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Model of successive granite sheet emplacement in
transtensional setting: integrated microstructural and
anisotropy of magnetic susceptibility study.
Z. Kratinova, K. Schulmann, J.B. Edel, J. Jezek, U. Schaltegger
To cite this version:
Z. Kratinova, K. Schulmann, J.B. Edel, J. Jezek, U. Schaltegger. Model of successive granite
sheet emplacement in transtensional setting: integrated microstructural and anisotropy of magnetic
susceptibility study.. Tectonics, American Geophysical Union (AGU), 2007, 26 (6), pp.TC6003.
Model of successive granite sheet emplacement in transtensional
setting: Integrated microstructural and anisotropy of magnetic
susceptibility study
Zuzana Kratinova´,1,2 Karel Schulmann,3 Jean-Bernard Edel,4 Josef Jezˇek,5 and Urs Schaltegger6
Received 11 August 2006; revised 22 June 2007; accepted 25 July 2007; published 9 November 2007.
[1] This study presents a model of successive emplacement of three granite plutons in transtensional deformation regime controlled by the preextensional synconvergent history of the lower and middle crust in the central Vosges Mountains (France). The complex compressional orogenic structure recorded a vertical exhumation-related fabric in midcrustal levels which was overprinted by the regional extension. Three successively emplaced granite sheets exploited the inherited vertical anisotropy obliquely oriented with respect to the applied tensile stress. This progressive opening of original steep fabric leads to the sequential generation of free spaces toward the south, and emplacement and deformation of granite sheets. Repeated granite intrusions are responsible for reheating of southern margins of the already partially solidified granites, which became reactivated under lower ambient thermal conditions related to overall 6 Ma cooling of rapidly exhumed crust. The results of anisotropy of magnetic susceptibility (AMS) fabric modeling suggest a highly partitioned oblique extension divided into pure shear-dominated deformation close to the central and northern margins of intrusions and wrench-dominated shear along the southern margins. The AMS modeling suggests the importance of overprinting of the intrusive fabrics by the transtensional deformation.
Citation: Kratinova´, Z., K. Schulmann, J.-B. Edel, J. Jezˇek, and U. Schaltegger (2007), Model of successive granite sheet emplacement in transtensional setting: Integrated microstructural and anisotropy of magnetic susceptibility study, Tectonics, 26, TC6003, doi:10.1029/2006TC002035.
1. Introduction
[2] Existing extensional models of granite emplacement
require an intimate link between magma emplacement and country rock displacement by faulting and ductile shearing [Paterson and Fowler, 1993]. In this context, numerous studies address the problem of significance of magmatic fabrics for regional kinematic reconstructions and exhuma-tion of deep crust [e.g., Lacroix et al., 1998; Wilson and Grocott, 1999]. Despite the existence of numerous studies dealing with emplacement of granites in an extensional and/ or transtensional context [e.g., Ayoa et al., 2005; Sadeghian et al., 2005], the interplay between magmatic and tectonic processes during pluton ascent and emplacement remains a subject of wide discussion [e.g., Zˇa´k and Paterson, 2005; Weinberg et al., 2004]. The review of Paterson et al. [1998] on magma fabrics in plutons argued that the preserved fabric pattern is mostly formed after chamber construction thereby providing little information about the ascent and emplacement of magma. These authors suggested that magmatic fabrics can easily be reset and often reflect only the last increments of deformation.
[3] The magmatic fabrics are due to rotations of rigid
crystals in melt in viscous flow, which itself is controlled by the shapes and movements of the rigid boundaries of the magmatic body [Jezˇek et al., 1996; Paterson et al., 1998]. Models of the behavior of multiparticle system in different types of viscous flow might explain magmatic fabrics [Fernandez and Laporte, 1991; Jezˇek et al., 1994], but the driving forces responsible for the magmatic fabrics remain unconstrained. This problem might be solved to some extent, if the degree of coupling of magmatic and host rock fabrics is known [Paterson et al., 1998]. According to this concept, magmatic fabrics which originated owing to internally driven flow are not mechanically coupled with the host rock structures, while syntectonic intrusions show continuity of magmatic fabrics with the host rock indicating a high degree of mechanical coupling [Gapais, 1989; Hutton, 1988]. The degree of mechanical coupling is commonly interpreted in terms of intrusion depth [Schofield and D’Lemos, 1998]; therefore magmatic fabrics associated with magma ascent may be preserved in granitoids intruding the upper crustal levels, where rapid cooling prevents crystallized magma from recording significant tectonic deformation. At greater intrusion depth, magma may record tectonic strain due to slow cooling and longer crystallization
1Geophysical Institute, Czech Academy of Sciences, Praha, Czech
Republic.
2Also at Institute of Petrology and Structural Geology, Charles
University, Prague, Czech Republic.
3
Centre de Ge´ochimie de la Surface, EOST, Universite´ Louis Pasteur, Strasbourg, France.
4
Institut Physique du Globe, EOST, Universite´ Louis Pasteur, Strasbourg, France.
5
Institute of Applied Mathematics and Computer Sciences, Faculty of Science, Charles University, Prague, Czech Republic.
6
De´partement de Mineralogie, Universite´ de Geneve, Geneva, Switzer-land.
Copyright 2007 by the American Geophysical Union. 0278-7407/07/2006TC002035
reflecting thermal environment of the host rocks during emplacement [Paterson et al., 1998].
[4] The magmatic or pre-Rheological Critical Melt
Per-centage (pre-RCMP) [Hutton, 1988] as well as solid-state fabrics can be regionally characterized by means of anisot-ropy of magnetic susceptibility (AMS) [Bouchez, 1997]. Such studies routinely provide maps of fabric intensities, symmetries and orientations of AMS fabric axes, often accompanied with systematic mapping of deformation microstructures [Bouchez et al., 1990; Ferre´ et al., 1995]. Quantitative microstructural and AMS fabric mapping yields important information about the geometry and kinematics of the deformation in syntectonically emplaced magmatic bod-ies and allow defining the time-temperature-deformation paths of such plutons. AMS fabrics can now be modeled for the known magnetic mineralogy using new software [Jezˇek and Hrouda, 2002]. Application of these simulations may also help to evaluate multiple superpositions of defor-mation fabrics resulting, for example, from oblique over-prints of tectonic deformations on intrusive fabrics.
[5] In this paper, we present field, microstructural, AMS
and geochronological data from three crustally derived
granite intrusions located in the central Vosges Mountains (France). We characterize the relationship between magmat-ic and subsolidus fabrmagmat-ics of the intrusions and structures of the host rocks in terms of mechanical coupling. We also focus on the spatial distribution of internal contacts between the three granites as well as on the AMS fabrics and deformation microstructures developed in the central and marginal parts of individual granite bodies. The structural and fabric study is complemented by isotopic U-Pb dating of granite crystallization age. Finally, we simulated the thermal evolution and modeled the AMS fabrics within the plutons to determine the direction of granite growth, as well as to characterize the transtensional partitioned defor-mation contemporaneous with and postdating emplacement.
