Introduction

Since Neoproterozoic, multi-stage tectono-magmatic activities were reported in South China Craton (Ren et al. 2013). In South China, magmatic activities were most developed during Jurassic, and a variety of granitic magma intruded South China, including I-type, S-type, and A-type granites. With strong magmatic activity, a large number of non-ferrous metal deposits were formed. W-Sn deposits are the most typical deposits in the Nanling metallogenic belt (Mao et al. 2009, 2008; Chen et al. 2014; Shu et al. 2021; Sun 2018; Xia et al. 2021). The outcropping area of the Zhuguangshan batholith is more than 5000 km2, it is exposed in the central part of the Nanling tectonic belt, and the batholith is composed of multiple Caledonian, Indosinian, and Yanshanian granitic plutons forming the Wanyangshan and Zhuguangshan at the boundary of Hunan, Jiangxi, and Guangdong (Zhou 2007). The Yanshanian period is an important metallogenic period, which consists of three stages of 150–160 Ma, 140–130 Ma, and 100–90 Ma (Mao et al. 2008; Fan et al. 2017). Minerals are developed in the southern part of the Zhuguangshan batholith, especially tungsten and tin minerals, and the research is detailed, while the northern part is relatively lacking due to the lack of minerals. Guangnan pluton is located in center east part of the Zhuguangshan batholith (Fig. 1). Researchers have little research on the Guangnan pluton; they mainly focused on the division of intrusive rock units and chronology (Zheng 1988; Li 1990; Guo et al. 2017). In this paper, we carried out comprehensive investigations and selected typical samples on field observation (Fig. 2), zircon laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U–Pb age dating, the characteristics of Hf isotopic, and whole-rock major and trace elements analysis, and combined with previous research results, to determine the formation age, geochemical characteristics, and geological significance of Guangnan pluton and provide conditions for the mineralogy in the northern part.

Fig. 1
figure 1

Geological schematic diagram of Zhuguangshan (modified after 1: 500,000 scale geological and mineral map of the Nanling area (Hunan Geological Survey, 2002) and distribution map of granite in south China (Zhou 2007). The pluton in the dark blue area is the Guangnan pluton. The data of the green stars quoted from Guo et al. (2017), and the yellow stars is this paper

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Fig. 2
figure 2

The typical sample of Guangnan pluton. Figures (a), (b), and (c) are for biotite granite (H0405-3 and H0403-10), two-mica granite (H0405-10), and garnet biotite granite (H0405-7), respectively. Figures (a2), (b2), and (c2) are images of the biotite granite, two-mica granite, and garnet biotite granite under a single polarizer. Figures (a3), (b3), and (c3) are crossed polarizers, respectively

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Geological background

During the Cambrian to Quaternary, the South China block has experienced the combined effect of multiple tectonic–magmatic cycles (Ren et al. 1990), including Caledonian cycle, Indosinian cycle, Yanshanian cycle, and the Himalayan cycle. During the Cambrian-Ordovician, the South China block was rifted, and the subsidence reached the strongest in the Early-Middle Ordovician, forming a huge thick marine sandy and muddy terrigenous clastic source clastic rock series with graptolite sheets and siliceous shale simultaneously. During the late Ordovician to early Silurian period, South China was involved in the Caledonian orogenic movement and developed large-scale granitic magmatic intrusions. The Caledonian orogenic movement led to the uplift of the South China fold and caused a regional angular unconformity between the Early Devonian or the later strata and the underlying strata, which including the Early Paleozoic or older strata. Then, the South China tectonic evolution entered the Post-Caledonian quasi-platform development stage (Ren and Li 2016). During the Indosinian, the basement of South China is influenced by the orogenic movement and caused the folds and fractures deformation, and accompanied by the syn-orogenic granitic magmatism simultaneously. On the Early Jurassic, South China was affected by local rifting and bimodal magmatic activity occurred in the region of southwestern Fujian-Southern to Jiangxi-Southern and Hunan (Chen et al. 1999; He et al. 2010). The sedimentary environment was generally transformed from marine facies to terrestrial facies in South China, except for the coast of Fujian and Guangdong. During middle Jurassic to the Early Cretaceous, the collision orogeny of the western Pacific ancient continent and the eastern margin of the Asian continent led to strong fold uplift and reverse faulting activities in South China, and large-scale granitic magma intrusions. At the same time, the materials of the Indosinian or earlier occurred extrusion folds, which were further strengthened, resulting in numerous reverse faults and related structures. During the Early Cretaceous, strike-slip and shear-extension deformations were developed and formed a series of faulted basins. Large-scale volcanic rocks developed in the Zhejiang-Fujian and Lower Yangtze regions, and A-type granite started to intrude at the coastal areas (Xing et al. 2008; Shu et al. 2009; Shu 2012; Liu et al. 2020). Since the Paleogene, the tectonic background of South China turned into the stage of extensional deformation. Because of the affection of the collision between the Philippine Sea plate and Taiwan, South China developed extrusion and deformation during the Neogene. Strong compression formed a reverse fault in the region and the red bed folded slightly, whereas the coast of Fujian and Guangdong and the continental shelf of the South China Sea continued to extend, rifts, with newly production of the basalt.

