Petrogenesis of high heat producing granites and their contribution to geothermal resource in the Huangshadong geothermal field, South China

Abstract

The Huangshadong geothermal field (HGF), situated in the contact zone between Mesozoic granites and NE-striking dominant faults in South China, has great geothermal potential. Petrogenesis of reservoir rock plays an important role in understandings its genetic mechanism and assessing geothermal potential. However, due to the lack of rock sample at depth collected from the geothermal reservoir, the petrogenesis of granites in the geothermal reservoirs of the HGF, remains an enigma. This study elucidated the petrogenetic characteristics of these granites sampled directly from geothermal reservoir at the depth of ∼3,000 km and their geothermal implications through zircon U-Pb dating, geochemical analysis, and Hf isotopic analysis. The zircon U-Pb ages indicate that the magmatism evolution of HGF contains three eras, namely, Cretaceous (135 ± 4 to 143.6 ± 2.8 Ma), Jurassic (152.7 ± 2.7 to 176.7 ± 1.8 Ma), and Permian granites (251 ± 9.1 to 251 ± 5 Ma) from the youngest to oldest. The reservoir granites were emplaced during the latest stage of Cretaceous intrusion, as indicated by the zircon U-Pb ages (135 ± 4 Ma and 135.3 ± 2.4 Ma) of rock samples from the deep part of well HR-1. These Cretaceous rocks are highly fractionated I-type granites, featuring high SiO₂, K₂O, and Na₂O contents, high Rb/Sr ratios, low Zr/Hf, Nb/Ta, and Th/U ratios, and A/CNK values of 1.05–1.13. Compared to other Cretaceous granites outcropping on the margin of the HGF, these granites have undergone the strongest fractional differentiation. The Cretaceous granites in the HGF are high-heat-producing rocks (>5 μW/m³), with an average heat production rate of 6.63 μW/m³. Notably, the Cretaceous reservoir granites (as reservoir rocks) serve as an important heat source for the formation of geothermal resources in the HGF. In addition, the zircon Hf isotopic composition indicates that the reservoir Cretaceous granites originated from Meso-to Paleo-Proterozoic lower crustal materials (T_DM2: 1,385 to 1907 Ma).

1 Introduction

Geothermal resources have attracted wide attention worldwide owing to their clean and sustainable nature (Tester et al., 2006; Lin et al., 2016; Lu, 2018; Lin et al., 2022; Ma et al., 2022; Chen et al., 2023). The geothermal energy utilized directly was up to 107,727 MWt in 2020, increasing by 52% since 2015 (Lu, 2018; Lund and Toth, 2021). The petrological characteristics of geothermal reservoir rocks serve as key parameters used to evaluate the geothermal potential and analyze the genesis of geothermal resources, especially for a granite-hosted geothermal system (Zhang et al., 2022; Alqahtani et al., 2023a; Alqahtani et al., 2023b; Ullah et al., 2023; Wang et al., 2023).

The heat for thermal anomalies originates from the radioactive decay of heat-producing elements (U, Th, and K) (Hasterok and Chapman, 2011; Wang et al., 2023). The upper crust has average U, Th, and K₂O contents of 2.7 ppm, 10.5 ppm, and 2.8%, respectively, with a radioactive heat production rate of 1.75 μW/m³ (Kemp and Hawkesworth, 2003; Kromkhun, 2010). In contrast, the granites are relatively rich in incompatible elements such as U, Th, and K, with average contents of 3.9 ppm, 16 ppm, and 3.6%, respectively. This enrichment results in a radioactive heat production rate of 2.53 μW/m³. Due to their incompatibility, these heat-producing elements are usually concentrated in the shallow crust, diminishing with depth. The granitic upper crust has depths of mostly 10–20 km, and its radioactive heat production contributes surface heat flow of 10–20 μW/m², which accounts for 50% of the surface heat in the stable continental crust (Artemieva and Thybo, 2013). High-heat-producing granites, with radioactive heat production rates >5 μW/m³, play a critical role in the formation of geothermal resources (Marshall, 2014; Artemieva et al., 2017). For instance, the Cooper Basin, which is subjected to the most recent intrusion of Carboniferous-Permian granodiorites rather than present-day magmatic events (Big Lake Suite, 310–327 Ma; (Marshall, 2014), records the terrestrial heat flow values measuring >100 mW/m². However, the minimum heat flow value is only 33 mW/m², which occurs at the edge of the rock mass in a borehole. This finding indicates that the temperature anomalies are primarily derived from the high-heat-producing granites in the basement, which have a maximum radioactive heat production rate of up to 10 μW/m³ (Roth and Littke, 2022).

