Quantitative structural analyses and numerical modelling of ...

ORIGIN OF FELSIC MIGMATITES 33during horizontal shortening of the thickened orogenicroot (Schulmann et al., 2005).The second deformation (D 2 ) is associated withreworking and folding of the S 1 compositional layeringin banded orthogneiss, so that S 1 is only preservedlocally in elongate relict domains (Fig. 2a). The D 2shearing is attributed to horizontal flow of hot lowercrust in a zone up to 10 km wide at a mid-crustal levelabove the Brunia promontory over distances of severaltens of kilometres (Schulmann et al., 2005). Relicdomains with gently folded S 1 fabric are surrounded byhighly deformed zones with tightly folded S 1 fabric(Fig. 2). The composite S 1)2 fabric is characterized bya banded structure with diffuse boundaries betweenpolymineralic K-feldspar- and plagioclase-richdomains similar to a stromatic migmatite structure(type II) (Fig. 2b). Locally the S 1 fabric is completelytransposed and a new S 2 foliation is dipping gently tothe SW (Fig. 1b). A sub-horizontal, gently S–SWplunging L 2 lineation (Fig. 1b) is mostly defined bypreferred orientation of sillimanite.Detailed field observations reveal that with ongoingdeformation the type II rock gradually pass into moreisotropic rock (type III) composed of K-feldspar–quartz and plagioclase–quartz aggregates (Fig. 2c) andcontaining rootless folds of the deformed S 1 fabric.This rock type alternates with irregular bodies orelongate lenses of fine-grained isotropic felsic rock(type IV, Fig. 2d), which in this region traditionallyhas been described as a nebulitic migmatite (Matějovska´,1974). Such a structural sequence originatedthrough intense D 2 deformation superimposed on anolder steep anisotropy and was identified in outcropscale along several sections. These observations aresupported by the existence of macroscopically visibleleucosomes or granitic veins that are also parallel to S 2and form isolated elongate pockets and lock-up shearbands.This area has been extensively studied by Mateˇjovska´(1974) and Dudek et al. (1974) who used theclassical migmatite terminology of Mehnert (1971) forthe above-described rock types. These authors identifiedtype I rock as banded orthogneiss, rock type II asstromatic migmatite and rock type IV as nebuliticmigmatite. Rock type III resembles the schlierenmigmatite of Mehnert (1971). Because the Gfo¨ hl Unitis considered as one of the largest migmatitic terranesof the Variscan belt, the traditional migmatite terminologywas adapted to these rocks.MICROSTRUCTURAL OBSERVATIONSThe microstructural characteristics including grainsize, grain shape and grain boundary geometry werestudied in each of the four rock types and inK-feldspar- and plagioclase-rich domains. Thin sectionswere cut perpendicular to the foliation andparallel to L 2 lineation (XZ section). To discriminateK-feldspar from plagioclase, the thin sections werestained according to the method of Bailey & Stevens(1960).Type I: banded orthogneissThis rock type is a fine-grained orthogneiss with 0.25-to 2.0-mm-thick layers of recrystallized plagioclase(30 modal%), K-feldspar (40 modal%) and quartz(20 modal%), separated by discrete layers of biotite(10 modal%) commonly associated with minor sillimaniteand garnet (Table 1, Fig. 3a).K-feldspar forms completely recrystallized aggregates(0.2–0.8 mm grain size) with straight grainboundaries locally meeting in triple point junctions at120° (Fig. 4a). Numerous rounded inclusions of quartz(0.05 mm) occur preferentially at triple points, alongplanar boundaries or in cores of feldspar (Fig. 4a).Plagioclase (An 10)20 ) is present in K-feldspar aggregatesas small interstitial grains or forms thin filmspreferentially tracing those K-feldspar boundaries thatare oriented at a high angle to the foliation (Fig. 5a).Rarely, tiny interstitial biotite is present in the K-feldspar-rich bands.Plagioclase aggregates (0.2–0.5 mm) are composedof an equidimensional polygonal mosaic with straightboundaries, and minor interstitial quartz and biotite(Fig. 4b). The plagioclase grains show abundanttwinning and form a foam-like texture with a perfecttriple point network of grain boundaries. Plagioclaseexhibits normal zoning with homogeneous oligoclasecores (An 24)28 ) and more sodic (An 10)18 ), clear,2to10lm-thick rims at boundaries with K-feldspar.Plagioclase grain size continuously decreases from thecentre of an aggregate towards its borders. Quartzoccurs as small (0.01–0.05 mm) rounded inclusions orinterstitial grains, whereas K-feldspar exhibits characteristiccuspate shapes (Fig. 5b). Tiny biotite grains(0.1–0.5 mm in length; X Fe ¼ 0.42–0.48, Ti ¼ 0.2–0.27 p.f.u.) commonly occur along the plagioclaseboundaries that are sub-parallel to the foliation(Fig. 3a).Quartz ribbons 0.3–1.0 mm wide are composed ofelongate grains with straight grain boundaries perpendicularto the ribbon margin (Fig. 3a). Quartz–feldspar boundaries are gently curved, with cusps thatpoint from feldspar to quartz. Biotite-rich layerscommonly show decussate microstructure, which is atextural equivalent of the foam-like texture of the felsicminerals (Vernon, 1976). Contacts between biotiteandplagioclase-rich layers are marked by numerous(