Plane-confined magnetic lineations in mingled mafic and felsic magmas, the Sázava pluton, Bohemian Massif

Journal of Volcanology and Geothermal Research 190 (2010) 312–324

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Journal of Volcanology and Geothermal Research j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j vo l g e o r e s

Plane-confined magnetic lineations in mingled mafic and felsic magmas, the Sázava pluton, Bohemian Massif Jiří Žák a,b,⁎, František Hrouda c,d, František V. Holub c a

Institute of Geology and Paleontology, Faculty of Science, Charles University, Albertov 6, Prague, 12843, Czech Republic Czech Geological Survey, Klárov 3, Prague, 11821, Czech Republic Institute of Petrology and Structural Geology, Faculty of Science, Charles University, Albertov 6, Prague, 12843, Czech Republic d AGICO Inc., Ječná 29, Brno, 62100, Czech Republic b c

a r t i c l e

i n f o

Article history: Received 12 June 2009 Accepted 4 December 2009 Available online 16 December 2009 Keywords: anisotropy of magnetic susceptibility (AMS) Bohemian Massif enclave fabric mingling pluton

a b s t r a c t Multiple magnetic lineations were revealed using the anisotropy of magnetic susceptibility (AMS) and anisotropy of magnetic remanence (AMR) methods in a domain of mafic–felsic magma mingling at the northwestern margin of the Sázava pluton, Central Bohemian Plutonic Complex, Bohemian Massif. Gabbrodioritic enclave swarms and sheets are steeply oriented and exhibit magnetic lineations plunging at steep to moderate angles (mostly 40–70°) whereas in their tonalitic host lineations plunge from vertical to horizontal angles, but mostly less than 40°. The magnetic lineations in both rock types spread along a ~ NNESSW steep plane that could be simplified as representing an “average” margin-parallel magmatic foliation in the pluton concordant with an “average” regional cleavage in the wall-rock. The plane-confined lineations are interpreted as having recorded the heterogeneous superposition of two processes: (1) vertical stretching during emplacement and magma mingling which left behind the steep lineations; and (2) regional tectonic stretching, which progressively rotated the mineral grains in rheologically weaker domains (chiefly in the host tonalite) to form the sub-horizontal lineation. The average foliation bearing the multiple lineations is interpreted as a composite foliation that recorded both margin-perpendicular shortening during emplacement, overprinted by coaxial regional tectonic shortening. This example reaffirms that (1) magmatic fabrics in crystallizing magmas can record accumulated strain resulting from both emplacement and regional tectonic deformation; and that (2) separating magmatic (and also magnetic) fabrics related exclusively to the internal chamber processes from fabrics caused by regional tectonic deformation is problematic or even impossible in cases where composite fabrics are recognized. © 2009 Elsevier B.V. All rights reserved.

1. Introduction This paper is concerned with the strain accumulation and fabric acquisition in contrasting magmas during mingling in mafic–silicic plutonic systems. In the simplest case, fabric orientation in mingled magmas, for example in microgranular enclaves and their felsic host, is a function of their relative viscosities at the time of fabric formation (e.g., Hrouda et al., 1999; Smith, 2000; Blake and Fink, 2000). In more complex cases, however, fabric in enclaves may form at different times and record different processes than fabric in their hosts (Paterson et al., 2003, 2004). Consequently, a variety of possible relationships between the geometry of mingled magma pulses and their internal fabric may exist, ranging from the simplest end-member where the mingled magmas share a single fabric (e.g., Hrouda et al., 1999) to multi-pulse mingling domains in which each magma type possesses a different magmatic fabric (see overview in Paterson et al., 2004). ⁎ Corresponding author. Institute of Geology and Paleontology, Faculty of Science, Charles University, Albertov 6, Prague, 12843, Czech Republic. Fax: + 420 221951452. E-mail address: [email protected] (J. Žák). 0377-0273/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jvolgeores.2009.12.002

In this paper, we examine an intriguing example of magma mingling and fabric formation near the northwestern margin of the Sázava pluton, Bohemian Massif (Fig. 1), where multiple planeconfined lineations were revealed in the mingled gabbrodioritic and tonalitic magma pulses. After briefly introducing the geological setting and structural pattern of the pluton, we describe structures resulting from magma mingling and their magnetic fabric, which has been investigated through the anisotropy of magnetic susceptibility (AMS) and anisotropy of magnetic remanence (AMR). Finally, we interpret the formation of multiple lineations in the mingled magmas and outline some broader implications for the interpretation of magmatic fabrics in plutons. 2. The Sázava pluton 2.1. Geologic setting The syntectonic ~354 Ma Sázava pluton, which is one of the earliest units of the Central Bohemian Plutonic Complex (Fig. 1; Janoušek et al., 1995; Holub et al., 1997a,b; Janoušek et al., 2000; Janoušek and Gerdes,

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Fig. 1. Simplified geologic and structural map of the Sázava pluton; index map shows its position in the central part of the Bohemian Massif. The magma mingling domain is exposed in the Teletín quarry at the northwestern margin of the pluton. Geology based on the Czech Geological Survey 1:200,000 map, sheet Tábor, U–Pb zircon age according to Janoušek and Gerdes (2003). Stereonets (lower hemisphere, equal area projection) show orientations of host rock regional cleavage and stretching lineation, and magmatic foliation and lineation in the tonalite.

