Prograde and retrograde metamorphic fabrics a key for understanding burial and exhumation in orogens (Bohemian Massif)

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1 J. metamorphic Geol., 011, 9, doi: /j x Prograde and retrograde metamorphic fabrics a key for understanding burial and exhumation in orogens (Bohemian Massif) E. SKRZYPEK, 1 P. ŠTÍPSKÁ, 1 K. SCHULMANN, 1 O. LEXA,3 AND M. LEXOVÁ 4 1 Ecole et Observatoire des Sciences de la Terre UMR 7516, Université de Strasbourg, 1, rue Blessig, Strasbourg, France (etienne.skrzypek@eost.u-strasbg.fr) Institute of Petrology and Structural Geology, Charles University, Albertov 6, 1843 Prague, Czech Republic 3 Czech Geological Survey, Klárov 3, Prague, Czech Republic 4 Mykuna, Nad Hercovkou 15, Prague 8, Czech Republic ABSTRACT In the Orlica S nie_znik Dome (NE Bohemian massif), alternating belts of orthogneiss with high-pressure rocks and belts of mid-crustal metasedimentary metavolcanic rocks commonly display a dominant subvertical fabric deformed into a subhorizontal foliation. The first macroscopic foliation is subvertical, strikes NE SW and is heterogeneously folded by open to isoclinal folds with subhorizontal axial planes parallel to the heterogeneously developed flat-lying foliation. The metamorphic evolution of the midcrustal metasedimentary rocks involved successive crystallization of chlorite muscovite ilmenite plagioclase garnet, followed by staurolite-bearing and then kyanite-bearing assemblages in the subvertical fabric. This was followed by garnet retrogression, with syntectonic crystallization of sillimanite and andalusite parallel to the shallow-dipping foliation. Elsewhere, andalusite and cordierite statically overgrew the flat-lying fabric. With reference to a P T pseudosection for a representative sample, the prograde succession of mineral assemblages and the garnet zoning pattern with decreasing grossular, spessartine and X Fe are compatible with a P T path from kbar C to peak conditions of 6 7 kbar 630 C suggesting burial from 1 to 5 km with increasing temperature. Using the same pseudosection, the retrograde succession of minerals shows decompression to sillimanite stability at 4 kbar 630 C and to andalusite cordierite stability at 3 kbar indicating exhumation from 5 km to around 9 1 km. Subsequent exhumation to 6 km occurred without apparent formation of a deformation fabric. The structure and petrology together with the spatial distribution of the metasedimentary metavolcanic rocks, and gneissic and high-pressure belts are compatible with a model of burial of limited parts of the upper and middle crust in narrow cusp-like synclines, synchronous with the exhumation of orogenic lower crust represented by the gneissic and high-pressure rocks in lobe-shaped and volumetrically more important anticlines. Converging P T D paths for the metasedimentary rocks and the adjacent high-pressure rocks are due to vertical exchanges between cold and hot vertically moving masses. Finally, the retrograde shallow-dipping fabric affects both the metasedimentary metavolcanic rocks and the gneissic and high-pressure rocks, and indicates that the 15-km exhumation was mostly accommodated by heterogeneous ductile thinning associated with unroofing of a buoyant crustal root. Key words: Bohemian Massif; burial and exhumation; prograde and retrograde fabrics; P T D path; P T pseudosection. INTRODUCTION Combined petrological and geochronological studies that constrain P T t paths have contributed significantly to the understanding of the thermal processes related to exhumation of deep-seated rocks in orogens (e.g. Duchêne et al., 1997; Rubatto & Hermann, 001; Carswell et al., 003). However, these studies are commonly not combined with petrological and microstructural investigations of metamorphic fabrics, precluding fuller understanding of tectonic mechanisms of exhumation. Several studies show that shallow-dipping fabrics are commonly associated with retrograde metamorphic assemblages and telescoped cooling ages which are interpreted as a result of exhumation processes driven by extension or gravitational spreading of thickened crust (e.g. Dewey et al., 1993; Koyi et al., 1999; Vanderhaeghe et al., 1999). By contrast, other petrological and structural studies show that the retrograde P T evolution in orogenic lower-crustal rocks may be connected with subvertical fabrics suggesting vertical extrusion flow to be a major exhumation mechanism (e.g. Sˇtı pska et al., 004). 451

2 45 E. SKRZYPEK ET AL. Both concepts are deduced from studies of deepseated rocks of the orogenic lower crust, but the link between structural and metamorphic records in nearby mid- to supracrustal rocks is rarely concurrently explored (e.g. Racek et al., 006). Once lower- and mid-crustal rocks with convergent P T paths reach lower-grade conditions, they commonly show the same deformation pattern (e.g. Sˇtı pska et al., 006) indicating that they shared the same tectonic evolution during the last part of the exhumation process. Because exhumed lower-crustal rocks lack sensitive records of the late P T evolution, it may be possible to use the retrograde fabrics and associated metamorphic records of mid-crustal rocks to infer the exhumation evolution of the whole crust. Moreover, in mid-crustal units the tectono-metamorphic record related to the thickening process is commonly better preserved because the exhumation structures are heterogeneous and less intense. Therefore, the petrological and structural evolution of mid-crustal rocks is also likely to provide information about the burial process that is not preserved by the orogenic lower crust. An area where these burial and exhumation processes can be deciphered occurs in the internal part of the Bohemian Massif (Fig. 1). There, recent determinations of the P T D paths in lower- and mid-crustal rocks led to the development of an exhumation model involving vertical extrusion of deep-seated rocks and subsequent subhorizontal deformation at a mid-crustal level (Sˇtı pska et al., 004; Franeˇk et al., 006; Tajcˇmanova et al., 006). This concept differs from diapiric models (Warren & Ellis, 1996; Lexa et al., 010) in invoking the predominance of lateral forces (e.g. Bell & Johnson, 1989) instead of buoyancy forces (Whitney et al., 004) as the driving mechanism for exhumation. However, the tectono-metamorphic (a) E 51 N 51 N West Sudetes Rhenohercynian Saxothuringian Cadomian (Armor-Teplá- Barrandian) Cadomian Moravo-Silesian Brunovistulian Moldanubian- Lugian Post-Variscan cover Devono-Carboniferous sedimentary rocks Granitoid Granodiorite sill Proterozoic & Palaeozoic sedimentary rocks Brunovistulian Sudetic Fault Gneiss Krkonoše Complex Intra-Sudetic Fault Góry Sowie block Mica schist Orlica-Śnieżnik Dome N Intra-Sudetic Basin Orlica-Śnieżnik Dome 17 E Fig. Stronie formation (metasedimentary & metavolcanic rocks) Orthogneiss Granulite 50 N 5 km Nové Město formation Staré Město formation Brunovistulian 50 N Eclogite lenses Zábřeh & Staré Město formations Metasedimentary rocks Utrabasic & metabasic rocks (b) 16 E Zábřeh formation 17 E Thrust Fig. 1. Tectonic setting of the area of study. (a) Position of the area in the framework of the European Variscides (modified after Edel et al., 003). (b) Geological map of the West Sudetes (modified after Mazur et al., 005).

