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The Guitiriz granite, Variscan belt of northern Spain: extension-controlled emplacement of magma during tectonic escape
EPSL ELSEVIER
Earth and Planetary
Science Letters 139 ( 1996) 165- 176
The Guitiriz granite, Variscan belt of northern Spain: extension-controlled emplacement of magma during tectonic escape Aitor Aranguren a**, Jo& Maria Tubia a, Jean Luc Bouchez b, Jean Louis Vigneresse ’ a Departamento b Laboratoire
de Geodindmica,
Universidad
de Pe’trophysique. URA CNRS no. 67, Universiti ’ CREGU
de1 Pais Vasco, Ap. 644, Bilbao 48080, Spain Paul Sabatier, 38 rue des 36 Ponts, F-31400
and ENS Gt!ologie Nancy, BP23, F-54501
Received 2 August
Toulouse, France
Vandoeuvre / Nancy, France
1995; accepted 4 December
1995
Abstract The 0110 de Sapo domain of the northern part of the Variscan belt of Spain, contains Precambrian and Ordovician metamorphic rocks intruded by the Guitiriz granite. The domain is bounded by two N-S transcurrent shear zones. Detailed structural and anisotropy of magnetic susceptibility studies of the Guitiriz pluton reveal a syntectonic emplacement, relative to movement on the two N-S trending bounding shear zones, characterized by: (1) development of N-S trending structural and magnetic fabrics; (2) concordant structures in granites and country rocks; and (3) development of shear zones along the east and west granite margins. Gravity data show two roots trending 110”E extending down to 4.5 km from an otherwise thin (< 3 km), slab-like body. The Guitiriz pluton was emplaced in a magma chamber developed by local N-S extension induced by an overall E-W shortening. The proposed emplacement model involves the northwards tectonic escape of a crustal wedge - the 0110 de Sapo Domain - bounded by two shear zones acting as conjugate strike-slip zones. Keywords:
0110 de Sapo domain;
Hercynian
Orogeny; granite; extension; magnetic susceptibility
1. Introduction Granitic plutons intrude in different erogenic contexts [ 11, including transcurrent [2-41, contractional [5-71 and extensional [g-lo] settings. Studies of granitic plutons often use petrological and geochemical approaches to provide information about the nature and origin of the granitic magmas. However, this type of study does not shed light on emplacement mechanisms, another key question about gran-
* Corresponding author. Tel: 34 4 464 77 00; Fax: 34 4 464 85 00, E-mail: [email protected]
0012-821X/96/$12.00 0 1996 Elsevier Science B.V. All rights reserved SSDl 0012-821X(95)00239-1
ites. Studies of the rheology of partial melts [l l-131, laboratory models on the interaction between soft materials bounded by rigid walls, and models on migration and deformation of diapiric bodies [ 14- 171 also help to constrain the kinematic and dynamic behaviour of granitic magmas, but field structural studies are still needed to constrain pluton emplacement models. The deformation of the country rocks controls the shape and emplacement of granites during intrusion [8]. Consequently, structures from both the granite and the country rocks are required to establish emplacement models. The granite provides information
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only about the microstructures formed during crystallization and subsequent deformation (e.g., magmatic versus solid-state deformation fabrics [ 18,193). The anisotropy of magnetic susceptibility CAMS) makes it possible to detect otherwise invisible foliations and lineations in granites [ 10,20,2 11. Gravity surveys provide the most efficient data for investigating the shape of plutons at depth [22,23]. These are complementary geophysical approaches, since gravity data give us the three-dimensional shape of the pluton whereas AMS provides a two-dimensional picture of its internal structure. This paper presents the results of a combined structural, AMS and gravity study of the Guitiriz pluton, in northern Spain (Fig. 1B). This multidisciplinary approach is used to determine the three-dimensional geometry and the emplacement history of
Science Letters 139 (1996) 165-176
the Guitiriz pluton. This pluton is of great interest because it is located at the edge of the IberoArmorican Arc, where several ductile shear zones have long been recognized [24] (Fig. 1). This area provides a good opportunity for analysing the relationships between the development of shear zones and the emplacement of granitic rocks.
2. Geological setting The synorogenic Guitiriz granite [25] intrudes Ordovician and Silurian schists and Precambrian metavolcanic rocks of the northern part of the 0110 de Sapo Domain (OSD). This N-S trending, 200 km long domain belongs to the Central Iberian Zone of the Iberian Massif 126,271(Fig. 1). The OSD records
7&l
[vivero[
7
Mondoiiedo she ar zone
1
200
II H
IiI $$$:: lIzI
km
Cabo Ortega1 Complex (C.O.C.) 0110 de Sapo Domain a)Lower Palaeozoic b) Precambrian (0110 de Saps) Mondofiedo Nappe Domain a)Lower Palaeozoic b) Precambrian Synkinematic Granitoids
++++ Late-postkinematic Granitoids lzl Fig. 1. The Guitiriz granite in northern Spain. (A) Location of the studied area in the frame of the Ibero-Armorican arc (from Burg et al. [27]). C.C.S.Z. = Coimbra-Cbrdoba shear zone; N.A.S.Z. = North Armorican shear zone; S.A.S.Z.= South Armorican shear zone. (B) Geological sketch map of the Variscan belt in northern Spain. We Guitiriz granite is outlined. The cross-section shows the main structural features of the area studied (simplified after Martinez-Cat&in et al. [28] and Bastida et al. 1301).
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three main phases of deformation, which form part of a progressive deformation process developed at mid to shallow crustal levels (ca. 15-9 km deep) in response to Variscan continental collision and crustal thickening [24,28]. The earliest deformation led to recumbent, E-vergent folds. The second phase (D,) resulted in E-directed overthrusts, such as the Mondoiiedo nappe or the allochthonous Cabo Ortega1 complex [29] (Fig. 1). Finally, the third phase of deformation (D,) caused a penetrative cleavage related to E-vergent inclined folds. D, is a retrograde phase and medium pressure metamorphism reached amphibolite facies conditions (600°C; 5OOMPa) during D, near the Guitiriz granite [30]. The structure of the OSD contrasts with that of the adjacent Mondoiledo Nappe Domain, where only a weak D, deformation, represented by open folds, overprint broad D, shear zones [30,31] (Fig. 1). As suggested by previous works [30,32], the Mondoiiedo Nappe is
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at upper levels of a crustal wedge formed by an antiformal stack of E-vergent thrust sheets. This interpretation has been confirmed by seismic studies recently completed in this part of the Iberian Variscan belt [33] and also by the analysis of the pronounced magnetic anomaly found in eastern Galicia 1341.The OSD is located at the rear of the tectonic crustal wedge. In the region studied the OSD is bounded by two major, roughly N-S trending, ductile shear zones. The Valdoviilo fault, to the west, is a sinistral strike-slip fault [35], whereas the Vivero fault, to the east (Fig. l), is a dextral, transtensional shear zone [28,30]. These block-boundary shear zones cut the metamorphic isograds of the 0110 de Sapo and the Mondoiiedo Nappe domains, respectively. These two faults converge towards the south, narrowing the OSD from a width of about 45 km, along the coastal traverse, down to 8 km at the southern end of the
LEGEND Trajectories of the magmatic foliation\ Trajectories of the myloniticfoliation \ m
Mariz (two-mica fine grained leucogranitej
stretchinglineation ---_----_ poles to C-planes
------------------
StNCtUreS
lineation poles to foliation
(9
;olld-state deformation
Trajectories of the S3 - schistosity \
I
magmatlc structures
Fig. 2. Structural map and petrographic zoning of the Guitiriz pluton. The trajectories of mylonitic foliation underline the two ductile, strike-slip shear zones developed along the eastern and western edges of the Guitiriz pluton. Orientation data of magmatic structures and solid-state deformation structures are shown on the corresponding stereograms (Schmidt lower hemisphere projection).
