Introduction

Besides the understanding of structural and geochemical issues, the itemisation of the geodynamic history within polyphase-reactivated basement domains requires detailed geochronological information on the intrusive, metamorphic and deformational history. Constraints on the deformational history imply the knowledge on the textural relation of the dated mineral with its host rock. At high temperature conditions, potential candidates are kyanite, staurolite or garnet that have been shown to be suitable for stepwise lead leaching experiments (Frei and Kamber 1995), while in greenschist facies and lower-grade metamorphic conditions the Ar systematic in K-bearing minerals such as mica becomes of particular interest (e.g. Hunziker et al. 1986; Wemmer and Ahrendt 1997; Clauer and Chaudhuri 1998). In pelitic rocks, K-bearing white micas are formed by prograde dynamo-thermal metamorphic reactions that involve chlorite and illite minerals (Frey et al. 1980), whereas fluid-assisted retrogression of high-grade psammopelitic assemblages will result in phyllitic rocks.

The rheological properties of phyllitic rocks make them highly sensitive even to minor variations of the regional stress field. Hence, K/Ar dating on fine fractions of newly formed or recrystallised white micas can provide insight into the final reactivation of low-grade metamorphic shear zones (Weaver and Wampler 1970; Hunziker 1979; Kligfield et al. 1986; Wemmer 1991; Clauer et al. 1995; Zhao et al. 1997, 1999; Glasmacher et al. 2003 and references therein). The interpretation of the Ar systematic in illite crystals from low-grade shear zones has to be corroborated by X-ray diffraction–based calculation of the Kübler index, in order to distinguish between newly grown metamorphic crystals or detrital material (e.g. Reuter 1987; Reuter and Dallmeyer 1989; Ahrendt et al. 1991; Clauer and Chaudhuri 1998; Wemmer and Ahrendt 1997).

Within the metamorphic basement of the Sierras Pampeanas in central and northern Argentina, the amount of reliable K/Ar fine-fraction ages is of minor significance, even though low-grade metapelitic rocks are widespread. Attempts to date the metamorphic history of the Puncoviscana Formation in the northern parts of the Sierras Pampeanas were done by Adams et al. (1990). Ages between 540 and 535 Ma have been related to the Pampean deformation. Within the Sierra de Pocho, i.e. the northwestern range of the Sierras de Córdoba, Rapela et al. (1998) presented a single K/Ar age of 512 ± 19 Ma on the grain size fraction <0.5 μm that they interpreted as the metamorphic peak in the low-grade successions. K/Ar fine-fraction ages from the Sierras de Quilmes (Salta) in the range between 404–400 Ma (<2 μm) and 392–379 Ma (<0.2 μm) represent metamorphic retrogression and have been correlated with the lower limit of greenschist facies conditions (Büttner et al. 2005).

In the Sierra de San Luis, phyllitic rocks are associated with metaquartz arenites and volcanites (i.e. the San Luis Formation of Prozzi and Ramos 1988) forming the envelope of a medium- to high-grade metamorphic basement (Fig. 1, Sims et al. 1998; von Gosen 1998). The contact relationship between the San Luis Formation and the higher-grade metamorphic rocks in the centre of this basement complex, i.e. the Pringles Metamorphic Complex of Sims et al. (1998), is under controversial discussion (von Gosen 1998; Ortiz Suárez and Casquet 2005; Steenken et al. 2008), even though most of the descriptions indicate that the low-grade metapelitic sequences underwent a common D2 folding event and metamorphism with those of higher grade at ~470 Ma during the Famatinian tectonic cycle (Sims et al. 1998; Steenken et al. 2008).

Fig. 1
figure 1

Geological overview on the different basement domains of the Eastern Sierras Pampeanas in Argentina (Modified from von Gosen and Prozzi 1998, Martino 2003 and Steenken et al. 2004). The dashed line indicates the eastern margin of the Paganzo Basin. Ages for the metasedimentary sequences refer to the timing of metamorphism

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In this paper, we present K/Ar mica fine-fraction ages from the phyllites of the San Luis Formation in the Sierra de San Luis. Fresh phyllitic material from eleven sample localities was subjected to K/Ar analysis and X-ray diffraction in order to define a minimum age for the deformation history of the phyllitic rocks under lower greenschist facies conditions. Previous efforts to date the timing of mylonitic deformation were carried out on mica separates from the more competent arenitic members of the low-grade successions yielding Ar/Ar ages of ~360 Ma (Sims et al. 1998) corresponding to the Achalian event in the Sierra de San Luis. Age determinations of the last deformational event will help to delineate between the Achalian and the Toco tectonic events; the latter one was recognised up to now only in northern Chile and southern Bolivia (Bahlburg and Breitkreuz 1991). The presented results are considered within the frame of the regional tectonics acting on the southwestern margin of Gondwana.