Nanling is the most developed Phanerozoic magmatism area in the South China orogenic belt. The granitic pluton at Nanling area is widely distributed and is characterized of multi-stage intrusion. It is also a mining area for rare and non-ferrous metals such as tungsten, tin, niobium, and tantalum (Zhou et al. 2006; Mao et al. 2009, 2008; Chen et al. 2014). The Zhuguangshan batholith, which is an important batholith that concentrated non-ferrous metal and uranium deposits in the Nanling magma-metallogenic belt, is located at the core of the South China orogenic belt. The southern half of the Zhuguangshan batholith (i.e., the southern body) outcropped in east–west direction, and exposed many Yanshanian plutons such as Jiufeng pluton, and also exposed the Caledonian and Indosinian plutons. The long axis direction of the northern half of the base (namely the northern body) outcropped in north–south direction, and the northern body is mainly composed of Caledonian and Indosinian intrusions, but Yanshanian granite plutons are less exposed compared to southern body.

Guangnan pluton is located at the junction of the Southern and the Northern bodies of the Zhuguangshan batholith, and extended into northeast in long axis under 2D view with an irregular shape pluton (Fig. 1). Its surrounding rocks are dominated by Caledonian granodiorite and biotite granite in the northwest and Sinian-Cambrian strata in the southwest, featuring superimposed fold structures in NE and NW direction. The southeast part of the pluton intruded into the Cambrian-Ordovician strata.

Petrography characteristics

Guangnan pluton is mainly composed of biotite granite, garnet biotite granite, and muscovite-bearing granite (Fig. 2).

Biotite granite (H0403-10) is of gray-white in color and porphyritic structure with medium coarse in sizes; these rocks are often weathered and loose in shape. The phenocrysts are mainly gray-white colorless K-feldspar (8%) with 2–4 cm in size, which can tell Castellin double crystal combined pattern clearly by naked eyes. The matrix minerals are 0.1–0.3 cm in size with medium- and fine-grained unequal grain structure and are mainly composed of quartz, biotite, K-feldspar, and plagioclase with accessory minerals including appatite, zircon, and spinel. The quartz (25% of the matrix) is anhedral in shape and 0.1–0.2 cm in size, and appears as quartz aggregation in some section. The biotite (10%) is dark in color and euhedral in shape with around 0.2–0.8 cm in size. The K-feldspar (40%) is euhedral plate in shape and 0.2–0.3 cm in size. The Plagioclase (15%) is euhedral off-white wide plate in shape and 0.2–0.3 cm in size.

Garnet biotite granite is of gray-white in color and porphyritic structure with medium fine in sizes. The phenocrysts of the rocks are mainly gray-white colorless K-feldspar (5–8%) with 0.2–0.8 cm in size. The matrix minerals are 0.01–0.02 cm in size with fine-grained structure and are mainly composed of quartz, biotite, K-feldspar, and plagioclase with accessory minerals including appetite, zircon, and spinel. The quartz (25%) of the matrix is anhedral in shape and 0.01 cm in size. The biotite (~ 5%) is dark brown in color and euhedral in shape with around 0.05 – 0.07 cm in size. The K-feldspar (~ 35%) is euhedral plate in shape and 0.2–0.3 cm in size. The plagioclase (~ 25%) is euhedral off-white wide plate in shape and 0.2–0.3 cm in size. The garnet (1 ~ 3%) is colorless with 0.02–0.08 cm in size.

Two-mica granite is of light red to gray-white in color and medium coarse in mineral sizes, porphyritic structure. The rocks weathered strongly in the field, and often loose in shape. The phenocrysts are mainly gray-white colorless K-feldspar (10%) and plagioclase (1–3%) with 2–4 cm in size, which can tell Castellin double crystal combined pattern clearly for K-feldspar and ring-shaped structure for plagioclase by naked eyes. The matrix minerals are 0.5–1.0 cm in size with coarse-grained unequal grain structure and are mainly composed of quartz, biotite, K-feldspar, and plagioclase with accessory minerals including appatite, zircon, and spinel. The quartz (30% of the matrix) is anhedral in shape and ~ 0.6–0.8 cm in size. The biotite (5–8%) is dark brown in color and euhedral in shape with around 0.6–0.8 cm in size. The muscovite (~ 1%) is colorless flake aggregates in shape with around 0.5–0.6 cm in size. The K-feldspar (30%) is euhedral plate in shape and 0.8–1.0 cm in size. The plagioclase (20%) is euhedral off-white wide plate in shape and 0.8–1.0 cm in size.

