Variation of radiogenic heat production rate in 2.8–1.8 Ga old rocks in the central Fennoscandian shield

Abstract

We present results of a geochemical survey based on 1150 outcrop samples for heat production studies in a E–W oriented band ( 120 km×500 km) located approximately between 62 and 63°N in Finland, the central Fennoscandian shield. The results provide representative averages of heat production rate in the studied tectonic units, but also demonstrate the heterogeneity and spatial variation in radiogenic heat production rate of the Precambrian lithosphere. The study area covers formations from the Archaean granite-greenstone terrain in the east via a Palaeoproterozoic mobile belt to a major granitoid complex surrounded by schist belts in central and western Finland. In this study we review the heat production rates as calculated from the U, Th and K total analyses and densities of the samples, and the relations of heat production rate values with major tectonic setting, lithological types, major composition and petrophysical properties of rocks. The data is further correlated with heat flow values measured in boreholes and applied in a lithospheric heat production model in the central part of the shield. Generally, heat production rate increases from Archaean to Proterozoic rocks in E–W direction but this trend is relatively weak and often overrun by lithological variations. The arithmetic means are 1.1±1.3 μW m −3 in the Archaean domain, 1.3±0.6 μW m −3 in the Höytiäinen autochthonous Proterozoic domain, 1.2±0.5 μW m −3 in the Suvasvesi domain characterized by allochthonous Proterozoic cover, 1.0±0.6 μW m −3 in the Proterozoic Raahe–Ladoga Mobile belt, 1.3±1.1 μW m −3 in the Proterozoic Rantasalmi–Haukivuori domain, 1.6±0.8 μW m −3 in the Central Finland Granitoid Complex and 1.4±0.8 μW m −3 in the Bothnian Schist Belt. The standard deviations of the mean values are considerable and reflect the heterogeneity of heat production. All distributions overlap. Heat production rate shows spatial variations which range in scale from kilometres to tens of kilometres. Well-defined systematic variations of heat production rate values with either SiO 2 content, density or P-wave velocity could not be found, although the correlations are statistically significant at 1% risk level. The correlations are weak and scattered, and sometimes opposite in sign in the major rock groups (plutonic, metavolcanic and metasedimentary). Heat flow density measured in boreholes increases with increasing heat production rate, but the present data do not support a strict linear dependence between the variables, and the regression line parameters vary depending on the applied heat production values (drill core analyses versus hole site averages from the present study). This indicates that the heat production rate is spatially heterogeneously distributed in the scale of the boreholes and their formations. A lithospheric heat production model was constructed using data from the present study for the upper crust, literature data for lower crustal xenoliths in the study area and a xenolith-derived heat flow density value for the mantle. The total crustal contribution to the surface heat flow density (37 mW m −2) from radiogenic heat sources is calculated as 26 mW m −2, and the contributions from the lower, middle and upper crust are 4, 11 and 11 mW m −2, respectively.