High-Temperature Geothermal Source in the Northeastern Datong Basin, North China: Evidence from the Drilled Rhyolite

0. INTRODUCTION

The high-temperature (HT) geothermal resources from the earth interior have been considered as a kind of clean and regenerated energy to resolve the growing issues of energy consumption and environmental pollution (e.g., Lund et al., 2022). Based on the geological background and forming mechanism, geothermal resources are commonly divided into the shallow, hydrothermal, dry hot rock and magmatic types (e.g., Zhang et al., 2024; Mao et al., 2019). The Datong Basin, a representative Cenozoic rift basin in northern Shanxi Province, contains abundant shallow geothermal resources in the North China Craton (NCC) (e.g., Zhou et al., 2020). Recently, the HT geothermal fluid with wellhead temperature of ~160 ℃ was acquired from the drillhole depth of ~1 620 m in Tianzhen area, northeastern Datong Basin. The obtained fluid is characterized by the highest temperature and the maximum discharge (~230 m³/h) at the similar depth in the NCC, representing an enormous breakthrough of the deep geothermal resources exploring in mid-eastern China. Due to the lack of detailed deep geological investigation, geothermal-forming mechanism as well as nature of heat source around the Tianzhen area in Datong Basin is unclear, which greatly limits our understanding on formation of HT geothermal resources within Cenozoic rift basin in the NCC as well as the geothermal utilization.

The rock samples collected from geological drill core provide valuable information on formation of the HT geothermal resources. The crystallized temperature of the rhyolitic sample is calculated based on the ternary-feldspar geothermometer. The whole rock compositions of the rhyolite, tuff, and their surrounding rocks, including tonalitic gneiss and basalt, are also analyzed to constrain the origin of rhyolite. In this contribution, we firstly present the petrological features, mineral composition, and geochemical data of rhyolitic samples from a geothermal drill core in the Tianzhen area. Combined with the tectonic background of the Datong Basin during the Cenozoic, we discuss the possible petrogenesis of the investigated rhyolitic magma and related subvolcanic system. These investigations provide some key evidences to constrain the geothermal-forming mechanism of large-scale Cenozoic basin in the NCC. The result is of important practical value in the drillhole and utilization of HT geothermal resources in the Tianzhen area, Datong Basin.

1. GEOLOGICAL SETTING AND SAMPLING

The NCC, one of the world's oldest cratons, is commonly divided into three litho-tectonic units, including Western Block, Eastern Block and Trans-North China Orogen (TNCO) (e.g., Zhao et al., 2005). The Datong Basin occurs as a representative large-scale Cenozoic basin in the northern part of TNCO. This study focuses on the Tianzhen area, the northeastern Datong Basin, were Archean gneissic complexes and Quaternary strata are distributed as the basement and sedimentary cover, respectively (Figure S1). The Archean gneissic complexes, locally referred to as the Gehuyao Formation, mainly consists of tonalitic-granodioritic gneisses and some mafic rocks. The protolith of the Gehuyao Formation was formed at ca. 2.5 Ga by U-Pb dating of magmatic zircons, and went through a Paleoproterozoic (ca. 1.8–1.9 Ga) high-pressure granulite-facies metamorphism (e.g., Zhang et al., 2023; Guo et al., 2002). Quaternary sediments mainly consist of loess-paleosols and lacustrine clay. In addition, some Cenozoic basaltic units also occur in the southeastern Tianzhen area, locally corresponding to the Xujiayao Formation, were determined to be ~200 000 years by paleomagnetism and K-Ar dating (e.g., Ao et al., 2017).

