Time constraints for the tectono-thermal evolution of the Cantabrian Zone in NW Spain by illite K–Ar dating

Tectonophysics 623 (2014) 39–51

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Time constraints for the tectono-thermal evolution of the Cantabrian Zone in NW Spain by illite K–Ar dating Norbert Clauer a,⁎, Alexander Weh b a b

Laboratoire d'Hydrologie et de Géochimie de Strasbourg (CNRS/UdS), 1 rue Blessig, 67084 Strasbourg, France Department of Geography and Geosciences, Friedrich-Alexander University of Erlangen-Nürnberg, Erwin-Rommel Strasse 1, 91058 Erlangen, Germany

a r t i c l e

i n f o

Article history: Received 27 November 2013 Received in revised form 14 March 2014 Accepted 16 March 2014 Available online 22 March 2014 Keywords: Variscan orogen Cantabria NW Spain Clay mineralogy Vitrinite reflectance Illite K–Ar dating

a b s t r a c t Contrasting sedimentary facies and tectonic structures diversely distributed and affected by diagenetic to lowgrade metamorphic activity form a complex pattern in the northwestern Spanish Cantabrian area of the Variscan orogen. The distribution of these metamorphic features is based on field observations, clay mineralogy, mineral crystallinity indices, vitrinite reflectance data, and observation of rock microtextures. K–Ar dating of fine-grained illite-rich size fractions provides a complete set of ages for the episodic deformation and thermal overprints in the southeastern Cantabrian area. The usual K–Ar isochron plotting was combined with the bench-type age distribution to unravel the successive illite crystallization episodes. Several tectono-thermal episodes occurred in the southeastern Pisuerga–Carrion Province: initially during the Early Permian at 293 ± 4 Ma in the Ruesga Line, Valsurvio Dome and Sierra del Brezo area. A further mid-Permian event was detected at 268 ± 6 Ma in the Palentine Nappes, and during the mid-Triassic at 243 ± 5 Ma, again in the Valsurvio and Sierra del Brezo areas, and along the Ruesga Line. A later reactivation of the Ruesga Line was detected during mid-Jurassic time at 175 ± 6 Ma. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Many studies have significantly contributed during the recent decades to the understanding of tectonic settings at the transition between diagenetic and incipient metamorphism by examining the tectonothermal evolution of the external parts of orogenic zones. These studies were often based on independent thermal indicators such as specific mineral assemblages, presence or absence of cleavage features, illite crystallinity, fluid inclusion microthermometry, and conodont color indices, but only a few studies combined several of these parameters. Episodes of regional metamorphism could be identified and described (e.g., Árkai et al., 1996; Bastida et al., 1999; Frey, 1987; Hesse and Dalton, 1991), some of which occurred at the same time as thrusting and faulting, therefore contributing to complex structural settings (Bevins and Robinson, 1995; Jaboyedoff and Thélin, 1996; Warr et al., 1996). However, distinction between motion of thrust belts and activation or reactivation of faults, and identification of these activities with a pre-tectonic diagenetic evolution, are generally not straightforward as timing is often difficult to constrain.

⁎ Corresponding author at: Laboratoire d'Hydrologie et de Géochimie de Strasbourg (CNRS/UdS), Université de Strasbourg, 1 rue Blessig, 67084 Strasbourg, France. Tel.: +33 3 90 24 04 33 (office), +33 6 80 01 80 49 (cell); fax: +33 3 90 24 04 02. E-mail address: [email protected] (N. Clauer).

http://dx.doi.org/10.1016/j.tecto.2014.03.013 0040-1951/© 2014 Elsevier B.V. All rights reserved.

Recent studies applied to diagenetic and low-grade metamorphic shales and slates from foreland folds and thrust belts included geochronological aspects, which turned out to be helpful in identifying recurrent tectonic activity and plutonic impacts in complex structural areas (Glasmacher et al., 2004; Nierhoff et al., 2011; Sasseville et al., 2008, 2012; Surace et al., 2011; Warr et al., 2007; Zwing et al., 2009). Isotopic dating of such diagenetic to low-grade metamorphic events, combining grain-size separation, X-ray diffraction (XRD), scanningand transmission-electron microscopy (SEM and TEM), with K–Ar dating of constitutive clay size fractions, appeared appropriate to study the complex Cantabrian Zone of the Spanish Variscan orogenic region that has already been extensively studied for its tectono-thermal history (Bastida et al., 2002; Julivert, 1981; Keller and Krumm, 1993; Marcos, 1979; Raven and Van der Pluijm, 1986; Wagner and Winkler Prins, 2000), but remains to be further explored for its historical evolution. Based on an extensive collection of shale- or slate-type samples representative of the metamorphic and folding/faulting activities of the Cantabrian Zone, the challenge was (1) separation of fine, clayrich fractions from rocks of the southeastern Pisuerga–Carrion Province (Fig. 1) that are representative of regional repetitive tectono-thermal events, (2) detailed characterization of the size separates by XRD, and (3) dating of these separates by the K–Ar method. This multimethodological approach has been used on purpose as an argument against the general belief that fine clay-rich size fractions do not provide reliable geologically meaningful ages, even if consistently selected and

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Fig. 1. Regional geologic map of the study area framed by the bold rectangle. The three areas inside are: (I) the Valsurvio Dome and the Sierra del Brozo area, (II) the Ruesga Line of La Pernia Domain, and (III) the Palentine Nappe. The framed insert in the upper right corner indicates the extent of the Variscan belt in the Iberian Peninsula in gray and the Cantabrian Zone in black. After Brower (1967) and Julivert et al. (1983).

carefully interpreted, because of potential mixings of detrital and authigenic materials (e.g., discussions by Clauer and Chaudhuri, 1995, 1998; Clauer and Lerman, 2012).

