Quantitative structural analyses and numerical modelling of ...

1378J. KonopaÂsek et al. / Journal of Structural Geology 23 2001) 1373±1392et al., 1997) with peak PT conditions differing considerablyfrom those of the adjacent metasediments RoÈtzler et al.,1998).Metamorphic conditions in the study area are similar tothose de®ned in Germany. A high dP/dT gradient wasobserved in metasedimentary rocks KonopaÂsek, 1998,2001) which host ma®c eclogites. The peak PT conditionsof the eclogites are not consistent with those of the adjacentparautochthonous metasediments, however, suggesting thatthey have been juxtaposed tectonically KlaÂpova et al.,1998).3. Lithological zonation of the studied areaThe area studied involves four main large-scale structuresexposed in the vicinity of the village of MeÏdeÏnecÐtheMeÏdeÏnec synform, the Oberwiesenthal structure, theMeÏdeÏnec antiformand the KlõÂnovec antiformSÏkvor,1975) Fig. 2). These large structures are composed of thefollowing rocks:1. Plagioclase schists are described by KonopaÂsek 1998)as characteristic metasediments forming the Saxothuringianparautochthon. Typical samples containreversely zoned plagioclase porphyroblasts envelopingnumerous garnet inclusions. As these rocks show a polyphasemetamorphic history, they can be sampled atdifferent stages of their evolution KonopaÂsek, 1998).These rocks are intercalated with metagreywackes'Dichte Gneisse' of Pietzsch 1914)) and metaconglomeratesMehnert, 1939; Sattran, 1963).2. Orthogneisses appear either as ®ne-grained equigranularvarieties or as deformed porphyritic granite 'RoteGneisse' of Scheumann 1935)). Porphyriticorthogneisses are heterogeneously deformed and occurin all deformation stages ranging from protomyloniticmetagranites up to banded ultramylonites.3. Garnetiferous micaschists are characterised by theappearance of numerous garnet porphyroblasts, sometimesup to 2 cm in diameter, surrounded by a whitemica and quartz matrix. In the southern limb of theKlõÂnovec antiform, garnetiferous micaschists are associatedwith a layer of amphibolites Fig. 2).4. Ma®c eclogites bear a typical HP mineral assemblageconsisting of omphacite 1 garnet 1 zoisite 1amphibole 1 rutile 1 quartz and hand specimens arecharacterised by well-developed planar and linear fabricKlaÂpova et al., 1998).The rock-types described above occur in all the largescalestructures, but the lithological zonation within thesestructures is not uniform. In the MeÏdeÏnec synform, plagioclaseschists with metagreywackes represent structurally thedeepest level. These schists pass upward into the layer ofgarnetiferous micaschists associated with the ma®ceclogites. In the hanging wall of garnetiferous micaschist,there occurs a large slab of orthogneisses with a ®ne-grainedvariety at the bottomand a porphyritic type at the top Fig.3aÐnorthern part of the pro®le 1±1 0 ). Locally, fragmentsof the allochthonous orthogneiss slab can be observed alsowithin the parautochthonous micaschists Fig. 3b). Thelithological zonation of the KlõÂnovec antiformis morecomplicated. In its eastern part, the zonation of the southernlimb is the same as that in the MeÏdeÏnec synformFig. 3cÐsouthern part of the pro®le 3±3 0 ). However, farther to thewest, the lithotectonic zonation becomes progressivelyreversed with orthogneisses in the lowermost positionpassing in the garnetiferous micaschists and with plagioclaseschists in the uppermost position Fig. 3dÐsouthernpart of the pro®le 4±4 0 ). Moreover, in the core of theKlõÂnovec antiform, large bodies of eclogites are situated inthe centre of the orthogneiss body Fig. 2).Field observations show that ma®c eclogites, togetherwith a layer of garnetiferous micaschists, are in mostcases exposed along the boundary between the orthogneissesand plagioclase schists. The location of these unitsalong this boundary is critical for understanding the tectonicevolution of the studied area.4. PT evolution of the studied area and the de®nition ofthe allochthonous and parautochthonous domainsThe PT conditions of orthogneiss formation are notknown, but the stability of plagioclase in all the studiedsamples excludes the possibility of them being deformedunder eclogite facies conditions. The peak pressure conditionsof the plagioclase schists were established to be 13±15 kbar at a temperature of 580±6308C KonopaÂsek, 1998).These pressure conditions are in contrast with those reportedby KlaÂpova et al. 1998) fromthe ma®c eclogites wherepeak pressures were estimated to be 26 kbar at 650±7008C. The PT conditions of the associated garnetiferousmicaschists approach those of the eclogites 6408C,22 kbarÐ KonopaÂsek, 2001). The difference between PTestimates from parautochthonous metasediments and eclogiteswas explained in terms of emplacement of previouslysubducted oceanic crust into continental rocks during theearly stages of collision KlaÂpova et al., 1998).As noted earlier, the ma®c eclogite bodies associated withthe garnetiferous micaschists occur systematically at theboundary between plagioclase schists and orthogneisses.This observation suggests that this boundary represents amajor crustal boundary along which ma®c eclogies wereexhumed and incorporated into the middle crust. Accordingto the interpretation of KonopaÂsek 1998) and KlaÂpova et al.1998), the plagioclase schists represent a Saxothuringianparautochthon, which was overthrust by middle-crustalorthogneisses derived froman orogenic root domain,today exposed in the eastern Moldanubian zone. Based onthis interpretation, these allochthonous orthogneisses,54