Paleomagnetism, rock-magnetism and geochemical aspects of early Cretaceous basalts of the Paraná Magmatic Province, Misiones, Argentina

Abstract

The basalts of the Posadas Formation were extruded during the huge continental volcanism that affected the Paraná Basin in the Lower Cretaceous. We have carried out a paleomagnetic and rock-magnetic study on samples collected along a basalt outcrop section in Misiones, Argentina and determined that rocks classified as tholeiitic basalts and andesi-basalts are characterized by a low to intermediate content of Ti. Paleomagnetic and rockmagnetic studies suggest that the main magnetic mineral is low-Ti titanomagnetite of superparamagnetic (SP) to single-domain (SD) sizes, and very low amounts of multi-domain (MD) particles. The stable magnetic remanence enabled us to define characteristic remanent magnetizations (ChRMs) with a maximum angular deviation (MAD) <5• in most cases; and in all the cases, a MAD <10• . The sequence has registered at least two polarity reversions, starting from a normal polarity at the base. The calculated virtual geomagnetic poles (VGPs) present an elongated distribution similar to other distributions of VGPs published for the Paraná Magmatic Province. The elongated distribution of the VGPs could be a real feature of the geomagnetic field at a time of frequent changes of polarity.

Paleosecular variation and absolute geomagnetic paleointensity records retrieved from the Early Cretaceous Posadas Formation (Misiones, Argentina)

“…Under this hypothesis, the volcanic sequence studied here, presents at least two polarity reversals. The occurrence of reverse and normal polarities in Parana Lavas were already detected by Ernesto et al (1990) and (2011) have been seen by several authors later (Alva-Valdivia et al, 2003;Mena et al, 2006 among others). Fig.…”

“…This pioneering study however presents a major limitation because only three oriented hand samples per site are investigated and thus does not meet the basic criteria to study the fine characteristics of geomagnetic paleosecular variation (Biggin et al, 2008). The same is true for the study conducted by Mena et al (2006) who only used one or two hand samples on the Argentinean part of PMP (Posadas Formation). This later study should be considered as reconnaissance and thus cannot be used as supplementary material for present modern paleomagnetic survey.…”

“…If the hypothesis that flow 20SG is younger than 19SG is valid, then the sequence would record four changes of polarity (N-R-N-R-N). It worth noting that the lava flow sequence reported in Mena et al (2006) (1981) and McFadden and McElhinny (1990) are both rejected at the 95% confidence level ( c = 12.12;  o = 13.60). In fact the first test is not strictly valid because one of the distributions has less than 5 data, but the second is valid because statistically at 95% level have a common k parameter (k r /k n = 1.2990; F 44;4;0.05 = 5.7091).…”

“…The present study was carried out on lava flow outcrops in the central part of the PMP, in Misiones Province, Argentina. These rocks were classified as tholeiitic basalts and andesi-basalts characterized by a low to intermediate content of Ti (Mena et al, 2006). Paleomagnetic samples were extracted along the roads between San Ignacio, Eldorado and Bernardo de Irigoyen localities (Fig.…”

“…For these reasons, we investigated 28 independent lava flows (331 standard paleomagnetic cores) erupted during the Early Cretaceous in northeastern Argentina, which have several advantages: (1) they are widely distributed in a large volcanic province and easy to access; (2) they record faithfully the magnetic field that existed at the time of their eruption (Mena et al, 2006); (3) the age of these eruptions are well constrained.…”