Introduction

It is well known that the Earth’s internal thermal energy is an indispensable driving force of plate tectonics and also plays a crucial role in the formation and evolution of the continental crust (Morgan 1984; Rybach 1988). Heat is generated at different ways in the interior of the Earth, of which radioactive heat production accounts for a relatively higher proportion (Arevalo et al. 2009; Pollack and Chapman 1977; Zhao et al. 2015). However, just radioelements like U, Th and 40K with relatively high abundance and long life can release considerable heat. At the same time, the U, Th and K are easily mobile and incompatible during geological processes, so they are not evenly distributed. Therefore, exploring their behaviors and distribution features is beneficial to our understanding on the geochemical and geothermal field. For the solid lithosphere, rock thermal conductivity is another important thermophysical parameter to characterize its heat transmission capacity (Clauser and Huenges 1995). Combined radiogenic heat production rate and rock thermal conductivity data, together with other geophysics data, can provide rigorous constraints on surface and mantle heat flow data, lithosphere thermal thickness and its thermal regime (He 2015; Jaupart et al. 1998; Mareschal et al. 2004).

Zhangzhou geothermal field, located in SE Fujian province along SE China coastal belt, is an important and representative middle-low temperature geothermal field. In the past decades, a large number of studies have been carried out and the results provide strict constraints on its type and genesis (Pang 1987, 2005; Wang 1993). However, we have very little knowledge about the lithosphere thermal regime of Zhangzhou region, which is crucial for understanding on the formation and evolution of the geothermal field.

In the current study, a large number of fresh rock samples from surface and drilling holes in the Zhangzhou region, SE China have been collected and determined involving rock density, radioactive heat-producing element content and thermal conductivity. Our new data, in combination with previous geology, geophysics and geothermics results, shed new light on radiogenic heat production features and the lithospheric thermal regime of the studied area, which is good for geothermal development and utilization.

Regional setting and sample collection

Zhangzhou region located in SE China along the southeastern margin of Eurasian continent is occupied by massive late Mesozoic volcanic and intrusive rocks, belonging to an important part of circum-Pacific magmatic belt (Li et al. 2014; Wang and Zhou 2002). Our target Zhangzhou pluton (or Zhangzhou complex) is a large composite rock body with covering area being larger than 900 km2, which is bounded by Changle-Nan’ao fault and Zhenghe-Dapu fault in the east and northwest, respectively (Fig. 1). Zhangzhou complex is surrounded/covered by early Jurassic Lishan Formation sedimentary rocks, late Jurassic Nanyuan Formation volcanic rocks, and Neogene Fotan Formation volcanic-clastic sedimentary rocks, which is further developed to form a Quaternary graben basin. According to petrological, geochemical and isotopic data, Wang and Zhou (2002) suggested that the late Mesozoic volcanic and intrusive rocks are closely associated in time, space and source, i.e., both volcanic and intrusive rocks were formed contemporaneously from the same source materials but with different occurrences and subsequent evolution processes. It is a consensus that the Fujian coastal magmatic belt including Zhangzhou complex is related to the subduction of the Palaeo-Pacific Plate beneath Eurasian continent (Li et al. 2014; Wang and Zhou 2002; Zhou and Li 2000).

Fig. 1
figure 1

Simplified geological map showing late Mesozoic igneous rock distribution in the coastal region of Fujian (modified from Li et al. 2014). Rectangle with dotted lines in a represents studied area of Zhangzhou Pluton (Complex), and b showing the geographical position of studied area in China

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Zhangzhou region is also rich in geothermal resources, belonging to an indispensible part of the “southeastern coastal geothermal zone” in China (Pang 1987; Wang 1993). A substantial number of hot springs are widely distributed in the whole area with the record of the highest temperature ever measured in southeast of China (114 °C at the well-head and 122 °C at a depth of 90 m from surface) (Pang 2005). On the basis of SiO2 geothermometer (mixing model) and other geochemical data, Pang (1987) and Wang (1993) thought that the reservoir temperature is about 140 °C with circulating depth at about 4 km, and they further suggested that the Zhangzhou geothermal system is a medium–low temperature convective type geothermal field, which is developed within a granitic batholith through the deep-circulating of meteoric water. However, Xiong et al. (1991) speculated that there is probably a partial melting magma body at the depth of Zhangzhou region in terms of seismic refraction profile observation.