2. Geological setting and sampling The westernmost part of the Variscan orogen in Europe consists of a typical thin-skinned tectonic arc whose center is the Cantabrian Zone in northwestern Spain with a structure and evolution that conform to models of most fold belts (Fig. 1). The folds and thrusts face toward the concavity of the arc, where external zones were described (PérezEstaún et al., 1988). The entire Cantabrian Zone represents the frontal part of the fold belt, consisting of shallow-water terrigenous and carbonate sediments that were detached from the underlying basement to form a thrust belt of nappes and slices with multi-generation folds (Julivert et al., 1980). Cleavage is often absent or only local, and the rocks are almost unstrained. The metamorphic gradient increases westwards, together with granitoid occurrences (Julivert, 1978). The Cantabrian Zone is generally divided into several major units differing as much in their structure as in their stratigraphic positions. Varied rock sequences were also subjected to contact metamorphism related to emplacement of the mentioned granitoid bodies in the southern area of the belt (Julivert and Arboleya, 1986). A regional syntectonic Variscan metamorphism that occurred close to the Carboniferous– Permian boundary during an extensional tectonic regime, and a Permian heat flow due to hydrothermal fluids migrating along pre-existing faults are among the described features (Aller et al., 2002; Brime et al., 2001; García-López et al., 1997). From W to E, the following wider regions were identified: (1) the Fold and Nappe Province, (2) the Central Basin, (3) the Ponga Nappe Province, (4) the Picos de Europa Province,

and (5) the Pisuerga–Carrion Province, which is the selected area for the present study. The Paleozoic sediments that unconformably overlie Proterozoic rocks reflect the evolution from rifting to a passive margin and further to a foreland fold and thrust belt (Aramburu and Bastida, 1995; Dallmeyer and Martinez-Garcia, 1990). The Variscan tectonics began in the late Carboniferous with the emplacement of regional thrusts and folds, followed by fracturing. The migration of the main thrusts from hinterland in the W towards the foreland in the E could be followed due to thin-skinned tectonic features (Pérez-Estaún and Bastida, 1990). After the main Variscan compression, continental Stephanian sediments were deposited in isolated intramontane basins. Diagenetic conditions were dominant with anchizonal to epizonal metamorphic grades described in the coal basins and in eastern Cantabria (Colmenero and Prado, 1993). This study concerns diagenetic to epimetamorphic, clastic sedimentary rocks of three areas of the southeastern Pisuerga–Carrion Province of the Cantabrian Zone (Fig. 1), namely: (1) the Valsurvio Dome and Sierra del Brezo area to the SSW, (2) the Palentine Nappes outcropping to the N of the Valsurvio and Sierra del Brezo domains, and (3) the Ruesga Line in between as the eastern continuation of the Ventaniella tectonic line. The Valsurvio and Sierra del Brezo regions are characterized by varied deformation features, and by different rock facies depending on a varied extension of the Upper Devonian clastics. These features induced cataclastic domains during Mesozoic and Cenozoic polyphased deformational faulting episodes. The Palentine Nappes consist of synorogenic Namurian to Westphalian sediments, including the Ruega faulting system to the S. The folded sequence is covered by varied synorogenic sediments of Westphalian age, and it outlines a sinistral shearing that was affected by successive deformations. These synorogenic sediments are polyphased in some cases,

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but were not transported over long distances. They are considered as parautochthonous, whereas the sub-Carboniferous deposits of the Palentine Nappes were interpreted as allochthonous units (Wagner, 1971). 3. Sample preparation and analytical procedure More than 1000 samples were collected and analyzed by XRD to assess the thermal context of the samples selected for K–Ar dating. The locations of the 20 samples selected for K–Ar dating of their size fractions are shown on maps of the three regions (Fig. 2A to D), and each rock facies description is provided with a geographic location (Table 1). This database was supplemented by expandability determinations of illite– smectite mixed layers (labeled I–S hereafter), by analysis of oriented and random clay powder slides, and by microscopic investigation of thin sections. About 150 polished sections were also examined for vitrinite reflectance data (Weh, 2004).

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degrees: ICI b0.25 for the epizonal degree, from 0.25 to 0.42 for the anchizonal degree, and N 0.42 for the diagenetic degree. The data were also normalized to the standard crystallinity scale of Warr and Rice (1994). The smectite content of the I–S minerals and the illite ordering degree were determined by the d002/d003 plots following Środoń (1984) and Moore and Reynolds (1997), and were checked with the Expert System software of Plançon and Drits (2000), which is able to identify up to three-component mixed-layer minerals consisting of any layer type and without any limitation in their order–disorder relationship. 3.2. Clay-particle morphology To identify the shapes of illite particles, size fractions were observed by TEM, by dispersing powders on a Cu grid and gold-coating them. The analytical conditions were set at an acceleration voltage of 15 kV, a probe current of 0.66 nA and a working distance of 20 mm. 3.3. Vitrinite reflectance

3.1. Clay mineralogy The samples were cleaned for all weathering signs at the rock surfaces, crushed, washed through a 2 mm sieve and ground in a mortar mill for 5 min in 0.01 N NH4 solution. The b 2 μm fraction was then separated by differential settling in deionized water following Stokes' Law. The suspensions were adjusted using a photometer to give highly oriented clay smear slides having a homogeneous density of 0.25 mg/cm2. The samples were X-rayed using a Siemens D5000 diffractometer (Cu-LFFtube, 40 kV, 35 mA, automatic theta-compensating slits, graphite monochromator). Peak width was determined with Krumm's (1994) WinFit software, with the following limits between the usual crystallization

Vitrinite reflectance data were obtained from sediment and coal samples. Coals were prepared as randomly oriented, resin-mounted preparations, while the sediments were cut perpendicular to bedding. After high-grade polishing, the samples were investigated using a Zeiss-reflection microscope with a 50 magnification oil-immersion lens and a Zeiss reflection measurement device. The standards used were garnet (= 0.883% R) and SiAl (= 7.42% R). The Rmax and Rmin values were determined on a ca. 100-point counting of each sample, and Rmean was calculated following Davis' (1978) formula: Rmean ¼ ð2Rmax þ Rmin Þ=3:

Fig. 2. Geologic maps with sample locations identified as VS1 to VS6, PN1 to PN7 and RL1 to RL7: (A) in the Valsurvio Dome (in the central part of zone I), (B) in the Sierra del Brezo area (in the eastern part of zone I), (C) in the Ruesga Line of La Pernia Domain (in the zone II), and (D) in the Palentine Nappe (in the central part of the zone III). Explanations of the symbols in the C and D maps are available in the referred publications. Panels A and B are both modified after Rodríguez-Fernández (1983). Panels C and D are both modified after Rodríguez-Fernández (1983, 1985, 1994) and Wagner (1984a,b).

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Fig. 2 (continued).

3.4. K–Ar dating The samples selected for K–Ar dating were crushed to chips of 5–10 mm and thoroughly washed with deionized water. Sample powdering was obtained by freezing and thawing cycles of the wet samples to avoid over-grinding of the initial framework minerals (e.g., Liewig et al., 1987). The separation of the b2 μm fractions was completed by differential settling in glass tubes filled with deionized water. Further size fractionation (b 0.2, 0.2–0.4, 0.4–0.6 and 0.6–2 μm) was obtained by differential settling in a temperature-monitored centrifuge.