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Guitiriz pluton (Fig. 1). However, they are not connected and one is not a second order of the other. The Guitiriz pluton shows two main facies (Fig. 2). The Puebla de Parga facies is composed of porphyritic biotite granite, with potassic feldspar porphyrocrysts (2-5 cm). The Friol facies is composed of two-mica granite with a medium to coarse-grained equigranular texture. They were initially considered as two different petrogenetic series of different ages [36]. However, our field criteria show that the transition between facies is gradual [25]. A co-genetic relationship is supported by the recent petrological and geochemical data of Bellido Mulas et al. [37]. A third facies, the Mariz facies, occurs in the southwestern part of the pluton. It is a two-mica, finegrained leucogranite. It post-dates the Friol and Puebla de Parga facies, as evidenced from conventional field evidence (sharp contacts and presence of enclaves of the other rock types), but it is still broadly co-genetic with the other two facies [37]. The Guitiriz pluton intruded at ca. 318.7 k 4.2 Ma, according to whole rock Rb/Sr isotopic dating [37].
3. Field structures At map scale, the structure of the country rocks is dominated by D, folds. They have kink geometries and show a vertical to steeply west-dipping axial planar cleavage. Fold axes are subhorizontal and trend N-S. Cleavage trajectories, which usually parallel the elongation direction of the Guitiriz pluton, are disturbed at the northern and southern edges of the granitic body, where cleavage triple points developed (Fig. 2). Such triple points represent the interaction of the regional stress field, with the local stress field related to the magma intrusion, as proposed by theoretical studies about the dynamics of granitic domes [16]. Wall rock deformation concentrates in two shear zones dipping to the west. The shear zones contain a subhorizontal and N-S trending stretching lineation and show opposite shear senses (Fig. 2). These shear senses are like those documented within the adjacent block-boundary shear zones (compare Figs. 1 and 2). Consequently, they can also be interpreted as boundary faults at local scale [38].
3.1. Structures in the Guitiriz granite The granite shows a steeply dipping, N-S striking, magmatic foliation that is parallel to the main planar structure (S,) of the country rocks (Fig. 2). This orientation deviates only in the southern part of the pluton, where flat-dipping foliations predominate. This suggests a flat shape for that part of the pluton. The foliation in the granite was produced by magmatic flow, as evidenced by the following microstructural observations: (I) the foliation is defined by the parallel arrangement of unstrained subhedral micas and feldspars; (2) quartz aggregates display equant shapes and are composed of large, optically strain-free grains lacking lattice-preferred orientations. The two main facies (Puebla de Parga and Friol) of the Guitiriz pluton have similar orientation patterns of foliations and lineations (Fig. 2). Solid-state deformation structures progressively replace magmatic structures on the eastern and western borders. Solid-state deformation features include widespread S-C structures, grain size reduction and elongation of quartz aggregates. C planes and shear senses are compatible with those of shear bands developed in the adjacent wall rocks (Fig. 2). Both the magmatic and solid-state foliations carry subhorizontal and N-S trending lineations (Fig. 2). The southernmost part of the pluton and its country rocks contain shear bands that dip approximately 10”N. These shear bands also carry N-S stretching lineations and are systematically related to top-to-the-north motions. These structural features suggest that the igneous rocks and its country rocks have shared a common deformational history during pluton emplacement, involving strike-slip shearing and a component of northward extensional deformation in the crustal domain comprised between these shear zones.
4. Magnetic susceptibility AMS allows us to detect systematic changes of several magnetic parameters [39,40] which provide valuable information on kinematic reconstructions of granite emplacement [ 10,20,2 11. Magnetic susceptibility and anisotropy measurements of oriented sam-
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ples from the Guitiriz pluton were performed with a MINISEP (MOLSPIN) susceptometer, operating at + 10 kHz in a low magnetic field of 7 X 10M4 T, with a precision of 5 X 10e7 SI. Samples for this study were collected from 112 stations. In order to avoid local perturbations of the magnetic properties, at least four oriented samples per station were taken using the same procedure as that used for paleomagnetic analyses. A sample subjected to an external magnetic field, H, develops an induced magnetic moment, J. The magnetic susceptibility, K, is a second-rank tensor which relates these two vectorial magnetic properties, and can be geometrically represented as an ellipsoid with three perpendicular axes, Kl 2 K2 2 K3 [39,40]. The bulk susceptibility magnitude corresponds to K = (Kl + K2 + K3)/3. All samples measured provide K values between 2.2 X 10e5 SI and i6.1 x lo-’ SI (Fig. 3). Magnetite has never been found, an observation that agrees with the very high Fe’+/Fe,,,,, ratio in the geochemical data of Bellido et al. [37]. The low-field magnetic susceptibility of the Guitiriz granite is, therefore, mainly due to biotite, a paramagnetic Fe( + Mn)-bearing silicate commonly found in granites with low magnetic susceptibility [41,42]. The linear relationship found in the Guitiriz granite between the measured susceptibility magnitudes and the Fe*+ content [38] also supports the overall paramagnetic nature of this pluton. 4.1. Magnetic
susceptibility
and AMS
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directional
data The diamagnetic contribution of the rock matrix, mainly due to quartz and feldspar, is considered isotropic and has a constant magnitude of KD = - 1.4 X lo-’ SI [41,43]. All measurements have been corrected for this value. Fig. 3 shows that the Puebla de Parga facies has the highest susceptibilities, whereas the Mariz facies provides the lowest ones. Intermediate values are observed for the Friol facies. The magnetic zoning correlates with the petrographic zoning already established [25,37,38] and is characterized by a roughly N-S trending pattern, parallel to the elongation of the pluton as well as to the foliation trajectories. The magnetic foliation trends N-S in both the
I
4
io
iS
Kpara ( lO%I ) Fig. 3. Histograms of the bulk susceptibility magnitudes of the Guitiriz pluton in the three main petrographic facies.
Friol and the Puebla de Parga facies (Fig. 4A). Magnetic foliations are more dispersed than planar fabrics measured in the field (cf. Fig. 2 b and d). This fact probably reflects the fact that different foliation markers, potassic feldspar and biotite, respectively, are used to define field and magnetic structures. This observation is consistent with the theoretical and experimental results of Femarrdez et al. [44] on the development of subfabrics as a consequence of different shapes of structural markers. The magnetic lineations (Kl) are concordant in the different petrographic facies (Fig. 4B) and also with stretching lineations within the country rocks. The magnetic lineations are subhorizontal and trend N-S, In contrast, on the southwestern side of the pluton, within the Mariz facies, the magnetic lineations show variable trends and steeper plunges (Fig. 4B). 4.2. AMS scalar data The anisotropy and the shape of the magnetic ellipsoid can be described with the aid of several
A. Aranguren et al./ Earth and Planetary Science Letters I39 (1996) 165-176
Friol
Puebla de Parga
q Magnetic Foliations
Magnetic Lineations
Fig. 4. Magnetic structmal maps of the Guitiriz granite. (A) Magnetic foliation map (normal to K3) and stereonets of K3. (B) Magnetic lineation map and stereonets of K 1. Dots indicate sampling stations with linear magnetic fabrics. These stations group in two zones (shaded areas) located over the two granite roots located at depth by the gravity survey (see Fig. 8).
parameters defined from the values of Kl , K2 and K3 [39]. Fig. 5 indicates that the majority of AMS ellipsoids are oblate on a Jelinek [45] diagram. There is no correlation between the anisotropy degree and the shape of the AMS ellipsoid. The total anisotropy percentage due to paramagnetic minerals is defined as [39]: Ppara% = [ ((Kl - KD)/(K3
- KD)) - l] x 100
Ppara % ranges from 2.3 to 16.4 with a high mean value (7.56) in the pluton as a whole. Ppara % increases from core to contacts (Fig. 6). This feature suggests that Ppara % is strongly controlled by the solid-state deformation along the ductile shear zones developed on the eastern and western contacts (Figs. 2 and 6). Another interesting fact is that there is no correlation between the Ppara % values and the
1.0 .
.
0.
0
q
q_.#
0.5 ‘_---s2;--p--*~----___-__.
c( -R
Oblate
.
*’ a;$i$;e= . 04.-0 m r_ 0.0 D e8 *.*I . B so - _ 0 ‘. N be . a* vL--_o_---___. L -0.5 ._-_‘--a_ !z
q
Prolate
*
lMPP ~Priol =Mariz -1.0 1.00
1.10
1.05
1.15
1.20
P=Kl/K3
Fig. 5. Diagram of the shape parameter(T) of Jelinek 14.5) versus the bulk anisotropy magnetic fabrics.
