Preliminary paleomagnetic results from Early to Middle Ordovician volcanic rocks of the Famatina System, NW Argentina

SPAGNUOLO, C.M.; RAPALINI, A.E.; ASTINI, R.A.

Mendoza

Congreso; Gondwana 12; 2005

Academia Nac. de Ciencias

The Paleozoic paleogeographic and tectonic history of Gondwana has been characterized by the wander of the supercontinent over the South pole and the accretion of allochthonous and para-autochthonous terranes. Controversy hold regarding the tectonic evolution of the Famatina System (west of Argentina) in the Early Paleozoic. It is characterized by an important but short-lived magmatism developed in the Early to Middle Ordovician. According to different authors (Saavedra et al., 1998; Rapela et al., 1998; Quenardelle and Ramos, 1999; Astini, 2003 and others), it evolved either as an “Andean type” magmatic arc in the SW continental margin of Gondwana or it consisted of a separate para-autochthonous tectonic block accreted to the Gondwana margin at some time in the Ordovician. Early Ordovician paleomagnetic poles reported by Conti et al. (1996) of the Famatina System and coeval volcanic rocks on the Eastern Puna Eruptive Belt were interpreted as evidence of a peri-Gondwana para-autochthonous terrane that underwent a rigid body clockwise rotation of near 50° before its accretion to the margin in the Middle-Late Ordovician. Since then, this proposal has been widely disputed pointing out the need for further paleomagnetic data to test it. With this aim, a paleomagnetic study was carried out on the Llanvirn (c.a. 470 Ma) Cerro Morado Group (Turner, 1967) exposed in the Famatina Ranges (28°44´–28°43´S and 67°80´–67°49´W). This unit is composed of ignimbrite and rhyolite flows. Preliminary results from 20 sites, analyzed with stepwise thermal and alternating field demagnetization techniques, indicate that magnetite is the main carrier of the magnetic remanence while hematite is also present in few cases. In most sites two magnetic components were found. The characteristic remanence has reversed polarity in all samples and the statistical parameters improve significantly when bedding correction is done (declination=104.3°; inclination=47.0°; N=18; α95=6.2°). The obtained pole falls on 24.7°S, 2.3°E (dp=5.2°; dm=8°) in South American coordinates. It indicates a paleolatitude around 28° for the deposition of Cerro Morado Group. The pole does not coincide with the Gondwana apparent polar wander path and falls very close to previous results from Famatina System and Eastern Puna Eruptive Belt, indicating a very large clockwise rotation of the study locality, which must have taken place in pre-Permian times according to preliminary data from Late Paleozoic red beds at the same locations. The localities involved in this study are far away from the ones analyzed by Conti et al. (1996), and the preliminary results obtained confirm on paleomagnetic grounds the model proposed by those authors for the Famatina System as a large para-autochthonous rotated terrane.