3 PROGRADE AND RETROGRADE FABRICS 453 evolution of the middle crust probably results from crustal-scale folding (Racek et al., 006), which is a process already proposed by e.g. Chamberlain (1986). Here, we present an example of a structural succession in a metasedimentary sequence of the Orlica S nie_znik Dome (OSD) (NE Bohemian Massif), and decipher the prograde and retrograde fabrics, based on the relationships of fabric development and porphyroblast growth. The prograde and retrograde parts of the P T path, determined via succession of porphyroblast growth and garnet zoning combined with pseudosection modelling are correlated with fabric development into a P T D path. The structural and metamorphic succession in the metasedimentary belt is correlated with the tectono-metamorphic evolution of an omphacite-bearing and felsic granulite belt that occurs only 5 km to the east. The implications for the burial and exhumation mechanisms of the continental crust are further discussed using existing conceptual models and associated P T D paths. GEOLOGICAL SETTING In the West Sudetes (Fig. 1), the OSD is interpreted as a remnant of the lower and middle crust of the Variscan orogenic root. The OSD is surrounded by narrow belts of low grade rocks (the Nove Meˇsto and Za brˇeh formations) thought to represent the upper orogenic crust (Mazur et al., 005; Schulmann et al., 008). To the west, the presence of blueschists and oceanic sedimentary rocks in the western Krkonosˇe Complex (e.g. Maluski & Patocˇka, 1997) suggests a Late Devonian Early Carboniferous east-dipping oceanic and continental subduction of the Saxothuringian zone (Collins et al., 000). To the east, the boundary is marked by the Stare Meˇsto formation interpreted as a remnant of a Cambro Ordovician intracontinental rift (Parry et al., 1997; Kro ner et al., 000; Sˇtı pska et al., 001) and by the Neoproterozoic Brunovistulian continental blocks (Schulmann & Gayer, 000; Kosˇulicˇova & Sˇtípska, 007) that collided during Carboniferous compression. Geology of the Orlica Śnie_znik Dome (OSD) The OSD is composed of orthogneisses, belts of metamorphosed volcano-sedimentary rocks of the Stronie ( Młynowiec) formation and bodies of high-pressure rocks (Don et al., 1990; Fig. 1b). The orthogneisses vary from an augen to a banded S nie_znik type and a finegrained mylonitic to migmatitic Gierałto w type. The Stronie formation is dominated by mica schist and paragneiss containing intercalations of acid to basic metavolcanic rocks (Floyd et al., 1996), calcitic and dolomitic marble and quartzitic schist (Wojciechowska, 197). High-pressure rocks are represented by eclogite (Smulikowski, 1967) and omphacite-bearing granulite (Kozłowski, 1965) that form isolated bodies or N S striking belts within the orthogneiss. U Pb zircon ages of Ma are generally accepted to reflect the emplacement of granitic precursors of the Gierałto w and S nie_znik gneisses (Turniak et al., 000; Kro ner et al., 001; Lange et al., 005b). Rare U Pb zircon ages between 370 and 360 Ma from high-grade lithologies have been attributed to an Early Variscan HT event (Lange et al., 005a), whereas Anczkiewicz et al. (007) interpreted a c. 370 Ma Lu Hf age of garnet from felsic granulites as a vestige of prograde UHP metamorphism. However, Sˇtípska et al. (004) and Bro cker et al. (009) considered Early Carboniferous metamorphism between 345 and 330 Ma as a dominant event as also confirmed by Sm Nd ages of Ma on garnet and clinopyroxene in eclogites (Brueckner et al., 1991). In addition, recent Sm Nd Garnet WR dating from the Stronie formation provided an age of ± 4.4 Ma interpreted as reflecting peak metamorphic conditions of mid-crustal rocks (Jastrzębski, 008). Rb Sr data from orthogneisses and 40 Ar 39 Ar dating on biotite, muscovite and hornblende from metapelites, orthogneisses and retrogressed eclogites yield Ma ages documenting cooling of the massif (Steltenpohl et al., 1993; Lange et al., 00; Schneider et al., 006). Major structural features recognized by Don et al. (1990) across the OSD involve folds, mylonitic zones in orthogneiss and east-directed thrust faults. Dumicz (1979) observed a subvertical schistosity deformed by folds with flat-lying axial planes and interpreted these structures as a result of Carboniferous tangential compression followed by vertical shortening due to gravitational loading. This fabric succession was confirmed by Sˇtı pska et al. (004) around a granulite belt in the NE part of the OSD (Fig. ). Apart from relics of a shallow-dipping fabric which have been locally preserved in the subvertical foliation, the vertical fabric linked with HP granulite facies metamorphism is deformed with variable intensity by a shallow-dipping fabric under amphibolite facies conditions. A similar fabric succession was reported by Jastrzębski (005, 008) in rocks from the Stronie formation where subvertical planes with remnants of a shallow-dipping structure are deformed into flat-lying structures displaying top-to-the-n sense of shear. According to this work, the horizontal compression at greenschist facies conditions is followed by prograde syn-burial subvertical shortening and subsequent decompression with later northward thrusting reactivating the flat-lying anisotropy. Some authors report peak metamorphic conditions of eclogites and granulites of 18 kbar and C (Steltenpohl et al., 1993; Sˇtípska et al., 004), whereas other studies suggest UHP conditions of 7 kbar (Bakun-Czubarow, 1991; Bro cker & Klemd, 1996). Retrogression in the amphibolite facies is constrained between 4 and 11 kbar at C (Steltenpohl et al., 1993; Bro cker & Klemd, 1996; Sˇtı pska et al., 004) for both eclogites and granulites.

