Emsian age (Early Devonian time)

Rodeo de la Bordalesa tonalite

The Rodeo de la Bordalesa tonalite was first described as intruded in the ‘La Horqueta Series’ by Dessanti (1956) and mentioned by Davicino and Sabalúa (1990) as tonalite dikes (‘trondjhemites’) emplaced in La Horqueta sequence. After González Díaz (1964, 1981), Cuerda and Cingolani (1998) and Cingolani et al. (2003) works, the area was remapped and tonalites host rocks were assigned to the Río Seco de los Castaños Formation (Manassero et al. 2009; Cingolani et al. 2011).

The Río Seco de los Castaños Formation (RSC) outcrops at
a. Road 144-Rodeo de la Bordalesa: locations, where Rubinstein (1997) found Silurian acritarchs and other microfossils, and trace fossils were mentioned by several authors (Criado Roqué and Ibáñez 1979; Poiré et al. 1998; Pazos et al.2015). Cingolani et al. (2003) as preliminary work, constraints the isotopic age and
composition of the tonalitic intrusive body;

b. Atuel River creek: this is the type section of the sequence, near Valle Grande area (González Díaz 1964). The beds are folded and show dipping of 50º–72º to the SE or NE. c. El Nihuil area: comprise a sedimentary sequence close to the Mesoproterozoic basement and to the Ordovician mafic rocks called ‘El Nihuil Mafic Unit’ (Cingolani et al. 2003).

d. Lomitas Negras and Agua del Blanco areas: comprise the southern outcrops of RSC, where Di Persia (1972) mentioned a coral (Pleurodyctium) of Devonian age and conglomerates with limestone clasts bearing Ordovician fossils.

The Rodeo de la Bordalesa intrusive rock crops out near the deactivated railroad tracks (‘Ferrocarril General San Martín’), as a gray tonalitic body with abundant mafic enclaves (less than 30 cm) and comprising 10–30 cm thick late magmatic aplite veins. At this area the RSC is a folded sequence of feldspathic sandstones, wackes, and shales (Cingolani et al. 2003; Manassero et al. 2009).

Previous geochronological data yielded biotite K–Ar ages of 475 ± 17 Ma and 452 ± 8 Ma (González 1971; González Díaz 1981) for the intrusive rocks, which are in disagreement with the intrusive character into Silurian-Lower Devonian country rocks.

Petrography and Geochemistry Aspects
The Rodeo de la Bordalesa tonalite consists of small laminar bodies intruded into the RSC unit; these intrusives are either parallel or crosscut the stratification, and although their composition is similar, they have different textures. The largest one (ca. 70 m thick) is close to the old railroad tracks and its country rocks (feldspathic sandstones, wackes and shales) develop a metamorphic contact characterized by recrystallized biotite and minor muscovite. The tonalite shows a medium-grained equigranular texture and it is composed of zoned plagioclase (average An40), green amphibole (sometimes with a core of clinopyroxene), biotite, and interstitial quartz. Zircon and apatite are present as accessory minerals.
The other body also intrudes the RSC and crops out northward of the previously described; it consists of dykes and small irregular bodies of porphyritic tonalite. Phenocrysts consist of zoned plagioclase (average An50), scarce clinopyroxene surrounded by amphibole and biotite. The groundmass is composed of plagioclase, scarce biotite, and interstitial quartz.
Five samples were analyzed for major, trace and rare earth elements. They plot in the TAS diagram adapted to plutonic rocks by Bellieni et al. (1995) into the field of tonalites.
Modal composition indicates an I-type signature and in the AFM diagram (Irvine and Baragar 1971) samples show a calc-alkaline trend. They are characterized by high to medium potassium concentrations (after Peccerillo and Taylor1976); with A/CNK index ranging from 0.90 to 0.95 they are regarded as metaluminous rocks. The extended multielement diagram normalized to primitive mantle (Taylor and McLennan 1985) show depression of Nb and Ti and low enrichment of HFSE, typical of calc-alkaline series. The REE patterns show LREE enrichment and flat HREE behavior, also characteristic of calc-alkaline rocks.
To constrain the tectonic environment of emplacement three discrimination diagrams were applied (Fig. 8), from which it is deduced that the tonalites intruded within an active continental margin since they plot in the field of volcanic arc granitoids (Fig. 8a) from Pearce et al. (1984), while in the Whalen et al. (1987) diagram plot into the I-type field.

Furthermore, Harris et al. (1986) diagram allows the discrimination of pre-collisional calc-alkaline arc-related granitoids from syn- to post-collisional intrusions and within plate intrusions. In this regard, the late- and post-collisional character of samples from the Rodeo de la Bordalesa agree well with an emplacement within the RSC folded sedimentary rocks afterwards the ‘Chanic’ tectonic phase.

All these characteristics allowed us to differentiate the Rodeo de la Bordalesa tonalite from the tholeiitic mafic rocks (mainly gabbros, amphibolites and porphyritic dolerites) exposed at the El Nihuil area (Cingolani et al. 2000).

Isotopic Data
To constrain the age of the Rodeo de la Bordalesa intrusive rocks new U –Pb, K–Ar, Rb–Sr and Sm–Nd data have been obtained, in addition to the Ordovician biotite K–Ar dates reported by González (1971) and González Díaz (1981) and Middle Devonian age (whole rock 380 ± 20 Ma) by Linares et al. (1987).

a. U–Pb (ID-TIMS):

As we can see on the Concordia diagram (Fig. 9) the U–Pb average age obtained in four zircon fractions by ID-TIMS is 401 ± 4 Ma and that corresponds to Early Devonian (Emsian) time (IUGS International Stratigraphic Chart 2015).

b. K–Ar:

The biotite gave an age of 401 ± 17 Ma. This value is very close and confirms the zircon U–Pb (ID-TIMS) age.

c. Rb–Sr:

The biotite separate from one whole rock was also used.

As it is shown in Table 4, the samples show low Rb (40–60 ppm) and
high Sr contents (300–500 ppm), with a low Rb/Sr ratio (0.10–0.20). Rb–Sr whole
rock diagram (Fig. 10) shows an alignment of five samples within a very low range
of 87Rb/86Sr (0.24–0.61), and defines an ‘age’ of 600 ± 100 Ma with an IR:
0.7043. Because the error is too high we utilized a biotite separation as Rb-rich
mineral. For the biotite sample the Rb/Sr ratio is 26. The age obtained with the five
whole rocks and the biotite is 374 ± 4 Ma, with an IR: 0.7056 ± 0.0006 as we can
see on the diagram from Fig. 10.

d. Sm–Nd:

Five whole rock tonalitic samples (RB1–RB 5) were used.

The samples do not define an acceptable alignment. The model ages (TDM) calculated according to DePaolo (1981) for the whole rock samples are in the range of 1 and 1.6 Ga. The εNd (400 Ma) for these samples is in between -4.45 and -10.20, indicating crustal source.

