Microstructural evidences for mineralogical ... - GeoScienceWorld

Bull. Soc. géol. France, 2015, t. 186, no 2-3, pp. 131-143

Microstructural evidences for mineralogical inheritance in partially molten rocks: example from the Vosges Mts PAVLÍNA HASALOVÁ1, KAREL SCHULMANN1,2, ANNE SOPHIE TABAUD 2,

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and EMILIEN OLIOT2

Keywords. – Vosges, Granite, Melt pathway, Magmatism, Magma mixing, Granite re-melting

Abstract. – During orogenic processes continental crust experiences significant partial melting. Repeated thermal pulses or fluctuation in fluid content can even cause multiple anatectic events that result in complex intrusion suits. In the Vosges mountains, France, two main generations of magmatic rocks are recorded. The first magmatic event occurred at ca. 340 Ma, and is represented by extensive K-Mg granitoids magmatism. The second magmatic event occurred at ca. 325 Ma and produced large quantity of felsic anatectic melts which further pervasively intruded and compositionally and texturally reworked previously formed granitoids. Detailed field and microstructural observations revealed continuous transitions from porphyritic granite with large euhedral Kfs and Pl phenocrysts (Type I granite) via intermediate granite (Type II) to fine-grained apparently isotropic granite (Type III) dominated by the neo-crystallized melt. The Type I granite preserves the original magmatic assemblage and has only incipient amount of the newly crystallized melt. The new melt-crystallized material forms narrow, fine-grained pathways along grain boundaries or cuts across pre-existing magmatic grains and forms an interlinked network. With increasing amount of the newly crystallized material the original magmatic grains are resorbed and show highly corroded shapes. The early formed feldspars grains have strong compositional zoning, with oscillatory zoned cores reflecting range of original magmatic compositions and rims showing later melt overgrowths. Original magmatic feldspars have different composition from the new phases crystallizing in the partially molten granite. We interpret the fine-grained microscopic corridors as melt pathways that were exploited by the new magma. We suggest that this melt pervasively migrated through the older granitoids resulting in mixture of inherited “xenocrysts” and of new melt-derived crystals. The interaction between the new melt and previously crystallized granitoids results in variety of granite textures and fabrics. These reflect different degrees of equilibration between the bulk rock and the passing melt. Finally, Type III granite carries mixed isotopic signature intermediate between the type I granite and the surrounding metasediments and granulites, suggesting mixing of the original granite with new later magma with source in these rocks.

Evidences microstructurales de l’héritage minéralogique dans les roches partiellement fondues : exemple du Massif vosgien Mots-clés. – Massif Vosgien, Granite, Voie d’infiltration du liquide silicaté, Refusion, Mélanges magmatiques

Résumé. – Lors des processus orogéniques, la croûte continentale subit une fusion partielle significative. Des pulses thermiques répétés ou une variation de la composition des fluides peuvent même mener à de multiples événements anatectiques qui conduisent à un ensemble d’intrusions complexes. Dans les Vosges, à l’est de la France, deux générations majeures de roches magmatiques sont observées. Le premier événement magmatique s’est déroulé vers 340 Ma, et il est associé à un magmatisme K-Mg extensif lié à la formation d’une épaisse racine orogénique et à l’exhumation subséquente de la croûte profonde vers un niveau crustal supérieur. Ces granitoïdes ont injecté tous les niveaux crustaux. Le second événement magmatique a eu lieu vers 325 Ma et a affecté exclusivement la croûte moyenne. Il a produit une grande quantité de roches anatectiques felsiques qui ont pénétré de manière diffuse et ont retravaillé la composition et la texture des magmas précédemment solidifiés. Les microstructures suggèrent que la seconde anatexie a été contemporaine à la déformation. Les observations de terrain et l’analyse microstructurale détaillée révèlent une transition continue du granite porphyrique avec de grands phénocristaux de feldspath-K et de plagioclase automorphes (granite type I), via le granite intermédiaire (Type II), au granite isotrope à grain fin (type III) qui a été entièrement retravaillé par le magma infiltré. Les granites de type I préservent l’ancien assemblage magmatique et ne contiennent qu’une petite proportion du magma nouvellement cristallisé. Ce nouveau matériel cristallisé suit des voies d’infiltration étroites et à grain fin le long des limites de grains ou recoupe les grains magmatiques préexistants. Ils forment ainsi un réseau étroit qui suit généralement la fabrique S-C définie par les grains magmatiques préexistants. L’augmentation de la quantité de matériel nouvellement cristallisé fait que les grains initiaux sont résorbés et montrent des morphologies très corrodées. Les grains de feldspath et de quartz initialement formés présentent une zonation compositionnelle importante, leurs cœurs reflétant la composition magmatique originelle et leurs bords montrant de multiples phases de refusion périphérique. Les feldspaths magmatiques originels ont une composition différente des phases néoformées qui ont cristallisé dans le granite partiellement fondu. Nous interprétons tous les couloirs microsco-

1. Centre for Lithospheric Research, Czech Geological Survey, Klárov 3, Prague 1, 11821, Czech Republic 2. Institut de Physique du Globe de Strasbourg, UMR 7516, University of Strasbourg/EOST, CNRS, 1 rue Blessig, F-67084 Strasbourg cedex, France 3. Chrono-environnement, UMR6249, University of Franche-Comté, CNRS, 16 route de Gray, 25030 Besançon cedex, France Emails : [email protected]; [email protected]; [email protected]; [email protected] Manuscript received on April 21, 2014; accepted on October 11, 2014 Bull. Soc. géol. Fr., 2015, no 2-3

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piques à grain fin comme les voies d’infiltrations empruntées par le nouveau magma. Nous suggérons que ce nouveau matériel en fusion a migré très largement à travers les granitoïdes plus vieux en suivant ce réseau. Il en a résulté un mélange de xénocristaux hérités et de nouveaux cristaux dérivés du matériel en fusion. L’interaction entre le nouveau magma et le granite précédemment cristallisé produit une variété de textures et de fabriques qui reflètent différents états d’équilibre entre la roche-hôte et le matériel en fusion injecté. Enfin, les granites de type II et III montrent des signatures isotopiques intermédiaires entre le granite de type I et les métasédiments et les granulites encaissants, ce qui suggère un mélange du granite initial avec le nouveau magma dont la source réside dans cet encaissant.

