Palaeomagnetism and K--Ar ages of Lower Ordovician and Upper Silurian--Lower Devonian rocks from north-west Argentina

Abstract

In this paper we present palaeomagnetic data from 87 hand samples collected in a sequence of tuffs and shales (Suri Formation) of Llanvirnian age, exposed in north-western Argentina (27" 47'S, 68" 06'W). After cleaning, the majority of samples showed reversed polarity and yielded a palaeomagnetic pole at 5.9" E, 8.5" S (ag5 = 5.9'). They also showed reversals of declination and inclination at the top of the sequence, which we have associated with geomagnetic excursions. Whole rock K-Ar age determinations suggest an age older than 416 2 25 Myr for the Suri rocks. The predominant reversed stable remanence of these rocks is consistent with the reversed polarity reported for Early Llanvirnian rocks from USSR. The palaeomagnetic pole for the Suri Formation is consistent with the interpretation that Gondwana was a single unit in Early Palaeozoic times.Palaeomagnetic data from 27 hand samples collected from 10 igneous units of Late Silurian-Early Devonian age (fluzorco Formation), exposed in the same area, are also given. The majority of the igneous units showed reversed polarity after cleaning. The positions of VGP's for the fiuEorco igneous units are scattered and they are not used for geodynamic interpretations. Whole rock K-Ar age determinations suggest ages of 416 k 25 and 360 k 10 Myr for two igneous units of the fiuGorco Formation.

Reinterpretation of the Ordovician rotations in NW Argentina and Northern Chile: a consequence of the Precordillera collision?

“…Clockwise rotations around vertical axis were observed in Ordovician rocks of the Famatina Ranges (Valencio et al 1980;Conti et al 1996;Spagnuolo et al 2008), the NW Pampean Ranges and the Eastern Puna (Conti et al 1996), whereas counter-clockwise rotations have been found in the Western Puna (Forsythe et al 1993;Rapalini et al 2002). The Famatinian magmatic arc, created by subduction of the Iapetus oceanic plate under the southwestern Gondwana margin that eventually led to the collision of Precordillera probably developed a negative roll back that caused the formation of an important back arc basin with significant extension that led to production of quasi-oceanic crust at some places.…”

“…Clockwise rotations (Famatina Range, NW Pampean Ranges and Eastern Puna) Conti et al (1996) carried out paleomagnetic studies in Lower-Middle Ordovician sedimentary and volcanic units (Susques and Acoite Fms) of the Eastern Puna, in the Early Ordovician Cuchiyaco Granodiorite of the NW Pampean Ranges (Table 1), and reassessed and reinterpreted the results of Valencio et al (1980) in the late Early Ordovician volcanic and volcanoclastic Suri and Las Planchadas formations in the Famatina Ranges. All these rocks provided similar paleomagnetic results, despite the different lithologies, magnetic carriers and locations separated by several 100 km.…”

“…Paleomagnetism is an excellent method to establish the paleolatitudinal position and former orientation of crustal blocks. Early Paleozoic paleomagnetic data from NW Argentina and N Chile have shown large systematic rotations with limited to no paleolatitude anomaly (Valencio et al 1980;Forsythe et al 1993;Conti et al 1996;Rapalini et al 1999Rapalini et al , 2002Rakotolofo 2004;Spagnuolo et al 2008;Spagnuolo 2009). These paleomagnetic poles were obtained in the Western Puna of Chile and Argentina, the Eastern Puna, the NW Pampean Ranges and the Famatina Ranges.…”

“…2), clockwise rotation of Ordovician rocks in the Famatina Range, NW Pampean Ranges and Eastern Puna (Valencio et al 1980;Conti et al 1996;Spagnuolo et al 2008), and counter-clockwise rotations of those in the Western Puna (Forsythe et al 1993;Rapalini et al 2002), we propose a model of systematic crustal block rotations produced by crustal shortening and tectonic escape during and immediately after the collision of Precordillera in the MiddleLate Ordovician (Astini and Dávila 2004;Ramos 2004). The time of collision of the Precordillera terrane has been independently suggested from cooling ages of the metamorphic basement of ca.…”

The Early Ordovician bellerophontoidPeelerophon oehlerti(Bergeron) from Argentina, Australia and Thailand

“…The reconstruction of Gondwana for the Early Ordovician, based on palaeomagnetic evidence (Vilas & Valencio, 1978;Valencio et aL, 1980), placed Argentina at a latitude of about 20°-30°S; calculated from data in Valencio et al, 1980 (Fig. 3).…”