Quantitative structural analyses and numerical modelling of ...

PRECURSOR AND RHEOLOGY OF VARISCAN GRANULITES 125cannot be fully excluded that the large perthitic exsolutionsin alkali feldspar porphyroclasts originatedduring later stages of cooling and decompression alongthe exhumation path.The recrystallization must have initiated by deformation-inducedconversion of the (perthitic) parentalalkali feldspar directly into small K-feldspar grains byan uncommon process, which is discussed in the nextparagraphs. Plagioclase was then gradually redistributedalong the new K-feldspar grain boundaries (e.g.Fig. 9b,f), presumably via grain-boundary diffusion.The sharp compositional boundaries between An 23cores and An 12 rims of the Type I plagioclase(Fig. 10a) indicate that they did not develop by continuousgrowth and that they were not significantlyaffected by later diffusion. We suggest that at first, theAn 23 plagioclase grains coalesced in triple junctions ofthe new K-feldspar grains by grain-boundary diffusionmechanism. They originated by reduction of surfaceand coalescence of preexisting coarse and compositionallysimilar An 24 plagioclase perthitic exsolutions,assuming perthitic nature of the parental alkali feldspar.The new grain boundaries were subsequentlyovergrown by An 12 plagioclase, the components forwhich were probably released from surroundingK-feldspar by volume diffusion. This process is indicatedby gradual zoning at rims of adjacent K-feldspargrains (Fig. 10a). The CSD patterns (Fig. 13) showthat the formation of the Type I K-feldspar–plagioclasemosaic originated by a nucleation-dominatedprocess accompanied by limited crystal growth, comparedwith the later Types II and III microstructure(Lexa et al., 2005; Hasalova´ et al., 2008b). The nucleationdensity of plagioclase was significantly higherthan that of K-feldspar.The most important manifestation of the perthiterecrystallization is redistribution of Or, Ab and Ancomponents on a micro-scale in a chemically closedsystem, resulting in development of the fine-grainedgranular matrix (Fig. 16, 3D block diagram 3). Thehigh amount of unlike grain boundaries in the granularmatrix preserved inside parental perthite indicates atendency to regular distribution and thus active intermixingof the K-feldspar and plagioclase. The regulardistribution represents the energetically lowest state ofsuch a polyphase material (Seng, 1936; DeVore, 1959)and progressive straightening of grain boundarygeometries of K-feldspar and plagioclase further minimizesthe surface energy by grain shape simplificationtypical for grain boundary reduction (Passchier et al.,1992). According to this model the recrystallizationthen appears to be substantially driven by a decrease ofchemical and surface energy in the metastable (perthitic)alkali feldspar. The concept of surface energyminimization was suggested by Flin (1969), who proposedthat the regular distribution is a consequence ofa smaller interfacial energy of unlike boundaries incomparison with like–like boundaries. However,Ramberg (1952) considered the interfacial energies tobe too small to drive diffusional mass transfer ingranulites.The weak but systematic scatter of lattice orientationsof new plagioclase and K-feldspar grains withrespect to the parental perthite (Fig. 14) points tocontribution of stress to the bulk recrystallizationprocess. Such a process of non-coherent decompositionof metastable alkali feldspar may be triggeredeither by water addition or by the application ofexternal stress (Brown & Parsons, 1989 and referencestherein). As a lack of hydrous phases and presumablylack of water is typical for the granulite facies evolutionof these rocks, the activity of external stressseems to be better candidate to trigger the recrystallizationprocess.The almost identical LPO of plagioclase andK-feldspar, regular grain distribution and high nucleationdensity CSD pattern of the polymineralicaggregate developed by recrystallization of solidsolution crystal has not been previously described innature. The sharp recrystallization front advancinginto parental perthites represents a moving grainboundary driven by both chemical and deformationalprocesses. According to Stu¨ nitz (1998), such recrystallizationcan be considered as a combination ofstrain- and chemically induced grain boundarymigration processes. At the temperature and chemicalconditions given during the D2, the activation energyof the grain boundary migration process was probablylower than that of dynamic recrystallization of themetastable (perthitic) alkali feldspar. Material scienceliterature (e.g. Saheb et al., 1995; Sennour et al., 2004)has examples of sharp recrystallization fronts advancinggradually into intact grains of alloys (Fig. 9a,b,f)explained by the chemically induced grain boundarymigration or a similar discontinuous precipitationprocess, which in many aspects resembles the alkalifeldspar recrystallization. According to Yoon (1995),the discontinuous precipitation in metastable alloysrepresents largely autocatalytic recrystallization drivenmainly by coherency strain energy. Activation of thisprocess depends upon composition, temperature, stressand strain, elastic anisotropy, crystallographic relationsand boundary curvature of involved grains(Yoon, 1995). Once triggered, e.g. by external strain, itcauses very efficient heterogeneous decomposition ofoversaturated solid solutions (like alkali feldspar) via(mainly chemically driven) grain boundary migration(Hay & Evans, 1987; Saheb et al., 1995; Sennour et al.,2004).In conclusion, the formation of the granular matrixresembles the discontinuous precipitation triggered bystraining of the probably perthitic alkali feldspargrains during D2 deformation (Fig. 16). The processstarts preferably on pre-existing grain boundaries withother feldspar porphyroclasts or with garnet. Themigration of the recrystallization front is driven mainlyby the metastability of the (perthitic) alkali feldspar. Inturn, the metastability of parental feldspar controls theÓ 2010 Blackwell Publishing Ltd363