Petrological and geochemical constraints on the petrogenesis of granitoids in the Gonghe geothermal basin, western Qinling (China)

Abstract

The Gonghe geothermal basin is situated in the westernmost part of the western Qinling orogen and possesses the supreme potentials to explore and develop hot dry rock (HDR) geothermal resources in China. The basal rock of HDR reservoir in Gonghe basin is composed of granitoids. In this contribution, in order to constrain the formation mechanism of the Gonghe HDR, granitic samples from outcropping have been collected and drilling core samples data from the archives also been compiled. The studied results show that the obtained zircon U–Pb ages of 255.1 ± 1.9 Ma and 253.9 ± 2.6 Ma for granodiorite and monzogranite, respectively. All granitoids from both outcropping and drilling core have the similar rock associations and their major and trace elements possess the consistent evolved patterns, such as relative enrichment of LILEs (e.g., K, Rb), depletion of HFSEs (e.g., Nb, Ti), and negative Eu anomalies (0.32–0.66), indicating that they share the same magma sources and evolutional processes. And they have the affinity of I-type granites. They were derived from the mixture of partial melting of metabasaltic rocks in the lower crust and a small amount of mantle-derived magma and formed in a subduction-related setting relative to the southward subduction of the Zongwulong oceanic crust beneath the west Qinling terrane. The heat production values of rocks around the Gonghe basin were calculated and all basal granitic rocks ranges from 0.35 to 8.51 μW/m3, yielding an arithmetic mean value of 1.97 μW/m3, indicating that the radioactive heat generation capacity of granitoids in the Gonghe basin is slightly lower. The HDR reservoir in the Gonghe basin was probably due to an allied thermal effect of the radiogenic heat production in the thickening continental crust and the heat contributed by a deep magma chamber below the basin. The early Triassic granitoids with a higher thermal conductivity serve as the conductive medium of the geothermal heat; whilst the sedimentary rocks marked by lower thermal conductivity above the granitic batholiths act as the reservoir cap. This contribution provides a relatively rational understanding for the mechanism of the HDR reservoir in Gonghe geothermal basin from the petrological and geochemical perspectives, which is conducive to the geothermal potential assessment and estimation, and to-be implementation of the Enhanced Geothermal System demonstration project in China.