Thermal Properties and Geothermal Effects of Magmatic Rocks in Jiangsu Province, China

Abstract

(1) Background: Geothermal resources are enriched in Jiangsu Province, particularly in its mid-deep geothermal reservoirs. The thermal properties and thermal effects of magmatic rocks, which are largely unknown in Jiangsu Province, are fundamental for analyzing the genetic mechanisms of geothermal resources and evaluating resource potential. (2) Methods: Representative magmatic rock samples from different geological periods and different tectonic settings are collected from the main tectonic units of Jiangsu Province. Key thermophysical parameters such as thermal conductivity, heat production rate, rock density, and porosity are systematically tested. (3) Results: The variation patterns of these thermal property parameters are analyzed, and the sources and spatiotemporal evolution characteristics of radiogenic heat production, and the thermal effects of magmatic rocks, are specifically explored. (4) Conclusions: Magmatic rock lithology from acidic to basic is negatively correlated with thermal conductivity, thermal diffusivity, and radiogenic heat production rate, and positively correlated with volumetric heat capacity. The radiogenic heat production of magmatic rocks is primarily controlled by the contents of U and Th, increasing with the increasing SiO₂ content. The formation of geothermal anomalies in areas with thin or absent sedimentary cover is significantly influenced by the thermal effect of magmatic rocks, especially by the high heat-producing granites. The radioactive thermal contribution of the Taolin and Suzhou plutons was calculated.

1. Introduction

Geothermal energy is a clean and renewable energy source characterized by wide distribution and large reserves. Compared with other clean energy sources such as wind and solar, it has the advantage of being unaffected by weather and climate [1]. The development and utilization of geothermal energy are significant for addressing the energy crisis and environmental challenges faced by human society, and have garnered increasing global attention [2,3]. China leads the world in both the total installed capacity and annual energy utilization for direct use of geothermal energy [4]. A notable example is Sinopec’s project in Xiong County, Hebei Province, which has achieved comprehensive geothermal heating coverage and established the nation’s first “smoke-free city” via clean heating. This initiative has realized a dual benefit of economic and social gains. Jiangsu Province, located in the economically developed Yangtze River Delta region of eastern China, has a strong demand for energy and faces substantial pressure for the low-carbon transformation, creating a bottleneck for high-quality socio-economic development. Simultaneously, Jiangsu Province is located on the Eastern China Geothermal Anomaly Belt, with widely distributed and large reserves of mid-deep geothermal resources, indicating broad prospects for development [5,6,7,8]. Geothermal exploration results show that the Subei Basin is the core area of geothermal resource enrichment in the region [9,10,11]. Medium-temperature hydrothermal geothermal fields (with bottom-hole temperatures of 93–113 °C)exist within the basin, such as in Baoying County, Shenju Mountain in Gaoyou County, and Xiaoyangkou in Rudong County where a comprehensive utilization demonstration station integrating drying, heating, cooling, wellness, and planting has been established [12]. Notably, the first high-temperature carbonate rock-type geothermal resource in eastern China was discovered in Xinghua County, Taizhou City. In addition, significant progress has been made in utilizing geothermal energy for building heating in Xuzhou City, northern Jiangsu Province, and the hot spring tourism and wellness industry in southern Jiangsu Province has begun to generate substantial economic benefits. These achievements preliminarily demonstrate the significant development potential and value of geothermal resources in Jiangsu Province. However, previous work has predominantly focused on local geothermal fields, and research on regional geothermal geological conditions and background remains lacking, which constrains the overall planning for resource exploration and development.

Rock thermal properties (including density, thermal conductivity, thermal diffusivity, volumetric heat capacity, and radiogenic heat production rate) are important parameters that characterize the ability of geological bodies to generate, store, and transfer heat [13,14], providing an essential basis for calculating terrestrial heat flow, simulating spatiotemporal distribution of temperature, understanding the thermal structure of the lithosphere, and modeling geothermal systems [15,16,17,18,19]. Magmatic rocks, which are widely distributed in the upper crust, form and emplace through processes that effectively transfer material and energy from the Earth’s deep interior to shallow levels, thereby are capable of controlling geothermal field distribution and the formation of high-temperature geothermal resources [20,21,22]. Particularly, intermediate-acidic magmatic rocks enriched in radioactive heat-producing elements such as U, Th, and K can provide a long-term stable heat source through radioactive decay, which is crucial for the formation of some high-quality geothermal resources [23,24]. Therefore, research on the thermal properties (e.g., thermal conductivity, heat production) and thermal effects of magmatic rocks is essential for elucidating the genesis, distribution patterns, potential assessment, and exploitation of geothermal resources [25].

Previous efforts have been made to understand the thermal properties of sedimentary strata from various periods in Jiangsu Province (including Paleogene-Neogene, Cenozoic, Mesozoic, Paleozoic, and even Neoproterozoic) and thermal property parameter columns were established [8,11,26]. However, research on the thermal properties of magmatic rocks, particularly the widely distributed intermediate-acidic magmatic rocks within the province, remains notably scarce. The available data are limited and lack representativeness, which severely hinders the analysis of regional geothermal genetic mechanisms and the accurate assessment of resource potential.

This study focuses on magmatic rocks in Jiangsu Province, with the core objective of providing indispensable fundamental data for investigating the genetic mechanisms of regional geothermal resources, as well as the exploration, evaluation, and utilization of geothermal resources.