A large quantity of Mesozozic granites in South China are genetically related to geothermal energy (Lin et al., 2016; Qiu et al., 2018; Liu et al., 2021; Lin et al., 2022; Lin et al., 2023; Pang and Huang, 2023). The granites in South China have an average heat production rate of 3 μW/m³ (Zhao and Luo, 1995). Among them, granites in Guangdong Province have an average heat production rate of 5.7 μW/m³, classified as high-heat-producing rocks. Many thermal springs are distributed in the HGF, with temperatures of 56–63.7°C and a flow rate of 0.38 L/s (Lin et al., 2016). Geothermal wells ZK 8 and HR-1 revealed high temperatures of 118.3°C and 127.5°C, respectively at depths of 591.5 m and 2,900 m (Xiao et al., 2019; Li et al., 2020a).

Previous studies of granites in the HGF focus on their regional petrogeological characteristics, subjected to rock samples collected from outcrops or shallow buried plutons (∼300 m below the surface; Xiao et al., 2019). Most of the samples were collected from outcrops at a great distance from HGF (G1, G2, G3, and G5 in Figure 2). As a result, the petrogenesis of geothermal reservoir granites buried at depth in the areas where a bunch of hot springs are exposed (locations of wells HR-1 and ZK8), as well as the genetic relationship between them and geothermal energy, remains unclear. This study conducted geochronological, geochemical, and Hf isotopic analyses of granites in the HGF (especially the reservoir granites dredged by HR-1 from the depth of ∼3,000 km), aiming to promote the understanding of the genetic relationship between the geothermal resources and high-heat-producing granites.

2 Geological setting

The South China Block (SCB) was formed by the amalgamation of the Yangtze Block in the northwest and the Cathaysia Block in the southeast during 1.0–0.9 Ga (Zhou et al., 2006) (Figure 1). The NE-striking Qinzhou Bay - Hangzhou Bay (Qin-Hang) suture zone between the latter two blocks was reactivated during the Middle-Late Mesozoic, leading to the formation of numerous plutons (Li et al., 2009; Mao et al., 2011; Hu et al., 2017). Regionally, there are six primary faults striking NE to NEE, namely, the Jiangshan-Shaoxing-Pingxiang, Zhenghe-Dapu, Changle-Nan’ao, Chenzhou-Linwu faults, and the Changlin-Guangchang and the Wuzhou-Sihui concealed faults. The HGF is located approximately 20 km northeast of Huizhou City, Guangdong Province (Figure 1).

FIGURE 1

Chen et al. (2014), copyright@Elsevier, 2016). Faults: (1) Jiangshan-Shaoxing-Pingxiang Fault; (2) Zhenghe-Dapu Fault; (3) Changle-NanaoFault; (4) Chenzhou-Linwu Fault; (5) Changlin-Guangchang buried Fault; (6) Wuzhou-Sihui buried Fault. NCB: North China Craton. TARIM: Tarim Basin.

The stratigraphy in the HGF consists primarily of Sinian and Cambrian metasedimentary strata, as well as Devonian, Carboniferous, Triassic, and Quaternary sedimentary strata (Figure 2). The Sinian metasedimentary strata contain two formations, namely, Laohukuang (Zlh, 725 m thick) and Bali (Zb, >500 m thick). The former is located in the eastern part of this gas field, comprising metasandstones and slates. In contrast, the latter is present in the northwestern part of the study area, consisting of metasandstones interbedded with slates. The Cambrian metasedimentary strata (Єn, 600 m thick) comprise metasandstones occasionally interbedded with phyllites. The Devonian sedimentary strata include the Yangxi (Dy, 370–750 m thick), Laohutou (Dl, 470–620 m thick), and Maozifeng (DCm, 65–410 m thick) formations. They occur in the southeastern part, comprising conglomerates, sandstones, and mudstones. The Carboniferous sedimentary strata are mainly present in the southwestern parts, consisting of three formations, namely, Dasaiba (Cds, 340–400 m thick), Ceshui (Cc, 300–400 m thick), and Hutian (CH, >150 m thick). Among them, the Dasaiba and Ceshui formations comprise quartz sandstones and slates, while the Hutian Formation is composed of limestones and dolomites. The Triassic Genkou Group (TG >1,250 m thick), which consists of conglomerates, sandstones, and mudstones, is found at the eastern and western edges of the HGF. The Quaternary sedimentary strata (Qdw, 10–25 m thick), primarily comprising clay, are distributed in the southwestern part.