INTRODUCTION It is not necessarily straightforward to infer former melt presence in felsic rocks at microscale [e.g. Sawyer, 2001; Hasalová et al., 2008; Hasalová et al., 2011]. Criteria for recognition of former melt have been summarized by many authors [for review see Sawyer, 1999, 2001]. These include: (i) interstitial grains/films or cuspate pools between adjacent original grains [e.g. Jurewitz and Watson, 1984; Sawyer, 1999; Holness and Sawyer, 2008]; (ii) plagioclase zoning in contact with K-feldspar [e.g. Marchildon and Brown, 2001; Hasalová et al., 2011]; and (iii) low Kfs-Kfs and Kfs-Kfs-Pl dihedral angles, typical for granitic melts [Laporte et al., 1997]. This becomes even more difficult in granitic rocks. Hasalová et al. [2008 a, b, c] developed a new concept of melt infiltration in solid state rocks based on microstructural analysis, thermodynamic and geochemistry modeling. This work shows a possibility to distinguish between different magmatic pulses in partially molten rocks. However, these works do not allow for precise distinction of phases belonging to “original” magmatic assemblage and to those newly crystallized from melt. Recently, cathodoluminiscence (CL) imaging of magmatic grains was used in order to demonstrate multiple magma recharges [e.g. Müller et al., 2005; Davidson et al., 2001] or melt overgrowths in leucosomes [e.g. Hasalová et al., 2011] and to difference between xenocrystic phases and fluid/melt derived phases in granites [Oliot et al., 2010; Goncalves et al., 2012]. Combination of such a detail CL imaging with the microstructural analysis result in better distinction between the inherited “xenocrysts” and of new melt derived crystals in granitoids. This then allows for better understanding of hybridization processes in granitoids. Recently, magma hybridization by equilibration of distinct magma batches already in the source was described [Hasalová et al., 2011]. They suggest strong contrast between inherited features of earlier crystal phases and neocrystallized grains. Ultimately, the residual/inherited as well as peritectic phases can be transported passively into magma chamber and hybridize the granitic magma as proposed by the peritectic entrainment model [Stevens et al., 2007; Taylor and Stevens, 2010; Farina et al., 2012]. In the Vosges Mts. two major syn-orogenic magmatic episodes, different in age and chemistry, have been recognized [see Fluck, 1980 for review]: the earlier (ca. 340 Ma) magmatic event producing K-Mg granitoids recycled by younger (ca. 325 Ma) anatectic leucogranite together with surrounding granulites and metasedimentary rocks. Regions of magma recycling show the original sub-vertical foliation either completely or partially reworked restored by growth of new magmatic phases [Schulmann et al; Kratinová et al., 2012]. Although this evolution is clearly recorded by the existing structural data, AMS data and Sr-Nd isotopic Bull. Soc. géol. Fr., 2015, no 2-3

geochemistry [Tabaud et al., 2015], the microstructural record and mechanisms of such magmatic reworking process are poorly understood. In this manuscript we investigate microstructural features related to reworking of the Vosges granitoids. We have distinguished three textural types (type I-III) characterizing progressive modification of original K-Mg granitoids microstructure (type I) by infiltrating anatectic melts producing final fine-grained central Vosges anatectic granite (type III) locally with fragments of original K-Mg fragments of original K-Mg magmatic assemblage. Microstructural observations reveal narrow corridors of fine-grained granitic material cutting across or lining the boundaries of larger grains in the first two textural types. These corridors link together and form an interconnected extensive network in type III. We discuss a number of possible ways in which these microstructures may have formed and suggest that they represent intrusive pathways of the anatectic melts to infiltrate previously crystallized granitoid. Finally we discuss possible micro-mechanisms of magma infiltration as well as possible heat sources for the late anatexis and formation of the central Vosges granitoids. VOSGES MTS COMPLEX ANATECTIC AND MAGMATIC TERRANE The Vosges Mts are part of European Variscan belt in western Europe (fig. 1a) and consist of three parts: (i) the northern Vosges consisting of low grade Palaeozoic units; (ii) the central high-grade zone; and (iii) the southern low grade volcano-sedimentary sequences Devonian to Carboniferous in age (fig. 1b) [e.g. Fluck, 1980]. The central Vosges zone consists of several metamorphic and magmatic units (fig. 1b). It contains high-grade gneisses, migmatites, felsic granulites with associated garnet and spinel peridotites and eclogites with voluminous magmatic intrusions. The peak P-T conditions have been estimated at 700-900 oC and 12-15 kbar in the felsic granulites and at 600-700oC and 6-7 kbar in the medium grade metasedimentary rocks [Latouche et al., 1992; Skrzypek et al., 2012]. The retrogression of both rock types occurred at 600-700 oC and 2-5 kbar [Rey et al., 1989, 1992]. These two contrasting metamorphic complexes were juxtaposed during ca. 350-340 Ma collision characterized by major vertical material transfer between the lower crust and upper crust. This event was accompanied by massive magmatism of K-Mg granitoids ranging from ultrapotassic and ultramagnesium magmas (the Durbachite suite) to more felsic equivalents “the Crêtes and Ballons intrusions” [Skrzypek et al., 2014]. The whole metamorphic association is affected by Carboniferous extensional tectonics at ca. 325 Ma accompanied

MINERALOGICAL INHERITANCE IN PARTIALLY MOLTEN ROCKS

by LP-HT massive anatexis [Schaltegger et al., 1999; Schulmann et al., 2009]. This resulted into extensive intrusion of leucogranites [Kratinová et al., 2007], remelting and mechanical reworking of dismembered gneisses, granulites, schists and Mg-K granitoids along a subhorizontal ~10 km thick transitional zone gently dipping to the south [Kratinová et al., 2012]. This study focuses on magmatic rocks forming this transitional zone of partial melting and leucogranite intrusions (fig. 1b). In detail, according to their petrography the magmatic rocks of central and southern Vosges were divided into three groups: (i) the “Crêtes” K-Mg granite [Groth, 1877] or so-called “Central Vosges K-Mg granitoids” [Tabaud, 2012] represented by Hbl-Bt porphyritic monzonite to granite; (ii) the “Ballons” plutonic K-Mg complex [Michel-Levy, 1910] or so-called “southern Vosges K-Mg granitoids” [Pagel and Leterrier, 1980; Tabaud, 2012] that consist of Hbl-Bt porphyritic granite, granite and small intrusions of diorites, gabbros, syenites and monzonites; (iii) the most voluminous “Fundamental granite” [Hameurt, 1967] or so-called “Central Vosges Granite (CVG)” [Tabaud, 2012] that consists of Bt-Ms peraluminous granite with numerous xenoliths of gneisses and sedimentary rocks. In this manuscript we adopt the terminology of Tabaud [2012]. There are different petrogenetical models to account for origin of the older central and southern K-Mg granitoids [e.g. Gagny, 1968; André, 1983; Fluck et al., 1991]. Recently, their generation is explained by dehydration melting of relaminated radiogenic material together with underlying metasomatized mantle. This was later (~10-15 m.y.) followed by a widespread mid-crustal anatexis, producing Central

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Vosges Granite from mixed crustal sources formed by paragneisses and/or immature felsic-intermediate metaigneous rocks [Tabaud et al., 2015]. Field observations Schulmann et al. [2009] studied the origin of large orthogneiss and granulite bodies (few kilometres in size) and described their progressive destruction by infiltrating anatectic melt from surrounding migmatites and granites. These authors have shown that the cores of these bodies were intruded by granitic veins. However, the margins of these bodies were completely migmatized due to pervasive infiltration of externally derived melts. The study of Schulmann et al. [2009] suggests that these granulites and gneisses were texturally modified during joint mechanical and magmatic process leading to their complete dissolution in the surrounding Central Vosges Granite (CVG). In this work we focus on process of reworking of the K-Mg magmatic suites related to formation of CVG. All three studied Vosges granitoid types reveal mineral assemblage of Kfs + Pl + Qtz + Bt + Ms + Hbl in different proportions, with accessory Ilm ± Mgt ± Ttn ± Zrc ± Mnz ± Ap (mineral abbreviations after [Kretz, 1983]). Our detailed field observations show that the Vosges granitoids can be divided into three major texturally different granite types (fig. 2): (i) Type I porphyritic granite which consists of large euhedral grains of Kfs and minor Pl with medium sized matrix of Kfs, Pl and Qtz (fig. 2a-b); (ii) Type II intermediate granite with euhedral to irregularly shaped relicts of the original magmatic Kfs and Pl grains with fine- to