Sampling and analytical procedures

Zircon U–Pb dating, Lu–Hf isotope, and major and trace element compositions were conducted by LA-ICP-MS at Beijing Createch Testing Technology Co., Ltd.

Zircon U–Pb dating of the detailed operating conditions for the laser ablation system, the ICP-MS instrument, and data reduction are the same as described by Hou et al. (2009). Laser sampling was performed using a RESOlution 193 nm laser ablation system. The instrument used to acquire ion signal intensities was Analytik Jena PlasmaQuant MS Elite ICP-MS. Helium was applied as a carrier gas. Each analysis data acquisition from the sample incorporated a background acquisition of approximately 15–20 s (gas blank) followed by 45 s. Off-line raw data selection and integration of background and analyte signals and time drift correction and quantitative calibration for U–Pb dating were performed by ICPMSDataCal (Liu et al. 2010). Zircon GJ-1 was used as external standard for U–Pb dating, and was analyzed twice every 5 ~ 10 analyses. Time-dependent drifts of U-Th-Pb isotopic ratios were corrected using a linear interpolation (with time) for every 5–10 analyses according to the variations of GJ-1 (i.e., 2 zircon GJ-1 + 5–10 samples + 2 zircon GJ-1) (Liu et al. 2010). Uncertainty of preferred values for the external standard GJ-1 was propagated to the ultimate results of the samples. In all analyzed zircon grains, the common Pb correction was not necessary due to the low signal of common 204Pb and high 206Pb/204Pb. U, Th, and Pb concentration was calibrated by NIST 610. Concordia diagrams and weighted mean calculations were made using Isoplot/Ex_ver3.

Zircon in situ Hf isotope analysis was carried out using a RESOlution SE 193 nm laser ablation system attached to a Thermo Fisher Scientific Neptune Plus multi collector (MC-ICP-MS). Instrumental conditions and data acquisition protocols were described by Hou et al. (2009). A stationary spot used a beam diameter of ~ 38 μm. As the carrier gas, helim was used to transport the ablated sample aerosol mixed with argon from the laser ablation cell to the MC-ICP-MS torch by a mixing chamber. 176Lu/175Lu = 0.02658 and 172Yb/173Yb = 0.796218 ratios were determined to correct for the isobaric interferences of 176Lu and 176Yb on 176Hf. For instrumental mass bias correction, Yb isotope ratios were normalized to 172Yb/173Yb = 1.35274 and Hf isotope ratios to 179Hf/177Hf = 0.7325 using an exponential law. The mass bias behavior of Lu was assumed to follow that of Yb; the mass bias correction protocol was described by Hou et al. (2007).

The major and trace element compositions were determined by X-ray fluorescence (XRF-1800, SHIMADZU) on fused glasses and inductively coupled plasma mass spectrometry (Element XR, Thermo) after acid digestion of samples in Teflon bombs. Loss on ignition was measured after heating to 1000℃ for 3 h in a muffle furnace. The precision of the XRF analyses is within ± 2% for the oxides greater than 0.5 wt.% and within ± 5% for the oxides greater than 0.1 wt.%. Sample powders (about 50 mg) were dissolved in Teflon bombs using a HF + HNO3 mixture for 48 h at about 190℃. The solution was evaporated to incipient dryness, dissolved by concentrated HNO3 and evaporated at 150℃ to dispel the fluorides. The samples were diluted to about 100 g for analysis after redissolved in 30% HNO3 overnight. An internal standard solution containing the element Rh was used to monitor signal drift during analysis. Analytical results for USGS standards indicated that the uncertainties for most elements were within 5%.

Fig. 3
figure 3

Figures of the Zircon test site of samples H0403-10, H0405-10, and H0405-8. Yellow rings are the U–Pb test site and the red rings are the Hf isotope test site

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Results

Zircon U–Pb dating

Coarse-grained porphyritic biotite granite (sample H0405-8)

