- Société géologique de France Éditeur, Paris
The combined use of the illite “crystallinity” Kübler index (KI) and the conodont colour alteration index (CAI) has revealed the existence of three thermal episodes in an area affected by thin-skin tectonics, close to the internal zones of the Variscan orogen in NW Spain. In the southernmost part of the study area, the first episode gave rise to a regional syntectonic Variscan metamorphism. The associated deformation involves the development of a slaty cleavage, which is mainly recognized in Precambrian rocks. Towards the foreland, the syntectonic metamorphism disappears and only an incipient burial metamorphism, giving rise to anchizonal conditions in the basal part of the thrust units, is observed.
Another metamorphic episode occurred close to the Carboniferous-Permian boundary in an extensional tectonic regime. This metamorphism is restricted to the northern part of the study area, where it reached anchizonal or epizonal conditions. It is associated with a subhorizontal or moderately north-dipping cleavage and can be considered as a late-Variscan episode.
The last thermal episode occurred during the Permian. It was produced by heat flow due to hydrothermal fluids, whose migration was favoured by faults. The effects of this episode are irregularly distributed, and they are apparent in the unconformable Stephanian rocks in which anchizonal or epizonal conditions were reached. It is interpreted as a post-Variscan episode.
The study of the thermal evolution of areas in the transition from diagenesis to metamorphism has been made possible through the development of techniques based on the analysis of clay minerals, mainly the Kübler index (KI) [Kübler, 1967], and the study of the transformation of organic matter with increase in temperature, largely using the conodont colour alteration index (CAI) [Epstein et al., 1977; Rejebian et al., 1987] and vitrinite reflectance [Stach et al., 1975; Tissot and Espitalié, 1975]. These techniques have allowed researchers to study the thermal evolution in the external zones of orogenic belts, where low or very low grade metamorphism can develop before, during or after deformation. The thermal evolution is related to mechanisms such as the simple burial of the series, tectonic superposition, emplacement of thrust units at higher temperatures, intrusion of igneous rocks, etc.
The Cantabrian Zone is the external zone of the Variscan belt in NW Spain, and presents a complex oroclinal structure of folds and thrusts that constitutes a good scenario for the study of the lowest metamorphic grades and their relation with regional geology. The thermal evolution in this area has been studied with KI methodology by Galán et al. , Pérez-Estaún , Brime and Pérez-Estaún , Brime [1981, 1985], Aller et al. , Alonso and Brime , Keller and Krumm [1992, 1993] and Gutiérrez-Alonso and Nieto . CAI data were incorporated to the analysis by Raven and van der Pluijm . The combined use of KI and CAI in the western and northwestern parts of the Cantabrian Zone [García-López et al., 1997; Bastida et al., 1999; Brime et al., 2001] has led to the establishment of models of tectonothermal evolution for these areas. Preliminary reviews on the tectonothermal evolution of the Cantabrian Zone, based on CAI and KI data, can be found in García-López et al.  and Bastida et al. .
Despite recent advances in our understanding of the tectono-thermal evolution of the Cantabrian Zone, some areas are still poorly known. One of them is the southern part of the Cantabrian Zone, which presents a complex structure and common late-orogenic unconformable series, whose study can be useful to constrain the age and tectonic implications of the late-Variscan metamorphism. The aim of this paper is the analysis, using KI and CAI methodology, of the thermal evolution in the southern part of the Cantabrian Zone. To achieve this goal, all the available data from this area have been compiled and new samples of shale and limestone have been collected to determine KI and CAI. The results obtained are analysed in the context of the whole Cantabrian Zone in order to improve the general model of tectono-thermal evolution of this zone.