Application of the K–Ar dating method follows Bonhomme et al. (1975). The samples were heated at 100 °C under vacuum for at least 12 h prior to Ar extraction to remove the atmospheric Ar potentially adsorbed onto the particles during sample preparation, size fractionation and handling. The K content was measured by flame spectrophotometry with a periodically controlled internal reproducibility better than 1.5% by analysis of the international basalt B-EN and glauconite GL-O standards. The amount of residual radiogenic 40Ar in the volume of the extraction line and the connected mass spectrometer was measured routinely after every 5-6 Ar determinations. Consistently in the

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Table 1 Location, rock type, stratigraphic age and geographic coordinates of the samples selected for the K–Ar study, XRD mineral composition, and illite and chlorite crystallinity indices of the b2 μm fractions. Sample ID

Location

Rock type

Stratigraphy

Position

Mineralogy

%Sm

ICI

CCI

Valsurvio and Sierra del Brezo VS1 N42°52′37.2″; W4°25′28.0″

Silty mudstone

Carboniferous (Westph. A)

Ill + Kaol + Chl + Pyr + Sm

12

0.10

0.21

VS2 VS3

N42°52′12.2″; W4°47′8.5″ N42°51′7.2″; W4°43′42.2″

Schist Clay-rich siltstone

Middle Devonian (Eifelian) Upper Devonian (Frasnian)

3

0.24 0.11

0.46 0.22

VS4

N42°52′27.9″; W4°36′28.5″

Black slate

Carboniferous (Westph. A)

0.16

0.32

VS5 VS6

N42°52′32.6″; W4°36′20.7″ N42°53′11.2″; W4°35′46.7″

Slate Black slate

Carboniferous (Westph. A) Middle Devonian (Eifelian)

Western polyphased Valsurvio Dome Id.; 1.4 km ESE of previous Southern Valsurvio Dome; 4.5 km ESE of previous Sierra del Brezo; thrust dominated area Id.; 0.5 km ENE of previous Id.; 2 km ENE of previous; two deformations

Palentine nappes PN1 N42°59′27.2″; W4°49′28.9″ PN2 N42°59′3.9″; W4°49′16.7″

Silty slate Silty slate

Upper Devonian (Famenian) Upper Devonian (Frasnian)

PN3

N42°57′22.0″; W4°49′2.5″

Conglomerate

Carboniferous (Westph. B)

PN4 PN5

nd N42°59′54.5″; W4°33′52.9″

Carboniferous (Westph. A) Lower Devonian (Siegenian)

PN6 PN7

N42°59′45.4″; W4°32′37.0″ N42°56′34.9″; W4°32′14.5″

Cataclastite in fault Fine-grained sandstone Claystone Mudstone

Ruesga Line RL1 N42°58′49.6″; W4°29′26.9″ RL2 RL3 RL4 RL5 RL6 RL7

Ill + Chl Ill + Ch + Sm Ill + Chl + Par + Mar Ill + Chl + Par + Mar + Sm Ill + Chl + Sm

8 5

0.16 0.10

0.32 0.21

Western part of Palentine nappe Id.; 0.5 km to S of previous; strongly faulted Id.; 2.5 km to S of previous; near thrust Id.; 6 km to ESE of previous Eastern part of Palentine nappe

Ill + Chl Ill + Chl + Sm

8

0.14 0.21

0.29 0.49

Ill + Sm

8

0.10

0.21

Ill + Chl Ill + Kaol + Sm

21

0.19 0.62

0.37 1.10

Lower Devonian (Siegenian) Lower Devonian (Siegenian)

Id.; 1.5 km to the E of previous Id.; 5 km S of previous; discordant cover

Ill + Kaol Ill + Sm

15

0.49 0.17

0.89 0.33

Schist

Carbononiferous (Westph. A)

Ill + Kaol + Sm

10

0.23

0.44

N42°58′55.1″; W4°29′19.2″

Siltstone

Carboniferous (Westph. A)

0.23

0.44

N42°59′11.1″; W4°28′16.8″ N42°55′9.4″; W4°30′52.8″ N42°55′34.3″; W4°29′14.3″ N42°52′57.3″; W4°26′45.0″ N42°52′49.2″; W4°48′46.6″

Claystone Mudstone Claystone Mudstone Clay-rich siltstone

Carboniferous (Westph. A) Lower Devonian (Siegenian) Carboniferous (Westph. A) Carboniferous (Westph. A) Lower Devonian (Emsian)

In Ruesga Line; SW of Castilleria area Id.; about 1 km to NNE of RL1; near major fault Id.; about 1.3 km to ENE of RL1 Id.; about 6 km to SWS of RL1 Id.; about 2.0 km to the E of RL4 Id.; about 10 km to S of RL3 Id.; about 1.5 km to the ESE of previous

0.50 0.13 0.32 0.21 0.09

0.91 0.27 0.59 0.40 0.20

Ill + Kaol Ill + Kaol + Chl Kaol + Ill + Sm Ill + Sm Ill + Kaol + Sm Ill

18 10 12

Ill stands for illite, Kaol for kaolinite, Sm for smectite, Chl for chlorite, Pyr for pyrophyllite, Par for paragonite, Mar for margarite, ICI for illite crystallinity index, CCI for chlorite crystallinity index, nd for not determined.

10−9 cm3 range, this volume of radiogenic 40Ar was two orders of magnitude lower than that of the analyzed size fractions, which was considered parochial enough to be neglected in the age calculation. The analytical external reproducibility was also controlled periodically by measuring the international standard mineral GL-O, which averaged 24.65 ± 0.09 (2σ) × 10− 6 cm3/g (in the STP system) of radiogenic 40Ar for five independent determinations during the course of the study, and the atmospheric 40 Ar/36 Ar ratio, which averaged 303.1 ± 3.1 (2σ) for eleven independent determinations of the atmospheric 40Ar/36Ar ratio during the course of the study. The recommended values being respectively 24.85 ± 0.24 × 10− 6 cm3/g for the amount of radiogenic 40Ar of the glauconite standard (Odin et al., 1982), and 298.6 ± 0.4 for the atmospheric 40Ar/36Ar ratio (Lee et al., 2006), the analyzed values were internally consistent and close enough to the theoretical ones that there was no need to apply discrimination corrections to the individual determinations. The K–Ar data were calculated with the usual decay constants (Steiger and Jäger, 1977), and with an overall error of ±2%. 4. Results 4.1. Clay mineralogy Most separated b 2 μm size fractions consist of illite, smectite sensu stricto, I–S, kaolinite and chlorite, as well as some more complicated assemblages including pyrophyllite, margarite and/or paragonite in a few samples (Table 1). To be mentioned is the occurrence of smectite that characterizes very-low temperature diagenetic or even continental

