Absolute Thellier paleointensities from Ponta Grossa dikes (southern Brazil) and the early Cretaceous geomagnetic field strength

Abstract

We report a detailed rock magnetic and Thellier paleointensity study from  130.5 Ma Ponta Grossa Dike Swarms in Southern Brazil. Twenty–nine samples from seven cooling units were pre–selected for paleointensity experiments based on their low viscosity index, stable remanent magnetization and close to reversible continuous thermomagnetic curves. 19 samples characterized by negative pTRM tests, Arai concave–up curves or positive pTRM tests with NRM loss uncorrelated with TRM acquisition were rejected. High quality reliable paleointensity determinations are determined from detailed evaluation criteria, with 10 samples belonging to three dikes passing the tests. The site–mean paleointensity values obtained in this study range from 25.6 ± 4.3 to 11.3 ± 2.1 µT and the corresponding VDMs range from 5.7 ± 0.9 to 2.5 ± 0.5 (10²² Am²). These data yield a VDM mean value of 4.1 ± 1.6 x 10²² Am². Significant variability of Earths magnetic field strength is observed for Ponta Grossa Dikes with the mean value being significantly lower as compared to the mean VDM obtained from the nearby Paraná Magmatic Province. The paleointensities for the Ponta Grossa Dikes are in agreement with absolute paleointensities retrieved from the submarine basaltic glasses from 130 to 120 Ma. It seems that a relatively low field prevailed just before the Cretaceous Normal Superchron.