Geological setting

The Eastern Sierras Pampeanas comprise the southern tip of the proto-Andean metamorphic basement outcropping in northern and central Argentina (Fig. 1, Caminos 1979). This basement has registered the polyphase accretional history in the east of the Río de La Plata craton starting in the early Cambrian with the collision of the Pampean Terrane with the craton (Ramos et al. 1986; Rapela et al. 1998; Stuart-Smith et al. 1999) followed by the amalgamation of the Precordillera/Cuyania Terrane to the newly formed margin of Gondwana during the Famatinian cycle (Astini and Thomas 1999; Rapalini et al. 1999; Casquet et al. 2001; Thomas et al. 2002; Astini and Dávila 2004; Steenken et al. 2006). The mode of emplacement of voluminous discordant granodioritic to syenogranitic batholiths throughout the Devonian is a matter of ongoing discussion. A post-tectonic emplacement with respect to the Famatinian cycle was favoured (Llambías et al. 1998; Pinotti et al. 2006), whereas the report of high-temperature shear zone fabrics provides evidence for their syntectonic emplacement (Siegesmund et al. 2004; López de Luchi et al. 2004). The latter supports the hypothesis of a separated Achalian tectonic cycle (Sims et al. 1998) probably related to the approach of the Chilenia Terrane (Ramos and Basei 1997). Relative tectonic quiescence commenced during the Late Devonian albeit subduction was active (Ramos et al. 1986). Extensional arc and retroarc basins formed from the Late Devonian on acted as recipients of alluvial and minor marine sediments (Limarino and Spalletti 2006). An interlude of compressional tectonics, the “Toco-orogeny”, in the Late Mississippian is recognised in northern Chile and southern Bolivia (Bahlburg and Breitkreuz 1991). This compressional period is documented by a hiatus in the stratigraphic record between the Late Devonian/early Mississippian successions and the Pennsylvanian to Permian Paganzo Formation (Limarino and Spalletti 2006). Following this tectonic stage, the deposition of the Paganzo Formation commenced in the latest Mississippian overlaying unconformably the Angualasto group (Salfity and Gorustovich 1983; Azcuy et al. 1999). In proximity to the Andean Cordillera, the resumption of compression started by the late Cisuralian, the San Rafael phase during which rhyolitic volcanites of the Choiyoi Group were discordantly deposited on top of the Paganzo Formation (Mpodozis and Kay 1992).

Among the basement domains of the Eastern Sierras Pampeanas, the Sierra de San Luis forms the southernmost extreme (Fig. 1). Three NNE trending medium- to high-grade metamorphic domains of predominantly metaclastic composition have been recognised: those are from west to east the Nogolí, Pringles and Conlara Metamorphic Complexes (Sims et al. 1998). They are predominantly constituted by metapelites and metapsammites with layers of amphibolites and deformed crustal derived granites. Only in the Nogolí Metamorphic Complex, the appearance of large volumes of orthogneisses has been described (Ortíz Suárez et al. 1992; von Gosen and Prozzi 1998; Sims et al. 1998). In the central and southern part of the sierra, the basement domains are separated by the low-grade metamorphic sequences of the San Luis Formation (Fig. 1). Dominant tight fold structures developed during the metamorphic peak of the Famatinian cycle. NNE trending compressive shear zones located within the low-grade successions accommodated the displacement related to the differential exhumation of the basement domains. Macroscopic shear fabrics generally indicate an “east-side-up” displacement with a subordinated sinistral component (von Gosen 1998; Steenken et al. 2008). Besides phyllitic rocks and metaquartz arenites, the San Luis Formation comprises minor occurrences of acidic and intermediate metavolcanoclastic deposits (Hack et al. 1991; Ortíz Suárez et al. 1992; von Gosen 1998). Remarkable is the appearance of a concordant level of poorly sorted matrix-supported polymictic metaconglomerates in the central part of the eastern phyllite belt (Prozzi 1990) that have been interpreted as channel deposits. The San Luis Formation forms the host for a series of mainly tonalitic intrusions of crustal origin (López de Luchi et al. 2007). Those were syntectonically emplaced approximately between 480 and 470 Ma (Sims et al. 1998; Sato et al. 2005; Steenken et al. 2006) providing a minimum time constraint for the formation of the open to tight folds in the San Luis Formation, corresponding to D2 event of the Famatinian cycle (Steenken et al. 2008). The best approximation for the time of deposition of the sequence is based on an U/Pb zircon age of 529 ± 12 Ma yielded from a corcordant volcanoclastic deposit (Söllner et al. 2000).

