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A preliminary AMS study in some tertiary granitoids from Northern Greece: integration of tectonic and paleomagnetic data
Physics and Chemistry of the Earth 27 (2002) 1289–1297 www.elsevier.com/locate/pce
A preliminary AMS study in some tertiary granitoids from Northern Greece: integration of tectonic and paleomagnetic data I. Zananiri a
a,*
, S. Dimitriadis b, D. Kondopoulou a, A. Atzemoglou
c
Department of Geophysics, Aristotle University of Thessaloniki, P.O. Box 352-1, Thessaloniki 54124, Greece Department of Mineralogy and Petrology, Aristotle University of Thessaloniki, Thessaloniki 54124, Greece c IGME, Frangon 1, Thessaloniki 54626, Greece
b
Abstract The present study focuses on some of the Tertiary granitoids of the Rhodope Massif, Northern Greece, namely the Early Miocene plutons of Symvolon, W. Vrondou and the Oligocene Xanthi pluton. Their low-field anisotropy of magnetic susceptibility (AMS) was studied in a number of stations and was combined with existing paleomagnetic and tectonic data in order to check their mutual consistency and to assess to what extent they can help clarifying the tectonic regime prevailing during emplacement of these granitoids. The bulk susceptibility magnitude is generally high, as well as the mean anisotropy degree which reaches 1.23, pointing to a dominant ferromagnetic control of the magnetic properties. Microscopic observation, isothermal remanent magnetization (IRM), and thermomagnetic analysis reveal that the magnetic mineralogy is controlled mainly by magnetite. The magnetic fabrics are welldefined in the plutons, with the Kmax axes (magnetic lineations) varying from gently to moderate plunges. Paleomagnetic results obtained from the same bodies display clear clockwise rotations for all the plutons, with minor or no tilt during their emplacement. Despite of the age differences, the tectonic and magnetic fabrics have mostly similar directions. The prevailing NE–SW-trending linear fabrics, visible mainly in the Early Miocene plutons are also clearly imprinted magnetically in the Early Oligocene pluton, in which no macroscopic fabric is visible. Finally, a preliminary attempt to correct the paleomagnetic results from the magnetic anisotropy effect showed that only qualitative conclusions can be reached from the AMS data, and that complementary AIRM measurements are required. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Paleomagnetism; AMS; Deformation; Tectonics; Tertiary granitoids; N. Greece
1. Introduction The study of the anisotropy of magnetic susceptibility (AMS) with its scalar and directional parameters is starting to become in recent years a standard and valuable technique, which, in addition to macro and microtectonic fabrics and microscopic textural data, can contribute to the unraveling of the emplacement, cooling and unroofing history of a pluton (e.g. Bouchez et al., 1990; Archanjo et al., 1994; Bouchez and Gleizes, 1995; Trindade et al., 1999). AMS has the merit of revealing very faint anisotropies in rock masses subjected to imperceptible tectonic imprints. Plutonic bodies emplaced in the Mesozoic and Tertiary are abundant in N. Greece. For some of them reliable radiometric ages, tectonic and petrographic data, *
Corresponding author. Fax: +30-31-998528. E-mail address: [email protected] (I. Zananiri).
and paleomagnetic and rock magnetic studies are available (Atzemoglou et al., 1994; Dimitriadis et al., 1998, and references therein). A first set of AMS measurements in three of these plutons, namely the Early Miocene Symvolon and West Vrondou, and the Early Oligocene Xanthi pluton (Fig. 1), are reported here. The present work was undertaken in order to see whether, using the available data, AMS, tectonic and microtextural features are mutually consistent in these granitoids, and if they can contribute to clarify the tectonic regime during their emplacement. It is well-known that AMS may deflect the vectors of remanence magnetization (Hrouda, 1982), but even if the mechanism and magnitude of this deflection is not yet fully clear, it certainly has to be taken into account in paleomagnetic studies. We make an attempt here to correct the existing paleomagnetic data for the three studied granitoids, taking into account a new set of AMS measurements.
1474-7065/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII: S 1 4 7 4 - 7 0 6 5 ( 0 2 ) 0 0 1 1 5 - 8
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Fig. 1. Southern Rhodope and adjacent areas. The main tectonic units are shown (PZ: Pelagonian zone; VZ: Vardar zone; PRZ: peri-Rhodopean zone; RCC.UPL: upper plate of the Rhodope core complex (Serbomacedonian element); RCC.LPL: lower plate of the Rhodope core complex; RBD: Rhodope beyond the Strymon valley detachment; TSVD: trace of the Strymon valley detachment fault; BSDF: breakaway of the Strymon detachment fault. The arrows indicate the paleomagnetically derived rotation values of igneous formations. X: Xanthi, SV: Symvolon, EV: E. Vrondou, WV: W. Vrondou (modified map from Dimitriadis et al., 1998).
