Palaeomagnetism of Some Igneous Rocks of the Middle Jurassic Chon-Aike Formation from Estancia La Reconquista, Province of Santa Cruz, Argentina

Abstract

Summary New palaeomagnetic data for the Middle Jurassic Chon-Aike Formation from 15 different geological units of rhyolitic porphyries, keratophyres, ignimbrites, pumicites and associated tuffs exposed at the Estancia La Reconquista, Province of Santa Cruz (4810'S, 6850' W), Argentina, are given. After cleaning, all the units show normally polarized magnetic remanence and the mean of their virtual geomagnetic poles is not coincident with the mean of the virtual geomagnetic poles derived from the Puerto Deseado (Valencio & Vilas) and Camarones (Creer et al.) areas. It is proposed that the most reliable palaeomagnetic pole for the Chon-Aike Formation is that given by the mean of the 54 virtual geomagnetic poles derived from all three areas, namely Puerto Deseado, Camarones and Estancia La Reconquista. It is situated at 85 S, 163 W, with ag5 = 6. Comparison of the position of this palaeomagnetic pole with the mean of the African Jurassic palaeomagnetic poles suggests that the separation of South America and Africa occurred after the Middle Jurassic. Palaeomagnetic studies of Jurassic rocks of South America are of basic importance in defining the beginning of the fragmentation of the South America-Africa block. This was noted by Valencio & Vilas (1969) who, on the basis of the available palaeomagnetic data, indicated the importance of knowing the Jurassic polar position for South America. These authors started the palaeomagnetic studies of lavas of the Chon-Aike Formation of Middle Jurassic age from Argentina. The first Jurassic palaeomagnetic pole for South America was computed in the study carried out with igneous rocks of this formation from Puerto Deseado, Province of Santa Cruz (4T47' S, 66 W). Comparison of the position of this pole with those of the Jurassic palaeomagnetic poles for Africa (Briden 1967) suggested that the opening of the South Atlantic occurred before the Middle Jurassic (Valencio & Vilas 1970). Later on Valencio (1970a, b, c) indicated the necessity of verifying the position of this pole; local reasons (tectonics, secular variation) might affect it. In particular, the interpretation concerning the time of opening of the South Atlantic conflicts with