1 INGEODAV, Departamento Ciencias Geológicas, Fac. Cs. Exactas y Naturales, Universidad de Buenos Aires-CONICET, Pabellón 2, Ciudad Universitaria, Buenos Aires, C1428EHA, Argentina. E–mail: rapalini@gl.fcen.uba.ar

2 CIG – Departamento de Geología, Univ. Nac. La Plata, calle 1 n. 644, 1900-La Plata, Argentina.

Key words: Paleomagnetism. Pavón Formation. San Rafael Block. Late Ordovician. Cuyania Terrane.

The Argentine Precordillera has been interpreted as an Early Paleozoic Laurentian derived exotic terrane (e.g., Dalla Salda et al., 1992; Astini et al., 1995; Mahlburgh Kay et al., 1996; Thomas and Astini, 1996; Dalziel, 1997; Keller et al., 1998; Benedetto, 1998). Ramos (1995) proposed that the Precordillera is part of a larger composite terrane (Cuyania) that includes also the San Rafael Block and the Pie de Palo Range in the Western Sierras Pampeanas. In all of these areas Grenvillian–age basement is exposed. For most authors, several lines of evidence coincide in that Cuyania most likely originated from the Ouachita embayment of North America. This reconstruction has been strongly supported by paleomagnetic data from the Early Cambrian Cerro Totora Formation (Rapalini and Astini, 1998). However, no consensus exists on how this terrane was transferred from Laurentia to Gondwana and the timing of its accretion, which varies in different models from Mid–Ordovician through Devonian. Although, there are models which interpret the origin of the Precordillera to be autochthonous (González Bonorino and González Bonorino, 1991) or that it represents a parautochthonous displaced terrane (Baldis et al., 1989; Aceñolaza et al., 2002). Recently, Finney et al. (2002) interpreted an Early Gondwanan provenance of this terrane based on new U–Pb ages from detrital zircons. All these antecedents stimulate new research regarding the paleogeographical evolution of Precordillera during the Early Paleozoic.

Since paleomagnetism is the only quantitative tool for paleogeographic reconstructions in pre–Jurassic times (e.g., Van der Voo, 1993), it could play a significant role in testing contrasting geodynamical hypotheses. Unfortunately, with the exception of the paleomagnetic study of the Cerro Totora Formation, no primary magnetizations have yet been recovered from Early Paleozoic rocks in the Argentine Precordillera or the San Rafael Block (Rapalini and Tarling, 1993; Truco and Rapalini, 1996; Rapalini et al., 2000). This is due to a pervasive remagnetizing event linked to the Late Paleozoic San Rafaelic tectonic phase that apparently affected all carbonatic rocks. In order to constrain the paleogeographic evolution of the Cuyania terrane a paleomagnetic study was attempted on the Early Caradoc Pavón Formation (Cuerda and Cingolani, 1998) exposed at Cerro Bola (34.6°S, 68.6°W) in the San Rafael Block, Mendoza Province (Figure 1). As a result of this study we could determine for the first time the paleolatitude of the Cuyania terrane during the Late Ordovician.

The San Rafael Block (Figure 1a) as the southern extension of the Precordillera terrane (Figure 1a) is located 200 km southwards of the Precordillera in the Mendoza province. Diverse igneous–metamorphic and sedimentary units of Precambrian to Middle Paleozoic age are present and known as ‘pre–Carboniferous units’ due to their clear separation from the Upper Paleozoic beds by a regional unconformity. One of these units, the Pavón Formation, is exposed in the central portion of the San Rafael Block at the eastern slope of the Cerro Bola (Figure 1b). Outcrops of the Pavón Formation cover an area which is 3,5 km long and 1,2 km wide and is composed of folded siliciclastic sediments, covered by Lower Permian volcaniclastics and intruded by Permian–Triassic rhyolitic rocks. The base of the 700 m thick Pavón Formation is not exposed. It consists of massive green–reddish–gray sandstones, wackes, quartz–sandstones, siltstones and interbedded black shales. The sedimentological features suggest gravity flows in a relatively deep marine sedimentary environment. The presence of a rich Ordovician graptolite fauna (Climacograptus bicornis Biozone) indicates a Lower Caradoc age for the Pavón Formation. Composition, provenance and tectonic setting of the Pavón Formation were discussed by Cingolani et al. (2002). The rhyolitic dome exposed at Cerro Bola is Permo–Triassic in age (Linares et al., 1978).

