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Article

Zircon U-Pb Age, Sr-Nd-Hf Isotopic Characteristics of Baiyinhushuo Adakite in Inner Mongolia: Implications for Tectonic Evolution

by
Yuxu Fan
1,2,*,
Qinghui Xiao
2,
Yang Cheng
3 and
Yan Li
3
1
General Prospecting Institute of China National Administration of Coal Geology, Beijing 100039, China
2
Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China
3
Institute of Mineral Resources Research, China Metallurgical Geology Bureau, Beijing 101300, China
*
Author to whom correspondence should be addressed.
Appl. Sci. 2024, 14(14), 6058; https://doi.org/10.3390/app14146058
Submission received: 6 June 2024 / Revised: 4 July 2024 / Accepted: 5 July 2024 / Published: 11 July 2024
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Abstract

:
The petrogenesis and geodynamic implications of the Early Paleozoic adakitic rocks in the east of Inner Mongolia remain topics of debate. This study presents new petrology data through zircon U-Pb age and Lu-Hf isotopic composition, whole-rock major-trace element geochemistry, and Sr-Nd isotopes from adakitic rocks. The zircon U-Pb dating results demonstrate that the formation age is 242.8 ± 1.0 Ma, which is the product of Early Triassic magmatism. Petrology and geochemical study have shown that the granodiorite have high SiO2 (66.93~69.40%), Al2O3 (15.37~15.43%), and MgO (1.35~1.55%), with LREE enrichment and HREE deficit, and they have high Sr, low Y, and high Sr/Y ratios, showing typical signatures of adakitic rocks. The εHf(t) values of zircon vary between +11.3 and +13.8, with low whole-rock (87Sr/86Sr)i (0.703382) and positive εNd(t) values, and the average Mg# of the rock is 56.14, suggesting that adakite derived from partial melting of MORB materials and magma interaction with the mantle. Comprehensive analysis suggests that during the Late Permian to Early Triassic, the subducted slab of the Paleo-Asian Ocean broke off, and the residual oceanic slab preserved in the mantle beneath the subduction zone underwent partial melting, generating adakitic magma.

1. Introduction

The southeastern segment of the Central Asian Orogenic Belt (CAOB) is sandwiched between the Siberian, North China, and Pacific plates (Figure 1a). It is the longest-known Phanerozoic accretion orogenic belt, with complex tectonic–magmatic processes [1,2,3,4,5]. The widespread occurrence of Paleozoic ophiolite, subduction–accretion mélanges, island arcs, micro-continental blocks, and complex magmatic rocks within the belt is closely related to the evolution of the Paleo-Asian Ocean. This region has long been the focus of attention from the domestic and international geological community and has achieved a series of important results [1,2,3,4,5,6,7,8,9,10]. The Baiyinhushuo area in Inner Mongolia, located in the southeastern part of the CAOB, is characterized by the development of multiple ophiolites and abundant magmatic rocks. It is considered the place where the Paleo-Asian Ocean trench–arc–basin system developed and where the final closure of the CAOB occurred [11,12,13]. The region has undergone a complex tectonic evolution and serves as a natural laboratory for reaching the accretionary orogenesis of the CAOB.
At present, a large number of Late Permian to Middle Triassic adakites have been found on both sides of the Xar-Moron River suture zone and are considered to be the magmatism of crustal thickening and melting during the evolution of the accretionary orogeny to the final collision [7,8,9,10,14,15]; some experts believe that it was formed by the melting of residual Paleo-Asian Ocean subducted slabs [16,17]. Previous studies have shown that the CAOB experienced the evolutionary stages from subduction to collision during the Late Permian to Early Triassic [14,18,19] and that it is a transitional period in the tectonic evolution of the eastern part of the CAOB. In this paper, detailed zircon U-Pb dating and zircon in situ Lu-Hf isotope and whole-rock Sr-Nd isotope analysis were carried out on the granodiorite intrusion in Baiyinhushuo. The genetic types and tectonic environment of the rocks were discussed; they provided constraints for the Late Paleozoic tectonic evolution and provided important information for the study of the tectonic–magmatic evolution process from subduction to collision.
Applsci 14 06058 g001
Figure 1. (a) Distribution range of the Central Asian Orogenic Belt (CAOB; modified from [9]). (b) Sketched tectonic map of the adakite in Baiyinhushuo, Inner Mongolia (modified from [20,21]). (c) Simplified geologic map of the study area [15].
Figure 1. (a) Distribution range of the Central Asian Orogenic Belt (CAOB; modified from [9]). (b) Sketched tectonic map of the adakite in Baiyinhushuo, Inner Mongolia (modified from [20,21]). (c) Simplified geologic map of the study area [15].
Applsci 14 06058 g001