4 454 E. SKRZYPEK ET AL. (a) Metamorphism Garnet-staurolite ky(s)/sil(s3) Cordierite Sample location Structures 75 S S vertical S trend 0 S3 S3 trend N 1 km S A Staré Město formation Javorník Mica schist sample M1B n = S A' n = Lądek Zdrój Granulite sample Štípská et al., S n = 191 S-3 n = 18 L n = S3 n = 150 (b) Metasediment Orthogneiss Granulite S S3 S S-3 S S3 L -3 L -3 L -3 S1 n = 41 A n = 48 n = 66 n = 6 n = 48 n = 46 n = 3 n = 43 n = 54 A' S3 S 500 m Orlica-Śnieżnik Dome-Stronie formation Mica schist, paragneiss, quartzite, metavolcanics Marble Orlica-Śnieżnik Dome Orthogneiss to migmatite Omphacite- & felsic granulite Staré Město formation Amphibolite Gneiss Granitoid Quaternary cover Amphibolite Amphibolite Mica schist Fig.. (a) Simplified geological and structural map of the Lądek Javornı k area. Occurrences of metamorphic minerals, location of sample used for petrology and position of the structural profile are indicated. Plots show the orientation of S, S3 and L 3 structures (Schmidt net, lower-hemisphere projection). (b) Interpretative geological cross-section showing the early subvertical structure S and the style of superimposed D3 deformation in the rocks of the Stronie belt, the surrounding orthogneiss and the granulites. Equal-area, lower-hemisphere stereoplots show D and D3 planar and linear structures. Vertical axis is not to scale. Eastern part of the section is modified after Sˇtı pska et al. (004).

5 PROGRADE AND RETROGRADE FABRICS 455 The metamorphic studies on marbles, metapelites (Jastrzębski, 005, 008) and metavolcanics (Murtezi, 006) from the Stronie formation show increasing metamorphic grade from the west to the east ranging from the garnet to the kyanite zone and peak pressures of 9 10 kbar achieved at C in the subvertical fabric. In addition, Jastrzębski (008) suggested that in metapelites, a temperature increase from 510 to 60 C at 7 8 kbar occurred in the shallow-dipping fabric. STRUCTURAL EVOLUTION IN THE LĄDEK JAVORNÍKAREA The investigated part of the Stronie formation forms an ENE WSW striking belt surrounded by augen to banded orthogneiss at the NE margin of the OSD (Fig. a). To the east, where the structural succession was documented by Sˇtı pska et al. (004), a granulite belt is mantled by migmatitic orthogneiss. Subvertical structure S The earliest mesoscopic structure in the metasedimentary rocks is a subvertical NE SW striking metamorphic foliation S that is best preserved in the north-eastern termination of the belt. It is defined by compositional banding in marble, amphibolite and quartzitic schist (Figs 3a,b & 4a) and by alternation of mica-, quartz- and plagioclase-rich layers in the mica schist (Fig. 4c). In the surrounding orthogneiss, recrystallized quartz and feldspar augen and ribbons alternating with biotite- and muscovite-rich layers define the S foliation. In the granulite belt located to the SE, this dominant NE SW striking S fabric dips steeply to the NW or SE (Sˇtípska et al., 004; Fig. a). Subhorizontal structure S3 The subvertical S fabric is heterogeneously deformed by metre- to millimetre-scale open to isoclinal folds F3 (Fig. 3b,c) with subhorizontal axial planes and subhorizontal, NE SW trending hinges (Fig. a). In the eastern part, in places rich in marble and amphibolite, the F3 folds are commonly open to close, whereas in areas where mica schist dominates, the folds tend to be close to isoclinal (Fig. 4a,c). The hinge zones of the folds are commonly crenulated on the centimetre to millimetre-scale (Fig. 4c), and in the mica schist, the S3 cleavage defined by millimetre-spaced alternation of mica- and quartz-rich layers develops parallel to the axial planes. The open to close F3 folds show a varying asymmetry from symmetric to highly asymmetric folds with weakly deformed subvertical short limb and stretched, subhorizontal long limb nearly parallel to the axial plane (Fig. 4b). In the central and western part of the belt, the F3 folding commonly leads to almost complete transposition of the S foliation into the shallow-dipping S3 cleavage (Fig. 3c). There, the remnants of the S fabric are present only in low strain areas, as rare open and more abundant almost close to isoclinal folds in marble and amphibolite, and as frequent isoclinal and rootless isoclinal folds in the mica schist. The shallow-dipping S3 foliation is the dominant structure in orthogneiss located on both sides of the metasedimentary belt. Close to isoclinal metre-scale folds with shallow-dipping axial planes are locally preserved, and hinge zones show isoclinally microfolded quartz-, feldspar- and biotite-rich bands. These hinge zones with axial-plane cleavage are in many places the only witness of the earlier steeply dipping S foliation. The D3 deformation is weaker in the granulites, resulting mostly in open F3 folds and localized shallow-dipping shear-zones (Sˇtı pska et al., 004). The orientation of the S3 cleavage varies slightly. It is moderately dipping towards the NW in the northern part of the area, towards the N to NE in the centre and towards the SE near the granulite belt (Fig. a,b). Superposition of the S and S3 fabrics leads also to the development of a NE SW subhorizontal intersection lineation L 3 that is parallel to the F3 fold hinge (Fig. ). PETROGRAPHY To determine the thermal and tectonic evolution of the structural succession, mica schist samples from the Stronie formation were used in this study. It is assumed that the orientation of pre-s3 minerals is preserved in the hinge zone during F3 fold development whereas minerals in the limbs may rotate and could acquire a different orientation. This assumption is supported by a quantitative microstructural study of garnet porphyroblasts from the same area (Skrzypek et al., 010). Observing relationships in the hinge zone facilitates interpretation of the relative growth of the minerals with respect to the structural development. Therefore, samples with well-preserved S fabric as well as from hinge zones of the F3 folds with variable intensity of the S3 cleavage development were investigated. The petrography is illustrated in Figs 5 and 6 and the interpretation of the relative mineral growth with respect to the deformational phases is summarized in Fig. 7. Mineral abbreviations used are defined as follows: and, andalusite; bt, biotite; chl, chlorite; crd, cordierite; czo, clinozoisite; g, garnet (with end-members: Alm, almandine; Grs, grossular; Prp, pyrope; Sps, spessartine) ; ilm, ilmenite; kfs, K-feldspar; ky, kyanite; ms, muscovite; pg, paragonite; pl, plagioclase (Ab, albite; An, anorthite; Or, orthoclase); qtz, quartz; sil, sillimanite; st, staurolite. Samples within the S foliation Samples collected within the well-preserved S fabric in the eastern termination of the Stronie belt display