Concluding Remarks
Based on the newly obtained data the following statements can be made:
– The Rodeo de la Bordalesa tonalite dykes at San Rafael Block are characterized by high to medium potassium contents, with a metaluminous character and I-type calc-alkaline signature. It forms part of a magmatism that could be related to a post-collisional tectonic event.
– We interpret the ca. 400 Ma U–Pb zircon age obtained within a concordia diagram, as the crystallization age which corresponds to the emplacement time. This data are confirmed by the K–Ar biotite age. The Ordovician K–Ar ages (González 1971) are not supported by our geochronological data and are also not consistent with the RSC paleontological record

– The Rb–Sr whole rocks and biotite age of 374 ± 4 Ma, could be linked to the‘Chanic’ tectonic phase, in agreement with other geochronological data (Toubes and Spikermann 1976, 1979).

Cingolani and Varela (2008) presented a Rb–Sr isochronic whole rock age of 336 ± 23 Ma for the anchimetamorfic event that affected the Río Seco de los Castaños unit, implying an Early Carboniferous (Mississipian) low-grade metamorphism for the RSC.

Tickyj et al. (2001), based on similar isotopic studies determined isochronic whole rock ages ranging from 371 ± 62 to 379 ± 15 Ma for the La Horqueta sequence, from which suggested an Upper Devonian low-grade metamorphism.

Similar data were obtained in metasedimentary rocks from Precordillera (Cucchi 1971; Buggish et al. 1994; Ramos et al. 1998; Davis et al. 1999) that strongly suggests Upper Devonian-Lower Carboniferous age for the synmetamorphic ductile deformation in connection with the ‘Chanic’ tectonic phase.

– Nd model ages (TDM) show an interval between 1 and 1.6 Ga that corresponds to Mesoproterozoic age derivation and the negative εNd is in accordance to crustal sources.
– The crystallization age for the Rodeo de la Bordalesa tonalite dykes corresponds to a Lower Devonian time (Pragian-Emsian boundary) according to IUGS time scale and suggests that part of the Late Famatinian magmatic event is present in the San Rafael Block.

The tonalite rocks are contemporaneous with the large peraluminous batholith exposed in Pampean Ranges (Rapela et al. 1992; Dahlquist et al. 2014), with the transpressional shear belts during ‘Achalian’ event (Sims et al. 1998); it could be as well correlated with the Devonian magmatism present in Pampa de los Avestruces (Tickyj et al. 2009) in the southern part of the Frontal Cordillera and some places studied recently by Tickyj et al. (2015) near Agua Escondida Mine District in the southern sector of the SRB.
– The geochemical and geochronological data allow us to differentiate the Rodeo de la Bordalesa tonalite from the mafic rocks (mainly porphyritic dolerites with tholeiitic signature) exposed at the El Nihuil area.

Bloque de Chadí Leuvú

(falta completar la información)

Post Wenlock age (Middle Silurian-Early Devonian times)

Río Seco de los Castaños Formation

The Río Seco de los Castaños Formation (González Díaz 1972, 1981) is one of the‘pre-Carboniferous units’ outcropping within the San Rafael block. This sequence was first part of the La Horqueta metasedimentary unit (Dessanti 1956), but it was redefined based on its sedimentary characteristics by González Díaz (1981) and was assigned to the Devonian by Di Persia (1972).

Contributions by González Díaz (1972), Nuñez (1976) and Criado Roqué and Ibañez (1979) described other sedimentary features of this foreland marine sequence. Rubinstein (1997) found acritarchs and other microfossils assigned to the Upper Silurian age near the 144 Road (km 702) outcrops. Poiré et al. (1998, 2002) recognized some trace fossil associations that helped to interpret different sub-environments of deposition within a wide siliciclastic marine platform.

More recently Pazos et al. (2015) record the presence of relevant ichnogenus along the Atuel River outcrops. Manassero et al. (2009) presented a sedimentary description and stratigraphy, geochemical and provenance facies analysis of this unit. Rapid deposition and storm action on the platform are suggested by the presence of hummocks and swaleys facies. Furthermore, plant debris indicates that the continental source was not far away.

It is well known that the Upper Silurian–Lower Devonian is a time of great changes not only of ecosystems but of climates as well, caused probably by complex interactions between the fast-developing terrestrial biosphere, marine ecosystems and the atmosphere. Within this framework the Río Seco de los Castaños Formation (RSC) was deposited within a basin influenced by both, land and sea environments.

Based on these records, the main focus of the present paper is to review the provenance data obtained by petrography and geochemical-isotope analyses as well as to describe the recently acquired U–Pb detrital zircon ages. The data comparison with La Horqueta Formation is also discussed here.

Neither the base nor the top of the RSC are exposed. At the Loma Alta section this unit is separated by an unconformity or tectonic contact from the Mesoproterozoic mafic rocks (basement) and the Ordovician dolerite rocks. In other regions it is separated by unconformity from the Carboniferous-Lower Permian (El Imperial Formation) a fossiliferous marine-glacial/continental sedimentary unit locally forming deeply incised channels.

The great angular unconformity is clearly showed at the Atuel River creek. The outcrops are rather isolated since they have been dismembered by Mesozoic and Cenozoic tectonism, according to Cuerda and Cingolani (1998) and Cingolani et al. (2003a) they are located at Road 144-Rodeo de la Bordalesa

Trace fossils such as the Nereites-Mermia facies were mentioned (Poiré et al. 1998,2002). Microfossils were described by Rubinstein (1997), although they were assigned to “La Horqueta Formation”. In this region, the Rodeo de la Bordalesa Tonalite intruded the RSC. It has a magmatic arc geochemical signature and a crystallization age of 401 ± 4 Ma (Lower Devonian; Cingolani et al. 2003b,this volume), which also constrain the depositional age of the RSC.

Atuel River Creek
González Díaz (1972, 1981) described the type-section of the RSC in this region.
Two main outcrops are recorded, one located about 12 km to the NE of the El Nihuil town (Fig. 3a) and the other near the Valle Grande dam, where the Seco de los Castaños River becomes an affluent of the Atuel River (González Díaz1972).

In the first locality the Formation comprises more than 600–700 m of tabular, green sandstones and mudstones with sharp contacts. It shows regional folding and dippings between 50° and 72° to the SE or NE. Above RSC inclined strata lay Upper Paleozoic horizontally bedded sedimentary rocks, displaying, therefore, a remarkable angular unconformity. In the Atuel Creek area fragments of primitive vascular plants are described and assigned to the Lower Devonian and marine microfossils such as prasinophytes, spores and acritarchs were found by D. Pöthe de Baldis in the RSC, indicating shallow water conditions near the coastline.

