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.