The Cantabrian Zone is an area of thin-skinned tectonics with thrusts and folds affecting rocks ranging in age from Precambrian to Carboniferous. The Palaeozoic pre-orogenic sequence is present up to near the Devonian-Carboniferous boundary, and consists of mainly siliciclastic Lower Palaeozoic series and alternating siliciclastic and carbonate Devonian formations. Carboniferous synorogenic deposits have been interpreted as deposited in a foreland basin in which a transition exists from dominant carbonates in the lower part to mainly siliciclastic coal-bearing series in the upper part [Julivert, 1978]. The most common structures in the Cantabrian Zone are thrusts and folds with an arcuate pattern that defines the Asturian arc (fig. 1⇓). Two sets of folds have been distinguished in this zone [Julivert and Marcos, 1973], a longitudinal set parallel to the thrusts and a radial set. These two sets are mainly thrust-related folds associated to the frontal and lateral ramps of the thrusts, respectively [Pérez-Estaún et al., 1988]. The Cantabrian Zone Palaeozoic sequence rests unconformably on the Precambrian rocks of the Narcea antiform, an area transitional to the more internal Westasturian-Leonese Zone, where cleavage and regional metamorphism are widespread. Stephanian (late Carboniferous) siliciclastic unconformable series with conglomerates and coal beds crop out in basins, commonly fault-bounded, distributed throughout the Cantabrian Zone.
The southern part of the Cantabrian Zone, studied in this paper, is part of the Nappe and Fold province [Julivert, 1971] and has a structure of thrusts and associated folds in which two main thrust units can be distinguished: the Bodón Unit to the north and the Correcilla Unit to the south (fig. 1⇑). The latter unit contains Precambrian rocks of the Narcea antiform in its southern part [Alonso et al., 1989]. These two units can be traced into the western part of the Cantabrian Zone, where they are named La Sobia Unit and Somiedo Unit, respectively. An important stratigraphical difference can be found in the Middle and Upper Devonian sequence, which is complete in the Correcilla Unit and missing in the Bodón Unit (fig. 2⇓). The post-Visean Carboniferous succession is also different, since the Correcilla Unit only presents Namurian turbidites and limestones, whereas the Bodón Unit has mainly paralic Westfalian facies over Namurian limestones. As it is common in the Cantabrian Zone, the thrusts usually show a décollement level located in the carbonate rocks of the Láncara Formation (Lower-Middle Cambrian), and folds from both the longitudinal and the radial sets are frequent in the area.
Unconformable Stephanian B-C rocks crop out in two areas corresponding to intermontane basins: the Ciñera-Matallana and the La Magdalena coal fields (fig. 3⇓). Basic dykes and sills can be found intruded in these rocks. The structure of these basins is mainly characterized by upright folds and high-angle faults concentrated near the boundaries of the basins. An important fracture, the León Fault, appears in the northernmost part of the study zone and constitutes the boundary with the thick successsion of Carboniferous rocks cropping out to the north (Central Coal basin). In the southern part of this basin, there is low-grade metamorphism and subhorizontal cleavage cutting through longitudinal and radial folds [Aller and Brime, 1985; Aller, 1986]. Another important fracture is the Sabero-Gordón fault, which is also oriented E-W and in its eastern part constitutes the southern boundary of the Ciñera-Matallana basin (fig. 3⇓).
MATERIALS AND METHODS
We have compiled all the available information about conodonts from the study area, corresponding to collections in the National Museum of Natural History at Leiden (The Netherlands) and the University of Oviedo (Spain). The CAI determinations of Raven and van der Pluijm  were used for the samples from Leiden, whereas for the Oviedo samples the CAI was determined by one of the authors (S.G.L.). This collection of data incorporated information from 107 localities in the study zone. These data were complemented with 36 new localities sampled for this study, of which 16 yielded conodonts and enabled CAI determination. Neighbouring localities have been plotted on the maps (figs. 3⇑ and 4⇓) in a single point. Due to the lack of conodont-bearing carbonate rocks in the Lower Palaeozoic, the information given by CAI data is restricted to the zones with outcrops of Devonian and Carboniferous rocks. The methodology used for sample treatment and CAI determination has been described in García-López et al.  and Bastida et al. . For the metamorphic zonation from CAI data, we use the terms defined by García-López et al. : diacaizone (CAI < 4), ancaizone (4 ≤ CAI ≤ 5.5) and epicaizone (CAI > 5.5). At each locality, the mean CAI value has been used for the contouring of the CAI data on the map.