Nihuil Area
The RSC is developed close to the Mesoproterozoic basement and Ordovician MORB-type dolerite rocks called ‘El Nihuil mafic body’ at the Loma Alta region (Cingolani et al. 2000)

Lomitas Negras and Agua del Blanco Areas
This region comprises the southernmost outcrops. A Devonian coral known as ‘Pleurodyctium’ was mentioned at Agua del Blanco, while conglomerates with limestone clasts bearing Ordovician fossils are described from the Lomitas Negras region (Di Persia 1972). Both successions are clearly folded and show substratal structures and wave ripples at the base of the sandy beds. After Di Persia (1972) the Lomitas Negras succession reaches a thickness of 2550 m.

Sedimentological Analysis
The main components of this marine fine-grained siliciclastic platform are sandstones and mudstones (Manassero et al. 2009). The conglomerates are restricted to channel fill deposits located mainly at the Lomitas Negras section. Main lithotypes recognized in the RSC platform are,
Mudstones: Comprise 50 –90% of thin-beds, greenish in colour, usually with lamination and slight bioturbation commonly in repetitive sequences. The dark tonality and the scarcity of organic activity suggest anoxic conditions in low energy environments.
Heterolithics: Comprises thin-bedded sandstones and intercalated mudstones, with good lateral continuity and tabular-planar beds of few centimetres thick and grey to green colours. It is a very common facies, that exhibit sharp contacts and in many cases wave and current ripple structures, and also climbing ripples.
Normal grading and bioturbation are the dominant internal structures. Represents a well-oxygenated environment interpreted as a proximal or shallow marine platform, with dominance of a sub-tidal environment. The trace fossils are developed over a soft substrate with moderate energy.
Laminated siltstones: These rocks comprise bedded siltstones that range in thickness from several tens of centimetres to 1 m. They are intercalated withfine-grained sandstones with sharp contacts. Some coarser grained beds show small-scale ripple cross-lamination.

Sandstones: Comprise fine to medium-grained, grey and green, medium-bedded (10–15 cm thick) sandstones. They not only show massive and sharp contacts but also current and wave ripple marks (wave index 12–20) suggesting seawater-depthsof 20 m. Deformational structures such as contorted beds and dish structures are present and scarce flute marks can develop to the base of the beds.
Rapid deposition and storm action on the platform is recognized by the presence of hummocks and swales in this facies. Furthermore, plant debris indicates that the continental source was not far away. The erosive base of some beds implies a high sedimentation rate and the dominance of thin-beds with fine sediments suggests the action of low density gravity flows in the platform.
Within the last described facies a charcoal bed (10–15 cm thick) that might be a marker horizon, was also found (Fig. 7a). It is composed of a mixture of silty-quartz, illite-kaolinite clays and amorphous organic matter with a TOC (total organic carbon) of 1%. Its presence is restricted to the section of Atuel creek.

Recently Pazos et al. (2015) record the ichnogenera Dictyodora Weiss, which constitutes one of the most diverse, documented outside Europe and North America. The ichnospecies recognised include D. scotica and D. tenuis—and a new ichnospecies, D. atuelica.

The succession studied by Pazos et al. (2015) contains abundant microbial mats (wrinkle marks) as either extended surfaces or patches. Wave-dominated deltas have facies sequences that coarsen upwards from shelf mud through silty-sand to wave and storm influenced sands, capped with lagoon or strand-plains where these peat beds can develop to the top of each cycle. This seems to be the case for the Atuel section (Manassero et al. 2009) where severalprograding sequences with intense wave action have been described.

Deformation by ‘Chanic’ tectonic phase is evident
Conglomerates: Both, clast- and matrix-supported conglomerates wtih erosive bases are usually restricted to 2–3 m wide and 1 m deep channels. This facies is only present at the Lomitas Negras section, developing lenticular and laterally discontinuous beds. They are poorly sorted and the matrix is medium to coarse sand. Clasts range from 2 to 10 cm long and show chaotic disposition without stratification; they are mainly composed of wackes, marls, limestones, siltstones, phyllites, quartz and feldspars.

Some limestone clasts bear Ordovician fossils (Nuñez 1976; Criado Roqué and Ibañez 1979).
To the top, the channels could reach several metres wide and two or three metres thick. The conglomerates tend to have a sub-vertical position, due to the regional folding of the sequence (Fig. 4). As they are harder than the associatedfine-to-medium-grained sedimentary rocks, they result into a strong geomorphologic control. The thickness of sandstones and mudstones associated to this facies suggests high energy, a relatively instability of the coastline and close continental source areas bearing plant remains (Fig. 8).

Sm–Nd Data
As were presented in Manassero et al. (2009) the RSC samples (n = 7) shows εNd(t) values (where t = 420 Ma is the proxy age of sedimentation) ranging from -2.5 to -7.7 (average -4.5 ± 1.7).

εNd values are between those typical for the upper continental crust or older crust and those typical for a juvenile component (Fig. 11). Samples with the less negative εNd (t) display the lowest Th/Sc ratios, indicating that the more juvenile the source the more depleted its geochemical signature. TheƒSm/Nd against εNd (t) diagram shows a data cluster between fields of arc-rocks and old crust. ƒSm/Nd values out of the range of variation of the upper crust (-0.4 to-0.5) could be indicating Sm–Nd fractionations due to secondary processes.

TDM ages are within the range of the Mesoproterozoic basement and Palaeozoic supracrustal rocks of the Precordillera. εNd values of the RSC are similar to those from sedimentary rocks from the Lower Palaeozoic carbonate-siliciclastic platform of the San Rafael Block, which show εNd (t) between -0.4 and -4.9 (Cingolani et al. 2003a) and they are also in the range of variation of εNd values of the Mesoproterozoic basement of the San Rafael Block (the Cerro La Ventana Formation; Cingolani et al. 2005) recalculated at 420 Ma.
Although some ƒSm/Nd values are below or above average values for the upper crust, all samples but one has ƒSm/Nd values in the range of variation of the Cerro La Ventana Formation (Cingolani et al. 2005)


Rb–Sr Whole Rock Data
The results were plotted on isochron diagram, using the Isoplot model after Ludwig (2001).
Eight fine-grained samples were selected for analysis by Rb–Sr systematic; their location (all from the Atuel river type-section) is shown in Fig. 3. The Rb content varies between 165 and 312 ppm, while the Sr concentration ranges from 29 to 88 ppm. The 87Rb/86Sr ratios are between 7.5 and 24.4 in agreement with the relative high concentration of Rb and low contents of Sr.

