Radiogenic heat production of Variscan granites from the Western Bohemian Massif, Germany

Abstract

Much of the Mid-European basement has been consolidated during the Variscan Orogeny and includes large volumes of granitic intrusions. Gamma radiation spectroscopic measurements in three study areas along the western margin of the Bohemian Massif give a record of radiogenic element concentrations in the Variscan granites. Most intrusions of the Fichtelgebirge (except for the Tin Granite) and intrusive complexes in the Bavarian Forest show Th/U ratios exceeding unity, most likely related to abundance of monazite. In contrast, some of the Oberpfalz granites located near the Saxothuringian–Moldanubian boundary (Flossenbürg, Steinwald and Friedenfels types) are characterized by higher uranium concentrations and thus Th/U < 1. The low Th/U ratios here are in agreement with a possible U mobilisation along the Saxothuringian–Moldanubian contact zone observed in previous studies. Heat production rates of granites in the three study areas vary between 3.9 and 8.9 µW/m 3 , with a mean of 4.9 µW/m 3 . This classifies the intrusions as moderate- to high-heat-producing granites. Considering the huge volume of granitic bodies in the Variscan crust of the Bohemian Massif, the contribution of in situ radiogenic heat production had to have a major impact and should be considered in further thermal modeling. contains on the surface heat flow. Hasterok and Chapman (2011) estimated that 26 % of the surface heat flow is generated by the upper continental crust. The new database GRAN-ITE2017 has been set up to compile available data on worldwide granitic terrains of all ages (Artemieva al. Based on statistical analysis, first conclusions have been made on variation of bulk heat production of granites through geological time. These results show a peak in the Middle Proterozoic times and a gradual decrease towards younger (Cenozoic) intrusions.