Quantitative structural analyses and numerical modelling of ...

EXHUMATION IN LARGE HOT OROGEN 289Thermochronology of Moldanubian and Lugian domainsThe distribution of isotopic ages reflects a two-stagecooling, related to exhumation processes. An olderstage is marked by cooling ages from minerals withhigh blocking temperatures (Sm–Nd system in garnet,700)750 °C, Hensen & Zhou, 1995;40 Ar ⁄ 39 Ar inhornblende, around 480 °C, Harrison et al., 1985).Systematically, younger cooling ages are retrievedfrom minerals with low blocking temperatures( 40 Ar ⁄ 39 Ar in muscovite, around 350 °C, and in biotite,around 280 °C, Harrison et al., 1985). Peridotitesand associated eclogites yield Sm–Nd whole-rock–garnet ages between 350 and 325 Ma (Brueckner et al.,1991; Beard et al., 1992; Medaris et al., 1995). Thepeak for Sm–Nd ages for both types of rocks is around336 Ma (Fig. 9e). The 40 Ar ⁄ 39 Ar hornblende data arecompatible with the distribution of Sm–Nd ages,whereas the 40 Ar ⁄ 39 Ar muscovite data record systematicallyyounger ages ranging from 331 to 325 Ma(Fig. 9e; Matte et al., 1990; Dallmeyer et al., 1992;Fritz et al., 1996).The Variscan cooling history of the Lugian eclogitesis documented by Sm–Nd garnet–clinopyroxene–whole-rock ages of 340)330 Ma (Brueckner et al.,1991). Recently, Lange et al. (2005) provided Sm–Ndisochrons for the Lugian granulites that define agesbetween 357 and 337 Ma, which fit the existing40 Ar ⁄ 39 Ar hornblende and biotite cooling ages fromthis region (Schneider et al., 2006). The 40 Ar ⁄ 39 Armuscovite ages from all lithologies of the Lugiandomain are similar to Sm–Nd garnet,40 Ar ⁄ 39 Arhornblende and 40 Ar ⁄ 39 Ar biotite data from granulitesand eclogites, indicating a rapid and monocycliccooling history (Fig. 9f).Polyphase fabric and metamorphic evolution of theorogenic beltThe structural pattern in the Lugian domain reveals awell-preserved D 2 vertical fabric in the orogenic lowerand middle crust as well as coherency of these units,defined by sub-parallel alternations of continuous beltsof orogenic lower and middle crust on the geologicalmap. Here, the geometry of the geological units andmap patterns are fully controlled by the D 2 deformation.The D 3 deformation was generally weak andheterogeneous, being mostly concentrated close to thethrust of the Lugian domain over the Ordovician leptyno-amphiboliteunit to the east and close to the areaof flat fabric and the normal-sense shear zone boundingthe Lugian domain in the west (Figs 7 & 8).Normal-sense non-coaxial shearing developed continuouslyfrom the earlier pure shear deformation andprobably this was responsible for displacement ofschists of the orogenic middle crust to the NW andunroofing of the orogenic lower crust.In the orogenic lower and middle crust of the Lugiandomain, petrology indicates that retrograde conditionsof omphacite-bearing granulites and peak pressureconditions of adjacent micaschists are similar. In theorogenic middle crust, the prograde mineral growth isclearly related to the steep S 2 fabric (Romanova´ &Sˇtı´ pska´ , 2001), whereas in the granulites from theorogenic lower crust, development of the S 2 fabric isrelated to their retrogression (Sˇtípska´ et al., 2004). Theremarkable differences in prograde and retrogradeP–T paths of rocks from the orogenic middle andlower crust, respectively, related to steep S 2 foliationsmakes this region the best example of vertical materialand heat transfer during D 2 .Sˇtípska´ et al. (2001) further discussed the significanceof similar Carboniferous and Ordovicianmetamorphic conditions for the easterly lower crustalleptyno-amphibolite complex. They concluded thatthis complex cooled after Ordovician rifting andremained stable in the crust before the onset ofVariscan deformation. Heterogeneous Variscanamphibolite facies deformation was localized in thewestern margin of the Ordovician block and sharesthe same P–T conditions and kinematics withamphibolite facies retrograde fabric of the easternmostorogenic lower crust. Consequently, Sˇtı´ pska´et al. (2004) suggested that the transition from thesteep-to-the-west moderately dipping fabrics in theorogenic lower crust was linked to thrusting of theserocks over a rigid lower crustal block made up ofleptyno-amphibolite unit rocks at a depth equivalentto about 10 kbar (Fig. 5c).In contrast, the horizontal S 3 fabrics affectingrocks of the orogenic lower crust and the normal-senseshear zone developed in the west were linked to thedevelopment of retrograde and syntectonic mineralassemblages indicating an important decrease of temperatureand pressure typical for detachment zones(e.g. Vanderhaeghe & Teyssier, 2001). The Lugian rootrecords a single-stage cooling history characterized bytelescoping of ages for minerals with different blockingtemperatures.The structural development of the NE terminationof the Moldanubian domain shares a number offeatures with the Lugian domain. In this area, the S 2foliation was well preserved in felsic granulites inconjunction with a weak D 3 reworking, whichemphasizes a coherency of the lower and middleorogenic crust and indicates that the map patterndeveloped during D 2 vertical movements, similar tothat in the Lugian domain. The steep fabric in theorogenic lower crust records retrogression to the sillimanitestability field, suggesting that the orogeniclower crust was exhumed to middle crustal conditionsalong the S 2 fabric. The HP characteristics of thisfabric are not necessarily always preserved, preservationbeing dependent mainly on whether the crustavoids re-hydration during exhumation. However, incontrast with the Lugian domain, the heterogeneousD 3 deformation does not show any normal-sensecomponent of shear and is exclusively associated withÓ 2007 Blackwell Publishing Ltd149