weathering conditions, together with paragonite, margarite and pyrophyllite that are more typical for lower anchizonal to epizonal metamorphic conditions (Frey, 1987). Used routinely as an indicator of the thermal evolution of rock samples, the illite crystallinity index (ICI) of the b2 μm clay fraction is indicative, in the Valsurvio and Sierra del Brezo domains, of the highest crystallization degree of the study area, ranging from upper (~ 0.24 Δ°2θ) to lower epizone (~0.09 Δ°2θ). In the Palentine Nappes, the ICI ranges widely from middle diagenesis (~ 0.62 Δ°2θ) to lower epizone (~0.10 Δ°2θ), whereas it is slightly less scattered in the samples from the Ruesga Line with values ranging from lower diagenesis (~ 0.50 Δ°2θ) to lower epizone (~ 0.10 Δ°2θ). The chlorite crystallinity index (CCI) has been less commonly used as a similar measure of crystallization degree (Árkai, 1991). Here the ICI and CCI correlate perfectly (Table 1), indicating that both minerals crystallized at identical thermal conditions, and that therefore they are most probably of the same generation, when observed together. The smectite content, which will be discussed in a further section, has been quantified from 1–2% to 21% with most fractions containing between 5% and 15% (Table 1; Fig. 3). Illite occurs generally as the better crystallized R3 Reichweite type (e.g., degree of ordering of the illite and smectite layers of I–S), but when the smectite content is higher than 15%, as is the case for three samples, illite is either of the R1 or the R2 type. Higher smectite contents correlate with higher ICI values for the I–S, as reported by Velde (1995) and Kübler and Goy-Eggenberger (2001), which appears as a kind of autocorrelation. The shape of the d001 peak of illite is also modified by the occurrence of NH4 (=tobelite), Na (= paragonite) and Ca (= margarite) in the mineral structure

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Fig. 3. Correlation between contents of smectite mixed with illite–smectite mixed layers and degree of illite ordering (=Reichweite) of the samples analyzed by XRD, based on the Expert System of Plançon and Drits (2000). The Δ°2θ 001 and Δ°2θ 002 values represent the difference in the mean position of the peaks (after Watanabe, 1981). The full width at half height maximum of the d001 (2θ) and d002 (2θ) peaks are compared with amounts of smectite given for the R = 0 to R = 3 illite Reichweite, which outlines the increasing layer stacking of illite in illite–smectite mixed layers.

(Fig. 4), especially in clay fractions from Valsurvio and Sierra del Brezo regions for the two latter minerals (Table 1). Based on the Mudmaster program (Eberl et al., 1996), the coherent scattering domains (CSD) of illite do not provide clear lognormal or asymptotic patterns. All clay fractions consist of mixtures of varied lognormal distributions with four mean CSDs: the thinnest at 6–7 nm followed by one at 9–12 nm, another at 18–20 nm and the last one at N24 nm. The smallest CSD occur mainly in samples of the eastern Palentine Nappes and the Ruesga Line, where they were probably mixed with coarser 9–12 and 18–20 nm particles. The parameters α (e.g., mean of the natural logarithms of the particle thickness) and β2 (e.g., variance of the natural logarithms of the same particle thickness) were calculated from crystal thickness distribution. The α–β2 plot of the particles shows that the data points are off the usual patterns of the β2 values with only the α values increasing with increasing particle size (Fig. 5). Less common minerals were also observed, such as chloritoid, pyrophyllite, zeolite, calcite, wollastonite, rutile, siderite, goethite and

Fig. 5. The parameters α (= mean of the natural logarithms of the particle thickness) and β2 (= variance of the natural logarithms of the particle thickness) were plotted in the α–β2 diagram of Eberl et al. (1996), which shows the progressive increase of the factor α that outlines random ripening of increasing grain size of the samples analyzed by XRD.

andalusite that depend on the type and chemistry of the rocks, the recrystallization degree and possibly the chemistry of the interfering migrating fluids. 4.2. Clay-particle morphology TEM studies are well suited for examination of particle shapes and for verification of the presence or absence of minute amounts of contaminant detrital minerals (e.g., Hamilton et al., 1992). Straight particle edges are often indicative of authigenic sheet silicates, whereas irregular edges are rather typical for detrital particles variously affected by dissolution processes or grinding impacts (e.g., Hunziker et al., 1986; Reuter and Dallmeyer, 1987). TEM observations may, therefore, provide useful information on the origin of the separated mineral assemblages. Here, the b0.2 μm fraction of sample RL6 from Ruesga Line consists mainly of idiomorphic particles having similar size and morphology (Fig. 6A and B). In Fig. 6B, the particles appear very thin, which is a

Fig. 4. XRD diagram showing the occurrence of margarite and paragonite in a b2 μm fraction. chl stands for chlorite, ill for illite, par for paragonite, and mar for margarite. From Weh (2004).

N. Clauer, A. Weh / Tectonophysics 623 (2014) 39–51

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Fig. 6. Transmission electron microscopic images: A and B correspond to the b0.2 μm fraction of sample RL6, with the white bars representing 1 and 2 μm, respectively, C and D show the b0.2 μm fraction of sample PN7 with the white bars representing 1 μm, and E and F outline the 0.2–0.4 μm fraction of sample PN7 with the white bars representing 2 μm. The arrows point to some odd, partly dissolved, particle edges.

reliable criterion for authigenesis together with the idiomorphic shape. The b0.2 and 0.2–0.4 m fractions of sample PN7 from the Palentine Nappes are also shown (Fig. 6C to F). Again, the particles are very thin, suggesting that they are mostly authigenic in the different separated size fractions as only a few odd shapes could be detected (see arrows in Fig. 6F).

2% Rmean were determined in the Ruesga Line; higher values up to 4.5% Rmean being recorded in contact aureoles of intrusive lithologies. Low values at about 1% Rmean were also determined at the southern border of the nearby Castilleria syncline.