(PI showing
the predominance
of oblate
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(Fig. 4). Such corridors probably reflect the locations of two root zones at depth, even it they do not correspond to the sites with the steeper foliations as proposed in previous works [46]. Given that the present erosion level is close to the granite roof, the linear fabrics would correspond to former near-vertical flow lines reoriented as a result of stresses imposed by the subsequent magma supply.
1
Fig. 6. Variation in the bulk anisotropy (Ppara %) of the magnetic susceptibility ellipsoid across the Guitiriz pluton. High anisotropy values are systematically found along the borders of the granitic body, pointing to a magmatic flow of planar type, induced by the edge effect of the country rocks.
petrographic facies. Linear magnetic fabrics are concentrated in two E-W trending corridors in the northern and southern parts of the Guitiriz pluton
5. Gravity measurements We conducted a detailed gravity survey over the region around the Guitiriz granite. The survey produced 1058 new gravity measurements with a density coverage greater than 1 point per square kilometre. We used a Lacoste-Romberg Model G gravity meter, with a precision of fO.O1 mGa1. Elevations -r
-
43’10
J. 43’00
Fig. 7. (A) Bouguer gravity anomaly map of the region studied. A schematic geological map is shown which delineates the Valdoviiio and the Vivero faults and the main granitic bodies (see Fig. 1). The Guitiriz granite appears as a depression, with positive values of the Bouguer anomaly (from +25 to +5 mgai). (B) Map of the floor in the Guitiriz granitic pluton. Note the overall small depth to the floor at the southern part of the massif based on inversion of gravity data.
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were determined using elevation benchmarks, where available, or a precision baro-altimeter, with a precision of + 0.5 m, equivalent to * 0.1 mGa1. Bouguer, elevation, latitude and topography corrections were applied. We used a reduction density of 2.70 g/cm3 to perform the gravity corrections and used the international system of gravity. Earlier maps show a bulk difference of - 5.5 mGa1, due to their density correction value (2.67 g/cm3). In order to map the zone on a larger scale, the data from both preceding surveys were digitized and reduced to the level adopted for the present survey. The manually contoured Bouguer anomaly map emphasizes the main features related to the granite intrusions. Two major lows correspond to the Guitiriz and Hombreiro granites and a smaller one relates to the Lugo granite (Fig. 7A). The effect of the regional anomaly, estimated using a larger gravity map, corresponds to a gently southward-dipping plane. The estimated gradient to the southwest is 0.66 mGal/km, whereas the constant value is 7 mGa1 at a basis point of coordinates 600E and 4762N. This could correspond to the contact of continental Spanish microplate with the adjacent Bay of Biscay oceanic crust. Density measurements were carried out on selected samples considered to be characteristic of each facies using the three weights method. The
estimated precision on density measurements is variable. It mainly depends on the quality of the samples, and on porosity related to alteration. A three-dimensional inversion was performed, using the iterative technique modified from Cordell and Henderson [47], to determine the shape at depth on granitic intrusions. The results are presented in a map indicating the depth contours of the granitic units (Fig. 7B). The estimated precision on the shape of that map depends on the average spacing of gravity measurements. Since we deal with homogeneously distributed measurements, we estimate the resolution of the contour map to be better than 1 km. The precision at depth depends mainly on how closely the density measurements of surface samples represent deeper rocks. The gravity traverses across the pluton and the neighbouring country rocks show that the Guitiriz granitic pluton is at the centre of a gravity low with minimum values of about 5 mgal (Fig. 7A). Estimates of the depth to the base of the pluton, computed from the gravity data inversion, vary from < 500 m to > 4 km. The southern part of this granitic massif takes the shape of a thin sheet, with a thickness lower than 500 m, bounded by a flat basal contact. In contrast, its northern part shows two narrow, E-W trending conduits, where the pluton reaches its maximum 3.5 km to > 4.0 km depth
OLLO DE SAP0 DOMAIN
Fig. 8. Tectonic escape model proposed for the emplacement of the syntectonic Guitiriz pluton. This model is used to account for: (1) the wedge shape of the 0110 de Sapo domain; (2) the setting of the Guitiriz pluton in back of this crustal wedge and (3) the relative distribution of the D, structures within the adjacent 0110 de Sapo and Mondoikdo nappe domains. I = D, thrust faults; 2 = D, folds; 3 = D, cleavage; 4 = extensional shear zone; 5 = principal axes of the regional stress field; 6 = northwards motion of the 0110 de Sapo crustal wedge; 7 = N-S elongated magmatic chamber with an E-W trending root zone.
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(Fig. 7B). This 3-D geometry agrees with structural data (Fig. 4), since low-dipping foliations predominate at the southern part of the massif and steeper foliations concentrate at the norther part. Such conduits would correspond to feeder conduits and are interpreted as tension cracks within the country rocks, since they are perpendicular to the trend of D, folds, the main compressional structures developed in the country rocks during the granite emplacement (Fig. 8).
6. Emplacement
model
Gravity values suggest a sheeted geometry at the eastern and southern edges of the Guitiriz massif and a thicker mass of granite towards the northwest. This shape also implies differences in the orientation of the contact, which dips gently within the southern and eastern sectors and steeply along the western half (Fig. 7B). Any model for the emplacement of the Guitiriz pluton must take into account the following features: 1. structures of granitic rocks are concordant with the N-S trending D, structures in the wall-rocks, and foliation triple points are observed around the northern and southern tips of the pluton (Figs. 2 and 4); 2. solid-state deformation is concentrated in N-S trending strike-slip shear zones which are kinematically compatible with block-bounding shear zones related to D, deformation; 3. the Guitiriz pluton intruded into the OSD, where D, strain is more pronounced than in the adjacent Mondohedo Nappe Domain; 4. the Mondoiiedo Nappe Domain was also intruded by syntectonic granites (Hombreiro, Vivero; Fig. 1) which are petrologically similar to the Guitiriz granite. According to available radiometric data, emplacement ages of c. 310-3 15 Ma have been proposed from @Ar/ 39Ar dating of muscovite from these syntectonic granites [48]. While the first point demonstrates that the emplacement of the Guitiriz pluton is syntectonic with respect to D, deformation, the second one suggests that the granitic massif acted as an anisotropy that partitioned D, strain around the pluton. D, deformation corresponds to a retrometamor-
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phic phase subsequent to a crustal thickening event accomplished during D, deformation. As a result of E-directed thrusting, an antiformal stack of thrust sheets developed below the Mondofiedo Nappe [29,32]. The OSD lies at the rear of this antiformal stack (Fig. 1; cross-section). Theoretical and physical studies of thrust mechanics usually apply the critical-tapered, Coulomb wedge model to explain the formation of thrust wedges [49,50]. According to this model, when a thrust wedge attains a critical taper, the rear of the wedge undergoes strain hardening [5 l] and the subsequent deformation of the wedge involves the motion of the wedge along a basal thrust fault [50]. The Coulomb wedge theory provides a good explanation for the widespread occurrence of S, cleavage within the OSD and the scarcity of D, structures in the MondoZedo Nappe Domain (Fig. 1B). The formation of strike-slip shear zones associated with D, deformation can be explained in the same way, since they are also concentrated in the OSD. We propose a tectonic model dominated by an E-W compressional stress field in relation to the present day orientation of this sector of the Hercynian Orogen. Using this model the emplacement of the Guitiriz pluton and the main structures of both the OSD and the Mondofiedo Nappe domain can be understood (Fig. 8). In the first step, crustal shortening was accommodated by folding and thrusting related to D, and D, structures, respectively. The appearance of an antiformal stack below the Mondoiiedo Nappe Domain is a key element in this model, since it controls the style of subsequent deformations. At this new stage, crustal shortening was accomplished by D, folds in the rear (OSD) of the antiformal stack and by thrusting along basal decollements in the crustal wedge. This implies that D, and the last D, structures developed simultaneously in different parts of the orogen (Fig. 8). Finally, when crustal shortening could not be absorbed by D, folding and the strength to compressional stresses within the OSD was exceeded, D, shear zones, the Vivero and ValdoviEo faults, developed (Figs. 1 and 8). Th’IS model requires equivalent ages for the D, deformation in the OSD and the latethrusting in the Mondosedo Nappe Domain. The wedge geometry shown by the OSD, as well as the combined motion along the Valdoviiio and
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Vivero faults, suggest that this domain acted as a crustal block, translated to the north by tectonic escape induced by the regional E-W compressional stresses (Fig. 8). The Guitiriz pluton intruded just in the neck of the crustal wedge, where a local N-S trending extensional regime is expected in response to the overall northwards displacement of the whole crustal wedge. The emplacement of the Guitiriz pluton can be explained as a permitted intrusion in such an extensional context. The E-W elongated deep conduits detected by our gravity data agree with this extensional context, because they can be seen as magmatic roots which use tensional fractures (Fig. 7B and Fig. 8). This emplacement of the Guitiriz granite in a magma chamber, developed during northward tectonic escape, is consistent with the most recent tectonic models for the Variscan belt of Iberia [52], since it postulates the beginning of the tectonic escape process due to the development of the Ibero-Armorican arc during the middle Carboniferous ( = 320 Ma). Intrusion in active extensional shear zones has been recognized as a mechanism of granite emplacement [9]. The originality of our model for the Guitiriz granite emplacement is that the N-S extensional deformation is subordinated to an overall E-W compressional field stress. This tectonic setting has several other implications because the maintenance of E-W trending compressional stresses after the intrusion of the Guitiriz pluton produced the folding of the easternmost part of the massif, where the granite has a sheeted shape, and the development of the two strike-slip shear zones which bounded the pluton.