2. Geological Setting and Samples

Baiyinhushuo ploton is located in the northern part of Xi Ujimqin town, which is located on the southeastern margin of the CAOB (Figure 1a), north of the Solonker-Xar Moron River ophiolite belt and south of the Erenhot-Hegenshan ophiolite belt (Figure 1b). In the Paleozoic period, it experienced multi-stage subduction and the accretion of the Paleo-Asian Ocean, which is a typical accretionary orogenic belt [2,3]. The exposed strata are mainly the Late Paleozoic Amushan Formation and the Gegenaobao Formation, as well as the Early Cretaceous Baiyingaolao Formation and the Quaternary sediments [15]. The main lithology of the Gegenaobao Formation is siltstone and fine siltstone, which experienced strong metamorphism. The metamorphism of the clastic rocks is mainly carbonatization and silicification.
The magmatic rocks in the study area are abundant and have a large outcropping area; the ophiolite of Wusinihei is located in the western part of the study area. Baiyinhushuo granodiorite intrudes into the Gegenaobao Formation. The intermediate acidic dikes are widely distributed in the Baiyinhushuo granodiorite, in which the main lithologies are quartz-diorite porphyrite, granite porphyry, and quartz porphyry, and Early Cretaceous andesite and rhyolite and other volcanic rocks can be found in some places (Figure 1c).
The granodiorite is overall grayish-white and massive, with a medium–coarse-grained structure, and the rock can be seen as slightly fractured (Figure 2a,b). It is mainly composed of plagioclase (45%), quartz (±20%), K-feldspar (15%), biotite (5%), and hornblende (10%). Observed under the microscope, plagioclase is subhedral and haphazardly distributed; its size is generally 1–2 mm; some parts are 2–4 mm, with the development of polysynthetic twins, and some particles show an obvious zoning structure. The quartz crystals has a granular structure and generally has a size of 0.5–1 mm; parts of it are 0.2–0.5 mm, with a small portion being 0.1–0.2 mm. It has a gap-filling distribution, and the surface is fresh and clean; a wave-like extinction is common, and a band extinction can be seen. Potassium feldspar is subhedral and plate-like; it has a size of about 0.5–1 mm and parts are 1–1.5 mm; it has haphazard distribution and showing kaolin alteration. Hornblende is subhedral and columnar; its size is generally 0.5–1 mm; a few parts are 1–1.5 mm and haphazardly distributed. Biotite is flaky with a flake diameter of 0.5–1 mm; parts of it are 1–1.5 mm; showing epidote and chlorite alteration. Biotite, which is present in small amounts, exhibits a flaky habit and pronounced pleochroism, which is indicative of its color variation when viewed from different directions. The accessory minerals primarily consist of zircon and apatite. Zircon occurs as idiomorphic crystals, and the size ranges from approximately 0.012 to 0.20 mm. Apatite presents a more complete crystalline structure, appearing as tabular or short prismatic, with visible zoning characteristics.

3. Analytical Methods

3.1. Whole-Rock Major and Trace Element Geochemistry

The fresh rock samples were crushed into 200 mesh without pollution and the powder samples were sent to the State Key Laboratory of Mineral Deposits Research, Nanjing University (NJU) for major and trace element analysis. The major elements were determined by the XRF method using the ARL 9900 X-ray fluorescence spectrometer (XRF) (Thermo Fisher Scientific, Waltham, MA, USA). The working voltage of the X-ray was 40 kV, and the current was 75 mA. When the sample concentration was >1.0%, the relative error of the analysis was +1%. When the sample concentration was <1.0%, the relative error was +10%. Trace elements were determined by high-resolution inductively coupled plasma mass spectrometry(ICP-MS). The samples were first chemically dissolved in a high-pressure reactor and then analyzed by plasma mass spectrometer (Instrument Model: ELEMENT-2, Thermo Fisher Scientific, Waltham, MA, USA) with a relative error of approximately 2%.

3.2. Zircon U-Pb Dating

Strictly avoiding pollution, zircon single minerals were selected through crushing, sorting, and picking out under a binocular microscope with no cracks on the surface, and they were smooth and intact. Then, the samples were made into targets with Canadian resin and photographed with cathodoluminescence by a scanning electron microscope. Zircon U-Pb isotopic dating was completed at the China University of Geosciences (Beijing), using the experimental method of laser ablation LA-ICP-MS in situ dating and the experimental instruments of the 193 nm excimer laser and inductively coupled plasma mass spectrometer (Instrument Model: Thermo Xseries2, Thermo Fisher Scientific, Waltham, MA, USA). Zircon standard 91500 was used for U-Pb isotopic ratio correction, and zircon standard GJ-1 was used as an unknown. The mass spectrometer used He gas as the carrier gas, and the laser spot diameter used in the analysis was 32μm, with a frequency of 6 Hz. The data processing and concordia plot drawing of zircon U-Pb dating were obtained using the ICPMSDataCal program [22] and Isoplot 3.0 program [23].

