Preliminary paleomagnetic results from Early to Middle Ordovician volcanites of the Famatina System, NW Argentina.

SPAGNUOLO, CECILIA M.; RAPALINI, AUGUSTO E.; ASTINI, RICARDO A.

Mendoza

Simposio; Gondwana 12; 2005

Comité Internacional del Gondwana - Academia Nac. 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 allochthonos and para-authocthnous terranes. Controversy surrounds the tectotnic evolution of the Famatina System (west of Argentina) in the Early Paleozoic. This is characterized by an abundant but short-lived magmatism that 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-authochtonous tectonic block acretted to the Gondwana margin at some time in the Ordovician. Early Ordovician paleomagnetic poles reported by Conti et al (1996) from the Famatina System and coeval volcanics on the Eastern Puna Eruptive Belt were interpreted as evidence of a peri-Gondwana para-authochtonous 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 of further paleomagnetic data to test it. With this aim, a paleomagnetic study was carried out on the Llanvirnian (c.a. 470 Ma) Cerro Morado Group (Turner, 1967) exposed in the Famatinian Ranges (28°44´-28°43´S and 67°80´-67°49´W). This unit is composed by ignimbritic outflows and ryolites. Preliminary results from 20 sites, analysed with stepwise thermal and alternating field demagnetization techniques indicate that magnetite is the principal 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. In a Gondwana reconstruction (Lottes and Rowley, 1990) the obtained pole falls on 30.6°S, 359.6°E (dp=5.6°; dm=8.2°). It indicates a paleolatitude around 28°S for the deposition of the 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. The localities involved in this study are far away from the ones by Conti et al. (1996), and the achieved results confirm on paleomagnetic grounds the model proposed by the latter authors for the Famatina System as a large para-authochtonous rotated terrane.