2. Geological Setting
2.1. Regional Geology and Lithology of Principal Units [6] The Vosges Mountains are divided into three major
domains [Fluck, 1980; Schulmann et al., 2002]: the south-ern Paleozoic basin, the central high-grade gneissic domain Figure 1. Geological map and the position of the Vosges Mountains (NE France) within the European
Variscides. (a) Location of the study area in the framework of the European Variscides (black square marks the Vosges Mountains), modified after Edel and Weber [1995]. (b) Simplified geological map of the Vosges Mountains after Schulmann et al. [2002] with the Thannenkirch, Bre´zouard, and Bilstein granites (BBTC complex) indicated within the black rectangle.
and the northern low-grade Paleozoic sedimentary sequen-ces (Figure 1). The north side of the central high-grade domain is limited by a dextral strike-slip shear zone (the Lalaye-Lubine zone, Figure 1) and the south side by the large calc-alkaline Ballon granite [Fluck, 1980], dated at 339 to 342 Ma (U-Pb zircon) [Schaltegger et al., 1996]. The high-grade domain is dissected by the NE-SW trending Saint Marie aux Mines fault zone (Figure 1). In the east, it is composed of a granulite facies unit including lenses of peridotites, felsic granulites and amphibolites [Fluck, 1980]. The footwall of the high-grade unit, composed of midcrustal monotonous paragneisses [Rey et al., 1989] (Figure 1), is further intruded by the three granitic sheets which are the focus of this study: the Bilstein, Bre´zouard and Thannen-kirch granite complexes (BBTC, Figure 1) [Fluck, 1980; Saavedra et al., 1973]. South of the three intrusions, the paragneisses are affected by partial melting and show progressive evolution into metatexites and diatexites and heterogeneous granites (so-called anatectic granite, Figure 1). This migmatitic unit contains several bodies of felsic migmatized orthogneiss, which may represent retro-grade granulites [Fluck, 1980] (Figure 1).
[7] This study is focused on the mechanisms of
emplace-ment of succession of three BBTC intrusions (168 km2) forming sheet-like bodies elongated in a WSW-ENE direc-tion parallel to the large-scale sinistral shear zone [Wickert and Eisbacher, 1988] (Figure 1). The Thannenkirch granite has a ratio between the width and length of about 1:2.5 while toward the south, pluton elongation in the WSW-ENE direction becomes more important, reaching the ratio1:7 for the Bre´zouard granite and1:16 for the Bilstein granite. [8] Generally, all three granite intrusions are relatively
homogenous without any evidence of internal zoning or sheeting. The main textural variety within the Thannenkirch granite is a medium-grained porphyritic biotite granite with rare xenoliths of host rock gneiss [Fluck, 1980]. The Bre´zouard and the Bilstein intrusions are represented by a fine-grained (2 mm) two-mica leucogranite, rich in quartz and muscovite, with a decreased ratio of K-feldspar with respect to plagioclase (+albite) [Saavedra et al., 1973]. The
Thannenkirch and the Bre´zouard granites are intruded by NW-SE or NE-SW striking granitic dike swarms of variable textures (aplites, granite porphyries).
2.2. Metamorphism and Existing Geochronology [9] The early high-grade stage in the granulite facies
rocks is characterized by Qtz-Grt-Kfs-Pl-Bt ± Ky (mineral abbreviation after Kretz [1983]) assemblage [Rey et al., 1989] and P-T conditions corresponding to 750° – 800°C at >9 kbar [Rey et al., 1989]. Subsequently, the granulitic assemblage was reequilibrated in LP-HT amphibolite fa-cies conditions estimated at 650°C and <5 kbar [Rey et al., 1989, 1992] or even at 2 – 3 kbar [Gayk and Kleinschrodt, 2000]. The underlying midcrustal monotonous gneiss unit, which hosts the BBTC, preserves relics of kyanite in garnet porphyroblasts [Rey et al., 1989] and underwent PT conditions of 7 kbar and 600°C [Rey et al., 1992]. Latouche et al. [1992] estimated 660° ± 50°C and 6 kbar for the dominant mineral assemblage comprising Bt-Sil-Grt ± Crd-Kfs-Pl in the same unit. The northernmost relatively narrow contact aureole of the Thannenkirch granite consists of hornfels-like paragneiss characterized by mineral assemblage dominated by biotite similar to the dominant assemblage. South of the BBTC, the adjacent Crd-Bt-Sil migmatized paragneiss is characterized by PT conditions of 640° ± 80°C and P 4kbar [Rey et al., 1992]. The degree of partial melting increases further to the south, where biotite diatexites and heterogeneous granites occur without the presence of temperature and pressure sensitive assemblages.
[10] The published U-Pb geochronological data on zircon
and monazite compiled in Table 1, show that the early peak metamorphism in the granulites occurred at around 335 Ma [Schaltegger et al., 1999]. Recrystallization of the high-grade orthogneiss in the southern part of the studied migmatitic domain at about 328 Ma [Schaltegger et al., 1999] is consistent with the crystallization ages of surround-ing anatectic granites and with the Bre´zouard granite [Schulmann et al., 2002]. The granulitic rocks north of
Table 1. Summary Table of Published and New Geochronological Dataa
U-Pb Zircon + Monazite K-Ar 39Ar-40Ar
South Migmatites
anatectic migmatites 326 ± 5Ma 333 ± 10Ma (biotite)
orthogneiss migmatites 328 ± 4Ma 336 ± 1Ma 328 ± 10Ma (biotite)
BBTC
Bilstein 323 ± 10Ma (muscovite) 327 ± 2Ma (muscovite)
Bre´zouard 329 ± 2Ma 323 ± 10Ma (muscovite) 332 ± 2Ma (muscovite)
Thannenkirch 326 ± 1Ma 314 ± 9Ma (chlorite)
North Gneiss Unit
metapelites 330 ± 7Ma (biotite)
granulite gneiss 335 ± 4Ma 327 ± 6Ma (biotite)
332 ± 3/-2 Ma (durbachite dike) 332 ± 4Ma (phlogopite)
337 ± 4Ma (amphibole)
aAr40-Ar39data from Boutin et al. [1995], the U-Pb zircon and monazite data from Schaltegger et al. [1996, 1999] and Schulmann et
the BBTC yield 39Ar-40Ar amphibole cooling age of 337 Ma, while biotite reveals ages between 327 and 332 Ma [Boutin et al., 1995]. K-Ar cooling ages from the southern migmatitic domain and from the BBTC yield consistent ages of323 Ma for muscovite and slightly older39Ar-40Ar
muscovite cooling ages similar to those in the granulite unit ranging between 327 and 332 Ma [Boutin et al., 1995]. To recapitulate, the peak metamorphism around 335 Ma related to the granulite facies conditions was followed by melting associated with crystallization of the BBTC intrusions at about 330 325 Ma. The subsequent rapid cooling occurred within 5 – 10 Ma time span.
3. New U-Pb Geochronology of the
Thannenkirch Granite
[11] In order to understand the mechanism and succession
of emplacement of the entire granitic suite, it is important to constrain the crystallization and emplacement age of the plutonic bodies. Whereas crystallization ages of the Bre´-zouard leucogranite and southern anatectic granite are well established by previous studies (Table 1), crystallization ages of the northern Thannenkirch granite remains un-known. This intrusion differs in geochemistry and mineral-ogy from the muscovite-bearing Bilstein and Bre´zouard leucogranites, and therefore was traditionally considered as a result of an earlier magmatic event [Fluck et al., 1987]. [12] New U-Pb age determinations of the Thannenkirch
granite (Table 2 and Figure 2) were carried out on zircon microfractions of 1 to 3 grains each, between 2 and 11 micrograms (see Appendix A). Three analyses of pris-matic zircon yielded a concordia age of 325.7 ± 0.7 Ma; two other analyses resulted in discordant points, indicating the presence of an inherited crustal component of Cadomian age (629 ± 82 Ma) in the analyzed zircons. The determina-tion of initial 176Hf/177Hf isotopic compositions of the analyzed zircon grains yielded eHf values of 0.7 and 1.3, indicating a crustal origin of the Thannenkirch granite magma [Kinny and Maas, 2003]. Hf depleted mantle model ages are 1.3 Ga, pointing to mixed Proterozoic-Paleozoic components in the source of the granitic magma. The age of 325.7 ± 0.7 Ma is interpreted as the time of magma crystallization of the Thannenkirch granite, contemporane-ous with the intrusion of the adjacent Bre´zouard granite [Schulmann et al., 2002], and within analytical errors also with the migmatites and the anatectic granite located to the south [Schaltegger et al., 1999]. These results suggest that all these intrusions represent S-type granites derived from a similar crustal source.
