1. Introduction
Beyond classic mineral exploration in hydrothermally altered or contact metamorphism rocks (e.g., Joly et al. [
1]; Dentith and Mudge [
2]) or as radioactive waste disposals (e.g., Wang [
3]), geophysical surveying of granite bodies is increasingly of interest due to, among other aspects, their potential in geothermal energy (Genter et al. [
4] Huenges and Ledru [
5]; Moore et al. [
6]; Zhang and Zhao [
7]). Potential fields geophysical techniques, both gravity and magnetism, have been of great help for 2D and 3D modeling of granitic bodies for a long time (Bott et al. [
8]; Bott and Smithson [
9], Henkel [
10] Vigneresse [
11] Ameglio et al. [
12]; Cruz et al. [
13]) inasmuch they are quick, repeatedly resolute and cost-effective methods for an initial characterization of those bodies at depth. As in any potential-field study, petrophysical data are an essential constraint to link the geophysical signal with the geological features, and thus a keystone to reduce the ambiguity and uncertainty in the interpretations (Henkel [
10,
14,
15]; Enkin et al. [
16]; Dentith et al. [
17]; Pueyo et al. [
18]).
Density and susceptibility relationships have been well-known in granitic rocks from a petrological and petrophysical point of view since the pioneering works by Henkel ([
10,
14]). Some works have demonstrated that δ
18O and the wt.% SiO
2 display a negative correlation to magnetic susceptibility in granites (Ellwood and Werner [
19], Villaseca et al. [
20] respectively). Moreover, it is well known that the magnetic properties of granites depend on their chemical and tectonic affinity (Kanaya and Ishihara [
21]; Ishihara [
22], 1981 [
23]) and they are classified as magnetic granites (
κ ranging from 10
−3 to 10
−2 S. I.) and non-magnetic granites (
κ from 10
−5 to 10
−4 S.I.) (Ellwood & Werner [
19]). Non-magnetic granites usually correspond to supracrustal sources, S-type (Chappell & White [
24]), where iron is fractionated mainly in ilmenite (paramagnetic at room temperature) and biotite (strongly paramagnetic; Martín-Hernández & Hirt [
25]). In magnetic granites, usually related to deeper and/or igneous sources (I-type), the iron mainly forms magnetite crystals (ferrimagnetic). Elming [
26] in the Caledonides of Jämtland (Sweden), or more recently Terrinha et al. [
27] in Sintra granite, showed coarse relations between density and magnetic susceptibility along five orders of magnitude (
κ from 10
−5 to 10
−1 S.I.). Criss and Champion [
28] measured the density in paleomagnetic samples of the Idaho granite and recognized that variations in susceptibility were mostly related to the rock density at a regional scale. Bourne [
29] stablished that density and magnetic susceptibility (separately) inversely correlate with SiO
2 concentrations in ferromagnetic granites (ca.
κ > 500 × 10
−6 S.I.; Bouchez [
30,
31]).
A big step further was given by Gleizes [
32]; Bouchez et al. [
33]; Gleizes et al. [
34] and in the Foix and Mont-Louis Andorra granites, respectively (in the Axial Zone of the Pyrenees). Gleizes et al. [
34] related the magnetic susceptibility with the iron composition [Fe
+2 and Fe
+3] and the expected susceptibility derived from theoretical estimates (after Rochette [
35] and Rochette et al. [
36]), and also proposed a correlation between magnetic susceptibility and magmatic facies in paramagnetic granites (ca.
κ < 500 × 10
−6 S.I). Their work has had a profound impact in the application of anisotropy of magnetic susceptibility (AMS) techniques to characterize the internal structure and composition of granitic bodies. Currently, hundreds of granitic bodies all over the world have been systematically studied by AMS, helping us to understand their emplacement models and generating vast databases of susceptibility data (Román-Berdiel et al. [
37]; Aranguren et al. [
38]; Trindade et al. [
39]; Sant’Ovaia et al. [
40]; Ferré et al. [
41]; Kratinová et al. [
42]; Joly et al. [
43]; Porquet et al. [
44] among many others).