6 456 E. SKRZYPEK ET AL. (a) (b) (c) (d) Fig. 3. Field photographs illustrating typical structural relationships. (a) Steeply dipping S foliation in interlayered marble and amphibolite. (b) Steeply dipping foliation folded by open F3 folds in amphibolite. (c) Isoclinal F3 folds in amphibolite. (d) Subhorizontal S3 foliation in mica schist.

7 PROGRADE AND RETROGRADE FABRICS 457 (a) (b) (c) (d) Fig. 4. Field photographs illustrating structural details. (a) Rheological contrast between marble with steeply dipping S foliation and surrounding mica schist transposed into the S3 fabric. (b) Asymmetric F3 fold in amphibolite with attenuated subhorizontal limbs. (c) Detail of subhorizontal crenulation cleavage S3 in the hinge of quartzitic mica schist. (d) Andalusite in quartz exudation parallel to the flat-lying S3 fabric in mica schist. coarse-grained quartz-rich ribbons that alternate with muscovite-rich layers containing a variable amount of biotite oriented parallel to the S foliation (Fig. 5a c). Garnet porphyroblasts ( 4 mm) are commonly elongated parallel to the S fabric and contain straight inclusion trails of quartz and ilmenite that are continuous with the external S foliation (Fig. 5a). Plagioclase forms large elongated porphyroblasts (1 4 mm) that commonly include oriented ilmenite, quartz, white mica and rare chlorite parallel to the external S fabric (Fig. 5b). Plagioclase also occurs as small grains in the matrix. Kyanite (up to 1 mm) is present only locally and is elongated parallel to the S foliation (Fig. 5c). Staurolite was not found in samples with the well-preserved subvertical S fabric but it is included in garnet that contains S inclusion trails perpendicular to the surrounding S3 foliation (Figs 5d & 8d). Other common accessory phases in the matrix are apatite and tourmaline and rare rutile is included in garnet. Samples within the hinges of the F3 folds Samples that were collected in the hinge zones of the F3 folds exhibit variable intensity of the S3 cleavage development and variable degree of preservation of the S structures. With increasing intensity of the F3 folding, three stages are distinguished based on the microstructural pattern of the matrix. First, the matrix composed of the quartz- and mica-rich S layers was microfolded (Fig. 5a c). Then, a spaced S3 cleavage developed, marked by the orientation of biotite and muscovite with some preserved isoclinal and rootless folds composed of quartz-rich ribbons (Fig. 5e). In the last stage, the matrix was completely deformed (Fig. 6); quartz- and mica-rich layers are subhorizontal and rarely contain rootless folds of quartz-rich layers. Porphyroblasts and inclusion trails in porphyroblasts of the hinge zone show variable orientation. In some cases, the inclusion trails of several neighbouring porphyroblasts are straight and oriented almost

8 458 E. SKRZYPEK ET AL. Fig. 5. Photomicrographs illustrating the relationships between the porphyroblasts and the S fabric (a d), and the common porphyroblast orientation within the hinge zones of the F3 macrofolds (e f). (a) Garnet and (b) plagioclase are elongated, and have inclusion trails parallel to the S metamorphic layering. (c) Kyanite crystals parallel to the S fabric. (d) Staurolite included in garnet with ilmenite inclusion trails perpendicular to the external S3 fabric. (e) Plagioclase elongation and orientation of the inclusion trails are perpendicular to the S3 fabric in the hinge of the F3 crenulation and oblique in the limb. (f) Garnet porphyroblasts with consistent inclusion trails perpendicular to the external S3 fabric. Full lines indicate the dominant foliation whereas dashed lines denote the inferred internal foliation in porphyroblasts or the orientation of the superimposed cleavage. Plane-polarized light except for (b).