4. Structural Pattern of the Host Rock
[13] In order to understand the fabrics of the BBTC
plutons and link them with the regional deformation events we have studied the structural record of the country rocks. The metamorphic rocks of the Central Vosges exhibit superposed Variscan deformations ranging from early com-pressional to late extensional fabrics [Rey et al., 1992].
T able 2 . Results of U-Pb Dating o f Z ircon From the Thannenkirch Granite a Numbe r Desc ription b W eigh t, mg N umber of Grains Conce ntrations Atomic Ratios Apparent Ages Error U Pb Rad iated, ppm Pb Non radiate d , pg Th/U c 206/2 04 d 207/2 3 5 e Error 2s, % 206/2 38 e Error 2s, % 207/2 0 6 e Error 2s, % 206/238 207/2 3 5 207/2 0 6 C o rrection 1 prism-tips 0 .00 3 3 3 244 12.4 6 5.5 0 .30 501 0.37 73 0.96 0.05172 0.60 0.05 290 0.84 325. 1 325. 0 324.5 0 .50 2 spr tips 0 .00 2 1 2 294 15.4 2 1.4 0 .37 1484 0.38 48 0.61 0.05194 0.42 0.05 373 0.41 326. 4 330. 6 359.9 0 .74 3 lge prism 0.01 12 1 241 12.4 1 1.6 0 .32 5603 0.37 78 0.39 0.05172 0.34 0.05 298 0.15 325. 1 325. 4 327.8 0 .92 4 prism-tips 0 .00 5 0 2 377 19.0 8 3.5 0 .24 1814 0.37 96 0.51 0.0520 0.43 0.05 530 0.32 326. 6 326. 7 327.7 0 .78 5 prism-tips 0 .00 6 6 2 370 30.9 6 1.8 0 .65 6624 0.61 37 0.41 0.0766 0.37 0.05 809 0.15 475. 9 485. 9 533.1 0 .93 aA nalytical data are related to Figu re 2. b Notation: ac , acicular; euh, euh edral, clrls, colorless, incl, inclu sions, pr , prism s; transp, transpa rent, frags, fragments; lpr , long prismati c; spr , shor t prismatic; z, zircon; G-type zircons are ac cording to Pu pin [1980]. c Cal culated on the basi s o f radioge nic 208 Pb/ 206 Pb ratios, assu ming concor dancy . dCorrected for fractionatio n and spike . eCo rrected for fractionation, spike , b lank and com mon lea d (acco rding to Stac ey and Kra mers [1975]).
4.1. Structural Evolution North of the BBTC
[14] Structures recorded by the metamorphic complexes
north of the BBTC result from three successive compres-sional deformation events D1n,D2n,and D3n. D1n is
repre-sented by a high-grade gneissosity in granulites and penetrative schistosity and compositional banding in meta-sedimentary rocks and amphibolites. Characteristic orienta-tions of D1nwithin the monotonous schists in the east part
of the studied area is the steeply dipping and E-W trending S1nfoliation (Figures 3a and 3b). However, in the west, S1n
of the granulitic unit dips either to the NW or to the NE under moderate to shallow angles.
[15] The second deformation event D2n is characterized
by gently dipping S2nfoliation, associated with the
devel-opment of symmetrical folding of S1n fabric (Figure 4a).
The D2ndeformation is more penetrative and widespread in
the metasedimentary rocks than in the granulitic high-grade unit, where it shows discrete mylonitic zones. Locally, close to the contact between the granulitic and monotonous units, S1n foliation in the monotonous paragneiss unit is
com-pletely transposed leading to the development of new amphibolite facies schistosity. The axial planar cleavage S2n, defined by the alignment of Sil and Bt, dips moderately
to the NE. The S2nfabric is commonly associated with the
development of mineral lineation L2n that is parallel to
hinges of F2nfolds. The limbs of mesoscopic F2nfolds dip
gently either to the NE or to the NW and the fold axis commonly plunges gently to NW (Figure 3a).
[16] The third deformation phase D3nis associated with a
heterogeneous greenschist facies deformation developed during the late NW-SE compression. The entire sequence of the upper granulitic unit and the underlying midcrustal rocks west from the St. Marie aux Mines shear zone are
folded by meter-scale F3nfolds with subhorizontal NE-SW
trending hinges and NW-dipping axial planar cleavage (Figure 4b). D3n deformation is strongly developed in the
Bt-Sill schists close to boundary with the hornfels zone which rims the Thannenkirch granite, but does not affect the granite itself or the hornfels. This may indicate that the hornfels zone represents a rigid mantle (carapace), which protected the granite interior from the D3n deformation
implying that the greenschist deformation of the granulites and monotonous unit occurred after the solidification of the BTTC.
4.2. Structural Evolution South of the BBTC
[17] South of the BBTC, the early fabric D1s of the
migmatitic domain is pervasively overprinted by the Car-boniferous extensional deformation, D2s, which also
dom-inates the structural record of the Central Vosges high-grade rocks west of the Saint Marie aux Mines fault zone [Rey et al., 1992]. The early formed metamorphic foliation S1s,
represented by polycrystalline aggregates of K-feldspar, plagioclase and biotite are locally preserved in two large domains composed of strongly recrystallized felsic orthog-neisses [Fluck, 1980; K. Schulmann et al., Influence of mechanical anisotropy and melt proportion on recurrent brittle and ductile response of partially molten crust exempli-fied by structural and AMS study, central Vosges, France, submitted to Journal of Structural Geology, 2007] (Figure 3b). The S1s dips steeply S or SSW, i.e., discordantly with
respect to the gently dipping extensional foliation S2s in
surrounding migmatites and anatectic granites (Figure 3a). S1sis also preserved in large Crd-Bt-Sil schists adjacent to
the southeastern margin of the BBTC, where schistosity dips steeply to the south (Figure 3a). Here the direction of Figure 2. Concordia diagrams with zircon U-Pb age for the Thannenkirch granite. The age is calculated
as a lower intercept age of the best fit line and is interpreted as the age of magma crystallization.
Figure 3. Structural data of the studied area, close-up of the Figure 1. (a) Detailed structural map with the line A-A0 indicating the position of the interpretative cross section. Contoured stereonets (equal-area lower hemisphere projection) of poles to S1,2,3foliation planes for northern gneiss and southern migmatites. (b) Interpretative cross section (A-A0) showing
overprint of vertical D1nfabric by the flat D2ndeformation phase in the south and late heterogeneous D3ndeformation in the
S2s foliation of diatexites and anatectic granites gradually
turns into the E-W direction in parallelism with the Bilstein granite sheet. Similarly, the mineral lineation change direc-tion toward the BBTC margin from SSE plunges to E-W subhorizontal lineations (Figure 3a).
[18] The extensional deformation D2sis coeval with the
process of anatexis, intrusions of S-type granites [Blumenfeld and Bouchez, 1988] and development of migmatitic layering S2sin the metatexites. The S2s is also defined by
the preferred orientation of schlieren and layers of meta-graywackes in diatexites granites (Figure 4c), while in Figure 4. Field photographs of macroscopic structural features in the studied area. (a) Open F2nfolds
north of the Thannenkirch granite (road cut north of Haute Koenigsburg). (b) Asymmetrical F3n open
folds in the Saint Marie aux Mines fault zone. (c) Metagreywacke boudins surrounded by diatexites and heterogeneous granites, migmatite region south from the BBTC (road cut in the Kaysersberg town). (d) Subvertical magmatic fabrics along the southern boundary of the Thannenkirch granite (road cut north of Ribeauville´). (e) Subvertical solid-state fabrics along the southern boundary of the Bre´zouard granite (road cut north of Ribeauville´). (f) S-C fabrics within the Bilstein granite (St. Ulrich castle north of Ribeauville´).
anatectic granites it is marked by the alignment of biotite and feldspar phenocrysts. The S2sfoliation generally dips to the
SW or W at gentle to moderate angles and bears strong mineral and stretching lineation L2splunging under shallow
angles to the SSE (Figure 3a). The magmatic fabric in migmatites and granites progressively evolve into solid-state deformation characterized by strong recrystallization of quartz and micas, and by the fracturing of feldspars [Paterson et al., 1989]. The deformation is locally strongly noncoaxial with important transition from magmatic to solid-state de-formation leading to the development of C-S structures [Blumenfeld and Bouchez, 1988] indicating top to the SSE normal sense of shearing. Rey et al. [1992] argued that this deformation results from crustal-scale extension based on existence of normal shear zones separating the hanging wall granulites and footwall barrovian schists and on shape of retrograde PT path in these rocks. Our structural analysis confirms the extensional character of deformation south of the BBTC including transition from high-temperature con-ditions to greenschist facies normal shearing (Figure 3). Retrograde character of D2s deformation is manifested by
C-S mylonites and dynamic recrystallization features in quartz which are extensively developed in localized greenschist facies shear zones parallel to principal migma-titic fabric.