2. Geological setting The Early Oligocene Xanthi and the Early Miocene Symvolon and W. Vrondou plutons, each exposed for several hundreds of km2 , crop out among the metamorphics of the Rhodope massif in eastern Macedonia and Thrace (Fig. 1). Radiochronological data of these granitoids, mostly cooling ages, are summarized in Dimitriadis et al. (1998) and are also given in Table 1. The large pluton of Vrondou is composed of two different intrusions: the locally strongly mylonitized Early Miocene West Vrondou body, and the isotropiclooking Early Oligocene East Vrondou body (Dinter et al., 1995). According to Kolokotroni and Dixon (1991) and Kolokotroni (1992), the Vrondou pluton was emplaced syntectonically within an extensional ramp space. Their results were mostly obtained from the deformed western part of the pluton, here referred to as the Early Miocene intrusion. Since no data are available from the eastern part of the Vrondou pluton, the latter is not considered in the present study. A similar extensional regime has also been suggested by Koukouvelas and Pe-Piper (1991) for the emplacement of the Early Oligocene Xanthi pluton, which shows no macroscopic deformation features except a spaced set of parallel joints which acted as conduits for the intrusion of aplitic dykes. Dinter (1994) proposed that in the Rhodope area
Table 1 Available radiometric data for the studied plutons Granitoid
Dating method/ reference
Age (Ma)
Xanthi
K–Ar biot. (Meyer, 1968) K–Ar hornbl. (Liati, 1986) Rb–Sr whole rock (Kyriakopoulos, 1987) Rb–Sr whole rock (Kyriakopoulos, 1987)
27:1 0:4
Symvolon
K–Ar biot. (Harre et al., 1968) U–Pb, 40 Ar–39 Ar hornbl. (Dinter et al., 1995) Rb–Sr whole rock, mica (Kyriakopoulos, 1987)
30:4 0:6 28:8 0:7 26:3 0:1 13:8 0:2 19.1–20.1
15–26
W. Vrondou
U–Pb, Ar–Ar (Kaufman, 1995)
23:7 0:1
E. Vrondou
K–Ar hornbl. (Papadakis, 1965) K–Ar hornbl. (Marakis, 1969) K–Ar biot. (Durr et al., 1978)
53:5 4:2 29 1 to 33 2 30 3
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the Oligocene was a tectonically quiet period, post-dating an Early Tertiary compressional phase and predating a strong syn-extensional stretching phase during which the Early Miocene plutons (Symvolon and W. Vrondou) were emplaced. This agrees with the lack of perceptible tectonic fabrics in the Oligocene Xanthi pluton, in contrast to the strong tectonic fabric observed in the two Miocene granitoids. AMS measurements are expected to complement the above given geological data and to provide more structural information about the apparently isotropic Xanthi pluton.
3. Sampling and measurements Drilled cores and oriented hand samples were collected at several sites in W. Vrondou, Symvolon and Xanthi plutons. At each site, three to seven hand samples, or eight to ten cores were collected. Standard 25 22 mm cylinders were collected from the samples. AMS measurements on these specimens were carried out with the Kappabridge KLY-2 apparatus at the University Paul-Sabatier in Toulouse. Microscopic examination of thin sections, thermomagnetic analysis, AF-demagnetizations, as well as isothermal remanent magnetization (IRM) measurements were performed at the Paleomagnetic Laboratory of the University of Thessaloniki.
4. Magnetic mineralogy Thermomagnetic analyses were carried out in order to determine Curie temperatures. Representative curves for samples of the studied plutons (Fig. 2) show that the magnetic susceptibility (K) decreases near zero at 580 °C, the Curie temperature for magnetite, following a slight increase related to the Hopkinson effect. In Xanthi
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we observe an irreversible behavior suggesting mineralogical transformation during heating. The susceptibility increase after 580 °C in the cases of W. Vrondou and Symvolon implies hematization. Stepwise acquisition of the IRM indicates that in most cases saturation is reached in fields lower than 0.02 T (Fig. 3). The dominance of magnetite as the main magnetic carrier can thus be assumed. An estimation of the percentage of the various magnetic grains was done by reflexion-microscopy of thin sections. In Symvolon and Xanthi magnetite reaches 90– 95% of the total opaques, while hematite represents the remaining 5–10%. In W. Vrondou the two mineral species participate in almost equal amounts. The overall high magnitudes of the magnetic susceptibilities (see Section 5) also point to the dominance of ferromagnetic carriers in these granitoids. Finally, the Lowrie–Fuller and Cisowski tests (Lowrie and Fuller, 1971; Cisowski, 1981) suggest that the remanence carrier is coarse-grained (MD) magnetite. The only ambiguous case is W. Vrondou, where NRM appears harder than IRM in low fields above which the two demagnetization curves cross-cut, implying the presence of both MD and SD grains (Fig. 3).
5. Anisotropy of magnetic susceptibility Each AMS measurement yields the magnitudes of the three principal and mutually orthogonal axes of the AMS ellipsoid (K1 P K2 P K3 ), as well as their declinations and inclinations with respect to the geographical frame. The bulk magnetic susceptibility magnitude is given by K ¼ ðK1 þ K2 þ K3 Þ=3. The anisotropy degree P ¼ K1 =K3 , and the shape parameter of Jelinek (1981) T ¼ ðln F ln LÞ=ðln F þ ln LÞ, where F ¼ K2 =K3 and L ¼ K1 =K2 , were also calculated. The mean AMS data for each plutonic body are presented in Table 2.
Fig. 2. Representative thermomagnetic curves from the studied plutons, showing the magnetic susceptibility (K) changes with temperature. Continuous lines correspond to heating, while discontinuous ones indicate cooling.
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settles the upper limits of the paramagnetic contribution to K < 300–500 106 SI and P < 1:2, our granitoids are clearly ferromagnetic. The main magnetic carrier is therefore magnetite, as also confirmed by other laboratory techniques (Section 4). Overall positive correlations appear between K and P, as usually observed in magnetite-bearing plutons (Bouchez, 1997). The only exception is Xanthi (Fig. 4a), where all the (K; P ) values are well grouped. In the case of Symvolon (Fig. 4b) the (K; P ) correlation can be attributed to superimposed strain at the solid state, a phenomenon identified by microstructural observations (Plate 1a). However, the observed steep increase of P, and its large variation up to 35% at high K values, suggest that the magnetic anisotropy largely derives from magnetic interactions between adjacent magnetite grains. Finally, in Vrondou (Fig. 4c) the data from various sites are scattered with no clear trend. However, the low K data come from the easternmost sites. Since the geological limits between the undeformed East-Vrondou and the deformed WestVrondou are not yet established, those specific sites could belong to E. Vrondou. Although no systematic AMS measurements were performed throughout the plutons, their magnetic fabrics seem well-defined and consistent. Except for the southern contact of W. Vrondou (Fig. 5d), the magnetic lineations (Kmax ) have consistently NE-trends and low to moderate plunges (Fig. 5a–c).