FIGURE 2

Spatially, thermal springs in Guangdong Province are closely associated with the outcrops of intrusions (especially Yanshanian intrusions), mostly found in the contact zone between intrusions (granite complex) and host rocks (Xiao et al., 2019).

Granite complex in the HGF comprising three parts (Figure 2): 1) Cretaceous granite, dominant geothermal reservoir rock; 2) Jurassic granite, occurring in the northeastern part; 3) Permian granites exposed only in the central part. Geothermal surface manifestations occur in the contact zone between the Permian granites and the Cambrian metasedimentary rocks. Cretaceous granites were excavated at a depth of about 300 m by well ZK8 (Xiao et al., 2019), and concealed granites were revealed at a depth of 1,560 m by geothermal well HR-1 (Li et al., 2020a).

3 Sampling and petrography

Seven granite samples, namely, HSD-1, HSD-4, HSD-8, HSD-14, HZ-1-1, HZ-3-1, and HZ-3-2), were collected from the HGF for dating and geochemical analysis (Figure 3; Table 1). Each sample was 5–10 kg in weight. Samples HSD-1, HSD-4, HSD-8, and HSD-14 were taken from granite outcrops (Figures 2, 3). Among them, samples HSD-1 and HSD-4 were fresh massive rocks obtained from a strip pit in the southwestern part of the HGF, while sample HSD-8 was gathered from the northeastern part. Samples HSD-1 and HSD-8 were medium-to coarse-grained equigranular biotite granites, and sample HSD-4 consisted of fine-grained equigranular biotite granites. Sample HSD-14 was collected from the two-mica granite outcrops near geothermal well HR-1, with its weathered parts chipped off using a hammer. Samples HZ-1, HZ-3-1, and HZ-3-2 were collected at depths of 1831 m, 2,637 m, and 2,702 m, respectively in well HR-1 (Figure 3).

FIGURE 3

Fresh field samples were cut into blocks of ∼45×25×15 mm using a diamond blade. Afterward, they were planed and then mounted on a 28×48 mm standard petrographic slide using epoxy. The thin sections measured 30 μm in thickness after being polished with abrasive powder. To identify the mineralogy and texture of rocks, the polished thin sections were examined under a binocular microscope and a Leica DM2700P polarization microscope at the Institute of Hydrogeology and Environmental Geology under the Chinese Academy of Geological Sciences.

Figure 4 shows the microscopic photographs of the samples. Samples HSD-1 and HSD-8 were medium-to coarse-grained equigranular biotite granites, which contained quartz (∼37 vol.%), plagioclase (∼30 vol.%), alkali feldspar (∼23 vol.%), biotite/muscovite (∼2 vol.%), and accessory minerals (<3 vol.%), including zircon and apatite. Sample HSD-4 exhibited similar petrological features with samples HSD-1 and HSD-8, except for smaller grain sizes (fine-grained: 0.1–0.3 mm). Samples HZ-1 and HZ-3 were medium-to fine-grained equigranular biotite granites, which contained quartz (∼42 vol.%), plagioclase (∼29 vol.%), alkali feldspar (∼23 vol.%), biotite (∼2 vol.%), and muscovite (∼1 vol.%). Furthermore, zircon is a common accessory mineral in these samples. In contrast, sample HSD-14 was medium-grained two-mica granite, which consisted of quartz (∼40 vol.%), plagioclase (∼30 vol.%), alkali feldspar (∼21 vol.%), biotite (∼2 vol.%), and muscovite (∼2 vol.%).

FIGURE 4

4 Methods

4.1 LA-ICP-MS U-Pb dating and Hf isotopes of zircon

This study conducted zircon dating of granite samples HSD-1, HSD-4, HSD-8, HSD-14, HZ-1, and HZ-3. These six samples (∼5 kg each) were processed at the Hebei Institute of Regional Geology and Mineral Resources Survey.

