Quantitative structural analyses and numerical modelling of ...

EXHUMATION IN LARGE HOT OROGEN 287were obliterated by strong S 3 reworking. High-pressureconditions are also reported from the Ky–Kfs granulitebody to the east (15 kbar, 800 °C, no. 2 in Figs 5 &9a), also part of the orogenic lower crust, where theretrieved P–T is clearly related to the S 2 fabric.The retrograde path of rocks from the orogeniclower crust situated far from the Brunia margin ischaracterized by an almost isothermal decompressionto about 7 kbar, as recorded by the S 3 assemblagegarnet-hornblende-plagioclase in eclogite and garnet–sillimanite–biotite in migmatite (no. 5, 6 in Fig. 9a).Unlike the central part of the Moldanubian, the Ky–Kfs granulite forming the orogenic lower crust at theboundary with the Brunia margin shows cooling withdecompression (no. 2 in Fig. 9a). Modelling of zoningin garnet that is syntectonic with the S 2 fabrics inparagneisses of the orogenic middle crust is consistentwith increase of P–T conditions to about 9 kbar and700 °C (nos 3 & 4 in Figs 5 & 9a). The retrograde P–Tpath of the sillimanite-bearing S 3 fabric in rocks of theorogenic middle crust is associated with decrease ofpressure coupled with increase of temperature, toabout 7 kbar and 700–750 °C (nos 3 & 4 in Fig. 9a).Kolenovska´ et al. (1999), Sˇtı´ pska´ & Powell (2005a)and P. Sˇtípska´ & K. Schulmann (unpublished data)examined the P–T evolution of rocks and relics of boththe orogenic lower and middle crust over a large areadominated by flat S 3 fabric in the central Moldanubiandomain (Fig. 2). Here, Ky–Kfs granulites register peakconditions around 18 kbar and 850 °C with no clearindication of the prograde path (no. 8 in Figs 5 & 9b).In contrast, a MORB-type eclogite provides evidenceof the prograde path from about 10 kbar and 750 °Cto 17–18 kbar in the form of inclusions of hornblendeand plagioclase in a prograde garnet (no. 7 in Fig. 9b).Both samples from the orogenic lower crust showalmost isothermal decompression and strong D 3reworking at 7)10 kbar and 750–850 °C (nos 7 & 8 inFig. 9b). Associated paragneisses from the orogeniclower crust contain early staurolite and kyanite inclusionsin garnet that preserves prograde zoning, indicatingburial to about 10 kbar and 700 °C (no. 9 inFigs 5 & 9b; P. Sˇtípska´ & K. Schulmann, unpublisheddata). The matrix was completely transposed by the D 3deformation, as marked by sillimanite and biotitedeveloped during decompression (no. 9 in Figs 5& 9b). In another paragneiss the flat sillimanite–biotite-bearingfabric was overgrown by garnet, indicatingheating at around 7)8 kbar and 700)850 °C (no. 10 inFig. 9b; P. Sˇtípska´ & K. Schulmann, unpublisheddata). The prograde conditions and peak pressuresfrom samples of this area could not be directly linkedto the steep fabric because of almost complete transpositionof rocks by the S 3 fabric. However, the progradeinclusions in garnet from the eclogite and fromthe paragneiss form straight, sub-vertical internalfabrics oriented at a high angle to the external S 3foliation, indicating that these garnet have probablygrown in the steep fabric.The steep S 2 fabric in the granulite body rimming theNE margin of the Moldanubian domain is marked bygarnet, kyanite and K-feldspar indicating conditions ofabout 18 kbar at 850 °C, whereas the crystallization ofbiotite in the same fabric occurred during decompressionaccompanied by slight cooling (no. 11 in Figs 6& 9c; Tajcˇmanova´ et al., 2006). Within the S 2 fabric,sillimanite–biotite intergrowths replacing garnet indicatesignificant decompression and cooling (no. 11 inFig. 9c). The S 3 fabric is associated with hercyniterimmingmetastable kyanite in the granulite and withthe development of cordierite, sillimanite and biotite inadjacent gneisses and migmatites, indicating conditionsof around 4 kbar and 700 °C (nos 12, 25 in Figs 6& 9c). Muscovite–biotite schists of the adjacent orogenicmiddle crust contain sillimanite that overgrowsrelicts of kyanite in the matrix. The peak P–T conditionsestimated by Pitra & Guiraud (1996) are 8–9 kbarat 610–660 °C, which was followed by near-isothermaldecompression, as recorded in the whole middle crustalcomplex, to 4–6 kbar (no. 13 in Figs 5 & 9c).P–T evolution of the Lugian domainEarly petrological studies of granulite from the orogeniclower crust suggested peak conditions of about28 kbar at 1000 °C (Bro¨ cker & Klemd, 1996; Kryzaet al., 1996; Klemd & Bro¨ cker, 1999), based on twofeldsparthermometry and Grt–Ky–Qtz–Pl barometry.However, Sˇtípska´ et al. (2004) questioned these resultsbecause of the possibility of non-equilibrium compositions,which may have been involved in the calculations.Instead, these authors proposed that peakpressures were around 18–20 kbar at 800)900 °C (no.14 in Figs 7 & 9d). Sˇtípska´ et al. (2004) further proposedthat these conditions are characteristic for thesteep S 2 foliation. These authors also constrainedconditions to around 10 kbar and 700 °C for theamphibolite facies retrogression within the S 3 fabric(no. 14 in Fig. 9d). Petrological studies and modellingof prograde garnet zoning from rocks of the Ky–St–Grt micaschists revealed a prograde path up to about10 kbar and 650 °C (no. 15 in Figs 7 & 9d; Romanova´&Sˇtı´ pska´ , 2001; Jastrzębski, 2005). A microstructuralstudy confirmed that the growth of prograde garnetwas syntectonic with the steep S 2 fabric. The mainreworking in the horizontal S 3 fabrics occurred in thefield of sillimanite stability at about 7 kbar and 650 °C(no. 15 in Fig. 9d).The Ordovician metamorphic fabric in the leptynoamphiboliteunit developed at about 10 kbar and800 °C (no. 18 in Figs 7 & 9d; Sˇtípska´ et al., 2001;Lexa et al., 2005). Carboniferous metamorphismrelated to the S 3 reworking of rocks at the westernmargin of this unit, close to the Carboniferous sillintruded at around 7 kbar (no. 17 in Figs 7 & 9d; Lexaet al., 2005), occurred under similar conditions ofaround 8 kbar and 750 °C (no. 16 in Figs 7 & 9d;Baratoux et al., 2005).Ó 2007 Blackwell Publishing Ltd147