Quantitative structural analyses and numerical modelling of ...

DTD 5ARTICLE IN PRESSL. Baratoux et al. / Journal of Structural Geology xx (xxxx) 1–24 52.2. Metamorphic zonalityThe Variscan metamorphic evolution of the study areainvolves two main phases; an M 2 metamorphism ofBarrovian character with the intensity increasing westwardsand an M 3 metamorphism induced by the Žulová graniteintrusion in the western margin of the Desná dome. TheBarrovian M 2 metamorphic grade ranges from chlorite zonein the eastern margin of the Desná dome up to staurolite andpossibly sillimanite zone in the west. The M 3 periplutonicHT/LP overprint also attains its maximum in the westernpart of the studied area, where it is documented by thepresence of K-feldspar–cordierite migmatites (Rozkošnýand Souček, 1989; Cháb and Žáček, 1994), as well as by thegrowth of sillimanite and new garnet in the staurolitemicaschists in the pluton aureole (Fig. 2).Although the metabasites cannot be precisely dividedinto particular metamorphic zones once they have reachedamphibolite facies conditions, it is nevertheless necessaryfor the purpose of this work to establish a metamorphiczoning of the amphibolite massif based on the increasingmetamorphic conditions. The metamorphic zones determinedin intercalated metasedimentary rocks by Souček(1978) are therefore extended into the adjacent metabasitesand used as a reference for the definition of the metamorphicgrade therein. The justification for this procedure is based onthe mutual field relations of the amphibolites and metasediments,which indicate that both lithologies have experiencedthe same tectonic and metamorphic history(Schulmann and Gayer, 2000). Therefore, PT conditionswere determined in metasediments applying variousthermobarometry methods (Baratoux, 2004). The degreeof metamorphism in the east of the metabasite massifcorresponds to the garnet zone in the contiguous metasedimentsand the mineral assemblage in the metabasitesincludes hornblende, plagioclase, actinolite, chlorite, andepidote corresponding to PT conditions of 540G10 8C and5G1 kbar. Further to the west actinolite and epidotedisappear and HblCPlGQtzCIlmCTtn, which correspondsto the staurolite zone, appears with metamorphicconditions of M 2 estimated to be 570G30 8C and 5.5G1 kbar (Baratoux, 2004). The mineral assemblage in themetabasites corresponding to the sillimanite zone isrepresented by HblCPlGQtzCIlmCTtn and AmpCPlGQtzGCpxGCalCIlmCTtn in calcium-rich lithologies andmetamorphic conditions of this zone were estimated to590G20 8C and 5.5G1 kbar. In the pluton aureole,sillimanite–cordierite–K-feldspar assemblage occur inmetapelites and the PT conditions of periplutonic M 3metamorphism reached 700G15 8C and 4.2G0.8 kbar (Fig.2b). The mineral assemblage in the amphibolites correspondsto that of the sillimanite zone.The age of the main fabric-forming M 2 metamorphicevent is difficult to establish in the studied area, but it issupposed to have occurred during the main collisionalevent, which is dated elsewhere at w340 Ma (Schulmannand Gayer, 2000; Štípská et al., 2004). The termination ofthe D 2 –M 2 event can be constrained by Rb–Sr dating (335G7.5 Ma) of the crystallization of the Žulová granite(Jehlička, 1995). However, 40 Ar/ 39 Ar dating of muscoviteand biotite from mylonitic gneisses of the Desná dome andof the Žulová granite (Maluski et al., 1995) suggest thatcooling through the white mica and biotite closuretemperatures (350 and 300 8C, respectively) occurredbetween 310 and 300 Ma. Therefore, this age maycorrespond to the greenschist facies F 3 folding activity inthe east and to the cooling of the Žulová pluton to the west.3. D 2 and D 3 amphibolite microstructures acrossmetamorphic zones3.1. Eastern part of the massif (the garnet zone)In the eastern part of the massif, i.e. in the garnet zone,some weakly deformed metagabbros with large grains ofhornblende and plagioclase (1–2 mm) are still present. Themain metamorphic fabric S 2 of amphibolites is preserved indomains unaffected by F 3 folding or in F 3 fold limbs and it ischaracterized by a strong mineral shape preferred orientation(SPO) of amphibole 0.2–3 mm in size (Fig. 3a). Thesegrains often show a strong chemical zonality with actinoliticcores and tschermakitic rims (Fig. 4a). Fine-grained (30–60 mm) sub-equant plagioclase (An 25–35 ) forms elongatepolycrystalline aggregates surrounded by laths of amphibole.These new grains develop from large relict clasts(An 40–50 ) (Fig. 4d). The plagioclase in these domainsexhibits features typical for dynamic recrystallization (in thesense of Poirier and Guillopé (1979)) such as undulatoryextinction, the development of sub-grain boundaries, and auniform grain-size distribution. However, a contribution ofmetamorphic nucleation is evidenced by the differentchemical composition of the host and the new grains(Rosenberg and Stünitz, 2003).In the hinge zones of F 3 microfolds, amphibole grainswith irregular boundaries are commonly bent and broken(Fig. 3b). Microfractures associated with domainal undulatoryextinction fragment large grains into smaller elongatesegments. There is no difference in plagioclase microstructurein the non-folded foliation and in the crenulateddomains. It is concluded, therefore, that the main D 3deformation mechanism for amphibole is fracturing (in thesense of Nyman et al. (1992) and Stünitz (1993)) andpassive grain rotation within the plagioclase matrix.3.2. Central part of the massif (the staurolite zone)In the eastern part of the central zone of the massifactinolite is still locally present in the cores of dark greengrains of magnesio-hornblende. Amphibole compositionfollows the pargasitic line (Fig. 4b) within the S 2 foliation.The size of the dark green grains of magnesio-hornblende113