The expansion in the isochronic diagram is acceptable for metasedimentary rocks. We interpret that during the low-metamorphic event Rb and Sr underwent isotopic homogenization, and therefore whole rock alignment is present (MSWD = 7.4).
The high value of the initial isotopic ratio 87Sr/86Sr (0.7243) suggests a provenance for the sedimentary detritus from an evolved continental crust source. If we reject the sample A-02-04 that is out of the main alignment, the obtained Rb–Sr age is 336 ± 23 Ma.

U–Pb Detrital Zircon Age Data
U–Pb analyses of detrital zircons have been intensively used as an important tool to study the sedimentary provenance and the age of source(s) of detritus (Cingolani et al. 2014). Three samples from the RSC were analyzed by the U–Pb zircon systematic

Lu –Hf Systematic
It is known that zircon preserves the initial 176Hf/177Hf ratio of the original magma, providing record of the Hf composition of their source environment at the time of crystallization. This ratio can be used to determine Hf model ages. Thus, the Hf isotopic composition of zircons can be utilized as a petrological tracer of a host rocks origin.
17 zircon grains were selected from the sample A-11-04 for Lu–Hf analysis The εNd(t) values of Ordovician and Neoproterozoic zircons range between -14.78 and -4.20 which reveal zircon derived from recycling old crust. Only one sample (Mesoproterozoic age zircon) show positive value 5.40 that linked with juvenile crust.

Discussion and Interpretation
As we concluded in Manassero et al. (2009) the relatively scarce diversity of sub-environments, dominance of fine-to-medium detrital grain sizes, lack of tractive sedimentary structures, and the important thickness of the beds associated with gravity flow processes are typical of a distal (below wave base) to proximal, silty-siliciclastic, marine platform-deltaic system. In this case, the sedimentary input was continuous, due to the absence of internal discontinuities. The dominant processes acting on this palaeo-environment were wave and storm action, prevailing the settling of fine material over the tractive processes.

The presence of primitive vascular plant debris in the Atuel and Lomitas Negras sections suggests closely related vegetated areas. The hydraulic regimes were moderate and the sea level changes in this sequence have generated very few sedimentary unconformities, but widespread lateral bed continuity.
Similar siliciclastic environments (and probably equivalent from a stratigraphical point of view), are interpreted as overfeed sedimentary foreland systems with great thickness (high sedimentary rates) and low textural maturity, e.g. the Villavicencio and Punta Negra Formations both from Precordillera. They have been described by other authors (González Bonorino 1975; Edwards et al. 2001, 2009; Peralta 2005, 2013; Cingolani et al. 2013).

However, the channelled conglomerates and organic matter-rich beds lithofacies (charcoal) present in the RSC, allow us to distinguish this unit from other similar environments found in Precordillera.
The main detrital zircon age populations found in the RSC, indicate that two main sources are responsible for the vast majority of observed ages. The main peak corresponds to Ordovician ages that could have been derived from the Famatinian orogenic belt which is developed easwards

A second group of sources is characterized by Mesoproterozoic ages between 1000 and 1150 Ma that may confirm partial derivation from the easternmost igneous-metamorphic complex (Cerro La Ventana Formation).

The Sm–Nd signature of the Río Seco de los Castaños Formation agree well with the Mesoproterozoic basement and the carbonate-siliciclastic platform (same range of variation of the εNd (t) and TDM ages), supporting both provenances.
The continental source areas (Cerro La Ventana Formation and the Ordovician sedimentary units) were located not far away towards the east within the San Rafael block.

The detrital material was westwards funnelled (conglomerate channels) from these positive areas into the outer platform areas also laterally associated with a progradating deltaic system along coastal sectors. The basin was deepening towards the west (open sea). Short transport is deduced from petrographical and sedimentological features. The limestone conglomerate-clasts support a provenance from rocks that belong to an Ordovician carbonate-siliciclastic platform, which is also located to the east.

Post Lochkovian age (Lower Devonian time)

La Horqueta Formation

This sedimentary unit was originally mapped and described by Dessanti (1945,1956) who called it ‘Serie de la Horqueta’. Later, it was renamed as La Horqueta Group (Dessanti and Caminos 1967), and then considered as a Formation by several authors (e.g. González Díaz 1981; Criado Roqué and Ibañez 1979).

It is a sandy-dominated meta-sedimentary sequence deposited in a marine environment.
The base of the sequence is not exposed. The metamorphic conditions were estimated to range from very low grade in the southernmost outcrops (González Díaz1972; Criado Roque and Ibáñez 1979; Tickyj and Cingolani 2000) to amphibolite facies in the northern area (Polanski 1964).

The sequence was affected by deformational events that developed folding with cleavage. In the area crossed by the Diamante River, Dessanti (1956) described a tight folding with similar, recumbent to asymmetric folds. The northernmost outcrops show folded rocks characterized by tight to isoclinal gently plunging, upright folds with N-S trending axial planes and rare recumbent folds (Polanski 1964).

The La Horqueta Formation was affected by a tectonic phase that put the metasedimentary sequence in contact with the Carboniferous continental to shallow marine (glacial) deposits of the El Imperial Formation. Some faults could have been reactivated during the Cenozoic Andean Orogeny (Moreno Peral and Salvarredi 1984; Cortés and Kleiman 1999; Japas and Kleiman 2004).
It is important to note that the “La Horqueta” unit initially comprised all ‘pre-Carboniferous’ sedimentary rocks of the San Rafael Block, exposed between the Los Gateados area and the Lomitas Negras and Agua del Blanco localities. Due to the lack of diagnostic fossils, an uncertain Precambrian to Devonian age was assigned (Dessanti 1956; Polanski 1964). At a later date, a fossil record including a Devonian coral similar to Pleurodyctium in Agua del Blanco exposures (Di Persia1972), microfossils (acritarchs) of the Upper Silurian age in outcrops near the road and ichnofossils like Nereites-Mermia facies in several outcrops (Rubinstein1997; Poiré et al. 2002) were mentioned.

More recently Morel et al. (2006) found herbaceous Lycophytes in the Atuel River section. These data support an Upper Silurian-Lower Devonian sedimentation age for part of the rocks included originally in the “La Horqueta” unit, now assigned to the Río Seco de los Castaños Formation.

According to Manassero et al. (2009) this formation was deposited in a marine platform-deltaic system, the dominant sedimentary processes were wave and storm action, whereas source areas were located mainly to the east. However, K-Ar geochronological data of two magmatic complexes (originally described as intrusive bodies) yielded Lower Paleozoic ages and suggested that the “La Horqueta”unit could be Lower Paleozoic in age (González Díaz 1981).

U-Pb age on zircons of 401 ± 4 Ma was obtained for the intrusive Rodeo de la Bordalesa Tonalite (Cingolani et al. 2003a), which is in according with mentioned fossil record, at least for a part of the unit now called Río Seco de los Castaños Formation.