The KT-8 geothermal drillhole is located on the northern side of the Tianzhen area(Figure S1). Notably, the measured temperature was greater than 150 ℃ at a depth of ~120 m, showing the highest temperature area at the similar depth within the Datong Basin. The samples of tuff and rhyolite were collected from the KT-8 at depth of 76 and 120 m, respectively. The tuff (KT-8-7) exhibits gray-white color, and volcanic tuff texture. The crystal grains are composed of quartz, plagioclase, and K-feldspar. Calcite grains are occasionally observed. The rhyolite (KT-8-12) displays purplish-gray color and volcanoclastic texture. The phenocryst mainly includes quartz, perthite, and biotite, whereas predominant substrate consists of cryptocrystalline to microcrystalline felsic minerals (e.g., plagioclase and quartz) and volcanic glass (Figure 1a). The quartz grains show rounded shape with grain size ranging from 100 to 500 μm, and biotite occurs in platy or columnar forms. The perthites, 50–300 μm in diameter, exhibit the hypidiomorphic granular texture, in which K-feldspar and plagioclase occur as the host crystal and the accessory crystal, respectively (Figure 1b). Based on the occurrence and petrological features, we suggest that the investigated rhyolite and tuff are generated at the same magmatic event. In addition, in order to determine the petrogenesis relationship between the investigated rhyolite and its wall rocks, two typical rock units, Archean tonalitic gneiss (Gehuyao Formation) (GR-1-803) and Quaternary basalt (Xujiayao Formation) (XXY-1, XXY-3), are also selected and analyzed in this study.

Figure  1.  (a) Photomicrographs of rhyolitic samples; (b) BSE image of the representative grain of perthite; (c) plots of ternary-feldspar thermometer; (d) ⁸⁷Sr/⁸⁶Sr versus ¹⁴³Nd/¹⁴⁴Nd of the investigated samples. Compositions of basalt from Datong Basin are from Zhou et al. (2024) and data of Huai'an complex is based on Zhang et al. (2023).

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2. ANALYTICAL TECHNIQUES

Major element analyses of perthite were analyzed by a JEOL JXA-8230 electron microprobe housed at the Wuhan Microbeam Analysis Technology Co., Ltd. The major elements and trace elements of whole-rock were determined by a wavelength-dispersion XRF-1800 spectrometer and an Agilent 7700e inductive coupled plasma mass spectrometry (ICP-MS) at the Wuhan Sample Solution Analytical Technology Ltd., respectively. Sr-Nd isotopic compositions of bulk rock were undertaken by a MC-ICP-MS analytical instrument (Neptune Plus) housed at the Wuhan Sample Solution Analytical Technology Ltd.

Details of the analytical methods are showed in the Supplementary Material Appendix A.

3. RESULTS

3.1 Whole-Rock Geochemistry

The tuff (KT-8-7) has 65.75 wt.% SiO₂, 10.89 wt.% Al₂O₃, 3.32 wt.% Fe₂O₃, 5.37 wt.% CaO and 6.78 wt.% Na₂O + K₂O, corresponding to the intermediate-acid volcanics (Table S1). This sample is characterized by enrichment in light rare earth element (LREE) content and depletion in heavy rare earth element (HREE) content with (La/Yb)_N ratio of 18.7. The rhyolite (KT-8-12) contains 67.57 wt.% SiO₂, 12.92 wt.% Al₂O₃, 5.54 wt.% Fe₂O₃, 2.19 wt.% CaO and 6.36 wt.% Na₂O+K₂O, showing an acid volcanoclastic rock with high-K calc-alkaline and peraluminous features (mol. Al₂O₃ /[CaO + Na₂O + K₂O] =1.11) (Figure S2). This sample is enriched in LREEs (Rb and Pb) and depleted in HFSEs (Ta, Nb, and Ti) (Figure S3).

The tonalitic gneiss (GR-1-803) has high contents of Al₂O₃ and Na₂O and shows significant LREE depletion with (La/Yb)_N = 16.9 and enrichment of LILEs (e.g., Rb, Pb and U). The basalt samples (XXY-1, XXY-2) have high alkali contents and enrichment of LREE. They are also marked by enrichment in Nb, Ta and large ion lithophile elements (LILEs) (Figure S3).

3.2 Whole-Rock Sr-Nd Isotopes

Whole-rock Sr-Nd isotopic compositions of the rhyolite, gneiss, basalt and tuff samples are shown in Table S1. The rhyolite has high ⁸⁷Sr/⁸⁶Sr ratio of 0.713 423 and ¹⁴³Nd/¹⁴⁴Nd ratio of 0.511 454 with negative ε_Nd (assuming t = 0.2 Ma) value of -23.1 and t_DM2 age of 2 690 Ma. The tuff has ⁸⁷Sr/⁸⁶Sr ratio of 0.712 986 and ¹⁴³Nd/¹⁴⁴Nd ratio of 0.511 249. They are similar to that of the tonalitic gneiss, such as ⁸⁷Sr/⁸⁶Sr ratio of 0.705 395 and ¹⁴³Nd/¹⁴⁴Nd ratio of 0.511 241. However, two samples of the basalt have the different Sr-Nd isotopic compositions with ⁸⁷Sr/⁸⁶Sr ratio of 0.703 620 and ¹⁴³Nd/¹⁴⁴Nd ratio of 0.512 962 (Figure 1d).