Focusing only on paramagnetic granites (in terms of Bouchez [
30], or on the paramagnetic trend (in terms of Henkel [
15] and Enkin et al. [
16]), very little has been done comparing these petrophysical properties. Ameglio et al. [
12] and references therein) found fine correlations among density and magnetic susceptibility in a few samples from three calc-alkaline S-type (paramagnetic) granites from French and Spanish Variscan massifs (namely Sidobre, Cabeza de Araya and Mont Louis-Andorra).
In this paper, the main goal is to build reliable correlations between magnetic susceptibility and density in paramagnetic granites. Three plutons from the Pyrenees are the study-cases: Mont Louis-Andorra, Maladeta and Marimanha. Previous AMS studies in these bodies (Gleizes et al. [
34]; Leblanc et al. [
45]; Antolín-Tomás et al. [
46], respectively) allow the conversion of a vast susceptibility database into robust and reliable density data ready to be modelled together with gravity signals.
2. Geological Setting
The granitic massifs of Maladeta (MAL), Marimanha (MAR) and Mount-Louis Andorra (MLA) crop out in the Axial Zone of the Pyrenees and belong to the tardi-tectonic granitic bodies of the Variscan Orogen (
Figure 1). All three plutons were intruded in the upper crust during the latest stages of the Variscan orogeny (Gleizes and Bouchez [
47]; Gleizes [
32]; Leblanc et al. [
45]; Antolín-Tomás [
48]; Antolín-Tomás et al. [
46]).
The Variscan belt of Europe is part of a vast Paleozoic chain built between 500 and 250 Ma, due to the convergence and collision of two large continental masses, Laurentia-Baltica and Africa (Matte. [
49]). It is a sinuous orogen that can be followed discontinuously from the south of Spain (Martínez-Catalán [
50]; Pastor-Galán et al. [
51]) to the Bohemian massif, and which very probably extends under the Carpathians to the Variscan Caucasus (
Figure 1A). It is therefore a Paleozoic chain of nearly 5000 km in length that constitutes the southwest edge of stable Europe (Matte [
52]); 250 Ma ago, most of the Variscan belt was completely eroded and subsequent extension and formation of basins occurred during the Mesozoic (Pangea breakup). Then, in the Cenozoic, the Alpine orogeny developed a new collisional chain between Iberia and the SW margin of the Eurasian plate between 65 and 20 M.a. (e.g., Muñoz [
52]) resulting in the current Pyrenean Mountain range. The Alpine orogeny gave rise to the uplift of Paleozoic units in the core of the Pyrenean range (i.e., the Axial zone) during the collision by developing a central antiformal stack of several southward-facing basement-involved thrust sheets (Muñoz [
53]; Martínez-Peña and Casas-Sainz [
54]; Casas et al. [
55]). Recently, some authors claim that the persistence of a relatively flat envelope for the Paleozoic sedimentary pile and Variscan isograds, and the absence of Alpine crustal-scale faults in the core of the Axial Zone, suggest that the Axial Zone constitutes a large Variscan structural unit preserved during Pyrenean orogeny (Cochelin et al. [
56]).
In this context, the Marimanha granite crops out within the Alpine Gavarnie/Nogueras thrust sheet, the uppermost unit of the south-verging piggyback-sequence in this part of the Pyrenees, whereas the Maladeta and Andorra-Mont-Louis granites, two of the largest granitic bodies of the Axial Pyrenees, crop out in the underlying Bielsa/Orri thrust sheet (
Figure 1B).