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development of two N-S trending ductile shear zones. The structures and kinematics of the country rocks are compatible with a regional E-W compressional stress field, leading to crustal thickening. The proposed emplacement model suggests that, during prolonged contractional conditions, a crustal wedge was generated by northwards tectonic escape in the rear of the thickened region. The Guitiriz pluton was intruded into the back of the crustal wedge, where local N-S extensional conditions prevailed. Gravity results are compatible with this interpretation, since two E-W directed deep zones parallel to the compression direction have been observed. These are interpreted as the magma feeding zones. One major point of the interpretation is that both gravity and AMS data were necessary to infer the mode of emplacement of the massif. Whereas structural, field and AMS data indicate a bulk N-S direction, gravity data provide information on deeper structures. Neither technique used by itself could have provided all the data necessary to infer the emplacement mechanism.
Acknowledgements This work was financed by the research project UPV 121.3 IO-EB 182/92, of the Universidad de1 Pais Vasco. We thank K. Benn, E. Duebendotfer, K.E. Karlstrom and an anonymous reviewer for careful reviews and useful suggestions for improvement of the manuscript. IRV]
References 7. Conclusions [I] W.S. a structural, magnetic susceptibility and gravity study, we propose that the Guitiriz granitic pluton (Variscan belt, northern Spain) is a synkinematic granite relative to D, E-W shortening. Its emplacement was controlled by N-S extension in an overall compressional stress field. The structural and magnetic observations systematically show N-S trending lineations, representing the local flow direction during magma emplacement. Near the edges of the pluton pre-full crystallization fabrics are replaced by solid-state deformational fabrics related to the From
[2]
[3]
[4] [5]
Pitcher, Granite type and tectonic environment, in: Mountain Building Processes, K.J. Hsu, ed., pp. 19-40, Academic Press, 1982. P. Guillet, J.L. Bouchez and J.L. Vigneresse, Le granite de Plouaret (Bretagne): mise en evidence structurale et gravimttrique de diapirs emboites, Bull. Sot. GCol. Fr. I. 503-513, 1985. A. Castro, Structural pattern and ascent model in the Central Extremadura batholith, Hercynian belt, Spain. J. Struct. Geol. 8. 633-645, 1986. D.H.W. Hutton and R.J. Reavy, Strike-slip tectonics and granite petrogenesis, Tectonics I I, 960-967. 1992. O.T. Tobisch and S.R. Paterson, The Yarra granite: An intradeformational pluton associated with ductile thrusting,
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Lachlan Fold BeIt, southeastern Australia, Geol. Sot. Am. Bull. 102, 693-703, 1990. [6] A. Aranguren and J.M. Tubia, Structural evidence for the relationship between thrusts, extensional faults and granite intrusions in the Variscan belt of Galicia (Spain), J. Struct. Geol. 14, 1229- 1237, 1992. [7] K.E. Karlstrom, CF. Miller, J.A. Kingsbury and J.L. Wooden, Phtton emplacement along an active ductile thrust zone, Piute Mountains, southeastern California: Interaction between deformational and solidification processes, Geol. Sot. Am. Bull. 105, 213-230, 1993. [8] D.H.W. Hutton, Granite emplacement mechanism and tectonic controls: inferences from deformation studies, Trans. R. Sot. Edinburgh 79, 245-255, 1988. [9] D.H.W. Hutton, T.J. Dumpster, P.E. Brown and S.D. Becker, A new mechanism of granite emplacement: intrusion in active extensional shear zones, Nature 343, 452-455, 1990. [lo] J.P. Bouillin, J.L. Bouchez, P. Lespinasse and A. P&her, Granite emplacement in an extensional setting: an AMS study of the magmatic structures of Monte Capanne (Elba, Italy), Earth Planet. Sci. Lett. 118, 263-279, 1993. [ 1 l] A.A. Arzi, Critical phenomena in the rheology of partially melted rocks, Tectonophysics 44, 173- 184, 1978. [ 121 I. Van der Molen and M.S. Paterson, Experimental deformation of partially melted granite, Contrib. Mineral. Petrol. 70, 299-318, 1979. [13] A.R. McBimey and T. Murase, Rheological properties of magmas, Annu. Rev. Earth Planet. Sci. 12, 337-357, 1984. [14] H. Ramberg, Model studies in relation to intrusion of plutonic bodies, Geol. J. 2, 261-286, 1970. [ 151 B.D. Marsh, Gn the mechanics of igneous diapirism, stoping and zone melting, Am. J. Sci. 282, 808-855, 1982. [ 161 J.P. Brun, L’origine des domes gneissiques: modules et tests, Bull. Sot. GCol. Fr. 7, 219-228, 1983. [17] R. Bateman. On the role of diapirism in the segregation, ascent and final emplacement of granitoids, Tectonophysics I IO, 211-231, 1984. [I81 B. Guineberteau, J.L. Bouchez and J.L. Vigneresse, The Mortagne granite pluton (France) emplaced by pull-apart along a shear zone: structural and gravimetric arguments and regional implications, Geol. Sot. Am. Bull. 99, 763-770, 1987. 1191 S.R. Paterson, R.H. Vernon and O.T. Tobisch, A review of criteria for the identification of magmatic and tectonic foliations in granitoids, J. Struct. Geol. 1 1, 349-363, 1989. 1201 F. Heller, Magnetic anisotropy of granitic rocks of the Bergell massif (Switzerland), Earth Planet. Sci. Lett. 20, 180-188, 1973. [21] P. Guillet, J.L. Bouchez and J.J. Wagner, Anisotropy of magnetic susceptibility and magnetic structures in the Guerande granite massif (France), Tectonics 2, 419-429, 1983. [22] LB. Ramberg, Gravity interpretation of the Oslo graben and associated igneous rocks, Norsge Geol. Undersokelse 325, 194 pp. (231 J.L. Vigneresse, Use and misuse of geophysical data to
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25, 249-260, 1990. [24] P. Matte, La structure de la virgation hercynienne de Galice (espagne), Rev. Gtol. Alp. 44, l- 128. 1968. [25] A. Aranguren and J.M. Tubia, Caracterfsticas estructurales y modelo de emplazamiento de1 pluton de Guitiriz, Rev. Sot. Geol. Espaiia 7, 63-73, 1994. [26] M. Julivert, J.M. Fontbotk, A. Ribeiro and L. Conde, Mapa tecdnico de la Peninsula Ibtrica y Baleares, Inst. Geol. Min. Espaha, Madrid, 1972. [27] J.P. Burg, M. Iglesias, P. Laurent, P. Matte and A. Ribeiro, Variscan intracontinental deformation: the CoimbraC6rdoba shear zone (SW Iberian peninsula), Tectonophysics 78, 161177, 1981. [28] J.R. Martinez-Cata& F. Gonzalez-Lodeiro. M. lglesias and M.A. Dfez-Balda, La estructura del Domo de Lugo y de1 anticlinorio de1 0110 de Sapo, Stud. Geol. Univ. Salamanca 12, 109-122, 1977. [29] F. Bastida, J.R. Martinez-Catalan and J.A. Pulgar, Structural, metamorphic and magmatic history of the Mondofiedo nappe (Hercynian belt, NW Spam), J. Struct. Geol. 8, 415-430, 1986. [30] R. Arenas, Opposite P, T, t paths of the Hercynian metamorphism between the upper units of the Cabo Ortega1 Complex and their substratum (northwest of the lberian Massif), Tectonophysics 191, 347-364, 1991. [31] J. Aller and F. Bastida, Anatomy of the Mondohedo Nappe basal shear zone (NW Spain), J. Struct. Geol. 15, 1405-1419, 1993. [32] A. Perez-Estaun, J.R. Martinez-Catalan and F. Bastida, Crustal thickening and deformation sequence in the footwall to the suture of the Variscan belt of northwest Spain, Tectonophysics 191, 243-253, 1991. 1331 A. Perez Estaun, J.A. Pulgar, E. Banda and J. Alvarez Marron, ESCI-N Research Group, Crustal structure of the external variscides in northwest Spain from deep seismic reflection profiling, Tectonophysics 232, 91 -I 18, 1994. [34] J. Aller, H.J. Zeyen, A. Perez Estaun, J.A. Pulgar and J.M. Pares, A 2.5D interpretation of the eastern Galicia magnetic anomaly (northwestern Spain): geodynamical implications, Tectonophysics 237, 201-213, 1994. [35] G. Courrioux, Exemple de mise en place d’un leucogranite pendant le foncionnement d’une zone de cisaillement: le granite hercynien de Puentedeume (Galice, Espagne), Bull. Sot. GCol. Fr. 7. 301-307, 1983. [36] R. Capdevila and P. Floor, Les differents types de granites hercyniens et leur distribution darts le Nord-Ouest de I’Espagne, Bol. Geol. Min. 81, 215-225, 1970. [37] F. Bellido, J. Garcia Garz6n and J. Reyes, Estudio petroldgico y geocronologico (Rb-Sr) de 10s granitoides de Friol y Puebla de Parga (NO de Lugo), Bol. Geol. Min. Espalia 101, 621-631, 1990. 1381 A. Aranguren, Estructura y cinemitica de1 emplazamiento de 10s granitoides de1 domo de Lugo y de1 antiforme del 0110 de Sapo, Lab. Xeoloxico de Laxe, Ser. Nova Terra 10, 237 pp., 1994.