3.3. Whole-Rock Sr-Nd Isotopes and Zircon Lu-Hf Isotopes

Whole-rock Sr-Nd isotope analyses were conducted by Triton TI surface thermal ionization mass spectrometry (TIMS) at the State Key Laboratory of Mineral Deposits Research, Nanjing University. A 200-mesh rock powder sample was taken, dried, and completely dissolved in a mixed acid of HF + HNO3, and Sr and Nd were separated and purified using a cation exchange resin. The mass fractionation of the 87Sr/86Sr ratio was corrected by normalization to a constant 86Sr/88Sr ratio of 0.1194, using an exponential law. The isobaric interference of 87Rb on 87Sr was corrected using a natural 87Rb/85Rb ratio of 0.3857. The mass bias correction of the 143Nd/144Nd ratio was performed by normalizing to 146Nd/144Nd = 0.7219, using an exponential fractionation law. The isobaric interference of 144Sm on 144Nd was corrected with a 147Sm/144Sm ratio of 4.8387.
The analysis of the zircon Lu-Hf isotopes was completed in the Key Laboratory of Continental Dynamics of Ministry of Natural Resources, using the Neptune Plus multi-reception plasma mass spectrometer (Thermo Fisher Scientific, Waltham, MA, USA) and the Geo Las Pro 193 nm laser ablation system (LA-MC-ICP-MS). Depending on the size of the zircon, the diameter of the laser ablation system was 44 μm, and the international standard for zircon, GJ-1, was used as the reference material. Detailed operation conditions and analysis procedures of the relevant instruments can be found in Hou et al. [24]. The weighted average values of the n(176Hf)/n(177Hf) tests for the zircon standard GJ-1 during the analysis were 0.282008 ± 0.000050 (2σ), respectively. For the calculation of the initial 176Hf/177Hf, a decay constant of 1.865 × 10−11a−1 [25] was used for Lu, and the εHf(t) values were calculated using the spherical meteorite Hf isotope values of n(176Lu)/n(177Hf) = 0.0332 and n(176Hf)/n(177Hf) = 0.282772 [26].

4. Analytical Results

4.1. U-Pb Zircon Ages

The zircons of the Baiyinhushuo granodiorite are highly crystalline, with zircon grain sizes ranging from 0.08 to 0.25 mm; they are generally euhedral and are colorless to light yellow. The Th/U ratios of the zircons range from 0.40 to 1.08, with an average of 0.55 (Table 1), and the cathodoluminescence photographs show the obvious oscillation zones and fan-shaped ring zones (Figure 3), indicating that they were all magmatic crystallization zircons [27,28]. Among the 25 zircon points, 2 zircons were punctured during the experimental process and thus lack credibility. Four zircons showed low concordance in terms of age and were excluded. Nineteen zircons exhibited relatively concordant ages and were located near the concordia curve. The individual zircon 206Pb/238U ages are listed in Table 1, and a weighted average age plot and a concordia diagram are constructed (Figure 4).The zircon concordant age is 242.8 ± 1.0 Ma (MSWD = 2.0), indicating that they are products of Middle Triassic magmatic activity.

4.2. Bulk-Rock Major and Trace Elements

The major and trace element compositions of the studied Baiyinhushuo granodiorite are shown in Table 2. This study analyzed two samples and collected data from four samples. The SiO2 content of the granodiorite ranges between 66.93% and 69.40%, with an average of 67.75%. The Na2O content is 4.35–5.45%, with an average of 4.63%. The K2O content is 2.48–2.95%, with an average of 2.60%, and the Na2O/K2O ranges from 1.56 to 2.05. The Al2O3 content was 15.13–15.53%, with an average of 15.39%. In the TAS diagram, the samples fall in the granodiorite fields, subalkaline magmatic series (Figure 5a,d). The total alkali (Na2O + K2O) amounts are 6.82–8.12% and show a calc-alkaline to high-K calc-alkaline series (Figure 5b). The Al2O3 content (11.45–14.11%) is moderate to low, reflecting A/CNK values of about 0.94–1.01 and A/NK values of 1.30–1.54 (Figure 5c), and is known for the metaluminous granitoids (Figure 5c).
The granodiorite samples showed low rare earth element (REE) abundances, with ΣREE ranging from 54.39 to 76.84 ppm; they showed a right-leaning chondrite distribution curve with a strong differentiation of light REE (LREE) and heavy REE (HREE) (Figure 6a). The LREE/HREE value was 7.38–10.68, with an average of 8.53; the (La/Yb)N value was 8.14–13.93, with an average of 10.10; the δEu value was 0.79–1.02, with an overall weak negative Eu anomaly. The trace element primitive mantle spider diagram is enriched in large ionic lithophilic elements (LILEs), such as Rb, Ba, Th, and U, and depleted in high-field-strength elements (HFSEs), such as Nb, P, and Ti. (Figure 6b) The trace elements have high Sr (424.00~570.14 ppm) and high Sr/Y (42.79~62.91) (Table 2).
The geochemical composition of the granodiorite sampled in this study is consistent with the findings of Xiong et al. [15]. The SiO2 varies from 66.93% to 69.40%; the MgO ranges from 1.35% to 1.55%; and the Al2O3 is between 15.37% and 15.43%; the rock is characterized by high Sr, low Y, and high Sr/Y, which is consistent with the geochemical characteristics of typical adakites.

4.3. Zircon Hf and Whole-Rock Sr-Nd Isotopic Compositions

The Lu-Hf isotope analysis of zircon shows (Table 3) that the 176Lu/177Hf ratio is less than 0.002, indicating that the zircons had extremely low amounts of radiotracer Hf and that the measured 176Hf/177Hf ratios basically represent the Hf isotopic composition system during crystallization [34]. The variation in 176Hf/177Hf of the samples ranges from 0.282943 to 0.283014, with an average of 0.282985. The corresponding εHf(t) values range from +11.3 to +13.8, all of which are positive. The depleted mantle model age (TDM) is 334–436 Ma, and the two-stage model age (T2DM) is 389–553 Ma. The 87Rb/86Sr and (87Sr/86Sr)i of the Baiyinhushuo granodiorite sample are low: the 87Rb/86Sr is 0.346558, and the (87Sr/86Sr)i is 0.703882. 147Sm/144Nd is 0.112238, and εNd(t) is +7.72, which is a positive value. The two-stage mode age of T2DM is 387 Ma (Table 4).