Six zircon grains were separated from these samples using conventional heavy-liquid and magnetic separation techniques. Then, about 100 zircon grains with the highest quality were hand-picked under a binocular microscope. These grains were mounted in epoxy and then polished. Afterward, the grains were photographed using optical microscopy and cathodoluminescence (CL) imaging to reveal their internal morphologies, which were used to select grains and analytical spots. CL images were obtained using a HITACHI S3000-N scanning electron microscope equipped with a Robinson backscattered electron detector and a Gatan Chroma CL imaging system.

To determine zircon U-Pb ages, these above samples were analyzed using an Agilent 7,900 quadrupole ICP-MS equipped with a 193 nm coherent Ar-F laser and a Resonetics S155 ablation cell at the Tianjin Institute of Geology and Mineral Resources. NIST SRM 610 glass used for Pb correction was analyzed after every 15 test samples. The mass bias of Th/U and Pb/Th ratios caused by downhole fractionation and instrumental drift was corrected using zircon reference material 91,500 following instructions of Wiedenbeck et al. (1995). Each zircon analysis involved 15 s blank gas measurement and 50 s analysis. The analytical spots were 29 μm each in size, and laser was emitted at a frequency of 7 Hz, yielding an energy density of approximately 3 J/cm². The grains ablated by laser were carried by He gas into the chamber at a flow rate of 0.35 L/min, where they were mixed with argon gas before being transported to the plasma torch. Zircon standards Temora (Black et al., 2003) and Plesovice (Sláma et al., 2008) were employed in this study. Data processing was performed using Concordia intercept ages on the Tera-Wasserburg plot which was prepared using ISOPLOT (Ludwig, v. 3.75, 2012). Data reduction was conducted using ICPMSDataCal (Liu et al., 2010). The random and systematic uncertainties were estimated using the method proposed by Paton et al. (2010).

Based on LA-ICP-MS zircon U-Pb dating, the cathodoluminescence (CL) diagram of zircon was plotted. The laser denudation was conducted under a beam spot diameter of 40 μm and a duration of 30 s, with GJ-1 used as the external reference material for the calculation of Hf (Geng et al., 2011). In the solution injection mode, a 200×10⁻⁹ standard solution was used. It was fed into a nebulizer at a flow rate of 50 μL/min using a peristaltic pump. Nine Faraday cups simultaneously received ¹⁷²Yb, ¹⁷³Yb, ¹⁷⁵Lu, ¹⁷⁶(Yb+Hf+Lu), ¹⁷⁷Hf, ¹⁷⁸Hf, ¹⁷⁹Hf, ¹⁸⁰Hf, and ¹⁸²W. Data acquisition was performed using the virtual amplifier technology of NEPTUNE (MC-ICPMS). The software automatically replaced the amplifier circuits of the analyzer after the acquisition of a set of data. After nine sets of data were acquired, the amplifier circuits were identical to those of the original analyzer. This technique can be used to effectively eliminate the isotope ratio error caused by the difference in the gain of each Faraday cup receiver, thus improving the accuracy of isotope ratio determination.

The laser ablation test began with the targeting, polishing, and photographing of the staghornite to be tested. Samples were arranged on the double-sided adhesive of the slide and put on the PVC ring. Then, epoxy resin and hardener were mixed thoroughly and injected into the PVC ring. After the resin was fully cured, sample holders were peeled off from the slide and then polished, followed by the taking of the cathodoluminescence, reflected light, and transmitted light photographs of the samples on the target. Based on these three kinds of photographs, the appropriate area of the struvite was selected, and the struvite was stripped using a 193 nm FX laser with a spot diameter of 35, 50, or 75 (um), an energy density of 10–11 J/cm², and a frequency of 8–10 Hz. The laser-stripped materials were fed into the Neptune (MC-ICPMS) with high-purity He as the carrier gas. The receiver configuration was the same as that of the solution injection.

4.2 Whole-rock geochemical analyses

Seven fresh samples were cut into blocks, each measuring ∼70×50×30 mm in size. These blocks were washed with tap water and ground to <200 meshes using an agate mill. Both major and trace element analyses were performed using the method proposed by Liu et al. (2016) at the Central Laboratory of China Railway Resources Group. The contents of major elements were measured using an X-ray fluorescence (XRF) spectrometer (Thermo Fisher ARL Advant'X), with the loss on ignition (LOI) being determined with an electronic analytical balance (CPA225D). The FeO content in the samples was measured using conventional wet chemical titration. The contents of trace elements, including rare earth elements (REEs), were determined using ICP-MS (Thermo Fisher X-series 2) after complete digestion. A detailed account of the test procedures and the lower detection limit were documented in Chinese national standard GB/T14506-2010 (Lv, 2010). The analytical errors varied in a range of 1%–3% of the present values. The whole-rock geochemical analyses were conducted under temperatures of 20–27°C and humidity of 30%–58%.