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[47] L. Cordell and R.G. Henderson, Iterative three dimensional
in geology and geophysics, Geophys. Surv. 5, 37-82, 1982. [401 G.J. Borradaile, Magnetic susceptibility, petrofabrics and strain, Tectonophysics 1.56, l-20, 1988. 1411P. Rochette. Magnetic susceptibility of the rock matrix telated to magnetic fabric studies, J. Struct. Geol. 9, lOl51020, 1987. [42] 0. Jover. P. Rochette, J.P. Lorand, M. Maeder and J.L. Bouchez. Magnetic mineralogy of some granites from the French Massif Central: origin of their low-field susceptibility, Phys. Earth Planet. Inter. 55, 79-92, 1989. [43] F. Hrouda, The effect of quartz on the magnetic anisotropy of quartzite, Stud. Geophys. Geod. 30, 39-45, 1986. [44] A. Fem&tdez, J.L. Feybesse and J.F. Mezure, Theoretical and experimental study of fabrics developed by different shaped markers in two dimensional simple shear, Bull. See. Gbol. Fr. 3, 157-158, 1983. [45] V. Jelinek, Characterization of the magnetic fabric of rocks, Tectonophysics 79, 563-567, I98 I. [46] M. Amice and J.L. Bouchez, Susceptibility magnttique et zonation du bathohte granitique de Cabeza de Araya (Extremadura, Espagne), C.R. Acad. Sci. Paris 308, I I71 - I 178, 1989.
solution of gravity anomaly using a digital computer, Geophysics 33, 5%~601, 1968. [48] R.D. Dallmeyer, J.I. Gil Ibarguchi and A. Ribeiro, Age, palinspatic derivation and tectonothermal evolution of exotic terranes in northwest Iberia, in: IGCP 233 Int. Conf. Paleozoic Grogens in Central Europe, Abstract Vol., pp. 46-49, 1990. [49] W.L. Zhao, D.M. Davis, F.A. Dahlen and J. Suppe. Origin of convex accretionary wedges: Evidence from Barbados, J. Geophys. Res. 91, 10246- 10258, 1986. [50] F.A. Dahlen. Critical taper model of fold-and-thrust belts and accretionary wedges, Annu. Rev. Earth. Planet. Sci. 18, 55-99, 1990. 1511 G. Mulugeta, Modelling the geometry of Coulomb tbtust wedges, J. Struct. Geol. IO, 847-859, 1988. [52] J.R. Martinez Catalan, A non-cylindrical model for the northwest Iberian allochthonous terranes and their equivalents in the Hercynian belt of Western Europe, Tectonophysics 179, 253-272. 1990.
Earth and Planetary
Science Letters 139 ( 1996) 165- 176
The Guitiriz granite, Variscan belt of northern Spain: extension-controlled emplacement of magma during tectonic escape Aitor Aranguren a**, Jo& Maria Tubia a, Jean Luc Bouchez b, Jean Louis Vigneresse ’ a Departamento b Laboratoire
de Geodindmica,
Universidad
de Pe’trophysique. URA CNRS no. 67, Universiti ’ CREGU
de1 Pais Vasco, Ap. 644, Bilbao 48080, Spain Paul Sabatier, 38 rue des 36 Ponts, F-31400
and ENS Gt!ologie Nancy, BP23, F-54501
Received 2 August
Toulouse, France
Vandoeuvre / Nancy, France
1995; accepted 4 December
1995
Abstract The 0110 de Sapo domain of the northern part of the Variscan belt of Spain, contains Precambrian and Ordovician metamorphic rocks intruded by the Guitiriz granite. The domain is bounded by two N-S transcurrent shear zones. Detailed structural and anisotropy of magnetic susceptibility studies of the Guitiriz pluton reveal a syntectonic emplacement, relative to movement on the two N-S trending bounding shear zones, characterized by: (1) development of N-S trending structural and magnetic fabrics; (2) concordant structures in granites and country rocks; and (3) development of shear zones along the east and west granite margins. Gravity data show two roots trending 110”E extending down to 4.5 km from an otherwise thin (< 3 km), slab-like body. The Guitiriz pluton was emplaced in a magma chamber developed by local N-S extension induced by an overall E-W shortening. The proposed emplacement model involves the northwards tectonic escape of a crustal wedge - the 0110 de Sapo Domain - bounded by two shear zones acting as conjugate strike-slip zones. Keywords:
0110 de Sapo domain;
Hercynian
Orogeny; granite; extension; magnetic susceptibility
1. Introduction Granitic plutons intrude in different erogenic contexts [ 11, including transcurrent [2-41, contractional [5-71 and extensional [g-lo] settings. Studies of granitic plutons often use petrological and geochemical approaches to provide information about the nature and origin of the granitic magmas. However, this type of study does not shed light on emplacement mechanisms, another key question about gran-
* Corresponding author. Tel: 34 4 464 77 00; Fax: 34 4 464 85 00, E-mail: [email protected]
0012-821X/96/$12.00 0 1996 Elsevier Science B.V. All rights reserved SSDl 0012-821X(95)00239-1
ites. Studies of the rheology of partial melts [l l-131, laboratory models on the interaction between soft materials bounded by rigid walls, and models on migration and deformation of diapiric bodies [ 14- 171 also help to constrain the kinematic and dynamic behaviour of granitic magmas, but field structural studies are still needed to constrain pluton emplacement models. The deformation of the country rocks controls the shape and emplacement of granites during intrusion [8]. Consequently, structures from both the granite and the country rocks are required to establish emplacement models. The granite provides information
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only about the microstructures formed during crystallization and subsequent deformation (e.g., magmatic versus solid-state deformation fabrics [ 18,193). The anisotropy of magnetic susceptibility CAMS) makes it possible to detect otherwise invisible foliations and lineations in granites [ 10,20,2 11. Gravity surveys provide the most efficient data for investigating the shape of plutons at depth [22,23]. These are complementary geophysical approaches, since gravity data give us the three-dimensional shape of the pluton whereas AMS provides a two-dimensional picture of its internal structure. This paper presents the results of a combined structural, AMS and gravity study of the Guitiriz pluton, in northern Spain (Fig. 1B). This multidisciplinary approach is used to determine the three-dimensional geometry and the emplacement history of
Science Letters 139 (1996) 165-176
the Guitiriz pluton. This pluton is of great interest because it is located at the edge of the IberoArmorican Arc, where several ductile shear zones have long been recognized [24] (Fig. 1). This area provides a good opportunity for analysing the relationships between the development of shear zones and the emplacement of granitic rocks.