FIG. 1. – (a) General location of the Vosges Mts within the European Variscides. (b) Simplified geological map of the Vosges Mts [modified after Tabaud, 2012]. The three major granite types are emphasized. Bull. Soc. géol. Fr., 2015, no 2-3

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medium-sized Kfs, Pl and Qtz matrix (fig. 2c-d); (iii) Type III fine-grained granite consisting dominantly of fine- to medium grained isotropic matrix of Kfs, Pl and Qtz with only few relicts of the original magmatic grains (fig. 2e-f). From Type I granite towards Type III granite there is a continuous disappearance of the large magmatic feldspars and increasing proportion of the fine-grained matrix resulting into apparently isotropic granite (fig. 2). These granite types alternate and continuously grade into each other at outcrop- to kilometre-scale and there are no macroscopically visible dykes or leucosomes. This textural sequence possibly records pervasive, microscopic and heterogeneous transposition of porphyritic granite into apparently isotropic fine-grained granite. Importantly, this process is also accompanied by change in isotopic composition (fig. 3) and anisotropy of magnetic susceptibility fabric [Kratinová et al., 2012]. Later in the article we detail on microstructural

changes following this process and discuss the possible processes involved. Isotopic signature of the different granite types Tabaud [2012] showed that the Vosges granitoids reveal mixed isotopic signature suggesting multiple intrusion history or possible granite remelting. All the studied granitic rocks have a distinct crustal signature with negative eNd and initial 87Sr/86Sr between 0.705-0.720 (fig. 3). Importantly, there is a clear difference in isotopic composition between the Type I granite and the other granite types (fig. 3). Type I porphyritic K-Mg granite samples have well-defined clustered isotopic composition with high initial 87Sr/86Sr of 0.708-0.714 and negative e Nd of –4 to –7. In contrast, Type III granites show very variable 87Sr/86Sr and eNd values: from lower initial 87Sr/86Sr of 0.705-0.708 and negative

FIG. 2. – Rock photographs illustrating different granite types in the Vosges Mts and their characteristics. (a-b) Type I porphyritic granite with many large euhedral Kfs and Pl phenocrysts (white arrows); (c-d) Type II intermediate granite with corroded relics of the original phenocrysts. Locally, euhedral phenocrysts are preserved; (e-f) Type III fine-grained apparently isotropic granite with no visible relicts of the original Kfs and Pl phenocrysts. Note grain size decrease and phenocrysts disappearance towards the Type III. Bull. Soc. géol. Fr., 2015, no 2-3

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eNd of –2 to –5 to high initial 87Sr/86Sr of 0.715-0.720 and eNd of –6 to –7. The surrounding metasedimentary rocks have initial 87Sr/86Sr of 0.712-0.720 similarly to Vosges granulites and to Type III granites with 87Sr/86Sr of 0.714-0.719 (fig. 3) [Skrzypek, 2011]. In contrast, surrounding orthogneisses have initial 87Sr/86Sr of 0.707-0.710 (fig. 3) [Bonhomme and Fluck, 1981]. Thus, Tabaud [2012] concluded that the original granite (Type I) was mixed with anatectic melts derived from mid- to upper continental crust (metasedimentary and/or metaigneous rocks). The isotopic signature suggests mixing of Type I granite with magma sourced either in the surrounding orthogneisses or the metasedimentary rocks. This is probably controlled by proximity of the granites to the surrounding units. Arguments for structural inheritance in textural CVG sequence The deformation and emplacement history of Central Vosges K-Mg granitoids and CVG was extensively studied using anisotropy of magnetic susceptibility (AMS) method [e.g. Kratinová et al., 2007; 2012; Schulmann et al., 2009; Tabaud, 2012]. These authors demonstrated that the magmatic fabric of Central Vosges K-Mg granitoids is characterized by dominantly vertical foliation resulting from collision during Early Carboniferous. In contrast, the CVG leucogranites reveal subhorizontal foliation and lineation associated with plane strain fabric ellipsoid typical for

FIG. 3. – Sr-Nd isotopic compositions for source rocks and different granites (data from Tabaud [2012]). 87Sr/86Sri and eeNdi values are distinct for the Type I porphyritic granite (black triangles; calculated for initial age of 340 Ma), and two main magma sources: orthogneisses (grey line; data from Bonhomme and Fluck [1981]) and metasedimentary rocks (black line; data recalculated at 340 Ma). Type III granites from western (open circles) and southern Vosges (grey circles) reveal mixed signature and can be modelled by mixing of both source rocks with the Type I porphyritic granite. Also plotted for comparison are Vosges granulites (dashed rectangle; data from Skrzypek [2011]).

non-coaxial deformation [Schulmann et al., 2009; Kratinová et al., 2007]. Interestingly, the Central Vosges K-Mg granitoids with strong vertical foliation correspond to our textural Type I granite, while the CVG correlate with Type III in our textural sequence. All together, in our view, the AMS and geochemical data suggest that the three textural granite types are linked and represent different stages of inheritance of formerly crystallized granitoid in newly derived granite. MICROSTRUCTURAL OBSERVATIONS Detailed microstructural work has been done in order to describe melt distribution, proportion and appearance at microscale in the three granite types. Thin sections were cut perpendicular to the foliation and parallel to lineation (XZ section). Backscattered electron images (BSE) and cathodoluminescence images (CL) were collected using Scanning Electron TESCAN with attached Oxford Inca EDX microanalytical system at the Charles University, Prague, Czech Republic, at 20 kV acceleration voltage. Mineral compositions and compositional maps of feldspars were obtained at the Cameca SX100 at the Czech Geological Survey, Brno. All the granite types are variable in grain size from fine-grained to coarse-grained and consist dominantly of plagioclase, K-feldspar and quartz (> 90%) in different proportions (fig. 4) with minor biotite, muscovite and hornblende (< 10%). The felsic minerals (Qtz, Pl, Kfs) have two distinct appearances: they either form medium to large, euhedral to irregular grains or small to medium-sized interstitial grains (figs. 4 and 5). We have adapted the terminology of Hasalová et al. [2011], and use the terms “fine-grained material” or “neo-crystallized material” for the fine-grained interstitial grains, and the terms “paleocrystals” or “paleograins” for the large older magmatic grains. The large K-feldspar, plagioclase and quartz paleo-crystals have grain size between 0.1 cm and 5 cm (average grain size ~ 0.5 cm). In contrast, the fine-grained material has a significantly smaller grain size of 20-200 µm (fig. 5). The fine-grained material consists of plagioclase, K-feldspar and quartz and either explores perthite exsolutions, oscillatory zoning planes, twinning planes, cuts across large crystals or follows grain boundaries (figs 5 and 7). Its distribution is heterogeneous and it appears in range of textural positions from isolated to semi continuous “string of beads” microstructures [Sawyer, 2010], via narrow corridors of granitic composition that extend for length of a single or few grains (fig. 5a-b) to irregular semi-continuous or continuous corridors of different and varying widths (fig. 5). These narrow corridors tend to form linked arrays around and through large crystals (fig. 5d). In contrast the wider corridors of plagioclase, K-feldspar, quartz (> 0.5 mm wide) tend to be oriented sub-parallel to the main magmatic foliation (fig. 5c-d). Some of the large palaeocrystals reveal ductile shearing along the fine-grained corridors. All these continuous or semi-continuous finegrained granitic tracks link together and form an extensive network branching across and around large grains (fig. 5d). Major difference between the three granite types is proportion and appearance of the neocrystallized material and Bull. Soc. géol. Fr., 2015, no 2-3