6. Microstructures of the plutons
Fig. 3. Examples of Cisowski (1981) and Lowrie and Fuller (1971) tests: acquisition and AF-demagnetization of IRM, and AF-demagnetization of NRM.
All massives have high (Symvolon) to very high (Xanthi) susceptibility magnitudes. Their anisotropy degree is rather high, ranging from 10% (Xanthi) to 31% (Symvolon). According to Rochette (1987), who roughly
The microstructures were determined from thin sections prepared from representative samples. In the following we summarize our observations: (i) Xanthi: this quartz–feldspar–biotite–hornblende granitoid, has no preferred orientation perceptible in the field, with a medium-grained, equigranular texture, apparently frozen at the magmatic state, i.e. not reworked in the solid state (Plate 1a); (ii) Symvolon: a penetrative planar fabric is perceptible in the field (strike: 199° and dip 14° ESE) in this mostly porphyritic quartz–plagioclase– hornblende–biotite granitoid. Typical magmatic textures were not found. Textures range from slightly deformed (subgrains in quartz) to strongly deformed, gneissic and mylonitic (plastic flow of quartz, broken feldspars, and smoothed and partly altered ferromagnetic minerals), indicating deformation in subsolidus conditions, at high to low temperatures (Plate 1b); (iii) W. Vrondou: in the western, deformed parts of this feldspar–quartz–biotite–amphibole pluton, the textures are commonly mylonitic to ultramylonitic within some strongly deformed decimetric bands (Plate 1c). Magmatic textures, or textures slightly affected in the solid state, are restricted to the eastern part. It appears that
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Table 2 Mean AMS measurements of the various magmatic bodies of the Rhodope massif Area
N (sites) 6–8 samples/site
Km (103 SI)
P
T
K1
K2
K3
Xanthi Symvolon W. Vrondou (northern margin) W. Vrondou (southern margin)
9 11 2
41.92 6.86 14.32
1.103 1.311 1.284
0.395 0.112 0.100
254°/26° 49°/14° 52°/53°
100°/64° 143°/24° 152°/4°
348°/11° 299°/65° 246°/35°
3
9.91
1.184
0.122
121°/16°
218°/27°
356°/60°
Fig. 4. Anisotropy degree (P) versus magnetic susceptibility (K) plots.
the degree of deformation in this pluton was concentrated in shear zones.
7. Implications of AMS on paleomagnetic measurements The paleomagnetic results obtained from Symvolo, Xanthi and W. Vrondou, summarized in Table 3, have been thoroughly presented by Atzemoglou et al. (1994) and Dimitriadis et al. (1998). These plutons are concluded to rotate eastwards about vertical axes with an-
gles ranging between 8° and 20° (Fig. 1). Inclination values are grouped close to the expected ones for the area (54°). Together with other criteria discussed in Dimitriadis et al. (1998), absence of substantial tilting can be safely concluded. The magnetic anisotropy in minerals can deviate from the ambient field the direction of thermo- or chemicalremanent magnetization acquired by minerals. This deviation is expected to be small for anisotropy degrees less than 5%, but can be substantial for larger values of P. This problem has been treated at length by Stacey
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Plate 1. Microphotographs of representative samples showing the microstructures of each pluton. (a) Xanthi, (b) Symvolon, (c) W. Vrondou.
(1960), Uyeda et al. (1963), Irving (1964), Hrouda (1982), and Stephenson et al. (1986). Most of the above workers state that the anisotropies of natural remanence (NRM) and IRM give a more precise measurement of the rock mineral anisotropy than AMS, since this latter also contains a contribution of superparamagnetic grains. In such conditions, Hrouda (1982), the magnetization vector may deflect from the vector of the external field towards the maximum susceptibility direction, the deflection being small for current anisotropies. In an effort to better establish the relationship between magnitude and shape of TRM and AMS ellipsoids, Stephenson et al. (1986) distinguish several cases, considering the MD or SD nature of magnetic grains. Given that the anisotropy degrees of the present study are high (between 1.1 and 1.3) the possible deviations of the paleomagnetic directions due to magnetic anisotropy are worth of investigation. At a first look, no deviation appears, since all paleomagnetic directions were not only grouped within the same pluton but also very consistent with all the paleomagnetic data obtained from both a broader area for the same time span and from volcanic rocks. However, by plotting the paleomagnetic directions together with the AMS axes and available tectonic data (Fig. 5) we remark the following. For the Xanthi and Symvolon plutons, the paleomagnetic and tectonic directions are reasonably close to K1 , but a 10–20° deviation in inclination is observed in Xanthi, and a still larger one for Symvolon. The same
Fig. 5. Equal-area lower hemisphere projections of the principal susceptibility axes (n: average K1 ; the field; F: pole of the foliation plane measured in the field; P: paleomagnetic direction).
average K ; H: stretching lineation measured in 3
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Table 3 Paleomagnetic data used in this study Area
N (sites)
Mean D (°)
Mean I (°)
a95
D exp (°)
R (°)
Xanthia Symvolona W. Vrondoub
10 9 4
207.5 24 16
)55 54 54
8 10 8
8 8 8
19.5 16 8
a b
Atzemoglou et al. (1994). Atzemoglou (1997).
holds for the northern W. Vrondou margin (Fig. 5c) and a more complicated pattern appears only for southern W. Vrondou margin (Fig. 5d). We have used the technique of Hargraves and Burt (1967) to eventually correct paleomagnetic directions for the AMS. The method assumes that the remanence vector migrates towards the K1 –K2 plane along a great circle passing through the minimum susceptibility axis and the ancient magnetizing field. When this method was applied graphically (Fig. 6) to the three plutons (with 4–7 site means each) the following data have been obtained: Uncorrected for AMS Xanthi: D ¼ 30° I ¼ 60° Symvolon: D ¼ 027° I ¼ 51° W. Vrondou: D ¼ 008° I ¼ 58°
Corrected for AMS D ¼ 093° I ¼ 63° D ¼ 003° I ¼ 61° D ¼ 337° I ¼ 69°
In order to make this correction we recalculated the paleomagnetic and K1 axes means for the sites used, that is those sites with both AMS and paleomagnetic data. For Xanthi the corrected results imply an unrealistic rotation for the pluton. Given that P ¼ 10%, we suggest that the method is not appropriate for moderate anisotropy factors. For Symvolon, the corrected remanence differs substantially from the uncorrected one, altering thus the implied rotation for the pluton (Fig. 6).