5. Fabric Pattern in the BBTC
[19] In order to characterize the internal fabric pattern of
the BBTC granites, thermal conditions and kinematics of deformation, we have examined microstructures and pre-ferred orientation of the quartz c-axis within the granites in addition to the field structural study.
5.1. Mesoscopic Fabrics and Field Structures
[20] The map shape of the BBTC consists of sigmoidal
granite bodies elongated in the E-W to WSW-ENE direction (Figure 3a). The present-day localization of the roof and the deeper parts of the magmatic system cannot be established owing to steeply dipping contacts (80°) between the individual granitic sheets. However, relatively abundant stoped blocks in the northern part of the BBTC indicate that the present erosion level in the north is close to the pluton roof (Figure 3a).
[21] The northernmost margin of the Thannenkirch granite
dips moderately (50°) to the NW forming a more or less conformable contact with the host rocks. The contact is characterized by numerous granite dykes crosscutting the metamorphic fabric of the host rock or intruding it in the form of wide sills parallel to the S1nand S2nfoliations. In addition,
this domain incorporates abundant stoped blocks (several meters to several hundreds of meters in size) of Bt-Sil hornfels. Similar crosscutting patterns are developed along the northern contact of the Bre´zouard granite. The southern margin of the BBTC has a host rock – pluton contact dipping 80° to S, parallel to the foliation in the migmatites and anatectic granites (Figure 3b). Here the magmatic to solid-state fabrics in the Bilstein granite are in agreement with the fabric pattern of the adjacent migmatites.
[22] The Thannenkirch granite preserves a complex
mag-matic foliation pattern, which is defined by the preferred orientation of large (5 cm) K-feldspar phenocrysts, igne-ous plagioclase and biotite and by the alignment of mafic enclaves [Fluck, 1980]. The core of the pluton reveals nonsystematic mesoscopic foliation orientations, whereas in the southern margin the magmatic fabric becomes sub-vertical and E-W trending, parallel to the pluton margin (Figure 4d). In the southern margin of the granite, the E-W trending subvertical magmatic fabric is progressively over-printed by a parallel solid-state foliation, which results in the development of fine-grained S-C mylonites indicating a sinistral sense of shear. Here the feldspar phenocrysts are perfectly aligned and show a transition from LS toward L fabric with E-W subhorizontal stretching axis.
[23] The Bre´zouard granite shows similar structural
pat-terns as the Thannenkirch intrusion. In the northern and central parts, the patterns are characterized by a fine-grained (2 mm) to weakly porphyritic isotropic granite that sharply develops a subsolidus fabrics (Figure 4e) and finally toward a hundred meters wide band of mylonites separating the Bre´zouard granite from the southerly Bilstein granite. The southern subsolidus fabric increases to form S-dipping steep foliation and subhorizontal ENE trending lineation without well-defined mesoscopic shear strain indicators. The mylonitic zone locally contains lenses of host rock which underwent a high deformation, so that their original features could be hardly identified.
[24] The whole body of the Bilstein granite is strongly
overprinted by the pervasive heterogeneous S-C mylonitic fabrics (Figure 4f), which show rarely preserved magmatic domains. Mylonitic foliation is steeply dipping to the south and bears stretching and mineral subhorizontal E-W trend-ing lineation (Figures 3a and 3b). Remarkably consistent S-C relationships indicate sinistral sense of shearing. 5.2. Deformation Microstructures and Quartz Crystallographic Preferred Orientations
[25] The microstructures in syntectonically deformed
granitoids provide important information about the history of decreasing-temperature overprint developed during the pluton crystallization and cooling [Paterson et al., 1989; Tribe and D’Lemos, 1996] and indicate the thermal envi-ronment of the host rock during granite emplacement. The nature and distribution of preserved fabrics indicate relative time of cooling through a distinct temperature interval assuming a continuous deformation [Schofield and D’Lemos, 1998].
[26] Deformation microstructures have been studied using
optical and SEM microscopy. Electron backscattered dif-fraction (EBSD) was employed to determine the crystallo-graphic preferred orientation (CPO) of quartz using the scanning electron microscope Cam Scan S4 fitted with HKL backscattered diffraction detector of Institute of Petrology and Structural Geology, Charles University, Prague. The data were acquired under operating conditions of 20 kV accelerating voltage, 5.6 nA beam current, working distance 39 mm, 2- to 5-mm beam diameter and were collected by manual identification of diffraction bands.
The diffraction patterns were indexed using the Channel5 software and data were treated using software of Mainprice ftp://ftp.dstu.univ-montp2.fr/pub/TPHY/david/pc).
5.2.1. Magmatic Microstructures
[27] The magmatic microstructure is well-developed in
the central and northern part of the Thannenkirch and Bre´zouard granites (Figure 5). The groundmass of these granitoids is formed by subhedral feldspars, biotite and interstitial anhedral quartz. Euhedral plagioclase phenoc-rysts up to 5 cm in size display typical magmatic growth zoning [Vernon, 2004]. Strongly aligned K-feldspar phe-nocrysts in southern parts of each intrusion and all of the Bilstein granite are affected by transgranular cracks healed by fine-grained quartz, minor orthoclase and plagioclase indicating magmatic fracturing [Bouchez et al., 1992]. Quartz grains show lobate grain boundaries and chessboard pattern of subgrain boundaries, which is indicative of high temperatures [Kruhl, 1996].
[28] The CPO of the quartz grains from the central part of
the Bre´zouard granite shows generally weak maxima with-out any preferred orientation (Figure 6b). However, the c-axis pattern from the southern part of the Thannenkirch granite with the strong magmatic fabric, exhibits incomplete crossed girdle distribution with weak maximum (Figure 6a). Several other weak submaxima are located around the periphery of the diagram approximately at 45° from the Z axis of the finite strain. Such c-axis distribution is typical of noncoaxial deformation and dominant activity of prism [c] slip system and subordinate activities of prismhai and basal hai slips. These textures and presence of chessboard microstructures are characteristic of high temperatures of deformation close to granite solidus700°C [Kruhl, 1996; Paterson et al., 1989]. However, recent papers discussing the effect of fluids on recrystallization mechanisms [Morgan and Law, 2004; Okudaira et al., 1998], revealed that the temperature related to active slip systems may dramatically decrease with respect to the physico-chemical conditions so that the geothermobarometer based on hai-[c] quartz transition [Kruhl, 1996] provides only an approximate information.
5.2.2. High-Temperature Solid-State Microstructures [29] High-temperature microstructure locally developed
in the Thannenkirch and Bre´zouard granites is charac-terized by layers of biotite flakes and strongly recrystallized fine-grained monomineralic quartz ribbons (Figure 5c). K-feldspar grains show dynamic recrystallization resulting from subgrain recrystallization mechanisms along the edges
of the porphyroclasts and transgranular shear zones (Figure 5b). Plagioclase grains showing bent twin-lamellae with sharp terminations, attest to intracrystalline deformation [e.g., Vernon, 2004]. Quartz show irregular grain shapes, lobate boundaries and numerous ‘‘left over grains’’ implying grain boundary migration recrystallization mechanisms [Guillope and Poirier, 1980]. Locally, the monomineralic ribbons of polygonal quartz grains with straight boundaries meeting in triple point junctions indicate postdeformational annealing.