To achieve this objective, the study adopted the following methodologies: (1) systematically collecting representative magmatic rock samples from different geological periods in Jiangsu Province; (2) measuring key thermal property parameters of these samples, including thermal conductivity, specific heat capacity, density, thermal diffusivity, and radiogenic heat production rate; (3) analyzing the characteristics of these thermal properties and their influencing factors; (4) discussing the sources and spatiotemporal evolution patterns of radiogenic heat production in magmatic rocks, as well as the thermal effects of magmatic activities; and (5) calculating the contribution ratios of crust-mantle heat sources for the Taolin and Suzhou plutons.

2. Geological Background

Jiangsu Province, located in eastern China, comprises three major tectonic units with a complex evolutionary history: the North China Block, the Sulu Orogenic Belt, and the Yangtze Block. The Yangtze Block is further subdivided into the Subei Basin and the South Jiangsu Uplift (Figure 1). After the Indosinian orogenic movement, the North China and Yangtze blocks collided and amalgamated along the Sulu Orogenic Belt into a unified whole. Subsequently, the region experienced intense Mesozoic tectono-magmatic-mineralization thermal events and strong Meso-Cenozoic lithospheric stretching and thinning events, both of which are controlled by the Pacific tectonic domain [27]. This process provided the key tectonic-thermal background for the formation of geothermal resources [28].

Magmatic activities in Jiangsu Province are intense and widely distributed (Figure 1), most notably during the Mesozoic Yanshanian period, followed by the Proterozoic Jinning and Cenozoic Himalayan periods. The outcrop area of intrusive rocks is limited due to extensive coverage by Quaternary sediments [29], but geophysical data (regional aeromagnetic, gravity, etc.) infer the presence of numerous concealed to semi-concealed intrusive rock masses [30,31], which are important for studying regional geothermal geological conditions.

Proterozoic Jinningian magmatism is distributed within the Sulu Orogenic Belt, mainly consisting of intermediate-basic to acidic submarine flood volcanic rocks, accompanied by extensive acidic magmatic intrusions. After subsequent ultrahigh-pressure (UHP) metamorphism caused by the collision and amalgamation of the South China and North China blocks in the Triassic period (240–220 Ma), the former formed amphibolite, schist, and leptynite, and the latter formed granitic gneisses, represented by the Niushan, Hushan, Chengtou, and Tuofeng units of the Donghai Group [32,33]. The two together constitute the regional metamorphic basement [27].

Mesozoic magmatic rocks are distributed across various regions (Figure 1B), manifesting as continental volcanic eruptions and intermediate-acidic magmatic intrusions. The former formed intermediate-acidic lavas and subvolcanic rocks, mainly distributed in the Ningwu, Lishui, and Shangdang volcanic basins [34,35,36]. The latter formed intermediate-acidic granites, granodiorites, and diorites, characterized by multi-phase, multi-center activity, often forming larger-scale complex plutons along regional deep faults and their intersections, such as the Taolin, Anjishan, Shima, and Suzhou intrusive complexes [37,38]. These intermediate-acidic intrusive rocks, enriched in U, Th, and K, possess high radiogenic heat production, directly influencing the regional geothermal field [39].

Cenozoic magmatism is relatively weak, with simple rock types, mainly composed of basalt eruptions and basic intrusions of gabbro and diabase dykes. It started in the Paleocene, with intense eruptions during the Miocene to Pliocene, and no activity has been recorded since the Pleistocene [27]. Given their predominantly basic composition and limited scale, these Cenozoic magmatic rocks have a minimal impact on the present-day geothermal field.

3. Sample Collection and Testing

3.1. Sample Collection

Focusing on the Jinningian and Yanshanian periods, 34 representative magmatic rock samples were collected from different tectonic units in Jiangsu Province via surface outcrops and drill cores. Sampling locations are shown in Figure 1 and

Table 1. Brief introduction to the magmatic rock samples in Jiangsu Province.

Sample Number	Tectonic Unit	Pluton Name	Longitude	Latitude	Lithology	Geological Age	Sample Type
			(°)	(°)
RWX-1	North China Block	Banjing Pluton	117.072	34.149	Gray diorite porphyry	Cretaceous (~130 Ma)	Outcrop
RWX-2		Liguo Pluton	117.293	34.563	Pale flesh-red granite
RWX-3			117.35	34.559	Gray granodiorite porphyry
RWX-4	Sulu Orogenic Belt	Taolin Pluton	118.423	34.273	Light gray granite	Cretaceous (~130 Ma)
RWX-5			118.423	34.293	Pale flesh-red granite
RWX-6			118.507	34.45	Flesh-red granite porphyry
RWX-7			118.518	34.477	Pale flesh-red granite porphyry
RWX-8			118.507	34.449	Gray granodiorite
RWX-9		Banzhuang Pluton	118.863	34.878	Light gray granodiorite	Neoproterozoic (~700–800 Ma)
RWX-10		Taolin Pluton	118.526	34.683	Pale flesh-red quartz monzonite
RWX-11		Niushan Gneiss	118.449	34.289	Light gray granitic gneiss
RWX-12		Moshan Gneiss	118.776	34.734	Pale flesh-red granitic gneiss
RWX-13		Hushan Gneiss	118.865	34.454	Gray granitic gneiss
RWX-14			118.898	34.491	Pale flesh-red granitic gneiss
RWX-15		Jushan Gneiss	119.146	34.530	Grayish-green granitic gneiss
RWX-16	South Jiangsu Uplift	Anjishn Pluton	119.170	32.127	Light gray granodiorite	Cretaceous (~130–105 Ma)	Outcrop
RWX-17			119.170	32.127	Dark gray quartz diorite porphyry
RWX-18		Yeshan Pluton	118.935	32.526	Gray quartz diorite		Drill Core
RWX-19		Shima Pluton	119.276	32.138	Light gray granodiorite
RWX-20			119.267	32.142	Light gray porphyritic granodiorite
RWX-21		Shangdang Volcanic Rock	119.451	32.074	Flesh-red dacite		Outcrop
RWX-22		Suzhou Pluton	120.477	31.298	Light gray granite
RWX-23			120.482	31.326	Buff-colored granite
RWX-24			120.435	31.293	Pale flesh-red granite porphyry
RWX-25		Miaoxi Pluton	119.428	31.267	Flesh-red granite porphyry
RWX-26		Lishui Volcanic Rock	119.000	31.746	Grayish-green brecciated tuff		Drill Core
RWX-27			118.993	31.749	Grayish-green tuff
RWX-28			118.962	31.617	Grayish-purple brecciated tuff
RWX-29			119.064	31.739	Grayish-white andesitic breccia lava
RWX-30			119.064	31.739	Grayish-white trachyandesite
RWX-31			119.059	31.745	Light gray andesite
RWX-32			118.993	31.749	Light gray andesite
RWX-33			118.995	31.752	Dark gray gabbro
RWX-34			118.985	31.76	Dark gray gabbro