There are many types of granitic rocks occurring in the Zhangzhou Complex, including coarse- to medium-grained granites, fine-grained granites, monzonitic granites, K-feldspar granites, biotite granites, granodiorite, granite porphyries, miarolitic granites, spherical weathered granites and hornblende-bearing granites. The field contact relationship between different types of granites (Fig. 2a), MMEs (mafic microgranular enclaves) occurring in the granitic rocks (Fig. 2b), and mafic dykes cutting the granitic rocks (Fig. 2c) has been observed in the field trip. Additionally, the direct contact relationship between nearly coeval intrusive and volcanic rocks has also been found (Fig. 2d). In the current study, 188 fresh surface granitic rocks, 11 and 27 granitic samples from drilling holes at depth range of 10–120 and 30–120 m within Liren School and Zhishan Park, and 99 surface non-granitic rocks including middle-acid volcanic rocks, basalts, mafic dykes, argillites and conglomerates have been collected.

Fig. 2
figure 2

Field photographs showing the features of Zhangzhou Complex

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Analytical methods

With the aim to calculate radiogenic heat production rates, 325 rock samples including 188 surface granites, 11 drilling hole granites from Liren School, 27 drilling hole granites from Zhishan Park, 51 middle-acid volcanic rocks and other 48 rock samples including basalts, mafic dykes, argillites and conglomerates have been carried out for rock density and radioactive heat-producing element content determination (Table 1). The rock density has been measured in the State Key Laboratory Breeding Base of Nuclear Resources and Environment, East China University of Technology by means of Archimedes’ principle (buoyancy method) with error margin being smaller than ±2 %. The radioactive Th, U and K concentrations are determined in the Guangzhou ALS Laboratory Group by ICP-MS and XRF, respectively. Analytical uncertainties range from ±1 to ±2 % for K, from ±1 to ±10 % for Th and U. The radiogenic heat production rates of the determined rocks are calculated according to Rybach (1988).

Table 1 The statistical rock density and radiogenic geochemistry features of the rocks from Zhangzhou region
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One hundred and thirty representative rock samples including 59 granitic rocks and 71 non-granitic rocks have been conducted for thermal conductivity determination in the State Key Laboratory Breeding Base of Nuclear Resources and Environment, East China University of Technology by TCi thermal conductivity analyzer with error margin smaller than ±5 %.

Result and discussion

The features of radiogenic heat production rates

The main statistical results of rock density, radiogenic heat-producing element contents are listed in Table 1. As shown in Table 1, the average densities of granite samples from surface and drilling holes (2.58–2.62 g/cm3) are slightly higher than those of middle-acid volcanic rocks and other non-granitic rocks (2.56–2.57 g/cm3). With the exception of K contents of drilling hole granites from Liren School, the radioelement U, Th and K contents of granitic rocks irrespective of surface or drilling holes are remarkably higher than those of middle-acid volcanic rocks and other non-granitic rocks. Our new data are in good agreement with previous results, which were determined by different analytical methods (Pang 1987; Zhao et al. 1995). Consequently, the radiogenic heat production rates of granitic rocks both from surface and drilling holes with average values of 3.52, 4.05 and 4.39 μW/m3 are systematically higher than those of middle-acid volcanic rocks and other non-granitic rocks with average values of 2.82 and 1.62 μW/m3 (Fig. 3). One explanation for the relatively higher radiogenic heat production rates of drilling hole granites may be the fact that circulating fluid bring some heat-producing elements. Although the radiogenic heat production rates of drilling hole granites are slightly higher than those of surface granites, yet the average value 3.70 μW/m3 of all granites is considered to be the radiogenic heat production rate of Zhangzhou Complex. Our new data confirm that the radiogenic heat production rates of granites from southeastern coastal region is markedly lower than those of granites from Nanling Region of South China (Sun et al. 2015; Wang et al. 2014; Zhao et al. 1995), but 1.5 times that of global average granite (McLaren et al. 2003).