[30] The quartz CPOs reveal strong single maximum (up
to 10 multiples of uniform distribution) in the centre of the pole figure (Figures 6c, 6d and 6e) indicating activity of prismhai slip system, related to plane strain and noncoaxial deformation [Schmid and Casey, 1986]. The type of quartz texture and grain boundary migration recrystallization mechanism suggest activity of high-temperature grain boundary migration dislocation creep regime of quartz (Type 3 of Hirth and Tullis [1992]) [Gleason and Tullis, 1995] at relatively high temperature (570° – 620°C after Stipp et al. [2002]) and/or high strain rate deformation [Handy, 1990; Schmid and Casey, 1986].
5.2.3. Low-Temperature Mylonitic Deformation [31] Low-temperature mylonitic deformation is
charac-teristic of the southernmost margins of the Thannenkirch and Bre´zouard granites and of the whole Bilstein granite sheet. Numerous microscopic shear sense indicators (S-C fabrics, bookshelf structure, mica fish, and oblique quartz foliation) indicate sinistral sense of shear. Recrystallized quartz together with the oriented biotite form continuous fine-grained ribbons wrapping around dismembered mag-matic feldspar porphyroclasts that are microfractured and boudinaged with neck zones filled with fine-grained quartz (Figures 5d and 5e). Relics of old quartz grains show cores with deformation lamellae and intense undulatory extinction surrounded by equal subgrains. They are often converted into matrix composed of new finely recrystallized grains up to 15 mm large indicating combined subgrain rotation and bulging recrystallization [Hirth and Tullis, 1992; Stipp et al., 2002].
[32] The pole figures show asymmetrically distributed
[c]-axis maxima or type I crossed girdle pattern of Lister [1977] (Figure 6h). The maxima are distributed either close to the centre of the pole figure (Figure 6g) or in between the Z and Y axes (samples Figures 6f, 6g and 6i) or close to periphery of the diagram (Figure 6h). These c-axes patterns indicate combined activity of prismhai, rhomb ha + ci and sometimes basal hai slip systems and noncoaxial
deforma-Figure 5. (a) Schematic map depicting the microstructural zonation of the BBTC and southerly migmatites and anatectic granites from the magmatic isotropic fabrics through the zone of magmatic fabrics marked by an important preferred orientation of K-feldspar phenocrysts (up to 10 cm long), narrow zone of high-temperature (HT) solid-state microstructures and low-temperature (LT) mylonites. The photomicrographs compare different deformation microstructures within the K-feldspar and quartz for HT and LT mylonites. (b) HT microstructure characterized by the dynamic recrystallization of K-feldspar. (c) HT microstructure of quartz showing grain boundary migration recrystallization organized into polycrystalline ribbons. (d) LT mylonite characterized by fragmentation of K-feldspar. (e) LT deformation of quartz characterized by subgrain rotation and bulging recrystallization. Relics of old quartz grains disclose intense undulatory extinction and core and mantle microstructures. (f) Magmatic S2sfabric in the southerly anatectic granite. (g) LT mylonitic
tion [Passchier and Trow, 1996]. The CPO of sample Figure 6f is characterized by incomplete great circles perpendicular to the lineation and close to the YZ plane. This type of pole figure is typical of prolate strain and
activity of basalhai slip system [Passchier and Trow, 1996]. The quartz c-axes patterns and deformation microstructures indicate mixed activity of subgrain rotation recrystallization and strain-induced grain boundary migration dislocation Figure 6. EBSD data showing the preferred crystallographic orientation of quartz c axis measured
within three microstructural zones of the BBTC. See text for pole figures description and interpretation. Orientations of quartz are plotted in a pole contoured diagram of individual c-axes in a reference frame defined by the foliation and lineation. Lower hemisphere, equal area projection, contoured at multiples of uniform distribution.
creep regimes (Types 1 and 2 of Hirth and Tullis [1992]) [Gleason et al., 1993]. At natural conditions, the transition between Type 1 and 2 dislocation creep regimes occurs around a temperature of 400° [Stipp et al., 2002].
[33] The anatectic granites south of Bilstein intrusion
reveal similar microstructural evolution as the BBTC suite. This is manifested by the presence of magmatic micro-structures dominantly developed in S2sfoliation (Figure 5f).
This fabric is reactivated by the anastomose network of normal shear zones developed under greeenshict facies conditions (Figure 5g).
6. AMS
[34] The technique of anisotropy of magnetic
suscepti-bility (AMS) [Tarling and Hrouda, 1993] was used in the BBTC, as well as in the southern host rock migmatites and granites of the anatectic crust, to complement the structural data acquired in the field. Samples were drilled with a portable drilling machine and locally some large hand specimens were collected, plastered and examined in the laboratory. The low field AMS were measured with KLY 3S Kappabridge [Jelı´nek and Pokorny´, 1997] and also in part, with DIGICO instruments in Ecole et Observatoire des Sciences de la Terre, Strasbourg. The data were statistically evaluated using the ANISOFT package of programs [Hrouda et al., 1990; Jelı´nek, 1978] and MOYASM1 (M. Westphal, unpublished report, 1992). In order to char-acterize the intensity and shape of the magnetic fabric ellipsoid, two parameters were calculated [Jelı´nek, 1978]: the degree of anisotropy P = k1/k3, and the shape factor T = 2ln(k2/k3)/ln(k1/k3) 1, where 0 < T < 1 indicates oblate
and 1 < T < 0 prolate shapes of magnetic susceptibility ellipsoids. The k1 k2 k3 are the principal susceptibility axes. Orientation of AMS is characterized by the planar structures (magnetic foliations, defined as planes perpen-dicular to k3) and linear structures (magnetic lineations, defined as directions parallel to k1).
6.1. Magnetic Mineralogy
[35] We have investigated the magnetic mineralogy to
characterize magnetic carriers and interpret the overall magnetic data. The contribution of particular minerals to the bulk rock susceptibility was established by analysis of the bulk susceptibility variations with temperature. The powder specimens were measured with a temperature inter-val of 25° – 700°C, using the CS-3 Apparatus and KLY 3S Kappabridge [Hrouda, 1994; Parma and Zapletal, 1991]. The bulk susceptibility (Km) of analyzed rocks was rela-tively low, ranging from 30 106 to 350 106 [SI] (Figure 7a). The Km data reveal very similar median values for both leucogranites (Bilstein granite: 91 106, Bre´-zouard: 75 106) and slightly greater median value for the Thannenkirch granite (150 106).
[36] The overall thermomagnetic curves reveal hyperbolic
courses (Figure 7b) indicating the importance of a paramag-netic phase in all plutons. Some samples of the Thannenkirch granite have pronounced peaks followed by acute decrease in the proximity of 560° – 570°C (Curie temperature of magnetite). These samples show K values greater than 350 106 [SI] suggesting minor contribution of ferro-magnetic minerals [Rochette, 1987]. The resolution into components after Hrouda [1994] has shown that the bulk susceptibility in all three plutons is dominantly controlled Figure 7. Magnetic mineralogy diagrams. (a) Histogram of mean magnetic susceptibility, Km, for the
BBTC granites reveals bimodal distribution with slightly higher values of Km for the Thannenkirch granite compared with the Bre´zouard and Bilstein granites. (b) Representative diagram of granite total susceptibility versus temperature from a sample from the Bre´zouard shows hyperbolic heating curve documenting dominant presence of paramagnetic phases within the three studied granites.
by paramagnetic (iron-bearing) silicate component (80%) represented by biotite and a minor component carried by magnetite (20%).
