Quantitative structural analyses and numerical modelling of ...

B10210ZÁVADA ET AL.: EXTREME DUCTILITY OF FELDSPAR AGGREGATESB10210Figure 7. Block diagram of the mylonite sample microstructure.The melt field in the legend represents grains ofphases assumed to have crystallized from interstitial melt.Note the melt topology; grain face pockets in XZ sectionand triple-point pockets in YZ section. Mineral abbreviationsare after Kretz [1983].(Figure 6c). Numerous interstitial round quartz grains withhigh dihedral angle occur in triple point junctions of thehost plagioclase aggregates often close to adjacent K-feldspar bands (Figure 6c). We suggest that these grainsdo not represent crystallized melt but resulted from annealingof disintegrated myrmekite fronts [Hanmer, 1982]. Locally,triangular K-feldspar grains with thin albitic ‘‘exsolution’’rim adjacent to oligoclase grains occur in plagioclasebands (Figure 6b). Thin wedge-shaped albite grains aredeveloped along (001) cleavage planes of muscovite grainsin plagioclase aggregates (Figure 6d). Plagioclase grainsadjacent to muscovite grains exhibit a rectangular networkof K-feldspar and albite, which is interpreted in terms ofmelt penetration along the cleavage planes of plagioclase(Figure 6d) [Mehnert et al., 1973; Dell’Angelo and Tullis,1988]. Quartz–K-feldspar boundaries show cuspate-lobateshapes and these boundaries are decorated with numeroussmall amoeboid grains of K-feldspar with high dihedral angleadjacent to the K-feldspar cusps (Figure 5).[19] Quartz and albite grains in K-feldspar aggregate,albite rims and scarce K-feldspar ‘‘pools’’ with albite‘‘exsolutions’’ in plagioclase bands, K-feldspar in plagioclasegrains, together with albite grains within mica crystalsand amoeboid grains of K-feldspar in quartz bands satisfythe criteria for presence of crystallized mineral phasesassumed to mimic residual melt topology [Rosenberg andRiller, 2000; Sawyer, 2001] and will be designated as ‘‘themelt’’ for simplicity. K-feldspar component crystallizedfrom melt probably grew mostly onto the older grainswithin K-feldspar bands, which would also explain theirperthite-free margins [Zulauf et al., 2002]. The crystallizationof melt as unlike phases in residual aggregates commonlyresults in disappearance of the banded rock texture.Transitions from a banded orthogneiss into a homogeneousnonfoliated rock can be observed on a single outcrop in thestudied area. However, in type 2 and type 3 samples, themelt distributes preferentially in K-feldspar bands.[20] The SEM imagery allowed identifying melt topologyalong intergranular voids as well as in intragranular fractures(Figures 5 and 6a). Quantitative analysis of meltpockets and their orientations allows creating a threedimensional(3-D) geometrical reconstruction of melt distributionin K-feldspar aggregates and analysis. Furthermore,we can define the geometrical relationship of intragranularfractures with crystal orientation and rock fabric.5.1. Intergranular Melt Topology[21] In order to depict and evaluate in detail the grainscalemelt distribution, grain boundaries on backscatteredelectron (BSE) images were traced and digitized from bothXZ and YZ sections (Figure 7). In moderately deformedorthogneiss type 2, melt in K-feldspar aggregate in XZsection occupies triple point junctions and extends into thinwedge-shaped melt films (maximum 30 mm in width) alonggrain faces perpendicular to the stretching lineation. Inhighly deformed orthogneiss type 3, the intergranular meltpockets (or seams [Rosenberg and Riller, 2000]) of aspectratio 2–3 show two submaxima in R f /f graph inclined at±20° relative to the apparent Z axis of the rock fabricellipsoid (Figure 8). Melt topology in YZ section of type3 orthogneiss is characterized by presence of melt ‘‘droplets’’at triple point junctions of K-feldspar grains. Rarely,the melt forms seams (30 mm in width) that line grainboundaries perpendicular to the foliation. These wide pocketsare likely to result from oblique sections of seamsoriented at high angle to the principle stretching direction.The melt topology from two perpendicular sections allowscreating a simplified 3-D geometrical model of interstitialmelt distribution in K-feldspar aggregate. This model ischaracterized as an interconnected network marked bymelt walls parallel to YZ plane, connected by tubes parallelto X direction along triple point boundaries between threegrains (Figure 9).5.2. Intragranular Fractures Locally Filled by Melt[22] A closer observation of XZ section BSE images ofthe type 3 (mylonite) sample revealed that some K-feldspargrains are crosscut by intragranular fractures (Figure 10a)and some of these fractures are filled with wedge-shapedmelt films. Orientations of traces in XZ section of theseFigure 8. R f /f diagram of the intergranular melt pocketswithin the K-feldspar aggregate from the XZ section.7of15285