Paleoproterozoic Crust–Mantle Interaction in the Khondalite Belt, North China Craton: Constraints from Geochronology, Elements, and Hf-O-Sr-Nd Isotopes of the Layered Complex in the Jining Terrane

Abstract

The Paleoproterozoic Khondalite Belt, located in the northwestern segment of North China Craton (NCC), is characterized by widespread high-temperature/ultrahigh-temperature (UHT) granulite/gneiss and large-scale magmatic activity. The tectonic evolution is still controversial. Here, we report new geochronological, elemental, and Hf-O-Sr-Nd isotopic data for a Paleoproterozoic layered complex in the Jining terrane to constrain the tectonic evolution of the Khondalite Belt. In situ zircon U-Pb dating indicates that the Sanchakou gabbros were emplaced between ~1.94 Ga and ~1.82 Ga, which might be the heat source of UHT metamorphism. The elemental and Hf-O-Sr-Nd isotopic analysis shows that the formation of Sanchakou gabbros is consistent with the assimilation and fractional crystallization (AFC) process. The magma originates from the 10%~20% partial melting of the spinel + garnet lherzolite mantle. The Sanchakou gabbros are magmatic crystallization products mixed with crustal wallrocks in the magma chamber. We have established a tectonic evolution model involving asthenosphere upwelling after the amalgamation of the Ordos and Yinshan Blocks at ~1.95 Ga.

1. Introduction

The concept of “layered complex” was first recorded in Layered Igneous Rocks [1]. Most of the layered complex is close to gabbros, mainly composed of basic or ultrabasic rocks. The most notable feature is the well-developed layered rhythmic structure. The layered complexes formed mainly in Archean and Proterozoic [2,3,4,5,6], with a lesser amount in Phanerozoic [7,8,9]. The tectonic environment is related to mantle plume or intracontinental rifting [10,11,12,13], controlled by regional fractures. Crustal contamination plays an important role in the formation of layered complexes, which can greatly change the composition of magma and result in the difference in products [14,15]. Therefore, the compositional changes in layered complexes in the open magmatic system are significant for revealing crust-mantle interaction [16,17,18,19].

The North China Craton (NCC) is a fundamental geological unit of the early Precambrian in China [20]. The Jining terrane, located in the eastern Khondalite Belt of the NCC, has been widely studied by researchers in past decades. A large number of ultrahigh-temperature (UHT) metamorphic rocks have been reported in this area [21,22,23,24,25,26,27,28,29,30,31,32]. In addition, there are small amounts of basic intrusive rocks dominated by gabbros, some of which occurred as multiple sets of layered complexes [33]. The episodic crystallization ages are 2.45~2.10 Ga, 1.97~1.92 Ga, and 1.85~1.84 Ga [34,35,36,37]. The magma may have originated from a deep mantle of ~3.0 GPa, and ~1550 °C [35]. However, the genesis of these basic intrusive rocks and tectonic environment have not been determined yet.

In this paper, we focus on a Paleoproterozoic layered complex in the Jining area, northwestern margin of the NCC, which is accompanied by extensive granulites/gneisses. Through the chronological, elemental, and Hf-O-Sr-Nd isotopic analysis methods, we have found that the layered complex is a product of crust–mantle interaction, which reflects the large-scale thermal fluctuation under the influence of the upwelling asthenosphere.

2. Geological Setting

The NCC is one of the rare ancient continental blocks with ~3.8 Ga crustal rock in the world [38,39,40]. There are different views about the formation and evolution of the NCC [20,41,42]. The Khondalite Belt is in the western part of the NCC (Figure 1a), considered to be formed via a collision between the Ordos and Yinshan Blocks at ~1.95 Ga [20,43]. From west to east, there are Qianlishan-Helanshan terrane, Daqingshan-Helanshan terrane, and Jining terrane. The Jining terrane is located in the eastern segment of the Khondalite Belt (Figure 1b), where UHT granulites/gneisses, S-type granites, and a small amount of basic intrusive rocks are mainly exposed (Figure 1c).

