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

1. Introduction

The southeastern segment of the Central Asian Orogenic Belt (CAOB) is sandwiched between the Siberian, North China, and Pacific plates. It is the longest-known Phanerozoic accretion orogenic belt, with the complex tectonic-magmatic processes. 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 [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 early Triassic adakites have been found on both sides of the Xar-Moron River suture zone, which are considered to be magmatism of crustal thickening and melting during the evolution of the accretionary orogeny to the final collision [7,8,9,10,14,15], and 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 Central Asian orogenic belt have experienced the evolutionary stages from subduction to collision during the late Permian to the early Triassic [14,18,19], is a transitional period in tectonic evolution of the eastern part of the CAOB. In this paper, detailed zircon U-Pb dating, 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, which 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.

2. Geological Setting and Samples

Baiyinhushuo is located in the northern part of Xi Ujimqin, 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 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 Gegenaobao Formation, as well as the Early Cretaceous Baiyingaolao Formation and the Quaternary sediments. The main lithology of Gegenaobao Formation is siltstone and fine siltstone, which experienced strong metamorphism. The metamorphism of the clastic rocks is mainly carbonatization, silicification and hornblende.

The magmatic rocks in the study area are relatively developed and have a large outcropping area., the ophiolite of Wusinihei is located in the western part of the study area. Baiyinhushuo granodiorite intrude into Gegenaobao Formation. The intermediate-acidic dikes are widely distributed in the Baiyinhushuo granodiorite, Which the main lithologies are quartz-diorite porphyrite, granite porphyry and quartz porphyry, and early Cretaceous andesite, rhyolite and other volcanic rocks can be found in some places.

The granodiorite is overall grayish-white, massive, medium-coarse-grained structure, and the rock can be seen slight fractured (Figure 2). It is mainly composed of plagioclase (75-80%), quartz (15%±), K-feldspar (5-10%), smectite (2-3%), hornblende (2-3%). Observed under the microscope, plagioclase is subhedral and haphazardly distributed,, which the size is generally 1-2mm, part of the 2-4mm, with the development of lamellar double crystals, and some particles showed an obvious ring structure. Quartz for it shape granular structure, the size of the general 0.5-1mm, part of 0.2-0.5mm, a small portion of 0.1-0.2mm, gap-filling distribution, the surface is fresh and clean, wave-like extinction is common, can be seen band extinction. Potassium feldspar semiautomorphic plate-like, size of about 0.5-1mm, part 1-1.5mm, haphazard distribution, light earthy, dirty surface. Hornblende semi-autonomous columnar, size generally 0.5-1mm, a few 1-1.5mm, haphazardly distributed. Black mica is flaky, flake diameter 0.5-1mm, part 1-1.5mm, haphazard distribution, local account of alteration, visible green cordite, chlorite.

3. Analytical Methods

3.1. Whole-Rock Major and Trace Element

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 tested by XRF method using ARL 9900 X-ray fluorescence spectrometer (XRF). The working voltage of the X-ray was 40 kV and the current was 75mA. When the sample concentration was >1.0%, the relative error of analysis was +1%. When the sample concentration was <1.0%, the relative error was +10%. Trace elements and rare earth elements were determined by high-resolution inductively coupled plasma mass spectr ometry. The samples were first chemically dissolved in a high-pressure reactor and then analyzed by plasma mass spectrometer (Instrument Model: ELEMENT-2) with a relative error of approximately 2%.

3.2. Zircon LA-ICP-MS 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, 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 LA-ICP-MS in-situ dating and the experimental instruments of 193nm excimer laser and inductively coupled plasma mass spectrometer (Instrument Model: Thermo Xseries2). 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 6Hz. The data processing and concordia plot drawing of zircon U-Pb dating were obtained by 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 determined 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 Sr and Nd isotope ratios were corrected by mass fractionation using ⁸⁶Sr/⁸⁸Sr=0.1194 and ¹⁴⁶Nd/¹⁴⁴Nd=0.7219.

The analysis of zircon Lu-Hf isotopes was completed in the Key Laboratory of Continental Dynamics of Ministry of Natural Resources, using Neptune Plus multi-reception plasma mass spectrometer and Geo Las Pro193nm 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 relevant instruments can be found in Hou et al. [24]. The weighted average values of n(¹⁷⁶Hf )/ n(¹⁷⁷Hf ) tests for 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 ε_Hf(t) values were calculated using the spherical meteorite Hf isotope values of n(¹⁷⁶Lu )/ n(¹⁷⁷Hf ) = 0.0332, n(¹⁷⁶Hf )/ n(¹⁷⁷Hf ) = 0.282772 [26].

4. Analytical Results

4.1. Chronological Characteristics

The zircons of the Baiyinhushuo granodiorite are well crystalline, with zircon grain sizes ranging from 0.08 to 0.25 mm, generally authomorphic-semi-authomorphic, and colorless-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 (Fig. 3), indicating that they were all magmatic crystallization zircons. [27,28]. Among the 25 zircons points, 19 zircons exhibit relatively concentrated ages and are located near the concordance line, while a few samples show poor concordance in their age data. The individual zircon ²⁰⁶Pb/²³⁸U ages are listed in Table 1, and the zircon concordant age is 242.8 ± 1.0 Ma (MSWD=2.0), indicating they are products of Early Triassic magmatic activity (Figure 4).

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.

4.2. Major and Trace Elements

The petrochemical data of Baiyinhushuo granodiorite are shown in Table 2. The SiO₂ content of the granodiorite is 66.93%-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. 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) were 6.82-8.12%, showed calc–alkaline to high-K calc–alkaline series (Figure 5b). The Al₂O₃ content of 11.45-14.11% were low, which the A/CNK value were 0.94-1.01, and the A/NK value ranges from 1.30-1.54, in the A/NK–A/CNK diagram (Figure 5c), the samples fall in the quasialuminum field (Figure 5c).