At this point it is important to mention some stratigraphic changes

(a) González Díaz (1981) splitted up the La Horqueta unit in the sense of Dessanti (1956) into two units: the La Horqueta and Río Seco de los Castaños formations. The latter lacks the regional metamorphic overprint as well as the mafic rock mentioned by Dessanti (1956) in the La Horqueta Formation.

Furthermore, the Río Seco de los Castaños Formation preserved some diagnostic fossils as we mentioned before (acritarchs, lycophytes, coral). This suggestion was followed by Cuerda and Cingolani (1998), Cingolani et al. (2005) and Manassero et al. (2009) who also included in the Río Seco de los Castaños Formation the outcrops placed near road 144 where Rubinstein (1997) found Upper Silurian microfossils (acritarchs), and Rodeo de la Bordalesa section with the intrusive tonalite; and

(b) the Caradocian graptolite-rich sedimentary rocks located on the eastern slope of the Cerro Bola and originally comprising the “La Horqueta” unit, are now know as the siliciclastic Pavón Formation (Holmberg 1948 emend; Cuerda and Cingolani 1998).

Summarizing, we agree with the suggestions of Cuerda and Cingolani (1998) and Cingolani et al. (2003b) that the La Horqueta Formation (sensu stricto) should be restricted to the outcrops located on a strip reaching from the Seco de las Peñas River in the North to the Agua de la Piedra creek in the South. Where the best section is exposed—at the Diamante river area these outcrops are 12 km wide.

The La Horqueta Formation is bounded by reverse faults that bring this unit in contact with the Carboniferous El Imperial sedimentary sequence (Dessanti1956; Giudici 1971) but in some outcrops like at Punta del Agua area, the Carboniferous rocks overlay the La Horqueta Formation separated by an angular unconformity.

The La Horqueta folded metasedimentary sequence is intruded by the Permian granitic stocks like Agua de la Chilena (Cingolani et al.2005). All these rocks are overlain by Permian-Triassic volcano-sedimentary sequences related to the Choiyoi Gondwanian magmatism (Llambías 1999; Rocha Campos et al. 2011).
Previous geochronological data of the metamorphic event that affected the “La Horqueta Formation” are K-Ar whole rock ages of 320 ± 20, 390 ± 15 and 395 ± 15 Ma (Toubes and Spikermann 1976, 1979; Linares and González 1990)

Two separate and very well exposed areas—as previously stated—were selected to perform metamorphic and isotopic studies: La Horqueta type section and Los Gateados area.

Sedimentological and petrographical aspects:

At the La Horqueta type area the sequence consists of alternate beds of metawackes, metasiltstones, metapelites, and rare metaconglomerates, deposited in a marine environment. The metasandstones are the commonest rock type. They show tabular layers of variable thickness—between 0.1 and 6 m—which usually preserve sedimentary structures such as graded bedding, lamination and cross-bedding. The meta-sandstones show metaclastic textures with a matrix recrystallized into chlorite, illite, quartz, albite and minor smectite.

The original texture has been modified to variable degrees.
Thick layers usually present rough foliation with recrystallized matrix, while other layers show penetrative foliation with ductile deformed clasts and pseudo-matrix development. In less deformed metawackes, clasts are mainly composed of quartz (mono and polycrystals), and sedimentary and metasedimentary, with scarce volcanic and limestone lithoclasts, and minor feldspars. The presence of carbonaceous material (0.5–2%) and authigenic pyrite is common.

Fine grained sediments are less abundant and they have been mostly metamorphosed to phyllites. They are mainly composed of well oriented crystals of illite and chlorite (up to 10μ wide) with a minor proportion of quartz and feldspar. Abundant thin veins of quartz and calcite cut the phyllites. The meta-conglomerates are scarce and usually appear at the base of graded sandy layers.
At the Los Gateados river area, the unit consists of intercalated layers of muscovite-biotite schists and quartzitic schists. They have granolepidoblastic textures with a typical mineral association of chlorite + muscovite + quartz ± biotite, with accessory tourmaline, zircon and opaque minerals. Its structure is characterized by a continuous penetrative foliation, with a NNE trend and dips of 35–40° to the East.

Structural characteristics

A structural profile was described between the Puesto La Horqueta and Loma Colorada del Infierno at the La Horqueta River area. In this section the La Horqueta Formation is in tectonic contact by areverse faulting with the mainly Carboniferous El Imperial Formation at the Northwestern tip of the profile.

In the SE outcrops the La Horqueta Formation is covered by the Loma Colorada del Infierno sub-volcanic rocks (Dessanti 1956; Giudici 1971; Rubinstein et al. 2013). The whole sequence is characterized by asymmetric, open to similar folds, with straight limbs and rounded hinges. These folds have axial planes striking to the NE and dipping to the NW, and axes plunging a few degrees to the NE or SW.

The fold vergence of the whole unit is towards SE. The main mesoscopic structure is a secondary foliation S1, usually defined by aligned illite and chlorite. It has a consistent orientation with a north strike and moderate dip to the west. The S1 foliation is continuous in metapelites, whereas it is anastomosed and spaced in metasandstones. Two types of lineations have been recognized linked to the folding. On S1-planes a first mineral lineation is indicated by aligned illite + chlorite and tails of quartz on clasts, whereas another lineation is determined by the intersection of bedding planes and cleavage surfaces.
Several faults have been recognized, some of them are in the limbs of large folds, suggesting that they could be reverse faults related to the asymmetric folding.
However, they may have been reactivated, or even generated, by post-Carboniferous tectonic events. These folds have axial planes striking to the NE and dipping to the NW, and axes plunging a few degrees to the NE or SW. The fold vergence of the whole unit is towards SE.

Six samples of micaschists from the Los Gateados section were analyzed. The Rb contents vary between 57 and 151 ppm, while the Sr contents vary from 50 to 83 ppm. The age obtained from the isochron calculated with using Isoplot/Ex Model 1 (Ludwig 1998) is 371 ± 62 Ma, initial 87Sr/86Sr 0.7165 ± 0.0034 and MSWD: 3.7.

Furthermore, seven samples of metapelites from the La Horqueta type section were analyzed. The Rb contents vary between 116 and 290 ppm, whereas the Sr contents from 29 to 57 ppm.
The Rb-Sr isochron calculated with Isoplot/Ex Model 1 (Ludwig 1998) yielded an age of 379 ± 15 Ma, with IR: 0.7151 ± 0.0026, and MSWD: 1.4 (Tickyj et al. 2001).

The Rb-Sr data pointed out that the low-grade metamorphism and folding events of the La Horqueta Formation are Late Devonian.
These data agree with previous K-Ar ages reported by Linares and González (1979). Similar data were obtained on low-grade metamorphic units from the western and south-western sections of the Precordillera (Cucchi 1971; Buggisch et al. 1994; Gerbi et al. 2002).