The UHT granulites/gneisses are scattered in Tuguishan, Dajing, Tianpishan, Xuwujia, Xumayao, Helinger, Hongsigou, Zhaojiayao, Liangcheng, and Hongshaba [23,24,25,26,28,29,30,31,44,45,46,47,48,49,50], continuously forming a UHT metamorphic zone of about 250 × 150 square kilometers. Most of these UHT granulites/gneisses are sillimanite-garnet gneisses, and only a few contain spinel/sapphirine + quartz assemblages. They have average metamorphic ages of 1.92~1.91 Ga [30,31,44,50] and ~1.88 Ga [28]. The peak temperature of UHT metamorphism is mainly concentrated at 950~1050 °C [23,24,25,26,28,29,30,31,44,45,46,47,48,49,50].

The vast majority of the S-type granites are garnet granites, which intruded into the khondalite and covered more than 40% of the Jining terrane. The crystallization age is about 1.94~1.90 Ga [51,52]. One view [51] believed that garnet granite is a mixture of the mantle-derived basic magma and the melt produced by the anatexis of metasedimentary rocks, formed in the process of UHT metamorphism (1.93~1.92 Ga). The other view [52] believed that the garnet granite was formed before UHT metamorphism (1.94~1.93 Ga) and underwent UHT metamorphism (~1.92 Ga) with metasedimentary rocks.

A few basic intrusive rocks are mainly gabbros with an average crystallization age of ~1.93 Ga, believed to originate from the mantle plume or mantle upwelling under the background of the mid-ocean ridge [35].

3. Samples and Methods

3.1. Sample Description

The studied sample is from a drilling core of the layered complex, taken from Sanchakou town, Jining District, Ulanqab City, Inner Mongolia Autonomous Region (Figure 1c). We selected a section in which the overall lithology is gabbro, with obvious changes in feldspar content in the vertical direction (Figure 2a). Therefore, the layered complex was named the Sanchakou gabbros. We chose typical positions of the drilling core, cut into rock thin slices, ground the rock into powder, and numbered SCK-1 to SCK-6, respectively.

The rock has a full-crystalline unequal granular texture. The main minerals include clinopyroxene, amphibole, and plagioclase. The altered minerals mainly include zoisite and epidote. The representative accessory minerals include ilmenite, magnetite, apatite, and zircon. The clinopyroxene is mostly irregular prismatic, widely developing a group of parallel cleavage (Figure 2b). The amphibole has obvious pleochroism, and its interference color is lower than that of the clinopyroxene. The primary amphibole is generally subhedral plate-prismatic. The secondary amphibole is in a xenomorphic long-prismatic shape, growing along the edge of other minerals. The plagioclase has undergone intense zoisitization and idolization alteration. The opaque minerals mainly include ilmenite and magnetite, some of which form the equilibrium assemblages (Figure 2c). The apatite is transparent and round. The zircon is subhedral long-prismatic, developing a higher white interference color.

3.2. Analytical Methods

3.2.1. Mineral Major Elements Analysis

Mineral major elemental analysis was conducted by EPMA at the MOE Key Laboratory of Orogenic Belts and Crustal Evolution, School of Earth and Space Sciences, Peking University, Beijing, China. We used JEOL JXA-8230 for in situ analysis, equipped with four-channel spectrometers (CH₁-CH₄). PETJ crystal (CH₁) was used to determine K, Ca, and Ti. Two TAP crystals (CH₂, CH₄) were used to determine Na, Si, Mg, and Al. LIFH crystal (CH₃) was used to determine Cr, Mn, Fe, and Ni. The acceleration voltage, the beam current, and the beam spot were set to 15 kV, 10 nA, and 2 μm. The counting times for the background and peak values of Ca, Mg, Al, and Fe are 5 s and 20 s, and those of other elements are 5 s and 10 s. The standard samples are 53 kinds of minerals from American Structure Probe Inc. Supplies^TM Company. We used the stoichiometric method to calculate and the PRZ method to correct. Detailed information for the configuration of crystal spectrometers and the mineral standard samples of related elements was the same as described by [53].

