Quantitative structural analyses and numerical modelling of ...

656 O. LEXA ET AL.Table 2. Summarized compositions of minerals used for thermobarometry.Sample Rock type Locality Grt Pl Amp Cpx Bt IlmXMg Alm Py Grs Sps An Compositional name a XMg Si (pfu) XMg XMg XIlmSamples associated with D1 Cambro-Ordovician structureLAC4a Grt-amphibolite 66 g 0.16–0.18 62–65 12–14 11–21 4–5 31–33 Ts, Fe-Ts 0.47–0.54 6.27–6.37 – – –LAC4b Cpx-amphibolite S149c – – – – – 44–50 Mg-Hbl, Ts, Ed, Prg 0.59–0.68 6.36–6.71 0.67 b – –LAC4c Melt patche within amphibolite S149f – – – – – 38–43 Ts 0.63–0.74 6.25–6.50 – – –TG1a Tonalitic migmatite S24-1b 0.10–0.11 55 (51) 7 (8) 28 (29) 6 (13) 33–35 Fe-Prg, Hs 0.34–0.37 5.8–6.2 – 0.34–0.36 –TG1b Tonalitic migmatite S24-5a 0.11–0.12 58 (50) 8 (6) 25 (31) 9 (14) 31–34 Fe-Prg, Hs 0.25–0.40 5.8–6.2 – 0.33–0.35 –TG1c Melt patche within migmatite S24-6e – – – – – 32–35 Fe-Prg, Hs 0.34–0.38 5.95–6.25 – 0.32–0.33 –MP1 Metapelite Heg3 0.15 (0.26–0.28) b 75 (69) 15 (26) 4 (4) 5 (1) 24–26 – – – – 0.50–0.53 0.95–0.98– – –6.1–6.37.7–7.96.0–6.35.30.46–0.670.48–0.500.82–0.880.64Samples associated with D2 Variscan structureGAHT Grt-amphibolite S33b 0.15 (0.05) 70 (57) 12 (3) 15 (27) 3 (12) 15–27 (15) Fe-Ts, TsCum– – –GHT3 Metagabbro S151 – – – – – 37–60 Ts,CumGLT2 Metagabbro S130 – – – – – 48–62 (25) Mg-Hbl, Ts 0.74–0.88 6.24–6.64 – – –T3 Granodiorite – – – – – 34–37 Mg-Hbl, Ts 0.54–0.64 6.3–6.7 – – –Values in brackets represent core compositions of zoned minerals. Notes: Alm ¼ 100 · Fe 2+ /(Mg + Ca + Mn + Fe 2+ ), An ¼ 100 · Ca/(Ca + Na + K), XMg ¼ Mg/(Mg + Fe 2+ ).a Amphibole compositional name from classification of Leake et al. (1997): Prg, pargasite; Ts, tschermakite; Ed, edenite; Hs, hastingsite; Tr, tremolite; Mg-Hbl, magnesio-hornblende.b XMg ¼ Mg/(Mg + Fe tot ).tine from the core to the rim (i.e. Alm 57 fi 70Grs 2 fi 15 Py 3 fi 12 Sps 12 fi 3 ; X Mg ¼ 0.05 fi 0.15).A relic magmatic assemblage in the eastern myloniticmetagabbro is represented by Pl + Hbl ±Cpx ± Ttn ± Ilm. Magmatic plagioclase (0.5–5 mm)shows zoning marked by a rimward increase ofanorthite from 50 to 60%, whereas recrystallization ofthe plagioclase (0.05–0.1 mm) is accompanied by adecrease in anorthite content (i.e. An 45 ). Magmaticamphibole (0.5–5 mm) with pyroxene relics in thecores is magnesiohornblende (6.8–7.0 Si p.f.u., X Mg ¼0.72–0.77). Recrystallized grains (0.02–0.1 mm) correspondto magnesiohornblende with a slightly lowercontent of Si and X Mg (6.6–6.7 Si p.f.u., X Mg ¼ 0.69–0.74) in comparison with original grains.P–T conditions of Variscan metamorphismThe pressure of crystallization of the granodiorite (T3)was calculated as 6.5–7.0 kbar using the Al-content ofamphibole (Schmidt, 1992) at a temperature of 670–725 °C inferred from the Pl–Hbl thermometer ofHolland & Blundy (1994). In the western mylonitemetagabbro the temperature was estimated as770 ± 50 °C using the thermometer by Holland &Blundy (1994) in metagabbro (GHT3), while in garnetamphibolitethe temperature was estimated as 710–740 °C (GAHT). A similar temperature of 750 ±50 °C is reported by Baratoux et al. (2005) for 40amphibole–plagioclase couples. Pressure conditionsbetween 8 and 10 kbar were calculated only in Qtzbearinggarnet-amphibolite by means of the Kohn &Spear (1990) barometer. Based on 30 amphibole–plagioclase couples from the eastern mylonite metagabbro,Baratoux et al. (2005) estimated temperatureas 650 ± 50 °C using the Pl–Hbl thermometer ofHolland & Blundy (1994). The pressure could not becalculated because of the lack of garnet.Mineral textures and mineral compositions of rocks of thebanded amphibolite complexMineral assemblages and selected mineral compositionsof all study rocks are given in Table 1 and summarizedin Table 2, respectively.Banded amphibolites are characterized by the mineralassemblage Amp + Pl + Qtz ± Cpx ± Grt ±Mag ± Ilm. Three types of microstructures marked bystraightened grain boundaries without indications ofdynamic recrystallization are distinguished: (1) Equigranularaggregates (grain size 0.05–1.0 mm) ofamphibole, plagioclase and quartz (Fig. 5a); (2)amphibole-rich layers alternating with elongateaggregates (grain size 0.05–1.0 mm) of quartz andplagioclase; and (3) coarse-grained aggregates (grainsize 1–5 mm) of randomly distributed plagioclaseand quartz sometimes with amphibole occur in meltpatches. In types (1) and (2) the mineral grains arealigned and elongated parallel to the compositionalÓ 2005 Blackwell Publishing Ltd238