5. Discussion

5.1. Emplacement Age of the Baiyinhushuo Granitoid

The emplacement age of the granodiorite has been controversial. In the 1:250,000 area survey report, the Baiyinhushuo granite was classified as Early Permian. Bao et al. (2007) [35] measured the age from 246 to 216 Ma by the SHRIMP method, and Xiong et al. (2015) [15] reported the zircon U-Pb age of 236.1 ± 3.4 Ma. The present age obtained by zircon LA-ICP-MS in the Baiyinhushuo granodiorite is 242.8 ± 1.0 Ma (MSDW = 2.0), which is Middle Triassic.
In the adjacent area, the magmatic activity was more frequent during the Late Permian–Early Triassic (Table 5). Late Permian granodiorites have been found in Seerbeng and Nuhetingshala in the southwest of Xi Ujimqin [16]; the Longtoushan pluton and the Zhuanshanzi pluton in the Linxi area (245.6 ± 0.9 Ma, 241 ± 3 Ma, [7]); and the Baiyinnuoer Triassic granodiorite (245.4 ± 1.8 Ma, [36]) and the andesites of the Xingfuzhilu Formation in the Linxi area (247 Ma, [17]); all of them are characterized by typical adakite, suggesting that there was a period of adakitic magma eruption during the Late Permian–Early Triassic.

5.2. Petrogenesis of Baiyinhushuo Granitoid

The Baiyinhushuo granodiorite, with high Sr content and Sr/Y and La/Yb ratios, is a typical adakite (Figure 7), which has special geodynamic significance [37]. In recent years, a large number of adakites of the Late Permian–Early Triassic have been discovered in the southeastern section of the Central Asian Orogenic Belt; they are distributed along the Xar Moron River suture zone [7,14,19]. Currently, there are three views on the geological background of the adakite on both sides of the Xar Moron suture zone: one suggests that they derived from the remelting of the juvenile lower crust [7,8,14,15]; one suggests that subducted slabs broke off, and the remaining slabs in the mantle melted and interacted with the mantle [16]; and another viewpoint suggests that the Triassic adakites were caused by plate subduction of the oceanic crust [17].
The Mg# of adakite is an important parameter for determining the origin of magma, and experimental petrological studies have confirmed that the Mg# formed by the partial melting of dehydrated basaltic rocks is usually lower than 45 [38]. High-Mg adakitic rocks are generally believed to be formed by partial melting of the thickened lower crust after detachment and interaction with mantle peridotite [39], or by melting of the subducted oceanic crust and interaction with the mantle wedge [40]; some scholars also believe that they are formed by the mixing of mantle-derived basic magma and crustal acidic magma [41].
The Mg# of the Baiyinhushuo adakite ranges from 46.73 to 61.78, with an average value of 56.14, which is higher than that of the adakite formed by the partial melting of the MORB, and is also higher than that of the adakite derived from lower crustal melting, such as the Mg# of the adakites, which ranges from about 35.72 to 46.16 and is formed by underplating in the North China Craton [42,43,44]. In the MgO–SiO2 diagram (Figure 8), the samples fall in the field of subducted slab-derived adakitic rocks, indicating that the source of Baiyinhushuo adakite is subducted slab of CAOB. In addition, the adakites derived from the thickened lower crust are often Na-rich and K-poor and belong to the peraluminous magma [45], while the adakites of Baiyinhushuo have K-rich and Na-poor characteristics, making them different from the adakites derived from the lower crust.
The high-Mg adakitic andesite of the Middle Triassic (244 Ma) was also discovered in the adjacent Linxi region, which derived from the partial melting of the remnant oceanic slab preserved in the mantle of the subduction zone [46] and is similar to the geochemical characteristics of the Baiyinhushuo adakite. Therefore, we believe that the Baiyinhushuo adakite was most likely generated by the partial melting of subducted oceanic crust.
The adakitic rocks were enriched in large ion lithophile elements (LILEs, such as Th and Rb) and depleted in Nb, Ta, Ti, and P, reflecting the geochemical characteristics of the subduction zone. In the (Yb + Ta)-Rb diagram and (Y + Nb)-Rb diagram, all the samples also fall into the field of volcanic arc granite, indicating that the adakites were most likely emplaced in the island arc background (Figure 9).
The igneous rocks derived from the melting of the lower crust exhibit distinct crustal characteristics in terms of isotopes, while the magma rock derived from the melting of subducted oceanic crust inevitably carries the signature of the oceanic crust. It is generally believed that the magma with Sr-Nd isotopic characteristics close to MORB is directly derived from the melting of the oceanic crust.
Generally, the Sr-Nd isotopic signatures of magmas close to MORB are directly derived from the melting of the mantle-derived oceanic crust (MORB), which is related to the island arc setting of the subduction zone. If (87Sr/86Sr)i > 0.704, typically between 0.706 and 0.710, it is considered that the magma derived from the partial melting of the basalt of the juvenile lower crust is caused by underplating, which is closely related to the intracontinental post-collision tectonic environment during the thickening lithosphere delamination and mantle magma underplating [48,49,50]. The Baiyinhushuo adakite has low (87Sr/86Sr)i (0.703382) and high 143Nd/144Nd (0.512721) and εNd(t) values (+7.72). In the Sr-Nd isotope diagram (Figure 10), the samples fall in the field of subducted oceanic adakite, far from the fields that are derived from the thickened lower crust. The Sr-Nd isotopes show that the source region is close to the depleted mantle, with a two-stage model age of 387 Ma, It exhibits a close affinity to the Sr-Nd isotopic values of the ophiolite in the adjacent Xi Ujimqin region, suggesting that the isotopic characteristics are similar to those of the subducted oceanic crust. Therefore, the Baiyinhushuo adakite is derived from the partial melting of the subducted oceanic crust, with its source potentially being the break-off subducted oceanic crust of CAOB.
The high stability of zircon, along with its high Hf mass fraction and very low 176Lu/177Hf, and the fact that it crystallizes with little or no radiogenic Hf accumulation, make zircon Hf isotope studies one of the most important means of tracing magma source at present [34,54]. The εHf(t) values in the Baiyinhushuo adakite are all positive, with a small range of variation between 11.3 and 13.8, and all the points are located between the evolutionary line of depleted mantle and the chondrite (Figure 10); in addition, the Hf isotopic signature is consistent with the evolution of Hf in the mantle-derived magma in the eastern region of the Xing-Meng Orogenic Belt (Figure 11) [8,55], showing similar isotopic composition characteristics to Phanerozoic magmatic rocks in the eastern part of the Xing-Meng Orogenic Belt. The two-stage model age of zircon Hf isotopes is 323–459 Ma, which is relatively young, implying that the magma may have originated from the partial melting of the Early Paleozoic subducted oceanic crust.