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the paleocrystals. Neocrystallized material ranges from 1-5% in type I granite to 5-15% in type II granite and up to ~ 80-90% in type III granite (fig. 4). Increase in the proportion of fine-grained material is accompanied by the widening of the corridors and increased corrosion and break-up of the original paleocrystals (fig. 4). In type III granite with the highest proportion of the fine-grained material the

distinction between the two components becomes increasingly more difficult or almost impossible as the large grains are being destroyed and the neo-crystallized material is increasing in size (fig. 4c). Microstructures of the paleocrystals K-feldspar paleocrystals in Type I granite are medium to large in size (up to 5 cm), euhedral to slightly irregular in shape (fig. 6a-c). They preserve igneous textures as clear oscillatory zoning, euhedral shapes and perthitic exsolutions (fig. 6a, c). K-feldspar grains contain numerous Qtz, Bt and Pl inclusions (fig. 6b). K-feldspar boundaries not traced by fine-grained material are straight to slightly lobate (fig. 6a-c). In contrast, the boundaries traced by the fine-grained interstitial material show embayed high energy shapes (figs. 6d-f and 7e-f) indicating that they have been resorbed [e.g. Hasalová et al., 2008; Hasalová et al., 2011]. K-feldspar grains are not visibly plastically deformed (fig. 6). In Types II and III K-feldspars form large irregularly-shaped grains with embayed, high-energy boundaries, mostly cross-cut by the fine-grained interstitial material (comprised of plagioclase and quartz; fig. 6d-f; and 7e-f). The fine-grained material often explores perthitic exsolution planes and oscillatory zoning planes causing complete destruction of the large paleocrystals (fig. 7d). The degree of embayments is highest in Type III granite as the number of embayments correlates with amount of fine-grained material along and across the large grains. In Type III extreme cases of resorption of large grains of K-feldspar represented by skeletal-like relics are present (fig. 6d). Plagioclase occurs as large euhedral to irregular, commonly twinned grains with numerous Qtz and Bt inclusions. Like K-feldspar, it is not visibly plastically deformed and commonly sericitized. The paleograins are traced by the fine-grained K-feldspar and quartz, but not plagioclase. Plagioclase in contact with the fine-grained material show embayed, high-energy boundaries (fig. 7f). Similarly to K-feldspar the plagioclase shapes are the most irregular and corroded in Type III granite. In general, plagioclase is less corroded than K-feldspar paleocrystals. Quartz in all three granite types forms large irregular, weakly deformed grains, showing undulatory extinction, and straight to slightly lobate boundaries suggesting incipient grain boundary migration recrystallization. Amphibole grains are variable in size (0.1-0.8 cm) euhedral in shape with common Qtz and Ilm inclusions, locally being retrogressed along their margins to biotite and chlorite. Biotite grains are variable in size and shape. They form euhedral to highly corroded cuspate grains filled with quartz, K-feldspar and plagioclase. Often they also form clusters with decussate microstructure. Microstructures of the neo-crystallized grains

FIG. 4. – Photomicrographs showing typical microscopic appearance of the three granite types: (a) Type I porphyritic granite; (b) Type II intermediate granite; (c) Type III fine-grained granite. All granite types show two distinct grain populations – large palaeocrystals and fine-grained neocrystallized material. Bull. Soc. géol. Fr., 2015, no 2-3

The neo-crystallized material is granitic in composition, consisting of Pl, Kfs and Qtz (fig. 7). Fine-grained feldspars and quartz range in size from 10 µm in thin films (fig. 7a and d) up to ~200 µm in wider corridors (fig. 7h). Fine interstitial plagioclase and K-feldspar grains have preferentially elongated shapes with low dihedral angles against coarse quartz and feldspars grains. Quartz, in contrast, forms small interstitial rounded shapes with high


dihedral angles against feldspars. Feldspars and quartz in narrow, well-defined corridors have lobate boundaries against larger grains causing embayments in feldspar paleocrystals (fig. 7e-f). The fine plagioclase, K-feldspar and quartz grains are not visibly plastically deformed. The fine-grained corridors were locally used by later brittle fractures (fig. 7g), which also served as good low temperature fluid influx channels causing complete homogenization in feldspar composition, diminishing the difference between the paleocrystals and neocrystallized material. In summary, coarse-grained crystals of quartz, plagioclase or K-feldspar are cross-cut or partly surrounded by finegrained granitic material (figs. 5-7). Contacts with the fine grains are typically corroded and irregular (fig. 7e-h), and except for quartz, large grains are not obviously deformed indicating weak to negligible ductile solid-state, post anatectic deformation [e.g. Sawyer, 1998].

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perthites and antiperthites, in contrast to fine-grained feldspars that do not reveal any exsolution (fig. 6). The plagioclase paleograins are generally oligoclase and have normal zoning (core = An10-20; rim = An 0-5) at boundaries lined with fine-grained material or when cross-cut by fine-grained material. Plagioclase composition in fine-grained material is albitic, reaching An 0-5 in the thinnest films (fig. 7e-f). Plagioclase in wider fine-grained corridors also exhibits the same normal zoning as the large grains, with An 5-20 cores and more sodic (An0-5), thick rims when in contact with coarse grains (fig. 7f). K-feldspar in these fine-grained corridors is more potassic and Ba-poor than the large Kfspaleocrystals. In summary, compositional zoning of finegrained feldspars indicates chemical disequilibrium with coarse grains. Together, these features indicate that early formed crystals might have been out of equilibrium with the magma during later stages of crystallization.

Mineral chemistry of neo-crystallized grains and paleograins There is a difference in composition between the coarse paleocrystals and the neo-crystallized fine-grained plagioclase and K-feldspar grains. Composition of the two grain-types remains similar between the three types of granites. Paleo-K-feldspar grains are orthoclase and exhibit clear zoning with more potassic and lower Ba rims at boundaries lined with fine-grained material (fig. 6e). Some large euhedral Kfs grains show strong magmatic oscillatory zoning, alternating Ba-rich and Ba-poor zones (fig. 6a). Both, K-feldspar and plagioclase palaeocrystals show

DISCUSSION Vosges granitoids have isotopic signatures suggesting mixing of previously crystallized granite with anatectic melts derived from surrounding metasedimentary and metaigneous sources [Tabaud et al., 2015]. The microstructural data suggest that large volumes of K-Mg granitoids were infiltrated by granitic melt that interacted physically and chemically with crystalline material from the former granite. These features and hypothesis are discussed below.

FIG. 5. – Photomicrographs showing distinct appearance and width of the fine-grained corridors. (a-b) Narrow corridors of fine-grained material (< 100 µm wide; white arrows) along most of large grain boundaries. (c) Wider, irregular aggregate of fine to medium grained material (100-500 µm wide) coating large K-feldspar grain in optical continuity on both sides of the fine-grained corridor. (d) Interconnected network of the granitic fine-grained corridors. Bull. Soc. géol. Fr., 2015, no 2-3

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Pervasive melt infiltration? Texturally different types of granite were identified at an outcrop-scale (fig. 2). There is gradual transition between these three granite types. We have described the granite transformation on microscale and identify large paleocrystals between fine-grained granitic material (figs 5-7). Different

granite types reveal different proportions of the large paleocrystals and the fine-grained material (figs. 4). We suggest that the large paleocrystals represent inherited component from previously crystallized granitoid whereas the fine-grained material represents anatectic melt pathways through formerly solidified granite. The documented sequence therefore records progressive pervasive granitization of an existing granitoid.

