Quantitative structural analyses and numerical modelling of ...

EXHUMATION IN LARGE HOT OROGEN 285and medium- to high-pressure recrystallization ofcoarse-grained granite (Hasalova´ et al., 2008a). Inareas inferred to be a part of the orogenic lower crust,based on the presence of HP relicts and the high degreeof anatexis, the S 2 fabric is defined by steep compositionallayering in amphibolites, felsic metavolcanicrocks, orthogneisses and paragneisses.In the orogenic middle crust the S 2 fabric is characterizedby a steep lithological alternation of paragneisseswith amphibolites, felsic metavolcanic rocks,quartzites, marbles and calcsilicate rocks (Romanova´&Sˇtípska´ , 2001; Racek et al., 2006). In the amphibolites,the steep fabric is characterized by alignedhornblende and recrystallized plagioclase ribbons inassociation with garnet that indicates medium-pressureconditions for formation of this fabric. Evidence of theprograde fabrics in the paragneisses of the Moldanubiandomain occurs only as rare oriented inclusions ofstaurolite and kyanite in garnet and remnants of theseminerals in the matrix (Racek et al., 2006). Even insome areas of the Moldanubian domain where themacroscopic fabric of the paragneisses remained steep,the higher-grade assemblages were commonly overprintedpassively by lower-grade assemblages with sillimaniteor cordierite, which we ascribe to the highreactivity of the paragneisses in the presence of melt.As the other lithologies do not show variation of theassemblage with changing P–T conditions, the mainguide in characterizing the steep fabric in the orogenicmiddle crust of the Moldanubian domain is the steeplithological alternation combined with peak P–T conditions.In the Lugian domain, in the orogenic middle crustthe characterization of the steep fabric is much betterconstrained because of the absence of anatexis in themetasedimentary rocks and the absence or limitedextent of anatexis in the orthogneisses. In the orthogneisses,the S 2 fabric is marked by steep alternation ofrecrystallized augen and ribbons of quartz and feldsparand by the orientation of biotite and muscovite. Steeplocalized ultramylonitic zones are developed locally.The prograde and peak P–T conditions of the steepfabrics in Lugian metapelites are constrained by thesuccessive growth of garnet, staurolite and kyanitein the micaschists (Romanova´ &Sˇtípska´ , 2001; Jastrzębski,2005; Murtezi, 2006).As described above, the flat fabric in the orogeniclower and middle crust commonly originates throughhorizontal folds affecting the steep fabric and is onlyrarely associated with sub-horizontal or gently dipping,thrust-related shear zones. Highly anisotropicrocks, such as banded orthogneisses, show strong axialplanar crenulation cleavage, which, in places, passesgradually into a sub-horizontal foliation with evidenceof the early vertical anisotropy occurring only as a fewrootless folds (Fig. 8h). The intensity and characteristicmineralogy of the flat reworking is variabledepending on the specific area, lithology and degree ofmetamorphism. However, some mineralogical featuresare common for the Ky–Kfs granulites, the orthogneissesand the paragneisses. These include thewidespread development of the assemblages garnet–sillimanite–K-feldspar, in the Moldanubian domain,and garnet–muscovite ± sillimanite, in the Lugiandomain, showing the overall moderate pressure characterof the S 3 reworking and the higher temperature inthe Moldanubian domain compared with the Lugiandomain. Because of the variability of the S 3 structuresalong the orogen, the detailed mineralogical characteristicsand P–T conditions for both the S 2 and S 3fabrics for the four areas identified above are describedbelow.P–T evolution of the Moldanubian domainRacek et al. (2006) examined petrological relationshipsbetween the steep and flat fabrics in the orogenic lowerand middle crust of the southern Moldanubian domain(Figs 5 & 9a). Modelling of garnet zoning in eclogitefrom the western part of the orogenic lower crust (no. 5in Figs 5 & 9a) indicates burial process to have takenplace from around 10 kbar and 700 °C to around 15–16 kbar and 800 °C. Similar peak conditions aredetermined for kyanite–sillimanite migmatite structurallyabove the eclogite (around 14 kbar at 750 °C;no. 6 in Figs 5 & 9a). However, these prograde andpeak conditions are associated with the steep S 2 fabriconly as the early structural relations of relict mineralsFig. 7. (a) Structural map of a critical area showing thin vertical belt of the orogenic lower crust surrounded by orogenic middlecrust in the eastern part of the Lugian domain with regional S 2 and S 3 fabrics as in Fig. 5 (after Sˇtı´ pska´ et al., 2004). See geological mapin Fig. 2 for regional location. The map shows a narrow body of the orogenic lower crustal granulites, preserving relicts of the S 2fabrics, and the surrounding fabric of the orogenic middle crust. The fabric in the mafic lower crust and the syntectonic tonalite is alsoshown. The structural trends (thin layers – S 3 fabrics, thick layers – S 2 fabrics) indicate extrapolations of major orientations ofstructural fabrics in the field. Density of trend lines indicates the homogeneity of fabric elements in the field. Stereograms:A – stereogram of poles to S 2 fabrics in the garnet–omphacite-bearing granulites; B – stereogram of poles to S 3 fabrics from theorogenic lower crust and surrounding orogenic middle crust rocks represented by thin dots and contoured. Thick dots in the stereogramsshow S 3 foliations typical for the Ordovician metagabbros and Carboniferous tonalite sill of the Lugian mafic complex.C – Stereogram of L 3 directions from the granulite belt and surrounding orogenic middle crust (thin dots and contoured data).Black squares show mineral and stretching lineations from mylonitized gabbros and granodiorite sill. Note strong discrepancy inorientations of S 3 foliations and L 3 lineations between granulite belt and adjacent Ordovician mafic complex. Equal area projection,lower hemisphere, contoured at multiples of uniform distribution. (c) Simplified cross-section shows the vertical central granulitebelt showing thrusting of the orogenic lower crust over the Ordovician leptyno-amphibolite complex. Note the syntectonic D 3intrusion of the 340 Ma granodiorite sill and discordant fabrics in the 510 Ma leptyno-amphibolite complex.Ó 2007 Blackwell Publishing Ltd145