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The western margin of Gondwana corresponding to the north-western Argentinian Andes was the location of the Puncoviscana Basin in Late

Proterozoic-Early Cambrian times (Aceñolaza & Duran, 1986; Aceñolaza et

al., 1990). The Puncoviscana Basin is variably interpreted to be a marginal

continental basin that developed along the western border of the Pampia craton (Ramos & Vujovich, 1993), or a foreland basin formed during the

collision of the Pampia and Rio de la Plata cratons (Kramer et al., 1995).

Palaeontological and geochronoligical data from the oldest sediments suggest that this basin existed between 700 Ma and 535 Ma (Ramos; 2000). The Puncoviscana Formation was deformed during the Early Cambrian Pampean orogeny, which was associated with the collision of the Mejillonia and Arequipa-Antofalla terranes (Ramos, 2000). Subduction-related magmatic activity occurred in this South American margin of Gondwana in Middle Cambrian to Early Ordovician times (510-470 Ma; Astini, 1996; Ramos, 2000).

The Puncoviscana Formation is overlain by the Late Cambrian shallow marine deposits (Méson Group) and the Early Ordovician Santa Victoria

Group (Coira et al., 1982). The Santa Victoria Group is variably interpreted

to be oceanic back-arc sediments (Ramos, 1988) or island arc sediments

(Coira et al., 1999). The Early Middle Ordovician time is characterised by

the Ocloyíc tectonic phase, thought by many authors to be related to the accretion of the Famatina and Precordillera terranes. The Ocloyíc phase resulted in progressive closing of the Ordovician Puncoviscana Basin and

development of the volcanic arc in the Puna region (Balburg et al., 2000;

Ramos, 2000). The Late Ordovician clastic marine sediments overlie these Early Ordovician volcano-clastic sediments. Marine sedimentation continued

in Silurian until the Middle Devonian Chaňic deformational phase.

Continental alluvial deposits followed from Early Carboniferous through to Permian-Triassic times.

Palaeomagnetic investigations of the Palaeozoic rocks of the Andes of NW Argentina were carried out in the Pampean Range, the Argentina Precordillera, the Famatina Range, and the Paganzo Basin. Various models of the tectonic evolution of these morpho-tectonic units have been published. However, the proposed geological interpretations are contradicting in some cases and the evolution of this southern part of the Central Andes is not clear yet. The current knowledge concerning the evolution of these units are presented in the following sections.

a) The Pampean Ranges

Ramos (1994) defined the Pampean Ranges(Sierras Pampeanas) as a series of Precambrian crystalline blocks that accreted to the Rio de la Plata craton during the Late Proterozoic-Early Cambrian. The Pampean continental blocks (Fig. 2.2a) is interpreted to have rifted off, and partially detached from the Gondwana margin after the Late Proterozoic break-up of Rodinia

(Rapela, 1998; Jaillard et al., 2000; Gosen, 2001). Rifting between the Rio de

La Plata craton and the Pampean terrane is thought to have resulted in the opening of the Puncoviscana Basin. Northeastward-directed subduction and progressive closure of this basin followed in Early Cambrian and continued until the collision of the Pampean block with the Rio de la Plata craton. The final collision of the Pampean block is thought to occur in late Early Cambrian and marked by the end of clastic sedimentation within the Puncoviscana Basin (Gosen, 2001) and compressive high grade

metamorphism followed by isothermal uplift at 520 Ma (Rapela et al., 1998).

The Pampean Ranges was affected by compressive deformation related to the eastward-directed subduction of the proto-Pacific Oceanic crust during the Late Cambrian-Early Ordovician.

b) The Argentine Precordillera

The Argentine Precordillera is widely interpreted as an exotic terrane derived from Laurentia and accreted to the western margin of Gondwana in

Palaeozoic times (e.g. Astini et al., 1995; Dalziel, 1997; Pankhurst et al.,

1998). Different models, however, have been presented concerning the

timing and its accretion scenario. On one hand, Dalla Salda et al. (1992) and

Dalziel et al. (1994) proposed that during Middle Ordovician time, Laurentia

collided with the Gondwana margin and transferred the Precordillera to Argentina during a Late Ordovician rifting event.