9 PROGRADE AND RETROGRADE FABRICS 459 Fig. 6. Photomicrographs illustrating the relationships between the metamorphic minerals and the S3 fabric. (a) Garnet with inclusion trails oblique to the external S3 foliation and sillimanite biotite intergrowths in the pressure shadows parallel to S3. (b) Garnet with inclusion trails at high angle to the external S3 foliation in the core and parallel to the S3 fabric at its outermost rim. (c) Plagioclase porphyroblasts with inclusion trails in the core at high angle to the external S3 fabric and inclusion trails in the rim passing continuously into the anastomosing S3 foliation. (d) Large and randomly oriented andalusite crystals with aligned ilmenite passing continuously into the external S3 fabric. Full lines indicate the dominant foliation whereas dashed lines denote the inferred foliation in porphyroblasts or relics of older fabrics. Plane-polarized light. perpendicular with respect to the external foliation (Fig. 5f). They are therefore interpreted as porphyroblasts that grew during or after the formation of the S fabric and that did not rotate during the F3 folding while the matrix was deformed into the S3 fabric. In some samples, neighbouring porphyroblasts show straight inclusion trails that are oriented at a high angle with respect to each other and at variable angle to the S3 cleavage. In these samples, the different orientation of the inclusion trails is probably caused by rotation of the grains with the limbs of the microfolds (Fig. 5e). The exact orientation of the S fabric cannot be inferred from such porphyroblasts, but their growth can still be assigned to the S fabric. Garnet (up to 6 mm) includes numerous ilmenite and quartz grains, and rare chlorite, staurolite and rutile grains. Ilmenite and quartz generally form straight inclusion trails that are oriented at high angle to the external S3 foliation (Fig. 6a). Some garnet porphyroblasts show straight inclusion trails in the core and curved inclusion trails in the outermost rim passing continuously into the S3 fabric (Fig. 6b). The microstructural relationships suggest that the major garnet growth occurred during development of the S fabric, but in some samples a small amount of garnet crystallized during development of the S3 fabric. Skrzypek et al. (010) reported garnet porphyroblasts with inclusion trails in the inner core oriented at high angle with respect to the S fabric, and suggested the presence of an earlier foliation S1 that is not preserved macroscopically. In a few cases, this inferred S1 foliation seems crenulated and preserved in garnet (Fig. 8d). Plagioclase forms porphyroblasts ( mm) and small grains (0.3 mm) within the matrix. Porphyroblasts contain numerous ilmenite, white mica, some quartz

10 460 E. SKRZYPEK ET AL. Structure Mineral ms chl bt pl1 pl g st ky sil and crd S 1 (subhorizontal) S 1- S S 3 (subvertical) S -3 (subhorizontal) Post-S 3 Fig. 7. Crystallization deformation relationships. Quartz and ilmenite are always present. For details see text. Fig. 8. Photomicrographs and BSE image of sample M1B selected for mineral chemistry and mineral equilibria modelling. (a) Garnet, staurolite, kyanite and sillimanite in muscovite biotite schist. (b) Location of the garnet profile presented in Fig. 9 (BSE). (c) Sillimanite in the D3 pressure shadow of a garnet that contains staurolite and crenulated ilmenite inclusions. (d) Detail of the garnet core shows crenulated S1 inclusion trails and S cleavage that is perpendicular to the external S3 fabric. Plane-polarized light.

11 PROGRADE AND RETROGRADE FABRICS 461 and rare chlorite. Ilmenite needles usually form straight inclusion trails oriented at high angle to the external foliation and rarely, the inclusion trails are curved at porphyroblast rims and continue into the S3 foliation (Fig. 6c). Small recrystallized plagioclase occurs within micaceous layers of the S3 cleavage and in samples that show almost complete transposition of the S3 foliation, the majority of plagioclase is present as small recrystallized grains in the matrix. The major porphyroblast growth is therefore assigned to the S fabric with a small amount assigned to the beginning of the S3 fabric development. The recrystallized small grains are associated with the S3 cleavage development. Staurolite (1 mm) is sometimes included in garnet (Figs 5d & 8d) and commonly occurs in the matrix. In samples exhibiting relicts of the S quartz-rich layers, it is oriented parallel to the S fabric. Its orientation in the matrix of strongly deformed samples is random or parallel to the S3 fabric. Staurolite inclusions in garnet and oriented staurolite parallel to the S fabric indicate its growth during the D deformation. Its orientation parallel to the S3 fabric in strongly overprinted samples is assigned to reorientation during the D3 event. Kyanite (0.5 1 mm) is common in the matrix and was not found in garnet. It is oriented parallel to the S quartz-rich layers in microfolded samples but the elongated grains are generally rotated with the limbs of the microfolds. In samples exhibiting strong D3 deformation without relicts of folded quartz-rich domains, kyanite orientation is random or mostly parallel to the S3 cleavage (Fig. 5d). Kyanite growth is assigned to the S fabric only; its orientation parallel to the S3 fabric in some samples is explained by reorientation during F3 microfolding. Mica in weakly folded samples mostly occurs parallel to the S quartz-rich layers, and is slightly rotated with the limbs of the microfolds. A small amount of biotite and white mica occurs in the spaced S3 cleavage. In strongly overprinted samples, the mica is oriented parallel to the S3 fabric and biotite prevails over muscovite. In some samples with strong S3 fabric, biotite-rich pressure shadows commonly associated with sillimanite are developed around garnet (Fig. 6a) and occasional biotite ribbons with sillimanite are parallel to the S3 foliation. It is assumed that the majority of mica grew during the D event, but a significant amount also grew in the S3 spaced cleavage. In strongly overprinted samples, it is impossible to distinguish the S from the newly grown S3 mica, and it is likely that the mica chemically equilibrated during the D3 event. The biotite associated with sillimanite is assigned to the S3 fabric. Sillimanite occurs only in samples that show strong D3 deformation. It is associated with biotite in the pressure shadows around garnet and in biotite-rich layers, always oriented parallel to the S3 foliation. It was never found microfolded. Its growth is therefore assigned to the D3 deformation. Andalusite is commonly found only in samples with strong D3 deformation. It occurs in quartz andalusite segregations parallel to the S3 fabric (Fig. 4d), in some pressure shadows of garnet and it forms large, randomly oriented porphyroblasts in the matrix. Andalusite commonly includes ilmenite oriented parallel to the S3 fabric and continuously passing into the matrix (Fig. 6d). Andalusite in the pressure shadows indicates that its growth started probably during the D3 deformation, but the majority of porphyroblasts is interpreted as late andalusite overgrowths on the already developed S3 foliation. Cordierite was found in one sample as elongated stripes (1.5 mm), or small highly pinitized grains in the S3 matrix and around garnet porphyroblasts (Fig. 5d). Its growth is interpreted as post-tectonic with respect to the D3 deformation. Chlorite occurs in the cores of some garnet and plagioclase porphyroblasts and rare chlorite is randomly oriented around garnet porphyroblasts. Chlorite included in garnet and plagioclase is assigned to the S event while the matrix chlorite replacing garnet is assigned to the late, post-d3 evolution. CHEMISTRY AND P T ESTIMATES Analytical procedure Chemical analyses have been carried out in Centre de Ge ochimie de la Surface in Strasbourg on a TESCAN VEGA XMU electron microscope with operating conditions of 15 kv and 15 na. Mineral analyses are listed in Table 1. Sample M1B Sample M1B shows alternating quartz- and mica-rich bands corresponding to the S3 fabric (Fig. 8a,c). It is composed of garnet, staurolite, kyanite, sillimanite, biotite, muscovite, plagioclase and quartz with accessory ilmenite. Because samples from the area of study commonly preserve the same mineralogy (Fig. ), sample M1B is considered as representative of this part of the Stronie formation. Garnet (<5%) and plagioclase ( 3%) form porphyroblasts (1 4 mm) that sometimes display inclusion trails oblique to the external S3 foliation. Plagioclase porphyroblasts (pl1) have outermost rims recrystallized into fine-grained aggregate (pl) and tiny plagioclase grains (pl) occur within the S3 matrix. Relics of staurolite and kyanite appear as small grains (0.5 mm) randomly distributed within the matrix lying next to garnet porphyroblasts but staurolite is also included in garnet with inclusions trails perpendicular to S3 (Fig. 8d). Intergrowths of biotite and sillimanite commonly appear in the garnet pressure shadows or as thin layers in the S3 matrix. Garnet shows strong zoning from core (Alm 0.60 Prp 0.04 Grs 0.16 Sps 0.1 ; X Fe = 0.94) to rim (Alm 0.79