, with some samples illustrated in Figure 2. Lithologies include granite, granitic porphyry, granodiorite, quartz diorite, granitic gneiss, volcaniclastic rocks, dacite, andesite, trachyandesite, and gabbro. All samples showed no significant weathering or alteration, ensuring that the test results reflect the original thermal properties of the rocks.

3.2. Sample Testing

The sample testing was conducted in the Nanjing Mineral Resources Supervision and Testing Center of the Ministry of Land and Resources. The tested items include thermal conductivity, specific heat capacity, density, porosity, and the contents of U, Th, and K elements.

3.2.1. Thermal Conductivity and Specific Heat Capacity Testing

Based on the transient plane source method proposed by Silas Gustafsson [40], the thermal conductivity and thermal diffusivity of natural-state rocks were measured using a thermal constants analyzer (TPS 2500S, Hot Disk AB, Göteborg, Sweden) at room temperature and normal atmospheric pressure. The specific test method is described in Zhao et al. [41]. Then, the specific heat capacity of the samples was calculated using Formula (1).

$$c={\displaystyle \frac{\lambda }{\alpha {·\rho }_0}}$$

(1)

where c is the specific heat capacity (kJ/(kg·K)); λ is the thermal conductivity (W/(m·K)); α is the thermal diffusivity (mm²/s); ${\rho }_0$ is the natural density (g/cm³), which is obtained by Formula (2).

3.2.2. Density Testing

The wax-sealing method was used to measure the bulk density of rocks, based on Archimedes’ principle. Natural density and dry density were calculated using Formulas (2) and (3) respectively.

$${\rho }_0={\displaystyle \frac{m_0}{\begin{array}{c}\frac{m_1-m_2}{{\rho }_{wt}}-\frac{{m_1-m}_0}{{\rho }_n}\end{array}}}$$

(2)

$${\rho }_d={\displaystyle \frac{m_d}{\begin{array}{c}\frac{m_1-m_2}{{\rho }_{wt}}-\frac{{m_1-m}_d}{{\rho }_n}\end{array}}}$$

(3)

where ${\rho }_d$ is the dry density (g/cm³); $m_0$ is the mass of the natural rock sample (g); $m_d$ is the mass of the dried rock sample (g); $m_1$ is the mass of the sample coated with paraffin in air (g); $m_2$ is the mass of the sample coated with paraffin in water (g); ${\rho }_{wt}$ is the density of water at t °C (g/cm³); ${\rho }_n$ is the density of paraffin (g/cm³).

3.2.3. Porosity Testing

Porosity was calculated from the dry density and grain density using Formula (4).

$$n=\left(1-{\rho }_d/{\rho }_s\right)\times 100\%$$

(4)

where n is the porosity (%); ${\rho }_d$ is the dry density (g/cm³), which is obtained by Formula (3); ${\rho }_s$ is the particle density of the sample (g/cm³), which is measured by the pycnometer method. The principle involves dispersing a known mass of dry rock powder in water within a pycnometer. After removing air via vacuum extraction, the mass of the pycnometer filled with both rock powder and water is measured, along with the mass of the pycnometer filled solely with water under identical conditions. The grain density of the rock is then calculated.

3.2.4. Radioactive Heat-Producing Elements (U, Th, K) Testing

Major elements were analyzed using X-ray fluorescence spectrometry (XRF), with analysis precision (relative error) better than 1%. The K₂O content was determined as part of the major element analysis. Trace elements were analyzed using inductively coupled plasma mass spectrometry (ICP-MS) following acid digestion and sample preparation, with analytical precision and accuracy better than 10% [42].