5.3. Tectonic Implications

The Late Permian–Early Triassic was a key stage in the tectonic evolution of the Central Asian Orogenic Belt (CAOB), in which the paleo-Asian Ocean closed and changed from subduction accretion to collisional orogeny. Previous research on the sedimentology and paleogeography of the Late Permian Linxi Formation in the southern segment of the Daxing’anling Mountains show that the region was marine sediment in the early–middle stage of the Late Permian as a remnant basin, and it was not until the late stage that it transitioned to a terrestrial environment. In the Early Triassic, river lake facies of sedimentary layers with red layer properties appeared [56], indicating that the surface of the paleo-Asian Ocean had closed.
In the Early Triassic, although the surface of the Paleo-Asian Ocean was closed, a large number of adakitic igneous rocks were derived from the melting of the subducted slab in the area north of the Xar Moron suture zone. For example, in the Early Mesozoic (244 Ma) high-Mg adakitic andesite exposed in the Linxi region was derived from the partial melting of the residual oceanic crust preserved in the mantle beneath the paleo-subduction zone, followed by interaction with mantle peridotite [46]. Li et al. (2020) [17] concluded that the volcanic rock of the Lower Triassic Xingfuzhilu Formation (247 Ma) in the Linxi area was a typical adakite derived from the melting of oceanic slabs and that subducted oceanic slabs still existed during the Early–Middle Triassic after the closure of the paleo-Asian Ocean surface. According to a detailed zircon mineral study, Liu et al. (2007) [57] concluded that the adakitic rocks of the Longtoushan pluton (241 Ma) were hydrous mafic rocks of the subducted oceanic crust. Shao et al. (2002) [58] analyzed the Sr-Nd isotopic characteristics of various igneous rocks and combined them with geophysical data, suggesting that the Mesozoic “soft collision, weak orogenesis” tectonic environment allowed a considerable portion of the oceanic plates to remain in the crust–mantle transition zone rather than completely returning to the asthenosphere, indicating that lithospheric-scale subduction continued.
Additionally, in the Xilinhot-Xi Ujimqin region, the Early Triassic dacite-rhyolite has been discovered in the volcanic rock sections of the Dalinuoer Formation and the Baoeraobao Formation. These rocks’ ages of petrogenesis were 245.7 ± 1.8 Ma and 242.7 ± 4.0 Ma (K-Ar whole-rock ages, [59]), and they exhibit geochemical characteristics similar to A-type granites. They belong to the extrusive phase of the contemporaneous A-type calc-alkaline series of peraluminous-type granites, closely related to the continental crust extension process resulting from the underplating of mantle-derived magma.
During the Late Permian to Early Triassic, the subduction plate segment of the Paleo-Asian Ocean broke off, and the remnant oceanic crust preserved in the mantle of the Paleo-subduction zone still underwent partial melting, generating adakitic magma. Concurrently, the asthenospheric mantle upwelling occurred, causing the melting of materials at different depths within the lithosphere, resulting in the petrogenesis of both adakitic rocks with island arc characteristics and rock assemblages under an extensional setting in the subduction zone.

6. Conclusions

(1)
The emplacement age of the Baiyinhushuo adakite granodiorite in the southeast of the Central Asian Orogenic Belt is 242.8 ± Ma, corresponding to the Early Triassic, and its geochemistry exhibits the characteristics of adakite.
(2)
The Baiyinhushuo adakite has low (87Sr/86Sr)i and high εNd(t) values, and they may derive from the partial melting of the broken-off subducted oceanic crust, which interacted with the mantle in the process of uplift beneath the subduction zone.
(3)
The Late Permian–Middle Triassic period marks a crucial stage in the tectonic transition from subduction to collisional orogeny. In this region, due to the slab break-off and the asthenosphere upwelling, the materials at different depths melted, forming the rock assemblages in diverse tectonic environments.