2. Geological setting The synorogenic Guitiriz granite [25] intrudes Ordovician and Silurian schists and Precambrian metavolcanic rocks of the northern part of the 0110 de Sapo Domain (OSD). This N-S trending, 200 km long domain belongs to the Central Iberian Zone of the Iberian Massif 126,271(Fig. 1). The OSD records
7&l
[vivero[
7
Mondoiiedo she ar zone
1
200
II H
IiI $$$:: lIzI
km
Cabo Ortega1 Complex (C.O.C.) 0110 de Sapo Domain a)Lower Palaeozoic b) Precambrian (0110 de Saps) Mondofiedo Nappe Domain a)Lower Palaeozoic b) Precambrian Synkinematic Granitoids
++++ Late-postkinematic Granitoids lzl Fig. 1. The Guitiriz granite in northern Spain. (A) Location of the studied area in the frame of the Ibero-Armorican arc (from Burg et al. [27]). C.C.S.Z. = Coimbra-Cbrdoba shear zone; N.A.S.Z. = North Armorican shear zone; S.A.S.Z.= South Armorican shear zone. (B) Geological sketch map of the Variscan belt in northern Spain. We Guitiriz granite is outlined. The cross-section shows the main structural features of the area studied (simplified after Martinez-Cat&in et al. [28] and Bastida et al. 1301).
A. Aranguren et al./Earth
and Planetary Science Letters 139 (1996) 16S-I76
three main phases of deformation, which form part of a progressive deformation process developed at mid to shallow crustal levels (ca. 15-9 km deep) in response to Variscan continental collision and crustal thickening [24,28]. The earliest deformation led to recumbent, E-vergent folds. The second phase (D,) resulted in E-directed overthrusts, such as the Mondoiiedo nappe or the allochthonous Cabo Ortega1 complex [29] (Fig. 1). Finally, the third phase of deformation (D,) caused a penetrative cleavage related to E-vergent inclined folds. D, is a retrograde phase and medium pressure metamorphism reached amphibolite facies conditions (600°C; 5OOMPa) during D, near the Guitiriz granite [30]. The structure of the OSD contrasts with that of the adjacent Mondoiledo Nappe Domain, where only a weak D, deformation, represented by open folds, overprint broad D, shear zones [30,31] (Fig. 1). As suggested by previous works [30,32], the Mondoiiedo Nappe is
167
at upper levels of a crustal wedge formed by an antiformal stack of E-vergent thrust sheets. This interpretation has been confirmed by seismic studies recently completed in this part of the Iberian Variscan belt [33] and also by the analysis of the pronounced magnetic anomaly found in eastern Galicia 1341.The OSD is located at the rear of the tectonic crustal wedge. In the region studied the OSD is bounded by two major, roughly N-S trending, ductile shear zones. The Valdoviilo fault, to the west, is a sinistral strike-slip fault [35], whereas the Vivero fault, to the east (Fig. l), is a dextral, transtensional shear zone [28,30]. These block-boundary shear zones cut the metamorphic isograds of the 0110 de Sapo and the Mondoiiedo Nappe domains, respectively. These two faults converge towards the south, narrowing the OSD from a width of about 45 km, along the coastal traverse, down to 8 km at the southern end of the
LEGEND Trajectories of the magmatic foliation\ Trajectories of the myloniticfoliation \ m
Mariz (two-mica fine grained leucogranitej
stretchinglineation ---_----_ poles to C-planes
------------------
StNCtUreS
lineation poles to foliation
(9
;olld-state deformation
Trajectories of the S3 - schistosity \
I
magmatlc structures
Fig. 2. Structural map and petrographic zoning of the Guitiriz pluton. The trajectories of mylonitic foliation underline the two ductile, strike-slip shear zones developed along the eastern and western edges of the Guitiriz pluton. Orientation data of magmatic structures and solid-state deformation structures are shown on the corresponding stereograms (Schmidt lower hemisphere projection).
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and Planetary Science Letters I39 (1996) 165-176
Guitiriz pluton (Fig. 1). However, they are not connected and one is not a second order of the other. The Guitiriz pluton shows two main facies (Fig. 2). The Puebla de Parga facies is composed of porphyritic biotite granite, with potassic feldspar porphyrocrysts (2-5 cm). The Friol facies is composed of two-mica granite with a medium to coarse-grained equigranular texture. They were initially considered as two different petrogenetic series of different ages [36]. However, our field criteria show that the transition between facies is gradual [25]. A co-genetic relationship is supported by the recent petrological and geochemical data of Bellido Mulas et al. [37]. A third facies, the Mariz facies, occurs in the southwestern part of the pluton. It is a two-mica, finegrained leucogranite. It post-dates the Friol and Puebla de Parga facies, as evidenced from conventional field evidence (sharp contacts and presence of enclaves of the other rock types), but it is still broadly co-genetic with the other two facies [37]. The Guitiriz pluton intruded at ca. 318.7 k 4.2 Ma, according to whole rock Rb/Sr isotopic dating [37].
3. Field structures At map scale, the structure of the country rocks is dominated by D, folds. They have kink geometries and show a vertical to steeply west-dipping axial planar cleavage. Fold axes are subhorizontal and trend N-S. Cleavage trajectories, which usually parallel the elongation direction of the Guitiriz pluton, are disturbed at the northern and southern edges of the granitic body, where cleavage triple points developed (Fig. 2). Such triple points represent the interaction of the regional stress field, with the local stress field related to the magma intrusion, as proposed by theoretical studies about the dynamics of granitic domes [16]. Wall rock deformation concentrates in two shear zones dipping to the west. The shear zones contain a subhorizontal and N-S trending stretching lineation and show opposite shear senses (Fig. 2). These shear senses are like those documented within the adjacent block-boundary shear zones (compare Figs. 1 and 2). Consequently, they can also be interpreted as boundary faults at local scale [38].
3.1. Structures in the Guitiriz granite The granite shows a steeply dipping, N-S striking, magmatic foliation that is parallel to the main planar structure (S,) of the country rocks (Fig. 2). This orientation deviates only in the southern part of the pluton, where flat-dipping foliations predominate. This suggests a flat shape for that part of the pluton. The foliation in the granite was produced by magmatic flow, as evidenced by the following microstructural observations: (I) the foliation is defined by the parallel arrangement of unstrained subhedral micas and feldspars; (2) quartz aggregates display equant shapes and are composed of large, optically strain-free grains lacking lattice-preferred orientations. The two main facies (Puebla de Parga and Friol) of the Guitiriz pluton have similar orientation patterns of foliations and lineations (Fig. 2). Solid-state deformation structures progressively replace magmatic structures on the eastern and western borders. Solid-state deformation features include widespread S-C structures, grain size reduction and elongation of quartz aggregates. C planes and shear senses are compatible with those of shear bands developed in the adjacent wall rocks (Fig. 2). Both the magmatic and solid-state foliations carry subhorizontal and N-S trending lineations (Fig. 2). The southernmost part of the pluton and its country rocks contain shear bands that dip approximately 10”N. These shear bands also carry N-S stretching lineations and are systematically related to top-to-the-north motions. These structural features suggest that the igneous rocks and its country rocks have shared a common deformational history during pluton emplacement, involving strike-slip shearing and a component of northward extensional deformation in the crustal domain comprised between these shear zones.