12 46 E. SKRZYPEK ET AL. Table 1. Representative chemical analyses for minerals from sample M1B. Sample M1B Mineral position g core g rim st core st rim bt matrix ms matrix pl 1 core pl 1 rim pl matrix Wt% SiO TiO Al O FeO MnO MgO CaO Na O K O ZnO n.a. n.a n.a. n.a. n.a. n.a. n.a. Total Cations Si Ti Al Fe Fe Mn Mg Ca Na K Zn n.a. n.a n.a. n.a. n.a. n.a. n.a. Total X Fe An Alm Ab Prp Or Grs Sps Structural formulae calculated on the basis of 1 oxygen for garnet, 46 for staurolite, for biotite and muscovite and 8 for plagioclase. n.a., not analysed. Prp 0.13 Grs 0.06 Sps 0.04 ; X Fe = 0.85) (Figs 8b & 9a). Staurolite is not zoned and its X Fe varies from 0.81 to All plagioclase is albite, but porphyroblasts show lower anorthite content (An 3 ) than the rims or small grains in the matrix (An 7 10 ). Typical X Fe values for biotite are and titanium clusters around 0.15 p.f.u. Mineral equilibria modelling Calculation method A pseudosection in the MnO Na O CaO K O FeO MgO Al O 3 SiO H O (MnNCKFMASH) system has been calculated for the whole-rock composition of sample M1B (in wt%: SiO = 61.44; TiO = 0.99; Al O 3 = 19.76; Fe O 3 = 0.99; FeO = 4.8; MnO = 0.37; MgO = 1.73; CaO = 0.89; Na O = 1.00; K O = 3.96; P O 5 = 0.08; standard wet chemical methods) with THERMOCALC 3.6 (Powell et al., 1998) and the database 5.5 (November 003 update; Holland & Powell, 1998). The activity composition relationships of feldspar are from Holland & Powell (003), white mica from Coggon & Holland (00), silicate melt from White et al. (007), Mn-bearing models from Mahar et al. (1997), chlorite from Holland et al. (1998) and biotite as in Powell & Holland (1999). Abbreviations for composition isopleths are x(g, st, bt) = Fe (Fe + Mg); z(g) = Ca (Fe + Mg + Ca + Mn); m(g) = Mn (Fe + Mg + Ca + Mn). Quartz, muscovite and H O are set as excess phases. Preliminary tests of pseudosection calculation for Stronie samples taking into account the fractionation of elements in garnet have shown such fractionation to be of small importance. This is in agreement with the study of Zuluaga et al. (005) on pelitic schists with <5% modal garnet. The observed garnet zoning (Fig. 9a) suggests that cation diffusion was limited during the metamorphic evolution. Therefore, the calculations do not take into account element fractionation or intra-crystalline diffusion. Pseudosection for sample M1B and P T evolution The pseudosection was calculated up to the P T conditions where melt appears ( C). The major features of the pseudosection (Fig. 10a) include garnet stability over the range of calculated P T conditions, staurolite stability from 550 C (at 3.1 kbar) to 630 C (at 6. kbar), chlorite stability up to C, and biotite stability above C. Kyanite is stable above 610 C (between 6 and 8.6 kbar), sillimanite from 580 C (at 4 kbar) to 660 C (at 6 kbar), andalusite between 50 and 630 C at.4 4 kbar and