2003; Janoušek et al., 2004; Žák et al., 2005a,b), intrudes into the Neoproterozoic and Lower Paleozoic metavolcanic and metasedimentary rocks along the eastern flank of the supracrustal Teplá–Barrandian Unit. The northwestern pluton wall is a Neoproterozoic volcanic arc complex consisting of lavas and volcaniclastic rocks of basalt to rhyolite composition and shallow-level plagiogranite and trondhjemite intrusions, now deformed into an antiformal ~NNE-SSW-trending, ~65 kmlong and ~3–5 km-wide belt (the “Jílové Belt” in Fig. 1). The pluton is comprised of biotite–hornblende tonalites to hornblende–biotite granodiorites of metaluminous, calc-alkaline to high-K calc-alkaline composition and irregularly-shaped, up to kmscale bodies of (±olivine, pyroxene) amphibole gabbro, quartz gabbro, gabbrodiorite, and quartz diorite. The more mafic units are scattered throughout the pluton, but become particularly abundant as sheets, enclave swarms, and domains of magma mingling near the

pluton margins. These mingling domains dip steeply in general and are parallel to the steep pluton walls. 2.2. Structural pattern A steep ~ NNE-SSW foliation is the dominant regional structure in the wall-rock along the northwestern margin of the Sázava pluton (Fig. 1). This regional foliation develops as a low-temperature spaced cleavage grading into a high-temperature foliation in the pluton aureole (Žák et al., 2005b), and it is axial-planar to the large-scale, upright antiform of the Jílové Belt. Both the spaced cleavage and the high-temperature foliation are associated with a shallowly-plunging ~NNE-SSW lineation, which is parallel to the sub-horizontal to gently plunging fold axis of the Jílové Belt antiform. The lineation is defined either by cleavage/bedding intersection and stretched clasts in the

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Neoproterozoic sedimentary rocks or by stretched mineral aggregates, pillow lavas, and volcanic fragments in the meta-volcanic rocks. The fabric intensity, finite strain, and temperature conditions of deformation in the wall-rock generally increase towards the pluton margin (Žák et al., 2005b). This deformation was interpreted as a recording of a large-scale ductile flow around the Sázava pluton during the Early Carboniferous ~WNW-ESE regional shortening (see Žák et al., 2009 for details). In the pluton, a magmatic foliation, defined by planar shapepreferred orientation of hornblende, biotite, feldspar, and quartz grains, dips steeply to the ~ESE and ~WNW and is parallel both to the regional cleavage and to the pluton/wall-rock contact (Fig. 1). The foliation intensifies towards pluton margins where it grades into narrow zones of sub-solidus foliation (see Žák et al., 2005b for details). A magmatic lineation, typically defined by euhedral hornblende crystals of up to 1 cm in length, plunges shallowly to the ~NNE or ~SSW and is also parallel to the stretching lineation outside the pluton. 3. Magma mingling along the northwestern margin of the Sázava pluton A spectacular domain of magma mingling is exposed in the Teletín quarry ~0.5 km east of the northwestern margin of the pluton (Fig. 1). Field relations, microstructures, and rheological aspects of magma mingling observed in this quarry are described below as a background for the magnetic fabric study. 3.1. Field relations Structures resulting from magma mingling in the Teletín quarry comprise scattered microgranular enclaves, enclave swarms (Fig. 2a– b), gabbrodiorite and quartz diorite sheets (Fig. 2c), and transitional structures where enclaves or enclave swarms were formed by the break-up or net-veining of larger sheets (Fig. 2d–e). Microgranular enclaves are present in several compositional and textural types, display a wide range of shapes, and have a variable spatial distribution and characteristics of enclave–host interfaces (Fig. 2b). Typically, the enclaves are ~ 0.1–1.0 m in size, fine-grained and commonly exhibit no macroscopically discernible magmatic fabrics. Two main textural varieties of enclaves can be recognized macroscopically: porphyritic with plagioclase phenocrysts (3–5 mm in size) set in dark fine-grained groundmass, and equigranular, medium-grained. The latter type is compositionally and texturally similar to quartz gabbroic rocks in larger mafic bodies and sheets. However, phenocrystic plagioclase is present in almost all enclaves, and the macroscopic differences are rather in phenocryst frequency and size relative to the groundmass. Enclave shapes vary from highly irregular blobs with lobate boundaries (Fig. 2f) through rounded ellipsoids (some enclaves in Fig. 2b) to sub-angular blocks (Fig. 2d, g). Interfaces between the enclaves and host tonalite are mostly sharp and locally gradational. Fine-grained chilled margins are typically absent or are rarely developed along only one side of enclaves. These observations suggest that a great majority of enclaves resulted from a late disintegration of originally larger bodies of mafic magma after the temperature between the host granitoid and more mafic magma was equilibrated. Enclave swarms in the Teletín quarry dip steeply and strike ~ NNESSW (Fig. 2a), parallel to both the magmatic foliation in the host