4. Magnetic susceptibility AMS allows us to detect systematic changes of several magnetic parameters [39,40] which provide valuable information on kinematic reconstructions of granite emplacement [ 10,20,2 11. Magnetic susceptibility and anisotropy measurements of oriented sam-
A. Aranpren
er al./Earrh
ples from the Guitiriz pluton were performed with a MINISEP (MOLSPIN) susceptometer, operating at + 10 kHz in a low magnetic field of 7 X 10M4 T, with a precision of 5 X 10e7 SI. Samples for this study were collected from 112 stations. In order to avoid local perturbations of the magnetic properties, at least four oriented samples per station were taken using the same procedure as that used for paleomagnetic analyses. A sample subjected to an external magnetic field, H, develops an induced magnetic moment, J. The magnetic susceptibility, K, is a second-rank tensor which relates these two vectorial magnetic properties, and can be geometrically represented as an ellipsoid with three perpendicular axes, Kl 2 K2 2 K3 [39,40]. The bulk susceptibility magnitude corresponds to K = (Kl + K2 + K3)/3. All samples measured provide K values between 2.2 X 10e5 SI and i6.1 x lo-’ SI (Fig. 3). Magnetite has never been found, an observation that agrees with the very high Fe’+/Fe,,,,, ratio in the geochemical data of Bellido et al. [37]. The low-field magnetic susceptibility of the Guitiriz granite is, therefore, mainly due to biotite, a paramagnetic Fe( + Mn)-bearing silicate commonly found in granites with low magnetic susceptibility [41,42]. The linear relationship found in the Guitiriz granite between the measured susceptibility magnitudes and the Fe*+ content [38] also supports the overall paramagnetic nature of this pluton. 4.1. Magnetic
susceptibility
and AMS
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and Planetmy Science Lerters 139 (1996) 165-176
directional
data The diamagnetic contribution of the rock matrix, mainly due to quartz and feldspar, is considered isotropic and has a constant magnitude of KD = - 1.4 X lo-’ SI [41,43]. All measurements have been corrected for this value. Fig. 3 shows that the Puebla de Parga facies has the highest susceptibilities, whereas the Mariz facies provides the lowest ones. Intermediate values are observed for the Friol facies. The magnetic zoning correlates with the petrographic zoning already established [25,37,38] and is characterized by a roughly N-S trending pattern, parallel to the elongation of the pluton as well as to the foliation trajectories. The magnetic foliation trends N-S in both the
I
4
io
iS
Kpara ( lO%I ) Fig. 3. Histograms of the bulk susceptibility magnitudes of the Guitiriz pluton in the three main petrographic facies.
Friol and the Puebla de Parga facies (Fig. 4A). Magnetic foliations are more dispersed than planar fabrics measured in the field (cf. Fig. 2 b and d). This fact probably reflects the fact that different foliation markers, potassic feldspar and biotite, respectively, are used to define field and magnetic structures. This observation is consistent with the theoretical and experimental results of Femarrdez et al. [44] on the development of subfabrics as a consequence of different shapes of structural markers. The magnetic lineations (Kl) are concordant in the different petrographic facies (Fig. 4B) and also with stretching lineations within the country rocks. The magnetic lineations are subhorizontal and trend N-S, In contrast, on the southwestern side of the pluton, within the Mariz facies, the magnetic lineations show variable trends and steeper plunges (Fig. 4B). 4.2. AMS scalar data The anisotropy and the shape of the magnetic ellipsoid can be described with the aid of several
A. Aranguren et al./ Earth and Planetary Science Letters I39 (1996) 165-176
Friol
Puebla de Parga
q Magnetic Foliations
Magnetic Lineations
Fig. 4. Magnetic structmal maps of the Guitiriz granite. (A) Magnetic foliation map (normal to K3) and stereonets of K3. (B) Magnetic lineation map and stereonets of K 1. Dots indicate sampling stations with linear magnetic fabrics. These stations group in two zones (shaded areas) located over the two granite roots located at depth by the gravity survey (see Fig. 8).
parameters defined from the values of Kl , K2 and K3 [39]. Fig. 5 indicates that the majority of AMS ellipsoids are oblate on a Jelinek [45] diagram. There is no correlation between the anisotropy degree and the shape of the AMS ellipsoid. The total anisotropy percentage due to paramagnetic minerals is defined as [39]: Ppara% = [ ((Kl - KD)/(K3
- KD)) - l] x 100
Ppara % ranges from 2.3 to 16.4 with a high mean value (7.56) in the pluton as a whole. Ppara % increases from core to contacts (Fig. 6). This feature suggests that Ppara % is strongly controlled by the solid-state deformation along the ductile shear zones developed on the eastern and western contacts (Figs. 2 and 6). Another interesting fact is that there is no correlation between the Ppara % values and the
1.0 .
.
0.
0
q
q_.#
0.5 ‘_---s2;--p--*~----___-__.
c( -R
Oblate
.
*’ a;$i$;e= . 04.-0 m r_ 0.0 D e8 *.*I . B so - _ 0 ‘. N be . a* vL--_o_---___. L -0.5 ._-_‘--a_ !z
q
Prolate
*
lMPP ~Priol =Mariz -1.0 1.00
1.10
1.05
1.15
1.20
P=Kl/K3
Fig. 5. Diagram of the shape parameter(T) of Jelinek 14.5) versus the bulk anisotropy magnetic fabrics.
(PI showing
the predominance
of oblate
A. Aranguren et al./Earth
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I:, p. t \ .:
at
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Ppara% (l-l’)
1
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/-
-*\ ‘\./
I
I
.
:;
1
76 12
1. ‘.
(2-2’) -.-.A
.-•
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Science Letters 139 (19961 165-176
(Fig. 4). Such corridors probably reflect the locations of two root zones at depth, even it they do not correspond to the sites with the steeper foliations as proposed in previous works [46]. Given that the present erosion level is close to the granite roof, the linear fabrics would correspond to former near-vertical flow lines reoriented as a result of stresses imposed by the subsequent magma supply.
1
Fig. 6. Variation in the bulk anisotropy (Ppara %) of the magnetic susceptibility ellipsoid across the Guitiriz pluton. High anisotropy values are systematically found along the borders of the granitic body, pointing to a magmatic flow of planar type, induced by the edge effect of the country rocks.
petrographic facies. Linear magnetic fabrics are concentrated in two E-W trending corridors in the northern and southern parts of the Guitiriz pluton
5. Gravity measurements We conducted a detailed gravity survey over the region around the Guitiriz granite. The survey produced 1058 new gravity measurements with a density coverage greater than 1 point per square kilometre. We used a Lacoste-Romberg Model G gravity meter, with a precision of fO.O1 mGa1. Elevations -r
-
43’10
J. 43’00
Fig. 7. (A) Bouguer gravity anomaly map of the region studied. A schematic geological map is shown which delineates the Valdoviiio and the Vivero faults and the main granitic bodies (see Fig. 1). The Guitiriz granite appears as a depression, with positive values of the Bouguer anomaly (from +25 to +5 mgai). (B) Map of the floor in the Guitiriz granitic pluton. Note the overall small depth to the floor at the southern part of the massif based on inversion of gravity data.