4. Results

4.1. Density

The measured density of magmatic rocks ranges from 2.22 to 2.92 g/cm³. In descending order: Gabbro has the highest density, averaging 2.84 ± 0.12 g/cm³; followed by andesite at 2.72 ± 0.03 g/cm³; diorite averages 2.66 ± 0.09 g/cm³; granodiorite averages 2.64 ± 0.02 g/cm³; granitic gneiss averages 2.60 ± 0.02 g/cm³; volcaniclastic rock averages 2.57 ± 0.16 g/cm³; granite averages 2.57 ± 0.10 g/cm³; granitic porphyry averages 2.52 ± 0.03 g/cm³. Overall, the density of magmatic rocks increases continuously with the evolution of lithology from acidic (granite, granite porphyry) to basic (gabbro) (Figure 3a). The density of volcaniclastic rocks has a large interquartile range (2.39–2.73 g/cm³), reflecting complex mineral composition and variable texture/structure. The average density values of intermediate-acidic magmatic rocks such as granite, granodiorite, diorite, and andesite are close to the median, with symmetric and concentrated data distribution, indicating similar mineral composition and relatively homogeneous texture/structure.

4.2. Thermal Conductivity

The thermal conductivity of magmatic rocks ranges from 1.60 to 3.66 W/(m·K). Granite, granitic porphyry, granitic gneiss. Among them, granitic gneiss have an average value of 3.16 ± 0.30 W/(m·K); granite has an average thermal conductivity of 2.97 ± 0.44 W/(m·K), the same as the granite porphyry which has an average value of 2.97 ± 0.41 W/(m·K). The thermal conductivities of the remaining magmatic rocks are similar, with relatively lower average values (<2.50 W/(m·K)). Among them, granodiorite has an average thermal conductivity of 2.42 ± 0.07 W/(m·K), pyroclastic rock has an average value of 2.37 ± 0.51 W/(m·K), andesite has an average value of 2.28 ± 0.44 W/(m·K), diorite has an average value of 2.17 ± 0.25 W/(m·K), gabbro has an average value of 2.08 ± 0.11 W/(m·K) (Figure 3b).

4.3. Porosity

The porosity of magmatic rocks is influenced by the morphology, size, and intercalation characteristics of mineral grains. The more uniform the mineral grains, the smaller their size, and the tighter their intercalation, the smaller the porosity [14,43]. The porosity of magmatic rocks ranges from 0.68% to 15.47%. Among them, pyroclastic rocks exhibit high porosity characteristics (with an average value of 8.25 ± 6.97%), and their interquartile range is the widest. This is related to the complex mineral composition of pyroclastic rocks and the uneven structure caused by the differential compaction of clastic materials such as volcanic ash and breccia during the diagenetic process [44]. The porosity values of other magmatic rocks are relatively low (<7%) and generally similar. The detailed classification shows the following characteristics: granite and granite porphyry have relatively high porosities, with average values of 3.42 ± 2.00% and 4.57 ± 1.41% respectively; granodiorite, diorite, gabbro, and granitic gneiss have moderate porosities, with average values ranging from 1.59% to 2.31%; andesite exhibits the lowest porosity, averaging 0.92 ± 0.25%, and also has the narrowest interquartile range (Figure 3c). The above variation characteristics may be related to the differences in the mineral structure of magmatic rocks. Granite generally has a medium-to-coarse-grained structure, with a high euhedral degree of mineral crystals and coarse grains; this results in relatively large pores between different grains. Granodiorite, diorite, gabbro, and other rocks generally have a medium-to-fine-grained structure, with subhedral mineral crystals as the main form, and moderately developed pores between grains; while andesite generally has a microcrystalline-cryptocrystalline structure, with anhedral mineral crystals as the main form, the smallest grains, and the lowest porosity between grains (Figure 2).

4.4. Volumetric Heat Capacity

The volumetric heat capacity of rocks is a key parameter characterizing the heat storage capacity per unit volume of rocks, which is calculated from the density and specific heat capacity of rocks [45].

$$C_v=\rho ·c$$

(5)

where C_v is the volumetric heat capacity (MJ/(m³·K)), ρ is the rock density (g/cm³), and c is the rock specific heat capacity (MJ/(kg·K)).

The volumetric heat capacity of magmatic rocks in the study area ranges from 1.61 to 2.30 MJ/(m³·K). The average volumetric heat capacity in descending order is: gabbro (2.26 ± 0.06 MJ/(m³·K)), pyroclastic rocks (2.13 ± 0.15 MJ/(m³·K)), andesite (2.10 ± 0.09 MJ/(m³·K)), diorite (2.02 ± 0.06 MJ/(m³·K)), granite porphyry (2.01 ± 0.11 MJ/(m³·K)), granitic gneiss (1.98 ± 0.08 MJ/(m³·K)), granodiorite and granite have similar volumetric heat capacities with average values ranging from 1.96 to 1.93 MJ/(m³·K). The volumetric heat capacity values of magmatic rocks are generally similar and increase slightly as the composition of magmatic rocks evolves from acidic to basic (Figure 3d).

4.5. Thermal Diffusivity

Thermal diffusivity, an important thermophysical parameter characterizing the rate of temperature transfer inside a material, is calculated from thermal conductivity, density, and volumetric heat capacity. It is proportional to thermal conductivity and inversely proportional to the product of density and volumetric heat capacity [46].

$$\alpha =\lambda /(\rho ·C_p)$$

(6)

where α is the thermal diffusivity of the rock (mm²/s); λ is the thermal conductivity (W/(m·K)); ρ is the rock density (g/cm³); and C_p is the rock specific heat capacity (MJ/(kg·K)).