Several correlations of sedimentary succession of the San Luis Formation with the Puncoviscana Formation cropping out in the provinces of Salta, Jujuy and Tucumán have been proposed (Toselli 1990; Rapela et al. 1998; Söllner et al. 2000), although contrasting Nd systematics and whole-rock geochemistry do not support their common sedimentary history but denote a probable correlation of the Puncoviscana Formation with the Conlara Metamorphic Complex (Fig. 3) representing the eastern part of the sierra (Steenken et al. 2004; Zimmermann 2005). Major and trace elements of the San Luis Formation indicate an average upper crustal composition typical for shales, whose deposition took place either in a continental island arc or in an active margin setting (López de Luchi et al. 2003). A recent compilation of various geochemical data of all basement units of the Sierra de San Luis and the Puncoviscana Formation of the Cordillera Oriental is given in Drobe et al. (2009).

Structural and petrological observation

The NNE trending low-grade metamorphic phyllites and quartz-arenites of the San Luis Formation appear in two belts of approximately 55 km length and a width of up to 5 km, forming the metasedimentary envelope of the medium- to high-grade metamorphic successions of the Pringles Metamorphic Complex. The structural relationship between the different basement units is under controversial discussion (von Gosen and Prozzi 1998; von Gosen 1998; Sims et al. 1998; Steenken et al. 2008). SHRIMP and conventional U/Pb age constraints suggest that the entire sequence was deposited following the Pampean orogeny (Steenken et al. 2006). Gradual structural and petrological transitions between the differentially metamorphosed sequences of the Pringles Metamorphic Complex (including the San Luis Formation—Steenken et al. 2004) were observed (von Gosen 1998) if not obliterated as in the southwest of the sierra by younger intrusive rocks like the La Escalerilla batholith (Sims et al. 1998; Fig. 1). Large-scale tight folds with NNE trending and moderately plunging axes were described as belonging to the first deformation of the San Luis Formation (Fig. 2a) corresponding to the Famatinian D2 event recorded by other basement domains of the Sierra (von Gosen 1998; Steenken et al. 2008). However, the local appearance of isoclinally folded quartz layers within the phyllites (Fig. 2b) suggests a preceding folding phase equivalent to the Famatinian D1 deformation. Differences in the structural record do not necessarily result from deformation offset and coverage by new sediments but may be addressed also to the different rheological properties at different crustal levels during deformation (Steenken et al. 2008).

Fig. 2
figure 2

Field and microscopic aspects of the San Luis Formation. a Open fold structures of the San Luis Formation corresponding to the Famatinian D2 event (width of view ~300 m). b Isoclinal intrafolial fold of a quartz layer suggestive of the presence of a D1 folding phase in the San Luis Formation. Strain-free quartz crystals with high-angle grain boundaries are developed (width of view: 2.5 mm). c Fine-grained phyllite with quartz, biotite and chlorite. Large chlorites frequently enclose high angles with the S1 foliation (parallel polarised light, width of view: ~21 mm). d S–C shear indicator showing the near vertical displacement of the Conlara metamorphic complex against the San Luis Fm. e Microscopic aspect of a biotite-bearing phyllite variety indicating lower greenschist facies conditions. Large biotite porphyroblasts that are parallel to the S1 fabric made up by chlorite, illite (Ms) and quartz (parallel polarised light, width of view: ~0.5 mm). f Conglomerate clast in incipient rotation indicating a dextral sense of shear. g, h SEM images from the eastern belt of the San Luis Formation. A 32-01 (g) is the southern end whereas A 14-01 (h) marks the northern end. Both images are perpendicular to the cleavage plane. In the right image (h), the various cleavage planes indicate that the sample was polyphase deformed. This deformation is shown in the left image (g) only as the wide bending of the sericite planes