Layer silicate mineralogy and Kübler index
71 samples of pelitic rocks were collected (fig. 3⇑) for X-ray diffraction analyses of the mineral composition in the < 2 μm fraction and for determination of the Kübler index (KI = illite crystallinity [see Guggenheim et al. 2002]). Diffractograms were obtained using a Philips X’Pert diffractometer equipped with graphite monochromator and using CuKα radiation. These data were complemented with 102 data points from the work of Marschik ; however, the data he provided correspond only to air-dried mounts.
Preparation of samples and determinations of the Kübler Index followed the recommendations of the IGCP 294 working group [Kisch 1991]. The KI values obtained have been converted to the Kübler scale, with anchizone limits of 0.42°/0.25°Δ2𝛉, using a set of samples provided by H. Kisch, the same standards as used by Marschik , and therefore allowing comparison of the two sets of data. Since illite/smectite mixed-layer minerals are abundant in the study area, the measurement of the peak width includes both illite and illite/smectite basal reflections. It should be noted that the numerical values of the KI decreases as the ‘crystallinity’, and therefore the grade, increases. KI values corresponding to the Kübler scale can be converted to the CIS (Crystallinity Index Standard) scale [Warr and Rice 1994] using the equation of Brime :
where R is the correlation coefficient.
Mean CAI values from the localities of the study zone range from 1.5 to 5, indicating diachizonal to anchizonal conditions. Figures 4⇑ and 5⇓ show that CAI values in general increase with rock age towards the basal part of the thrust units, with a tendency to parallelism between bedding and CAI isovalues. CAI values in the Devonian rocks of the limbs of the La Robla syncline, where shallow anchizonal values are reached, are higher than those found in rocks of the same age immediately to the north, where values between 3 and 3.5 prevail (fig. 4⇑). Lateral variations of CAI values in rocks of the same age are also found in the Carboniferous carbonates of the Bodón Unit (fig. 4⇑), from common values of 3 to 4 in the western part to values ≤ 2 in the eastern part.
Layer silicate mineralogy and KI
Mineralogical analyses of the < 2 μm fraction show that illite is the most common phase. Other clay minerals present are kaolinite, mixed-layer illite/smectite, mixed-layer chlorite/vermiculite, pyrophyllite, and chlorite. The mixed-layer illite/smectite is always ordered, indicating that zone 3 of Eberl  was reached and therefore the temperature was > 100°C. Figure 6⇓ shows that KI increases with decreasing age from Precambrian to Middle-Upper Devonian rocks; however, the KI values of the Carboniferous rocks are lower than those of the Devonian rocks. The distribution of KI values (fig. 7⇓) indicates diagenetic conditions in most of the study area. Nevertheless, several higher-grade areas can be determined. There are anchizonal values in the Precambrian and Cambrian rocks of the southwestern part of the study area. In a few areas within the Precambrian, epizonal conditions were described by Perez-Estaún  using a different scale to standardise the KI. Similarly, anchizonal values are found in the Cambrian rocks of the basal part of the Bodón and Correcilla units, previously described by Brime  and Aller et al. . In the Carboniferous rocks of the Bodón Unit, close to Villamanín, there is also a narrow E-W strip with anchizonal KI values. Other anchizonal KI values are found in the Central Coal basin (fig. 1⇑), to the north of the León fault, where even epizonal conditions are found in the eastern part (fig. 7⇓) [Aller and Brime, 1985; Aller, 1986; Aller et al., 1987]. Finally, anchizonal and epizonal KI values have been found, respectively, in the unconformable Stephanian rocks of the Ciñera-Matallana and La Magdalena basins.