FIG. 6. – Photomicrographs, backscattered electron (BSE) and CL images showing typical features of the paleo-crystals and their relationships with fine-grained corridors. (a-c) Large oscillatory zoned euhedral to slightly corroded paleo-K-feldspar grains with numerous Qtz and Bt inclusions in Type I granite. Note irregular shapes and embayments along the boundaries traced by fine-grained material (white arrows). (d) Skeletal-like paleo-K-feldspar with highly corroded boundaries (black arrows) and relict of original oscillatory zoning in Type II granite. (e) Highly corroded and zoned paleo-Kfs at boundaries traced by neo-crystallized Pl and Qtz (grey arrow). Kfs rim is K-rich and Ba poor and represent Kfs component of the neo-crystallized melt (white arrows). (f) Originally one large paleo-Kfs that has been disaggregated by corridors of neo-crystallized grains (white arrows). Bull. Soc. géol. Fr., 2015, no 2-3

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Beyond our hypothesis, observed microstructures could be also explained by different possible mechanisms: (i) late melt crystallization in a closed system [Hibbard, 1987; Bouchez et al., 1992; Vernon, 2004]; (ii) hydrothermal alterations; or (iii) solid-state deformation/dynamic recrystallization [e.g. Stipp et al., 2002; Vernon et al., 2004]. We discuss these possibilities below. Late melt crystallization in a closed magmatic system Continuous crystallization of granitic magma commonly results in bimodal grain size distribution with large phenocrysts isolated in a mass of fine-grained crystals [e.g. Hibbard, 1987; Vernon, 2004]. Large grains in Type I and II granites reveal some typical features of porphyritic magmatic rocks, such as oscillatory zoning, perthite exsolutions, euhedral shapes or simple twinning (fig. 6) [e.g. Vernon, 1990]. However, in Types II and III granites these large grains have strongly corroded shapes and are commonly cross-cut or coated by corridors of fine-grained granitic material (figs. 6d-f and 7). Unlike typical porphyritic texture where phenocrysts are surrounded by a finer-grained matrix, in Vosges granites, the fine-grained granitic material form a continuous or semi-continuous extensive network cross-cutting and branching around the large grains (figs. 4 and 5). Embayments and corroded shapes in contact with the fine-grained granitic material indicate partial resorption of the large grains and imply disequilibrium between the grains and melt [e.g. Hasalová et al., 2008]. This disequilibrium textures indicate that the microstructure cannot be a result of closed crystallization of a magma giving rise to a porphyritic texture [Hasalová et al., 2011]. Hydrothermal alteration Late stage magmatic hydrous fluids can circulate along grain boundaries and fractures, causing local dissolution and precipitation, modifying the original magmatic texture. In such case, fine-grained Qtz, Pl and Kfs could be precipitated and large crystals corroded. We have documented locally extensive hydrothermal overprint (fig. 7g). These hydrothermal alterations either follow brittle fractures or use the large pre-existing fine-grained corridors (fig. 7g). They also often crosscut these fine-grained corridors. They change and homogenize composition of the surrounding minerals (e.g. Kfs in fig. 7g) without precipitation of any new feldspars or quartz. Biotite is locally, altered to chlorite along these alteration pathways. These documented features are clearly late without any genetic link to the fine-grained corridor. Solid-state deformation/dynamic recrystallization Metamorphic conditions during the HT-LP anatexis were estimated to have reached ca. 700oC [Rey et al., 1992], i.e. sufficient to enable dynamic recrystallization. Typically, dynamic recrystallization is producing fine, monomineralic aggregates compositionally similar to the original porphyroclasts [e.g. Vernon, 2004]. This is incompatible with our observations that fine-grained corridors consist of different proportions of all three minerals comprising a granitic rock (plagioclase, K-feldspar and quartz), or by two of these minerals with the third found as overgrowths on large

grains in the absence of evidence for alteration [Holness and Sawyer, 2008; Hasalová et al., 2011]. Moreover, the fine-grained material lacks microstructures indicating extensive grain boundary migration [Stipp et al., 2002]. In chemically open system the recrystallization can involve changes in chemical composition [Stünitz, 1998] and subsequently the recrystallized grains will have different composition. However, we lack any evidence for open-system during the metamorphism. In summary, the textures described are unlikely to represent a porphyritic texture resulting from closed system crystallization, or hydrothermal alteration, or to be result of solid-state deformation. We therefore suggest that these features represent new melt channels in previously solidified granitoid. Melt infiltration causing granitization of a former granite In our view, the observed textures result from an open-system, where newly formed anatectic melt pervasively infiltrated previously formed granite. In this process the different granite types: (i) reflect the different proportions of the newly crystallized melt and original paleocrystals, and (ii) represent different degrees of the granitization process. The anatectic melt exploits microfractures through and in between crystals, giving rise to an interconnected network. When an anatectic melt passes through a granitic rock, differences between the incoming magma and existing solid phases results in textural and chemical disequilibrium features. This is well documented along the interconnected networks formed by neo-crystallized granite (fig. 7). Evidence for textural disequilibrium includes paleograins with irregular, high energy, slightly to strongly corroded shapes at contact with newly crystallized material. Chemical disequilibrium is manifested by compositional zoning of feldspars. Granitic melts close to the eutectic composition are comprised almost entirely of combinations of Qtz + Kfs + Pl. As stated by Hasalová et al. [2011] “if a new melt batch invades a solid framework of feldspars and quartz, it either crystallizes all three felsic phases, or two felsic phases, forming the bulk of the dykelet, whereas the third phase either crystallizes as overgrowths on the neighbouring paleocrystal or elsewhere along the dykelet”. Both, plagioclase and K-feldspar paleograins in Vosges granites have compositional zoning at boundaries lined with neo-crystallized material: oligoclase is rimmed by albite, and orthoclase is rimmed by more potassic rims. In summary, the studied Vosges granites record, at microscale, process of pervasive granitization of previously solidified granitoid. Moreover, in the field, we do not observe any significant melt accumulations as dykes or leucosomes suggesting that this process is rather cryptic and pervasive. Mechanisms Cosgrove [1997] proposed a model of hydraulic fractures development at high and low differential stress in lower crustal conditions where the fluid is represented by silicate melts. This concept can be applied to the studied textural sequence and fits several observations in Types I and II granites. The melt exploits various types of intragranular discontinuities such as: oscillatory zoning surfaces, perthite Bull. Soc. géol. Fr., 2015, no 2-3