13 PROGRADE AND RETROGRADE FABRICS 463 (a) 1.00 Sps Alm Prp Grs X Fe (b) M1B (Length : 3630 mm) Rim Core Rim Modelled profile x(g) = Fe/(Fe + Mg) z(g) = Ca/(Fe + Mg + Ca + Mn) m(g) = Mn/(Fe + Mg + Ca + Mn) Fig. 9. (a) Garnet profile from sample M1B. The line of the profile is shown in Fig. 8b. (b) Modelled garnet profile for the P T path represented by the white arrow (stages 1 ) in Fig. 10. cordierite between 500 and 650 C with an upper pressure limit at kbar. Sequential growth observed in the thin section involves oriented ilmenite, chlorite and white mica in plagioclase porphyroblasts, and garnet porphyroblasts with straight inclusion trails of ilmenite. This is complemented by the presence of randomly oriented staurolite and kyanite in the S3 matrix and sillimanite in garnet S3 pressure shadows as well as parallel to the S3 matrix foliation. This suggests a medium pressure prograde path starting in the quinivariant g pl chl field, continuing across the quadrivariant bt g pl st field to the bt g pl ky field followed by a pressure drop into the bt g pl sil field. The beginning of the P T path is deduced from garnet core chemistry where grossular and X Fe values (X Fe = 0.94; Grs = 0.16) fit approximately the isopleths, but modelled m(g) = 0.3 is higher than the measured spessartine (Sps = 0.1) content (Fig. 9b). This discrepancy is probably due to excessive Mn incorporation in garnet. Indeed, in the present calculations garnet and chlorite are the only phases with Mn-bearing models present at low P T conditions. However, a significant amount of MnO can also be trapped in other minerals (Spear & Cheney, 1989), especially in ilmenite (Caddick & Thompson, 008) which is generally abundant in the matrix of pelitic samples and therefore, calculations taking into account TiO would be likely to decrease m(g) values at low P T conditions. This issue has been addressed by Skrzypek et al. (010), where MnNCKFMASHTO pseudosections yielded similar P T estimates for the onset of garnet growth but with a better match between m(g) and spessartine values. It is therefore proposed that the P T evolution starts at kbar and C (Fig. 10a c, stage 1). Inclusions in garnet porphyroblasts together with staurolite and kyanite occurrences in the matrix indicate further prograde P T evolution. This is confirmed by staurolite chemistry (X Fe = ) compatible with the calculated isopleths (Fig. 10b) and by observed grossular and spessartine content at the garnet rim (Grs = 0.06; Sps = 0.04) that lie very close to the calculated isopleths of z(g) and m(g) in the kyanite field. This indicates P T conditions of about 6 7 kbar and 630 C for the peak of metamorphism (Fig. 10a c, stage ), supported by the modelled garnet zoning closely reproducing the measured garnet profile (Fig. 9). In the absence of migmatization, the maximum temperature is bounded by the liquid-in line at 670 C (Fig. 9a). The interpretation of the sillimanite and biotite growing at the expense of garnet in the pressure shadows is compatible with pressure decrease and decrease in the modal proportions of garnet. This retrograde reaction is likely to modify the garnet rim chemistry. If Fe Mg exchange is considered to be dominant during this process, this could result in the observed difference between garnet rim X Fe and the calculated x(g) isopleths at the pressure peak (Figs 9b & 10b), while grossular and spessartine contents remain relatively unchanged. Therefore, the modelled values of x(bt) = and x(g) = 0.85 similar to X Fe of biotite and garnet rim suggest that garnet biotite re-equilibration occurred in the sillimanite stability field. This points to retrogression towards 4 kbar and 630 C (Fig. 10b, stage 3). Additional petrographic information from other samples is also correlated with the pseudosection to roughly estimate the P T evolution during further decompression. Because andalusite is widespread in samples with penetrative S3 foliation, and it is not stable in the pseudosection above 4 kbar, it is only possible to draw a cooling curve that lies below this upper pressure limit (Fig. 10a). Cordierite is present in one sample only; it is associated with andalusite, biotite, garnet, plagioclase, muscovite and quartz, which allows speculation that pressure decrease in the area of study, at least locally, continued to 3 kbar at

14 E. SKRZYPEK ET AL. (a) M1B : SiO Al O 3 CaO MgO FeO K O Na O MnO qtz +ms +H O bt g 13 P (kbar) g pg chl czo g pg pl chl czo g pl chl czo g pg chl 15 g pl chl pl chl pg chl 14 bt g + st bt g pl st + st st bt g pl and 3 bt g pl and crd 1 3 bt g pl crd : bt g pl st ky 10: bt g pl chl st ky 11 : bt g pl chl ky 1 : bt g pl pg 13 : bt g pg 14 : bt g pl pg chl 15 : g chl pg pl 16 : bt g pl sil kfs 1 : bt g pl crd chl : bt g pl crd chl and 3 : bt g pl chl and 4 : bt g pl chl and st 5 : bt g pl and st 6 : bt g pl chl st 7 : bt g pl crd sil 8 : bt g pl st sil bt g pl 3 bt g pl ky bt g pl sil + liquid % % 4% 3% 7% 6% 5% 1% % mol%(g) P (kbar) T ( C) 17 : bt g pl sil kfs -ms 18 : bt g pl sil kfs crd -ms 19 : bt g pl sil kfs crd 0 : bt g pl crd kfs -ms 1 : bt g pl and kfs crd : bt g pl and kfs crd -ms 3 : bt g pl crd kfs x(g) x(st) x(bt) Fig. 10. (a) P T pseudosection in the MnNCKFMASH system for sample M1B (in moles adjusted to 100%), contoured for garnet modal proportion. Full and dashed white lines indicate prograde and retrograde evolution derived by comparing the modelled assemblages and isopleths with observed assemblages, chemistry and zoning of minerals. Ellipses denote the probable P T ranges for the different stages of the P T path. See text for details. (b, c) Simplified part of the pseudosection with calculated isopleths of mineral composition. P (kbar) (b) (c) T ( C) z(g) m(g) T ( C) temperatures above C, where this assemblage occurs in the pseudosection. The widespread presence of chlorite indicates further cooling below 530 C. DISCUSSION In the following sections, structural and metamorphic observations are combined into a P T D path reflecting burial and subsequent exhumation of the metasedimentary rocks. This evolution is correlated with the P T D path from neighbouring lower-crustal granulites (Sˇtı pska et al., 004) and the tectonic significance of these P T D paths is discussed in the light of P T paths established for conceptual tectonic settings. This approach is finally used to suggest that the observed P T D paths may be diagnostic of crustalscale folding and ductile thinning. Mineral growth and P T conditions of the metamorphic fabrics in metasedimentary rocks Although the major garnet growth has been ascribed to the S foliation based on microstructural observations, there is no evidence for garnet crystallization being restricted to this fabric. Some garnet porphyroblasts have core inclusion trails (S1) at high angle to the S fabric and indicate crystallization in an older, probably shallow-dipping S1 fabric that was completely obliterated on the macroscopic scale (Fig. 11a). These polyphase garnet porphyroblasts are therefore likely to document the very early stage of the prograde evolution (Skrzypek et al., 010). The D deformation produced a NE SW striking subvertical fabric visible in all the lithologies. In metapelites, oriented inclusion trails of white mica, chlorite and ilmenite in plagioclase and garnet porphyroblasts together with shape preferred orientation of kyanite and staurolite document growth of these minerals during the formation of the S foliation (Fig. 11b). The Barrovian succession of mineral growth together with the character of garnet zoning (Fig. 10) points to a prograde P T path from kbar and C to 6 7 kbar and 630 C. The D3 deformation locally affects the S fabric by open to isoclinal folds but commonly entirely transposes the previous metamorphic banding into a new,

