Quantitative structural analyses and numerical modelling of ...

CONTRASTING TEXTURAL RECORD OF TWO METAMORPHIC EVENTS 655Table 1. Mineral assemblages and textures of samples used for thermobarometry.Sample Unit Locality Rock type Texture Grt Pl Hbl Cum Cpx Qtz Bt Kfs Sil Rt Ilm Ttn Mag OpxSamples associated with D1 Cambro-Ordovician structureLAC4a LAC 66 g Grt-amphibolite Preferred shape orientation,· · · – – · – – – – · – · –annealedLAC4b LAC S149e Cpx-amphibolite Weak shape preferred orientation, – · · – · · – – – – · – · –annealedLAC4c LAC S149f Melt patches within Random orientation – · · – – · – – – – – – – –amphiboliteTG1a LAC S24-1b Tonalitic migmatite Weak shape preferred orientation, · · · – – · · – – – – – – –annealedTG1b LAC S24-5a Tonalitic migmatite Weak shape preferred orientation, · · · – – · · – – – – – – –annealedTG1c LAC S24-6e Melt patches within Random orientation – · · – – · · · – – – · · –migmatiteMP1 LAC Heg3 Metapelite Weak shape preferredorientation, annealed· · – – – · · · · · · – – –Samples associated with D2 Variscan structureGAHT Western S33b Grt-amphibolite Strong preferred orientation, dynamic · · · · – · – – – – · – · –gabbroic beltrecrystalliation of Pl, QtzGHT3 Western S151 Metagabbro Strong preferred orientation of Amp, – · · · · – – – – – · · – ·gabbroic beltdynamic recrystalliation of Pl, AmpGLT2 Eastern S130 Metagabbro Strong preferred orientation of Amp, – · · – – – – – – – · · – –gabbroic beltdynamic recrystalliation of Pl, AmpT3 Sill Granodiorite Dynamic recrystallization of Pl, Qtz – · · – – · · · – – · · · –PETROLOGY, METAMORPHIC TEXTURES ANDP–T ESTIMATESSample selection for petrological study and analyticalproceduresThe study samples were selected according to theirstructural position with respect to assumed Cambro-Ordovician and Variscan fabrics. Mineral analyseswere carried out on a CAMECA SX 50 at ETH Zu¨ richand a JEOL microprobe at the University of Mainz.Operating conditions were 15 kV acceleration voltageand beam current of 20 nA. Representative sampleswith mineral assemblages are given in Table 1 andcorresponding mineral compositions are summarizedin Table 2. Mineral abbreviations used in text andtables follow Kretz (1983). Representative mineralanalyses are listed in Tables 3–6. Amphibole formulaewere calculated after Holland & Blundy (1994) andclassified according to Leake et al. (1997). The geothermometersand geobarometers used for calculationsare given in Table 7. For each sample, five to 10 sets ofanalyses were used for P–T calculations and the resultsare shown in Table 7 and Fig. 6.Metamorphic textures and mineral compositions of thegranodiorite sill and mylonitic metagabbrosThe granodiorite sill is composed of Pl + Qtz +Amp + Bt + Kfs + Ttn + Mag + Ilm + Ap. Itshows magmatic, sub-magmatic and solid-statemicrostructures marked by dynamic recrystallizationof plagioclase, amphibole and quartz (Parry et al.,1997). Amphibole correspond to magnesiohornblendeand tschermakite (i.e. Si p.f.u. ¼ 6.3–6.7, X Mg ¼ 0.54–0.64) and plagioclase is An 34)37 andesine.The proportions of plagioclase and amphibole in thewestern mylonitic metagabbro vary significantly andthe rock locally consists of up to 90% plagioclase or90% amphibole. Minor clinopyroxene occurs in thecore of large amphibole. Titanite is an abundantaccessory mineral. Non-recrystallized amphibole isinterpreted as magmatic in origin and the compositioncorresponds to a magnesiohornblende (i.e. Si p.f.u. ¼7.00–7.25, X Mg ¼ 0.84–0.89). The magmatic plagioclaseis a An 55)62 labradorite.The mineral assemblage of highly reworked metagabbrowith banded mylonitic structure (Figs 4c & 5c)includes Pl + Amp ± Cum ± Cpx ± Opx ± Grt ±Ttn ± Rt ± Ilm. A granulite facies mineral assemblage,comprising saphirine and corundum, isre-equilibrated during subsequent amphibolite faciesreworking (Baratoux et al., 2005). Both amphibole andplagioclase show strong compositional variationsbetween those of old magmatic and metamorphicgrains (Table 1). Rare garnet-bearing amphibolitedeveloped through the deformation and metamorphismof a tonalitic migmatitic gneiss associatedwith metagabbro in the lower part of the gabbroicsheet. It exhibits mineral assemblage Hbl ±Cum ± Grt + Pl + Qtz ± Mag ± Ilm (Table 1)which allows the pressure during the Variscan metamorphismto be estimated.Recrystallized plagioclase exhibits an increase in theanorthite content from the core towards the rim inthe metagabbro (An 37 fi 60 ) and garnet-amphibolite(An 15 fi 27 ). Recrystallized amphibole in the metagabbrocorresponds to tschermakite, and tschermakite orferrotschermakite in the garnet-amphibolite. Garnetshows an increase in almandine, grossular, pyrope andX Mg (Fig. 7d), accompanied by depletion of spessar-Ó 2005 Blackwell Publishing Ltd237