Quantitative structural analyses and numerical modelling of ...

48 P. HASALOVÁ ET AL.refractory. In the present case, Hasalova´ et al. (2008b)report continuous trends in whole-rock geochemistryand mineral compositions for the sequence of rocktypes, but different Nd isotopic composition for thetype I orthogneiss compared with the rest of the sequence,which precludes of in situ anatexis in a closedsystem.Melt infiltration modelThe discrepancies between the evolutionary trends wereport and generally accepted trends for anatecticterranes require an appropriate explanation that isconsistent with the structural, quantitative microstructuraland mineral compositional data. As a possibleexplanation, we introduce the concept of meltinfiltration from an external source, where melt passespervasively along grain boundaries through the wholerockvolume and changes macroscopic (Fig. 2) andmicroscopic (Fig. 3) appearance of the rock. Thisprocess is characterized by resorption of old phases,nucleation of new phases along high-energy like–likegrain boundaries and modification of mineral andwhole-rock compositions. These gradual changes areaccompanied by grain size reduction (Fig. 7) andprogressive disintegration of former aggregate (layered)distribution of original phases (Fig. 9). We suggestthat the individual migmatite types representdifferent degrees of equilibration between the host rockand migrating melt. It should be emphasized, that allthese processes occur along a retrograde path duringexhumation of the Gfo¨ hl Unit. We are aware that adecrease in P–T conditions during melt infiltration is afundamental and limiting factor for the model proposed.The amount of melt and its connectivity are criticalparameters controlling melt mobility and the rheologicalbehaviour of melt-present rocks. To constrainthese parameters both AMS and EBSD were used.Using AMS, it is possible to distinguish between solidstatedominated deformation mechanisms in themelanosome and free rigid body particle rotation in theleucosome (e.g. Ferré et al., 2003). On the other hand,using the EBSD technique enables us to distinguishdeformation mechanisms in the solid framework andto constrain the mechanical role of melt during thedeformation.AMS fabric origin: solid framework or melt controlleddeformationThe AMS study shows that the magnetic anisotropy isdominated by biotite. The oblate shape of magneticellipsoid and high degree of anisotropy of type I orthogneissand type II migmatite (Fig. 10a) are consistentwith strong preferred orientation of biotite and thefact that biotite has a intrinsically oblate shape of thesingle-grain magnetic ellipsoid (Zapletal, 1990; Martı´n-Hernandéz & Hirth, 2003). The type III and IV migmatitesreveal partly resorbed biotite flakes uniformlydispersed in the rock marked by slightly weaker degreeof magnetic anisotropy and less oblate fabric ellipsoidcompared with types I and II migmatite (Fig. 10a).This contrasts with common granites and diatexitesfrom other migmatitic terranes which show significantlylower values of degree of anisotropy and highlyvariable shapes of AMS ellipsoids (Fig. 10a; Bouchez,1997; Ferre´ et al., 2003).Numerous natural studies supported by numericalmodelling indicate that the magnetic susceptibility inviscously flowing magmas is characterized by a verylow degree of anisotropy, pulsatory fabrics and dominantlya plane strain AMS ellipsoid shape (Blumenfeld& Bouchez, 1988; Hrouda et al., 1994; Arbaretet al., 2000). A comparison of the AMS fabrics withthose of diatexites and results of numerical modelsindicate that the intensity of the AMS fabric of typesIII and IV migmatites does not originated from freelyrotated biotite in viscously flowing melt. On the contrary,we argue that the AMS fabric in all types ofmigmatites resembles fabrics usually acquired throughsolid-state deformation of a load-bearing framework,similar to melanosomes in migmatites (Ferre´ et al.,2003). To understand the mechanisms responsible fordevelopment of such fabrics the grain-scale deformationmechanisms and melt behaviour in individual rocktypes is discussed.Deformation mechanismsExperimental studies of low melt fraction rocks deformedunder high differential stress show that matrixminerals deform by grain boundary migrationaccommodated dislocation creep (DellÕ Angelo et al.,1987; Walte et al., 2005). Strong shape and GBPO offeldspar (Figs 8 & 9) as well as LPO of residual quartzgrains (Fig. 11a) in the type I orthogneiss may beinterpreted in terms of plastic deformation consistentwith a dislocation creep deformation mechanism(Rosenberg & Berger, 2001). However, the weak LPOof residual grains of both feldspars (Fig. 12a, e) in thetype I orthogneiss suggests a contribution of grainboundary sliding during the development of themicrostructure. In other words, the type I microstructurecorresponds to a transient microstructure interms of decreasing activity of dislocation creep andenhancement of diffusion controlled processes.Decrease in SPO and GBPO and constantly weakLPO in feldspar of type II migmatite (Figs 8 & 9) maybe interpreted as a result of melt-enhanced diffusioncreep (Garlick & Gromet, 2004). However, the largequartz grains reveal intense activity of basal Æaæ slipsuggesting important plastic yielding of this mineral(Fig. 11a). Elongate pockets inferred to represent formermelt oriented at a high angle to the stretchinglineation in the type I orthogneiss (Fig. 5a) and type IImigmatite indicate that the melt distribution wascontrolled by the deformation. This is supported by theÓ 2007 Blackwell Publishing Ltd334