Figure 1. A. Location of the San Rafael Block. B. Outcrops of the Pavón Fm. and locations of paleomagnetic samples sites.

Figure 2. A. Sample characteristic direction for the Pavón Fm. in situ and after restoration to paleohorizontal showing a positive fold test and antipodal polarities. B. Paleogeographic reconstrution of Gondwana supercontinent and Laurentia for the Late Ordovician (ca. 455 Ma). The paleomagnetically determined position of the Cuyania terrane during the interval of the deposition of the Pavón Formation is indicated. Note that the paleolatitude is both consistent with paleogeographic position of the Ouachita Embayment in SE Laurentia and the present location of Cuyania in SW South America.

Eighty–three oriented samples were collected at twelve sites in the Pavón Formation. One separate site (seven samples) was selected in the rhyolitic dome of Cerro Bola. One or two specimens were processed from each sample. All samples were submitted to detailed AF or thermal demagnetization (e.g., Butler, 1992) in twelve to seventeen steps up to 140 mT or 695°C. Magnetic components were determined by principal component analyses (Kirschvink, 1980), with MAD values under 10°. Most samples showed good magnetic stability with one to three magnetic components. Ten sites in the Pavón Formation presented a postectonic remanence consistent with that isolated from the single site at the Cerro Bola rhyolitic dome. This component shows dual polarity which indicates that magnetization is post–Kiaman (younger than 260 Ma, approximately). A paleomagnetic pole computed from this remagnetized directions is coincident with the Early Triassic mean pole for South America (McElhinny and MacFadden, 2000) supporting that the age for the remagnetization is coeval and therefore likely related with the extensive magmatic activity of the region.

At four sites (PV5, 6, 9 and 12) another component was isolated with unblocking temperatures higher than 660°C (hematite), in sites 5, 6 and 9, and of around 590°C (magnetite?) in PV12. This component also recorded dual polarities, even at a single site (PV6), and presents a positive fold test (Figure 2), indicating that the magnetization is pre–tectonic. Both characteristics strongly suggest that this magnetization is primary and was acquired by the sediments shortly after deposition or during early diagenesis. High within site consistency of directions reinforce the reliability of this component. However, due to the fact that both polarities are recorded at a single site a sample based mean direction was preferred. Restored to paleohorizontal, the mean direction is at Dec: 65.2° Inc: 44.1° a95: 3.7° n = 22 samples (4 sites).

A paleomagnetic pole was computed for the Pavón Formation. On the basis of the magnetic characteristics mentioned above, this pole is interpreted to represent a Late Ordovician (ca. 455 Ma) pole for the Cuyania terrane. When rotated into a Gondwana configuration, this pole falls on NE Africa. It indicates that this area of the Cuyania terrane during the time of depositon of the Pavón Formation was at a latitude of 25.7° ± 2.9°. Figure 2 shows a Late Ordovician paleogeographic reconstruction based on the Late Ordovician mean paleomagnetic poles for Laurentia and Gondwana (McElhinny and MacFadden, 2000). This figure illustrates the possible position of the Cuyania terrane on a paleolatitudinal band that corresponds to the data from the Pavón Formation based on the paleomagnetic pole obtained and its uncertainties. It displays that the paleolatitude is coincident with those correspondents to SE Laurentia (Ouachita Embayment) and the present position of Cuyania in southwestern South America. Therefore, our new paleomagnetic results cannot distinguish, on the basis of the paleolatitude alone, whether or not Cuyania was already accreted to Gondwana by the Caradoc, as both SE Laurentia and SW South America were at similar latitudes. However, our data are significant for the paleogeographic evolution of Cuyania in respect to the fact that the Pavón Formation was deposited at tropical latitudes (less than 30°) at around 455 Ma. A higher latitude for Cuyania at this time has been proposed on the base of sedimentological and paleontological evidence (Keller and Lehnert, 1998 and Lehnert et al., 1999). Our result suggests that these models should be modified. Some ten million years later, however, glacial deposits were recorded during the Hirnantian glaciation in several localities of South America, including the Cuyania terrane. Due to lack of reliable paleomagnetic data for Gondwana across the Ordovician–Silurian boundary, it will remain controversial whether or not this glaciation was associated with a very fast drift of Gondwana (and Cuyania?) towards polar latitudes.

Financial support from UBACyT (grant XO45) and CONICET permitted this study. Critical review by O. Lehnert and anonymous reviewer improved the final version. L. Ortiz kindly helped us during the fieldwork. Contribution to IGCP 436.