The spatial distribution of CAI and KI values is complex (figs. 4⇑ to 7⇑⇑⇑) with different features that must be considered independently. One of the basic features observed is the general decrease in metamorphic grade with decreasing rock age (figs. 4⇑ to 7⇑⇑⇑). CAI data record this decrease from the Lower Devonian rocks up to the lower Carboniferous rocks, whereas KI data record it from the Precambrian up to the Middle-Upper Devonian rocks. This pattern is characteristic of burial metamorphism, which is a primary contributor to the metamorphism of the study area. KI data indicate that burial metamorphism gave rise in several areas to anchizonal conditions in the Cambrian-Ordovician rocks of the basal part of the main thrust units (Correcilla and Bodón nappes). At several localities of the study area (fig. 7⇑) an inverted metamorphic pattern, with a metamorphic hiatus between metapelitic zones above and below the basal thrust, is observed, indicating that this metamorphic event was essentially prior to the thrusts of the area, in agreement with its burial character. The metamorphic hiatus suggests that the hanging-wall did not heat the footwall during the emplacement of the thrusts.
An exception to this general pattern has been described in a section of the Correcilla basal thrust, located in the western part of the study area [Brime, 1981; Aller et al., 1987], where KI data indicate that the hot basal part of the thrust unit caused some reheating of the Carboniferous rocks on the upper part of the footwall.
The anchi-epizonal Precambrian rocks in the southwestern part of the study area have a well-developed slaty cleavage. In this area, a continuous thermal trend is observed across the unconformity between the Cambrian and Precambrian rocks, suggesting the absence of pre-Palaeozoic thermal events in the area. This metamorphism is different from the burial type described above and must be considered as the regional orogenic Variscan metamorphism which is widespread, together with the associated cleavage (S1 in the Westasturian-Leonese Zone), throughout the internal zones of the Iberian Variscan belt. According to 40Ar/39Ar dating in micas by Dallmeyer et al. , the age of S1 in a locality of the Cantabrian coast in the eastern part of the Westasturian-Leonese Zone is 336 Ma. Since Variscan deformation migrated from the Westasturian-Leonese Zone towards the Cantabrian Zone [Pérez-Estaún et al., 1991], this age is considered a maximum age for S1 in the study area. On the other hand, the age of 321 Ma obtained for a S2 foliation in a shear zone in the Precambrian folds of the nearby Narcea antiform [Dallmeyer et al., 1997] would be a minimum age for S1 in the Cantabrian Zone. According to these data, the age of S1 and the associated regional metamorphism is early Carboniferous in the study area. This age is similar to that estimated for the climax of the very low-grade burial metamorphism, limited by the age of the younger sediments of the buried sequence (early to late Carboniferous) and that of the Variscan deformation in the area: late Carboniferous after Pérez-Estaún et al., . The age and disposition of the burial and the orogenic metamorphisms indicate that they could represent two manifestations of the regional metamorphism during the Carboniferous, being early episodes in the context of the Variscan deformation.
In addition to the metamorphism described above, development of late thermal episodes in the context of the tectonothermal evolution of the area is indicated by the following evidences:
existence of anchiepizonal KI values in the Westphalian rocks of the southern Central Coal basin. In this area, the low-grade metamorphism is accompanied by anthracites [Colmenero and Prado, 1993; Piedad-Sánchez et al., 2003], and has an associated subhorizontal cleavage that cuts through both longitudinal and radial folds [Aller, 1981, 1986; Aller et al., 1987]. The late character and the attitude of the cleavage suggest that this metamorphic episode might be associated with extensional tectonics. The southern limit of this metamorphic area with its associated cleavage roughly coincides with the León fault, which suggests a movement of this fault that raised the northern wall after the development of the metamorphism [Aller, 1986];
