Anisotropy of magnetic susceptibility of the Pyrenean granites

ABSTRACT In this paper, we report on a compilation of more than 2200 sites (more than 10,000 individual measurements) where anisotropy of magnetic susceptibility (AMS) was studied in granites from the Variscan Pyrenees. The standardization and homogenization of this information has allowed us to produce three Main Maps that synthesize all the information related with the AMS of the Pyrenean granites. We also describe the problems found during the construction of the database (variable geo-positioning, different published information, etc.). The information derived from 21 granite bodies, the database, and the synthesis maps (magnetic susceptibility, Km, and the orientation of the magnetic foliation, plane perpendicular to k3, and of the magnetic lineation, k1) allow us to see for the first time a complete image of this important kinematic and petrographic indicator.


Introduction
Anisotropy of magnetic susceptibility (AMS) is a sound and proven technique to determine the mineral-preferred orientation of rock volumes (Borradaile & Henry, 1997;Graham, 1954;Parés, 2015;Tarling & Hrouda, 1993). It is founded on the parallelism between the crystallographic and magnetic fabrics of some paramagnetic minerals, especially phyllosilicates (Martín-Hernández & Hirt, 2003). As long as some conditions are met, AMS is a quick, inexpensive, effective, and non-destructive way to determine rock fabric, and is able to obtain the mineral-preferred orientation in rocks with apparently absent macroscopic and even microscopic evidence.
Compared to measurements on sedimentary and metamorphic rocks (Graham, 1954), the application to granite started later (Heller, 1973;King, 1966). AMS in granite with a calc-alkaline affinity seems to display a better characterization of the rock fabric when compared to classic techniques such as those based on outcrop or microscope measurements of feldspar and biotite crystals, and so on. The reason lies in the total content in iron and its mineral fractioning, a fact that promoted a classification of granites as either magnetic or non-magnetic (Ellwood & Wenner, 1981;Ishihara, 1977). For all these reasons, the application of AMS in calc-alkaline plutons (non-magnetic; iron is fractioned mostly in biotite) has represented a turning point in the interpretation of the kinematics of their emplacement modes (Bouchez, 1997). Apart from being able to precisely determine the preferred orientation of biotites in apparently isotropic rocks (a main marker of the rock fabric), AMS also yields a control of the deformation intensity (relative differences among the AMS ellipsoids) and can be used as a petrological mapping variable, being correlated with iron content (Gleizes, Nédélec, Bouchez, Autran, & Rochette, 1993).
The Geokin3Dpyr group (Communauté de Travail des Pyrénées; INTERREG III program) was formed in 2003 and integrated all Pyrenean universities and research centers in Earth Sciences. A main goal of this program was the development of electronic and public databases including the preliminary compilation of AMS data (López, Oliván, Oliva, & Pueyo, 2008;Pueyo et al., 2006). Recently, the AMS database of the Pyrenees has been completed and updated both in sedimentary rocks (Pocovi et al., 2014) and granites (Porquet, 2014). In this paper, we introduce the maps that synthesize information from the latter, where AMS data from 21 different granites and one gneissic dome (Aston) have been homogenized and compiled.
The final goal of this database is to be integrated, in the near future, in the Information Web Resources of the Geological Surveys of Spain and France (www.igme.es and www.brgm.fr, respectively).

Map specifications
The maps included in this paper use the ETRS 1989 datum and the UTM Zone 30 projected coordinated systems, although the eastern part of the map belongs to the 31T zone, it has been converted to 30T coordinates. All three maps fulfill the Mapping Standards of Aragón (Spain; http://idearagon.aragon.es/nca/).
The geologic map used as a background (Barnolas et al., 2008) is large scale (1:400,000) but displays enough structural features (fold axes, thrust traces, etc.). However, cartographic detail on rock ages and lithologies has been simplified considerably, following the style of Choukroune and Seguret's classic structural map (1973) of the Pyrenees.