Zircon grains in sample H0405-8 are euhedral, with the lengths of 100 to 150 μm and the length/width ratio of 1.5 to 4. It can be found that some grains are fine oscillatory growth zoning from the CL images (Fig. 4a). 232Th/238U ratios range from 0.14 to 1.06. All the characteristics indicate they are formed by the magmatic origin. There were 23 zircons tested, and all of them were analyzed at the edge of single-grain zircon (Fig. 3). The test data results are presented in Table 1. The results of H0405-8–06, H0405-8–13, and H0405-8–15 were abandoned because of their serious signal loss leading harmony degree less than 90%. The results of H0405-8–01 (481.5 Ma) and H0405-8–12 (534.9 Ma) are significantly older; these features can be interpreted as captured Caledonian magmatic zircons during Jurassic magmatic activity. The results of H0405-8–05 (204.3 Ma), H0405-8–11 (266.2 Ma), and H0405-8–12 (218.4 Ma) can be interpreted as captured Caledonian and Indosinian magmatic zircons during Jurassic magmatic activity. The results data showed that the 15 points are plotted on or close to the coordination line in the coordination diagram (Fig. 4a), with weighted mean age at 164.4 ± 3.4 Ma (MSWD = 2.1, n = 15, confidence 95% (Fig. 4c)), which can be interpreted as crystallization age of the rock.

Fig. 4
figure 4

Zircon U–Pb concordia diagram for samples H0403-10, H0405-10, and H0405-8 from the Guangnan pluton

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Table 1 Results of Zircon U–Pb dating in granite
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Two-mica granite (sample H0405-10)

Zircon grains of sample H0405-10 are euhedral, with the lengths of 80 to 150 μm and the length/width ratio of 1.5 to 3. It can be found that some grains are fine oscillatory growth zoning from the CL images (Fig. 4a). 232Th/238U ratios range from 0.53 to 1.59. There were 22 zircons tested, and all of them were analyzed at the edge of single-grain zircon. H0405-10–1, H0405-10–2, and H0405-10–14 were abandoned because of their serious signal loss leading harmony degree less than 90%. The results data showed that the 19 points are plotted on or close to the coordination line in the coordination diagram (Fig. 4b). Based on the age results, the zircon ages can be divided into two groups. The 206Pb/238U ages of the first group yield between 151.1 and 168.4 Ma, with weighted mean age at 159.1 ± 4.1 Ma (MSWD = 5.4, n = 10.95% confidence) (Fig. 4a), which can be interpreted as crystallization age of the rock. The second group yield between 483.7 and 415.9 Ma including spots H0405-10–03, H0405-10–04, H0405-10–07, H0405-10–09, H0405-10–10, H0405-10–13, H0405-10–16, H0405-10–17, and H0405-10–20. These ages, compared with the first group, are significantly older, and these zircons showed many fine oscillation rings. The features can be interpreted as captured Caledonian magmatic zircons during Jurassic magmatic activity.

Biotite granite (sample H0403-10)

Zircon grains of sample H0405-10 are euhedral, with the lengths of 200 to 500 μm and the length/width ratio of 1.5 to 3. It can be found that some grains are fine oscillatory growth zoning from the CL images (Fig. 4a). 232Th/238U ratios range from 0.23 to 1.43. There were 20 zircons tested and were analyzed at the edge of single-grain zircon. H0403-10–03, H0403-10–09, H0403-10–19, and H0403-10–20 were abandoned because of their serious signal loss leading harmony degree less than 90%. The results data showed that the 16 points are plotted on or close to the coordination line in the coordination diagram (Fig. 4c). Based on the age results, the 206Pb/238U ages of the 16 points yield between 150.7 and 163.8 Ma, with weighted mean age at 153.9 ± 1.8 Ma (MSWD = 1.5, n = 16, the reliability is 95% confidence (Fig. 4b)), which can be interpreted as crystallization age of the rock. The second group yield between 483.7 and 415.9 Ma including spots H0405-10–03, H0405-10–04, H0405-10–07, H0405-10–09, H0405-10–10, H0405-10–13, H0405-10–16, H0405-10–17, and H0405-10–20. These ages, compared with the first group, are significantly older, and these zircons showed many fine oscillation rings. The features can be interpreted as captured Caledonian magmatic zircons during Jurassic magmatic activity (Figs. 5,6,7,8,9).

Fig. 5
figure 5

a Diagrams of TAS (modified after Middlemost 1994), b A/CNK vs. A/NK (after Maniar and Piccolli 1989), c Amazonia silica-rich granite bodies (modified after Frost et al. 2016), d CaO/Al2O3 vs. 1/TiO2, e CaO/Na2O vs. 1/TiO2, and f (Na2O + K2O)/CaO vs. 1/TiO2 (modified after Sun 2018. Exing 1 to Exing 5 based on 1:200,000 regional geological survey report 1969)

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Fig. 6
figure 6

a primitive mantle-normalized element spider diagram, normalizing values from Sun and McDonough (1989); b Chondrite-normalized REE patterns for the Guangnan pluton, normalizing values from Sun and McDonough (1989)