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flames and Carlsbad twin planes in large feldspar phenocrysts (figs 6 and 7d), or coats grain boundaries. Ultimately, the intragranular fracturing results in complete disintegration of the paleocrystals (fig. 7d), thereby producing angular breccia-like aggregates (fig. 7c). Such fine-grained melt pathways in the Type I and II granites may be interpreted as intragranular fractures i.e. fractures developed at moderate or very low differential stress and melt pressure [Cosgrove, 1997]. Slightly different model can be proposed to explain origin of polycrystalline veinlets. These fine-grained zones are preferentially oriented and locally displace the fragmented feldspar paleocrystals (fig. 5c-d). Therefore, the veinlets are both tensional and shear fractures and satisfy requirements for hybrid fracture. Fair preferred orientation of these microfractures indicates relatively high differential stress while undeformed melt infill is an argument for high melt (fluid) pressure [Price and Cosgrove, 1990]. The intergranular fractures oriented at variable angle to principal compressive stress were also observed; suggesting that pre-existing anisotropic planes can be also dilated [Závada et al., 2007]. In theory, surfaces with low tensional strength can be dilated even if they are oriented at high angle to main compressive stress at high fluid pressure provided the differential stress is lower than the difference in tensional strength of variably oriented planes [Cosgrove et al., 1997]. In conclusion, the original granitoid can be dismembered at microscopic scale thanks to high melt pressure and low to moderate differential stress during the contemporaneous extensional shearing and melting as already proposed by Schulmann et al. [2009]. The fine-grained Type III granite originates from complete disintegration of the original solid framework of K-Mg granitoids, resulting in formation of matrix composed of paleocrystal and neo-crystal mixture. At this stage the new sub-horizontal AMS fabric as documented by Kratinová et al. [2012] is dominant, which implies partial to complete resetting of original mineral fabric. The possible mechanism is granular flow involving passive rotation of paleocrystals and their reorientation parallel to new fabric as it was demonstrated by Hasalová et al. [2008a]. Heat source responsible for formation of CVG anatectic melts The origin of CVG involves major and localized melting of crust ca. 15 m.y. after the main collisional event [Tabaud et al., 2015]. There are several possible heat sources which can produce such a massive melting of middle crust as upwelling of asthenospheric mantle, radioactive heat production in overthickened crust and emplacement of heatproducing element rich granitoids in the middle crust.

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The upwelling of the asthenospheric mantle associated with thinning of the lithosphere may result in elevation of hot geotherm and melting of fertile middle crust [e.g. Thompson et al., 2001] as exemplified by Vosges K-Mg granitoids magmatism [Schulmann et al., 2002]. This roughly corresponds to thermal effects of mantle lithospheric delamination or lithospheric mantle ablation [Faure et al., 2002; 2010]. However, lack of contemporaneous basic igneous activity in the Vosges Mts rules out any mantlerelated heat sources. Radioactive heat production in overthickened crust is a popular mechanism described for example in the Massif Central, to explain a major magmatic event associated with extensional tectonics [e.g. Faure et al., 2009; Choulet et al., 2012]. According to this model the radioactive heat production in overthickened crust is responsible for progressive heating, subsequent melting and rheological collapse of collisional orogen associated with emplacement of S-type granites in the upper crust [e.g. Vanderhaeghe and Teyssier, 2001]. However, this model may not be appropriate in the case of the Vosges Mts as the felsic lower crust has been depleted in LILE and radioactive elements already at ~340 Ma during generation of Mg-K magmas and complementary felsic HP–HT granulites [Skrzypek et al., 2012]. Other possibility to increase temperature seems an emplacement of heat-producing element (HPE) rich granitoids directly in the mid-crustal levels [e.g. Sandiford et al., 2002; Lexa et al., 2011]. The central and southern Vosges K-Mg granitoids are characterized by exceptionally high amounts of radioactive elements [Tabaud et al., 2015]. The time- and length-scales of transient heating period in middle crust will depend on amount of HPE-rich granitoids, their emplacement depth and efficiency with which the HPE were extracted from the deep crustal source. Therefore, emplacement of HPE-rich K-Mg granitoids at ~340 Ma in the mid-crustal levels might play an important role in temperature increase. This can potentially trigger partial melting of the surrounding metasedimentary rocks and subsequent production of CVG after a ~10-15 m.y. time lag. However, the K-Mg granitoids itself lack any evidence of in-situ partial melting, implying that either we are not able to see this granitic melt or that this extensive anatexis affects only the surrounding units. All together the possible heat source causing such extensive melting in Vosges middle crust remains enigmatic and needs to be answered. This is also true for other large magmatic areas throughout the Variscan belt [e.g. western Iberia – Gutierrez-Alonso et al., 2011; Bohemian massif – ðák et al., 2014].

FIG. 7. – Photomicrographs and backscattered electron (BSE) images showing main features of the neo-crystallized grains. (a-c) Semi-continuous to continuous intracrystalline corridors of fine-grained granitic material forming interconnected network. Note in (c) wide completely interconnected network of breccias-like appearance. (d) Originally one large paleo-Kfs that has been completely disrupted by continuous corridors of fine-grained Pl and Qtz (white arrows). Note common perthite exsolutions. These were used by the fine-grained material. (e-f) BSE images of semi-continuous narrow corridors of quartz and plagioclase cutting across coarse oscillatory zoned K-feldspar. Plagioclase grains in (f) are zoned, with more albitic rims (black arrows). Note high-energy corroded boundaries of coarse K-feldspar (white arrows). (g) Kfs paleo-crystal traced and cross-cut by neocrystallized material (white arrows) overprinted by late hydrothermal alteration zone. (h) Wider corridor of fine-grained Kfs + Pl + Qtz along large paleo-Kfs. Bull. Soc. géol. Fr., 2015, no 2-3

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CONCLUSIONS Isotopic signature of Vosges granitoids suggests that the K-Mg granitoids are mixed with anatectic melts from surrounding metasedimentary rocks, orthogneisses and granulites. This is further supported by microstructural data showing that large volumes of K-Mg granitoids were infiltrated by granitic melt that interacted physically and chemically with crystalline material from the granitoid. This resulted into formation of texturally distinct granite types (Type I-III) that reveal different proportions of the newly melt-crystallized material and the original paleocrystals. The new anatectic melt is granitic in composition and exploits microfractures through and in between crystals, giving rise to narrow, fine-grained pathways forming an interlinked network. Such fine-grained melt pathways are interpreted as intragranular fractures i.e. fractures developed at moderate or very low differential stress and melt pressure. Larger melt corridors behave as hybrid fractures supporting hydraulic micro-fracturing at high differential stress and high fluid/melt pressure. The original paleocrystals have slightly to strongly corroded shapes in contact with the fine-grained granitic material,

indicating their partial resorption. This implies textural disequilibrium between these grains and inflowing melt. Chemical disequilibrium is manifested by compositional zoning of feldspars. Such disequilibrium results from differences between the inflowing magma and existing solid phases. In summary, the large paleocrystals represent inherited component from previously crystallized granitoid whereas the fine-grained material represents anatectic melt pathways through formerly solidified granite. The documented sequence records progressive pervasive granitization of an existing granitoid. Possible heat source triggering the melting might be heat producing elements-rich K-Mg granitoids emplaced at ~340 Ma at mid-crustal levels.

Acknowledgements. – This work was financially supported by Internal Funding of Czech Geological Survey (grant No. 321250). P. Hasalová also acknowledges Funding of Grant Agency of Czech Republic (No. 14-25995S). We also gratefully acknowledge support of BRGM (project PDR13DGR43: Démonstrateur Vosges – Fossé Rhénan, RGF program). We would like to thank M. Faure and P. Olivier for the constructive reviews, R. Weinberg for stimulating discussions and the journal guest editor P. Barbey for all his extensive editorial work on the manuscript.