5 Results

5.1 Zircon dating

As presented in Figure 5, zircons from sample HZ-1 were typically transparent, ranging from colorless to slightly brown, and exhibited rectangular to prismatic crystal structures. They were 80–180 μm long, with aspect ratios ranging from 1.5:1 to 3:1. Furthermore, these crystals commonly exhibited oscillatory zoning (Figure 5). Zircons from sample HZ-3 were predominantly transparent, clear to pale yellow, and euhedral to subhedral crystals. They were 70–170 μm long, with aspect ratios varying in the range of 2:1–3:1. Among them, euhedral grains showed concentric zoning with relatively bright cores, as shown in CL images (Figure 5). Zircons from samples HSD-1 and HSD-8 were similar in shape and color. They were slightly larger (typical length: 100–200 μm), with aspect ratios between 1:1 and 3:1, and exhibited weakly oscillatory zoning. Furthermore, zircons from sample HSD-4 were considerably smaller (length: 40–100 μm), with aspect ratios of 1:1–4:1.

FIGURE 5

The LA-ICP-MS zircon U-Pb ages of the HGF are shown in Figure 6; Table 2. Fifteen analyses showed that sample HZ-1 had a²³⁸U/²⁰⁶Pb age of 135 ± 4 Ma (MSWD = 5.7), while 20 analyses revealed that sample HZ-3 had a weighted average ²³⁸U/²⁰⁶Pb age of 135.3 ± 2.4 Ma (MSWD = 3.2). The two ages, which were consistent within the given error range, indicate that both samples formed during the Cretaceous. In contrast, samples HSD-1, HSD-4, and HSD-8 exhibited significantly older weighted average ²³⁸U/²⁰⁶Pb ages of 152.7 ± 2.7 Ma (MSWD = 4.8), 155.8 ± 3.3 Ma (MSWD = 5.2), and 153 ± 3 Ma (MSWD = 3.8), respectively. These ages, which were also consistent within the given error range, suggest that these three samples intruded during the Jurassic. Sample HSD-14 was emplaced during the Indosinian, as indicated by four data points yielding a²³⁸U/²⁰⁶Pb age of 153.7 ± 1.2 (MSWD = 2.2; probability = 0.002). As shown in Figure 7; Table 3, sample HSD-14 exhibited the highest Ti-in-zircon temperatures from 684°C to 781°C (Ferry and Watson, 2007), with an average of 742°C, while the other samples had lower Ti-in-zircon temperatures, with averages ranging from 617°C to 687°C.

FIGURE 6

FIGURE 7

5.2 Geochemical characteristics

We analyzed the major and trace element compositions of seven representative samples (Table 5). These samples featured high SiO₂ (74.4–76.56 wt.%) and K₂O (3.76–4.84 wt.%) contents. All the samples fell within the high-K calc-alkaline zone (Figure 8A). Samples HSD-4, HSD-8, and HZ-1 exhibited aluminum saturation index (ASI) values of 1.02, 1.05, and 1.05, respectively, all of which were less than 1.1 (Figure 8B). In comparison, samples HSD-1, HZ-3-1, and HZ-3-2 had ASI values of 1.13, 1.13, and 1.11, respectively, all of which were greater than 1.1.

FIGURE 8

Compared to the other samples, samples HSD-1, HSD-8, and HSD-14 had higher K₂O, CaO, MgO, TiO₂, Zr, Sr, and Ba contents but lower Na₂O and Al₂O₃ contents (Figures 8–11; Table 5). All these samples were depleted in P₂O₅ (Figure 11).

FIGURE 9

FIGURE 10

FIGURE 11

As shown in the primitive mantle-normalized diagrams (Figure 12; Table 5), sample HSD-14 exhibited the strongest depletion in Ti, Sr, La, and Ce but notable enrichment in Rb, U, and Nd. Compared with samples HSD-1 and HSD-8, samples HSD-4, HZ-1, and HZ-3 were relatively depleted in Ti, Sr, and Ba but enriched in Rb, U, Nb, and Nd.

FIGURE 12

Sun and McDonough, 1989).