tonalite and to the pluton/wall-rock contact. The swarms commonly alternate with sub-vertical, ~ NNE-SSW sheets of fine-grained quartz diorites (Fig. 2a, c). The sheets are parallel to both the magmatic foliation in the host tonalite and to the enclave swarms. The sheet/ host contacts are sharp, only locally gradational as a result of limited hybridization. Some of the sheets clearly intrude the tonalite, but they are also locally boudinaged, irregularly broken-up, or net-veined by the tonalitic magma (Fig. 2a, d, e). As with the enclaves, the sheets are fine-grained and usually do not exhibit any mesoscopically discernible fabric. 3.2. Microstructures The host granitoids (the “Sázava type”) are composed of plagioclase, hornblende (magnesio- to ferrohornblende), biotite, quartz and commonly also K-feldspar. Plagioclase has a largely oligoclase to andesine composition (An20–40) and locally contains more calcic cores and spots of labradorite to bytownite. Accessory minerals are prismatic apatite, euhedral zircon, sphene, magnetite, pyrite, and allanite. Mineral assemblage in the microgranular enclaves and sheets is generally similar to that of the host granitoids. Modal content of biotite and hornblende is significantly higher compared to the host granitoids; typically, hornblende prevails over biotite and both minerals form subhedral to almost anhedral grains (Fig. 3a, b). Plagioclase tends to form elongate euhedral shapes and contains bytownite to labradorite cores, zones, and spots surrounded by or infilled with andesine (Fig. 3c). Quartz is either absent in the enclaves or forms interstitial to poikilitic grains. Accessory minerals are acicular apatite (Fig. 3d), scarce euhedral zircon (Fig. 3d), allanite, magnetite, pyrite, and ilmenite. The shape-preferred orientation of plagioclase laths and ferromagnesian mineral grains is generally weak in the enclaves and sheets (Fig. 3a). Mineral grains are undeformed or only weakly deformed in contrast to the elongated and flattened shapes of enclaves. This suggests that the final enclave shapes were acquired during the presence of a significant amount of melt. The micro-scale plastic deformation of mineral grains, such as bending and deformation twinning of phenocrystic plagioclase (Fig. 3b), was observed in the finer-grained, irregularly-shaped porphyritic enclaves and grades locally into microcracks filled with quartz, chlorite, and calcite. The mafic enclaves display specific textural features indicative of magma mixing, such as corroded calcic cores of plagioclase (Fig. 3c), acicular apatite (Fig. 3d), and interstitial poikilitic quartz (see Janoušek et al., 2004 for details). The complex field relations, large compositional and textural variability, and microstructures thus indicate that the mafic enclaves and sheets were derived from a number of coevally emplaced and mingled magma pulses. 3.3. Enclave shapes and orientations Enclave shapes and orientations were measured on two closely spaced sub-vertical joint surfaces in the Teletín quarry. One joint strikes ~NNE-SSW and is parallel to the magmatic foliation and lineation in the host tonalite (Fig. 4a), while the other joint strikes ~WNW-ESE and is perpendicular to the foliation and lineation in the host tonalite (Fig. 4b). The analyzed population consists of 21 enclaves

Fig. 2. Structures resulting from magma mingling in the Teletín quarry. (a) Steeply-dipping ~ NNE-SSW quartz diorite sheets alternating with enclave swarms; view on a steep quarry wall. (b) Close-up to show variably-shaped microgranular enclaves aligned parallel to magmatic foliation in the host tonalite. (c) Steeply-dipping ~ NNE-SSW diorite sheets alternating with an enclave swarm and thin screens of tonalite; hammer for scale. (d) Disrupted (boudinaged) diorite sheets form rectangular enclaves in the host tonalite; hammer for scale. (e) Fractures both inside the dioritic sheets and at the sheet margins indicate cracking (net-veining) of high-viscosity dioritic magma during intrusion of the tonalite; penknife (9 cm long) for scale. (f) Irregularly-shaped microgranular enclave with lobate contacts against the host tonalite. (g) Angular enclaves with sharp, fracture-like contacts against the host tonalite; coin for scale.

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Fig. 3. Microtextural features of microgranular enclaves of quartz-gabbrodiorite to gabbrodiorite composition. (a) Typical magmatic texture with shape-preferred orientation of plagioclase crystals; crossed polars. (b) Magmatic texture modified by a rare weak deformation of plagioclase (bent twin lamellae in lower left center); crossed polars. (c) Irregularly corroded highly calcic core (bottom left) and calcic spike zone within intermediate plagioclase (center); crossed polars. (d) Abundant acicular apatite (Ap) and rare zircon (Zr) enclosed in quartz; plane-polarized light.