The thermal diffusivity of magmatic rocks in the study area generally ranges from 0.79 to 1.75 mm²/s, which can be roughly divided into three groups: acidic magmatic rocks (granite, granite porphyry, granitic gneiss) have relatively high thermal diffusivity, with average values ranging from 1.48 to 1.59 mm²/s; intermediate-acidic magmatic rocks (granodiorite, diorite, andesite, etc.) have moderate thermal diffusivity, with average values ranging from 1.07 to 1.25 mm²/s; basic gabbro has the lowest thermal diffusivity, with an average value of 0.92 ± 0.03 mm²/s (Figure 3e).

4.6. Radiogenic Heat Production

The radiogenic heat production was calculated based on the rock density and the contents of U, Th, and K using Formula (7) [47]:

$$A=0.01\rho (9.52C_u+2.56C_{Th}+3.48C_k)$$

(7)

where A is the radiogenic heat production (μW/m³); $\rho$ is the rock density (g/cm³); $C_u$ and $C_{Th}$ are the contents of radioactive heat-producing elements U and Th in the rock (μg/g) respectively; and $C_k$ is the weight percentage of K in the rock (%).

The U content in magmatic rocks ranges from 0.48 to 11.90 μg/g, with an average of 2.29 ± 2.22 μg/g; the Th content ranges from 1.53 to 53.8 μg/g, with an average of 14.22 ± 12.59 μg/g; the Th/U ratio ranges from 1.73 to 18.13, with an average of 6.46 ± 4.01; the K₂O content ranges from 0.19 to 6.18%, with an average of 3.19 ± 1.45%; and the radiogenic heat production ranges from 0.60 to 5.56 μW/m³, with an average of 1.74 ± 1.32 μW/m³ (Appendix A).

Statistics on the heat production of different types of rocks show (Figure 3f) that the radiogenic heat production of magmatic rocks gradually decreases as the composition of magmatic rocks evolves from acidic to basic. Among them, granite has the highest radiogenic heat production, with an average value of 3.85 ± 1.01 μW/m³ and a maximum value of 5.29 μW/m³; granite porphyry is next, with an average value of 3.52 ± 1.44 μW/m³ and a maximum value of 5.56 μW/m³. Gabbro has the lowest radiogenic heat production, with an average value of 0.37 ± 0.05 μW/m³. The radiogenic heat production of other magmatic rocks is similar, with average values ranging from 0.70 to 1.69 μW/m³, which are basically consistent with the heat production of sedimentary rocks in the Subei Basin [28]. This indicates that, except for acidic granite (porphyry), the contribution of the radiogenic heat production of other magmatic rocks is comparable to that of sedimentary strata and cannot provide an anomalous heat source for the formation of geothermal resources.

High heat-producing granite (HHP) generally refers to granite with a radiogenic heat production greater than 5 μW/m³ [19]. The heat generated by its radioactive decay is significantly higher than other types of rocks, and can provide an important heat source for the formation of geothermal resources, especially in areas lacking modern volcanic or magmatic activities [48,49]. The average heat production of granite (and granite porphyry) in the study area was 3.31 μW/m³. This value is slightly higher than the global average for Mesozoic-Cenozoic granites (3.09 μW/m³ [50]), but comparable to that of granites in the Zhangzhou geothermal field in Fujian Province, China (3.32 μW/m³ [51]) and the granite in the Gonghe hot dry rock geothermal field in Qinghai Province, China (3.20 ± 1.07 μW/m³ [13]). However, it is significantly lower than the high heat-producing granite in the Fogang and Huizhou areas of Guangdong Province, China (6.77 μW/m³ and 5.81 μW/m³ [24,52,53]) and the Cooper Basin in Australia (7.50–10.30 μW/m³ [23]). Notably, however, the radiogenic heat production of two granite samples from the Suzhou pluton (5.22 μW/m³ and 5.56 μW/m³) meets the standard for HHP granite, which may have an important impact on the formation of regional geothermal resources.

5. Discussion

5.1. Influencing Factors on Rock Thermal Conductivity

The thermal conductivity of rocks is affected by mineral composition, porosity, water saturation, temperature, pressure, etc. [19,28], but is primarily controlled by mineral composition, porosity, and water saturation [53,54]. The thermal conductivity of magmatic rocks in the study area shows a significant positive correlation with the SiO₂ content (Figure 4a), reflecting that the thermal conductivity is mainly controlled by the content of quartz. As the content of minerals with high thermal conductivity (e.g., quartz) in magmatic rocks increases, the thermal conductivity also increases rapidly [14,55,56]; while the contents of FeO and MgO have a negative correlation with thermal conductivity (Figure 4b,c), reflecting that the increase in the content of mafic minerals lead to a continuous decrease in the overall thermal conductivity of the rock [43,57]. Porosity and thermal conductivity show an overall negative correlation (Figure 4d). Increased porosity raises the air proportion in the heat transfer path, forming thermal barriers and reducing rock thermal conductivity. However, when porosity is low (<7%), its relationship with thermal conductivity is not obvious, indicating that mineral composition dominates the influence on thermal conductivity under low-porosity conditions.

5.2. Source of Radiogenic Heat Production

Combining this study with previously published geochemical data of magmatic rocks [58,59,60,61,62,63,64,65], the radiogenic heat production of magmatic rocks is summarized. The results show a clear linear relationship between heat production and U and Th content (Figure 5a,b), indicating that U and Th are the primary elements for radiogenic heat production, with U being slightly more sensitive than Th. The correlation with K₂O is weaker and more scattered (Figure 5c), only slightly stronger when K₂O < 4.0%. A ternary diagram further shows (Figure 5d) that heat production in magmatic rocks mainly originates from U and Th decay, with K contributing the least.