References ANDRÉ F. (1983). – Pétrologie structurale et pétrogenèse des formations plutoniques septentrionales du massif des Ballons (Vosges, France). – Thèse doct., Université de Nancy. BONHOMME M. & FLUCK P. (1981). – Nouvelles données isotopiques Rb-Sr obtenues sur les granulites des Vosges. Age protérozoïque terminal de la série volcanique calco-alcaline et âge acadien du métamorphisme régional. – C. R. Acad. Sci., 293, 771-774. BOUCHEZ J.-L., DELAS C., GLEIZES G., NÉDELEC A. & CUNEY M. (1992). – Submagmatic microfractures in granites. – Geology, 20, 35-38. CHOULET F., FAURE M., FABRRI O. & MONIÉ P. (2012). – Relationships between magmatism and extension along the Autun-La Serre fault system in the Variscan belt of the eastern French Massif Central. – Internat. J. Earth Sci., 101, 393-413. COSGROVE J.W. (1997). – The influence of mechanical anisotropy on the behaviour of the lower crust. – Tectonophysics, 280, 1-14. DAVIDSON J., TEPLEY F., PALACZ Z. & MEFFAN-MAIN S. (2001). – Magma recharge, contamination and residence times revealed by in situ laser ablation isotopic analysis of feldspar in volcanic rocks. – Earth Planet. Sci. Lett., 184, 427-442. FARINA F., STEVENS G., DINI A. & ROCCHI S. (2012). – Peritectic phase entrainment and magma mixing in the late Miocene Elba island laccolith-pluton-dyke complex (Italy). – Lithos, 153, 243-260. FAURE M., MONIÉ P., MALUSKI H., PIN C. & LELOIX C. (2002). – Late Visean thermal event in the northern part of the French Massif Central. New 40Ar/39Ar and Rb-Sr isotopic constrains on the Hercynian syn-orogenic extension. – Internat. J. Earth Sci., 91, 53-75. FAURE M., LARDEAUX J.-M. & LEDRU P. (2009). – A review of the pre-Permian geology of the Variscan French Massif Central. – C. R. Geosciences, 341, 202-213. FAURE M., COCHERIE A., MÉZÈME E.B., CHARLES N. & ROSSI P. (2010). – Middle Carboniferous crustal melting in the Variscan belt: New insights from U-Th-Pbtot. monazite and U-Pb zircon ages of the Montagne Noire axial zone (southern French Massif Central). – Gondwana Res., 18, 653-673. FLUCK P. (1980). – Métamorphisme et magmatisme dans les Vosges moyennes d’Alsace. Contribution à l’histoire de la chaîne Varisque. – Mém. Sci. Géol., 62, 248. Bull. Soc. géol. Fr., 2015, no 2-3

FLUCK P., PIQUÉ A., SCHNEIDER J.-L. & WHITECHURCH H. (1991). – Le socle vosgien. – Sci. Géol. Bull., 44, 207-235. GAGNY C. (1968). – Pétrogenèse du granite des Crêtes. Vosges méridionales, France. – Thèse d’Etat, Université de Nantes. GONCALVES P., OLIOT E., MARQUER D. & CONNOLLY J.A.D. (2012). – Role of chemical processes on shear zone formation: an example from the Grimsel metagranodiorite (Aar massif, Central Alps). – J. metam. Geol., 30, 703-722. GROTH P. (1877). – Das Gneissgebiet von Markirch im Oberelsass. – Abh. Geol. Spezialkarte Elsass-Lothringen, 1, 395-488. GUTIÉRREZ-ALONSO G., FERNÁNDEZ-SUÁREZ J., JEFFRIES T.J., JOHNSTON S.T., PASTOR-GALÁN D., MURPHY J.B., FRANCO M.P. & GONZALO J.C. (2011). – Diachronous post-orogenic magmatism within a developing orocline in Iberia, European Variscides. – Tectonics, 30 (5), TC5008. HAMEURT J. (1967). – Les terrains cristallins et crystallophylliens du versant occidental des Vosges moyennes. – Mém. Serv. Carte géol. Alsace-Lorraine, 26, 402. HASALOVÁ P., JANOUÒEK V., SCHULMANN K., ÒTÍPSKÁ P. & ERBAN V. (2008c). – From orthogneiss to migmatite: Geochemical assessment of the melt infiltration model in the Gföhl Unit (Moldanubian zone, Bohemian massif). – Lithos, 102, 508-537. HASALOVÁ P., SCHULMANN K., LEXA O., ÒTÍPSKÁ P., HROUDA F., ULRICH S., HALODA J. & TÝCOVÁ P. (2008a). – Origin of migmatites by deformation-enhanced melt infiltration of orthogneiss: A new model based on quantitative microstructural analysis. – J. metam. Geol., 26, 29-53. HASALOVÁ P., ÒTÍPSKÁ P., POWELL R., SCHULMANN K., JANOUÒEK V. & LEXA O. (2008b). – Transforming mylonitic metagranite by open-system interactions during melt flow. – J. metam. Geol., 26, 55-80. HASALOVÁ P., W EINBERG R.F. & MACRAE C. (2011). – Microstructural evidence for magma confluence and reusage of magma pathways: implications for magma hybridization, Karakoram shear zone in NW India. – J. metam. Geol., 29, 875-900. HIBBARD M.J. (1987). – Deformation of incompletely crystallized magma systems: granitic gneisses and their tectonic implications. – J. Geol., 95, 543-561.


HOLNESS M.B. & SAWYER E.W. (2008). – On the pseudomorphing of meltfilled pores during the crystallization of migmatites. – J. Petrol., 49, 1343-1363. JUREWITZ S.R. & WATSON E.B. (1984). – Distribution of partial melt in a felsic system – the importance of surface energy. – Contrib. Mineral. Petrol., 85, 25-29. KRATINOVÁ Z., SCHULMANN K., EDEL J.B. & JEðEK J. (2007). – Model of successive granite sheet emplacement in transtensional setting: integrated microstructural and anisotropy of magnetic susceptibility study. – Tectonics, 26, TC6003. KRATINOVÁ Z., SCHULMANN K., EDEL J.B. & TABAUD A.S. (2012). – AMS record of brittle dilation, viscous stretching and gravity driven magma ascent in area of magma rich crustal extension (Vosges Mts, NE France). – Internat. J. Earth Sci., 101, 803-817. KRETZ R. (1969). – On the spatial distribution of crystals in rocks. – Lithos, 2, 39-66. KRETZ R. (1983). – Symbols for rock-forming minerals. – Am. Mineral., 68, 277-279. LAPORTE D., RAPAILLE C. & PROVOST A. (1997). – Wetting angle, equilibrium melt geometry, and the permeability threshold of partially molten crustal protoliths. In: J.-L. BOUCHEZ, D.H.W. HUTTON and W.E. STEPHENS , Eds, Granite: from segregation of melt to emplacement fabrics. – Kluwer, Dordrecht, 31-54. LATOUCHE L., FABRIES J. & GUIRAUD M. (1992). – Retrograde evolution in the Central Vosges mountains (northeastern France): implications for the metamorphic history of high-grade rocks during the Variscan orogeny. – Tectonophysics, 205, 387-407. LEXA O., SCHULMANN K., JANOUÒEK V., ÒTÍPSKÁ P., GUY A. & RACEK M. (2011). – Heat sources and trigger mechanisms of exhumation of HP granulites in Variscan orogenic root. – J. metam. Geol., 29, 79-102. MARCHILDON N. & BROWN M. (2001). – Melt segregation in late syn-tectonic anatectic migmatites: an example from the Onawa contact aureole, Maine, USA. – Phys. Chem. Earth, Part A: Solid Earth and Geodesy, 26, 225-229. MICHEL-LEVY A. (1910). – Analogie des terrains primaires du Sud des Vosges et de ceux du Morvan. – Bull. Soc. géol. Fr., X, 816-828. MÜLLER A., BREITER K., SELTMANN R. & PÉCSKAY Z. (2005). – Quartz and feldspar zoning in the eastern Erzgebirge volcano-plutonic complex (Germany, Czech Republic): evidence of multiple magma mixing. – Lithos, 80, 201-227. OLIOT E., GONCALVES P. & MARQUER D. (2010). – Role of plagioclase and reaction softening in a metagranite shear zone at mid crustal conditions (Gotthard massif, Swiss Central Alps). – J. metam. Geol., 28, 849-871. PAGEL M. & LETERRIER J. (1980). – The subalkaline potassic magmatism of the Ballons massif (southern Vosges, France): shoshonitic affinity. – Lithos, 13, 1-10. PRICE M.J. & COSGROVE J.W. (1990). – Analysis of geological structures. – Cambridge University Press, Cambridge. REY P., BURG J.-P., LARDEAUX J.-M. & FLUCK P. (1989). – Evolutions métamorphiques contrastées dans les Vosges orientales: témoins d’un charriage dans la chaîne varisque. – C. R. Acad. Sci., 309, 815-821. REY P., BURG J.-P. & CARON J.-M. (1992). – Middle and late Carboniferous extension in the Variscan belt: structural and petrological evidences from Vosges massif (eastern France). – Geodin. Acta, 5, 17-36. SANDIFORD M., MCLAREN S. & NEUMANN N. (2002). – Long-term thermal consequences of the redistribution of heat-producing elements associated with large-scale granitic complexes. – J. metam. Geol., 20, 87-98. SAWYER E.W. (1998). – Formation and evolution of granite magmas during crustal reworking: the significance of diatexites. – J. Petrol., 39, 1147-1167. SAWYER E.W. (1999). – Criteria for the recognition of partial melting. – Phys. Chem. Earth, 24, 269-279. SAWYER E.W. (2001). – Melt segregation in the continental crust: distribution and movement of melt in anatectic rocks. – J. metam. Geol., 19, 291-309. SAWYER E.W. (2010). – Migmatites formed by water-fluxed partial melting of a leucogranodiorite protolith: Microstructures in the residual rocks and source of the fluid. – Lithos, 116, 273-286. SCHALTEGGER U., SCHNEIDER J.-L., MAURIN J.-C. & CORFU F. (1996). – Precise U-Pb chronometry of 345-340 Ma old magmatism related to syn-convergence extension in the southern Vosges (Central Variscan belt). – Earth Planet. Sci. Lett., 144, 403-419.