in the foliation-parallel section and 25 enclaves in the lineationperpendicular section. Two parameters of each enclave were measured: aspect ratio and the angle between long axis of the enclave and a horizontal reference line. The angles range from 0° to 90° and have a negative or positive value depending on whether the long axis of the enclave was measured clockwise or anticlockwise with respect to the reference line (Fig. 4a). The results are summarized in Fig. 4 as plots of aspect ratio vs. angle and as frequency histograms of angles for each section. In the foliation-parallel section, most of the enclaves have low aspect ratios ranging from 1 to 2.5 and their long axes have variable plunges (Fig. 4a). In the lineation-perpendicular section, enclaves have generally higher aspect ratios ranging from 2 to 6, and their long axes plunge steeply (60°–80°). These data indicate that the enclave shapes are oblate, the shortest axes of the enclaves are perpendicular to the magmatic foliation in the host tonalite, and their long axes plunge moderately to steeply in both measured sections. 3.4. Rheological aspects of mingling structures During and after magma mingling, viscosities of coeval magmas change up to ten orders of magnitude as a result of thermal exchange and progressive crystallization. The change in relative viscosities of mingled magmas represents an important control on their geometry, behavior during deformation, and fabric acquisition (e.g., Sparks and Marshall, 1986; Frost and Mahood, 1987; Williams and Tobisch, 1994; Fernandez and Gasquet, 1994; Paterson and Vernon, 1995; Fernandez

et al., 1997; Hrouda et al., 1999; Blake and Fink, 2000; Smith, 2000; Paterson et al., 2004). In this study, we avoid quantifying the relative viscosities of mingled magmas, because the specific magma viscosities in such a complex mingling domain are difficult to calculate due to their dependence on multiple poorly constrained parameters (composition, content of volatiles, temperature, crystallinity, grain size, geometrical parameters of suspension, proportion of mafic and felsic magmas, and strain rate; e.g., Scaillet et al., 1997, 1998, 2000; Petford, 2003). However, the range of enclave shapes and the presence of viscosity-dependent structures provide at least some qualitative clues regarding the relative viscosities of mingled magma pulses with respect to the formation of magmatic fabric. The ellipsoidal shapes of enclaves and their smooth outer contacts (Fig. 2b) suggest that the enclaves behaved as weak pockets in stronger tonalite when the viscosity of the mafic magma was lower than that of the tonalitic host (Hrouda et al., 1999). In contrast, subangular to angular enclaves, bounded by fracture-like contacts (Fig. 2d, g), indicate that the enclaves behaved as rigid blocks in a less viscous tonalitic matrix (Hrouda et al., 1999). Several types of the enclave/host interfaces were observed: gradational and irregular, fold-like (cuspate-lobate), and sharp planar. Gradational and irregular boundaries point to low viscosity contrasts of basic and felsic magmas, both being largely molten to allow their hybridization and interfingering. Many enclaves show evidence of interface folding, developed as cuspate-lobate structures at the enclave–host contacts (Fig. 5a, b). The cuspate-lobate boundaries are perpendicular to the magmatic foliation in the host tonalite, with

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Fig. 4. Diagrams of enclave aspect ratios and orientations of their long axes with respect to a horizontal reference line. In the foliation-parallel section (a) the enclaves have lower aspect ratios and are weakly aligned. In the lineation-perpendicular section (b) the enclaves have high aspect ratios, show strong alignment, and have steeply plunging axes. In general, the enclaves tend to have oblate shapes.

the tonalite prongs pointing into the enclaves. This structure indicates a higher viscosity of the dioritic than the tonalitic magma during cuspperpendicular shortening (Smith, 2000). Other features also point to generally higher viscosities of the basic magma during mingling: (1) net-veining, taking place along margins of dioritic sheets, where basic magma rapidly cooled near contacts and was fractured and intruded by the less viscous tonalitic magma (Fig. 2e); and (2) pinch-and-swell structures and boudinage of basic sheets within tonalitic magma (Fig. 2d).

4. Anisotropy of magnetic susceptibility (AMS) The main method of fabric analysis used in this study is the anisotropy of magnetic susceptibility (AMS). This method is extremely rapid; measurement of a specimen takes two-three minutes, allowing large collections of specimens to be easily investigated. In addition, modern instruments for measuring the AMS are sensitive enough to measure almost all rock types with sufficient accuracy.

Fig. 5. (a, b) Cuspate-lobate structures at the enclave/host interfaces indicate higher viscosities of dioritic magma with respect to the host tonalite during interface folding; penknife (9 cm long) for scale.

