CONICET | Buscador de Institutos y Recursos Humanos

The origin of Patagonia has long called the attention of South American earth scientists. In recent years a dispute over whether it is an accreted crustal block that collided with Gondwana in Late Paleozoic times or an autochthonous part of South America has taken place. Scarce paleomagnetic data mostly younger than Devonian, preclude a definite paleomagnetic test on the origin of this terrane. The presence of well-dated and undeformed Ordovician granitoids discordantly covered by the Silurian-Devonian Sierra Grande Fm in the northeastern corner of the North Patagonian Massif (NPM) are a suitable target to undertake such test. The plutonic rocks belong to the Punta Sierra and related granitoids which have recently yielded U-Pb crystallization ages between 472-476 Ma (Pankhurst et al, 2006, Varela et al., 2009 and references therein). A paleomagnetic study of these intrusives (41.5°S, 65.0°W) was carried out. As part of a multidisciplinary study, anisotropy of magnetic susceptibility (AMS) measurements, systematic analyses of petrographic thin sections and rock magnetic analyses, have also been performed. About one hundred specimens were processed, comprising ten sites on the granite and two sites on the quartzites and sandstones of the Early Devonian Sierra Grande Formation, with five cores each. Demagnetization at high temperatures isolated a reverse characteristic remanent magnetization, suggestive of being acquired during the Kiaman reverse superchron. Structural correction of paleomagnetic data from plutons was available only at a few sites from bedding attitudes of the Devonian or Tertiary sedimentary rocks. Seven out of twelve sites, all in the granites, provided consistent remanence directions. Structural correction could be applied at a single site worsening the statistical parameters of the mean site direction and suggesting a secondary magnetization. A paleomagnetic pole (PP) was computed from the mean of the seven site directions. The position of this PP on the apparent polar wander path of South America is at: 11.5°E, 65.0°S; A95=12°, K=24.5 suggesting that magnetization was acquired during the Early Permian, being this pole consistent with previous poles of that age from South America (Tomezzoli, 2009). A remagnetization during the late Early Permian has been already reported on some outcrops of the Devonian Sierra Grande Fm. in the same area (Rapalini and Vilas, 1991). Our data suggests that the remagnetization was pervasive and affected the Ordovician granitoids as well. Whether this remagnetization is due to the widespread Permian magmatism that affected the NPM or to the deformational phase ascribed in some models to the collision of Patagonia against the Gondwana margin is to be determined. However, it is significant that several South American Lower Permian paleomagnetic poles have been computed from syntectonic magnetizations (e.g. Ponón Trehue; Tunas I PP; Cochico PP; Río Curacó PP; and Sierra Chica PP, among others). All these PPs come from localities along a 500 km long WNW-ESE orogenic belt that extends from the San Rafael block in the province of Mendoza to the Ventana System in the province of Buenos Aires. Deformation along it has been assigned to the San Rafaelic orogenic phase and dated at approximately 290 Ma. This phase has been recognized mainly in the western areas of Argentina and has been linked to remagnetization of a regional scale (Rapalini and Astini, 2005). Time coincidence of remagnetizations suggests that causal links for all of them including those in the NPM are likely. Regional remagnetization associated to this major orogenic phase and its geotectonic framework should be explored.

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