Author Contributions

Conceptualization and methodology, Y.F. and Q.X.; formal analysis, Y.F. and Q.X.; investigation, Y.F., Y.C. and Y.L.; resources, Y.C. and Q.X.; data curation, Y.F.; writing—original draft preparation, Y.F.; writing—review and editing, Y.F. and Q.X.; funding acquisition, Q.X. All authors have read and agreed to the published version of the manuscript.

Funding

This work was financially supported by the Geological Prospecting Foundation of Inner Mongolia Autonomous Region of China (Project No. 2017-YS01) and (Project No. 2020-YS01).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.

Acknowledgments

We sincerely appreciate the State Key Laboratory of Mineral Deposits Research, Nanjing University and the Key Laboratory of Continental Dynamics of the Ministry of Natural Resources for helping with the experimental analysis.

Conflicts of Interest

The authors declare no conflicts of interest.

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Applsci 14 06058 g002
Figure 2. Field outcrop (a,b) and photomicrograph (c,d) of granodiorite.
Figure 2. Field outcrop (a,b) and photomicrograph (c,d) of granodiorite.
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Applsci 14 06058 g003
Figure 3. Representative CL images of analyzed zircons. Ages (Ma) and analysis spots (cycle) are marked on the zircons.
Figure 3. Representative CL images of analyzed zircons. Ages (Ma) and analysis spots (cycle) are marked on the zircons.
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Figure 4. Concordia and weight diagrams of U-Pb ages of zircons for the granodiorite.
Figure 4. Concordia and weight diagrams of U-Pb ages of zircons for the granodiorite.
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Figure 5. (a) Total alkalis—SiO2 (TAS) [29]; (b) ANK-ACNK plot [30]; (c) K2O-SiO2 [31]; (d) TFeO/MgO-SiO2 diagram [15,32].
Figure 5. (a) Total alkalis—SiO2 (TAS) [29]; (b) ANK-ACNK plot [30]; (c) K2O-SiO2 [31]; (d) TFeO/MgO-SiO2 diagram [15,32].
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Applsci 14 06058 g006
Figure 6. (a) Chondrite-normalized REE pattern and (b) primitive mantle-normalized trace element spider diagram [15,33].
Figure 6. (a) Chondrite-normalized REE pattern and (b) primitive mantle-normalized trace element spider diagram [15,33].
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Applsci 14 06058 g007
Figure 7. Diagrams of (a) Sr/Y–Y [15] and (b) (La/Yb)N–(Yb)N for granitoids in Baiyinhushuo (after [37]).
Figure 7. Diagrams of (a) Sr/Y–Y [15] and (b) (La/Yb)N–(Yb)N for granitoids in Baiyinhushuo (after [37]).
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Figure 8. SiO2-Mg# (a) diagram and SiO2-MgO diagram (b) for the in Baiyinhushuo area (after [15,45]).
Figure 8. SiO2-Mg# (a) diagram and SiO2-MgO diagram (b) for the in Baiyinhushuo area (after [15,45]).
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Applsci 14 06058 g009
Figure 9. (a) Rb-(Y + Nb) and (b) Rb-(Yb + Ta) tectonic discriminant diagrams of the Baiyinhushuo adakite (modified from [15,47]).
Figure 9. (a) Rb-(Y + Nb) and (b) Rb-(Yb + Ta) tectonic discriminant diagrams of the Baiyinhushuo adakite (modified from [15,47]).
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Figure 10. (87Sr/86Sr)i—εNd(t) isotope composition of Baiyinhushuo (after [51]). Date sources: intra-oceanic adakite of Tibet is from [49]; adakite of modern intra-oceanic arc is from [52,53]; adakite in Linxi is from [8].
Figure 10. (87Sr/86Sr)i—εNd(t) isotope composition of Baiyinhushuo (after [51]). Date sources: intra-oceanic adakite of Tibet is from [49]; adakite of modern intra-oceanic arc is from [52,53]; adakite in Linxi is from [8].
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Figure 11. εHf(t)-T diagram of zircon (after [55]).
Figure 11. εHf(t)-T diagram of zircon (after [55]).