anchizonal KI values in the Ciñera-Matallana Stephanian basin and epizonal values in the La Magdalena Stephanian basin are associated with high coal ranks. Vitrinite reflectance values corresponding to the anchizone (% Rr > 2) have been measured in some areas of the Ciñera-Matallana basin [Frings, 2003; Frings et al., 2004]. These data indicate a temperature peak subsequent to the Stephanian B-C. KI values in the rocks from the Ciñera-Matallana and La Magdalena basins are lower than those found in the pre-Stephanian surrounding rocks (fig. 7⇑). This can be explained by the fact that the Stephanian sediments were deposited in areas bounded by faults where localised extension occurred. This tectonic setting enabled the rise of the hydrothermal fluids and magma and favoured development of unusually high geothermal gradients of 85°C/km [Frings et al., 2004]. Recent studies of fluid inclusions in the Ciñera-Matallana basin [Ayllon, 2003; Ayllón et al., 2003] indicate that, at peak temperatures, fluid temperature and pressure conditions were about 160–290°C and 3–48 MPa. In addition, at the time of the peak temperatures, the Stephanian sediments were probably less consolidated than the older rocks, which could have enhanced hydrothermal fluids motion and heat transfer. Colmenero and Prado  refer to this fact to explain a comparable situation in a Stephanian basin located near Puerto Ventana, to the northwest of the study area;
CAI values ranging from 2 to 4.5 in the Middle-Upper Devonian rocks of the northern limb of the La Robla syncline and to the west of the Ciñera-Matallana basin present an anomalously higher mean and dispersion than the CAI values found in rocks of the same age in other sectors of the investigated area (mainly 2–3.5) (figs. 4⇑ and 5⇑). On the other hand, these anomalous values are developed on conodont elements that sometimes present sugary textures. The transformations could have been favoured by the intense faulting of the area, and could be a result of high heat flow associated with the ascent of hydrothermal fluids during the same metamorphic event that affected Stephanian B-C rocks. Analogous features have been interpreted in a similar way in other regions of the Variscan belt [Königshof, 2003]. The area with relatively high CAI values in the southern limb of the La Robla syncline could have been affected by the post-Stephanian B-C metamorphism observed in the La Magdalena basin, although an effect of the neighbouring regional metamorphism cannot be discarded. In agreement with the above discussion, the apparent folded pattern of the ancaizonal bands along the limbs of the La Robla syncline could be a consequence of the convergence of two bands following different trends, and not the result of folding of a single initial band (fig. 4⇑). The development of these low-ancaizonal areas is not associated with anchizonal conditions in the neighbouring pelitic rocks (fig. 7⇑). This is probably due to the different kinetics of the illitization and the carbonization of organic matter, as the former process is less sensitive to the time spent at a certain temperature [Hillier et al., 1995; Brime et al., 2001 Arkai et al., 2002 ]. This suggests that the faster and shorter the heating, the higher the CAI value corresponding to the KI in the diagenesis-anchizone boundary;
existence of relatively high diacaizonal CAI values in the western part of the Bodón Unit. Although in this area the cartographic trend of CAI isogrades is not well defined, CAI distribution does not follow a burial pattern (fig. 4⇑) and could be interpreted as another result of the late thermal events.
It is difficult to know at present whether the post-Stephanian B-C metamorphism of the Ciñera-Matallana and La Magdalena basins, and that of the southern part of the Central Coal basin, are different expressions of the same thermal event. The main analogy between them is that in both basins the metamorphism is associated with faults. Nevertheless, in the Central Coal basin the metamorphism has an associated subhorizontal cleavage, whereas in the Stephanian rocks this cleavage is absent and there are igneous rocks and evidence of associated hydrothermal fluids. In any case, the metamorphism developed in these areas is a result of late- or post-orogenic events.
The anchizonal values in the pelitic Carboniferous rocks of the Bodón Unit (fig. 7⇑) are located in the upper part of the footwall of a thrust sheet in which Cambrian rocks thrust over Westphalian rocks. These anchizonal values could be attributed to the late metamorphic events, but the fact that there are some diachizonal CAI values in the proximity and their restriction to the Carboniferous rocks of the footwall make this explanation unlikely. Discrepancies between KI values and thermal organic indicators have been described in Upper Carboniferous rocks from several areas of the Cantabrian Zone, and related to the presence of high-grade illite detrital crystals inherited from the source area, based on microprobe and diffractometry of illite [Castro et al., 2000; Brime et al., 2001; Brime et al., 2002; Castro and Brime, 2003]. This could be a possible explanation for the present case, and agrees with the absence of cleavage, which is usually present in the anchizonal Carboniferous rocks affected by the extensional late-Variscan thermal episode in the southern Central Coal basin. This detrital material, together with the late Palaeozoic thermal events, accounts for the mean KI values being lower in the Carboniferous than in the Devonian rocks (fig. 6⇑).