Fig. 6. Graphical representation (equal-area plot) using the technique of Hargraves and Burt (1967) for the correction of remanence from the anisotropy effect, using site means. The great circle for each site passes through the minimum susceptibility axis and the ancient magnetizing field. The mean intersection provides the corrected paleomagnetic direction. P represents the mean paleomagnetic direction, and the square the mean K1 axis.
Finally, in W. Vrondou there seems to be no important vectorial deviation, though the individual values appear to be different numerically.
8. Discussion The regional mineral stretching lineations collected by Kolokotroni and Dixon (1991), Kolokotroni (1992), Dinter et al. (1995) and Koukouvelas and Pe-Piper (1991) from a wide area around the studied plutons, have been plotted in Fig. 5. A good correlation is observed between the regional tectonic lineation and the mean Kmax plots for the syntectonic Xanthi and the highly deformed Symvolon plutons. By introducing our microstructural observations, the latter correlation suggests that Xanthi was affected by the regional tectonics while still in the magmatic state (not completely crystallized); as for Symvolon, it was affected by the regional tectonics after its full crystallization. The case of W. Vrondou is more complex: in its northern contact Kmax and Kmin axes appear to be inverted, with respect to Symvolon, while in its southern contact, an interchange of Kmax and Kint is observed. This behavior can be interpreted in terms of local tectonics and geometrics of the contacts. The northern contact area is undeformed and variable pre-full crystallization fabrics are observed, while the southern contact is a shear zone, with a top-tothe-SW sense of shear. Except for SW. Vrondou, and despite the age differences, the tectonic and magnetic fabrics have similar directions. The prevailing NE–SW-trending linear fabric that characterizes the Early Miocene plutons (Symvolon, W. Vrondou) is clearly imprinted magnetically in the apparent isotropic Oligocene Xanthi pluton which is concluded to have been deformed according to the same tectonics in the magmatic to late-magmatic state. The preliminary results suggest that the Early Oligocene in the Rhodope area was a period during which a slow extensional deformation was initiating following the formation of the Early Tertiary orogen. It culminated in a post-orogenic collapse during the regional extension of the Early Miocene. A new phase of extension started after the activation of the North Anatolian Fault, detachment of part of the Rhodope and subsequent clockwise rotation of parts of the Greek Rhodope
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(Haubold et al., 1997; Dimitriadis et al., 1998). As a consequence, the latter post-early Miocene clockwise rotation had rotated not only the paleomagnetic directions but also the tectonic and AMS vectors. If these vectors are rotated back to their original directions, the tectonic lineations and corresponding AMS axes become closer to N–S, hence closer to the dipole direction at that time. We suggest that the paleomagnetic directions could have been influenced by the AMS directions. This may modify the implied rotations for the area. However, further measurements of the anisotropy of TRM/IRM are required along with a more thorough sampling of the plutons. Acknowledgements The first author thanks the National Grant Foundation of Greece for financial support and is indebted to Jean-Luc Bouchez for allowing the use of the Laboratoire de Petrophysique, in Toulouse and enlightening many aspects concerning the theory of magnetic fabrics. Some of the thermomagnetic curves have been obtained in the paleomagnetic laboratory of the ENS (Paris). Critical comments from the two referees have greatly contributed to the substantial improvement of the manuscript. Finally, R. Scholger is warmly thanked for his patience and efficiency. This paper is a Geophysical Laboratory of the Aristotle University of Thessaloniki contribution number #604/2002. References Archanjo, C.J., Launeau, P., Bouchez, J.-L., 1994. Magnetic fabric vs. magnetite and biotite shape fabrics of the magnetite-bearing granite pluton of Gameleiras (Northeast Brazil). Phys. Earth Planet. Int. 89, 63–75. Atzemoglou, A., 1997. Paleomagnetic results from Northern Greece and their contribution to the interpretation of the geodynamic evolution of the area during the Tertiary. Ph.D. Thesis, Aristotle University of Thessaloniki, 319pp. Atzemoglou, A., Kondopoulou, D., Papamarinopoulos, S., Dimitriadis, S., 1994. Palaeomagnetic evidence for block rotations in the western Greek Rhodope. Geophys. J. Int. 118, 221– 230. Bouchez, J.-L., 1997. Granite is never isotropic: an introduction to AMS studies in granitic rocks. In: Bouchez, J.-L., Hutton, D.H.W., Stephens, W.E. (Eds.), Granite: From segregation of melt to emplacement fabrics. Kluwer Academic Pubishers, Dortrecht, pp. 95–112. Bouchez, J.-L., Gleizes, G., 1995. Two-stage deformation of the MontLouis-Andorra granite pluton (Variscan Pyrenees) inferred from magnetic susceptibility anisotropy. J. Geol. Soc. London 152, 669– 679. Bouchez, J.-L., Gleizes, G., Djouadi, T., Rochette, P., 1990. Microstructure and magnetic susceptibility applied to emplacement kinematics of granites: the example of the Foix pluton (French Pyrenees). Tectonophysics 184, 157–171. Cisowski, S., 1981. Interacting vs. non-interacting single domain behavior in natural and synthetic samples. Phys. Earth Planet. Int. 26, 56–62.