3.3 Mineral Major Element Compositions

Major element composition of plagioclase and K-feldspar was analyzed from representative grains of perthite. The K-feldspars have 14.04 wt.%–15.49 wt.% K₂O and 0.94 wt.%–1.75 wt.% Na₂O, with Or content ranging from 0.82 to 0.91. The plagioclases contain 6.57 wt.%–8.12 wt.% Na₂O and 3.61 wt.%–7.58 wt.% CaO, which have compositions of An_0.17–0.37Ab_0.56–0.71Or_0.09–0.19, thus corresponding to andesine and oligoclase (Table S2).

4. DISCUSSION

4.1 Crystallized Temperature of Rhyolitic Magma

Perthite occurs as phenocryst in the rhyolitic sample (KT-8-12), providing excellent tool to constrain its crystallized temperature through the geothermometer of ternary-feldspar (e.g., Wu et al., 2023). By mean of SEM images and photoshop software, area ratios (equivalent to volume ratio) of K-feldspar and plagioclase within perthite grains are estimated as 91%–96% and 4%–9%, respectively. Combined with area ratio and mineral density (ρ_pl = 2.67 g/cm³ and ρ_kfs = 2.57 g/cm³), mass fractions of plagioclase and K-feldspar in original solid solution of feldspars vary from 0.04 to 0.09 and 0.91 to 0.96, respectively. And then, the reconstructed compositions of original feldspars are An_0.02–0.04Ab_0.13–0.18Or_0.78–0.85. Geophysical investigations show that some high resistance bodies present beneath the basin at depth of ~10 km, representing occurrences of the younger igneous intrusion. Thus, assuming 0.3 GPa as the intrusive pressure for the rhyolitic magma, crystallized temperatures of feldspar ranging from 600 to 800 ℃ are obtained using Solvcalc2 software (e.g., Wen and Nekvasil, 1994) and the model proposed by Fuhrman and Lindsley (1988) (Figure 1c). It agrees with the lowest crystallized temperatures of fluid-present granitic magma, known as the range of 650–700 ℃. Considering the crystallized sequence of feldspar in the granitic magma, a temperatures range of 600 to 800 ℃ with average value of 700 ℃ may represent crystallized temperature of rhyolitic magma beneath the studying area.

4.2 Origin of the Rhyolitic Magma

Formation of rhyolite in the continental rift basin is primarily attributed to the following mechanisms (e.g., Halder et al., 2021): (1) assimilation-fractional crystallization (AFC) of mantle-derived melt; (2) crust-mantle magma mixing; (3) partial melting of lower continental crust. As to the investigated sample of rhyolite and tuff in the studying area, petrological features and geochemical data are used to distinguish these possible petrogenesis.

Firstly, rhyolite and tuff have high content of SiO₂, low content of MgO and TFe₂O₃, and show relatively closer compositions of trace elements and Sr-Nd isotope. It suggests that these rocks may derive from crustal sources or AFC of mantle-derived melt, rather than from crust-mantle magma mixing with geochemical variations (e.g., Li et al., 2022). It is agreement with our petrological observation that no mafic mineral presents.

Secondly, rhyolite and tuff show significant depletion of Nb and Ta, which is inconsistent with characteristics of mantle-derived magma with enrichment of Nb and Ta. It indicates that these samples are unlikely to have originated from AFC of mantle-derived magma (e.g., He et al., 2022).