The interpretation about the thermal episodes presented above improves the understanding of the tectonothermal evolution of the Cantabrian Zone. At the same time, available data about low grade metamorphism in the Cantabrian Zone casts light on the age and meaning of the late thermal episodes of the study area.
The orogenic regional metamorphism, widespread in the hinterland of the belt, slightly penetrates in the northwestern, western and southwestern Cantabrian Zone, where the metamorphic front has been mapped [García-López et al., 1997; Bastida et al., 1999; Brime et al., 2001]. This metamorphism has an associated cleavage (S1 in the Westasturian-Leonese Zone). The epizone-anchizone boundary has been found in the Precambrian rocks of the Narcea antiform located within the Cantabrian Zone, whereas the anchizone-diagenetic zone boundary appears within lower Palaeozoic rocks [Pérez Estaún, 1978; Brime, 1985; Aller et al., 1987; Gutiérrez-Alonso and Nieto, 1996; Brime et al., 2001]. In the northern part of this antiform, the metamorphic front coincides with the thrust that places epizonal Precambrian rocks of the Westasturian-Leonese Zone over diagenetic Lower Devonian rocks of the Cantabrian Zone [Brime et al., 2001]. This inverted metamorphic pattern with metamorphic hiatus indicates that the orogenic metamorphism predates thrusting. Very low-grade burial metamorphism is also observed in other parts of the Cantabrian Zone, mainly in the Somiedo Unit [Bastida et al., 1999; Brime et al., 2001]. Breaks in thermal trend across the basal thrusts also indicate that this metamorphism basically predates the motion of the thrust units.
The metamorphism in extensional regime of the Carboniferous rocks in the southern part of the Central Coal basin is similar to the one that developed in the eastern part of the Cantabrian Zone, mainly in the Pisuerga-Carrión Unit (fig. 1⇑), where there is also a well-developed late-Variscan subhorizontal or north-dipping cleavage associated with the metamorphism. This metamorphism has a continuous trend through the late Carboniferous thrust that placed the Picos de Europa Unit over the Pisuerga-Carrión Unit [Bastida et al., 2004]. On the other hand, this cleavage is prior to the contact metamorphism associated with the Peña Prieta granodiorite in the Pisuerga-Carrión Unit [Gallastegui et al., 1990; Rodríguez-Fernández, 1994], whose age is 292 ± 2/3 Ma, according to the U-Pb geochronology by Valverde-Vaquero et al. . Hence, the age of the extensional late-Variscan metamorphism is close to the Carboniferous-Permian boundary.
In several areas of the Cantabrian Zone, the development of many ore deposits has been associated with moderate-temperature hydrothermal fluids, whose temperature is about 170–200°C after Gómez-Fernández et al. [1993, 2000]. These ore deposits have various morphologies, such as stratiform, vein deposits, karst fills, limestone breccias, mineralizations associated with faults, etc. [Martínez-García, 1981, 1983; Martínez-García and Tejerina, 1985; Martínez-García et al., 1991; García-Iglesias and Loredo-Pérez, 1992; Gómez-Fernández et al., 1993, 2000; Martín-Izard et al., 1995]. They can appear in post-orogenic Permian sediments, but never in the Mesozoic cover, and have been interpreted as originated at the beginning of the post-Variscan continental rifting [Martínez-García, 1981, 1983; Martínez-García and Tejerina, 1985; Martínez-García et al., 1991]. The existence of several anomalies in CAI values and textural alterations of conodonts in the Picos de Europa Unit (fig. 1⇑), in areas with a dense fault pattern and abundant mineral deposits, could be an indication of this hydrothermal episode [Bastida et al., 2004]. K-Ar ages in hydrothermally altered illites of about 270 Ma obtained by Weh et al.  in the Pisuerga-Carrión Unit probably provide the age of this Permian hydrothermal episode. In the study area, Stephanian rocks were affected by at least one of the two late Palaeozoic extensional thermal episodes described above i. e. the extensional late Variscan episode and the extensional Permian episode. The hydrothermal activity described in the Ciñera-Matallana basin [Ayllón et al., 2003] and the notable thermal variation shown by the different indicators throughout the Stephanian basins support the notion that the hydrothermal event is linked to Permian extension. The absence of subhorizontal cleavage, commonly associated with the late-Variscan extensional episode, supports this interpretation.