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Fig. 7
figure 7

Diagrams of the Guangnan and Amazonia silica-rich granite bodies (modified after Frost et al. 2016). (a), (b), (c), (d), (e), and (f) are the diagrams of Fe index vs. SiO2 (wt.%), ASI vs. SiO2 (wt.%), MALI vs. SiO2 (wt.%), Rb (ppm) vs. Sr (wt.%), Zr (ppm) vs. SiO2 (wt.%), and Sr/Y vs. Y (ppm), respectively

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Fig. 8
figure 8

Diagrams of CaO/Al2O3 vs. (CaO + Al2O3) (a) and (Na2O + K2O)/CaO vs. (Na2O + K2O + CaO) (b) (modified after Sun 2018)

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Fig. 9
figure 9

Diagrams of the Zr vs. 10000*Ga/Al from Guangnan pluton (modified after Sami et al. 2017)

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Zircon Lu–Hf isotope analysis

The zircon Lu–Hf isotopic test results of the Guangnan pluton are shown in Table 2. The measured 176Lu/177Hf ratio (< 0.002) of the three samples can represent the Hf isotope composition characteristics during the formation of zircon.

Table 2 Results of Zircon Lu–Hf isotopes in granite
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Sample H0405-10 tested 12 spots; the initial ratio of (176Hf/177Hf), the εHf(t) value, age of the first-stage Hf model TDM1, and the age of second-stage Hf model TDM2 yielded at 0.282294–0.282426, − 3.94– − 11.67, 1157–1344 Ma, and 1729–2060 Ma, respectively. Sample H0403-10 tested 13 spots. The initial ratio of (176Hf/177Hf), the εHf(t) value, age of the first-stage Hf model TDM1, and the age of second-stage Hf model TDM2 yielded at 0.282364–0.282469, − 7.45– − 11.13, 1115–1268 Ma, and 1677–1910 Ma, respectively. Sample H0405-8 tested 13 spots. The initial ratio of (176Hf/177Hf), the εHf(t) value, age of the first-stage Hf model TDM1, and the age of second-stage Hf model TDM2 yielded at 0.282373–0.282446, − 8.41– − 10.75, 1212–1289 Ma, and 1740–1889 Ma, respectively. The Hf isotope two-stage model age TDM2 values of the three samples fall between Early Proterozoic and Mesoproterozoic (Fig. 10).

Fig. 10
figure 10

Diagrams of the εHf (t)-t (5a, 5b) from Guangnan pluton (modified after Zhou 2007)

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Major and trace elements

The major and trace elements of the Guangnan pluton are shown in Table 3.

Table 3 Results of major and trace elements
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The major elements show they are higher SiO2 (72.82–76.16 wt.%), K2O (4.91–5.85 wt.%), Na2O (2.68–3.50 wt.%), Al2O3 (12.49–13.92 wt.%), lower ZrO2 (83.5–118 ug/g), TiO2 (0.09–0.52 wt.%), Fe2O3 (0.03–0.93 wt.%), FeO (0.35–2.64 wt.%), MgO (0.10–0.42 wt.%), MnO (0.03–0.12 wt.%), and CaO (0.42–1.21 wt.%). K2O/Na2O ratios range from 1.41 to 2.01; total alkali contents range from 7.95 to 8.82%. The values of DI range from 88.46 to 94.84, and the AR range from 2.43 to 3.07. The samples are located in the granite area on the diagram of TAS (Fig. 5a), and located in the high potassium calcium-alkaline series area on the SiO2-K2O diagram (Fig. 5b). The value of A/CNK ranges from 0.98 to 1.12 (Table 3, Fig. 5b), and most of them are greater than 1, belonging to peraluminous granite. The Guangnan pluton further classified as ferrous, calcareous-alkali-calcium sample in the SiO2-FeO* diagram and SiO2-alkali-calcium index of magma (MALI) diagram, respectively (Fig. 7a and c).

The rare earth element distribution pattern of the Guangnan pluton sample has obvious negative Eu anomalies, showing a right-leaning “V”-shaped characteristic, and the middle rare earth elements are not enriched relative to the heavy rare earth elements (Fig. 6a). In the spider web diagram of trace elements, the Guangnan granite samples are enriched in Rb, Th, U, Nb, and light rare earth elements, and are obviously depleted in Ba, Sr, P, and Ti (Fig. 6b).

