Quantitative structural analyses and numerical modelling of ...

108 J. FRANĚK ET AL.(a)perthite porphyroclasts were oriented with respect tothe penetrative S2 foliation and L2 lineation, representingXY, XZ and YZ sections.(b)(c)Petrography of the granulites with the S1 fabricRocks at outcrop H296 are white-grey, fine-grainedgranulites composed of alkali feldspar, quartz,plagioclase and garnet (0.2 mm), with minor biotite,kyanite (0.3 mm) and porphyroclasts (up to 17 mm)of perthitic alkali feldspar. The rocks record evidenceof the complete granulite facies structural evolutiondescribed above. Microscopically, S1 contains discontinuousbands or lenses dominated by plagioclase thatcontain numerous garnet, kyanite, some quartz andbiotite (Fig. 5b,d). The almost monomineralic S1quartz bands are recrystallized into elongated S2ribbons and only quartz accumulation into stripesindicates the original S1 layering (Fig. 5c). Less elongatedquartz grains are rarely preserved in pressureshadows of perthite porphyroclasts. Large perthiteporphyroclasts with numerous lensoidal to lamellaroligoclase exsolutions are recrystallized at their grainboundaries to a mixture of small K-feldspar grains(0.063 mm) with rare perthitic exsolution lamellaeand oligoclase grains (0.047 mm). The feldspardominatedbands with rare garnet are predominantlycomposed of this K-feldspar–plagioclase mixture withminor quartz (0.055 mm), ascribed to the D2recrystallization process (Figs 5a & 6a,b). In therecrystallized matrix composed of K-feldspar, plagioclaseand quartz, minor garnet, kyanite, biotite, rutile,ilmenite, zircon, monazite and apatite also occur(Fig. 6g,h).The perthitic porphyroclasts (up to 17 mm across)contain inclusions of quartz, garnet and kyanite, andmore rarely biotite, rutile, ilmenite, zircon, monazite,apatite and Fe-sulphide (Fig. 6a–f). Quartz inclusions(up to 1 mm across) commonly consist of a singlecrystal with oval or euhedral shape. Garnet (up to1.2 mm across) enclosed in perthite is euhedral and iscommonly surrounded by a thin corona of plagioclase.Subhedral kyanite inclusions are also separated fromperthite by a thick plagioclase corona. Biotite inclusionsin perthite have short prismatic habits, and are inplaces partially retrogressed to chlorite.Fig. 4. Field photographs of (a and b) passive F2 folds depictingpenetrative development of S2 axial cleavage across the foldedS1 compositional layering. (c) F3 folds reworking at amphibolitefacies conditions the granulite facies S2 mylonite.back-scattered diffraction (EBSD). To study the transitionfrom the S1 fabric into the S2 cleavage andthe associated P–T path, the rocks were collected fromone locality within the elliptical relict structuraldomain (outcrop H296; Fig. 2; 48°51¢52.343¢¢N, 14°19¢14.135¢¢E). Sixty thin sections containing 350 largeMineral chemistryTo specify the P–T path for the S1 and S2 fabrics, onesample (H296-S1A) with the S1 layering affected bythe S2 cleavage, as described above, was analysed indetail. It contains garnet, kyanite, perthitic K-feldspar,plagioclase, quartz, biotite, rutile, ilmenite, apatite andzircon. The composition of the minerals in the individualbands is similar. Large perthite grains includequartz, garnet, kyanite, rutile, ilmenite, apatite andzircon. Garnet included in large perthite and garnetfrom the matrix are zoned from core to rim withÓ 2010 Blackwell Publishing Ltd346