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In general, the mineralogy of the phyllites of the San Luis Formation comprises illite + chlorite + biotite + quartz ± albite. Ubiquitous biotite with a diameter of up to 1 mm documents a minimum temperature of 400°C during its formation (Nitsch 1970) characterising lower greenschist facies metamorphism (Fig. 2e). Even higher metamorphic conditions may be indicated by the local presence of small garnets, which usually crystallise at temperatures >450°C as long as no Mn is present stabilising the garnet at lower temperature (Bucher and Frey 1994). Accessory minerals are tourmaline and ore minerals. The schistosity plane is frequently patched with needle-shaped chlorite crystals of less than 2 mm length. Within the proximity to the tonalitic intrusions, the phyllites have turned into hornfels with relict sericite-chlorite ± biotite aggregates suggesting the former existence of andalusite (von Gosen 1998).

The phyllites exhibit a pervasive alignment of micas and flattened quartz (Fig. 2c). Intracrystalline deformation of the quartz is indicated by parallel subgrain boundaries although contacts between larger crystals show triple junctions of 120°. Only in the vicinity of the tonalitic intrusions, a non-oriented growth of illite and biotite is observed related to contact metamorphism.

The eastern belt of the San Luis Formation hosts the transpressional Río Guzmán shear zone that accommodated the “east-side-up” displacement of the Conlara Metamorphic Complex. S–C fabrics document this vertical displacement (Fig. 2d). Steeply dipping stretching lineations on the NNE-SSW striking mylonitic foliation are south plunging indicating a minor sinistral component (von Gosen 1998; Steenken et al. 2008). However, the tectonic activation of the basement was not uniform since normal faulting with a dextral component has been reported from the conglomerate layers in the eastern belt of the succession (Fig. 2f, Martino et al. 2005). This extensional movement was associated with the formation of a discontinuous planar fabric that is characterised by chlorite (Fig. 2c).

Within the western belt, the “east-side-up” displacement is consistent with the vertical displacement that is recorded by the La Arenilla shear zone, which is located in the eastern part of the Pringles Metamorphic Complex (Ortíz Suárez et al. 1992; Hauzenberger et al. 2001).

Sample description

In order to access the latest stages of the Palaeozoic deformation within the Sierra de San Luis, eleven samples of the phyllitic sequences of the San Luis Formation have been subjected to the K/Ar fine-fraction dating technique (cf. Wemmer 1991). Of those samples, eight were taken from the eastern belt, whereas the other three represent the western belt of the San Luis Formation (Fig. 3). The sampling style avoided the contact metamorphic overprinted areas around the tonalitic intrusions. All samples are of pelitic grain size and show a closely spaced schistosity (Fig. 2g, h). The schistosity planes have a silver-grey gloss whereas perpendicular to the planar fabric the phyllites appear in an overall grey. The alternation between pure pelitic layers and more psammitic layers at the mm to cm scale is documented by the variation in colour intensity from dark grey to medium grey, respectively. SEM images from samples of the eastern belt of the San Luis Formation (Samples A 14-01 and A 32-01) are shown in Fig. 2g, h. Both images taken perpendicular to the foliation display various cleavage planes indicating a polyphase deformational history. The tight and even composition of the cleavage planes is in good agreement with epimetamorphic conditions. No indication for a lower-grade overprint, e.g. irregular growth of illite, has been observed. Microscopically identified minerals are illite, chlorite, biotite and quartz. Additionally, the presence of albite is documented by X-ray diffraction in the samples A 14-01, A 17-01, A 30-01, A 31-01, A 32-01 and AH 5 (Table 1). Garnet appears in sample A 30-01.