Geological setting
The Pyrenees are a collision chain formed during the Late Cretaceous and Eocene epochs, up to the Miocene on the southern part of the range, due to convergence between the Iberian and Eurasian plates (Muñoz, 1992). They are located between the Gulf of Lion in the Mediterranean Sea and the Biscay Bay in the Atlantic Ocean (Muñoz, 1992;Vera, 2004). The Axial Zone of the Pyrenees (backbone of the chain), where most of the Hercynian bodies are located, depicts an antiformal geometry. It is made up of Precambrian and Paleozoic rocks affected by the Hercynian tectonic phases, reworked, to some extent, by Alpine deformations.
The Pyrenean Hercynian basement belongs to the southern part of the European Variscides (inset Figure 1), an orogenic belt mostly formed during Late Carboniferous times and partly reworked during the Alpine orogeny, and is composed of sedimentary, metamorphic, and igneous rocks, ranging in age from Upper Proterozoic to Permian epoch. The Alpine orogeny brought about an antiformal stack of basement nappes, and the subsequent exhumation of Paleozoic units in the core of the Pyrenean range during Eocene and Miocene times (Beamud et al., 2011;Fitzgerald, Muñoz, Coney, & Baldwin, 1999) allowed access to the actual outcrop, making observations of the Hercynian crust possible. Reactivation of previous Variscan structures constitutes an important deformation mechanism in Alpine tectonics, and precludes the accurate chronological and kinematic analysis of tectonic phases related to the first orogenic event.
The Variscan structure of the Pyrenean Axial Zone is the result of a polyphased structural evolution related to an oblique continental collision and crustal thickening. Early south-verging thrust sheets involve Silurian to Carboniferous rocks in the hanging wall and Cambro-Ordovician rocks in the footwall (e.g. Bodin & Ledru, 1986;Losantos, Sanz, & Palau, 1986;Majesté-Menjoulas, 1982;Raymond, 1986). The Silurian slates act commonly as the detachment level between the two units. Subsequent south-verging folds and thrusts are related to a widespread regional penetrative cleavage with steep to moderate dips (Carreras & Capella, 1994;Soula, Debat, Déramond, & Pouget, 1986), the so-called D2 phase (Zwart, 1986), which has been characterized as a compressional-transpressional regime with dextral shear motion at the final stages, accompanied by granite intrusions (e.g. Evans, Gleizes,  Leblanc, Gleizes, Roux, & Bouchez, 1996).

Methodology
The AMS technique is based on the measurement of the magnetic susceptibility (k) of a standard cylindrical sample (25 mm diameter and 22 mm height) in different directions, in order to calculate the magnetic ellipsoid of that sample. This method is based on the relation between the induced magnetization (M) with the magnetic field (H ), where M = kH. The magnetic susceptibility k is a third-order tensor that can be graphically described by an ellipsoid whose three axes k 1 , k 2 , and k 3 (also called k max , k int , and k min ) correspond to the maximum, intermediate, and minimum susceptibilities, respectively ( Figure 2).
Magnetic fabric analysis is a powerful approach for studying granite bodies, because it may provide magmatic information at a regional scale, in rocks where fabrics are difficult to measure optically or by other techniques (Bouchez, 2000). Providing that the paramagnetic content dominates the bulk susceptibility (i.e. biotite and amphibolite are the main carriers) and because of the correspondence between the crystallographic and susceptibility main directions of phyllosilicates (Martín-Hernández & Hirt, 2003) then, a direct comparison between the magmatic and magnetic fabrics can be established. In these cases, the AMS can be interpreted in terms of mineral-(phyllosilicate) preferred orientation; the principal AMS axis (k1) is then directly correlated to the magmatic lineation and the minimum axis (k3) can be considered as the pole of the magmatic foliation. However, recent studies suggest that during the emplacement (magmatic conditions) some deformation phases may overprint each other contributing to the finite strain ellipsoid and, thus, the study of AMS alone does not necessarily unravel the complete deformational history of the granite (Schulmann & Ježek, 2012). In addition, the correspondence between deformation axes from felspar and biotite is not always univocal (Kratinová et al., 2010;Román-Berdiel, Pueyo-Morer, & Casas-Sainz, 1995). In this sense, some work in weakly deformed sedimentary rocks suggests the importance of magnetic subfabric separation (Oliva-Urcia et al., 2009) to disentangle the deformation history. This problem is beyond the scope of this paper and we simply respect the original interpretation of the source papers to build the overview map. Studies of AMS measurements in calcalkaline granites were pioneer in the Pyrenees (Bouchez et al., 1990;García-Maiztegi et al., 1991;Gleizes & Bouchez, 1989;Santana & Tubía, 1988) and usually assume a simple deformational history during the emplacement; they hypothesized that AMS blocks an infinitesimal deformation ellipsoid coincident with the finite strain tensor.
The variables represented on the maps of this paper are briefly described below; the orientation of the magnetic foliation (plane perpendicular to k 3 ), lineation (k 1 ), and the magnetic bulk susceptibility (km). Using the magnitude of the k 1 , k 2, and k 3 axes, it is possible to obtain some simple parameters, as the average of the magnetic susceptibility is obtained from:

Raw data
More than 20 papers and PhD theses from different authors (see Table 1) have been compiled to merge the data necessary to build the synthetic maps. These data also required a homogenization process in order to represent all the variables in the same units and formats. Unfortunately, the raw data from the papers were mostly given in tables and represented on large-scale maps, without the corresponding georeferenced information (Figure 3). In this paper, every map and every measurement site has been georeferenced and all the information has been compiled into new tables.

Raw maps
Some cartographic resources to construct the maps were used as background layers in the GIS project: . The two main sources of geologic maps were the Geologic and Mining Spanish Institute (IGME) and the French Geological Survey (BRGM). On one side, the IGME provides the MAGNA maps (1:50,000 scale), on the other, the BRGM also provides the entire French territory at the same scale. As shown in Figure 4, we combine both sources depending on location of the granite (whether it is located in France or in Spain). On the left side, the figure shows two different layers: the Aya pluton shape, laid over the MAGNA map. On the right side, the figure shows Neouvielle pluton drawn on Google Earth. MAGNA maps are available online (http://info.igme.es/cartografia/magna50.asp). . Moreover, there are very few cartographic compilations in the Pyrenees. The recent map by Barnolas et al. (2008) has been used as a background map ( Figure 5(a)). However, the simplicity of the structural map of the Pyrenees by Choukroune and Séguret (1973, Figure 5(b)) inspired other authors (Ramón Ortiga, 2013) to modify the Barnolas map. In this paper, we have used the simplified version used by Ramón Ortiga (2013, Figure 5(c)).  Gleizes (1990) and Gleizes et al. (1993) 17 Néouvielle ( 6. Discussion

Georeferencing errors
The original maps that were contained in the papers had very poor geographic information (both quality and accuracy). This fact made the geo-positioning process difficult, and errors could not be fully avoided. The following table shows the mean errors found in each granite. No quantifiable units (e.g meters) are given for the errors because the original maps (extracted from the papers) do not supply such information (Table 2).

Standardization of information
In this paper, when discussing AMS spherical information, we had to face three main problems: (1) data were represented in scalar format, thus they had to be converted to directional format with hundreds of measurements; (2) in some cases magnetic foliation had to be converted into k 3 ; (3) most of the direction data needed to be changed into stereographic space (360°) instead of the 180°space used by some authors, which meant adding the N-S-E-W. This last step was of vital importance because, although the reader may easily understand that 210°= 30S, the software ArcGIS is unable to do so.