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The AMS characterizes preferred orientation of magnetic mineral grains in rocks called the magnetic fabric. In rocks where AMS is effectively controlled by one magnetic mineral, a close relationship exists between the AMS and the preferred orientation of the mineral (e.g., Uyeda et al., 1963; Owens and Rutter, 1978; Hrouda et al., 1985; Hrouda and Schulmann, 1990; Lüneburg et al., 1999), and the AMS can even be converted into the orientation tensor of the respective mineral (Hrouda and Schulmann, 1990; Ježek and Hrouda, 2000). Difficulties may arise if the AMS is effectively carried by two or more minerals. Fortunately, there are techniques for detecting whether the rock under consideration is magnetically mono-mineralic or multi-mineralic (e.g., Owens and Bamford, 1976; Hrouda and Jelínek, 1990; Friedrich et al., 1995; Martín-Hernández and Hirt, 2001, 2004; Ferré et al., 2004). In granitic rocks, the effective carrier of the AMS can be magnetite, paramagnetic mafic silicates such as hornblende and biotite, or both magnetite and paramagnetic minerals. The interpretation of the AMS is straightforward in the first two cases, complexities arise in the third case. Fortunately, the magnetic fabrics of magnetite and of paramagnetic fractions are frequently coaxial, allowing the interpretation of symmetry of the AMS ellipsoid and orientation of magnetic foliation and magnetic lineation. 4.1. Methodology Oriented samples for investigating the AMS were collected using a portable drill at 19 localities of the Sázava pluton (102 specimens) and in the Teletín quarry where enclaves (44 specimens) and tonalite (77 specimens) were sampled at 10 sites scattered all over the quarry. The AMS was measured with the KLY-3S Kappabridge (Jelínek and Pokorný, 1997) in the laboratories of AGICO Inc. (Brno, Czech Republic), and statistical analysis of the AMS data was carried out using the ANISOFT package of programs (written by M. Chadima and V. Jelínek; www.agico.com). The AMS data are presented as km, P, and T parameters and orientations of the magnetic foliation and magnetic lineation. The km, P, and T parameters are defined as: km = (k1 + k2 + k3)/3; P = k1/k3; T = 2 ln(k2/k3)/ln(k1/k3) − 1, where k1 ≥ k2 ≥ k3 are the principal susceptibilities. The km parameter represents the mean bulk susceptibility reflecting the qualitative and quantitative content of magnetic minerals in the rock. The P parameter (Nagata, 1961), called the degree of AMS, reflects the eccentricity of the AMS ellipsoid and thus indicates the intensity of the preferred orientation of the magnetic minerals. If the AMS is carried by one mineral, the higher is the P parameter the stronger is the preferred orientation. The T parameter (Jelínek, 1981) indicates the symmetry of the AMS ellipsoid. It varies from − 1 (perfectly linear fabric) to 0 (transition from linear to planar fabric) and to + 1 (perfectly planar fabric). The orientations of magnetic foliation poles and magnetic lineations are presented either as stereographic projections in the geographic coordinates or as locality means on a map. In order to determine the contribution of particular minerals to the rock susceptibility, the susceptibility variation with temperature was investigated on coarsely powdered specimens in the temperature interval of 25–700 °C, using the CS-3 Apparatus and KLY-3 S Kappabridge (Parma and Zapletal, 1991; Hrouda, 1994; Jelínek and Pokorný, 1997). The approach developed by Hrouda (1994), based on mathematical resolution of the initial part of the heating curve (usually in the temperature interval between 25 °C and 200 °C) into a paramagnetic hyperbola and a ferromagnetic straight line parallel to the abscissa, enables the contributions of paramagnetic and ferromagnetic fractions to the rock bulk susceptibility to be estimated. 4.2. Magnetic mineralogy The mean bulk susceptibility ranges from 10− 4 to 10− 3 [SI] both in the microgranular enclaves and in the host tonalite (Fig. 6a, b). The susceptibility values on the order of 10− 4 correspond to those

characteristic of paramagnetic granites (Bouchez, 2000), while the susceptibilities at the low end of the order of 10− 3 may indicate a transition from the paramagnetic to ferromagnetic granites. Consequently, the above bulk susceptibility values may indicate a combined effect of both paramagnetic and ferromagnetic minerals. The susceptibility variation with temperature is presented in Fig. 6c, d. Heating susceptibility vs. temperature curves for both enclaves and host tonalite show hyperbolic courses in their initial parts (up to the temperature of 450 °C) and pronounced peaks in the vicinity of 560–580 °C, followed by rapid susceptibility decreases. The hyperbolic course indicates the presence of paramagnetic minerals (biotite and hornblende), and the rapid decrease of susceptibility above 570 °C indicates the Curie temperature of magnetite. Resolution of the initial part of the heating curve into a paramagnetic hyperbola and a ferromagnetic straight line parallel to the abscissa (Hrouda, 1994) reveals that, in general, the paramagnetic silicates (hornblende and biotite) and magnetite contribute to the bulk susceptibility more or less equally. The curve for quartz diorite sheet (Fig. 6d) shows a hyperbolic course in the entire temperature interval (20–700 °C), indicating the dominant effect of paramagnetic silicates. Provided that the grain AMS of paramagnetic minerals and magnetite are not an order of magnitude different, and there is no reasonable argument for expecting such a difference, the results of the above resolution represent not only bulk susceptibility but also AMS, as revealed by mathematical modeling of the problem (Borradaile, 1987; extended by Hrouda, in press). Hence, we conclude that the AMS in both enclaves and host tonalite indicates the magnetic fabric carried by both mafic silicates and magnetite. In both enclaves and host tonalite, the degree of AMS varies according to the mean susceptibility (Fig. 7a, b). This variation can be easily explained by assuming variable contributions of the paramagnetic minerals and magnetite to the rock susceptibility, each of them showing different grain AMS or intensity of preferred orientation (Rochette, 1987; Hrouda, 1987). As the degree of AMS of the specimens with the highest mean susceptibility is relatively high, with the P parameter reaching almost 1.3 and 1.6 in tonalite, it can hardly be attributed to paramagnetic silicates only (Zapletal, 1990; Borradaile and Werner, 1994; Martín-Hernández and Hirt, 2003). It is thus likely that the AMS of the specimens with low susceptibility is controlled predominantly by paramagnetic minerals than by magnetite, while the AMS of the specimens with higher susceptibility is more affected by magnetite. On the other hand, the shape of the AMS ellipsoid and the orientations of magnetic foliation and magnetic lineation show virtually no variation with the mean susceptibility (Fig. 7c–h). This indicates that the paramagnetic mineral fabric and the magnetite fabric are coaxial. We conclude that the magnetic fabric in the enclaves and host tonalite is carried by both mafic silicates and magnetite and that the paramagnetic mineral fabric and the magnetite fabric are coaxial and can thus be treated as a single fabric element. This is a feasible assumption because our interpretation is based on the orientations of the principal susceptibilities and on the shape of the AMS ellipsoid, rather than on the degree of AMS. 4.3. Overall magnetic fabric of the Sázava pluton The Sázava pluton has a different magnetic fabric in the central part and near its margins. In the pluton center, magnetic lineations are generally steeper, plunging moderately to steeply to the ~SE (Fig. 8). Magnetic foliations dip steeply, as indicated by the k3 axes near the periphery of the stereonet defining a broad girdle around the lineations (Fig. 8). The central part of the pluton yields a low degree of anisotropy (P = 1.023–1.158) and both prolate and oblate magnetic ellipsoid shapes (Fig. 8). Along the pluton margins, magnetic foliations dip steeply to moderately to the ~ESE or ~WNW, and magnetic lineations plunge shallowly to moderately to the ~NNE-SSW to ~NE-SW (Fig. 8).