The contributions of radioactive elements in different types of magmatic rocks are slightly different. From acidic to basic rocks, the average heat production contributions of U and K₂O gradually increase. For example, the contributions of U and K₂O in acidic rocks are 32% and 16% respectively, and gradually increase to 39% and 30% in basic rocks, while the average contribution of Th significantly decreases.

In terms of geochemical correlation, the radiogenic heat production shows significant positive correlations with U, Th, K₂O, and SiO₂, and negative correlations with CaO, Fe₂O₃, MnO, MgO, and TiO₂ (Figure 6). This indicates that with the increase in SiO₂ content, incompatible elements U and Th tend to enrich in felsic melts, leading to higher heat production in intermediate-acidic magmatic rocks compared to intermediate-basic ones [55,66,67]. Notably, Jingning granitic gneiss (~700–800 Ma) distributed within the Sulu orogenic belt and mid-Early Cretaceous granitic rocks (~130 Ma) all have high SiO₂ content (>70%), but their heat production rates differ significantly, averaging 1.58 ± 0.52 μW/m³ and 4.21 ± 1.35 μW/m³, respectively (

Table 2. Radiogenic heat production of magmatic rocks in different tectonic units, Jiangsu Province.

Tectonic Unit	Pluton	Lithology	Geological Period	Age	SiO2	Heat Production	Average Heat Production (μW/m3)
				(Ma)	(%)	(μW/m3)	Pluton	Tectonic Unit
North China Block	Banjing Pluton	Diorite Porphyry	Mid-Early Cretaceous	127	57.47	0.68	0.68	0.75 ± 0.07
	Liguo Pluton	Granite		131	72.78	0.72	0.78
		Granodiorite Porphyry		130	65.40	0.85
Sulu Orogenic Belt	Taolin Pluton	Granite	Mid-Early Cretaceous	131	76.45	3.04	2.99	2.75 ± 0.64
		Granite		131	76.91	3.25
		Granite Porphyry		/	77.70	3.09
		Granite Porphyry		/	77.53	3.25
		Granodiorite		129	67.28	2.31
		Quartz Monzonite		135	67.59	2.98
	Banzhuang Pluton	Granodiorite		130	68.53	1.36	1.36
	Niushan Gneiss	Granitic Gneiss	Jinningian Period	700–800	77.12	1.50	1.50	1.58 ± 0.52
	Moshan Gneiss			700–800	77.67	1.67	1.67
	Hushan Gneiss			700–800	76.92	2.32	1.51
				700–800	78.18	0.69
	Jushan Gneiss			700–800	73.90	1.73	1.73
South Jiangsu Uplift	Anjishn Pluton	Granodiorite	late-Early Cretaceous	104	68.02	1.30	1.34	1.45 ± 0.27
		Quartz Diorite Porphyry		106	62.87	1.38
	Shima Pluton	Granodiorite		109	68.13	1.70	1.75
		Porphyritic Granodiorite		103	67.06	1.80
	Shangdang Volcanic Rock	Dacite		107	70.72	1.07	1.07
	Yeshan Pluton	Quartz Diorite	Mid-Early Cretaceous	124	64.11	0.88	0.88	3.54 ± 1.78
	Suzhou Pluton	Granite		130	76.25	5.29	4.88
		Granite Porphyry		128	74.90	3.81
		Granite		/	77.20	5.56
	Miaoxi Pluton	Granite Porphyry		/	74.56	2.18	2.18
	Lishui Volcanic Rock	Brecciated Tuff		128	58.57	0.77	0.73	0.70 ± 0.22
		Tuff		129	56.70	0.60
		Brecciated Tuff		/	54.83	0.60
		Andesitic Breccia Lava		/	56.42	0.95
		Trachyandesite		/	48.16	1.00
		Andesite		/	56.59	0.79
		Andesite		129	54.58	0.81
		Gabbro		127	45.42	0.33	0.37
		Gabbro		128	48.71	0.40

Note: The ages of magmatic rocks are cited from references [27,38,58,59,61,62,63,65,69,70,71].

). During the Sulu orogenic moment in the late Mesozoic, the Jinningian magmatic rocks experienced intense deformation and metamorphism, potentially causing activation and migration of mobile elements like U and Th [68], leading to significantly reduced element content, while K₂O content changed little (Appendix A). Loss of radioactive elements is the main reason for the difference in heat production.

5.3. Spatiotemporal Evolution of Radiogenic Heat Production

Samples can be divided into three periods based on diagenetic age: Jinningian magmatic rocks (~700–800 Ma [33,69,70]) are distributed within the Sulu Orogenic Belt; mid-Early Cretaceous (~130 Ma [58,59,63,71]) magmatic rocks exposed in tectonic uplift areas across the province; and late-Early Cretaceous (~105 Ma [38,62]) magmatic rocks are distributed within the Ningzhen (Nanjing-Zhenjiang) tectonic-magmatic belt, located near the northern boundary of the South Jiangsu Uplift (Figure 1, Table 2).