The REE patterns (Figure 13; Table 5) showed that sample HSD-14 exhibited a leftward REE pattern with the lowest La_N/Yb_N ratio (0.56); samples HZ-3-1 and HZ-3-2 showed relatively flat patterns with similar La_N/Yb_N ratios of 1.47 and 1.32, respectively; sample HZ-1 had a much higher La_N/Yb_N ratio of 9.7; among the remaining samples, HSD-1 displayed the highest La_N/Yb_N ratio (5.06), followed by HSD-8 (2.95), while HSD-4 had the lowest La_N/Yb_N ratio (1.19); samples HSD-4, HSD-14, HZ-3-1, and HZ-3-2 exhibited the lowest δEu values, which were 0.1, 0.1, 0.09, and 0.07, respectively, while sample HSD-1 had the highest δEu value, which was 0.32. The Indosinian granite (sample HSD-14) exhibited relatively low total REE contents (ΣREE; 78.38). In contrast, the Cretaceous granites showed the highest ΣREEs, with values of 188.55 ppm, 111.12 ppm, and 149.78 ppm for samples HZ-1, HZ-2, and HZ-3, respectively.

FIGURE 13

Sun and McDonough, 1989).

5.3 Hf isotopes

This study conducted the Lu-Hf isotope analyses for zircons that were subjected to U-Pb dating, as shown in Table 6; Figure 14. Specifically, Lu-Hf isotope analysis was conducted on 15 and 20 zircon grains from HZ-1 and HZ-3, respectively. These zircons show a wide range of¹⁷⁶Yb/¹⁷⁷Hf ratios from 0.017479 to 0.126740 (average: 0.050829) and ¹⁷⁶Lu/¹⁷⁷Hf ratios between 0.000509 and 0.003495 (average: 0.00132), indicating low content of radiogenic Hf. Furthermore, 35 analytical spots yielded ε_Hf(t) values of −11.4334 to −3.0396, which corresponded to two-stage Hf model ages (T_DM2) of 1,385–1907 Ma dominated by 1,500–1700 Ma, as depicted in Figure 15.

FIGURE 14

FIGURE 15

6 Discussion

6.1 Timing of granite emplacement

The zircon ages of the granites in the HGF are summarized in Table 4; Figure 16. These ages indicate three stages of granite emplacement: 1) the Indosinian stage, corresponding to Permian granites (251 ± 9.1 to 253 ± 5 Ma); 2) the Yanshanian stage, including Jurassic granites (152.7 ± 2.7–176.7 ± 1.8 Ma); 3) the Cretaceous stage (135 ± 4–143.6 ± 2.8 Ma) (Xiao et al., 2019). These ages indicate intense Mesozoic (Yanshanian) magmatic activity in the study area. The Cretaceous granites dominate the HGF, exhibiting the largest outcrop area (100 km²; Figure 2), while the Permian granites only occur in the northern part of the geothermal field (Xiao et al., 2019). As indicated by the zircon dating results of samples HZ-1 and HZ-3 taken from the deep part of well HR-1, the surrounding rocks of the geothermal reservoirs in the HGF consist of Cretaceous granites, although Permian granites are the closest to geothermal wells (HR-1 and ZK8).

FIGURE 16

6.2 Petrogenesis of granites

As shown in the Harker diagram, the samples collected in this study were the product of felsic magmas and had undergone significant fractionation. The Yanshanian granites, represented by the samples collected in this study, show a downward trend of Al, Mg, and Ca contents with an increase in the SiO₂ content (degree of fractionation; Figure 9). The depletion in Sr, Ba, and Ti, as well as prominent negative Eu anomalies (Figure 10), is indicative of the fractionation of plagioclase and Ti-Fe oxides. Figure 11 illustrates decreases in the Zr/Hf, Nb/Ta, and Th/U ratios but increases in the Rb/Sr ratio with an increase in the degree of magmatic fractionation (Stepanov et al., 2016; Breiter and Škoda, 2017; Cai et al., 2020). Furthermore, samples collected from or around geothermal wells exhibited significantly V-shaped REE patterns than those collected at a greater distance from geothermal wells, indicating that they underwent a higher degree of crystal fractionation.

