Characteristics of the radiogenic heat production in granitoids from southern Tibetan Plateau: Thermal and geodynamic implications

Abstract

The long-lived radioactive isotopes of U, Th, and K serve as the principal heat-producing elements in rocks. Investigating the concentrations of these radiogenic elements and their associated heat production rates within the extensively distributed granitoids of the southern Tibetan Plateau not only provides important insights into the heat source mechanisms and genetic models of the widely distributed high-temperature geothermal resources in the region but also offers valuable contributions to understanding the geodynamic characteristics of the area. In this study, we systematically compiled geochemical data for granitoids from the southern Tibetan Plateau spanning several decades, resulting in a dataset of 1293 radiogenic heat production rate (RHPR) values. Representing the first systematically RHPR dataset for the southern Tibetan Plateau so far. The statistical analysis reveals average concentrations of U, Th, and K of 3.87 ± 3.99 ppm (Range: 0.11 ∼ 53.87 ppm), 18.38 ± 14.46 ppm (Range: 0.19 ∼ 118 ppm), and 3.06 ± 1.21 % (Range: 0.02 ∼ 8.03 %), respectively. The average Th/U and K/U ratios were found to be 6 ± 4.12 (Range: 0.06 ∼ 64.09) and 1.27 ± 1.03 (× 10⁴) (Range: 0.01 × 10⁴ ∼ 9.93 × 10⁴), respectively. The average RHPR of granitoids from the southern Tibetan Plateau was calculated to be 2.57 ± 1.79 μW/m³ (Range: 0.05 ∼ 15.06 μW/m³), and the data display a well-defined normal distribution. The average concentrations of U, Th, K, and RHPR are lowest during the Mesozoic, with Th showing the highest average concentration in the Late Paleozoic, while the other elements peak during the Cenozoic. The Th/U ratio exhibits a gradual decline with younger geological ages, whereas the K/U ratio is highest in the Mesozoic and lowest in the Cenozoic. The spatial distribution of RHPR reveals a clear trend of gradually decreasing RHPR values from north to south across the southern Tibetan Plateau, with the Qiangtang, Lhasa, and Himalayan blocks exhibiting RHPR values of 3.1 ± 1.42 μW/m³, 2.46 ± 1.83 μW/m³, and 2.25 ± 1.84 μW/m³, respectively.

Although the RHPR values in the southern Tibetan Plateau are generally lower than those in other high-temperature geothermal fields where granitoid radiogenic heat predominates, elevated RHPR values are still observed in certain regions, such as Gonghe Basin, Cooper Basin which is located in Australia. Based on the RHPR data collected in this study and information on the enrichment layers of radiogenic heat-producing elements in the crust, the radiogenic heat contribution of granitoids to the terrestrial heat flow was estimated, The results show that the heat contribution of granitoid in the Qiangtang, Lhasa, and Himalayan blocks was estimated to be 29.4 ∼ 39.2 μW/m³, 23.6 ∼ 31.5 μW/m³, and 21.3 ∼ 28.4 μW/m³, respectively. The above results further emphasize the significant role of RHPR in granitoids in the formation of high-temperature geothermal resources in the region. Besides, the Th/U values in the southern Tibetan Plateau show significant variation, ranging from 0.06 to 64.09, with an increasing trend in data dispersion from north to south. This pattern may reflect the complex or heterogeneous magmatic processes occurring in this region. The fluctuations in K/U ratios further emphasize the intricate material migration processes within the magma source, driven by the subduction of the Indian Plate and the subsequent crustal thickening of the southern Tibetan Plateau. The elevated K/U values observed during the Cenozoic are likely the result of widespread lava flows that traversed thick, incompatible-element-enriched crustal sequences before eruption, causing chemical and isotopic mixing within the continental crust.