Discussion

Time of Guangnan pluton intrusion

The Zhuguangshan batholith is composed of multiple Caledonian, Indosinian, and Yanshanian granitic rocks (Li 1990), and the Yanshanian plutons are mainly distributed in the southern body; therefore, few chronological studies were done on the Yanshanian plutons exposed in the northern Zhuguangshan batholith. Zheng (1988) reported the Rb–Sr isochronal age of 157 Ma and the mica Ar–Ar age of 152 Ma from the Guangnan biotite granite in the Exing area. Li (1990) analyzed single-grain zircon U–Pb from the biotite granite in the Exing area yielded at 155.5 Ma. Guo et al. (2017) conducted SHRIMP U–Pb age dating on the two-mica granite from Dongluo county, and yielded at 148.2 Ma.

This paper studied the biotite granite and found that the pluton formed at 164.4 M in Exing area. Compared with the studies of Zheng (1988) and Li (1990), the result of this paper is slightly earlier. The two-mica granite of Exing area formed 159.1 Ma on the southwest, the biotite granite in the Shanggushi area formed 153.9 Ma, and the two-mica granite from the Dongluo area formed 148.2 Ma (Guo et al. 2017). The ages of Guangnan pluton show that the pluton is gradually getting younger from the northeast to the southwest. Considering analyzed multiple factors acted on previous studies, it is deduced that the Guangnan pluton may intrude during 164 Ma and 148 Ma, that is, the Guangnan pluton formed in the late Middle Jurassic to the Late Jurassic period.

In this study, the Zircon LA-ICP-MS U–Pb age characteristics of the pluton show that the granitic magma intrusive activity can be divided into four stages (Fig. 1c). The first stage was represented by the magma intruded into the northeast of the Exing area earlier than 164 Ma forming coarse-grained porphyritic biotite granite. The second stage can be recognized as the formation of the two-mica granite intrusion at 159 Ma in the southwest of the Exing area. The third stage was represented by the magma intruded into the Shanggushi area at 154 Ma and formed a biotite granite. The last stage magmatic activity occurred in the Dongluo area, forming two-mica granite 148 Ma (Guo et al. 2017). The time interval between each intrusion is about 5–6 Myr. Spatially, intrusive activities migrated from the northeast (Exing area) to the southwest (Shanggushi-Dongluo area). Therefore, the Guangnan pluton is a compound pluton finally formed after 4 pulsating magma intrusions and lasted about 17 Myr. According to lithological changes, it can be divided into early magmatic (Middle Jurassic 164–159 Ma) stage and late magmatic (153–148 Ma) stage. In each stage, the biotite granite is followed by two-mica granite.

Genesis of the Guangnan granites

The lack of “low-evolved” granite samples with SiO2 < 70 wt.% has brought great difficulties to the interpretation of high-silica granites genesis in Guangnan area. Although the Guangnan granite was plotted in the iron region of the SiO2-FeO* diagram (Fig. 7a), its composition is close to the “haplogranite” due to its high SiO2 content (almost all samples SiO2 > 73 wt.%). In this case, it is failed to use the S-I-M-A “letter” classification of granite (Wang 2008a; Frost et al. 2016; Garcia-Arias 2020; Miller et al. 2020). Besides, in view of the lower Zr content of the Guangnan pluton, the Guangnan high-silica granite was not considered A-type granite (Tong and Wang 2013; Wang 2008b; Wang et al. 2013, Sami et al. 2017). Guangnan high-silica granites can be classified high-silicon peraluminous light-colored granites or ferroan calc-alkaline (Ferroan calc-alkaline) high-silica granites, or muscovite-containing peraluminous granite (MPG) type granite classified by Barbarin (1999) because of its peraluminous (ASI > 1), high SiO2 (> 72 wt.%), and low total mafic oxides (Fe2O3 + FeO + MnO + MgO + TiO2 < 5 wt.%) characteristics (Garcia-Arias 2020; Frost et al. 2001, 2016) (Fig. 6). The rocks are located in the metamorphic mudstone-derived melt zone and the overlapping area among metapelitic rock-derived melt zone, biotite gneisses-derived melt zone, and granodiorite-dacite-derived melt zone on the diagrams of (CaO + Al2O3) vs. (CaO/Al2O3) and (Na2O + K2O + CaO) vs. (Na2O + K2O)/CaO (Fig. 8a and b). The characteristics show that the Guangnan high-silica granite may be derived from the partial melting of metapelitic rocks, but also cannot eliminate the possibility of being derived from the partial melting of biotite gneiss or granodiorite-diarrhea. The Zr vs. Ga/Al diagram (Fig. 9) confirms the rocks are I- or S-type. These results are consistent with that of Rb vs. Sr on Fig. 7d and Fig. 8a. The characteristics of low Sr and Eu negative anomalies show that plagioclase maintained in the residual phase or crystalline phase combination in equilibrium with the melt. Since amphibole was not found in the Guangnan biotite granite, it is deduced that the water content of the magma is less than 3–4 wt.% according to the phase diagram (Naney 1983; Lu et al. 2015), and the Guangnan pluton can be considered the product of partial melting of the lower crustal rocks.