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I. Zananiri et al. / Physics and Chemistry of the Earth 27 (2002) 1289–1297 bility and remanence in rocks and minerals. Geophys. J. R. Astr. Soc. 84, 185–200. Trindade, R., Raposo, I., Ernesto, M., Siqueira, R., 1999. Magnetic susceptibility and partial anhysteretic remanence anisotropies in
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A preliminary AMS study in some tertiary granitoids from Northern Greece: integration of tectonic and paleomagnetic data I. Zananiri a
a,*
, S. Dimitriadis b, D. Kondopoulou a, A. Atzemoglou
c
Department of Geophysics, Aristotle University of Thessaloniki, P.O. Box 352-1, Thessaloniki 54124, Greece Department of Mineralogy and Petrology, Aristotle University of Thessaloniki, Thessaloniki 54124, Greece c IGME, Frangon 1, Thessaloniki 54626, Greece
b
Abstract The present study focuses on some of the Tertiary granitoids of the Rhodope Massif, Northern Greece, namely the Early Miocene plutons of Symvolon, W. Vrondou and the Oligocene Xanthi pluton. Their low-field anisotropy of magnetic susceptibility (AMS) was studied in a number of stations and was combined with existing paleomagnetic and tectonic data in order to check their mutual consistency and to assess to what extent they can help clarifying the tectonic regime prevailing during emplacement of these granitoids. The bulk susceptibility magnitude is generally high, as well as the mean anisotropy degree which reaches 1.23, pointing to a dominant ferromagnetic control of the magnetic properties. Microscopic observation, isothermal remanent magnetization (IRM), and thermomagnetic analysis reveal that the magnetic mineralogy is controlled mainly by magnetite. The magnetic fabrics are welldefined in the plutons, with the Kmax axes (magnetic lineations) varying from gently to moderate plunges. Paleomagnetic results obtained from the same bodies display clear clockwise rotations for all the plutons, with minor or no tilt during their emplacement. Despite of the age differences, the tectonic and magnetic fabrics have mostly similar directions. The prevailing NE–SW-trending linear fabrics, visible mainly in the Early Miocene plutons are also clearly imprinted magnetically in the Early Oligocene pluton, in which no macroscopic fabric is visible. Finally, a preliminary attempt to correct the paleomagnetic results from the magnetic anisotropy effect showed that only qualitative conclusions can be reached from the AMS data, and that complementary AIRM measurements are required. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Paleomagnetism; AMS; Deformation; Tectonics; Tertiary granitoids; N. Greece
1. Introduction The study of the anisotropy of magnetic susceptibility (AMS) with its scalar and directional parameters is starting to become in recent years a standard and valuable technique, which, in addition to macro and microtectonic fabrics and microscopic textural data, can contribute to the unraveling of the emplacement, cooling and unroofing history of a pluton (e.g. Bouchez et al., 1990; Archanjo et al., 1994; Bouchez and Gleizes, 1995; Trindade et al., 1999). AMS has the merit of revealing very faint anisotropies in rock masses subjected to imperceptible tectonic imprints. Plutonic bodies emplaced in the Mesozoic and Tertiary are abundant in N. Greece. For some of them reliable radiometric ages, tectonic and petrographic data, *
Corresponding author. Fax: +30-31-998528. E-mail address: [email protected] (I. Zananiri).
and paleomagnetic and rock magnetic studies are available (Atzemoglou et al., 1994; Dimitriadis et al., 1998, and references therein). A first set of AMS measurements in three of these plutons, namely the Early Miocene Symvolon and West Vrondou, and the Early Oligocene Xanthi pluton (Fig. 1), are reported here. The present work was undertaken in order to see whether, using the available data, AMS, tectonic and microtextural features are mutually consistent in these granitoids, and if they can contribute to clarify the tectonic regime during their emplacement. It is well-known that AMS may deflect the vectors of remanence magnetization (Hrouda, 1982), but even if the mechanism and magnitude of this deflection is not yet fully clear, it certainly has to be taken into account in paleomagnetic studies. We make an attempt here to correct the existing paleomagnetic data for the three studied granitoids, taking into account a new set of AMS measurements.
1474-7065/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII: S 1 4 7 4 - 7 0 6 5 ( 0 2 ) 0 0 1 1 5 - 8
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Fig. 1. Southern Rhodope and adjacent areas. The main tectonic units are shown (PZ: Pelagonian zone; VZ: Vardar zone; PRZ: peri-Rhodopean zone; RCC.UPL: upper plate of the Rhodope core complex (Serbomacedonian element); RCC.LPL: lower plate of the Rhodope core complex; RBD: Rhodope beyond the Strymon valley detachment; TSVD: trace of the Strymon valley detachment fault; BSDF: breakaway of the Strymon detachment fault. The arrows indicate the paleomagnetically derived rotation values of igneous formations. X: Xanthi, SV: Symvolon, EV: E. Vrondou, WV: W. Vrondou (modified map from Dimitriadis et al., 1998).