6.2. Magnetic Fabrics
[37] The northern Bre´zouard and Thannenkirch granites
show more complex magnetic fabrics in comparison with the southernmost Bilstein intrusion. The central and northern parts of the Bre´zouard and Thannenkirch granites character-ized by the feeble magmatic fabrics exhibit a weakly dominant NW-SE trending magnetic foliations (pole to k1-k2 plane) with dips ranging from gently subhorizontal (15°) to moderate (50°) (Figure 8a). Locally, NE-SW trending, gently dipping magnetic foliations also occur. Most magnetic lineations (k1 directions) show subhorizontal or moderately plunging NW-SE trending directions (Figure 8b). In contrast, the southern margins of both intrusions where LT deformation was strong exhibit subvertical magnetic foliations with WNW-ESE trending subhorizontal magnetic lineations. The southernmost Bilstein granite exhibits ubiq-uitous steep E-W trending magnetic foliations and subhor-izontal magnetic lineations parallel to the macroscopic mylonitic fabrics (Figures 8a and 8b). The AMS foliation and lineation directions form an angle of 20° with respect to the WSW-ENE trending northern and southern boundaries of the granite body and its elongation direction.
[38] The southernmost margin of the Thannenkirch exhibit
relatively high degree of magnetic anisotropy expressed by values of the parameter P reaching up 1.1 (Figure 9), which has its maximum along its eastern part. It coincides with E-W-trending AMS foliations and lineations. In contrast, the regions weakly dominated by NW-SE-oriented AMS fabrics in the central domain of the Thannenkirch intrusion display relatively low degree of anisotropy represented by P values approaching 1 (Figure 9). The Bre´zouard granite shows higher values of P parameter within the narrower eastern part with a peak value along the southern margin around 1.06 (Figure 9). The broad western region is marked by the lower P values around 1.025 which correspond to the subhorizontal AMS fabrics.
[39] The Thannenkirch granite shows a prolate to plane
strain shape of T values ranging between 0.9 and 0.3 along the southern margin, whereas the centre reveals plane strain to oblate shapes with T values between 0.1 and 0.7 (Figure 9). In the Bre´zouard granite, the western zone is dominated by NW-SE trending fabrics, which yield mainly plane strain shapes (T = 0.4 to 0.5). In comparison, the southeastern margin shows the predominance of plane strain to oblate shapes (around T = 0.5) (Figure 9). The Bilstein granite exhibits overall plane strain shapes with T values between0.6 and 0.6 with slightly prolate shapes along the
southern margin and slightly oblate shapes toward the northern margin of the intrusion.
[40] The AMS fabrics within metatexites and diatexites
and granites of the southern migmatites region reveal a consistent pattern showing shallow SW dipping foliations and SSE-NNW trending subhorizontal lineations. In the northern part adjacent to the Bilstein granite, the magnetic fabric is parallel to the intrusion shape. The degree of anisotropy shows low values and the shape of AMS is generally plane strain to prolate (P = 1.004, T =0.072).
[41] The fabric pattern is summarized in Figure 10, which
shows the AMS trajectories of individual intrusions and southern migmatites. The data suggest that two distinct regions of AMS fabrics in the BBTC can be determined. In the central and northern parts of the granites, the trends of magnetic fabrics are subparallel to NW-SE trending folia-tions and lineafolia-tions in migmatites. On the contrary, along the southern margin of the BBTC intrusions, the magnetic foliations are vertical, subparallel to the pluton margins and parallel to the fabrics of adjacent migmatites.
7. Emplacement Model of the BBTC
[42] The metamorphism and deformation recorded in the
host rocks revealed that the studied magmatic bodies separate two distinct areas that preserve contrasting struc-tural patterns and metamorphic overprints. The schists and gneisses north of the BBTC suite exhibit exclusively compression-related polyphase fabrics. In contrast, the re-gion south of the BBTC reveals mainly extensional flat fabric, developed in migmatites and heterogeneous granites with local relics of early compressional structures preserved in competent orthogneisses. This rather exceptional struc-tural and metamorphic contrast between the northern and southern regions suggests that the granitic suite represents a major discontinuity, separating crustal domains of different tectonic histories.
[43] The northern intrusive margins of the studied plutons
display contrasting fabrics compared to D1n– D2nstructures
in the host rocks implying that the magma flows postdated these compressive deformations. However, granite fabrics are developed in geometrical and kinematical continuity with structures in southerly migmatites (Figure 10). In addition, the geochronological data (Table 1) support the contemporaneous crystallization of anatectic granites and migmatites in the south and crystallization of the BBTC intrusions between 330 and 325 Ma.
7.1. Crustal Extension and Emplacement Mechanisms of the BBTC
[44] The extensional deformation affecting the central
Vosges was explained using a model of low-angle normal
Figure 8. Map of AMS fabric data of the BBTC and southern migmatites. (a) Magnetic foliations. Contoured stereonets of poles to magnetic foliation (k3 direction) for northern central part and southern margin of each intrusion are discussed in the text (equal area, lower hemisphere projection, contoured at multiples of uniform distribution). (b) Magnetic lineations (k1 direction). Contoured stereonets of magnetic lineations for northern central part and southern margin of each intrusion are discussed in the text (equal area, lower hemisphere projection, contoured at multiples of uniform distribution).
shear zone responsible for exhumation of partially molten crust [Rey et al., 1992]. In this sense, the BBTC could be interpreted within the context of crustal extension zone as granitoid sheets emplaced along normal and transcurrent shear zones [Grocott et al., 1994].
[45] The structural, petrological and geochronological
data indicates that the lower crustal rocks north of the BBTC have been exhumed before the onset of extensional deformation to midcrustal levels during the early compres-sional stage [Schulmann et al., 2002]. The origin of the compressional fabrics S1n2n may be analogous to steep
metamorphic fabrics described in a similar regions of the Variscan belt by Sˇtı´pska´ et al. [2004], which are also related to exhumation of lower crustal rocks [Schulmann et al., 2005]. We suggest that the extensional tectonics in central
Vosges was superimposed on such inherited vertical oro-genic fabric at about 330 – 325 Ma.
[46] Here we propose a model where the extension has
three effects: (1) it causes heterogeneous crustal thinning associated with melting of lower crustal source zone, (2) progressive reactivation and segmentation of inherited vertical mechanical anisotropy defined by the S12 fabric
and, (3) emplacement of magma along progressively opened space between vertical metamorphic fabric, forming sheet-like intrusions. The elongate shapes of the three studied intrusions, their steep southern margins and the coupled nature of the host rock-magma deformation rela-tionships in the south are consistent with a sheet-like emplacement mechanism of successive magma batches along active fault zones [Hutton, 1988].
Figure 9. Contoured map of P parameter for the BBTC intrusions and P-T diagrams for selected areas (P, degree of AMS; T, shape of AMS ellipsoid; see text for parameter definition).
Figure 10. Interpretative maps of AMS fabric distribution within the BBTC and southern migmatites. The AMS measurements grouped into trends show the contrast between the central northern regions of the Thannenkirch and Bre´zouard granites, with dominant subhorizontal foliations bearing NW-SE lineations and the southernmost margins marked by steep foliations and margins-parallel horizontal lineations. The distinct fabric regions are explained in terms of partitioned transtensional regime with domains of wrench-dominated transtension (WD) along the southern margins and pure-shear dominated transtension (PSD) in the central and northern regions which is schematically illustrated in the sketch in the upper diagram (lower right sector). This simple sketch shows fabric orientations and their relationships to instantaneous strain axes.
[47] An important consequence of this model is the
existence of a strong anisotropic domain, which is subjected to horizontal extensional stress. Our structural data reveal remarkable coherence between the orientation of individual plutons and primary vertical E-W trending S1n fabric.
Another important observation is that the SSE trending stretching direction in the extensional anatectic domain south of the BBTC forms an angle of 60° – 70° with pluton margins (Figure 10). Consequently, the horizontal tensional stress, operating at high angle with respect to the S1
mechanical anisotropy, enhanced opening of space parallel to this metamorphic fabric and intrusion of granites [Cosgrove, 1997]. Therefore we suggest that the extension applied on inherited subvertical mechanical anisotropy S1
controlled the pluton emplacement in terms of space making process and ascent of magma.