The characteristics of the Middle and Late Jurassic granites in the Nanling area, including the Guangnan pluton, were high Sr/Y and La/Yb ratios. Its reason is the source rocks of these high-silica granites which have a high content of SiO2, and the high-silica granites are mainly composed of metamorphic rocks, biotite gneiss, and granodiorite-diarrhea. A large number of previous partial melting experimental results show that the stable pressure range of plagioclase is larger than that of basic source rocks during the intermediate-acid source rocks partial melting with water loss, that is, when the water content of the intermediate-acid source rock system is lower, the stable pressure range of plagioclase is larger than that of basic source rocks (Wang 2015, 2016). Therefore, when crystalized mineral assemblage contains plagioclase in equilibrium with high-silicon granite melts, the rocks show low Sr content and obvious negative Eu anomalies. According to the phase diagram, the upper limit of the origin pressure of these granites is 1–1.2 G Pa, which is equivalent to a depth of 35–40 km (Wang 2015).

The zircon Hf isotopic composition of the Guangnan pluton indicates that the source rock of the high-silica granite is the Precambrian crust rock. Zircon U–Pb age dating results show that the biotite granite in the first stage (164 Ma) contains Caledonian and Indosinian residual zircons, the two-mica granite in the second emplacement (159 Ma) contains Caledonian period residual zircon (Table 1, Fig. 4a, c), the biotite granite in the third emplacement (153 Ma) contains a small amount of residual zircon with Early Jurassic age (Table 1), and the fourth stage two-mica granite (148 Ma) contains a small amount of residual zircon from the Indosinian period (Guo Aimin et al., 2017, Table 1). This indicates that the early granitic rocks in the Zhuguangshan area, as part of the source rocks, participated in the formation of the Middle and Late Jurassic high-silica granites, including Caledonian, Indosinian, and Early Jurassic granites.

Field characteristics of the Guangnan pluton showed the lithological change from biotite granite to dimica granite, the intrusion direction changes from southeast to northwest, and the content of dark minerals decreases with the intrusion time. The content of the mafic microgranular enclaves gradually decreases from northwest to southeast in the pluton, the content is high in the Exing area, and the long axis of the mafic microgranular enclaves is distributed in a certain direction with different sizes. The granite from Dongluo area in southwestern also contains a small amount of dark mafic microparticle inclusions (Guo et al. 2017), and they are small. These imply that basic magmatism is one of the heat sources of the crust-derived granitic magma, while magmatic mixing has limited contribution to the material composition of granite formation (Clemens 2012).

Overall, it is deduced that the Guangnan high-silica granite is the product of partial melting of the lower crustal metamorphic pelitic rock and/or metamorphic acidic igneous rock heated by the underplated basic magma in the South China Orogen, and is the product of multiple remelting of the lower crustal materials.

Tectonic significance

Based on previous studies, the Jurassic magmatism in the Nanling area can be divided into two stages, namely, the early Jurassic stage (195–165 Ma) and the late Jurassic stage (165–145 Ma) (Zhou 2007). Some scholars believed that the granite was formed in the background of compression in the late stage of Early Yanshanian (170–150 Ma) in South China (Xu and Tu 1984; Xia and Liang 1991; Chen and Mao 1995; Mao et al. 2008). Others scholars considered that the Late Jurassic granites were mainly formed in the back-arc extensional environment, controlled by the Paleo-Pacific subduction process (Jiang et al. 2015; Liu et al. 2020).

Most discussions on the tectonic background of Jurassic magmatic activity in South China were based on the geochemical characteristics of magmatic rocks. In recent years, some scholars emphasized the methodology and viewpoints of “intrusive rock geotectonics,” but magmatic activity is the only one aspect of the tectonic movement process. “Identifying the tectonic background of magmatic activity should be based on regional geological research, not the geochemical characteristics of granite types or basalts” (Clemens 2003; Wang 2007); the tectonic background of magmatic rocks is the magmatic rock aspect of tectonic research. Therefore, we will discuss the tectonic background of the Guangnan pluton based on the understanding of the Yanshanian tectonic evolution in South China.