A comparison between the CAI values of the study area and those of other areas of the Fold and Nappe Province (western nappes and northwestern part of the Cantabrian Zone) is shown in figure 5⇑ for rocks of different ages. In all cases, the CAI values are higher in the southern part of the Cantabrian Zone than in other parts of the Fold and Nappe Province. This increase is probably the result of the late Palaeozoic metamorphic events described above, whose effect was more intense in the southern part of the Cantabrian Zone.
The existence of pelitic Westphalian rocks with metamorphic features inherited from the source area has also been described in the Aramo Unit and the northern part of the Central Coal basin (fig. 1⇑) [Bastida et al., 1999; Castro et al., 2000; Brime et al., 2002; Castro and Brime, 2003]. Their presence indicates an erosion of Palaeozoic and perhaps Precambrian rocks during the Westphalian in areas towards the hinterland where Variscan deformation was taking place. It is also in agreement with the presence of Stephanian rocks lying unconformably over Precambrian rocks, indicating that during the Stephanian the previous Palaeozoic succession had been entirely eroded in some areas.
The southern Cantabrian Zone is an area affected by thin-skinned tectonics and located close to the internal zones of the orogen, where regional metamorphism and cleavage are widespread. The complex distribution of CAI and KI data in this area is the expression of several Palaeozoic tectono-thermal episodes, whose timing, structural and magmatic relationships have been synthesized in figure 8⇓. The study of these episodes has allowed us to reach the following conclusions.
– The regional orogenic Variscan metamorphism with associated cleavage affected the southwestern part of the study area, close to the Westasturian-Leonese Zone, where this metamorphism is widespread. This orogenic metamorphism is early Carboniferous in age.
– A burial metamorphism occurred prior to the motion of the thrust units as evidenced by the break in the thermal trend, with apparent inversion of the metamorphic gradient, at the base of the thrust units. Cooling took place during the rise and erosion of the hanging walls.
– An extensional late-Variscan metamorphic episode, with subhorizontal cleavage associated that postdates most of the folds in the area, affected the northernmost part of the study area. The spatial distribution of this event is controlled by a long E-W fault (León Fault) and its highest grade (anchizone-epizone) is located in the northern wall of this fault; that is, in the Westphalian rocks of the Central Coal basin. This metamorphism is comparable to that observed in other areas in the eastern part of the Cantabrian Zone, mainly in the Pisuerga-Carrión Unit, where an age close to the Carboniferous-Permian boundary has been established for this metamorphic episode.
– A metamorphic event is registered in the unconformable Stephanian B-C rocks of the southern part of the study zone (Ciñera-Matallana and La Magdalena basins). It reaches the anchizone in the Ciñera-Matallana basin and the epizone in the La Magdalena basin. In these basins there are small igneous basic intrusions and coal beds with high rank. This thermal episode also affected pre-Stephanian rocks, at least in the northern limb of the La Robla syncline. The higher metamorphic grade found in the Stephanian rocks can be explained by the structural location of the basins, partly bounded by faults, which enabled local tectonic extension and the rising of hydrothermal fluids and magma. This metamorphism was a probable result of the hydrothermal Permian episode described in several areas of the Cantabrian Zone. This episode is responsible for many mineral deposits and has been interpreted as a result of post-Variscan extension.
Mean CAI values are higher in the southern part of the Cantabrian Zone than in other areas of the Fold and Nappe province for rocks of the same age. This is probably due to the more intense development of the late- and post-Variscan thermal episodes in the southern part of the Cantabrian Zone.
The present work was supported by the Spanish Project BTE2003-01609 of the Spanish Ministerio de Ciencia y Tecnología and Fondo Europeo de Desarrollo Regional (FEDER). We are grateful to Javier Sanz-López for his help, and to Richard J. Lisle and Peter Königshof for their valuable suggestions that notably improved the paper.
- Manuscript Received 4 January 2005.
- Manuscript Accepted 10 May 2005.