7.2. Interpretation of Internal Fabrics in the BBTC [48] It has been demonstrated that the extensional
defor-mation is responsible for the fabric record within the migmatites [Rey et al., 2002], coherent with the estimated regional extension direction in NW-SE direction. We have also shown that the emplacement of the BBTC granites is contemporaneous with this regional extension, which is interpreted to be responsible for development of flat fabrics in the central and northern parts of granites and steep fabrics along their southern margins.
[49] We suggest that the distribution of fabrics within the
BBTC and their angles with respect to the regional stretch-ing direction are consistent with the model of partitioned sinistral transtension [Dewey, 2003]. Teyssier and Tikoff [1994] have shown that the strain partitioning is an intrinsic characteristic of oblique convergence, and Schreurs and Colletta [1998] and Krabbendam and Dewey [1998] pro-posed the same concept for obliquely divergent boundaries. In addition, granite fabrics have been used to evaluate strain partitioning in transtensional settings [Archanjo et al., 2002; Sadeghian et al., 2005].
[50] Homogeneous transtensional deformation can be
divided into two main types: pure shear-dominated (PSD) transtension and wrench-dominated transtension (WD) [Fossen and Tikoff, 1993]. The boundary between these two major modes of transtension is constrained by a divergence angle a of 20° with respect to the steep walls of the system, where the WD occurs for a lower and the PSD higher than 20°, respectively [Dewey et al., 1998]. The concept of discrete strain partitioning suggests that the oblique deformation is partitioned into discrete zones of simple shear accommodating the lateral displacement component, while the rest of system accommodates the remaining pure shear-across-strike component [Teyssier and Tikoff, 1994]. Schulmann et al. [2003] introduced models of ductile partitioning in which the oblique displacement splits the deformed domain into the PSD zone and the WD zone. In this model, it was proposed that the pure shear-across-strike component is homogeneously distributed across the whole system, while simple shear-lateral displace-ment is accommodated only in the wrench-dominated zone. We suggest that the ductile partitioning is the most suitable
model for our case study, since it can explain the studied fabric patterns. We also assume that the partitioning of the deformation was induced by thermal and rheological bound-ary conditions of the obliquely extended system.
[51] The extensional pure shear-dominated zones (flat
fabrics and SSE-NNW lineation) are restricted to the central and northern parts of the plutonic bodies, while the wrench-dominated transtension (steep fabric and E-W trending line-ation) was active along the southern margins (Figure 10). In our model, the pure shear and simple shear dominated zones are characterized by the ratio between the pure and simple shear strain rate components [see also Schulmann et al., 2003], which is high for the former and low for the latter type of deformation.
[52] There are two major factors enhancing the strain
partitioning: (1) the existence of predeformational fabrics favorably orientated for reactivation by oblique-wrench-dominated deformation which may facilitate the strain partitioning [Jezˇek et al., 2002]; (2) continuous crystalliza-tion leading to the development of the rigid crystal network, which behaves as a non-Newtonian body initiating strain partitioning [Pawley and Collins, 2002; Schulmann et al., 1997]. Therefore it is fundamental to understand mecha-nisms explaining observed deformation partitioning within the individual granites of the BBTC.
7.3. Geological and Geochronological Arguments for Asymmetrical BBTC Cooling
[53] In our tectonic model, we consider a succession of
instantaneous intrusions emplaced along a thermal gradient between the northern gneiss unit and the southern migma-tites. The gradient is roughly estimated to exhibit lower temperatures in the north and higher in the south. This temperature distribution is based on the presence of fine-grained hornfels contact zone developed in sillimanite schists along the northern margin of the Thannenkirch granite, which indicates that the schists had undergone considerable cooling prior to the magma intrusion. Also similar U-Pb zircon age of 335 ± 3.6 Ma and 39Ar-40Ar plateau age 337 ± 4 Ma of amphibole from granulites indicate fast cooling of the order of 100°/Ma (or faster) through a temperature of 500°C during the early compres-sional exhumation. The integrated39Ar-40Ar cooling ages of biotite from the granulite unit around 327 Ma (Table 1) and U-Pb zircon crystallization age of the Thannenkirch pluton (326 ± 1 Ma, this work) show that the cooling of the metamorphic complex north of the BTTC through a tem-perature of 300°C is temporally related to the intrusion and cooling of this granitoid body. The absence of contact aureole and complete coupling of deformation between migmatized cordierite-bearing schists and the Bilstein gran-ite in the southern margin of the BBTC suggest high temperatures of host rocks during emplacement of the BBTC. In addition, exceptionally high cooling rates of 100° –50°/Ma estimated from similar zircon U-Pb (328 Ma, Table 1) and muscovite and biotite cooling ages of the BBTC (327 and 328 Ma, Table 1) indicate that both granitoids and migmatites cooled fast and together. The geochronological constrains of the BBTC argue for coeval
emplacement age of the BBTC with the regional extensional deformation in the time interval of 6 Ma, i.e., between 331 and 325 Ma, errors in U-Pb dating included.
[54] The geochronological data (Table 1) and petrological
zonation shown in structural and metamorphic map (Figure 3) indicate asymmetrical thermal and cooling his-tory of the BBTC and its host rock. However, neither U-Pb crystallization ages of granitoids nor39Ar-40Ar cooling ages from the entire area allow determination of the relative sequence of granite sheets intrusions. Therefore, instead to rely on microstructural differences between the two areas in order to determine the relative sequence of BBTC intrusions, it is necessary to simulate the possible thermal environments. 7.4. Thermal Modeling of Opposite Sequences of BBTC Intrusions
[55] In this section, we describe limits, set up, and results
of numerical models used to determine the thermal evolu-tion of the system as a funcevolu-tion of the order of pluton intrusion. We assume three instantaneous intrusions emplaced successively along the thermal gradient between the cooler northern gneiss unit and the hotter southern migmatites, roughly estimated to be 100°C hotter. The model of the thermal relaxation was calculated by solving 1D conduction equation in the horizontal direction.
[56] Prior to the granite emplacement, the thermal
evolu-tion is associated with two dominant processes: (1) horizontal heat exchange between the hotter and the cooler part and (2) cooling of the whole profile due to vertical heat loss as the local geotherm disturbed by rapid exhumation is relax-ing near the surface. The second process can be approxi-mated by a constant temporal decrease of temperatures in the whole profile, about 250°C in 6 My (4.2 105°C/a). This value is estimated using similar cooling rates of the BBTC and the southern migmatitic unit. A rapid (instanta-neous) emplacement of a tabular and vertical granite sheet can then be modeled by increasing temperatures at the position of the tabular pluton emplacement (Figure 11a, emplacement at the beginning of the thermal relaxation).
[57] Figure 11b summarizes the input data for the two
models, i.e., the northward and southward intrusion sequen-ces. The results of modeling are shown in Figure 11c, where the temperatures at the margins of each granite for the
northward versus southward emplacement sequences are compared. In addition, the temperature evolution is pre-sented also for the country rocks 2 km from the contact north and south of the BBTC.
[58] In the first case (Figure 11c, top), we assume that the
BBTC complex grew from south to north; that is, the Bilstein intrusion was followed by the Bre´zouard and Thannenkirch intrusions. The cooling curves show that the northern margin of the Bilstein granite cools similarly to the southern margin, down to approximately 600°C. The successive intrusions of the Bre´zouard and Thannenkirch granites are responsible for the stepwise temperature in-crease within the Bilstein granite (Figure 11c, left). The northern and southern margins of the Bre´zouard granite exhibit similar postintrusion cooling curves followed by an important heating along the northern margin due to the Thannenkirch granite emplacement. Thus, for the case of northward growth of the BBTC, the northern side of each early intrusion is more strongly heated than their southern sides, and the country rocks show similar reheating related to the intrusions of the smaller Bilstein and Bre´zouard granites. However, the northern gneiss unit experienced important thermal effect associated with the Thannenkirch granite intrusion compared to southern migmatites.