2. Geological setting The Early Oligocene Xanthi and the Early Miocene Symvolon and W. Vrondou plutons, each exposed for several hundreds of km2 , crop out among the metamorphics of the Rhodope massif in eastern Macedonia and Thrace (Fig. 1). Radiochronological data of these granitoids, mostly cooling ages, are summarized in Dimitriadis et al. (1998) and are also given in Table 1. The large pluton of Vrondou is composed of two different intrusions: the locally strongly mylonitized Early Miocene West Vrondou body, and the isotropiclooking Early Oligocene East Vrondou body (Dinter et al., 1995). According to Kolokotroni and Dixon (1991) and Kolokotroni (1992), the Vrondou pluton was emplaced syntectonically within an extensional ramp space. Their results were mostly obtained from the deformed western part of the pluton, here referred to as the Early Miocene intrusion. Since no data are available from the eastern part of the Vrondou pluton, the latter is not considered in the present study. A similar extensional regime has also been suggested by Koukouvelas and Pe-Piper (1991) for the emplacement of the Early Oligocene Xanthi pluton, which shows no macroscopic deformation features except a spaced set of parallel joints which acted as conduits for the intrusion of aplitic dykes. Dinter (1994) proposed that in the Rhodope area
Table 1 Available radiometric data for the studied plutons Granitoid
Dating method/ reference
Age (Ma)
Xanthi
K–Ar biot. (Meyer, 1968) K–Ar hornbl. (Liati, 1986) Rb–Sr whole rock (Kyriakopoulos, 1987) Rb–Sr whole rock (Kyriakopoulos, 1987)
27:1 0:4
Symvolon
K–Ar biot. (Harre et al., 1968) U–Pb, 40 Ar–39 Ar hornbl. (Dinter et al., 1995) Rb–Sr whole rock, mica (Kyriakopoulos, 1987)
30:4 0:6 28:8 0:7 26:3 0:1 13:8 0:2 19.1–20.1
15–26
W. Vrondou
U–Pb, Ar–Ar (Kaufman, 1995)
23:7 0:1
E. Vrondou
K–Ar hornbl. (Papadakis, 1965) K–Ar hornbl. (Marakis, 1969) K–Ar biot. (Durr et al., 1978)
53:5 4:2 29 1 to 33 2 30 3
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the Oligocene was a tectonically quiet period, post-dating an Early Tertiary compressional phase and predating a strong syn-extensional stretching phase during which the Early Miocene plutons (Symvolon and W. Vrondou) were emplaced. This agrees with the lack of perceptible tectonic fabrics in the Oligocene Xanthi pluton, in contrast to the strong tectonic fabric observed in the two Miocene granitoids. AMS measurements are expected to complement the above given geological data and to provide more structural information about the apparently isotropic Xanthi pluton.
3. Sampling and measurements Drilled cores and oriented hand samples were collected at several sites in W. Vrondou, Symvolon and Xanthi plutons. At each site, three to seven hand samples, or eight to ten cores were collected. Standard 25 22 mm cylinders were collected from the samples. AMS measurements on these specimens were carried out with the Kappabridge KLY-2 apparatus at the University Paul-Sabatier in Toulouse. Microscopic examination of thin sections, thermomagnetic analysis, AF-demagnetizations, as well as isothermal remanent magnetization (IRM) measurements were performed at the Paleomagnetic Laboratory of the University of Thessaloniki.
4. Magnetic mineralogy Thermomagnetic analyses were carried out in order to determine Curie temperatures. Representative curves for samples of the studied plutons (Fig. 2) show that the magnetic susceptibility (K) decreases near zero at 580 °C, the Curie temperature for magnetite, following a slight increase related to the Hopkinson effect. In Xanthi
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we observe an irreversible behavior suggesting mineralogical transformation during heating. The susceptibility increase after 580 °C in the cases of W. Vrondou and Symvolon implies hematization. Stepwise acquisition of the IRM indicates that in most cases saturation is reached in fields lower than 0.02 T (Fig. 3). The dominance of magnetite as the main magnetic carrier can thus be assumed. An estimation of the percentage of the various magnetic grains was done by reflexion-microscopy of thin sections. In Symvolon and Xanthi magnetite reaches 90– 95% of the total opaques, while hematite represents the remaining 5–10%. In W. Vrondou the two mineral species participate in almost equal amounts. The overall high magnitudes of the magnetic susceptibilities (see Section 5) also point to the dominance of ferromagnetic carriers in these granitoids. Finally, the Lowrie–Fuller and Cisowski tests (Lowrie and Fuller, 1971; Cisowski, 1981) suggest that the remanence carrier is coarse-grained (MD) magnetite. The only ambiguous case is W. Vrondou, where NRM appears harder than IRM in low fields above which the two demagnetization curves cross-cut, implying the presence of both MD and SD grains (Fig. 3).
5. Anisotropy of magnetic susceptibility Each AMS measurement yields the magnitudes of the three principal and mutually orthogonal axes of the AMS ellipsoid (K1 P K2 P K3 ), as well as their declinations and inclinations with respect to the geographical frame. The bulk magnetic susceptibility magnitude is given by K ¼ ðK1 þ K2 þ K3 Þ=3. The anisotropy degree P ¼ K1 =K3 , and the shape parameter of Jelinek (1981) T ¼ ðln F ln LÞ=ðln F þ ln LÞ, where F ¼ K2 =K3 and L ¼ K1 =K2 , were also calculated. The mean AMS data for each plutonic body are presented in Table 2.
Fig. 2. Representative thermomagnetic curves from the studied plutons, showing the magnetic susceptibility (K) changes with temperature. Continuous lines correspond to heating, while discontinuous ones indicate cooling.
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settles the upper limits of the paramagnetic contribution to K < 300–500 106 SI and P < 1:2, our granitoids are clearly ferromagnetic. The main magnetic carrier is therefore magnetite, as also confirmed by other laboratory techniques (Section 4). Overall positive correlations appear between K and P, as usually observed in magnetite-bearing plutons (Bouchez, 1997). The only exception is Xanthi (Fig. 4a), where all the (K; P ) values are well grouped. In the case of Symvolon (Fig. 4b) the (K; P ) correlation can be attributed to superimposed strain at the solid state, a phenomenon identified by microstructural observations (Plate 1a). However, the observed steep increase of P, and its large variation up to 35% at high K values, suggest that the magnetic anisotropy largely derives from magnetic interactions between adjacent magnetite grains. Finally, in Vrondou (Fig. 4c) the data from various sites are scattered with no clear trend. However, the low K data come from the easternmost sites. Since the geological limits between the undeformed East-Vrondou and the deformed WestVrondou are not yet established, those specific sites could belong to E. Vrondou. Although no systematic AMS measurements were performed throughout the plutons, their magnetic fabrics seem well-defined and consistent. Except for the southern contact of W. Vrondou (Fig. 5d), the magnetic lineations (Kmax ) have consistently NE-trends and low to moderate plunges (Fig. 5a–c).