Temporally, radiogenic heat production of Jinningian granitic gneiss averages 1.58 ± 0.52 μW/m³; mid-Early Cretaceous intrusive rocks from different tectonic units average 2.62 ± 1.50 μW/m³, with a large variation range (0.70–3.54 μW/m³); late-Early Cretaceous magmatic rocks average 1.45 ± 0.27 μW/m³. For intrusive rocks over a large age span, there is no consistent trend of heat production change with diagenetic age, likely due to superimposed influences from lithology, genetic mechanisms, and later alteration. However, within the South Jiangsu Uplift, late-Early Cretaceous magmatic rocks (Anjishan, Shima, ~105 Ma) show significantly lower heat production than mid-Early Cretaceous ones (Suzhou pluton, Miaoxi pluton, ~130 Ma), decreasing from 3.54 μW/m³ to 1.45 μW/m³. This may relate to differences in the deep geological processes of magmatic rock formation. Studies suggest that eastern China entered an extensional regime starting around ~135 Ma, divisible into early EW-SE extension (120–135 Ma) and late WE extension (100–120 Ma) stages [72,73]. During early extension, Asthenosphere upwelling heated the lithosphere, and lithospheric extension caused decompression melting, forming abundant bimodal magmatic rocks. Acidic intrusive rocks (SiO₂ > 75%) formed by the lower crust are rich in incompatible elements (U, Th, K). With intensified extension, strong crust-mantle interactions like lower crustal delamination and lithospheric thinning occurred, forming abundant crust-mantle mixed magmatic rocks (SiO₂ < 70%), typified by adakites in the Ningzhen area [74]. The addition of mantle-derived magma components diluted U, Th, and K content, leading to reduced heat production.

Spatially, mid-Early Cretaceous magmatic rocks (~130 Ma), products of contemporaneous tectonic-magmatic events, are exposed from the North China Block, Sulu Orogenic Belt, to the South Jiangsu Uplift. Their average heat production is 0.75 ± 0.07 μW/m³, 2.75 ± 0.64 μW/m³, and 3.54 ± 1.79 μW/m³, respectively, showing an increasing trend from north to south, possibly related to different formation mechanisms or source materials of magmatic rocks across tectonic units.

By tectonic unit, magmatic rocks in the North China Block have the lowest heat production, consistent with the relatively low geothermal gradient in this area, indicating their small contribution to the regional geothermal field. The Sulu Orogenic Belt contains magmatic activity from two periods. The radiogenic heat production of Jinningian granitic gneiss (~700–800 Ma)is similar to sedimentary rocks (Figure 5b), thus not an indicator for geothermal exploration. Mid-Early Cretaceous magmatic rocks (~130 Ma) formed several large complexes around intrusive centers, represented by the Taolin pluton with relatively high heat production (2.99 μW/m³), providing favorable conditions for geothermal resource formation. Mid-Early Cretaceous magmatic rocks (~130 Ma) in the South Jiangsu Uplift exhibit bimodal characteristics, with large differences in heat production: acidic granitic rocks have high heat production (average 3.54 μW/m³), contrasting sharply with intermediate-basic volcanic-subvolcanic rocks having low heat production (average 0.70 μW/m³). Granitic rocks are sporadically exposed but have large concealed batholiths at depth. Some granites meet HHP standards, providing important heat source conditions for regional geothermal resources.

5.4. Thermal Effect of Magmatic Rocks

The surface outcrop of magmatic rocks in Jiangsu Province is limited, but regional geophysical interpretations indicate numerous concealed plutons. Their impact on the present-day geothermal field and thermal effect merits discussion. Previous studies show that magmatic residual heat of shallow crustal intrusions generally dissipates within 10 Ma [75,76]. Magmatic intrusions in Jiangsu occurred mainly during the Jinningian and Yanshanian periods. Although intrusions are large, their formation age is relatively old; magmatic residual heat has transferred to the country rocks and reached thermal equilibrium, thus not constituting an additional heat source. However, intermediate-acidic intrusive rocks, especially HHP granites, have high radiogenic heat production. Heat accumulated via radioactive decay can serve as a reliable heat source for geothermal resources. In areas overlain by low thermal conductivity cover rocks, they can significantly affect the geothermal field distribution. For example, the high-heat-producing granite (7.50–10.30 μW/m³) in Australia’s Cooper Basin makes important contributions to the formation of high-temperature hot dry rock resources [23,77]. Abundant HHP granites are distributed in south China, which is an important terrestrial heat flow anomaly area [7], and numerous hot springs are closely related to HHP granites, indicating their importance for geothermal resource formation.

Studies show the Subei Basin is the most geothermal-rich area in Jiangsu Province, with an average geothermal gradient of 30 °C/km, ranking the highest in the province. This is attributed to the thick Cenozoic low thermal conductivity cover (several kilometers), which provides a stable thermal insulation layer [11]. The contribution of concealed plutons to geothermal anomalies is relatively weak, only correlating with geothermal gradient anomalies in a few areas like western Yancheng and Taizhou. In other tectonic uplift areas or shallow cover areas outside the Subei Basin, where Cenozoic sedimentary cover is thin (generally <2 km), the distribution of magmatic rocks exerts a more significant impact on the regional geothermal field, especially in the South Jiangsu Uplift. Geothermal gradient anomaly areas in Nanjing, Suzhou, and southeastern Nantong correspond well with the distributions of concealed magmatic rocks, and high terrestrial heat flow anomalies (>70 mW/m²) are also distributed around plutons, indicating clear control by plutons on regional geothermal anomalies (Figure 7).