Generally, granites can be categorized into S-, I-, A-, and M-types based on their genesis (Collins et al., 1982; Chappell and White, 2001; Chappell et al., 2012; Zhou et al., 2021). S-type granites originate from sedimentary rocks (Chappell et al., 1999), while I-type granites originate from ancient igneous rocks (Chappell and White, 2001). A-type granites are generally formed by the partial melting of dry granulitic residues in the lower crust, featuring a high abundance of large highly charged cations (Nb, Ga, and REEs) and low Mg, Ca, and Al content (Collins et al., 1982). M-type granites are derived from the subducted oceanic crust or overlying mantle (Whalen et al., 1987). A/CNK serves as an effective factor used to distinguish between S- and I-type granites (Chappell and White, 2001). Sample HSD-14 (Permian granites) had an A/CNK value of 1.2, suggesting that it was S-type granite. This result is consistent with the presence of primary muscovite (Figure 4). In contrast, Yanshanian granites (Jurassic and Cretaceous stages) exhibit A/CNK values of 1.05–1.13, ruling out the possibility of them being S-type granites. As presented in Figure 17, these Yanshanian granites fall within the zone of fractionated granites, suggesting that these granites are highly fractionated I-type granites rather than A-type granites. This finding is consistent with the downward trend of P content with an increase in the SiO₂ content (Figure 10) and can also be evidenced by low Ti-in-zircon temperatures of 475–781°C (Table 3; Figure 7).

FIGURE 17

Zircon Lu-Hf isotope compositions are an important tool for identifying the magma source of granites (Bhattacharya and Janwari, 2015; Bea et al., 2018). As indicated by the Hf isotope composition and T_DM2 ages (1,385–1907 Ma) of samples HZ-1 and HZ-3, collected at depths of 2,637 m and 2,702 m, respectively (Table 6; Figures 14, 15), the reservoir granites originated from the mixing of Meso-to Paleo-Proterozoic lower crust and juvenile mantle materials. Two theories have been proposed for the formation of highly fractionated I-type granites, namely, the partial melting of crustal materials (Chappell et al., 1999; Wyborn et al., 2001) and the fractional crystallization of mafic melts (Chappell et al., 1999; Chappell et al., 2012). The latter assumption can be excluded for the following reasons: 1) such a large volume of granitoids (80%) is difficult to form due to the too small quantity of mafic rocks within igneous rocks (Zhang et al., 2015); 2) the parent magmas (peraluminous melts) of I-type granites are unlikely to be produced through the fractional removal of hornblende (Chappell et al., 2012). Instead, Cretaceous I-type granites in the study area are likely derived from the partial melting of the lower crust. Since the Early Jurassic (∼190 Ma), the Cathaysia Block had experienced asthenospheric upwelling and intra-continental lithosphere extension after the Indosinian flat-slab subduction of the Paleo-Pacific plate (Li and Li, 2007; Chen et al., 2008). As presented in Figure 18, these granites fall within the plate granite (WPG) zone, indicating an extensional setting. This finding is consistent with the Jurassic-Cretaceous coastward migration of extensional magmatism in the South China Block (Li and Li, 2007; Qiu et al., 2017), during which mafic magmas produced by the partial melting of the asthenospheric mantle triggered the partial melting of lower crust materials and the following generation of felsic magmas. This conclusion accords with the presence of gabbro-basalt rocks (height: 5 km) in the middle crust (depth: 20 km) (Zhang et al., 2005; Zhang et al., 2013).

FIGURE 18

Pearce et al. (1984)). VAG. Volcanic-arc granite; syn-COLG. syn-collisional granite, WPG: within-plate granite; ORG: ocean-ridge granite.

6.3 Contributions of cretaceous granites to geothermal resources

Granites that have heat production rates >5 μW/m³ are classified as high-heat-producing granites, which have significant effects on the surface heat flow. The presence of high-heat-producing granites at depths of 1,650–2,980 m can increase the surface heat flow from the background value of 68 mW/m² to 93 mW/m², with an increase by 40% (Wang et al., 2023). Compared to magmas, radioactive heat causes a much lower increase in temperature but an extremely longer timespan of thermal anomalies. Mclaren et al. (1999) attributed the temperature increase of 40°C in the contact zone between plutons and host rocks to the decay of radiogenic elements. In the HGF, the Yanshanian granites exhibit heat production rates of 4.51–7.99 μW/m³, averaging 6.35 μW/m³ (Table 5). Therefore, they are high-heat-producing granites. Furthermore, the Cretaceous and Jurassic granites have average heat production rates of 6.63 μW/m³ and 6.06 μW/m³, respectively (Table 5).