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Fig. 6. Bulk susceptibility distribution and temperature variation of susceptibility. (a) Histogram of susceptibility distribution in microgranular enclaves; (b) histogram of susceptibility distribution in host tonalite; (c) examples of heating susceptibility vs. temperature curves in enclaves; (d) examples of heating susceptibility vs. temperature curves in tonalite and quartz diorite sheet.

The degree of anisotropy and shape parameter are increased at the margins compared to the pluton center. High P values (up to P = 1.725) correspond to the sub-solidus deformed tonalite at the northwestern margin, and slightly lower values (P = 1.044–1.270) were obtained near the eastern margin. Susceptibility ellipsoids exhibit oblate shapes (Fig. 8). 4.4. Magnetic fabric of the enclaves, sheets, and host tonalite in the Teletín quarry

Magnetic lineations, however, exhibit a different orientation distribution compared to those in the tonalite beyond the mingling domain (Fig. 9c; compare with Fig. 8). The lineations define a broad ~N–S girdle in the stereonet, and their plunges range from horizontal to vertical, with most of the lineations plunging at either <40° or 50–70° (Fig. 9c). The degree of magnetic anisotropy of the host tonalite varies from low to moderate (P = 1.013–1.534), and the magnetic susceptibility ellipsoid is predominantly oblate (T = −0.606–0.993). 5. Anisotropy of magnetic remanence (AMR)

In the microgranular enclaves, magnetic foliations dip steeply to the ENE to E and WSW to WNW (Fig. 9a). Magnetic lineations define a broad ~ NNE-SSW girdle in the stereonet (Fig. 9a). A histogram of lineation plunges (Fig. 9a) shows two subordinate maxima (20–30° and 80–90°) and most of the lineations plunging at 40–70°. Only two specimens have shallowly-plunging lineations (<20°). The degree of anisotropy in enclaves varies from low to moderate (P = 1.010–1.221) and magnetic ellipsoids are mostly oblate (T = −0.038–0.938). In the quartz diorite sheets, magnetic foliations and lineations exhibit a scattered pattern in stereonet (Fig. 9b). Some of the foliations dip steeply to the ~ NW, ~ WSW, or ~ ENE and some magnetic lineations plunge moderately to steeply to the ~ NW, ~ N, or ~SW. However, other foliations are oriented ~ E–W and lineations plunge shallowly to the ~ E. Magnetic fabric in the sheets is characterized by a low degree of anisotropy (P = 1.022–1.056) and an oblate magnetic ellipsoid (T = 0.213–0.909). In the host tonalite (Fig. 9c), magnetic foliations roughly correspond to those of measured near the margins of the pluton.

