Quantitative structural analyses and numerical modelling of ...

PRECURSOR AND RHEOLOGY OF VARISCAN GRANULITES 123Type III microstructureThe quartz LPO shows a high angle Type II crossgirdleof c-axes distribution with a maximum either inthe centre or near edges of the girdle. The LPO issimilar to those measured from the S2 matrix microstructureand implies a combination of prism and rhomb + slip (Fig. 15b). Plagioclaseshows the (010) and (001) planes subparallel to foliationand a girdle distribution of [100] and [201] alongthe foliation with single maxima parallel to the lineation.This pattern points to activity of three slip systems,namely [100] (010), [201] (010) and [100] (001). Inthe case of K-feldspar, all samples show a pronouncedmaximum of (010) planes parallel to foliation, whereasother common slip planes do not show such relationship.A girdle distribution is observed for the [001] and[100] directions, with a maximum of the [001] directionoriented parallel to the stretching lineation. Theseattributes indicate that the activity of [100](010) slipsystem is the characteristic for both feldspars.DISCUSSIONWe discuss the origin and petrological significance ofthe S1 fabric as a precursor for development of characteristicgranular microstructure of Variscan felsicgranulites. Subsequently, an attempt is made to discussthe origin of granular fabric, its specific quantitativemicrostructural characteristics and its importance forunderstanding deep crustal flow processes. Finally, thepetrology, microstructure and LPO of various types ofgranulites are discussed in terms of the rheologicalevolution of orogenic lower crust and its extrusion(Franěk et al., 2011) during the Variscan orogeny inEurope. Figure 16 puts the microstructural evolutionin context with the tectonic history.Interpretation of S1 fabric: HP orthogneissThe observed structural succession, field distributionof the S1 relicts and microstructural relations implythat the perthitic alkali feldspar, plagioclase and quartzaggregates preserved locally within the S1 compositionallayering represent a remnant of the precursor ofthe Blansky´ les felsic granulites. The best preservedrelicts of the S1 layering contain a substantial amountof large alkali feldspar (presently perthite) and largequartz grains in the microstructure. The inclusions ofkyanite, high-grossular garnet and rutile suggest thatthe large alkali feldspar crystallized at HP conditions(Figs 8 & 16). Despite diffusional re-equilibration ofmost minerals, the peak P–T conditions related to thegrowth of large alkali feldspar are estimated at1.6)1.8 GPa and 850)880 °C. The abundant idiomorphicor oval-shaped quartz inclusions in the largeperthite porphyroclasts point to a lack of plasticdeformation during alkali feldspar growth. Suchinclusions may develop either in granites, or duringmelt-assisted recrystallization in migmatites (Mehnert,1968, pp. 111, 192). The lenticular plagioclase-richaggregates are interpreted as completely recrystallizedremnants of large plagioclase grains complementary tothe alkali feldspar porphyroclasts (Figs 9c,d & 16, 3Dblock diagram 1). Consequently, the original rock was acoarse-grained granite ⁄ orthogneiss crystallized underHP conditions in the kyanite stability field. Existence oftwo complementary feldspars at peak conditions contrastswith laboratory experiments of Tropper et al.(2005) focused on the South-Bohemian felsic granulites,where only a single alkali feldspar existed above 850 °C.The S1 layering of initially coarse-grained quartz,K-feldspar and plagioclase-rich layers may be interpretedas a result of solid-state deformational segregationof minerals with different plasticity at hightemperatures, a process common in formation of, e.g.layered orthogneisses. The length of the bands thenindicates significant strain during the D1 phase.Assuming growth of large alkali feldspar during theD1 episode, the overgrowth of quartz, garnet andbiotite by the large feldspar indicates a dominance ofgrowth processes resulting in overall coarsening of thegneiss (Higgins, 1998; Lexa et al., 2005). Such crystalgrowth indicates that the temperature ⁄ strain rate ratiowas rather high and thermodynamic modelling (Fig. 8)suggests contemporaneous partial melting. High-Ppeak conditions (1.6–1.8 GPa) indicate that the flowoccurred at the bottom of thickened crust as alsosuggested by other studies (e.g. Sˇtípska´ & Powell, 2005;Racek et al., 2006; Tajcˇmanova´ et al., 2006). Therefore,the S1 fabric probably originated during lowercrustal flow at the bottom of a thickened crustal rootand at progressively increasing temperatures (Fig. 16).The inclusions within the perthite porphyroclasts,the S1 compositional layering and the S2 myloniticfoliation in the BLG are similar to the oldest structuresdescribed from the Saxonian Granulitgebirge (Behr,1961). In addition, all of the Variscan felsic granulitesreveal similar geochemistry and geochronology(340 Ma peak). Consequently, both the mentionedmassifs and other Variscan felsic granulite massifs aswell may have evolved from such HP coarse-grainedorthogneisses.Origin of granular matrix in granulitesThe microchemical and textural relations indicate thatthe initial large alkali feldspar underwent heterogeneousstep-by-step recrystallization during the D2event, which resulted in formation of a fine-grainedmatrix. This recrystallization probably postdatedexsolution of the coarse perthitic plagioclase because(i) the newly formed Type I K-feldspar in the granuliticmatrix is devoid of such large braided exsolutions, and(ii) the perthite exsolutions reveal 1–2% higheranorthite content than the cores of Type I matrixplagioclase indicating a bit higher temperature of formation(Fig. 16, 3D block diagram 2). Nevertheless, itÓ 2010 Blackwell Publishing Ltd361