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Table 1. LA-ICP-MS zircon U-Pb dating results of the Baiyinhushuo.
Table 1. LA-ICP-MS zircon U-Pb dating results of the Baiyinhushuo.
SpotPbThUTh/UIsotope RatioAgeConcordance
207Pb/206Pb1σ207Pb/235U1σ206Pb/238U1σ207Pb/206Pb1σ207Pb/235U1σ206Pb/238U1σ
HS17-05-19.40 84.0 198 0.420.0525 0.0053 0.2513 0.0211 0.0384 0.0010 306231.522817.12436.593%
HS17-05-214.06 148 267 0.550.0524 0.0045 0.2702 0.0211 0.0382 0.0007 306202.824316.82424.199%
HS17-05-312.02 104 262 0.400.0505 0.0051 0.2622 0.0281 0.0382 0.0008 220218.523622.62424.997%
HS17-05-414.361892860.660.04570.00450.24310.02170.03820.0009error 22117.72415.391%
HS17-05-537.64736450.730.06360.00340.33440.01780.03820.0008728114.829313.52424.780%
HS17-05-615.771552940.530.05190.00430.28840.02170.04120.0009280190.725717.12605.798%
HS17-05-718.32 233 356 0.650.0532 0.0046 0.2778 0.0229 0.0382 0.0009 339196.324918.22425.697%
HS17-05-812.571782350.760.04280.00400.22440.01790.03870.0008error20614.82455.282%
HS17-05-917.91 196 324 0.600.0496 0.0035 0.2759 0.0177 0.0403 0.0007 176−27.824714.12544.297%
HS17-05-109.85 102 206 0.500.0584 0.0049 0.2998 0.0213 0.0382 0.0010 546185.226616.72426.490%
HS17-05-119.01 73.4 184 0.400.0597 0.0055 0.3060 0.0259 0.0382 0.0008 592197.227120.12425.288%
HS17-05-1229.8 384 579 0.660.0531 0.0038 0.2764 0.0187 0.0380 0.0008 345167.624814.82404.896%
HS17-05-1317.40 193 342 0.570.0519 0.0038 0.2712 0.0191 0.0382 0.0007 283170.424415.32424.499%
HS17-05-1413.68 131 286 0.460.0535 0.0039 0.2775 0.0179 0.0380 0.0006 350166.624914.22414.096%
HS17-05-1513.59 118 298 0.390.0573 0.0033 0.3029 0.0177 0.0381 0.0008 506127.826913.82414.789%
HS17-05-1612.511082390.450.06280.00640.33640.03050.03820.0011702213.929423.12417.180%
HS17-05-1729.7 538 498 1.080.0534 0.0030 0.2744 0.0145 0.0382 0.0008 346132.424611.62425.098%
HS17-05-1823.0 239 456 0.520.0549 0.0035 0.2917 0.0185 0.0382 0.0007 409144.426014.62424.492%
HS17-05-1917.191853290.560.06340.00410.32780.01930.03810.0008724134.228814.82414.982%
HS17-05-2011.57 107 230 0.470.0606 0.0046 0.3107 0.0222 0.0382 0.0008 628164.827517.22424.987%
HS17-05-2125.5 330 487 0.680.0534 0.0036 0.2717 0.0167 0.0382 0.0008 346156.524413.32425.298%
HS17-05-2217.52 200 331 0.600.0567 0.0040 0.2963 0.0195 0.0381 0.0007 480155.526315.32414.191%
HS17-05-2337.2 427 721 0.590.0534 0.0026 0.2804 0.0132 0.0382 0.0006 346107.425110.42423.696%
HS17-05-2429.5 259 626 0.410.0464 0.0023 0.2463 0.0118 0.0382 0.0005 16.8114.82249.72423.392%
HS17-05-2521.46 243 427 0.570.0499 0.0041 0.2583 0.0194 0.0381 0.0007 191177.823315.72414.596%
Table 2. Composition of major elements (%), trace elements (ppm), and REE (ppm) of samples from the granodiorite.
Table 2. Composition of major elements (%), trace elements (ppm), and REE (ppm) of samples from the granodiorite.
SampleSiO2TiO2Al2O3Fe2O3FeOMnOMgOCaONa2OK2OP2O5LOITATOLMg#A/CNK
HS17-0569.400.4815.433.07/0.051.351.925.452.670.161.3999.2946.731.01
HS17-0968.760.4315.373.24/0.061.552.904.602.950.142.299.0648.870.96
DW11-b1 *67.060.5415.131.841.580.042.273.44.352.470.150.8298.8361.780.95
DW11-43 *66.930.5315.481.761.490.032.123.524.452.50.140.9598.9561.550.94
DW11-4 *67.220.515.531.381.710.0323.454.462.560.140.9898.9856.820.95
DW11-6 *67.150.5215.41.681.510.032.113.474.462.480.140.9298.9561.120.94
SampleRbBaThUNbSrNdZrHfYbLaCePrNdSm
HS17-0552657.554.170.884.21570.1413.961123.090.8015.47 33.55 3.85 13.96 2.73
HS17-0961625.284.441.173.86509.5415.231053.080.8013.96 30.53 3.77 15.23 2.83
DW11-b152.5401.44.350.984.65416.611.67118.54.920.9711.7523.12.7611.672.37
DW11-4351.9399.74.360.744.22421.113.15104.64.311.0512.8325.993.1413.152.53
DW11-452.9388.54.281.034.05425.710.89101.53.910.9210.4421.182.5710.892.14
DW11-651.3393.34.20.794.1542411.69109.74.530.9511.3922.672.7411.692.34
SampleEuGdTbDyHoErTmYbLuYΣREELREE/
HREE		LaN/YbNδEu