Global data information
In total, 21 different tables (one for each granitic body) with about 2210 sites and more than 12,000 different data were obtained. These results have been synthesized in three different maps:  The bulk susceptibility map contains site mean values and a mapping following fixed intervals of susceptibility for all bodies. This information allows us to rapidly assess the total iron distribution in the Pyrenean plutons, and therefore an approach to the petrologic facies. These intervals have been set according to Gleizes et al. (1993), who proposes a correspondence between bulk susceptibility and petrologic facies. Here, measures below 100 × 10 −6 SI correspond to leucogranites; measures from 100 to 200 × 10 −6 SI correspond to monzogranites; measures from 201 to 300 × 10 −6 SI correspond to granodiorites; and measures above 300 × 10 −6 SI correspond to quartz diorites. Figure 5. The addition of (A) the map by Barnolas et al. (2008) plus (B) the style and simplicity of the map by Choukroune and Séguret (1973) has given rise to (C) our background map. Taken from (Ramón Ortiga, 2013).
Interestingly, and apart from the data of the Aston gneissic dome (Denèle, Olivier, Gleizes, & Barbey, 2009), the magnetic susceptibility histogram (Figure 6(right)) displays values similar to those found in many sedimentary rocks (see data from the cover rocks by Pocovi et al., 2014). This robust observation (>10,000 individual measurements) contradicts the general assumption on the magnetic nature of the hercynian basement that has been used in many petrophysical and geophysical studies.
The Foliation map includes the magnetic foliation at every site, that is, the plane perpendicular to the k 3 (k min ) axis. As there is no deformation in the solid state, it is not possible to comment on pole of the cleavage, but it is true that the pole of k 3 and this pole of cleavage are very similar, which means that granites could have suffered this deformation in a passive way. It is worth noting that only raw data have been plotted, and we have not performed any kind of interpolation. Additionally, we have plotted together in the stereoplot Allmendinger, Cardozo, & Fisher, 2013) all mean data of k 3 orientation (Figure 7, right). Here some observations can be made: . Although the stereoplot looks slightly noisy, as many peculiarities from every pluton are assembled together, the main eigenvector fitted by Bingham statistics (1974) surprisingly resembles the pole of the Pyrenean cleavage (Choukroune & Séguret, 1973). This fact likely reflects, at least partially, a northward tilt in the plutons caused by alpine basement nappes. Bimodal subhorizontal distributions indicate that most of the bodies are batholites. . As expected, the main girdle is not coaxial to the present Pyrenean trending.
The Lineation map includes the mapping of individual magnetic lineations k 1 (k max ) axis. Regarding the overview stereonet, we can highlight some facts: . Magnetic lineation seems to display a bimodal distribution; ENE-WSW and E-W (Figure 7, left). . Similar to the possible reworking caused by the alpine tilting detected in the magnetic foliation, the lineations can be fitted to a girdle that could be produced by Pyrenean deformation (passive movement into the basement nappes). Note: Cauterets-Panticosa was georeferenced as one body. Figure 6. Susceptibility overview. Left: density of sampling of AMS sites per pluton. Larger bodies have a slightly lower density of sampling. Right: Susceptibility distribution following the same classification as on the map, and its correspondence with petrologic facies according to Gleizes et al. (1993). Only site means were considered in this graph. The Aston dome was not considered in these graphs.
Regarding the map overviews, both lineation and foliation symbols on the maps have been reproduced following the style used by Román-Berdiel et al. (2004), which is simple and easy to read. In the lineation case, the top of the arrow indicates the direction, and there are three different levels depending on the azimuth results: less than 30°, between 30 and 60°, and more than 60°. For the foliation, there are again three levels to display the dip in each case. In the bulk susceptibility case, we pick a simple green scale following 100 10 −6 S.I. steps, allowing for quick data interpretation.

Conclusions
In this paper, we have synthesized in three different maps the information derived from more than 25 papers and PhD theses on the anisotropy of the magnetic susceptibility of 21 granitic bodies, and one gneissic dome from the Pyrenees. The homogenous cartographic data, together with the standardized magnetic information, allow us to display a better characterization of the regional distribution of the variables; bulk susceptibility (km), magnetic lineation (k 1 ) and pole to magnetic foliation (k 3 ).
Specifically, 22 bodies (21 granites and a gneissic dome) have been redrawn, involving more than 2200 sites with more than 12,000 original AMS measurements. This information is now in a database which we plan to upload on the webGIS servers of the IGME and BRGM (Spanish and French geological surveys). This paper shows only the three final maps for the entire Pyrenees however, 22 different maps were generated, one for each granite, with their respective table of information.
Software Esri ArcGIS 10.2 was used for (1) gathering and visualizating the underlying data, (2) georeferencing granite bodies and sites, (3) digitizing and editing magnetic data layers and (4) producing the map layout and corresponding legend.

Disclosure statement
No potential conflict of interest was reported by the authors.