3.2.2. In Situ Zircon U-Pb Isotopic Analysis

In situ zircon U-Pb isotopic analysis was conducted by LA-ICP-MS at the Wuhan SampleSolution Analytical Technology Company Limited, Hubei, China. Detailed operating conditions for the laser ablation system and the ICP-MS instrument were the same as described by [54]. Laser sampling was performed using a GeolasPro laser ablation system that consists of a COMPexPro 102 ArF excimer laser (wavelength of 193 nm and maximum energy of 200 mJ) and a MicroLas optical system. We used an Agilent 7900 ICP-MS instrument to acquire ion-signal intensities. Helium was applied as a carrier gas. Argon was used as the make-up gas and mixed with the carrier gas via a T-connector before entering the ICP. A “wire” signal smoothing device is included in this laser ablation system [55]. The spot size and frequency of the laser were set to 24 µm and 5 Hz. International zircon standard 91500 and glass NIST610 were used as external standards for U-Pb dating and trace element calibration, respectively. TEMORA-2 was treated as a blind sample to monitor the working state of the instrument. Each analysis incorporated a background acquisition of approximately 20~30 s followed by 50 s of data acquisition from the sample. We used ICPMSDataCal to perform offline selection and integration of background and analyze signals, time-drift correction, and quantitative calibration [56,57]. Concordia diagrams, probability density diagrams, and weighted mean calculations were made using Isoplot 4.15 [58].

3.2.3. Whole-Rock Major and Trace Elements Analysis

Whole-rock chemical pretreatment and major and trace element analysis were conducted at Nanjing FocuMS Technology Company Limited, Jiangsu, China. For major elements, we adopt the acid dissolution method to dissolve the sample powder to analyze the major elements except for Si and the alkali fusion method to liquate the sample powder to analyze Si. The analytical instrument was Agilent 5110 ICP-OES. The analysis accuracy of SiO₂ is better than 1%; that of Al₂O₃, TFeO, MgO, K₂O, Na₂O, and CaO is better than 3%; and that of TiO₂, MnO, and P₂O₅ is better than 5%. For trace elements, we adopt the acid dissolution method to dissolve the sample powder. The analytical instrument was Agilent 7700x ICP-MS. Among them, the analysis accuracy of trace elements with content more than 50 × 10⁻⁶ is better than 5%; that of trace elements with content between 5 × 10⁻⁶ and 50 × 10⁻⁶ is better than 10%; and that of trace elements with content between 0.5 × 10⁻⁶ and 5 × 10⁻⁶ is better than 20%. Geochemical reference materials of BHVO-2 and AGV-2 were treated as blind samples for quality assurance of measurement, the measured values of which were compared with Geological and Environmental Reference Materials (GeoReM) [59]. The detailed operating process was the same as described by [60].

3.2.4. In Situ Zircon Hf-O Isotopic Analysis

In situ zircon Hf isotope ratio analysis was conducted using a Neptune Plus MC-ICP-MS in combination with a Geolas HD excimer ArF laser ablation system that was hosted at the Wuhan SampleSolution Analytical Technology Company Limited, Hubei, China. A “wire” signal smoothing device is included in this laser ablation system, by which smooth signals are produced even at very low laser repetition rates down to 1 Hz [55]. Helium was used as the carrier gas within the ablation cell and was merged with argon after the ablation cell. Small amounts of nitrogen were added to the argon makeup gas flow for sensitivity improvement [61]. The single spot size and energy density of the laser were set to 32 μm and ~7.0 J cm⁻². Each measurement included 20 s of background signal acquisition and 50 s of ablation signal acquisition. Detailed operating conditions for the laser ablation system, the MC-ICP-MS instrument, and the analytical method were the same as described by [61]. The interference of ¹⁷⁶Lu and ¹⁷⁶Yb on ¹⁷⁶Hf was corrected according to [62]. Off-line selection and integration of analytical signal and mass bias calibrations were performed using ICPMSDataCal [57]. To ensure the data reliability, three international zircon standards of Plešovice, 91500, and GJ-1 were analyzed simultaneously with the actual samples, the Hf isotopic compositions of which have been reported by [63]. The external accuracies (2σ) of Plešovice, 91500, and GJ-1 were better than 0.000020. The test values were consistent with the recommended value within the error range.