During the Variscan orogeny, low-grade Paleozoic metasedimentary rocks were affected by progressive polyphasic deformation (Druguet [
57], García-Sansegundo et al. [
58], Casas et al. [
59]), which mostly occurred during a compressional tectonic setting in which fold and thrust systems developed, giving rise to the crustal thickening of the Variscan cordillera in this region (Soula et al. [
60]; Carreras and Capella [
61], Gutierrez-Medina [
62]; Clariana and García-Sansegundo [
63]). Subsequently, in the last states of the variscan deformation, an extensional deformation event occurred. According to Soula [
64]) and Autran [
65]) the main deformation event is synchronous with the peak metamorphism, while other authors (Guitard [
66], Zwart [
67,
68]; Liesa [
69] and Aguilar [
70]) consider the deformation event to have continued during and after the metamorphic peak. Early south-verging thrust sheets involve Silurian to Carboniferous rocks in the hanging wall and Cambro-Ordovician rocks in the footwall (e.g., Majesté-Menjoulas [
71]; Raymond [
72]; Bodin and Ledru [
73]; Losantos et al. [
74]). The Silurian slates act commonly as a detachment level between the two units, and the deformation above it is characterized by fault propagation folds affecting Silurian and Devonian rocks (García-Sansegundo [
75], Clariana [
76], Margalef [
77]). The last stages of the variscan deformation have been characterized as dextral shear motion accompanied by granite intrusion (Leblanc et al. [
78]; Evans et al. [
79]; Gleizes et al. [
80,
81,
82]; Olivier et al. [
83]; Román-Berdiel et al. [
84,
85]; Auréjac et al. [
86]). Due to crustal thickening, the Variscan segment of the Pyrenees experienced crustal flow, gneiss dome formation and subsequent granitic massifs intrusion in the upper crust by in- situ ballooning favored by the boundary between the Cambro-Ordovician and Siluro-Devonian rocks (Antolín-Tomás et al. [
46]). The U–Pb ages published for the Pyrenean granites indicate that the Variscan plutonism of the Pyrenees is mainly Carboniferous (Romer and Soler [
87]; Paquette et al. [
88]; Guerrot [
89,
90]; Roberts et al. [
91]; Maurel et al. [
92]; Olivier et al. [
93]; Gleizes et al. [
82]) and Permian in age (Denèle et al. [
94]). Recent works claim that partial melting, crustal flow, gneiss dome formation and pluton emplacement occurred over a very short period at the time scale of the Variscan belt formation, on the order of 5 Ma, at ca. 304 Ma (Denèle et al. [
95]). In general, these magmatic bodies follow the dominant trend of the Variscan structure.
Figure 1.
(
A) European Variscan belt (Modified after Franke [
96]); CZ-Cantabrian Zone, WALZ-West Asturian-Leonese Zone, GTOMZ-Galicia Tras-os-Montes Zone, CIZ-Central Iberian Zone, OMZ-Ossa Morena Zone, SPZ-Southportuguese Zone, PAZ-Pyrenean Axial Zone, CCR-Catalonian Coastal Ranges, NPM-Nord-Pyrenean Massifs, MB-Massifs Basques. (
B) Geological sketch map of the Central part of the Pyrenees, showing the situation of the Pyrenean granites.
Figure 1.
(
A) European Variscan belt (Modified after Franke [
96]); CZ-Cantabrian Zone, WALZ-West Asturian-Leonese Zone, GTOMZ-Galicia Tras-os-Montes Zone, CIZ-Central Iberian Zone, OMZ-Ossa Morena Zone, SPZ-Southportuguese Zone, PAZ-Pyrenean Axial Zone, CCR-Catalonian Coastal Ranges, NPM-Nord-Pyrenean Massifs, MB-Massifs Basques. (
B) Geological sketch map of the Central part of the Pyrenees, showing the situation of the Pyrenean granites.
The Marimanha granite is a triangular pluton in map view, with an outcrop area of about 45 km
2. The available geochronological data for the Marimanha pluton consists of a poorly constrained Rb–Sr age (on whole rock) of 315–290 Ma (Palau i Ramirez [
96]). At the petrological level, this massif includes gabbro, diorite, granodiorite and leucogranite facies (Palau i Ramirez [
97]). Its intrusion occurred in the western end of the Pallaresa Dome, in Cambro–Ordovician siliciclastic metasediments, cutting the Silurian and Devonian limestones and slates involved in the Variscan Roca Blanca thrust (Losantos et al. [
74]; Bodin and Ledru [
73]). The metamorphic aureole overprints the Variscan structures, and the whole is cut by Late Variscan and/or Alpine WNW–ESE reverse faults.
