Granitoids indicating moderate to high heat production associated with oceanic subduction system in the NE Tibetan Plateau

Abstract

Regional heat flow provides a direct surface indication of the thermal state and energy balance of the lithosphere. Heat flux in the Tibetan Plateau remains poorly studied due to the lack of adequate information on heat flow. In this study, we investigated granitoids from the Gonghe-Guide district located in the NE Tibetan Plateau, which have important significance as hot and dry rocks with potential for geothermal resources. Samples from granitoid rocks as well as felsic dikes were collected from this area, and their geochemical data were combined with regional geophysical and drilling core data to evaluate a high-heat-flow anomaly. Our data show that granitoid rocks, including granodiorite, monzogranite, and syenogranite, crystallized at ca. 253−240 Ma, while felsic dikes formed ca. 230 Ma. The former were emplaced during the oceanic subduction process, whereas the latter formed in the syncollisional stage associated with the closure of the paleo-Zongwulong Ocean. Geochemically, these granitoid rocks are metaluminous to slightly peraluminous, with an average differentiation index (DI) value of 80, and they are classified as weakly to moderately fractionated I-type granites. The felsic dikes are peraluminous with high DI values (average of 95), typical of highly fractionated I-type granites. In terms of their high K and especially U and Th abundances, the calculated radioactive heat generation produced by the granodiorite, monzogranite, syenogranite, and felsic dikes is 1.85, 2.91, 6.85, and 3.02 μW/m3, respectively. Their moderate to high heat production generates a local heat-flow anomaly of 14.8−22.2 mW/m2 above the regional value of subduction zones, accounting for 11%−18% of the total regional heat flow. In combination with regional magnetotelluric data, the high-heat-flow anomaly may be attributed to an additional heat-flow contribution from a partial melt layer beneath this region. Furthermore, there is a significant positive correlation between the radioactive heat generation and magmatic fractionation, indicating that the enrichments of heat-producing elements are controlled by fractional crystallization and subsequently source composition. The granitoid rocks with high heat generation correspond to the large-scale intermediate-felsic magmatism during the subduction stage. We propose a model wherein a continuous subduction process during the closure of the paleo-Zongwulong Ocean and protracted cooling and crystallization processes resulted in the enrichment in heat-producing elements.