HS17-050.70 2.43 0.32 1.62 0.30 0.86 0.12 0.80 0.13 9.06 76.8410.6813.93 0.83
HS17-090.69 2.48 0.32 1.66 0.30 0.87 0.12 0.80 0.13 9.04 73.7110.0012.47 0.79
DW11-b10.682.080.321.830.340.990.150.970.169.4259.177.658.69 0.94
DW11-430.742.270.341.950.371.060.161.050.189.8465.767.918.76 0.94
DW11-40.681.960.31.740.330.950.140.920.158.6454.397.388.14 1.02
DW11-60.692.070.321.810.3410.150.950.179.0358.337.578.60 0.96
* data from Xiong et al., 2015 [15].
Table 3. Zircon Lu-Hf isotopic compositions of the granodiorite.
Table 3. Zircon Lu-Hf isotopic compositions of the granodiorite.
Spott (Ma)176Yb/177Hf2σ176Lu/177Hf2σ176Hf/177Hf2σεHf(0)εHf(t)TDM(Ma)T2DM(Ma)f(Lu/Hf)s
HS17-05-10242.80.012445 0.000207 0.000465 0.000006 0.283006 0.000017 8.28 13.5 343 405 −0.99
HS17-05-10 (2)242.80.020651 0.000135 0.000779 0.000007 0.282970 0.000020 7.00 12.2 397 491 −0.98
HS17-05-11242.80.022239 0.000562 0.000871 0.000018 0.282982 0.000015 7.44 12.6 381 464 −0.97
HS17-05-13242.80.022755 0.000505 0.000914 0.000021 0.283014 0.000016 8.57 13.8 336 391 −0.97
HS17-05-14242.80.011469 0.000265 0.000470 0.000012 0.282996 0.000016 7.93 13.2 357 428 −0.99
HS17-05-15242.80.019129 0.000328 0.000722 0.000008 0.282951 0.000016 6.34 11.6 423 533 −0.98
HS17-05-3242.80.028464 0.000230 0.001110 0.000007 0.282954 0.000016 6.42 11.6 424 532 −0.97
HS17-05-4242.80.016900 0.000988 0.000670 0.000038 0.283014 0.000018 8.57 13.8 334 389 −0.98
HS17-05-5242.80.017065 0.000089 0.000703 0.000004 0.282999 0.000017 8.02 13.2 356 425 −0.98
HS17-05-6242.80.010922 0.000090 0.000425 0.000003 0.282999 0.000015 8.03 13.3 353 421 −0.99
HS17-05-7242.80.019116 0.000357 0.000741 0.000010 0.282943 0.000015 6.03 11.3 436 553 −0.98
HS17-05-8242.80.021709 0.000173 0.000873 0.000008 0.282972 0.000016 7.09 12.3 395 487 −0.97
HS17-05-9242.80.016488 0.000241 0.000661 0.000008 0.283001 0.000016 8.11 13.3 352 419 −0.98
Table 4. Sr-Nd isotopic compositions of the representative samples for the Baiyinhushuo pluton.
Table 4. Sr-Nd isotopic compositions of the representative samples for the Baiyinhushuo pluton.
Scheme 87.Rb (μg/g)Sr (μg/g)87Rb/86Sr87Sr/86Sr (2σ)(87Sr/86Sr)iSm (μg/g)Nd (μg/g)147Sm/144Nds143Nd/144Nds143Nd/144Nd(t)εNd(0)εNd(t)TDM2
HS17-0960.98509.540.3466 0.70508 0.703882 2.83 15.23 0.11220.5129000.5127215.10 7.72 387
Table 5. Statistical table of age of zircon in Baiyinhushuo and adjacent areas.
Table 5. Statistical table of age of zircon in Baiyinhushuo and adjacent areas.
Area.LithologyAge (Ma)MethodData Source
Baiyinhushuoadakite242.8 ± 1.0LA-ICP-MSthis study
adakite236.1 ± 3.4 LA-ICP-MS[15]
adakite246–216SHRIMP[35]
Seerbengadakite255.3LA-ICP-MS[16]
Nuhetingshalaadakite254.4LA-ICP-MS
Longtousahnadakite241 ± 3 LA-ICP-MS[7]
Zhuansahnziadakite245.6 ± 0.9 LA-ICP-MS
Xingfuzhiluadakite247LA-ICP-MS[17]
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Fan, Y.; Xiao, Q.; Cheng, Y.; Li, Y. Zircon U-Pb Age, Sr-Nd-Hf Isotopic Characteristics of Baiyinhushuo Adakite in Inner Mongolia: Implications for Tectonic Evolution. Appl. Sci. 2024, 14, 6058. https://doi.org/10.3390/app14146058

AMA Style

Fan Y, Xiao Q, Cheng Y, Li Y. Zircon U-Pb Age, Sr-Nd-Hf Isotopic Characteristics of Baiyinhushuo Adakite in Inner Mongolia: Implications for Tectonic Evolution. Applied Sciences. 2024; 14(14):6058. https://doi.org/10.3390/app14146058

Chicago/Turabian Style

Fan, Yuxu, Qinghui Xiao, Yang Cheng, and Yan Li. 2024. "Zircon U-Pb Age, Sr-Nd-Hf Isotopic Characteristics of Baiyinhushuo Adakite in Inner Mongolia: Implications for Tectonic Evolution" Applied Sciences 14, no. 14: 6058. https://doi.org/10.3390/app14146058

APA Style

Fan, Y., Xiao, Q., Cheng, Y., & Li, Y. (2024). Zircon U-Pb Age, Sr-Nd-Hf Isotopic Characteristics of Baiyinhushuo Adakite in Inner Mongolia: Implications for Tectonic Evolution. Applied Sciences, 14(14), 6058. https://doi.org/10.3390/app14146058

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