4.3. Vitrinite reflectance

Two to four size fractions were dated by K–Ar for each sample depending on the available amounts of extracted powder (Table 2). In the samples from the Ruesga Line, the individual K–Ar ages range from 166.8 ± 5.7 to 392.0 ± 13.2 Ma. The age spectra of the different size fractions feature either inclined lines from finer and younger separates to coarser and older separates such as for samples RL2, RL3, RL6, and RL7, or “bench”-type age trends with the finer fractions of close to identical ages such as for sample RL1 at a mean age of 170 ± 10 Ma

The vitrinite reflectance decreases from 6.8% Rmean to the W to as low as 0.9% Rmean to the E, especially in the western Palentine Nappes. No coherent data spectrum could be generated for the samples from eastern Palentine Nappes. The values are about constant at 4–5% Rmean in the Valsurvio Dome, whereas they increase slightly to 6% Rmean in the Sierra del Brezo area. The lowest vitrinite reflectance values below

4.4. K–Ar dating

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Table 2 K–Ar results of the different size fractions from selected samples. The analytical age errors are shown in brackets. Size fractions (μm)

K2O (%)

Rad Ar (%)

Rad 40Ar (10−6 cm3/g)

40

40 36 K/ Ar (×10−6)

Age (Ma ± 2σ)

Smectite (%)

Valsurvio Dome and Sierra del Brezo VS1 b0.2 0.2–0.4 0.4–0.6 0.6–2 VS2 b0.2 0.2–0.4 0.4–0.6 0.6–2 VS3 b0.2 0.6–2 VS4 b0.2 0.2–0.4 0.6–2 VS5 b0.4 0.6–2 VS6 b0.2 0.4–0.6

5.06 5.28 5.33 4.96 4.20 4.10 1.54 4.34 3.34 5.96 4.24 4.82 4.18 4.04 4.27 4.52 4.13

85.30 55.51 90.32 91.34 78.50 76.40 39.21 70.43 26.40 92.44 65.70 47.60 93.50 42.34 91.64 79.80 90.13

51.99 56.98 59.87 62.05 30.80 41.30 10.53 29.43 12.69 56.01 34.47 43.27 43.48 39.31 43.08 38.00 39.54

2011 664 3053 3414 1372 1254 486 999 nd 3906 862 564 4546 513 3534 1459 2993

0.093 0.019 0.133 0.135 0.081 0.053 0.015 0.058 nd 0.213 0.038 0.017 0.226 0.012 0.174 0.076 0.156

293.6 (7.3) 307.2 (11.5) 318.7 (7.5) 351.4 (8.3) 214.2 (5.6) 288.1 (7.7) 200.5 (7.9) 198.9 (5.1) 114.2 (4.6) 270.2 (5.4) 236.0 (6.2) 258.9 (8.7) 296.8 (7.8) 279.1 (14.0) 288.7 (6.9) 243.5 (6.1) 274.8 (7.2)

12.5 10.0 7.0 2.5

0.4–0.6 0.2–0.4 0.4–0.6 b0.2 0.2–0.4 0.6–2 b0.2 0.2–0.4 b0.2 0.2–0.4 0.4–0.6 0.6–2 b0.2 0.2–0.4

4.80 6.84 9.48 4.92 5.44 5.04 5.64 3.98 4.02 4.64 4.47 4.37 3.71 3.55

88.11 44.40 97.17 81.80 71.50 95.31 78.80 70.30 85.96 78.75 92.58 94.84 85.70 61.10

26.18 26.88 78.85 29.14 32.68 40.49 37.24 37.48 35.10 42.42 45.97 51.85 32.77 36.68

2486 532 10,427 1625 1036 6303 1393 995 2104 1391 3981 5722 2065 760

0.223 0.033 0.674 0.123 0.168 0.413 0.091 0.041 0.115 0.066 0.194 0.244 0.110 0.025

161.7 (3.8) 117.9 (2.6) 241.1 (2.8) 174.9 (4.2) 177.3 (4.1) 233.4 (5.3) 194.0 (4.1) 270.7 (7.4) 252.4 (6.4) 263.5 (7.1) 293.7 (6.9) 334.9 (7.8) 255.1 (7.3) 294.9 (8.6)

b0.2 0.2–0.4 0.4–0.6 0.6–2 b0.2 0.4–0.6 b0.4 0.6–2 b0.2 0.2–0.4 0.4–0.6 0.6–2 b0.2 0.2–0.4 0.4–0.6 0.6–2 b0.2 0.6–2 b0.2 0.2–0.4 0.6–2

3.36 4.68 3.92 3.79 3.92 3.77 3.97 3.26 3.52 4.32 4.38 3.08 6.37 6.03 6.81 6.43 7.00 5.32 5.43 8.48 9.48

74.68 63.28 92.15 94.58 79.80 88.56 39.16 89.94 64.95 47.71 80.44 63.40 94.72 96.44 97.20 97.73 89.10 91.34 84.40 93.20 98.96

19.98 26.36 29.75 32.54 40.00 43.81 30.21 37.87 42.41 44.42 44.23 42.90 52.48 57.85 61.96 65.76 58.86 64.19 42.90 70.95 88.39

1167 805 3763 5453 1441 2584 486 2936 843 565 1511 807 5596 8295 10,552 13,008 2722 3114 1891 4344 28,524

0.081 0.050 0.247 0.322 0.062 0.109 0.014 0.123 0.025 0.015 0.066 0.020 0.356 0.461 0.585 0.635 0.158 0.143 0.111 0.264 1.67

175.7 (5.4) 166.8 (5.7) 221.3 (5.3) 248.5 (5.9) 291.6 (8.0) 328.6 (9.2) 221.9 (11.7) 329.4 (8.2) 239.7 (11.1) 293.8 (12.7) 288.9 (7.7) 392.0 (13.2) 239.1 (5.4) 275.6 (6.1) 262.3 (5.7) 292.3 (6.3) 243.6 (4.2) 340.0 (7.5) 229.7 (5.0) 242.4 (3.4) 268.2 (3.1)

Sample ID

Palentine nappes PN1 PN2 PN3 PN4

PN5 PN6

PN7

Ruesga Line RL1

RL2 RL3 RL4

RL5

RL6 RL7

Ar/36Ar

21.0 16.5 11.5 11.0

22.5 17.0 14.0 13.2

Rad stands for radiogenic, nd for not determined.