[59] The second case is calculated for the reverse sequence
of intrusions where the Thannenkirch intrusion is followed by the successive emplacements of the Bre´zouard and Bilstein granites to the south. The Thannenkirch granite shows first rapid cooling down to solidus temperature. Intrusion of the Bre´zouard granite is responsible for insig-nificant heating of the northern Thannenkirch margin, while the southern margin of this intrusion experiences a signif-icant reheating. The Bre´zouard granite exhibits similar evolution as the Thannenkirch intrusion in terms of its postemplacement cooling and thermal effect due to the southerly Bilstein intrusion. In this sequence, the southern margins of the Thannenkirch and Bre´zouard granites un-derwent slower cooling due to the intrusion of southerly granites. Only the Bilstein granite exhibits nearly identical cooling history for both margins. In this sequence, the thermal evolution of the northern gneiss unit and southern migmatites are different with the latter subjected to the periodical and significant reheating and the
temperature-Figure 11. Numerical model of the thermal relaxation of N-S horizontal profile. (a) Thermal relaxation caused by horizontal heat exchange between the hotter (700°C) southern migmatites and colder (600°C) northern gneiss together with the cooling of the whole profile by 250°C in 6 Ma caused by vertical heat loss due to the exhumation of geotherm. The initial peak in the middle corresponds to the instantaneous emplacement of a 6-km-wide tabular intrusion of hot magma (900°C) at the beginning of the successive intrusion process (time 0 Ma). Time step between temperature curves is 0.5 Ma; the initial and final curves are thick. (b) Input data for two models (northward and southward intrusion sequence) are summarized in the tables, where widths of individual granites are taken from the geological map. (c) Thermal relaxation of N-S horizontal profiles for the BBTC intrusion sequence and also for the host rocks directed (top) from the south to the north and (bottom) from the north to the south. Diagrams show the evolution of temperature in each pluton along their southern and northern margins and also in the host rocks 2 km from the BBTC contact toward the south and north. For each intrusion, temperatures at its northern boundary are plotted as thin line and at its southern boundary as thick line. Similarly, the temperature in the gneiss to the north from the BBTC is plotted as thin lines and in the migmatites south from the BBTC are plotted as thick lines. Peaks in the diagrams correspond to the temperature increase due to emplacement of individual intrusion marked by the vertical arrows and indicated name.
time curve reflects all the thermal peaks related to the three BBTC granite intrusions. In contrary, the northern para-gneiss exhibits nearly monotonous cooling.
7.5. Direction of the BBTC Growth: Correlation of Thermal Modeling and Microstructural Zonation
[60] The calculated thermal successions are compared
with the microstructural data to establish the presumable direction of pluton growth. In terms of microstructural and rheological evolution of the whole system, the southern margins of the BBTC intrusions was capable of accommo-dating viscous deformation for prolonged period of time as evidenced by microstructural analysis. The thermal model-ing indicates that the southward successive sequence of intrusions exhibits features that could most easily explain the preserved spectrum of microstructures in the BBTC ranging from the higher solid-state temperature fabrics to the low-temperature reworking. In this scenario, where intrusions progressed to the south, the described micro-structures could be most simply explained by granite intrusion, cooling, and local reheating in the south, by the next intrusion.
[61] The thermal modeling and microstructural zoning of
such a system can be used to explain the progression of segmentation of the vertically anisotropic crustal zone and successive emplacement of magma batches. The first space was created in the north where the Thannenkirch granite exploited the S1n metamorphic fabric. The microstructures
and quartz textures in the southern Thannenkirch submag-matic zone (Figure 5) indicate that the cooling of the entire system reached 600° at that early stage (Figure 12a).
[62] Continued extension facilitates the intrusion of the
Bre´zouard granite to the south. At this stage, the southern margin of the Thannenkirch granite was already solid and represented the rigid wall during the intrusion of eastern part of the Bre´zouard granite. The intrusion of the Bre´-zouard granite induced a temperature increase along the southern margin of the Thannenkirch granite and the continuous transtensional deformation led to reactivation of steep fabrics along this suitably oriented boundary and to development of HT solid-state strike-slip fabric observed in the field. Thermal modeling shows that at this time the temperature of the whole system might have decreased down to 500°C (Figure 12b). The southern margin of the Bre´zouard granite actively deformed during progressive cooling of the whole extended crust down to 500°, leading to the development of HT solid-state microfabrics.
[63] Finally, the narrow sheet of the Bilstein granite was
emplaced in between already solidified southern margin of the Bre´zouard granite and Bt-Sil schists and migmatites in the south. Geochronology data supported by the thermal modeling and deformation microstructures show that the temperature of the whole system decreased to 350° – 400°C approximately 2 Ma after the emplacement of the Bilstein granite (Figure 12c).
[64] The successive intrusions of the BBTC are
respon-sible for the periodical reheating of the adjacent country rocks to the south. These migmatites are characterized by higher initial temperature compared to northern gneisses at the beginning of extensional process and this difference was systematically accentuated by the southward growing plu-tonic complex.
8. Strain Partitioning in the BBTC
Syntectonically Deformed During Regional
Transtension
[65] As explained in the precedent section, the southern
and northern margins of individual intrusions and their host rocks exhibit contrasting thermal and microstructural evo-lutions. The originally hotter southern migmatites were systematically reheated during pluton intrusion events dur-ing the extensional history, which also means that they were rheologically weaker in comparison to northern gneiss unit which is documented by their different kinematical history. Analogue experimental studies have shown that boundaries of competent objects like boudins represent sites of highly localized shear instabilities propagating into progressively stretched interboudin neck zones [Kidan and Cosgrove, 1996]. In agreement to this model the abrupt ending of both extensional and strike-slip structures does support the interpretation that granites intruded at the high-strain bound-ary separating a strong and unstrained zone to the north, from a weak and intensely deformed terrain to the south. This idea is compatible with the concept of Weinberg et al. [2004] establishing close relationship between plutons shapes and shear zones, particularly where shear zones intersect major lithological/rheological boundaries.
[66] We have shown that the BBTC complex emplaced
during regional extension facilitating decoupling between the northern nonextended zone and southern extensional domain. We suggest that the thermal and rheological boundary conditions induced the limited strain partitioning where the extensional pure shear-dominated zones (PSD) Figure 12. Interpretative three-dimensional block-diagrams showing the evolution of internal fabrics in the BBTC in relation to the temperature evolution of exhumed crustal zone. (a) The first stage corresponds to the time step 3 Ma after the emplacement of the Thannenkirch granite. The granite records the magmatic subhorizontal extension-related fabrics in the centre and steep margin-parallel magmatic fabrics in the southern margin. Solid curves show ambient isotherm distribution after 3 Ma of cooling. (b) The second stage represents a time step immediately after the emplacement of the Bre´zouard granite, which is responsible for the important disturbance of the isotherms. Owing to thermal effect of granite intrusion, the southern margin of the Thannenkirch granite is reactivated at higher temperature. The whole zone is cooled down to 500°C. (c) The third stage demonstrates the evolution immediately after the emplacement of the Bilstein granite, which causes the thermal effect in the southern margins of the Bre´zouard and Thannekirch granites, respectively. The whole system is exhumed and cooled down to 400°C of ambient temperature while the southern margins of all the intrusions are progressively overprinted by the low-temperature mylonites.
![Figure 13. Results of numerical simulation of AMS fabrics during progressive extensional deformation using the code after Jezˇek and Hrouda [2002]](https://thumb-eu.123doks.com/thumbv2/123doknet/14799749.605610/24.918.105.808.139.813/figure-results-numerical-simulation-progressive-extensional-deformation-jezˇek.webp)