Moreover, the investigated rhyolite displays enrichment in Rb and U, low Nb/U ratio (9.04), negative anomalies of Ta, Nb, Ti and Sr, and strong positive Pb anomaly (Figure S3). These features are strikingly similar to that of crustal-derived melt (e.g., Halder et al., 2021), thus suggesting that the rhyolitic magma may be originated by partial melting of crustal source. In addition, whole-rock Sr-Nd isotope of rhyolite and tuff is very close to that of tonalitic-granodioritic gneisses. In the whole-rock Sr-Nd isotope concordial diagram (Figure 1d), samples of rhyolite and tuff are plotted in the region of basement rocks (Huai'an Complex) around the Tianzhen area, and align with of the Gehuyao Formation (e.g., Zhang et al., 2023). They have similar negative ε_Nd(t) values (-27.3 to -23.1) and t_DM2 age (2 690 to 3 025 Ma) (Table S1), further supporting such rhyolitic magma would be have derived from partial melting of Archean gneiss in the studying area.

4.3 Implication for Geothermal Source in Northeastern Datong Basin

It is well known that the development of volcanic group within the Datong Basin is a significant tectonomagmatic event in the NCC during the Cenozoic (e.g., Zhou et al., 2024; Xu et al., 2005), corresponding to the strongly crustal extension and lithospheric thinning. Although the numerous occurrences of geothermal anomalies in the northeastern Datong Basin and its adjacent areas, the components of the HT geothermal system, including the heating source, internal structure and geothermal reservoirs, are still obscure due to the overburden effect of the Quaternary layer. The HT geothermal resources beneath the Datong Basin was usually considered as the important result associated with eruption of basaltic magma from the asthenosphere upwelling (e.g., Zhou et al., 2020).

Here, samples of rhyolite and tuff is identified from the geothermal drillhole of KT-8, providing the direct evidence for the presence of subvolcanic rocks beneath the Quaternary layer. According to our knowledge, no similar rock units have reported in the northeastern Datong Basin. Geological survey shows that the Quaternary basalt of the Xujiayao Formation mainly occur around the southern segment of the Tianzhen area. Combined with geochemical analysis, there are no obvious petrogenesis relationship between the Xujiayao basalt and the investigated rhyolite, which show geochemical affinity with its country rocks, that is, Archean gneissic complexes. Therefore, one possible tectonomagmatic evolution that is responsible for the formation of the HT geothermal system has been proposed as follows: the basaltic volcanos, such as the Xujiayao Formation, were erupted during the crustal extension and lithospheric thinning during Quaternary, and then, partial melting of Archean gneissic complexes would be taken place through the deep emplacement of basaltic magma (Figure 2). As the result, the rhyolitic magma was produced and segregated to shallow level of the upper crust along the developed active faults in the Tianzhen area.

Figure  2.  The proposed tectonic model for the generation of rhyolitic magma, which can be as a possible heat source in the studying area.

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Furthermore, the geothermometer of ternary-feldspar in rhyolitic sample yields the crystallized temperatures ranging from 600 to 800 ℃, indicating that the cooling temperature of such rock can be compatible with crystallized temperature of normal granitic magma. Considering its HT condition as well as depth of emplacement, we thus suggest that such rhyolitic magma would be serves as the heat source for the HT geothermal system in the Tianzhen area. Meanwhile, the Quaternary sediments within the basin can be as the excellent overlying strata to keep the slow cooling of rhyolitic chamber due to its low thermal conductivity, and the Archean gneissic complexes can serve as the geothermal reservoirs for the HT geothermal system. Thus, our study suggests that magmatic evolution could be a key factor to facilitate formation of HT geothermal resources within the Cenozoic rift basin.

5. CONCLUSION

We have identified the rhyolitic samples from the HT geothermal drillhole KT-8 in the Tianzhen area, northeastern Datong Basin. Such rock consists of phenocryst grain of quartz, perthite, and biotite with minor volcanic glass. The geothermometer of ternary-feldspar yields the crystallized temperatures ranging from 600 to 800 ℃ with average value of 700 ℃, representing the crystallized temperature of rhyolitic magma. According to major and trace element characteristics as well as Sr-Nd isotope, such magma likely derived from partial melting of Archean gneissic complexes that was triggered by the segregation of mantle-derived basalts in the studying area. The magmatic evolution is important link between deeply magmatic source and shallow volcanic activity. Combined with tectonic background and geothermal investigation, the rhyolite and its surrounding gneissic complexes would be as the source and reservoir for the HT geothermal system in the Tianzhen area, respectively. The final publication is available at Springer via https://doi.org/10.1007/s12583-024-0035-y.