In situ zircon O isotopic analysis was conducted using a SHRIMP Ⅱe-MC that was hosted at the Beijing SHRIMP Center, Institute of Geology, Chinese Academy of Geological Sciences, Beijing, China. Detailed operating conditions were the same as described by [64,65]. Two groups of scans were set for each data analysis. Each group was scanned 6 times, and the integral time of each scan was 10 s. Between the two groups of scans, the instrument can automatically readjust the parameters of the primary ion current and the secondary ion current to achieve the best result. During the analysis process, the ratio of sample zircon data points to standard zircon data points was 1:2~1:4. The internal accuracy of single analysis data was generally better than ±0.3‰ (2σ). We used TEMORA-2 (δ¹⁸O = 8.20‰) [66] and 91500 (δ¹⁸O = 9.86‰) [67] to monitor the working state of the instrument and correct for the mass fractionation. Vienna standard mean ocean water (V-SMOW) was adopted to standardize the O isotopic analysis results.

In particular, the zircon number and position of in situ Hf-O isotopic analysis were consistent with those of in situ U-Pb isotopic analysis.

3.2.5. Whole-Rock Sr-Nd Isotopic Analysis

Whole-rock chemical pretreatment and Sr-Nd isotopic analysis were conducted at Nanjing FocuMS Technology Company Limited, Jiangsu, China. We adopt the acid dissolution method to dissolve the sample powder, followed by extraction and chromatographic separation according to the method of [68]. The detailed operating process was the same as described by [69]. The analytical instrument was Nu Plasma II MC-ICP-MS. Raw data of isotope ratios were internally corrected for the mass fractionation by normalizing ⁸⁶Sr/⁸⁸Sr = 0.1194 for Sr, ¹⁴⁶Nd/¹⁴⁴Nd = 0.7219 for Nd with exponential law. International isotopic standards (NIST SRM 987 for Sr, JNdi-1 for Nd) were periodically analyzed to correct instrumental drift. Geochemical reference materials of BCR-2 and BHVO-2 were treated as blind samples for quality assurance of measurement. These isotopic results agreed with previous publications regarding analytical uncertainty [70].

4. Results

4.1. Mineral Chemistry

The EPMA analytical results of clinopyroxene, amphibole, plagioclase, magnetite, and ilmenite in the Sanchakou gabbros are listed in Table 1.

4.1.1. Clinopyroxene

According to the classification of [71], the clinopyroxene of Sanchakou gabbros corresponds to the Ca-Mg-Fe pyroxenes (Figure 3a). Its composition is Wo_46.71~47.48En_36.26~39.35Fs_12.17~14.74, belonging to the diopside (Figure 3b). The Mg^# of the clinopyroxene ranges from 72.11 to 77.20, with an average value of 75.58. The Cr^# is mostly 0. The clinopyroxene has relatively low contents of TiO₂ (0.14~0.56 wt.%), Al₂O₃ (1.42~3.61 wt.%), TFeO (7.57~8.97 wt.%), and Na₂O (0.30~0.55 wt.%).

4.1.2. Amphibole

The amphibole of Sanchakou gabbros has relatively high contents of Al₂O₃ (10.14~10.97 wt.%), TFeO (12.12~14.12 wt.%), MgO (12.30~13.84 wt.%), and CaO (11.76~12.86 wt.%) and relatively low contents of SiO₂ (42.15~45.53 wt.%), Na₂O (1.21~1.61 wt.%), and K₂O (0.96~1.32 wt.%). In general, its composition has little change. According to the classification of [72], the amphibole belongs to the pargasite and edenite (Figure 3c).