This geochronological data let infer a Devonian age for the synmetamorphic ductile deformation, supported by 40Ar/39Ar plateau data on white micas (384 ± 0.5 and 378 ± 0.5 Ma) from low-grade metamorphic rocks from Bonilla and Portillo areas obtained by Davis et al. (1999).

U-Pb geochronology

U-Pb dating on detrital zircons of six metasandstone was performed in order to estimate maximum age of deposition and to accomplish a geochronological provenance study of the La Horqueta unit.

Results:

As we depicted the detrital zircon population of samples Hor 21 (n = 61), Hor46 (n = 60) and Hor10 (n = 60) show patterns dominated by grains of Mesoproterozoic ages, minor peaks corresponding to Neoproterozoic.

Lower Paleozoic age and few subordinate peaks in the Paleoproterozoic and Neoarchean.
As it is shown the sample Hor 21 notably record 85% of zircons derived from Mesoproterozoic sources, most of them from Upper Mesoproterozoic (“Grenvillian-age” or M3) in a polymodal detrital zircon-age pattern. The 9% correspond to the Neoproterozoic (Pampean-Brasiliano cycle), 6% of zircon grains were derived from cratonic domains (Paleoproterozoic ages).

The sample Hor46 could be described as bimodal that shows more than 80% of zircons of Mesoproterozoic sources, with 55% that correspond from the”Grenvillian-age” or M3; 11% are from the Pampean-Brasiliano cycle, 9% from cratonic sources and only 3% derived from the Famatinian belt.

The sample Hor 10 also presents main peaks (unimodal age pattern) in the Mesoproterozoic (72%) with 62% from the M3 or “Grenvillian-age”. Zircons of the Pampean-Brasiliano cycle are present with 18% and about 10% derived from cratonic sources (Paleoproterozoic).

The samples Hor15 (n = 59) and Hor 81 (n = 56) record a pattern with main peaks in the Neoproterozoic-Lower Paleozoic as well as in the Mesoproterozoic,
with minor peaks from Paleoproterozoic to Neoarchean ages.

The detrital zircon age pattern for sample Hor15 shows two major groups corresponding to Mesoproterozoic (62% of the grains, with 42% of M3), and Pampean (27% of zircons), with minor contribution from cratonic sources (11%), where 3% were Neoarchean.

For the sample Hor81 the zircon population is dominated by a strong peak corresponding to Pampean-Brasiliano ages (40%), then 37% from Mesoproterozoic ages (25% of M3), 13% from Paleoproterozoic ages (major percentage of cratonic sources without Neoarchean ages), whereas zircons from the Famatinian cycle represent a 10% of the analyzed grains.
The sample Hor27 (n = 64) show a quite different pattern from other studied samples. In this sample the detrital zircon age pattern is dominated by 54% of Famatinian zircon grains (23% Silurian and 31% Early Devonian in age), then a 35% derived from a source of Mesoproterozoic age (while 26% from M3), and subordinate contributions from Pampean-Brasiliano sources (8%) and cratonic areas (4%, with 2% from Neoarchean).

Constrains on provenance of the main detrital zircon sources could be as follows,
from older to younger age components:
(1) Archean to Paleoproterozoic: The source rocks of the obtained clusters (4–
13%) are probably derived from the erosion of the basement of the Río de la Plata craton located toward the East.

(2) Mesoproterozoic: Prominent clusters at M3 or“Grenvillian-age” 1.0–1.2 Ga were registered in all samples (26–62%), the most probable source of zircons of this age is the juvenile basement of Laurentian affinity of Precordillera-Cuyania, outcropping at the Pie de Palo, Umango ranges, Cerro La Ventana Formation at the San Rafael and Las Matras blocks (Sato et al. 2000).

(3) Neoproterozoic-Lower Paleozoic: Clusters of these ages were found in all studied samples (8–40%). Zircons of these ages are abundant in southern South America, evidencing the uplift and denudation of the Pampean-Brasiliano orogenic belts.

(4) Ordovician-Early Devonian: Zircon grains of these ages are recorded in samples Hor27 (with more than 50%), Hor46 and Hor81 (in between 3 and 10%). These grains probably derived from the erosion of the igneous rocks from the Late Famatinian magmatic arc, well known in western-central Argentina. The Devonian ages are abundantly registered on magmatic zircons from sample Hor27.
The younger detrital zircon ages (ca. 410 Ma) recorded on the sample Hor27 allows to constrain the age of deposition of the La Horqueta Formation to the
Silurian-Devonian limit.

Katian to Telychian age, (Upper Ordovician to Llandovery, Lower Silurian times)

El Nihuil Mafic Unit.

‘El Nihuil Mafic Unit’ comprises the Upper Ordovician-Lower Silurian undeformed porphyritic dolerites developed along the central and southern sector of the body (Cingolani et al. 2000). .

The sedimentary Upper Ordovician (Pavón Fm) that was the most probably country rocks of the dolerites were not recorded along the‘El Nihuil Mafic Unit.’ The Pavón Fm records detrital chromian spinels derived from mid-ocean ridge and continental flood basalts.

The El Nihuil Mafic Unit was considered as a probable source of these spinels (Abre et al. 2009), and although the dolerites were emplaced in the same tectonic setting as the source rocks of the spinels, such a derivation is not supported by isotopic ages, because dolerites are younger than the Pavón Fm.

Detrital zircon ages from Pavón Fm confirmed a main Mesoproterozoic source therefore a provenance of chromian spinels from the Mesoproterozoic section of the El Nihuil mafic body cannot be yet ruled out.

In hand specimen the color of undeformed porphyritic dolerites is dark, medium or dark-gray, or greenish. The porphyritic texture is clearly observed in several outcrops with fine and centimeter white/pale phenocrysts.
The dolerites show classical porphyritic texture, with elongated subhedral plagioclase (andesine) and clinopyroxenes phenocrysts; the finer matrix has a subophitic texture and it is composed of anhedral plagioclase enclosing clinopyroxenes or amphiboles. Hornblende and tremolite-actinolite amphiboles are commonly replacing pyroxenes. Accessory minerals are apatite, opaque minerals andfinely disseminated pyrite. Olivine crystals were also recognized in some samples.

Fifteen dolerite whole rock samples were selected to be analysed for major, minor, and trace elements (including REE). SiO2 concentrations varying from 46.07 to 49.34% and Na2O + K2O contents less than 4 wt% allow classifying the dolerites as basalts (Le Maitre 1989). MgO concentrations vary between 5.88 and 7.77%, while FeO ranges from 6.92 to 8.94%. Alkalis show low concentrations, ranging from 0.25 to 0.77% for K2O and between 2.11 and 3.37% for Na2O; wt% TiO2 is relatively high (1.12–2.19%) and wt% MnO and P2O5 values are less than 0.29%; Na2O concentration have an average of 2.58 wt%, that could indicate absence of albitization processes. Loss on ignition (LOI) is in between 1.09 and 4.5% indicating moderate to high alteration. Cr = 207, Sc = 45, and Ni = 101 ppm averages indicate moderate concentrations.