The ΣREE of the granodiorite are low, with ΣREE contents ranging from 54.39×10^-6 to 76.84×10^-6, they showed a right-leaning chondrite distribution curve with a strong differentiation of light and heavy rare earth elements. 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 spidergram is enriched in large ionic lithophilic elements (LILEs), such as Rb, Ba, Th, U., and depleted in high-field-strength elements (HFSEs), such as Nb, P, Ti. The trace elements have high Sr (424.00×10^-6~570.14×10^-6), and high Sr/Y (42.79~62.91) (Table 2).

The geochemical composition of the granodiorite sampled in this study is consistent with Xiong et al. [15]. The SiO₂ varies from 66.93% to 69.40%, MgO ranges from 1.35% to 1.55%, and Al₂O₃ is between 15.37% to 15.43%, and 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. Isotope Geochemical Characterization

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 Hf isotopic composition system during crystallization [34]. The variation of ¹⁷⁶Hf/¹⁷⁷Hf of the samples ranges from 0.282943 -0.283014, with an average of 0.282985. The corresponding ε_Hf (t) values range from +11.3 to +13.8, all of which are positive. The depleted mantle model age (T_DM) is 334-436 Ma, and the two-stage model age (T_2DM) is 389-553 Ma. The ⁸⁷Rb/⁸⁶Sr and (⁸⁷Sr/⁸⁶Sr)_i of the Baiyinhushuo granodiorite sample is low, which the ⁸⁷Rb/⁸⁶Sr is 0.346558, and the (⁸⁷Sr/⁸⁶Sr)_i is 0.703882. ¹⁴⁷Sm/¹⁴⁴Nd is 0.112238, ε_{Nd (t)} is +7.72, which is a positive value. The two-stage mode age of T_2DM is 387 Ma (Table 4).

5.1. Chronology

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 216Ma by the method of SHRIMP, 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 Early Triassic.

In the adjacent area, the magma activity was more frequent during the Late Permian Early Triassic. Late 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, [7]), the Baiyinnuoer Triassic granodiorite as (245.4 ± 1.8 Ma, [36]), and the andesites of Xingfuzhilu Formation in the Linxi area (247 Ma,[17]), and all of them are characterized by typical adakite, suggesting that there was a period of adakic magma eruption during the Late Late-Permian-Early Triassic.

5.2. Petrogenesis of Adakite

The Baiyinhushuo granodiorite, with high Sr content and Sr /Y and La /Yb ratios, is a typical adakite (Figure 7), which has special continental dynamics significance [37]. In recent years, a large number of adakites Late Permian-Early Triassic have been discovered in the southeastern section of the Central Asian orogenic belt, which are distributed along the Xar Moron River suture zone [7,14,19]. Currently, there are three views on the geological background of adakite both sides of the Xar Moron suture zone: one suggests that they drived from the remelting of the Juvenile lower crust [7,8,14,15]; one suggests that subducted slabs broke off, and remained slabs in the mantle were 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 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 of after detachment and interaction with mantle peridotite [39], or by melting of subducted oceanic crust and interacted with mantle wedge [40], and 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 partial melting of the MORB, and also higher than that of the adakite derived from lower crustal melting, such as the Mg# of the adakites range from about 35.72 to 46.16 formed by underplating in the North China Craton [43,44,45]. 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 thickened lower crust- are often Na-rich and K-poor, and belong to the peraluminous magma [42], while the adakite of Baiyinhushuo have the characteristics of K-rich and Na-poor, which is different from the adakites derived from the lower crust.

The high-Mg adakitic andesite of Early 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 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 samples also fall into the field of volcanic arc granite, indicating that the adakite are most likely emplaced at island arc background (Figure 9).

The igneous rocks derived from the melting of the lower crust exhibits 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 directly derived from the melting of oceanic crust.

Generally, Sr-Nd isotopic signatures of magmas close to MORB are directly derived from the melting of 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 juvenile lower crust 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 a low (⁸⁷Sr/⁸⁶Sr)i (0.703382), high ¹⁴³Nd/¹⁴⁴Nd (0.512721) and ε_Nd(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 thicked lower crust-derived. 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 isotopic characteristics are similar to those of subducted oceanic crust. Therefore, the Baiyinhushuo adakite derived from the partial melting of 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 ε_Hf(t) in the Baiyinhushuo adakite are all positive, with a small range of variation between 11.3 and 13.8, and all points are located between the evolutionary line of depleted mantle and the chondrite (Figure 10), and the Hf isotopic signature is consistent with the evolution of Hf in 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 Xing-Meng orogenic belt. The two-stage model age of zircon Hf isotopes is 323-459 Ma, which is relatively young, implying that the Early Paleozoic subducted oceanic crust. is important in the magma genesis.

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 transitioned to a terrestrial environment. In the early Triassic, river lake facies 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 adakites derived from the melting of subducted slab in the area north of the Xar Moron suture zone. For example, the Early Mesozoic (244 Ma) high-Mg adakitic andesite exposed in the Linxi region 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 Lower Triassic Xingfuzhilu Formation (247 Ma) in the Linxi area is 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 detailed zircon mineral study, Liu et al. (2007) [57] concluded that the adakitic rocks of the Longtoushan pluton (241 Ma) 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 region of 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 age of petrogenesis were 245.7±1.8Ma and 242.7±4.0Ma (K-Ar whole-rock ages, [59]), exhibiting 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, 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

5. 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 ε_Nd(t) values, and they may derived from 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-Early 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 melt, forming the rock assemblages in diverse tectonic environments.