5.5. Radioactive Heat Contribution of Granite Plutons

Plutons with heat production > 3 μW/m³ may contribute more than 80% to the crustal heat flow [78]. This study selected the Taolin pluton (2.99 μW/m³) and Suzhou pluton (4.88 μW/m³), which have large surface exposure and relatively high heat production, to calculate their radioactive heat contribution.

The heat production of granite is mainly related to the type of granite, the degree of crystallization differentiation, and alteration intensity [79]. With the enhancement of crystallization differentiation, the granite exposed near the surface shows a high heat production due to the enrichment of radioactive elements. The heat production inside the pluton shows a vertical gradient decrease, decaying exponentially with depth [46,80].

According to the regional tectonic background and geophysical data, the average crustal thickness in Jiangsu Province is ~32 km [81]. Integrated interpretations of regional seismic profiles, combined with drilling data and non-seismic geophysical surveys (including gravity, magnetic, and magnetotelluric methods), indicate that the thickness of some concealed intermediate-felsic intrusive rocks in the Subei Basin attains thicknesses of up to 10 km [82], In previous studies on the crustal thermal structure of southeastern China, the thickness of the radiogenic heat-producing layer was set to 10 km [51], therefore, the average thickness of the Taolin and Suzhou pluton complexes was set at 8–10 km. An exponential decay model was used to calculate the variation of heat production with depth [83]:

$$A_z=A_0\mathrm{e}\mathrm{x}\mathrm{p}(-Z/D)$$

(8)

where A₀ is the heat production of granite at the surface (μW/m³); Az is the heat production of granite at depth Z (km) (μW/m³); and D is the thickness of the granite mass (km).

Heat production at depth z (km) was calculated for 4 samples from Taolin and 7 samples from Suzhou (3 samples from this study, 4 samples from previous studies [65]). The average heat production for Taolin is 2.01 ± 0.65 μW/m³ (range 1.12–3.25 μW/m³); for Suzhou is 3.31 ± 1.47 μW/m³ (range 1.15–7.98 μW/m³) (Figure 8).

To quantitatively assess the vertical heat contribution of granite intrusions to the regional heat flow, the radiogenic heat production Az was vertically integrated over the depth intervals 0–8 km and 0–10 km separately. The vertical heat flow is given by:

$$Q_{vertical}={\int }_0^{z}A\left(z\right)dz$$

(9)

Combining (8) and (9) gives (10):

$$Q_{vertical}={\int }_0^z{A}_0\mathrm{e}\mathrm{x}\mathrm{p}(-z/D)dz$$

(10)

The calculation results show that the vertical heat flow from the Taolin pluton is 15.97–19.96 mW/m², and from the Suzhou pluton is 26.24–32.80 mW/m². The average terrestrial heat flow values for the Taolin and Suzhou areas are 64.00 mW/m² and 60.20 mW/m², respectively [5,7]. Thus, the heat flow generated by the Taolin granite accounts for about 25–31% of the terrestrial heat flow, while the heat from the Suzhou pluton accounts for about 44–54%. Therefore, in these areas, mantle-derived heat flow constitutes the primary source of terrestrial heat flow [84], particularly in the Taolin pluton where mantle heat flow plays an even more dominant role, which is consistent with the Tan-Lu Fault Zone being a regional asthenospheric upwelling area. The conclusions are subject to uncertainty due to factors such as sample representativeness, pluton thickness, and the depth-dependent variation model of radiogenic heat production.

6. Conclusions

In this study, the thermophysical parameters of magmatic rocks in Jiangsu Province were tested, the variation laws and influencing factors of the thermophysical parameters were analyzed, the source, controlling factors, spatiotemporal evolution laws, and geothermal effects of the radiogenic heat production were discussed, and the crust-mantle heat contributions of typical rock masses were quantitatively evaluated. The main conclusions are drawn below:

• The thermal conductivity of magmatic rocks is influenced by mineral composition, porosity, and texture/structure. When porosity is lower than 8%, mineral composition plays a decisive role. Thermal conductivity increases rapidly with higher felsic mineral content.
• The radiogenic heat production shows a significant positive correlation with U and Th content, with U slightly more sensitive than Th, while its relationship with K₂O is weaker. The thermal contribution of radioactive elements varies among rock types; Th dominates in intermediate-acidic rocks, while U dominates in basic rocks. The radiogenic heat production increases with SiO₂ content, consistent with the strong incompatibility of U and Th. The later deformation and metamorphism of magmatic rocks significantly reduce the radiogenic heat production, indicating that U and Th elements have active geochemical properties and are prone to migration. The spatiotemporal variation of the radiogenic heat production of magmatic rocks may be related to regional tectonic setting, the formation mechanism, and later deformation/metamorphism.
• The geothermal effect of magmatic rocks is affected by pluton scale, their thermophysical properties, and the sedimentary cover thickness. When radiogenic heat production is relatively low, thick, low thermal conductivity sedimentary cover plays an important role in geothermal resource formation. In tectonic uplift areas with relatively thin sedimentary cover, magmatic rocks with high heat production significantly influence the geothermal field. Calculations based on the exponential decay model indicate that the terrestrial heat flow is mainly mantle-derived both in the Taolin and Suzhou plutons.
• The average heat production of granite (and granitic porphyry) in Jiangsu Province is 3.31 μW/m³, comparable to that of granites in the Zhangzhou geothermal field (Fujian Province) and the Gonghe hot dry rock field (Qinghai Province), but lower than that of typical global HHP granites (>5 μW/m³). However, locally occurring HHP granites are significant for the formation and exploration of regional geothermal resources.