Palaeomagnetism and K--Ar age of the Upper Ordovician Alcaparrosa Formation, Argentina

Palaeomagnetic data from 71 hand samples of igneous rocks of Late Ordovician age exposed in western Argentina (31.3°S, 69.4°W, Alcaparrosa Formation) are given. Stable remanent magnetization was isolated in the majority of samples; they yield a palaeomagnetic pole at 56°S 33°E (N = 8, α95 = 16°). Whole rock K—Ar age determinations yield an age of 416 ± 10 Myr for a pillow lava of the Alcaparrosa Formation. Palaeomagnetic data for South America, Africa, Australia, Antarctica and India suggest that Gondwana was a unit at least as far back as 1000 Myr. The palaeomagnetic data define a rapid polar migration for Gondwana in Ordovician time which is consistent with the widespread occurrences of Late Ordovician glacial deposits across this supercontinent.