(Table 2). Such bench-type distributions relative to the size of the illiterich separates were initially described by Reuter (1987) and further discussed by Clauer and Chaudhuri (1998). Whenever the ages of the two smallest fractions are analytically identical, and those of the coarser fractions are older, the drafted result implies that the identical younger K–Ar ages are geologically meaningful, whereas the older is (or are if more than one fraction) biased by addition of either detrital material or material crystallized during an earlier event. Also, any inclined shape indicates heterogeneous size fractions and therefore ages that record mixings of K-bearing minerals of different initial ages. In plotting the K–Ar ages relative to the grain size of the fractions, most patterns feature inclined lines such as for sample RL7, as well as one horizontal

segment for samples RL1 and RL4 (Fig. 7). In the case of the Palentine Nappes, the individual K–Ar ages range from 117.9 ± 2.6 to 334.9 ± 7.8 Ma (Table 2), which is slightly narrower than the age range for the size fractions from Ruesga Line. Only the data points of the two finer size fractions of sample PN4 yield a similar mean age at 176 ± 2 Ma (Fig. 7). In the Valsurvio and Sierra del Brezo areas, the individual K– Ar ages range from 114.2 ± 4.6 to 351.4 ± 8.3 Ma (Table 2). No consistent horizontal segment was observed among the size fractions of the samples from these areas. Comparison of the TEM observations and K–Ar ages shows that the particles of the b0.2 μm fraction of sample RL6 and the two b 0.2 and 0.2–0.4 μm fractions of sample PN7 are mostly of authigenic origin as

N. Clauer, A. Weh / Tectonophysics 623 (2014) 39–51

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fractions are younger than the depositional ages of the samples, whatever the stratigraphy and rock facies, it implies repetitive crystallization processes in the clay-sized fractions of the selected samples. Crystallization of authigenic K-bearing clays is therefore to be expected periodically at 300 ± 10, 250 ± 10, 230 ± 10 and 180 ± 10 Ma during the tectono-thermal history of the Cantabrian Zone. These ages were already reported in claystones and siltstones of other regions characterized by Variscan orogenic activities, especially of the Rhenish Massif in Germany (Ahrendt et al., 1978, 1983; Lippolt and Hess, 1983; Lippolt et al., 1989; Nierhoff et al., 2011; Reuter, 1987; Reuter and Dallmeyer, 1987). 5.1. K–Ar age interpretation based on bench-type and isochron plots The multi-episodic regional tectono-thermal evolution, combined with the fact that the K–Ar ages have a tendency to increase relative to grain size, complicates the interpretation of the K–Ar data. Complex tectono-thermal evolutions of regional extent that integrate successive metamorphic and/or tectonic episodes have been reported (e.g., Sasseville et al., 2008, 2012; Surace et al., 2011). Such a complex evolution is supported here by changing paragenetic mineral associations that include smectite, pyrophyllite, margarite and paragonite and by the α–β2 plot (Fig. 5). In order to constrain the K–Ar results, two approaches were examined for their interpretation: the above mentioned bench-type age distribution of the varied size fractions of the different samples, and the more commonly used 40Ar/36Ar vs. 40 K/36Ar isochron plot. Only the b0.2 and b 0.4 μm fractions were selected for this latter graphic, as they most probably consist only of authigenic material, no contaminating mineral phases having been detected by XRD or observed by TEM (Table 1). Fig. 7. Bench-type presentation of K–Ar ages as a function of grain size in samples of the different study areas. The gray zones represent the geological meaningful average ages.

the K–Ar ages at 243.6 ± 4.2, 255.1 ± 7.3 and 294.9 ± 8.6 Ma, respectively, are within the age ranges of the different illitization episodes determined independently by the bench-type distribution (Fig. 7). In summary, combining the different horizontal distribution segments with the individual ages of similar size fractions shows that the K–Ar ages relative to the analyzed size fractions point to three or four episodes of illite crystallization at 300 ± 10 in the Ruesga Line and the Palentine Nappes, at 230 ± 10 Ma in the former and apparently slightly earlier at 250 ± 10 in the later, and at 175 ± 10 Ma again in both regions. 5. Discussion Progressive K–Ar age increase relative to grain size is generally considered a result of an increasing supply of detrital K-minerals in the progressively coarser fractions (e.g., Clauer and Chaudhuri, 1995). This distinction is less evident in samples that underwent a complex evolution integrating recurrent metamorphic and/or tectono-thermal episodes, and are characterized by specific paragenetic associations including pyrophyllite, margarite and paragonite. The unexpected α–β2 plot also suggests a mixing of several generations of illite with different structural features (Fig. 5). Most if not all of the samples studied here underwent a diagenetic to low-grade metamorphic overprint with illite crystallization temperatures in the range of 200–300 °C (Krumm, 1992), the higher conditions occurring probably in the hanging-wall rocks of the fault zones due to enhanced fluid movements (e.g., Raven and Van der Pluijm, 1986). Such a complex evolution is also apparent in the age distribution relative to the size fractions of samples such as RL1 and RL5. It is obvious that these mineral phases are not just either detrital or authigenic clay fractions; they could represent successive generations of the same illite type. As the K–Ar ages of the finer size

5.1.1. The bench-type distribution The bench-type method is based on the concept that two finegrained clay-rich size fractions from the same sample give the same age only when they are strictly homogeneous, therefore of the same generation, whereas coarser fractions yield older ages due to addition of detrital components (Clauer and Chaudhuri, 1998). This is because different size fractions cannot yield identical isotopic ages if they consist of mixtures of detrital and authigenic particles, unless containing precisely the same amounts of both mineral types having each strictly the same ages, which is almost never the case in nature. If this assumption of homogeneous fractions with the same age is fulfilled, the combination of the ages with the size of the different analyzed fractions will give a bench-type distribution pattern if correlating the K–Ar ages with the size of the separated clay fractions. Among the studied samples of the Ruesga Line, sample RL1 yields a bench-type distribution with a geologically meaningful K–Ar age at 160–180 Ma. In the Palentine Nappes, sample PN6 yields a similar bench-type pattern at 250– 270 Ma, whereas sample PN4 yields an incomplete pattern again at 160–180 Ma (Fig. 7). Sample VS5 of the Sierra del Brezo area provides also an incomplete pattern with a potentially useful age at 280– 290 Ma. Some size fractions with older K–Ar ages plot in the age ranges defined by other size fractions; it is the case for most coarse 0.6–2 μm size fractions. In summary, this bench-type method has the advantage of allowing distinction of several tectono-thermal episodes in the whole set of ages at 160–180, 220–240, 250–270, and 280–300 Ma (Fig. 7). The coarse size fractions (0.6–2 μm) that potentially are dominated by detrital material (e.g., samples RL2, RL3, RL6, RL7, and VS1), can also be dominated by authigenic clay material crystallized during earlier tectonic activities (e.g., samples RL1, RL4, PN1, PN2, and VS3). Additional information, which is less constrained, is also in the K–Ar ages of some 0.6–2 μm fractions that cluster at 340 ± 10 Ma, such as for samples RL3, RL6, and PN6. This age cluster records coarse fractions that could consist of illite- and mica-type material mixed with other K-bearing minerals such as feldspars potentially generated during the