The Maladeta granite massif (Evans [
98]; Leblanc et al. [
45]; Evans et al. [
79]) is elongated in the E-W direction with an area of 400 km
2. Available geochronological data provide a rather young date of 277 ± 7 Ma (Rb/Sr; whole-rock, Vitrac-Michard et al. [
99]). Petrographic zoning is defined in the western Aneto unit, with more basic rock-types surrounding others that appear to be more silicic, and the presence of gabbros is defined in the south-eastern part of the massif (Charlet [
100,
101]). The massif is penetratively deformed in some places near its southern edge, and major fault zones separate different units. It was emplaced into Cambro-Ordovician to Carboniferous rocks at a shallow level of about 6 km in depth (Pouget et al. [
102]). It developed a contact metamorphism aureole in which the sillimanite grade has been reached.
The Mont-Louis-Andorra massif (Autran et al. [
103]; Debon et al. [
104]; Gleizes & Bouchez [
47]; Gleizes et al. [
34]; Bouchez and Gleizes [
105]) is elongated in the E-W direction over 55 km and covers nearly 600 km
2. Recent available geochronological data provide an age of 301–303 Ma (U/Pb, Denèle et al. [
95]). Petrographically, this massif consists of leucogranites, monzogranites and granodiorites with dioritic microgranular enclaves. Its intrusion occurred mainly in Ediacaran-Cambrian metasediments (Laumonier et al. [
106]) and its top reaches Devonian sediments to the SW (Andorra). It developed a metamorphic contact aureole with isograds cutting across the regional metamorphic isograds (Guitard et al. [
107]; Laumonier et al. [
108]).
The bulk susceptibility from almost 600 studied AMS sites (
Figure 2) displays very similar distributions for the three plutons and non-significant differences in the mean values (
Table 1). These available and densely sampled nets of AMS data from the 90s and early 2000s (
Figure 2) are revisited with new data acquired during the development of this work in the three granites to establish reliable correlations between density and magnetic susceptibility.
3. Methodology
In total, both properties (density and magnetic susceptibility) were studied in 128 sites from the Mont Louis-Andorra (MLA), Maladeta (MAL) and Marimanha (MAR) plutons (71, 42 and 15 sites, respectively) in the Central Pyrenees (
Figure 3).
Four different types of samples were considered (
Figure 4): (A) large hand samples of a few kilograms (type 1) were taken in 92 new sites (MLA and MAL plutons), (B) from them, subsampling of smaller blocks (mini-blocks; type 2; 20 to 200 g) were measured in 48 of those sites. (C) several previous samples from AMS studies (paleomagnetic standard specimens, type 3, about 25–30 g) were remeasured in 36 sites from MLA (21 sites; Gleizes et al. [
33]) and MAR (15 sites; Antolín et al. [
45]). Finally, (D) subsampling of some standard cores with a 0.5 cm Ø non-magnetic drill bit (1 cm in length, type 4-minicores, below 1 g) in the MLA granite allowed for the determination of susceptibility and other magnetic properties using vibrating and superconducting magnetometers. All the studied samples from these 128 sites tried to evenly cover the entire susceptibility spectra of the studied plutons (
Figure 2).
3.1. Density Determinations
In type 1 samples (large hand blocks), the apparent density estimation was performed following the European standard UNE EN 1936:2006 (Natural stone test methods-Determination of real density and apparent density, and of total and open porosity [CEN/TC 246-Natural stones; CEN/TC 246/WG 2-Test methods]). More than two density data were taken in average per site (ranging between 1 and 5) at the IGME laboratories (Tres Cantos, Madrid), yielding a total of 95 determinations. Two main initial conditions must be honored: the specimens must be larger than 60 mL in volume, and their surface/volume ratio must range between 0.08 and 0.20 mm
−1 (regular cubes with sides between 30 and 75 mm). In the laboratory, the specimens are first dried in an oven at 70 ± 5 °C until a constant mass (m
D) is achieved; the difference between two consecutive weighing carried out after 24 h must be lower than 0.1% of the initial mass (this mass will also allow obtaining the dry bulk density). Then, the specimens are subjected to a vacuum of 2.0 (±0.7) kPa for 2 (±0.2) h. Distilled water is slowly added without losing the vacuum, and the samples are submerged more than 15 min. The water temperature should be 20 ± 5 °C. Atmospheric pressure is slowly restored and the samples are left 24 (±2) additional hours in water. Then, we proceed to the weighing of the specimens immersed in a hydrostatic balance (m
H) and subsequently dried with a damp cloth, and its saturated mass is determined (m
S). See additional details and instrumentation in Rubio et al. [
1]).