6. Microstructures of the plutons
Fig. 3. Examples of Cisowski (1981) and Lowrie and Fuller (1971) tests: acquisition and AF-demagnetization of IRM, and AF-demagnetization of NRM.
All massives have high (Symvolon) to very high (Xanthi) susceptibility magnitudes. Their anisotropy degree is rather high, ranging from 10% (Xanthi) to 31% (Symvolon). According to Rochette (1987), who roughly
The microstructures were determined from thin sections prepared from representative samples. In the following we summarize our observations: (i) Xanthi: this quartz–feldspar–biotite–hornblende granitoid, has no preferred orientation perceptible in the field, with a medium-grained, equigranular texture, apparently frozen at the magmatic state, i.e. not reworked in the solid state (Plate 1a); (ii) Symvolon: a penetrative planar fabric is perceptible in the field (strike: 199° and dip 14° ESE) in this mostly porphyritic quartz–plagioclase– hornblende–biotite granitoid. Typical magmatic textures were not found. Textures range from slightly deformed (subgrains in quartz) to strongly deformed, gneissic and mylonitic (plastic flow of quartz, broken feldspars, and smoothed and partly altered ferromagnetic minerals), indicating deformation in subsolidus conditions, at high to low temperatures (Plate 1b); (iii) W. Vrondou: in the western, deformed parts of this feldspar–quartz–biotite–amphibole pluton, the textures are commonly mylonitic to ultramylonitic within some strongly deformed decimetric bands (Plate 1c). Magmatic textures, or textures slightly affected in the solid state, are restricted to the eastern part. It appears that
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Table 2 Mean AMS measurements of the various magmatic bodies of the Rhodope massif Area
N (sites) 6–8 samples/site
Km (103 SI)
P
T
K1
K2
K3
Xanthi Symvolon W. Vrondou (northern margin) W. Vrondou (southern margin)
9 11 2
41.92 6.86 14.32
1.103 1.311 1.284
0.395 0.112 0.100
254°/26° 49°/14° 52°/53°
100°/64° 143°/24° 152°/4°
348°/11° 299°/65° 246°/35°
3
9.91
1.184
0.122
121°/16°
218°/27°
356°/60°
Fig. 4. Anisotropy degree (P) versus magnetic susceptibility (K) plots.
the degree of deformation in this pluton was concentrated in shear zones.
7. Implications of AMS on paleomagnetic measurements The paleomagnetic results obtained from Symvolo, Xanthi and W. Vrondou, summarized in Table 3, have been thoroughly presented by Atzemoglou et al. (1994) and Dimitriadis et al. (1998). These plutons are concluded to rotate eastwards about vertical axes with an-
gles ranging between 8° and 20° (Fig. 1). Inclination values are grouped close to the expected ones for the area (54°). Together with other criteria discussed in Dimitriadis et al. (1998), absence of substantial tilting can be safely concluded. The magnetic anisotropy in minerals can deviate from the ambient field the direction of thermo- or chemicalremanent magnetization acquired by minerals. This deviation is expected to be small for anisotropy degrees less than 5%, but can be substantial for larger values of P. This problem has been treated at length by Stacey
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Plate 1. Microphotographs of representative samples showing the microstructures of each pluton. (a) Xanthi, (b) Symvolon, (c) W. Vrondou.
(1960), Uyeda et al. (1963), Irving (1964), Hrouda (1982), and Stephenson et al. (1986). Most of the above workers state that the anisotropies of natural remanence (NRM) and IRM give a more precise measurement of the rock mineral anisotropy than AMS, since this latter also contains a contribution of superparamagnetic grains. In such conditions, Hrouda (1982), the magnetization vector may deflect from the vector of the external field towards the maximum susceptibility direction, the deflection being small for current anisotropies. In an effort to better establish the relationship between magnitude and shape of TRM and AMS ellipsoids, Stephenson et al. (1986) distinguish several cases, considering the MD or SD nature of magnetic grains. Given that the anisotropy degrees of the present study are high (between 1.1 and 1.3) the possible deviations of the paleomagnetic directions due to magnetic anisotropy are worth of investigation. At a first look, no deviation appears, since all paleomagnetic directions were not only grouped within the same pluton but also very consistent with all the paleomagnetic data obtained from both a broader area for the same time span and from volcanic rocks. However, by plotting the paleomagnetic directions together with the AMS axes and available tectonic data (Fig. 5) we remark the following. For the Xanthi and Symvolon plutons, the paleomagnetic and tectonic directions are reasonably close to K1 , but a 10–20° deviation in inclination is observed in Xanthi, and a still larger one for Symvolon. The same
Fig. 5. Equal-area lower hemisphere projections of the principal susceptibility axes (n: average K1 ; the field; F: pole of the foliation plane measured in the field; P: paleomagnetic direction).
average K ; H: stretching lineation measured in 3
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Table 3 Paleomagnetic data used in this study Area
N (sites)
Mean D (°)
Mean I (°)
a95
D exp (°)
R (°)
Xanthia Symvolona W. Vrondoub
10 9 4
207.5 24 16
)55 54 54
8 10 8
8 8 8
19.5 16 8
a b
Atzemoglou et al. (1994). Atzemoglou (1997).
holds for the northern W. Vrondou margin (Fig. 5c) and a more complicated pattern appears only for southern W. Vrondou margin (Fig. 5d). We have used the technique of Hargraves and Burt (1967) to eventually correct paleomagnetic directions for the AMS. The method assumes that the remanence vector migrates towards the K1 –K2 plane along a great circle passing through the minimum susceptibility axis and the ancient magnetizing field. When this method was applied graphically (Fig. 6) to the three plutons (with 4–7 site means each) the following data have been obtained: Uncorrected for AMS Xanthi: D ¼ 30° I ¼ 60° Symvolon: D ¼ 027° I ¼ 51° W. Vrondou: D ¼ 008° I ¼ 58°
Corrected for AMS D ¼ 093° I ¼ 63° D ¼ 003° I ¼ 61° D ¼ 337° I ¼ 69°
In order to make this correction we recalculated the paleomagnetic and K1 axes means for the sites used, that is those sites with both AMS and paleomagnetic data. For Xanthi the corrected results imply an unrealistic rotation for the pluton. Given that P ¼ 10%, we suggest that the method is not appropriate for moderate anisotropy factors. For Symvolon, the corrected remanence differs substantially from the uncorrected one, altering thus the implied rotation for the pluton (Fig. 6).