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N. Clauer, A. Weh / Tectonophysics 623 (2014) 39–51

mid-Carboniferous Variscan plutonic activity in this part of the orogen. Such an interpretation is supported by isotopic ages of regional plutonic intrusions (Almeida et al., 2002; Dallmeyer and Martinez-Garcia, 1990; Fernandez-Suarez et al., 2000; Valle Aguado et al., 2005). Some of the older illite material could have been generated during an earlier Variscan plutonic activity than that recorded in the metasedimentary rocks. 5.1.2. The isochron method The 40Ar/36Ar vs. 40 K/36Ar isochron pattern is used sometimes to constrain the individual K–Ar ages of clay-type material. However, this application is partly obscured here by a scattered plot of the data points between two lines that limit the age spread between about 290 and 170 Ma (Fig. 8A). Based only on the very fine fractions that are considered to consist mostly if not solely of authigenic illite and smectite, the data scatter is complicated, reflecting the complex regional tectonic and metamorphic evolution. If plotting only the data points of the Ruesga Line, three lines consisting respectively of two, four and three data points provide ages of 292.7 ± 0.3, 243.0 ± 4.7 and 175.0 ± 5.3 Ma (Fig. 8B). These lines yield initial 40Ar/36Ar ratios reasonably close to the atmospheric Ar ratio, and no data point off the lines. In the case of the Palentine Nappes, one line can be drawn through three data points; one data point plotting off the line (Fig. 8C). A mean age of 268.0 ± 5.8 Ma is obtained with an initial 40Ar/36Ar of about 400, which is slightly high relative to the atmospheric value. Two lines also fit through the data points of the Valsurvio and Sierra del Brezo areas, consisting both of three data points (Fig. 8D). The upper line yields an age of 293.4 ± 4.0 Ma and the lower an age of 243.4 ± 4.2 Ma. Both lines have initial 40Ar/36Ar ratios close to that of the atmospheric

value. In summary, the lines of the general isochron plot (Fig. 8A) are confirmed by the results in the different regions, with an age of 292.7 ± 0.3 Ma in the Ruesga Line and of 293.4 ± 4.0 Ma in the Valsurvio and Sierra del Brezo areas. Further episodes at 268.0 ± 5.8 Ma were detected in the Palentine Nappes, and of 243.4 ± 4.2 in both the Valsurvio and Sierra del Brezo areas, and of 243.0 ± 4.7 Ma in the Ruesga Line, the last event being set at an average age of 175.0 ± 5.3 Ma in the Ruesga Line. In summary, the bench-type method allowed distinction of tectonothermal episodes at 280–300, 250–270 and 160–180 Ma. When applied to the samples of the different regions, the isochron method confirms this distinction with additional details for the intermediate period at about 243 Ma. 5.2. Smectite occurrence and K–Ar ages Occurrence of smectite is not common in clay fractions of siltstones to claystones that crystallized during a thermal event, as it is generally indicative of low-temperature continental weathering and/or discrete diagenetic processes (e.g., Price and McDowell, 1993; Zhao et al., 1999). If smectite crystallization occurs at temperatures below ~100 °C, it may have crystallized in epimetamorphic K-depleted environments such as carbonates (e.g., Burkhard, 1988; Dunoyer de Segonzac and Bernoulli, 1976; Huon et al., 1987). The occurrence of I–S in folded and faulted rocks, together with smectite as is the case here, has been attributed in some occasions to fluid circulation in faulted systems with elevated water–rock ratios (e.g., Clauer et al., 1992, 1995; Sasseville et al., 2012), which could have been possible here. Two features are consistently noted here: (1) the negative correlation between increasing amount of smectite and decreasing K–Ar age and ICI of the associated I–S (Table 2), and (2) the correlation between increasing smectite content and decreasing size of the separated fractions (right-hand plots of Fig. 9), which is complemented by lower ICI of the I–S when particle size increases, confirming in turn lower smectite contents in coarser size fractions (left-hand plots of Fig. 9). The negative correlation between smectite content and ICI of the associated I–S supports a contemporaneous crystallization. Younger than the deposition age, the K–Ar ages, at least of the smaller size fractions, confirm the authigenic nature of the two clay components. The sample preparation including removal of the weathered rock parts as mentioned in a previous section precludes continental weathering for the observed smectite, whereas the absence of K-depleted rocks in the studied geologic environment rules out an epi-metamorphic crystallization. As all analyzed samples are either folded or faulted, it can be safely assumed that smectite resulted from interaction of the host rocks with K-depleted hydrothermal fluids at probable temperatures of ca. 150– 250 °C, as the most abundant authigenic clay minerals in the studied fault rocks are chlorite, kaolinite, smectite and smectite-rich I–S that are all depleted in K. 5.3. The regional structural evolution during Variscan and post-Variscan times

Fig. 8. 40Ar/36Ar vs. 40 K/36Ar isochron plots for the b0.2 and b0.4 μm size fractions from the different study areas. A = Fractions from all areas; B = from the Ruesga Line; C = from the Palentine Nappe; D = from the Valsurvio Dome and Sierra del Brezo area. The numbers in brackets stand respectively (1) for b 0.2 and (2) for 0.2–0.4 microns.

The Cantabrian arc was described first by Suess (1885–1909) about 128 years ago, and was classified as an “orocline” by Carey (1955), which corresponds to an orogenic belt with an imposed bending curvature resulting from a horizontal deformation of the crust. Several evolutionary models of the arc were constructed (Julivert and Arboleya, 1986; Matte, 1986; Matte and Ribero, 1975; Pérez-Estaún and Bastida, 1990; Pérez-Estaún et al., 1988; Ries and Shackelton, 1976). Weil et al. (2000, 2001) described two Variscan and one Mesozoic tectonic events on the basis of secondary deformations of the Cantabrian arc. The first pulses were considered to belong to an E–W compressive folding and thrusting that occurred in Namurian to Stephanian time. The following phase was identified as a Permian N–S compression oroclinal bending that developed radial folds with variably oriented axes.

N. Clauer, A. Weh / Tectonophysics 623 (2014) 39–51

Fig. 9. On the right side the K–Ar ages are plotted on the ordinate relative to the smectite amounts in the grain sizes of samples VS1, RL5 and PN6 on the abscissa. Are plotted on the left side: (1) the K–Ar ages relative to the increasing size fractions (gray symbols) with the same age ordinate than in the right-side graphics, and (2) the illite crystallinity indices (open symbols) on the ordinate relative to the same size fractions on the abscissa.