4.1.3. Plagioclase

The plagioclase of Sanchakou gabbros has relatively high contents of Al₂O₃ (27.46~29.03 wt.%) and CaO (10.49~11.94 wt.%) and a relatively low content of K₂O (0.14~0.35 wt.%). Its composition is An_50.20~57.16Ab_41.42~48.12Or_0.80~1.99. According to the classification of [73], the plagioclase belongs to the labradorite (Figure 3d). Figure 3

Mineral compositions of the Sanchakou gabbros from the Jining terrane. (a) The Q-J diagram of pyroxene according to the nomenclature of [71]; (b) The classification diagram of pyroxene according to the nomenclature of [71]; (c) The classification diagram of amphibole according to the nomenclature of [72]; (d) The classification diagram of feldspar according to the nomenclature of [73]. Mineral abbreviations: Ab, albite; An, anorthite; En, enstatite; Fs, ferrosilite; Or, potassium feldspar; Wo, wollastonite.

[Figure omitted. See PDF]

4.2. In Situ Zircon Isotopic Characteristics

4.2.1. Zircon U-Pb Dating

The in situ zircon U-Pb isotopic data of the Sanchakou gabbros are listed in Table 2. The zircons selected from the Sanchakou gabbros are irregularly rounded crystals, ranging from 70 μm to 180 μm in diameter. Almost all zircons have bright growth edges in CL images, which vary in width (Figure 4a). The core of zircons is generally dark. A few of the cores have obvious growth stripes inside. Small amounts of zircons have 2225~2375 Ma inherited cores. All characteristics reflect the crystallization history of zircons and the superposition of multi-stage magmatism. The zircon Th/U ratios range from 0.11 to 7.53, with an average value of 1.44. Based on these features, the zircon belongs to the magmatic zircon.

Except for the data of inheritance cores, the rest zircon grains show a “beaded” distribution along the concordia line (Figure 4b). Based on the principle of statistics, the data can be divided into two groups: the concordia age of the first group is 1942 ± 8 Ma (n = 7) and the concordia age of the second group is 1824 ± 17 Ma (n = 11). The distribution of corrected ²⁰⁷Pb/²⁰⁶Pb apparent ages ranges from 1802 ± 38 Ma to 2029 ± 39 Ma. The ²⁰⁷Pb/²⁰⁶Pb weighted average age is concentrated at 1923 ± 28 Ma (n = 18). The ²⁰⁷Pb/²⁰⁶Pb age probability density histogram shows that the data distribution is continuous (Figure 4c), and the peak value is 1941 ± 22 Ma (n = 18).

4.2.2. Zircon Hf-O Isotopes

The in situ zircon Hf-O isotopic data of the Sanchakou gabbros are listed in Table 3.

The zircon has a high Hf content and a very low Lu content, resulting in a very low ¹⁷⁶Lu/¹⁷⁷Hf ratio and a very low content of ¹⁷⁶Hf formed by Lu decay. So, there is no obvious radioactive accumulation after the formation of zircon. The ¹⁷⁶Hf/¹⁷⁷Hf measured in the samples can represent the Hf isotopic composition of the system when the zircons formed. The initial ¹⁷⁶Hf/¹⁷⁷Hf value of zircon in the samples is low (0.281383~0.281468), and the weighted average value is 0.281413 ± 0.000011. The ε_Hf(t) values are all negative, ranging from −8.4 to −3.8 (Figure 5a). The Lu-Hf T_DM age ranges from 2444 Ma to 2609 Ma, which belongs to the Paleoproterozoic and Archean, indicating that the magma may have originated from the enriched mantle or suffered from crustal contamination [74].

The zircon δ¹⁸O value of the samples ranges from 8.08 to 9.26, with a weighted average value of 8.75 ± 0.16‰ (Figure 5b), which is much higher than that of the mantle zircon (5.3 ± 0.6‰) [75]. Its genesis may be that the parent magmatic source of the Sanchakou gabbros was added with the high-δ¹⁸O crustal materials, or the parent magma suffered from contamination by high-δ¹⁸O crustal wallrocks during emplacement. Figure 5

Zircon Hf-O isotopic compositions for the Sanchakou gabbros. (a) The ε_Hf (t) versus ²⁰⁷Pb/²⁰⁶Pb age diagram; (b) The δ¹⁸O weighted average value distribution diagram. The Hf isotopic data of Xuwujia gabbronorites and Xigou gabbros are from [35,76].