To classify the dolerite rocks we employed diagrams based in immobile elements. According to the Zr/TiO2 versus Nb/Y rock classification diagram (Winchester and Floyd 1977) the El Nihuil Dolerite samples plotted in the field of andesite/basalts, although according to their maximum SiO2 content (anhydrous base) of 51.5% they are recorded as basalts. The distribution of major oxides indicates the effects of secondary processes (weathering), but despite this the samples have the chemical characteristics of the tholeiitic series, as shown in the AFM diagram (Irvine and Baragar 1971).

Following Pearce and Cann (1973), the Ti/100 – Zr - Y/3 and Ti/100 – Zr - Sr/2 diagrams indicate that the dolerites correspond to MORB-type rocks. La/Yb normalized ratios between 0.93 and 1.44 fall within the range of values occurring in mid-ocean ridge volcanic rocks (Gale et al. 2013). The Th/U ratios between 3.53 and 4.56 are close to E-MORB and Zr/Y ratios ranging from 2.15 to 2.86 suggest continental crust contamination (Arévalo and McDonough 2010).

In the P2O5 versus Zr diagram (Fig. 7a) for basalts classification after Winchester and Floyd (1976), the El Nihuil Dolerite samples plot in the tholeiitic field. In the Th–Hf/3–Ta tectonic diagram (Wood et al. 1979), that can discriminate silicic magmas derived from E-type MORB or WPB and those associated with destructive plate margins or remelted continental crust) the dolerite dykes are plotted mainly in E-MORB field (intraplate oceanic basalts).
The primordial mantle normalized trace elements show enrichment in Cs, Rb, Ba, and Nb compared to the average pattern of N-MORB. Chondrite-normalized REE diagrams show El Nihuil dolerite patterns parallel to average N-MORB, but enrichments of the lighter elements (LREE) are evident, particularly for La and Ce. The Lan/Lun ratio is between 0.99 and 1.48. The normalized ratios data are similar to those from the Precordilleran basalts (Ramos et al. 2000; Boedo et al. 2013). From the comparison to the N-MORB pattern, an enrichment of LIL (large ion lithophile) elements is evident, whereas the HFSE (high field strength elements) does not show such behavior. LIL elements could be enriched due to alteration of the rock or contamination with crustal material.

Ten whole rock dolerite samples were used for Sm–Nd isotopic analyses. The Nd model ages were calculated using single stage after DePaolo (1981) and second stage after Liew and Hofmann (1988). Epsilon Nd (0) values for the samples are in between +3.85 and +7.84 while εNd(t = 450 Ma) record positive values ranging from +4.27 to +12.42.

Nd model ages (two stage depleted mantle) are in between 0.51 and 0.80 Ga. These values are very close due to fractionation of Sm–Nd from-0.01 to -0.03. Sample N4-A has a higher εNd(t = 450 Ma) value and a NdTDM age younger than the crystallization age, but the ƒSm–Nd is indicating that these aberrant values are consequence of REE remobilization, probably due to alteration as deduced by LILE enrichments.

Dolerite samples were dated K-Ar (whole rock) systematic and the ages are 448.5 ± 10 and 434.2 ± 10 Ma (Katian to Telychian, Upper Ordovician to LLandovery, Lower Silurian.

The Western Precordillera Mafic Belt: A Comparison

Haller and Ramos (1984), Ramos et al. (1999) and Ramos (2004) were the authors that linked the mafic-type rocks and their extension to the tectonic suture zone. However, the ophiolitic signature of the mafic and ultramafic belt developed along western Precordillera was established by Borrello (1963, 1969).
This mafic belt that extends over ca. 1000 km from south to north was named by Haller and Ramos (1984, 1993) as the ‘Famatinian ophiolites’ emplaced as a disrupted ophiolite during the Early Paleozoic (Ramos et al. 1984, 1986)

Mahlburg Kay et al. (1984) based on geochemical data proposed that the western Precordillera mafic rocks could have formed in a broad back-arc basin or at a mid-ocean ridge with an enriched source or as an early oceanic rift next to a continental margin.
Davis et al. (1999) challenged this interpretation suggesting the occurrence of two different sections of an ophiolite assemblage in Western Precordillera.

A Proterozoic one and another of Early Paleozoic age. More recently Boedo et al. (2013) discussed the E-MORB (enriched-type MORB) like signature of the mafic dykes and sills studied along the western Precordillera mafic-ultramafic belt as part of a non-subduction related ophiolite. González Menéndez et al. (2013) argued that the studied mafic rocks related to subduction or either N-MORB or OIB environment derived from primordial garnet-spinel transition mantle sources.

The model supports a thinned continental margin between Chilenia and Cuyania terranes (‘Occidentalia terrane’ after Dalla Salda et al. 1998) during the Middle to Late Ordovician. Many authors (Alonso et al. 2008 and references therein) described an extensional regime developing in a passive margin environment during the Ordovician in the western margin of Precordillera. These data were discussed in several geotectonic models proposed by different authors such as Ramos et al. (1986), Dalla Salda et al. (1992), Rapela et al. (1998), Davis et al. (2000), Gerbi et al. (2002), Ramos (2004), Thomas et al. (2012), González Menéndez et al. (2013) and Boedo et al. (2013).

The El Nihuil Dolerites record a geochemical and isotopic signature similar to that exposed along the western Precordillera belt (Boedo et al. 2013) as a subduction unrelated type at continental margin (Dilek and Furnes 2011).

Formación Ponón Trehue:

The Darriwilian to Sandbian Ponón Trehué Formation crops out at the southern edge of the San Rafael Block Mendoza province, Argentina
It is an olistostromic carbonate–siliciclastic sequence unconformably overlying, the Mesoproterozoic basement known as the Cerro La Ventana Formation (Nuñez 1979; Criado Roqué and Ibañez 1979; Heredia 1996; Cingolani and Varela 1999; Beresi and Heredia 2003; Cingolani et al. 2005; Heredia 2006).

The unit comprises outcrops of the previously known Lindero Formation (Nuñez 1979 and see discussion in Heredia 1996, 2006 and Abre et al.2011).
The continental Carboniferous Pájaro Bobo Formation (correlated with El Imperial Formation towards the Northwest of San Rafael Block) overlies the Ponón Trehué sequence through either an unconformity or a fault contact.
As an Ordovician fossil-rich unit, it contains trilobites, brachiopods, ostracods, fragmentary crinoids, corals and conodonts (Nuñez 1962; Baldis and Blasco 1973; Rossi de García et al. 1974; Levy and Nullo 1975; Heredia 2006). The first fossiliferous record was made by Nuñez (1962). This material was preliminary classified by Armando F. Leanza as the brachiopods Obolus and Taffia, and the trilobite Lonchodomas cf salagastensis (Rusconi).

