Investigation of Archean gamma-ray fingerprint: Methodology and tectonic application in central Brazil

Abstract

Gamma-ray spectrometric responses of Archean rocks were studied in order to define a characteristic signature for these rocks. As the half-life of the most frequent Thorium isotope (232Th – half life of 14.05 Ga) is more than 3 times longer relative to the Uranium isotope (238U – half life of 4.50 Ga), a low response of radiation intensity in the spectral range of decay series of Uranium relative to that of Thorium is expected for Archean rocks. Based on this theoretical aspect, associated with gamma-ray spectrometric responses parametrically studied in several Archean areas of Brazil, gamma-ray and magnetometric structural interpretation, follow-up field studies, micropetrography, lithogeochemistry, and geochronology (U-Pb in zircon), it was possible to define and identify previously unidentified Archean rocks (2.82 Ga) tectonically imbricated amid Neoproterozoic magmatic arc terrains. This discovery has geotectonic implications regarding the magnitude of the Transbrasiliano Lineamento and its potential to fragment large Archean blocks and arrange them aligned within the Tocantins Province, Central Brazil.