[Figure omitted. See PDF]

4.3. Whole-Rock Geochemical Characteristics

4.3.1. Major Elements

The whole-rock major elemental data of the Sanchakou gabbros are listed in Table 4. The loss on ignition (LOI) of the Sanchakou gabbros is negative, indicating that the samples have suffered from a low degree of alteration. It is consistent with the relatively fresh characteristics observed in the field. The content of SiO₂ is 43.45~45.01 wt.%, indicating that the samples belong to the basic rocks. The samples contain 12.73~13.64 wt.% Al₂O₃, 0.14~0.23 wt.% K₂O, and 2.05~2.25 wt.% Na₂O. The content of Na₂O + K₂O is 2.21~2.47 wt.%, and the ratio of K₂O/Na₂O is 0.06~0.10, indicating that the samples belong to calc-alkaline gabbros and have not suffered obvious potassium metasomatism. The samples fall into the peridot-gabbro field in the (Na₂O + K₂O)-SiO₂ diagram (Figure 6a) and the medium-K calc-alkaline series field in the K₂O-SiO₂ diagram (Figure 6b). The samples contain 7.44~7.77 wt.% MgO and 17.17~20.27 wt.% TFeO. The Mg^# of the samples ranges from 40.08 to 44.64, with an average value of 41.78. The Mg^# is relatively low, indicating that the Sanchakou gabbros were not formed in primary magma. The content of TiO₂ is 2.06~2.65 wt.%, which is higher than that of the high-K alkaline and shoshonitic rocks, indicating that the Sanchakou gabbros should originate from the mantle.

4.3.2. Trace Elements

The whole-rock trace elemental data of the Sanchakou gabbros are listed in Table 5. In the primitive mantle-normalized trace elements patterns of the Sanchakou gabbros, it is generally shown that Ba, K, Nb, Ta, Sr, and Ti are enriched, while Th and U are depleted (Figure 7a). In the chondrite-normalized rare earth elements (REE) patterns of the Sanchakou gabbros, the curve has a slightly right-leaning trend (Figure 7b). The total amount of REE (ΣREE) is not high, ranging from 58.6 × 10⁻⁶ to 77.40 × 10⁻⁶, with an average value of 68.51 × 10⁻⁶. The LREE/HREE ratio is 2.30~2.84, and the (La/Yb)_N ratio is 1.37~1.88, indicating that the LREE and the HREE of the samples are not differentiated. The δEu value is 0.95~1.08, indicating that the samples have no obvious Eu anomaly.

4.3.3. Sr-Nd Isotopes

The whole-rock Sr-Nd isotopic data of the Sanchakou gabbros are listed in Table 6. The ⁸⁷Sr/⁸⁶Sr ratios range from 0.704313 to 0.704882, slightly higher than the mantle value (0.704) and lower than the average value of the continental crust (0.719). The calculated (⁸⁷Sr/⁸⁶Sr)_i ratios range from 0.703942 to 0.704702, lower than that of basaltic magma formed by mantle (0.706) and granite formed by partial melting of crust (0.718). The ¹⁴³Nd/¹⁴⁴Nd ratios range from 0.512364 to 0.512573, lower than the modern value of the primitive mantle (0.512638). The ε_Nd(t) values are all positive, ranging from 2.12 to 2.39. The Sm-Nd T_DM age ranges from 2535 Ma to 2834 Ma. The Sm-Nd T_DM2 age ranges from 2204 Ma to 2226 Ma. In the ε_Nd(t)-(⁸⁷Sr/⁸⁶Sr)_i diagram [80], the Sanchakou gabbros fall into the continental basalt or oceanic island basalt (OIB) field (Figure 8a).