While the AMS is controlled by all minerals in a rock, the anisotropy of magnetic remanence (AMR) is controlled solely by ferromagnetic minerals (e.g., Jackson and Tauxe, 1991; Hrouda, 2002). Therefore, a combination of both AMS and AMR methods is particularly suitable for comparison of magnetic fabrics in compositionally diverse rocks, such as microgranular enclaves and their host granitoids. In this study, we investigated the AMR of seven specimens of the microgranular enclaves and four specimens of the host tonalite using the AMU-1 Anhysteretic Magnetizer, LDA-3 AF Demagnetizer and JR-5A Spinner Magnetometer (all produced by Agico Inc., Brno) in the bias direct field of 50 μT and alternating field of 100 mT. The measurements were processed using the method of Jelínek (1993). The AMR results are presented in Fig. 10a, showing the mean orientations of magnetic foliation poles and magnetic lineations. Magnetic foliations in the enclaves dip steeply and strike ~ NNE-SSW to ~NE-SW, and magnetic lineations plunge steeply to moderately to the E. Magnetic lineations in the tonalite have orientations

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Fig. 7. (a–h) Variations of the AMS parameters (degree of anisotropy, shape parameter, trend and plunge of magnetic lineations and of poles to magnetic foliation) with the mean bulk susceptibility in the enclaves and host tonalite.

approximately similar to those in the enclaves. The same samples were measured also for the AMS (Fig. 10b); the AMR and AMS fabrics are roughly coaxial. The shape of the AMR ellipsoid in the enclaves is also comparable to that of the tonalite (Fig. 10c), while the degree of the AMR is clearly higher compared to the AMS (e.g., Stephenson et al., 1986; Hrouda et al., 1999).

The investigations of the AMR thus corroborate the conclusions drawn in Section 4.2. Magnetic mineralogy, as the AMR exhibits similar shape of the anisotropy ellipsoid to that of the AMS and the magnetic (AMR) lineations and foliations in both enclaves and host tonalite have approximately similar orientations as those of the AMS (Fig. 10b).

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Fig. 8. Map of magnetic foliations and magnetic lineations in the Sázava pluton with inset of magnetic anisotropy P–T plots and stereonets (lower hemisphere, equal area projection) showing orientations of maximum and minimum susceptibilities. Magnetic foliations and lineations in the tonalite along the pluton margins are generally concordant with regional cleavage and stretching lineation in the host rock. In contrast, the central part of the pluton exhibits steeper magnetic lineations associated with a prolate shape of the AMS ellipsoids.

6. Discussion The following points are particularly important for interpreting the formation of the multiple magnetic lineations in the mingled magmas of the Sázava pluton. (1) The magnetic lineations define a broad ~N–S to ~NNE-SSW girdle on the stereonets (Fig. 9a, c), that could be simplified to represent a single foliation parallel to the “average” magmatic foliation as measured in the pluton. (2) Microstructural indicators (Paterson et al., 1989; Vernon, 2000) for solid-state flow in the studied rocks are almost absent, implying that the fabrics are magmatic to submagmatic and formed during the presence of a melt (Fig. 3). (3) The steeply-plunging magnetic lineations are not seen anywhere in the Teplá–Barrandian host rock and are clearly decoupled from the regional tectonic fabric (Žák et al., 2005a,b, 2009). The sub-horizontal ~NNESSW stretching lineation in the Teplá–Barrandian wall-rock maintains its orientation across the regional strain gradient; that is, it has no relationship to the strain intensity and distance from the pluton margin (Fig. 1; see also Žák et al., 2005b for details). These observations thus preclude the possibility that the lineations record a switch from horizontal to vertical as a result of increasing strain and/or the angle of convergence during regional transpression (e.g., Robin and Cruden, 1994; Tikoff and Greene, 1997). Therefore, we interpret the steep orientation of the enclaves and sheets and the steep magnetic lineations as reflecting a bulk sub-vertical stretching during intrusion (emplacement) of the gabbrodioritic and tonalitic magmas (e.g., Naba et al., 2004; Vegas et al., 2008). Further, we

assume that the intrusive magnetic lineations were not initially vertical (90° plunge), but their plunges likely varied from sub-vertical to moderate as a result of non-uniform magma flow and mingling along the pluton margin. We also assume that, at this stage, the magma flow and the associated steep lineations were already confined to a sub-vertical ~NNE-SSW plane parallel to the pluton wall acting as a rigid backstop. The further evolution of the lineations was presumably controlled by cooling of the already emplaced and mingled magmas and by the overprinting tectonic deformation. Fast cooling of the gabbrodioritic enclaves and sheets (note that the quarry is ~500 m from the pluton margin) may have caused an increase in their viscosity by a few orders of magnitude, so they increasingly behaved as rigid bodies within the weaker tonalite (e.g., Hrouda et al., 1999). This interpretation is consistent with the viscosity-dependent structures, generally indicating higher viscosities of the gabbrodioritic magma relative to the tonalitic host (Fig. 2d, e, g). During cooling and progressive crystallization, the enclaves and sheets became trapped in the crystallizing and increasingly more viscous tonalitic crystal-rich framework maintaining their steep orientation and preserving the steeply to moderately plunging lineations (e.g., Paterson et al., 2003). The host tonalite also preserves steeply to moderately plunging lineations, indicating that some domains within the mush crystallized before others or were low-strain sites not overprinted by later deformation. Subsequently, the melt-lubricated grains in some enclaves and preferentially in the weaker tonalite began to rotate towards the principal stretching axis of the regional deformation to form

