Quantitative structural analyses and numerical modelling of ...

B10210ZÁVADA ET AL.: EXTREME DUCTILITY OF FELDSPAR AGGREGATESB10210Figure 15. Illustration of the dilatancy mechanism andfracturing of K-feldspar grains. Lineation parallel to s 3 isvertical in geographic coordinates. (a) Low proportion ofmelt shuffled in intergranular spaces during GBS accommodatedby melt-enhanced diffusion creep along grainboundaries and dislocation creep within grains in bothfeldspars. (b) Dilatancy driven by cavitation or by meltoverpressure in K-feldspar that extracts the melt fromplagioclase intergranular films. Arrows designate thedirection of melt flux.that melt was present in both plagioclase and K-feldsparaggregates during deformation. At first, origin of melt-filledseams and intragranular fractures developed within K-feldsparaggregates is critically discussed using petrologicaldata. We further consider the processes responsible forenhancement of solid-state flow, given by the presence ofmelt phase, and the role of its redistribution on the rheologyof polyphase and strongly deformed rock in terms ofdraining and accumulation of melt in different mineralaggregates. Finally, we discuss the apparent rheologicalrelationship between ‘‘weak’’ feldspars and ‘‘strong’’quartz.8.1. Cavitation Versus Fracturing Driven by MeltOverpressure[31] In studied rocks, the melt topology is characterizedby preferential distribution of melt in K-feldspar intergranularpockets, which is similar to experiments conductedwith low amounts of melt and at relatively high differentialstress [Daines and Kohlstedt, 1997; Gleason et al., 1999;Rosenberg, 2001; Holtzman et al., 2003]. In addition, K-feldspar grains are affected by fractures of the same orientation,which show affinity to the (001) cleavage. Ourpetrological investigations have shown that the melt wasproduced by metamorphic reaction within mica-plagioclasebands (source) and migrated into K-feldspar bands (sink).This implies that individual mineral aggregates building therock can be characterized as open systems (Figure 15).There are two possible mechanisms, which could haveproduced the observed melt topology and intragranularfractures in K-feldspar.[32] If a fluid is introduced to the rock, e.g., by meltproducing metamorphic reaction, its pressure may overcomethe least principal stress s 3 as well as cohesion ortensile strength of suitably oriented planes in the aggregate[Hubbert and Rubey, 1959; Price and Cosgrove, 1990].These planes would in our case be represented by grainboundaries and (001) crystallographic planes oriented perpendicularto the stretching lineation (s 3 direction). As aresult of melt overpressure, the K-feldspar aggregate dilatesand the melt is accumulated in intergranular pockets andsome intragranular fractures. The aggregate hardens due toincreased frictional stress on melt free boundaries, becausemodeling of melt productivity suggests that no significantamount of melt was gained from external sources (‘‘dilationhardening’’ of Renner et al. [2000]).[33] The second model, explaining production of intergranularmelt pockets and intragranular fractures is theprocess of cavitation. Cavitation, formation of submicroscopiccavities driven primarily by diffusion and accumulationof vacancies, is the direct consequence of GBS. WhenGBS cannot be fully compensated by diffusion and/ordislocation creep controlled grain shape change, cavitiesnucleate at first on grain boundaries at high angle to thetensional direction (direction of s 3 )[Čadek, 1988; Kassnerand Hayes, 2003]. Cavities then grow and coalesce witheach other to form intergranular voids. Further cavitycoalescence can be caused by nucleation and growth ofcavities on grain boundaries at low angle to the tensionaldirection due to (1) formation of tensile and compressiveledges, where boundaries are not straight due to inhomogeneousplastic deformation of the grains or (2) dislocationpileups at grain boundaries (Zener-Stroh mechanism) oraround impurities in crystal lattice (Figure 16) [Vollbrecht etal., 1999; Kassner and Hayes, 2003]. Plastic deformation isfurther being localized to the ‘‘bridges’’ (intact grain boundarysegments) affected by further cavitation. The bridgesloose its stability and locally contract to form finally afracture which is driven by coalescence of cavities at its tip(Figure 16). In this way, clear macroscopic intragranularfractures develop [Čadek, 1988].[34] According to the cavitation model, opening of intergranularvoids and intragranular fractures during GBSproduces local underpressure (the volume of the systemincreases due to creation of voids in K-feldspar and thegrains support the voids as a load bearing framework),Figure 16. (a) Illustration of the cavitation mechanismgenerated by GBS (grain boundary sliding) and formationof intergranular and intragranular fractures at the onset offinal creep failure of the aggregate. T and C designate thetensional and compressive sector of a grain boundary ledge.Z-S designates illustration of the Zener-Stroh mechanism ofcavity formation [Kassner and Hayes, 2003]. See descriptionin text.11 of 15289