15 PROGRADE AND RETROGRADE FABRICS 465 (a) Limestone 6 P (kbar) (b) S 6 P 4? 4 S1 Volcanics T ( C) qtz g bt T ky st pl g g? qtz ilm + ms pl qtz ms st chl? ilm pl g ky ilm ms (c) 6 P (d) 6 P 4 4 S3 pl ky Hinge zone g Totally reworked S3 g bt ky T ms st qtz S3 ilm crd g and T st crd qtz crd ms qtz bt ky ilm + ms pl sil ilm g sil pl ky and g chl bt sil ms pl Fig. 11. (a d) Sketches showing evolution of macrostructures, petrological succession and inferred P T path. The orientation of inclusion trails within porphyroblasts and orientation of minerals with respect to the external structures shows the interpretation of the structural development of mineral assemblages during successive deformation phases. For discussion, see text. flat-lying S3 structure. Some garnet and plagioclase rims parallel to the S3 fabric show that the shallowdipping fabric started to develop at the metamorphic peak, where garnet stopped growing (Fig. 11c). Microstructural evidence of sillimanite biotite growth in the garnet pressure shadows and in the shallowdipping foliation suggests a crossing of the kyanite sillimanite transition during the development of the S3 fabric and therefore indicates a drop in pressure towards 4 kbar at temperatures still above 600 C (Fig. 10). The andalusite in the pressure shadows of garnet and andalusite in quartz segregations oriented parallel to the S3 fabric (Fig. 11d) indicates that the D3 deformation and mineral growth continued down to 3 kbar. Some andalusite and cordierite overgrow post-tectonically the S3 foliation indicating further

16 466 E. SKRZYPEK ET AL. decrease in pressure to 3 kbar at temperatures between C after the D3 deformation (Fig. 10). Post-tectonic chlorite shows further cooling below 530 C. P T D t relationships between metasedimentary rocks and neighbouring granulites The P T D path (Figs 10 & 11) shows that rocks from the middle crust probably underwent early burial to 1 km under greenschist facies during formation of an early shallow-dipping fabric. Further burial from 1 km (3.5 kbar) to 5 km (6 7 kbar) occurred during development of the regionally well-preserved subvertical fabric together with an increase in temperature from 500 to C (Fig. 11). The age of the metamorphic peak may correspond to the c. 345 Ma Sm Nd Garnet WR isochron age from a kyanite-bearing metapelite located farther south where garnet growth was ascribed to the steeply dipping fabric by Jastrzębski (008). A subsequent exhumation stage within the late shallow-dipping foliation is responsible for a minimum of 10 km of vertical displacement towards a depth of 1 km (Fig. 1), evidenced by andalusite or cordierite growth, at still elevated temperatures of 580 C (Fig. 10). An identical succession of fabrics in pelites, as well as in orthogneiss and granulite (Sˇtı pska et al., 004), indicates a complete structural continuity between the investigated metasedimentary rocks and the highpressure granulites located only 1 5 km to the SE (Fig. ). Therefore, the tectono-metamorphic history of metasedimentary rocks can be compared to that of granulites. In granulites, an early, probably flat-lying, S1 fabric is folded into the subvertical S foliation that develops at a pressure peak of 18 kbar and 850 C and is followed by heterogeneous shallow-dipping D3 deformation at 6 10 kbar and 700 C (Fig. 1). The prograde character of the remnants of the early shallow-dipping fabric is not demonstrated, but it is likely that the metamorphic peak of granulites is achieved before the onset of the D deformation (Sˇtı pska et al., 004). The timing of the metamorphic peak is constrained by a SHRIMP U Pb age of football-shaped zircon yielding c. 34 Ma, and the time span of granulite exhumation was estimated by the age of a syntectonic granodiorite sill emplaced in the Stare Město formation (Fig. 1) at c. 339 Ma (Pb Pb zircon age) parallel to the S3 fabric in adjacent rocks (Parry et al., 1997; Sˇtípska et al., 001; Lexa et al., 005). This shallow-dipping S3 fabric affected the hinge zone of the orthogneiss granulite anticline, but because of the lack of pressure and temperature sensitive equilibria, the amount of vertical movement associated with the formation of the shallow-dipping fabric was not assessed in these rocks. Nevertheless, Sˇtı pska et al. Active buckling Passive amplification Passive upper crust Ductile thinning in the middle crust Underthrusting of Brunia below the lower crust Depth (km) Pre-D End of D Onset of D3 End of D3 Post-D3 Estimated depth 1 km (?) 60 km (Fig. 11a) 5 km 35 km (Fig. 11b) 5 km ~ 5 km (Fig. 11c) 1 km 1 km (Fig. 11d) ~ 7 km ~ 7 km Metasedimentary upper-middle crust Gneissic middle crust Granulitic lower crust Mantle Metasedimentary rocks (this work) Granulite (Štípská et al., 004) Passive upper-crustal lid Mid-crustal shear zone Underthrust Brunia crust Fig. 1. Schematic evolution of the orogenic crust from thickening to exhumation. Inferred position of the Stronie formation metasedimentary rocks (black star) and neighbouring granulites (black circle) are indicated.