Paleomagnetism of the Sierra de Las Animas Complex, southern Uruguay: its implications in the assembly of western Gondwana

Abstract

Global paleogeographies in the Neoproterozoic are poorly known. In particular, the assembly of Gondwana at the end of the Proterozoic is controversial. In order to constrain the kinematic evolution of the Rio de la Plata craton, a paleomagnetic study was carried out in the Vendian to Cambrian Sierra de Las Animas magmatic Complex, exposed in southeastern Uruguay (34.7°S, 55.3°W). One hundred and forty-seven samples from 23 sites were studied. These included two sites on the Vendian glaciogenic sediments of the Playa Hermosa Formation. After stepwise AF and thermal cleaning a characteristic remanence was isolated at several sites. According to the available isotopic data for the complex and the correlation between remanence directions and lithologic type and age, two paleomagnetic poles were computed for the Sierra de Las Animas complex. SA1 (338.1°E, 5.9°N, dp=19.6°, dm=26.7°, N=7 sites, mean declination: 44.5°, mean inclination: 58.0°, α 95: 18.1°), with a likely age around 520 Ma, shows a distribution of VGPs that suggests a remanence acquisition that spanned a long time during a period of fast apparent polar wander for Gondwana. SA2 (250.9°E, 16.9°S, dp=15.9°, dm=21.5°, N=6 sites, mean declination: 95.3°, mean inclination: −58.5°, α 95: 14.5°) corresponds to a likely age around 550 Ma and falls close to a pole of similar age from the Congo craton. This suggests that the Rio de la Plata craton was at that time part of Gondwana and confirms previous postulates that a single APWP can be defined for the supercontinent since 550 Ma. A preliminary mean geomagnetic pole obtained from the Playa Hermosa Formation (PH, 198.4°E, 43.0°S, dp=8.6°, dm=16.0°, n=6 samples, mean declination: 226.0°, mean inclination: 24.2°, α 95: 15.0°) is consistent with a previous pole of 595 Ma from the Campo Alegre (CA) lavas and may suggest, if confirmed, that the glaciogenic deposits were produced at low to intermediate latitudes. Despite high uncertainties, a new speculative model on the process of assembly of Gondwana is proposed.