These records have been used to correlate the Ponón Trehué unit with the Middle Ordovician San Juan Limestones, cropping out in the Precordillera region, as was first mentioned by Wichmann (1928) that considered the carbonates similar to those of Cerro de la Cal and Salagasta (near the city of Mendoza). Baldis and Blasco (1973) revised in detail the trilobite material and described the new genus Elbaspis (Odontopleuridae, Selenopeltinae) (? = Miraspis; Ramsköld 1991; Jell and Adrain 2003) and the new species Elbaspis pintadensis, Toernquistia chinchensis (Dimeropygidae) (reassigned to Paratoernquistia by Chatterton et al. 1998), Ampyx nunezi (Raphiophoridae), and Flexicalymene frontalis (Calymenidae). Undeterminable species of Monorakidae, Trinucleidae, and Illaeninae are also present in the assemblage.

At the northern sector (in the way to the Chinches Hill) in small outcrops of Ponón Trehué Formation, some stromatolite structures were recognized in limestone rocks preserved as olistoliths.

The Ponón Trehué Formation is subdivided into two members: the lowermost (Peletay Member) is composed of conglomerates and conglomeratic arkoses, limestones, quartz arenites, and black shales, whereas the uppermost (Los Leones Member) is composed of mudstones, siltstones, arenites, and conglomeratic arenites.
The extension undergone produced the brecciation of parts of the carbonate platform that slumped down the slope, forming this breccia-type deposit (Astini 2002) within a fine clastic matrix. The absence of blocks from the Cambrian carbonate platform indicates that for the time of deposition of the Ponón Trehué Formation the basement was exposed (Heredia 2006).

The provenance of the Ponón Trehué Formation was determined using petrography, whole-rock geochemistry and isotope geochemistry (including detrital zircon dating)
The Ponón Trehué Formation at the La Tortuga section (35° 10′ 53″S–68° 18′ 13″W) comprises claystones, siltstones and fine-grained sublith- and subfeldspathic arenites (Dott 1964). The arenites are moderately sorted with scarce matrix. The framework minerals include: subrounded to subangular monocrystalline (with low sphericity) and polycrystalline (less abundant) quartz as well as subrounded K-feldspar commonly totally replaced by chlorite or clay minerals; detrital muscovite lamellae are very scarce.

Sedimentary lithoclasts derived from siltstones, carbonates, mudstones, and cherts were also described. When present, the cement is composed of calcite. The heavy minerals fraction comprises zircon, apatite, chromian spinel, tourmaline, rutile, and iron oxides such as hematite. X-ray diffraction analyses indicate that clay minerals within the three lithotypes are mainly chlorite, sericite, and illite (Abre 2007; Abre et al. 2011).

Sandstones of the Ponón Trehué Formation had shown relatively textural immaturity and mineralogical maturity, which altogether may imply that the detritus had suffered low transport but a certain degree of chemical weathering. The composition of the lithoclasts indicates sedimentary rocks as part of the source, while the presence of detrital chromian spinels clearly points to a mafic source. The bulk of the mineralogical composition indicates felsic sources.
A well-crystallized illite, characteristic of a relatively high-temperature history, has sharp peaks, and therefore a low index, while low-temperature illite is more disordered, and has irregular peaks with large indexes. The ICI on four fine-grained clastic samples of the Ponón Trehué Formation shows that the unit was affected by very low-grade metamorphism

U–Pb detrital zircon

Detrital zircon dates (n = 38) of the Ponón Trehué Formation cluster between 1065 and 1277 Ma with a main peak at about 1200 Ma. Only one discordant grain has a younger age of 834 Ma (Abre et al. 2011). The very narrow range of detrital zircon ages implies a local and restricted provenance, most likely from the underlying Cerro La Ventana Formation and is in agreement with the low recycling deduced from petrographic and geochemical analyses. Th/U ratios measured in zircons along with cathodoluminescence images indicate a dominance of grains originated by magmatic processes rather than metamorphic.

The Cerro La Ventana Formation, with ages between 1.1 and 1.2 Ga (Cingolani and Varela 1999; Cingolani et al. 2005), matches the detrital zircon ages and was a source of detritus. Other Mesoproterozoic rocks within the basement of the Cuyania terrane are found at the Pie de Palo Range (1.0–1.2 Ga; McDonough et al. 1993) and the Umango, Maz and Espinal Ranges (1.0–1.2 Ga; Varela and Dalla Salda 1992; Varela et al. 1996; Casquet et al. 2006; Rapela et al. 2010; Varela et al. 2011).

Lower Paleozoic sedimentary cover

The Ordovician sedimentary cover of the San Jorge Formation appears in scattered outcrops, the most important ones to the west of Limay Mahuida where two members were defined, the sedimentary, San Jorge Member and the metamorphic, Cerro Rogaziano Member, which not only differ in their metamorphic character, but also in their structural attitude (Tickyj 1999, unpublished phD thesis; Melchor and Casadıo 1999). Pb–Pb and U–Pb isochrones and 87Sr/86Sr compositions constrain the depositional age of limestones most favorably to about 500 Ma (Melchor et al. 1999), supported by conodonts of late Tremadoc age (Tickyj et al. 2002; Albanesi et al. 2003), which allows their correlation with La Flecha and La Silla Formations of Precordillera. Calcite twinning features in the marbles of the metamorphic member suggest a lowgrade metamorphism between 150C and 300C (Tickyj 1999, unpublished phD thesis), similar to that obtained by conodont color alteration index CAI 5 (Albanesi et al. 2003).

The deformation and metamorphism are tentatively attributed to the Devonian Chanic phase (Melchor et al. 1999), on the basis of whole rock K–Ar dates between 392 and 382 Ma of the Las Matras pluton (Linares et al. 1980).

Miaolingian-Sheinwoodian (Middle Cambrian-Wenlock, Middle Silurian, 500 ± 27 Ma-431 ± 12 Ma)


Pichi Mahuida Group

Grupo Pichi Mahuida 

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Cingolani, C. A., Manassero, M. and Abre, P. 2003, Composition, provenance, and tectonic setting of Ordovician siliciclastic rocks in the San Rafael block: Southern extension of the Precordillera crustal fragment, Argentina. Journal of South American Earth Sciences 16: 91–106

Cingolani, C. A., Tickyj, H. y Chemale Jr. F., 2008? PROCEDENCIA SEDIMENTARIA DE LA FORMACION LA HORQUETA, BLOQUE DE SAN RAFAEL, MENDOZA (ARGENTINA): PRIMERAS EDADES U-Pb EN CIRCONES DETRITICOS

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