB10210Figure 9. Simplified 3-D melt topology model in K-feldspar aggregate of the mylonite type 3 orthogneiss. Seetext for explanation.fractures are represented by a rose diagram in Figure 10band show strong maximum subparallel to the Z direction ofthe rock fabric ellipsoid and perpendicular to the horizontalstretching lineation (X direction). Furthermore, we havemeasured the CPO of grains affected by intragranularfractures, which exhibit clustering of poles to (001) planesin stretching lineation direction, while poles to (010) revealmaximum in the Y direction of the sample coordinatesystem (Figure 11). In order to evaluate relationship betweenorientations of crystallographic directions and intragranularfractures within corresponding grains, theorientations of fractures measured by U stage and graincrystallographic orientations obtained by means of EBSDhave been compared. Poles to all measured fractures exhibitstrong maximum close to the X direction, which is extendedtoward the Z direction (Figure 11). Analysis of anglesbetween pole of fracture and pole to (001) of K-feldsparcrystal shows that small angles characterize grains with theirpoles to (001) planes subparallel to the stretching lineation.In contrast, this angle is high for grains with (001) poles athigh angle to the stretching lineation (Figure 11). Thestatistics of angles between fracture orientations and (001)planes clearly shows that the majority of fractures aresubparallel to the (001) plane (in 52% of the fracturedgrains the angle is less than 30°, see Figure 12). Morphologyof the fractures shows distinctive features for bothgroups of grains with low (60°) angle.In group 1 (60°), the grains show single curved fractures orinward tapering melt intrusions (Figure 10a). In addition,there exist intragranular curved fractures that were initiatedfrom triple point junctions and locally adjoin to the cleavageof the grain at their tips (Figure 10c).6. Melt Producing Reaction and Melt VolumeEstimates[23] An important issue is the origin of the interstitial meltdescribed in section 5. The melt could have been producedby in situ melting reaction or by infiltration from externalsources, which is reflected by the melt composition underestimated PT conditions and possible reaction textures[Sawyer, 2001]. Another important issue is the melt volumeproduced and volume change of the melting reaction, asboth these variables control the embrittlement of rock atdynamic conditions during breakdown reactions of somehydrous phase and/or at sufficient heating rates [Connolly etal., 1997; Rosenberg, 2001; Rushmer, 2001; Holyoke andRushmer, 2002]. These data have critical impact on thedescribed melt topology, deformation mechanisms andrheology of studied orthogneiss and will be further discussed.Figure 10. (a) Detail of two fractured grains. Arrows and numbers designate group 1 and 2 fracturedgrains. Scale bar 100 mm. (b) Rose diagram of traces of the intragranular fracture orientations in K-feldspar bands (XZ section of the sample coordinate system). (c) Sketch of curved and splittingintragranular fractures initiated from triple point junctions. Melt is gray; straight lines in the grains markthe cleavage.8of15286