On the other hand, the faunal affinities between Laurentia and the Precordillera during the Cambrian-Early Ordovician, and the palaeontological divergence throughout the Ordovician times are interpreted by Benedetto (1998) as the result of progressive drift of the Precordillera away from Laurentia and later accretion with Gondwana. This interpretation favours the model in which the Precordillera is considered to be an independent crustal plate that travelled across the Iapetus Ocean to reach and

collide with the western margin of Gondwana (Astini et al., 1995; Thomas &

Astini, 1996). In this scenario, the final accretion of the Precordillera is

thought to have occurred during the Late Ordovician times (Ramos et al.,

1986, 1993; Astini et al., 1995; Benedetto, 1998). However, based on pre-

Carboniferous lithostratigraphy and fauna, Keller et al. (1998), proposed that

the Precordillera was separated from Laurentia during the Ordovician, drifted slowly across the Iapetus and collided to Gondwana much later in Silurian- Devonian times.

Although, various scenarios involving the exotic origin of the Argentine

Precordillera have been widely published, some authors (e.g. Baldis et al.,

1989; Aceňolaza & Toselli, 2000; Aceňolaza et al. (2002) propose a para-

autochthonous origin of the Precordillera. Their models assume that the Precordillera was part of a hypothetical SAFRAN plateau, which developed between South America, Africa and Antarctica. A subsequent displacement along transcurrent faults is thought to have brought the Precordillera into its present location.

c) The Famatina Range

The central part of the Sierras Pampeanas is characterised by a magmatic arc

of Early to Middle Ordovician age (Pankhurst et al., 1998), which comprises

the Famatina system and the Faja Eruptive de la Puna Oriental (Conti et al.,

1996). Astini et al. (1995) consider this arc as an oceanic island arc, which

approached the Gondwana margin as a result of subduction and closure of an

(1983), and lithostratigraphic, volcanism, and palaeontological studies

reported by Rapela et al. (1992) support the presence of this Ordovician

ocean. However, based on geochemical data, Pankhurst et al. (1998)

suggested that the Famatina Arc is a continental arc that developed on this margin of Gondwana in Early to Middle Ordovician.

On the Other hand, Conti et al., (1996) suggested a para-autochthonous

model based on palaeomagnetic data from the Puna-Oriental and Famatina Range. These authors suggest that both entities formed a mobile terrane that has been rotated clockwise and displaced with respect to Gondwana after the Early Ordovician. Petrological and faunistic evidence presented in Rapalini (1998) support this model.

d) The Paganzo Basin

The Paganzo basin is a foreland basin, which developed in NW Argentina

during the Carboniferous (Lopez Gamundi et al., 1994; González Bonorino

& Ayles, 1995). The basin is situated east of the Argentine Precordillera, bounded by folded Early Palaeozoic to Early Carboniferous sediments of the Pampean range. Detailed studies of the sedimentary sequences and stratigraphic correlations indicate that sedimentation within the Paganzo basin started in the Early Upper Carboniferous (Namurian) and continued

until the Middle to Late Permian (Net & Limarino, 1999; Limarino et al.

(2002). The Late Carboniferous is interpreted as being a postglacial succession deposited during two main episodes of marine transgression, palaeotologically dated as Namurian-Westphalian and Stephanian in age. The Namurian-Westphalian transgressive sequence embraces the Guandacol, the Agua Colorada and the Malanzán Formations. The Stephanian transgression is well represented in the upper part of the Tupe Formation and in the Agua

de Jagüel Formation. Limarino et al. (2002) subdivided the Paganzo Basin

into different depositional settings, based on fossils and facies distribution. An open marine setting is suggested for the Western domain, while a marine- continental transition and a continental-dominated settings are attributed to the Central and Eastern domains, respectively.