The reconstructed structural models that were published over the recent decades do not necessarily agree in the description and timing of the successive deformational episodes. Alonso et al. (2009), for instance, suggested that breaching thrusts, such as the León fault, which is the western extension of the Ruesga Line, could have induced duplications of former thrust stacks. Such large breaching thrusts could have further dismantled initial paleogeographic patterns, producing apparent paleogeographic inversions at a regional scale. The León fault in the Variscan foreland fold–thrust belt of the Iberian Peninsula caused much controversy in the interpretation of the Iberian Paleozoic tectonic evolution, even implying a redefinition of the geological provinces of the Cantabrian Zone. It also has implications in discriminating between the various kinematic models proposed for the larger Ibero-Armorican Arc. The intention here was not to favor any of the structural models on the basis of the obtained K–Ar ages on illite fractions, but to constrain best the dominant tectono-metamorphic episodes that were described in the studied area. Ries (1979) published K–Ar ages of 41 mica separates from early Paleozoic rocks of the Variscan fold belt of southern Brittany and northwestern Spain that were distributed between 332 and 262 Ma, including results from low-grade metamorphic rocks. Fernandez-Suarez et al. (2000) distinguished two main magmatic syn- and post-Variscan episodes on the basis of U–Pb dating granitoid rock-types: an emplacement at ca. 325 Ma with a D2 crustal deformation and additional intrusions, a 320–310 Ma post-tectonic activity as well as minor mafic rock intrusions at 295–290 Ma and scarce leucogranite intrusions at 290–285 Ma. Also noteworthy are U–Pb zircon and monazite ages of granite plutons from northern and central-

49

northern Portugal that were emplaced from 314 to 308 Ma, and at 299 Ma (Almeida et al., 2002; Valle Aguado et al., 2005). Here, the K–Ar ages of illite-type clay material from metasedimentary rocks of three tectonic Cantabrian settings point to an Early Permian episode at about 293 ± 3 Ma in the Ruesga Line and mid-Permian episodes at about 268 ± 6 in the Ruesga Line and Palentine Nappes, and at about 243 ± 5 Ma in the Valsurvio and Sierra del Brezo areas. A Jurassic episode occurred at about 175 ± 6 Ma in the Ruesga Line. Low-grade metamorphism, folding and faulting appear contemporaneous to high-grade metamorphism and plutonism after the Variscan orogeny, at about 293 ± 3 and 268 ± 6 Ma. Alternatively, the low-grade metamorphism and folding/faulting at 268 ± 6 and 243 ± 5 Ma, and at 175 ± 6 Ma, were not recorded by high-grade metamorphic and plutonic activities. In the German Rhenish Massif, which was also subjected to a complex Variscan tectono-thermal evolution, especially in its eastern part (review in Nierhoff et al., 2011), the early Permian episode was at about 282 ± 12 Ma, as well as less-constrained Permian extensional post-orogenic activity at around 270 Ma in the western area. At a larger scale, metamorphic ages of about 270 Ma are widely reported in Western Europe, for instance in the French Massif Central (e.g., Alexandre, 2007), and in the Ivrea Zone of the Alps (e.g., Voshage et al., 1987). The occurrence of a Permian plume activity is also reported in many locations of Central Europe between Oslo and Bolzano (e.g., Breitkreuz and Kennedy, 1999; Timmerman, 2004). The mid-Jurassic episode at 175 ± 6 Ma determined in the Ruesga Line and the Valsurvio and Sierra del Brezo areas is also recognized as a widely spread hydrothermal imprint reported by varied methods on different minerals over all of Western Europe: by K–Ar illite dating of fault gouges in the northwestern Massif Central (Cathelineau et al., 2004) and the central Rhine Graben (Schleicher et al. (2006), and by U–Pb uraninite dating in southeastern Massif Central (Léveque et al., 1988). More Liassic hydrothermal activities were also reported in metasedimentary rocks of Western Europe and Northern Africa by illite K–Ar dating (Clauer et al., 1996; Schaltegger et al., 1995). Interestingly, migrating hydrothermal fluids that induced these clay and ore precipitations seem to have occurred in quiescent regions at significant distances (more than 500 km) from contemporaneous deformation areas (orogens, rifts). These hydrothermal fluid flows were often related to episodic rifting and opening episodes of the northern Atlantic Ocean, especially the Biscayne Bay rifting (Cathelineau et al., 2012; Clauer et al., 1996), which is closer to the Cantabrian Zone than the identified contemporaneous tectono-thermal episodes in the European and African metasediments. 6. Conclusions Diagenetic and low-grade metamorphic imprints in sedimentary rocks form, together with tectonic features, a complex pattern in the Cantabrian Zone of the Variscan orogen. The distribution of these metamorphic and tectono-thermal records has been based on illite and chlorite crystallinity indices, vitrinite reflectance data, clay mineralogy and microscopic observations of rock microtextures. K–Ar dating of finegrained, illite-rich size fractions provides a complete set of ages for the deformations and thermal overprints in the southeastern area. The structural pattern of the Cantabrian Zone with its thin-skinned geometry is complicated by more than one level of decollement, integrating an irregular map of thrust units. Because of this complexity, two K–Ar analytical approaches were combined to extend the results at a regional scale: the bench-type age distribution and the isochron method, in order to consolidate the obtained age by reciprocal control. The following tectono-thermal episodes could be identified in the southeastern Pisuerga–Carrion Province: (1) at 293 ± 3 Ma in the Ruesga Line, the Valsurvio Dome and the Sierra del Brezo area, (2) at 268 ± 6 Ma in the Palentine Nappes, (3) at 243 ± 5 in the Ruesga Line and the Valsurvio and Sierra del Brezo areas, and (4) at 175 ±

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6 Ma in the Ruesga Line. The highly deformed Ruesga Line was the most active area of the studied region. Acknowledgments We would like to sincerely thank Rob and Ray Wendling, D. Tisserant and R. Winkler who were the core of the isotopic technical team of the Centre de Géochimie de la Surface (CNRS/ULP), for their permanent involvement in the analytical aspects. NC would also like to extend his sincere thanks to Mrs. F. Whitehurst for improvement of the English presentation of this publication. Ray Ferrell Jr. of Louisiana State University is also sincerely thanked for his very constructive and helpful review. References Ahrendt, H., Hunziker, J.C., Weber, K., 1978. K–Ar-Altersbestimmungen an schwachmetamorphen Gesteinen des Rheinischen Schiefergebirges. Z. Dtsch. Geol. Ges. 129, 229–247. Ahrendt, H., Clauer, N., Hunziker, J.C., Weber, K., 1983. Migration of folding and metamorphism in the Rheinisches Schiefergebirge deduced from K–Ar and Rb–Sr-age determinations. 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