Abstract
Abstract 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 Cráter, Mojón and Moreniyeu Formations. The paleomagnetic sampling comprised 75 sites in lava flows and dikes from the Cráter Formation, three sites in a lava flow from the Mojón 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 Fermín and 1.9±0.4 Ma from outcrops at Cerro Negro, both at the Cráter Formation. These ages suggest an early-middle Pleistocene age for the lava flows from Cerro Fermín, and a late Pliocene to early Pleistocene age for the Cerro Negro lava flows. Based on the magnetic polarity temporal scale, the Cerro Fermín 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 Cráter Formation. The K-Ar radiometric age of the Mojón 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 Mojón and Cráter basalt extrusion suggest the presence in the area of a temporarily extensive thermal anomalies.
Highlights
Late Cenozoic volcanic rocks are extended on a huge surface in the northern Patagonia Extrandina
We report a paleomagnetic and rockmagnetic study carried out on basalts belonging to the Crater, Mojon and Moreniyeu Formations (Fig. 1)
Preliminary studies of magnetic fabric performed on these lava flows show that both the susceptibilities and anisotropy of the magnetic susceptibility (AMS) are controlled by titanomagnetites (Singer et al, 2005)
Summary
Late Cenozoic volcanic rocks are extended on a huge surface in the northern Patagonia Extrandina. The Mojon Formation, assigned to the late Pleistocene (Ravazzoli and Sesana, 1977), outcrops north of the Crater Formation volcanic field These volcanic rocks conform to an extensive lava flow located near Mamil Choique, (41◦46 S, 70◦08 W), Rıo Negro province. The only other paleomagnetic studies performed on coeval lavas correspond to volcanic rocks exposed 1000 km south of the study area (Brown et al, 2004; Baraldo et al, 2003; Mejia et al, 2004; Singer et al, 2004) In this contribution, we report a paleomagnetic and rockmagnetic study carried out on basalts belonging to the Crater, Mojon and Moreniyeu Formations (Fig. 1). The establishment of chronological relationships among the different lava flows and the location of their sources are very important for the knowledge of the tectomagmatic activity of the area
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