Paleomagnetism of Late Cenozoic Basalts from Northern Patagonia

M MENA; GH RÉ; MJ HALLER; SE SINGER; JF VILAS

Earth Planets Space

Terra Scientific Publishing

Lugar: Tokio; Año: 2006 vol. 58 p. 1273 - 1281

1880-5981

Late Cenozoic volcanic rocks outcrop in the northern Patagonia Extrandina. Lava flows, characterized as olivine and alkaline basalts, belong to intraplate volcanism. We report paleomagnetic and rock-magnetic studies carried out on Late Cenozoic basalts belonging to the Crater, Mojon and Moreniyeu Formations. The paleomagnetic sampling comprised 75 sites in lava flows and dikes from the Crater Formation, three sites in a lava flow from the Mojon Formation and three sites in a lava flow from the Moreniyeu Formation. Alternating field (AF) and thermal detailed demagnetization techniques were used. Most of the samples have a viscous component. The AF procedure was more effective than thermal demagnetization in destroying viscous components and in defining the characteristic remanent magnetizations. Demagnetization curves and rock-magnetic studies suggest that the main remanence carrier is Ti-poor magnetite. Radiometric K-Ar ages were performed on these basalts. The radiometric ages are 0.8±0.1 Ma from outcrops located at Cerro Fermin and 1.9±0.4 Ma from outcrops at Cerro Negro, both at the Crater Formation. These ages suggest an early-middle Pleistocene age for the lava flows from Cerro Fermin, and a late Pliocene to early Pleistocene age for the Cerro Negro lava flows. Based on the magnetic polarity temporal scale, the Cerro Fermin lava flows have registered the beginning of the Brunhes Chron, while the Cerro Negro basalts could have been extruded during the Olduvai Subchron. The K-Ar radiometric age of the Moreniyeu Formation (1.6±0.2 Ma) suggests an early Pleistocene age for this lava flow. The reverse polarity of its virtual geomagnetic poles (VGPs) is in agreement with the predominant one during the Matuyama Chron and suggests that the Moreniyeu Formation constitutes another volcanic event clearly separate from those of the Crater Formation. The K-Ar radiometric age of the Mojon Formation (3.3±0.4 Ma) locates it in the middle Pliocene. The VGP polarity would be correlated with some reverse subchron located in Gauss Chron or with the end of the Gilbert Chron. The petrographical and geochemical similarities between the studied basalt and the Somuncura plateau basalts (late Oligocene-early Miocene, located northern and eastern of the study area), together with the time lapsed among between the Mojon and Crater basalt extrusion suggest the presence in the area of a temporarily extensive thermal anomalies.±0.1 Ma from outcrops located at Cerro Fermin and 1.9±0.4 Ma from outcrops at Cerro Negro, both at the Crater Formation. These ages suggest an early-middle Pleistocene age for the lava flows from Cerro Fermin, and a late Pliocene to early Pleistocene age for the Cerro Negro lava flows. Based on the magnetic polarity temporal scale, the Cerro Fermin lava flows have registered the beginning of the Brunhes Chron, while the Cerro Negro basalts could have been extruded during the Olduvai Subchron. The K-Ar radiometric age of the Moreniyeu Formation (1.6±0.2 Ma) suggests an early Pleistocene age for this lava flow. The reverse polarity of its virtual geomagnetic poles (VGPs) is in agreement with the predominant one during the Matuyama Chron and suggests that the Moreniyeu Formation constitutes another volcanic event clearly separate from those of the Crater Formation. The K-Ar radiometric age of the Mojon Formation (3.3±0.4 Ma) locates it in the middle Pliocene. The VGP polarity would be correlated with some reverse subchron located in Gauss Chron or with the end of the Gilbert Chron. The petrographical and geochemical similarities between the studied basalt and the Somuncura plateau basalts (late Oligocene-early Miocene, located northern and eastern of the study area), together with the time lapsed among between the Mojon and Crater basalt extrusion suggest the presence in the area of a temporarily extensive thermal anomalies.