Ren (1990) pointed out that the Caledonian orogenic belt in South China is a miogeosyncline fold system, and later evolved into the Post-Caledonian Paraplatform and finally integrated with the Yangtze Paraplatform. The South China was involved into the collided orogenic process between the western Pacific ancient land and the eastern edge of Asia which converted from passive continental margin to active continental margin because of the Indosinian (Akiyoshi) orogenic activity. The Japan-Ryukyu-Taiwan-Palawan zone is a collision orogen between the ancient land of the western Pacific and the eastern part of mainland China. The Yanshanian orogeny stage was the strongest period of the collision activity between the western Pacific ancient land and the eastern edge of Asia (Ren 1990; Ren et al. 1990). Evidence of collision events during the Middle Jurassic (164–161 Ma) from detrital zircons has been found in Hong Kong recent years (Sewell et al. 2016). The Guangnan pluton was formed in 164–148 Ma, and the magma intruded during the early Yanshanian stage based on chronological data. Therefore, tectonically, they should belong to the “syn-collisional” and “post-collisional” granites. On the Y + Nb vs. Rb and the R1 vs. R2 diagram, the Guangnan pluton samples were plotted in the area of the “syn-collisional” and “post-collisional” granites (Fig. 11), which confirmed our understanding based on the tectonic evolution of South China.

Fig. 11
figure 11

The diagrams of (Y + Nb)/10−6 vs. Rb/10−6 (a) and R1 vs. R2 (b) (a, modified after Pearce et al. 1984; b, modified after Batchelor and Bowden (1985)). Data of Early Yanshanian in early stage quoted from Chen et al. (1999), Zhou (2007), Guo et al. (2017), Xue et al. (2011), Li (2019); late stage date quoted from Zhao et al. (2004), Zhou (2007), Deng et al. (2012), Xue et al. (2011), showing the fields of volcanic arc (VAG), ocean-ridge granite (ORG), within-plate granite (WPG), and syn-collisional granite (syn-COLG), mantle plagiogranite (MPG), destructive active slab edge (before plate collision) granite (DSEG), uplift granite after plate collision (CEUG), late-formation granite (POG), non-orogenic A-type granite(non-ORG), post-orogenic A-type granite (post-COLG)

Full size image

As mentioned earlier, the Guangnan pluton has experienced two stages of intrusive activities. In each stage, biotite granites were formed in the earlier, and followed by two-mica granites. Previous studies indicate that large granite bodies in the Nanling area intruded during the Middle and Late Jurassic generally have similar lithological pattern (Wang 2007, Table 1). Therefore, the Guangnan pluton can be used as a representative of the granitic magmatism in the Late Jurassic period in Nanling area. The Nanling area was widely intruded by “syn-collision” to “post-collision” granites during the Middle and Late Jurassic.

Many scholars considered the “post-collision” stage as an extensional structural environment (Shu 2012; Mahdy et al. 2020; Sami et al. 2017). At an extensional regional, the intrusive basic magma is easy to eject out of the surface, together with the acidic magma at the same time, which forms a “bimodal” igneous rock assemblage. A typical example is the bimodal magmatic rock combination developed in the “rift zone” along the east–west direction from western Fujian to southern Jiangxi to southern Hunan intruded during early Jurassic (190–170 Ma) (Chen et al. 1999; He et al. 2010). In the regional compression environment, most of the basic magma is trapped in the lower part of the crust (i.e.: underplating). The heat released from crystallization process of the basic magma provided energy for the lower crustal rocks leading to partial melting, which formed a wide range of acidic magma. During the Middle and Late Jurassic period (170–140 Ma), the Nanling area was characterized by repeated intrusions of large-scale acidic magma, with few basic rocks, and the appearance of magmatic activity was very different from the early Jurassic period (Zhou 2007).

Conclusion

Based on all information acquired and discussed above, combined with previous data, we suggested the following preliminary understanding:

  1. (1)

    The Guangnan pluton is a compound pluton which formed in the Late Jurassic (164.4–148.2 Ma). The pluton has experienced four batches of magma invasion with 5–6 Myr interval. The magma emplacement can be divided into two stages, and the biotite granite intruded at first, followed by two-mica granite in each stage. First stage is distributed in the Exing area in the northeast of the pluton, and second stage distributed in the Shanggushi-Dongluo in the southwest. The biotite granite is mainly outcropped in the southeastern part of the compound pluton, and gradually transformed into the two-mica granite to northwestern part.

  2. (2)

    In terms of petrochemistry, the Guangnan pluton is high-silica granite with peraluminum and high potassium and calcium alkalinity characteristics, and shows ferro- and alkali-calcium feature which is similar to the MPG, and the source rocks originated from the Precambrian lower crustal metamorphic pelitic rocks and/or metamorphic intermediate-acid igneous rocks, and the earlier granitic intrusions also provided material source for the Guangnan granite. The heat source for the Guangnan pluton comes from the underplating magma.

  3. (3)

    The Guangnan pluton was formed in the collisional orogeny stage between the Western Pacific ancient land and the South China orogenic belt during the Middle and Late Jurassic, and was the product of the compression orogeny stage of the Asian continent eastern margin in the early Yanshanian period.