Then the apparent (saturated) density is (in kg/m
3):
where
ρW is the density of distilled water at 20 °C: 988 kg/m
3.
Type 2 samples (more than 80 mini-blocks) were measured in the University of Barcelona laboratories applying the Archimedes principle (without a previous drying and without using paraffin); part of this set was previously measured in the IGME laboratories in Madrid (as type 1 samples). In type 3 AMS samples (111 specimens from MAR), density was estimated in three different ways: using the Archimedes principle (samples with and without paraffin) and estimation of the rock volume with a Vernier caliper on cylindrical samples. Using this third method, 21 additional estimates from MLA were also taken. For that purpose, only regular (cylindrical sections) and complete samples (whole, unbroken, etc.) were used. Any broken, incomplete, irregular or cracked specimen was ruled out for this purpose. Apart from the weighting of the sample (m), the maximum and minimum diameters (Ø) and heights of the specimen (H) were measured with a Vernier caliper. Afterwards both measurements were averaged out (Ø
m and H
m) and the volume was rapidly calculated: V = π (Ø
m/2)
2·H
m, as well as the density
ρ = m/(π [Ø
m/2]
2·H
m). Considering all types together, 310 density determinations were obtained; 69 in MAL, 130 in MLA and 111 in MAR plutons (
Table 2).
3.2. Susceptibility and Rock Magnetism Measurements
In total, more than 2600 susceptibility determinations were obtained in 128 studied sites (
Table 2). More than 80% were taken directly in 44 outcrops accompanying type-1 samples (21 selected sites from MAL and 23 ones from MLA plutons) using SM20 (1687 measurements) and KT20 (428 measurements) portable susceptometers (by GF Instruments and Terraplus, respectively). On average, more than 50 readings were measured in these outcrops (ca. 10–15 m
2), and at least one large hand sample (type 1) was taken approximately in the center of the outcrop surface. Type 2 samples (>200 readings) were measured in a KLY-2 kappabridge located in the Paleomagnetic laboratory of the CCiTUB and Geociencias Barcelona (CSIC). Type 3 samples (paleomagnetic standard) were measured in susceptibility bridges available at the laboratories of the universities of Toulouse and Zaragoza (AGICO kappameters models KLY-2 and KLY-3, respectively). Selected specimens from Toulouse were measured again (1 per selected site) and contrasted to previously published works in the Mont Louis-Andorra granite (Gleizes et al. [
34]). In the case of Marimanha, all available stored specimens at the Geotransfer Group (University of Zaragoza) were re-measured in some particular sites (Antolín et al. [
46]; Rubio et al. [
110]; Loi et al. [
111]).
Finally, some comparisons between high (κHF) and low field (κLF) susceptibilities were performed using a MPMS superconducting magnetometer (model 5S by Quantum Design Ltd.) of the University of Zaragoza to determine the para- and ferromagnetic s.l. fractions. Selected cores from MLA pluton were re-measured in the KLY-3 and then subsampled. Those minicores (type 4 samples) were measured in the MPMS instrument under the same conditions as the AGICO instruments (external magnetic field of 0.4 mT, 4 Oe in AC at about 900 Hz) to obtain the low-field value (κLF). Eventually, the same minicores were subjected to two high DC fields (0.9 and 2.5 T), after the total magnetic saturation, to estimate the high-field paramagnetic susceptibility (κHF). Since the κHF only reflects the paramagnetic contribution, meaning κLF ≈ κHF, then a dominant paramagnetic contribution can be assumed. Otherwise, if κLF > κHF, then the ferromagnetic contribution cannot be neglected. Additionally, the same collection of mini cores was re-measured in a MVSM (model 3900 by Princeton Measurements Corporation) hosted in the CEREGE paleomagnetic laboratory at Aix-en-Provence while trying to characterize the ferromagnetic hysteresis as well as the high field (up to 1 T) response (paramagnetic slope).