Fig. 3
figure 3

Radiogenic heat production rates of different types rocks. It is noted that other non-granites represent basalts, mafic dykes, argillites and conglomerates

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Another important aspect is to explore the U, Th and K contributions to the radiogenic heat production rates of granites. The 40K contribution to the radiogenic heat production rates is generally less than 10 %, which is consistent with the analytical results from Southeastern Nigeria (Joshua et al. 2008). The radiogenic heat production rates are mainly from the decay heat of radioelements U and Th. In most samples, the U contribution is nearly equal to that of Th (Fig. 4).

Fig. 4
figure 4

U thermal contribution to radiogenic heat production rates normalized to that of K versus Th thermal contribution to radiogenic heat production rates normalized to that of K

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The features of rock thermal conductivities

One hundred and thirty representative rock samples including 59 granite samples and 71 non-granite samples have been carried out for thermal conductivity determination. The granites and non-granites exhibit average values of 3.19 and 2.52 W/mK, respectively, indicating that the thermal conductivity of granites is higher than that of non-granites in the studied area. The average value for all 130 rock samples is 2.83 W/mK, which is slightly higher than the recommended value 2.70 W/mK of middle-upper crust (Smithson and Decker 1974), and the previously determined value 2.47 W/mK from the same region by different analytical method (Pang 1987).

Lithospheric thermal regime of Zhangzhou region

The lithospheric thermal regime is crucial for our understanding on the lithospheric thermal evolution, heat flow and continental geochemical composition (Arevalo et al. 2009; He 2015; McLennan and Taylor 1996; Morgan 1984). Our new data, in combination with previous geology, geochemistry, geophysics and geothermics data, can provide strong constraints on lithospheric thermal regime.

Morgan (1984) proposed that the surface heat flow is mainly composed of crust heat flow (Qc) and mantle heat flow (Qm). The Qc is a first-order function of radiogenic heat-producing element contents, and the Qm mainly depends on the lithospheric thickness. Previous data show that the thickness of radioelement-rich crustal layer in Zhangzhou region is 10–12 km (Ma et al. 2015; Pang 1987), the geothermal gradient is 30–40 °C/km (Ma et al. 2015 and references therein). Additionally, the radiogenic heat production rate and thermal conductivity obtained in this study is 3.70 μW/m3 and 2.83 W/mK, respectively. Therefore, the calculated surface heat flow is in the range of 85–113 mW/m2, in good agreement with previously determined value of 90–115 mW/m2 (Hu et al. 2000, 2001; Wang and Huang 1988, 1990). The crust heat flow can be approximatively considered to be the product of thickness of radioelement-rich crustal layer (10–12 km) and the radiogenic heat production rate (3.70 μW/m3), whose value is 37–45 mW/m2. Thus, the calculated Qm is 40–75 mW/m2, which is larger than Qc with Qm/Qc ratio being more than 1. The Qm contribution to surface heat flow is 55–65 %. Previous Sr–Nd isotopic geochemistry data also indicate that ca. 60 % mantle-derived materials are involved in the granite magma resource (Zhou et al. 1988). The high Qm value implies that the lithospheric thickness in Zhangzhou region is relatively thin. Geophysical data confirm that the crustal thickness is about 29 km (Xiong et al. 1991), which is smaller than the normal crust thickness. Briefly, relatively thinner lithosphere in Zhangzhou region triggers mantle upwelling, which contributes largely to the surface heat flow.

Conclusions

Some important conclusions can be addressed in the following:

  1. 1.

    The average radiogenic heat production rate for more than 200 granite samples from surface and drilling holes in Zhangzhou region, SE China, is 3.70 μW/m3, 1.5 times higher than that of global average granites.

  2. 2.

    Thermal conductivity of middle-upper crust rocks from Zhangzhou region is 2.83 W/mK, in consistent with the recommended value of global middle-upper crust.

  3. 3.

    The Qm/Qc ratio in the region is larger than 1, implying that the mantle heat flow contribution is larger than that of crust heat flow and the crust thickness is relatively thinner.