Fig. 3
figure 3

Geological overview of the southwestern portion of the Sierra de San Luis (modified after von Gosen 1998 and Steenken et al. 2006). Sample locations and results of K/Ar age analyses

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Table 1 Results of X-ray diffraction analyses
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Data acquisition

About 2 kg of previously washed sample material was crushed and ground in a shatter mill for 13–18 s until no graininess is registered by manual examination (Reuter 1985). The grain size fraction of <2 μm was gained by differential settling (Atterberg method) of the sieve fraction <63 μm. Enrichment of the grain size fraction <0.2 μm was carried out by ultracentrifugation (Wemmer 1991). The two grain size concentrates were subjected to isotope measurements as well as X-ray diffraction.

Kübler index (illite crystallinity)

The Kübler index was determined using a Phillips PW 1800 X-ray diffractometer. Texture compounds were prepared using 1.5–2.5 mg/cm2 of sample material (Weber 1972). The metamorphic grade has been inferred from the peak width at half height of the 10-Å peak (Kübler 1967) using the software algorithm IDEE provided by the University of Göttingen (Friedrich 1991). Digital measurement was taken by a step scan: 301 points, 7–10° 2Θ, scan step 0.010° 2Θ, integration time 4 s, receiving slit 0.1 mm, automatic divergence slit. The crystallinity values may range from 0.060° Δ2ΘCuKα for ideally ordered muscovite to >1° Δ2ΘCuKα for illite/smectite mixed layers (Ahrendt et al. 1991; Wemmer 1991). Limits between the fields of diagenesis, anchi- and epizone are given by ~0.420° Δ2ΘCuKα and 0.250° Δ2ΘCuKα, respectively (Kübler 1967; Marheine 1997). The presence of mixed layer clays that may obliterate the 10-Å peak has been tested by duplicate determination of the material under air-dried and glycolated conditions.

Mineralogy of grain size fraction

In a follow-up session, X-ray diffraction was extended to the range from 4° to 70° 2Θ in order to estimate the mineral phases present in the sample material and to rule out the possible interference of other K-bearing minerals (e.g. K-feldspar) in the isotope analysis (Table 1).

K/Ar analyses

The argon isotopic composition was measured in a Pyrex glass extraction and purification line coupled to a VG 1200 C noble gas mass spectrometer, operating in static mode. The amount of radiogenic 40Ar was determined by isotope dilution method using a highly enriched 38Ar spike from Schumacher, Bern (Schumacher 1975). The spike was calibrated against the biotite standard HD-B1 (Fuhrmann et al. 1987). The age calculations are based on the constants recommended by the IUGS quoted in Steiger and Jäger (1977).

Potassium content was determined in duplicate by flame photometry using an Eppendorf Elex 63/61. The samples were dissolved in a mixture of HF and HNO3. CsCl and LiCl were added as an ionisation buffer and internal standard, respectively. The analytical error for the K/Ar age calculations is given at a 95% confidence level (2σ). Analytical results are presented in Table 2. Details of argon and potassium analyses for the laboratory in Göttingen are given in Wemmer (1991).

Table 2 Compilation of the K/Ar ages and the Kübler Index of mineral fine fractions of <2 and <0.2 μm
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Results

The Kübler index of all analysed samples varies from 0.145 ± 0.005° Δ2ΘCuKα to 0.285 ± 0.005° Δ2ΘCuKα. The minor differences in the Kübler index between the glycolated and the air-dried measurements are not significant; therefore, the presence of retrogradely grown mixed layer clays can be excluded (Table 2). All but three obtained values are indicative for epimetamorphic conditions (<0.250° Δ2ΘCuKα). In case of sample A 30-01, anchimetamorphic conditions are indicated, whereas the samples A 7-01 and A 14-01 mark the limit between epi- and anchimetamorphic conditions (Fig. 4). No systematic variation with respect to sample localities has been observed. Samples A 14-01 and A 30-01 belong to the northern and southern ends of the eastern belt of the San Luis Formation, whereas the sample A 7-01 was taken in the immediate west of the La Escalerilla batholith. Epimetamorphic conditions of around 300–350°C are insufficient to allow the growth of the observed small biotite and local garnet crystals, requiring a temperature of 400°C (lower greenschist facies). Temperatures as high as 400°C will certainly erase any detrital memory in the investigated fine fractions, and the gained age information has to be interpreted in terms of a post-peak metamorphic deformational reactivation of the phyllites. Additionally, the mineralogical composition gained from the X-ray diffraction does not indicate any other potassium- or calcium-bearing mineral phase that would disturb the isotope system.