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were determined using elevation benchmarks, where available, or a precision baro-altimeter, with a precision of + 0.5 m, equivalent to * 0.1 mGa1. Bouguer, elevation, latitude and topography corrections were applied. We used a reduction density of 2.70 g/cm3 to perform the gravity corrections and used the international system of gravity. Earlier maps show a bulk difference of - 5.5 mGa1, due to their density correction value (2.67 g/cm3). In order to map the zone on a larger scale, the data from both preceding surveys were digitized and reduced to the level adopted for the present survey. The manually contoured Bouguer anomaly map emphasizes the main features related to the granite intrusions. Two major lows correspond to the Guitiriz and Hombreiro granites and a smaller one relates to the Lugo granite (Fig. 7A). The effect of the regional anomaly, estimated using a larger gravity map, corresponds to a gently southward-dipping plane. The estimated gradient to the southwest is 0.66 mGal/km, whereas the constant value is 7 mGa1 at a basis point of coordinates 600E and 4762N. This could correspond to the contact of continental Spanish microplate with the adjacent Bay of Biscay oceanic crust. Density measurements were carried out on selected samples considered to be characteristic of each facies using the three weights method. The
estimated precision on density measurements is variable. It mainly depends on the quality of the samples, and on porosity related to alteration. A three-dimensional inversion was performed, using the iterative technique modified from Cordell and Henderson [47], to determine the shape at depth on granitic intrusions. The results are presented in a map indicating the depth contours of the granitic units (Fig. 7B). The estimated precision on the shape of that map depends on the average spacing of gravity measurements. Since we deal with homogeneously distributed measurements, we estimate the resolution of the contour map to be better than 1 km. The precision at depth depends mainly on how closely the density measurements of surface samples represent deeper rocks. The gravity traverses across the pluton and the neighbouring country rocks show that the Guitiriz granitic pluton is at the centre of a gravity low with minimum values of about 5 mgal (Fig. 7A). Estimates of the depth to the base of the pluton, computed from the gravity data inversion, vary from < 500 m to > 4 km. The southern part of this granitic massif takes the shape of a thin sheet, with a thickness lower than 500 m, bounded by a flat basal contact. In contrast, its northern part shows two narrow, E-W trending conduits, where the pluton reaches its maximum 3.5 km to > 4.0 km depth
OLLO DE SAP0 DOMAIN
Fig. 8. Tectonic escape model proposed for the emplacement of the syntectonic Guitiriz pluton. This model is used to account for: (1) the wedge shape of the 0110 de Sapo domain; (2) the setting of the Guitiriz pluton in back of this crustal wedge and (3) the relative distribution of the D, structures within the adjacent 0110 de Sapo and Mondoikdo nappe domains. I = D, thrust faults; 2 = D, folds; 3 = D, cleavage; 4 = extensional shear zone; 5 = principal axes of the regional stress field; 6 = northwards motion of the 0110 de Sapo crustal wedge; 7 = N-S elongated magmatic chamber with an E-W trending root zone.
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(Fig. 7B). This 3-D geometry agrees with structural data (Fig. 4), since low-dipping foliations predominate at the southern part of the massif and steeper foliations concentrate at the norther part. Such conduits would correspond to feeder conduits and are interpreted as tension cracks within the country rocks, since they are perpendicular to the trend of D, folds, the main compressional structures developed in the country rocks during the granite emplacement (Fig. 8).
6. Emplacement
model
Gravity values suggest a sheeted geometry at the eastern and southern edges of the Guitiriz massif and a thicker mass of granite towards the northwest. This shape also implies differences in the orientation of the contact, which dips gently within the southern and eastern sectors and steeply along the western half (Fig. 7B). Any model for the emplacement of the Guitiriz pluton must take into account the following features: 1. structures of granitic rocks are concordant with the N-S trending D, structures in the wall-rocks, and foliation triple points are observed around the northern and southern tips of the pluton (Figs. 2 and 4); 2. solid-state deformation is concentrated in N-S trending strike-slip shear zones which are kinematically compatible with block-bounding shear zones related to D, deformation; 3. the Guitiriz pluton intruded into the OSD, where D, strain is more pronounced than in the adjacent Mondohedo Nappe Domain; 4. the Mondoiiedo Nappe Domain was also intruded by syntectonic granites (Hombreiro, Vivero; Fig. 1) which are petrologically similar to the Guitiriz granite. According to available radiometric data, emplacement ages of c. 310-3 15 Ma have been proposed from @Ar/ 39Ar dating of muscovite from these syntectonic granites [48]. While the first point demonstrates that the emplacement of the Guitiriz pluton is syntectonic with respect to D, deformation, the second one suggests that the granitic massif acted as an anisotropy that partitioned D, strain around the pluton. D, deformation corresponds to a retrometamor-
173
phic phase subsequent to a crustal thickening event accomplished during D, deformation. As a result of E-directed thrusting, an antiformal stack of thrust sheets developed below the Mondofiedo Nappe [29,32]. The OSD lies at the rear of this antiformal stack (Fig. 1; cross-section). Theoretical and physical studies of thrust mechanics usually apply the critical-tapered, Coulomb wedge model to explain the formation of thrust wedges [49,50]. According to this model, when a thrust wedge attains a critical taper, the rear of the wedge undergoes strain hardening [5 l] and the subsequent deformation of the wedge involves the motion of the wedge along a basal thrust fault [50]. The Coulomb wedge theory provides a good explanation for the widespread occurrence of S, cleavage within the OSD and the scarcity of D, structures in the MondoZedo Nappe Domain (Fig. 1B). The formation of strike-slip shear zones associated with D, deformation can be explained in the same way, since they are also concentrated in the OSD. We propose a tectonic model dominated by an E-W compressional stress field in relation to the present day orientation of this sector of the Hercynian Orogen. Using this model the emplacement of the Guitiriz pluton and the main structures of both the OSD and the Mondofiedo Nappe domain can be understood (Fig. 8). In the first step, crustal shortening was accommodated by folding and thrusting related to D, and D, structures, respectively. The appearance of an antiformal stack below the Mondoiiedo Nappe Domain is a key element in this model, since it controls the style of subsequent deformations. At this new stage, crustal shortening was accomplished by D, folds in the rear (OSD) of the antiformal stack and by thrusting along basal decollements in the crustal wedge. This implies that D, and the last D, structures developed simultaneously in different parts of the orogen (Fig. 8). Finally, when crustal shortening could not be absorbed by D, folding and the strength to compressional stresses within the OSD was exceeded, D, shear zones, the Vivero and ValdoviEo faults, developed (Figs. 1 and 8). Th’IS model requires equivalent ages for the D, deformation in the OSD and the latethrusting in the Mondosedo Nappe Domain. The wedge geometry shown by the OSD, as well as the combined motion along the Valdoviiio and
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Vivero faults, suggest that this domain acted as a crustal block, translated to the north by tectonic escape induced by the regional E-W compressional stresses (Fig. 8). The Guitiriz pluton intruded just in the neck of the crustal wedge, where a local N-S trending extensional regime is expected in response to the overall northwards displacement of the whole crustal wedge. The emplacement of the Guitiriz pluton can be explained as a permitted intrusion in such an extensional context. The E-W elongated deep conduits detected by our gravity data agree with this extensional context, because they can be seen as magmatic roots which use tensional fractures (Fig. 7B and Fig. 8). This emplacement of the Guitiriz granite in a magma chamber, developed during northward tectonic escape, is consistent with the most recent tectonic models for the Variscan belt of Iberia [52], since it postulates the beginning of the tectonic escape process due to the development of the Ibero-Armorican arc during the middle Carboniferous ( = 320 Ma). Intrusion in active extensional shear zones has been recognized as a mechanism of granite emplacement [9]. The originality of our model for the Guitiriz granite emplacement is that the N-S extensional deformation is subordinated to an overall E-W compressional field stress. This tectonic setting has several other implications because the maintenance of E-W trending compressional stresses after the intrusion of the Guitiriz pluton produced the folding of the easternmost part of the massif, where the granite has a sheeted shape, and the development of the two strike-slip shear zones which bounded the pluton.
Science Letters 139 (1996) 165-176
development of two N-S trending ductile shear zones. The structures and kinematics of the country rocks are compatible with a regional E-W compressional stress field, leading to crustal thickening. The proposed emplacement model suggests that, during prolonged contractional conditions, a crustal wedge was generated by northwards tectonic escape in the rear of the thickened region. The Guitiriz pluton was intruded into the back of the crustal wedge, where local N-S extensional conditions prevailed. Gravity results are compatible with this interpretation, since two E-W directed deep zones parallel to the compression direction have been observed. These are interpreted as the magma feeding zones. One major point of the interpretation is that both gravity and AMS data were necessary to infer the mode of emplacement of the massif. Whereas structural, field and AMS data indicate a bulk N-S direction, gravity data provide information on deeper structures. Neither technique used by itself could have provided all the data necessary to infer the emplacement mechanism.
Acknowledgements This work was financed by the research project UPV 121.3 IO-EB 182/92, of the Universidad de1 Pais Vasco. We thank K. Benn, E. Duebendotfer, K.E. Karlstrom and an anonymous reviewer for careful reviews and useful suggestions for improvement of the manuscript. IRV]
References 7. Conclusions [I] W.S. a structural, magnetic susceptibility and gravity study, we propose that the Guitiriz granitic pluton (Variscan belt, northern Spain) is a synkinematic granite relative to D, E-W shortening. Its emplacement was controlled by N-S extension in an overall compressional stress field. The structural and magnetic observations systematically show N-S trending lineations, representing the local flow direction during magma emplacement. Near the edges of the pluton pre-full crystallization fabrics are replaced by solid-state deformational fabrics related to the From
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