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Fig. 9. Stereographic projections (equal area, lower hemisphere) of magnetic foliations and lineations and magnetic anisotropy P–T plots of the microgranular enclaves (a), quartz diorite sheets (b), and tonalite (c) in the Teletín quarry. Histograms show the distribution of plunges of magnetic lineations in the enclaves and tonalite.

sub-horizontal ~NNE-SSW magmatic lineations (Žák et al., 2005b). This process is documented by the broad girdle-like pattern of magnetic lineations in the stereonet (Fig. 9a, c), what we interpret to

represent various stages of reorientation of lineations from initially sub-vertical (i.e., formed during construction of the mingling domain) to sub-horizontal (due to the regional stretching) orientations. This

Fig. 10. (a) Magnetic foliation poles and magnetic lineations of the AMR indicating the magnetite fabric in the microgranular enclaves (closed symbols) and the tonalite (open symbols). Orientations of magnetic foliations and lineations in the enclaves and tonalite are similar. (b) Magnetic foliation poles and magnetic lineations of the AMS in the same samples as measured for the AMR. Orientations of magnetic foliations and lineations are also similar in the enclaves and host tonalite. Note the steep to moderate plunges of magnetic lineations in both enclaves and host tonalite. Equal area projection on lower hemisphere. (c) P–T plot with the AMR data from the enclaves and the tonalite.

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reorientation was also confined to the ~ NNE-SSW-oriented steep plane parallel to the pluton margin. In some cases, the enclaves were further strained due to the regional ~ WNW-ESE shortening and ~ NNE-SSW stretching, as documented by the folding of their margins (Fig. 5) with axial planes of the folds parallel to the host magmatic foliation, and by the boudinage of some enclaves with boudin necks perpendicular to the sub-horizontal lineation in the host tonalite (Fig. 2d). Regardless of the orientation (vertical to horizontal), magnetic lineations are associated with the steep ~ NNE-SSW magmatic (and magnetic) foliation which is parallel both to the northwestern margin of the pluton and to the regional cleavage in the Teplá–Barrandian wall-rock. This implies that the ~ NNE-SSW foliation may also have a complex origin, involving earlier margin-perpendicular shortening during emplacement (associated with the sub-vertical stretching) and subsequent coaxial regional tectonic shortening (associated with the sub-horizontal stretching). We thus conclude that the ~ NNE-SSW magmatic foliation recorded accumulated strain and that this foliation is a composite structure making the separation of these two coaxial contributions impossible. 7. Conclusions and broader implications In the Teletín quarry, the steep orientation of the flattened enclaves and the moderately to steeply plunging magnetic lineations in the enclaves and in the host tonalite may reflect heterogeneous intrusive strain (sub-vertical stretching) during construction of the vertically oriented mingling domain. Subsequent cooling and solidification of the gabbrodioritic magma caused an increase in its viscosity by a few orders of magnitude relative to the tonalitic host, so that the enclaves became more rigid bodies trapped in the tonalite mush and preserved the steeply to moderately plunging lineations. Some domains in the host tonalite also preserved the steeply plunging lineations, which may indicate that these domains were crystallizing before others or represent low-strain sites not overprinted by the regional deformation. After the steeply to moderately plunging lineations were “locked in”, the mineral grains in the weaker tonalite could further reorient parallel to the regional stretching direction to form a sub-horizontal ~NNE-SSW lineation. The formation of the ~NNE-SSW magmatic and magnetic foliation may also be complex, involving both margin-parallel shortening during emplacement and construction of the mingling domain superposed by coaxial ~WNW-ESE regional tectonic shortening. As opposed to viewing magmatic fabrics (and magnetic fabric inferred from the AMS) as simple structures formed by a single process, a more general implication of the above results is that magmatic foliations and lineations in plutons may form as complex, composite structures recording prolonged magmatic history (e.g., Launeau and Cruden, 1998); that is, heterogeneous accumulated strains caused by a combination of different processes. The concept of composite fabrics has been elaborated, for example, by Housen et al. (1993), Aranguren et al. (1996), Lüneburg and Lebit (1998), and Miller et al. (2005) in sedimentary and metamorphic rocks and sheared granitoids. Based on the Teletín quarry example and our previous studies, we argue that it may also apply to magmatic fabrics in plutonic rocks (Paterson et al., 2003; Žák et al., 2007, 2008). As a consequence, in cases where composite magmatic fabrics are recognized, the distinction between fabrics as those solely related to internal magma chamber processes or emplacement from others strictly related to regional tectonic deformation is blurred, making the interpretation of magma flow or even fabricforming processes from the preserved rock record and from the AMS much more difficult or even impossible. Acknowledgements We thank Jean-Luc Bouchez and an anonymous reviewer for their constructive and detailed reviews, Karel Schulmann for collaboration

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during the initial stages of the research, and Scott Paterson for discussions on enclaves and fabrics in plutons. The research was supported by the Grant Agency of the Czech Republic through postdoctoral grant No. 205/07/P226 (to Jiří Žák) and grant No. 205/09/ 0630 (to František Holub), and by the Ministry of Education, Youth and Sports of the Czech Republic Research Plan No. MSM0021620855.

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