Fig. 4
figure 4

Chart showing the Kübler Index measured for the analysed samples. Similarity between ‘air-dried’ and ‘glycolated’ experiments is indicative for the absence of mixed layer clays

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K/Ar ages of the two grain size fractions, i.e. <2 and <0.2 μm, are in the range from 334.4 ± 7.1 to 299.1 ± 7.0 Ma and 323.0 ± 7.2 to 293.4 ± 7.2 Ma (2σ), respectively. The weighted means for the two fractions are at 321.1 ± 7.5 and 310.2 ± 7.6 Ma. Comparing the ages of both fractions within the same sample reveals an age gap of ~20 Ma in only three samples (A 11-01, A 13-01 and A 17-01), whereas for the majority of the samples the two obtained ages overlap within error yielding a maximum difference of 13 Ma. Such a distribution can be explained in terms of an undisturbed deformational and metamorphic history. If larger differences between the two analysed fractions would appear, two scenarios could be discussed: (i) a detrital component in the <2 μm fraction leading to older ages or (ii) a weak retrograde overprint resulting in higher diffusion in the <0.2 μm fraction (Hess et al. 1993, Glasmacher et al. 2001) and a rejuvenation of the K/Ar ages. Both scenarios can be excluded. Gradual differences in the ages between the western and eastern belt of San Luis Formation can be seen (Fig. 5). A correlation of the younger K/Ar results with the Kübler Index of the anchimetamorphic values is apparent in samples A 14-01 and A 30-01.

Fig. 5
figure 5

Results from K/Ar analyses for the western and eastern belt of the San Luis Formation. Results can be interpreted in relation to the hiatus between the Late Devonian/early Mississippian Angualasto group and the late Mississippian to Permian Paganzo Formation caused by the Toco orogeny

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Discussion

The interpretation of the K/Ar illite fine-fraction ages depends largely on the understanding of the closure behaviour of the small illite crystals. The importance of the effective diffusion radius on the closure temperature for the Ar system has been demonstrated throughout a large number of publications (Dahl 1996; Villa 1998; Hodges 2003). These publications focused on white micas with grain sizes >1 mm. In case of K/Ar fine-fraction ages, the available information is limited. Hunziker et al. (1986) reported a closure temperature interval for the mica fractions <2 μm of 260 ± 30°C. Wemmer and Ahrendt (1997) reported Cretaceous K/Ar ages on fine-grained white mica (sericite <2 μm) from the bottom of the German Deep Drilling Bore Hole (KTB). Here, in situ temperatures of 275°C did not reset the Ar system in the <2 μm serecites. Therefore, the closure temperature has to be estimated somewhere between 275–350°C for grain size fractions <2 μm, being somewhat lower than the closure temperature interval of 350 ± 50°C for coarse grained muscovite recommended by Purdy and Jäger (1976) and McDougall and Harrison (1999). Extensive discussion on the problems related to isotopic closure is given in Villa (1998).

The post-metamorphic cooling history of the different basement domains has been previously accessed by K/Ar dating on micas of various grain sizes (Varela et al. 1994; Sims et al. 1998; González and Sato 2000; Sosa et al. 2002; Steenken et al. 2004, 2008). Those results suggest that the cooling front as it is recorded by the Ar system in biotite commenced in the Nogolí Metamorphic Complex from where it migrated to the east. The recorded differences are small and it appears that the entire Sierra de San Luis has reached epimetamorphic conditions during the Middle Mississippian (345–328 Ma, ISC 2009). The internal distribution of the K/Ar ages suggests that the differential cooling of the metamorphic complexes (Nogolí, Pringles and Conlara) was not the result of a rotational ‘en-bloc’ exhumation but rather accommodated by the two belts of the San Luis Formation (Steenken et al. 2008). It appears plausible that part of this adjustment is recorded by the new K/Ar fine-fraction ages covering the time-span from the Middle Mississippian to the lowermost Cisuralian (345–299 Ma, ISC 2009).

The K/Ar fine-fraction age of 512 ± 19 Ma from the nearby Sierra de Pocho presented by Rapela et al. (1998) marks final steps of the Pampean orogeny. It is much older than the new results of ~330 Ma for the Sierra de San Luis, indicating that despite the close proximity of the two tectono-morphological terrains the late Palaeozoic history took place independently.