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SCHALTEGGER U., FANNING C.M., GÜNTHER D., MAURIN J.C., SCHULMANN K. & GEBAUER D. (1999). – Growth, annealing and recrystallization of zircon and preservation of monazite in high-grade metamorphism: conventional and in situ U-Pb isotope, cathodoluminiscence and microchemical evidence. – Contrib. Mineral. Petrol., 134, 186-201. SCHULMANN K., EDEL J.B., HASALOVÁ P., COSGROVE J., JEðEK J. & LEXA O. (2009). – Influence of melt induced mechanical anisotropy on the magnetic fabrics and rheology of deforming migmatites, Central Vosges, France. – J. Struct. Geol., 31, 1223-1237. SCHULMANN K., SCHALTEGGER U., JEðEK J., THOMPSON A.B. & EDEL J.B. (2002). – Rapid burial and exhumation during orogeny: thickening and synconvergent exhumation of thermally weakened and thinned crust (Variscan orogen in western Europe). – Am. J. Sci., 302, 856-879. SKRZYPEK E. (2011). – Contribution structurale, pétrologique et géochronologique à la tectonique intracontinentale de la chaîne hercynienne d’Europe (Sudètes, Vosges). – Thèse doct., Université de Strasbourg, 381p. SKRZYPEK E., TABAUD A.S., EDEL J.B., SCHULMANN K., COCHERIE A., GUERROT C. & ROSSI P. (2012). – The significance of Late Devonian ophiolites in the Variscan orogen: a record from the Vosges Klippen belt. – Internat. J. Earth Sci., 101, 951-972. SKRZYPEK E., SCHULMANN K., TABAUD A.S. & EDEL J.B. (2014). – Palaeozoic evolution of the Variscan Vosges mountains. In: SCHULMANN K., MARTINEZ CATALÁN J.R., LARDEAUX J.-M., JANOUÒEK V. and OGGIANO G., Eds, The Variscan Orogeny: extent, timescale and the formation of the European crust. – Geol. Soc., London, Sp. Publ., 405, 45-75. STEVENS G., VILLAROS A. & MOYEN J.F. (2007). – Selective peritectic garnet entrainment as the origin of geochemical diversity in S-type granites. – Geology, 35, 9-12. STIPP M., STÜNITZ H., HEILBRONNER R. & SCHMID S.M. (2002). – The eastern Tonale fault zone: a “natural laboratory” for crystal plastic deformation of quartz over a temperature range from 250 to 700oC. – J. Struct. Geol., 24, 1861-1884. STÜNITZ H. (1998). – Syndeformational recrystallization – dynamic or compositionally induced? – Contrib. Mineral. Petrol., 131, 219-236. TABAUD A.S. (2012). – Le magmatisme des Vosges: conséquence des subductions paléozoïques (datation, pétrologie, géochimie, ASM). – Thèse doct., Université de Strasbourg. TABAUD A.S., JANOUÒEK V., ROSSI P., W HITECHURCH H., SKRZYPEK E., SCHULMANN K., GUERROT C. & PAQUETTE J.-L. (2015). – Chronology, petrogenesis and heat sources for successive Carboniferous magmatic events in the southern-central Variscan Vosges Mts (NE France). – J. Geol. Soc. London (in press). TAYLOR J. & STEVENS G. (2010). – Selective entrainment of peritectic garnet into S-type granitic magmas: Evidence from Archaean midcrustal anatectites. – Lithos, 120, 277-292. THOMPSON A.B., SCHULMANN K., JEðEK J. & TOLAR V. (2001). – Thermally softened continental extensional zones (arcs and rifts) as precursors to thickened orogenic belts. – Tectonophysics, 332, 115-141. VANDERHAEGHE O. & TEYSSIER C. (2001). – Crustal-scale rheological transition during late-orogenic collapse. – Tectonophysics, 335, 211-228. VERNON R.H. (1990). – K-feldspar augen in felsic gneisses and mylonites – deformed phenocrysts or porphyroblasts? – J. Geol. Soc. Sweden, 112, 157-167. VERNON R.H. (2004). – A practical guide to rock microstructures. – Cambridge University Press, Cambridge. VON E LLER J.-P. (1964). – Dioritisation, granitisation et métamorphisme dans les Vosges cristallines du Nord. I – Région comprise entre la plaine d’Alsace, d’Andlau à Saint-Nabor et le Champ du Feu. – Bull. Serv. Carte Géol. Alsace-Lorraine, 17, 171-210. ZÁVADA P., SCHULMANN K., KONOPÁSEK J., ULRICH S. & LEXA O. (2007). – Extreme ductility of feldspar aggregates – Melt enhanced grain boundary sliding and creep failure: Rheological implications for felsic lower crust. – J. Geophys. Res., 112, B10210. ðÁK J., VERNER K., JANOUÒEK V., HOLUB F.V., KACHLÍK V., FINGER F., HAJNÁ J., TOMEK F., VONDROVIC L. & TRUBAC J. (2014). – A plate-kinematic model for the assembly of the Bohemian massif constrained b y structural relationships around granitoid plutons. In: K. SCHULMANN, J.R. MARTINEZ CATALÁN, J.-M. LARDEAUX, V. JANOUÒEK and G. OGGIANO, Eds, The Variscan orogeny: extent, timescale and the formation of the European crust. – Geol. Soc., London, Sp. Publ., 405, 45-75. Bull. Soc. géol. Fr., 2015, no 2-3