Fig. 6. Graphical representation (equal-area plot) using the technique of Hargraves and Burt (1967) for the correction of remanence from the anisotropy effect, using site means. The great circle for each site passes through the minimum susceptibility axis and the ancient magnetizing field. The mean intersection provides the corrected paleomagnetic direction. P represents the mean paleomagnetic direction, and the square the mean K1 axis.
Finally, in W. Vrondou there seems to be no important vectorial deviation, though the individual values appear to be different numerically.
8. Discussion The regional mineral stretching lineations collected by Kolokotroni and Dixon (1991), Kolokotroni (1992), Dinter et al. (1995) and Koukouvelas and Pe-Piper (1991) from a wide area around the studied plutons, have been plotted in Fig. 5. A good correlation is observed between the regional tectonic lineation and the mean Kmax plots for the syntectonic Xanthi and the highly deformed Symvolon plutons. By introducing our microstructural observations, the latter correlation suggests that Xanthi was affected by the regional tectonics while still in the magmatic state (not completely crystallized); as for Symvolon, it was affected by the regional tectonics after its full crystallization. The case of W. Vrondou is more complex: in its northern contact Kmax and Kmin axes appear to be inverted, with respect to Symvolon, while in its southern contact, an interchange of Kmax and Kint is observed. This behavior can be interpreted in terms of local tectonics and geometrics of the contacts. The northern contact area is undeformed and variable pre-full crystallization fabrics are observed, while the southern contact is a shear zone, with a top-tothe-SW sense of shear. Except for SW. Vrondou, and despite the age differences, the tectonic and magnetic fabrics have similar directions. The prevailing NE–SW-trending linear fabric that characterizes the Early Miocene plutons (Symvolon, W. Vrondou) is clearly imprinted magnetically in the apparent isotropic Oligocene Xanthi pluton which is concluded to have been deformed according to the same tectonics in the magmatic to late-magmatic state. The preliminary results suggest that the Early Oligocene in the Rhodope area was a period during which a slow extensional deformation was initiating following the formation of the Early Tertiary orogen. It culminated in a post-orogenic collapse during the regional extension of the Early Miocene. A new phase of extension started after the activation of the North Anatolian Fault, detachment of part of the Rhodope and subsequent clockwise rotation of parts of the Greek Rhodope
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(Haubold et al., 1997; Dimitriadis et al., 1998). As a consequence, the latter post-early Miocene clockwise rotation had rotated not only the paleomagnetic directions but also the tectonic and AMS vectors. If these vectors are rotated back to their original directions, the tectonic lineations and corresponding AMS axes become closer to N–S, hence closer to the dipole direction at that time. We suggest that the paleomagnetic directions could have been influenced by the AMS directions. This may modify the implied rotations for the area. However, further measurements of the anisotropy of TRM/IRM are required along with a more thorough sampling of the plutons. Acknowledgements The first author thanks the National Grant Foundation of Greece for financial support and is indebted to Jean-Luc Bouchez for allowing the use of the Laboratoire de Petrophysique, in Toulouse and enlightening many aspects concerning the theory of magnetic fabrics. Some of the thermomagnetic curves have been obtained in the paleomagnetic laboratory of the ENS (Paris). Critical comments from the two referees have greatly contributed to the substantial improvement of the manuscript. Finally, R. Scholger is warmly thanked for his patience and efficiency. This paper is a Geophysical Laboratory of the Aristotle University of Thessaloniki contribution number #604/2002. References Archanjo, C.J., Launeau, P., Bouchez, J.-L., 1994. Magnetic fabric vs. magnetite and biotite shape fabrics of the magnetite-bearing granite pluton of Gameleiras (Northeast Brazil). Phys. Earth Planet. Int. 89, 63–75. Atzemoglou, A., 1997. Paleomagnetic results from Northern Greece and their contribution to the interpretation of the geodynamic evolution of the area during the Tertiary. Ph.D. Thesis, Aristotle University of Thessaloniki, 319pp. Atzemoglou, A., Kondopoulou, D., Papamarinopoulos, S., Dimitriadis, S., 1994. Palaeomagnetic evidence for block rotations in the western Greek Rhodope. Geophys. J. Int. 118, 221– 230. Bouchez, J.-L., 1997. Granite is never isotropic: an introduction to AMS studies in granitic rocks. In: Bouchez, J.-L., Hutton, D.H.W., Stephens, W.E. (Eds.), Granite: From segregation of melt to emplacement fabrics. Kluwer Academic Pubishers, Dortrecht, pp. 95–112. Bouchez, J.-L., Gleizes, G., 1995. Two-stage deformation of the MontLouis-Andorra granite pluton (Variscan Pyrenees) inferred from magnetic susceptibility anisotropy. J. Geol. Soc. London 152, 669– 679. Bouchez, J.-L., Gleizes, G., Djouadi, T., Rochette, P., 1990. Microstructure and magnetic susceptibility applied to emplacement kinematics of granites: the example of the Foix pluton (French Pyrenees). Tectonophysics 184, 157–171. Cisowski, S., 1981. Interacting vs. non-interacting single domain behavior in natural and synthetic samples. Phys. Earth Planet. Int. 26, 56–62.
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