The new K/Ar illite fine-fraction ages (>320 Ma) are synchronous with a widespread period of intra-Carboniferous tectonism, e.g. the Toco orogeny of Bahlburg and Breitkreuz (1991). The younger ages correspond to stratigraphic ages known from the Paganzo Basin, which shows an almost continuous stratigraphic record from the latest Upper Mississippian (318 Ma, ISC 2009) to the late Permian (251 Ma, ISC 2009) in the vicinity of the Eastern Sierras Pampeanas. In this area, the Paganzo Basin extends from the cities of Catamarca down to San Luis (cf. Fig. 1, Salfity and Gorustovich 1983; Azcuy et al. 1999). The eastern limit of the basin largely follows the present-day eastern footwall of the Sierras de Córdoba. Relicts of this retro arc basin in the Sierra de San Luis are recognised in the Bajo de Veliz, a NNE-trending graben in the northeasternmost tip of the Sierra. Stratigraphic ages reported by Salfity and Gorustovich (1983) for the Bajo de Veliz Formation start at the Upper Pennsylvanian, (303 Ma, ISC 2009). Deposition of the Paganzo Formation was contemporary with the shallow-level emplacement of plutonic rocks in the metasedimentary cover of the Chilenia Terrane (Mendoza Province) due to the ongoing subduction along the western outboard of Gondwana (Azcuy et al. 1999).

According to these stratigraphic findings, it is suggested that the K/Ar fine-fraction ages have recorded the Upper Mississippian (328–318 Ma, ISC 2009) tectonic phase that led to the hiatus in the stratigraphic record between the Late Devonian/early Mississippian Angualasto Group and the Paganzo Formation whose deposition started in the latest Mississippian (Limarino and Spalletti 2006). The younger ages of the archived spectra are the result of cooling during the subsidence of the Paganzo Basin. Normal faulting with dextral oblique component has been documented for the conglomeratic layers in the eastern phyllite belt (Martino et al. 2005).

K/Ar fine-fraction ages in a similar range from 320 to 290 Ma have been reported for the Cordillera Oriental in southern Bolivia close to the Argentinean border, i.e. the Chaco-Tarija Basin (Jacobshagen et al. 2002). This foreland basin (Isaacson and Díaz Martínez 1995) developed in the east of the east-verging belt of the Toco orogeny (Bahlburg and Breitkreuz 1991) stressing the importance of this intra-Carboniferous event for the geodynamic history of the proto-Andean belt of southern South America. However, from a palaeotectonic point of view, the Upper Mississippian tectonism is poorly understood. A relation to the accretion of the Chilenia terrane has been proposed (Ramos et al. 1986; Gohrbandt 1992; Davis et al. 2000; Ramos 2001). This hypothesis contradicts the hypothesis of an Early Devonian docking of the Chilenia terrane to the southwestern margin of Gondwana (Sims et al. 1998; Siegesmund et al. 2004).

Conclusion

The K/Ar fine-fraction dating technique provides a powerful tool in the analysis of polyphase-reactivated basement domains. In the case of the Eastern Sierras Pampeanas in Argentina, the tectono-metamorphic record started in the early Palaeozoic and lasted until the extensional formation of the Paganzo Basin.

The final placement of the different basement domains of the Sierra de San Luis is recorded by K/Ar fine-fraction ages ranging from ~330 to 290 Ma. Ages >320 Ma correspond to a phase of compressional tectonism recognised as the Toco orogeny in northern Chile and southern Bolivia. Younger ages are the result of the subsequent subsidence of the basement consistent with the stratigraphic record of the Paganzo Basin. Evidence for the extension of this basin within the Sierra de San Luis is found in the graben structure of the Bajo de Veliz where sedimentation commenced during the Upper Pennsylvanian (307–299 Ma, ISC 2009).

The 320–290 Ma K/Ar fine-fraction ages from the Cordillera Oriental in the southern extreme of Bolivia (Chaco-Tarija basin, Jacobshagen et al. 2002) together with the set of data presented here put emphasis on the importance of compressional tectonism during the Pennsylvanian and Cisuralian in the proto-Andean basement of southern South America.