According to lithological records and zircon ages (Tables 2, 6; Figures 6, 16), the concealed Cretaceous granites in the HGF occur at depths of 1,560–3,000 m in geothermal well HR-1 (Li et al., 2020b). The large volume of high-heat-producing Cretaceous granites (geothermal reservoir rocks) is an important heat source of geothermal resources in the study area. The surface heat flow of the HGF is 115.5 mW/m², about 1.6 times the background heat flow (73 mW/m²) (Xi et al., 2021). In sum, the heat production of Cretaceous granites plays a major role in the formation of geothermal resources in the HGF.

6.4 Magmatism evolution model

Previous studies summarized the regional petrogeological characteristics of granites based on granite samples collected from outcrops at a greater distance from HGF (G1, G2, G3, and G5 in Figure 2; Xiao et al., 2019). By comparison, all the samples used in this study were collected from or around geothermal wells (e.g., HR-1) to figure out the petrogenesis of geothermal reservoir rocks buried at depth and the magmatism evolution model of the HGF. As shown by the sampling locations and zircon ages of the samples (Figures 2, 5; Table 4), the ages of the Yanshanian granites exhibit a downward trend from the periphery to the center of the HGF (geothermal wells HR-1 and ZK8). In other words, the geothermal resources are hosted by relatively younger Yanshanian granites. Most especially, for Cretaceous granites, Xiao et al. (2019) obtained the emplacement ages ranging from 138.4 to 143.6 Ma based on the zircon dating of granite samples from G1, G2, and 300 m at the depth of ZK8 (Figure 2; Table 4). This study discovered that the reservoir granites at depth (∼3,000 m) intruded later than the upper and outer Cretaceous granites, with an emplacement age of ∼135 Ma. The reservoir granites exhibit the flattest REE pattern, indicating the strongest fractional crystallization among Cretaceous granites. This result accords with their higher heat generation. Their average heat production (HZ-1 and HZ-3, Table 4) is 6.44 μW/m³, while that of the upper (Id-33, 3.16 μW/m³) and outer (4.51 μW/m³) granites is much lower (representative samples: G1, G2; Figures 10–13). Regarding the Jurassic granites, two zircon ages were obtained by Xiao et al. (2019): 155.8 Ma for G3 and 176.7 Ma for G5. These ages indicate two episodes of Jurassic magmatism in the HGF. The intrusion ages of the Jurassic granites (HSD-1, HSD-4) exposed around the geothermal wells are consistent with those of granites (HSD-8) collected from G3 within the error range. This suggests that the Jurassic granites around the geothermal wells should be formed during the later Jurassic magmatic event (JS 2 in Figure 16) recorded by Xiao et al. (2019). Their average heat production rate (5.99 μW/m³, average value from HSD-1, HSD-4, HSD-8, and Id-04) is twice that (2.62 μW/m³) of granodiorites formed during the former Jurassic magmatic event. It is noteworthy that fine-grained equigranular biotite granites (HSD-4) intruded into HSD-1 (Figure 3) exhibit the strongest depletion of Ba, Nb, Sr, and Ti (Figure 12) and significant Eu anomalies (Figure 13) in the Jurassic granites, suggesting that they have undergone the strongest fractional differentiation (Chen et al., 2014; Liao et al., 2021). This implies that these granites are the latest phase of Jurassic granites.

In sum, the Yanshanian granites around geothermal wells were formed at the late stage of both the Jurassic and Cretaceous. The geochronological data of the granites in the HGF were integrated with the evolution model for plutonism in this study (Figure 19), yielding findings as follows. The Permian two-mica granites (G4 in Figure 2; Figure 19) intruded into the Paleozoic sandstone at the center of the HGF at 253–251 Ma. During the Jurassic, two magmatic events occurred. At 176.7 Ma, granodiorites were emplaced in the southeast of this study area (Jurassic stage 1), while the intrusion of biotite granites (G3 and G6 in Figure 2; Figure 19) occurred at 155.8–153 Ma (Jurassic stage 2). During the Cretaceous, three stages of magmatism occurred: 1) stage 1: biotite granites (G1) were exposed in the northwest of the HGF, with ages ranging from 143.6 to 140.9 Ma; 2) stage 2: biotite granites (G1) were in intrusive contact with granodiorites (G5) in the southeast of the HGF; 3) stage 3: biotite granite intruded under the two-mica granites and Paleozoic sandstones at 135–135.3 Ma as the reservoir rocks of geothermal resources in the HGF.

FIGURE 19

7 Conclusion

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