5. Discussion

5.1. Duration of Magmatism and UHT Metamorphism

The Paleoproterozoic Khondalite Belt is characterized by widespread UHT granulites/gneisses and large-scale magmatic events. Combined with previous studies in the Jining terrane, the results of zircon U-Pb geochronology analysis in UHT granulites/gneisses show the extensive and continuous “beaded” distribution along the concordia line, with an average age of ~1.92 Ga [30,31,44,50]. The occurrence time of UHT metamorphism is still controversial. One view is that it occurred at approximately ~1.92 Ga [52]. The other view is that it occurred before ~1.94 Ga, and then the rock experienced a slow cooling process of ~40 Myr [32]. However, it is generally believed that the cooling duration of UHT metamorphism varies with the tectonic environment. The slow cooling is usually >30 Myr, while the rapid cooling is mostly <10 Myr [81,82]. The “beaded” concordia diagram is more likely to reflect the slow cooling process [29,83,84,85]. Therefore, the UHT metamorphism in the Jining terrane belongs to the slow cooling and long-term persistent type, which is similar to that in the Rogaland area of Norway [84] and the Napier complex in East Antarctica [83]. Through the zircon U-Pb geochronology analysis of the Sanchakou gabbros, we found that the magmatism occurred from ~1.94 Ga to ~1.82 Ga, with a duration over 100 Ma and a peak time of ~1.94 Ga. It is coupled with the peak period and slow cooling process of UHT metamorphism in the Jining terrane. Therefore, the magma of these basic intrusive rocks may be the heat source for the UHT metamorphism [30,35,52,86].

5.2. Genesis of Layered Complex

In previous studies, the layered complex is always considered to be the magmatic product undergone a special process of fractional crystallization. In the closed magmatic system, the layered complex can be formed by the magmatic differentiation with only one large-scale magma injection and little or no magma replenishment, such as the Skaergaard complex in Greenland [87]. In the open magmatic system, the layered complex can be formed by magmatic differentiation after the multiple magma injection, such as the Muskox complex in Canada [88], the Bushveld complex in South Africa [89], the Stillwater complex in America [90], and the Rum complex in Britain [91]. However, the fractional crystallization process is usually accompanied by the contamination of crustal wallrocks in many cases [92,93]. Crustal contamination can change the compositions of magmatic melts [94,95]. Here, we discuss the genesis of the Sanchakou gabbros.

5.2.1. Crustal Contamination and Fractional Crystallization

We often regard “Assimilation and Fractional Crystallization (AFC)” as a significant process during the magmatic evolution, which can modify the geochemical compositions of the initial magma [94,95,96]. It does not probably occur in the shallow crust, because there is not enough energy accumulation [97]. The binary diagrams of ε_Nd(t), Mg^# versus SiO₂ show roughly negative correlations, indicating the Sanchakou gabbros are formed in the AFC process and similar to experimental results from peridotite melts (Figure 8b,c). The occurrence of the AFC process is supported by the 2225~2375 Ma inherited zircon in CL images. The Mg^# is often taken to reveal the process of fractional crystallization. There is a positive correlation between CaO/Al₂O₃ ratio and Mg^#, further suggesting that the clinopyroxene is probably the main fractionating mineral phase (Figure 8d). The Sc/Y ratio is usually controlled by clinopyroxene crystallization [98]. The decreasing Sc/Y ratio with the decreasing Mg^# also indicates the crystallization of clinopyroxene (Figure 8e). The plagioclase does not play a vital role during the magmatic evolution, as shown by the nearly constant Eu^*/Eu with the decreasing Mg^# (Figure 8f). Figure 8

Geochemical diagrams of the Sanchakou gabbros. (a) The ε_Nd(t) versus I_Sr diagram (after [80]). (b) The ε_Nd(t) versus SiO₂ diagram. (c) The Mg^# versus SiO₂ diagram. Supposed peridotite melts and crust AFC curve are after [99]. (d) The CaO/Al₂O₃ versus Mg^# diagram. (e) The Sc/Y versus Mg^# diagram. (f) The Eu*/Eu versus Mg^# diagram. Mineral abbreviations: Cpx, clinopyroxene; Ol, olivine; Pl, plagioclase. The major and trace elemental data of Xuwujia gabbronorites and Xigou gabbros are from [35].

[Figure omitted. See PDF]

5.2.2. Magmatic Source and Fluid Metasomatism

The Sr-Nd and Hf-O isotopic systems are used to determine the source of magmatic rocks. The Sanchakou gabbros have a positive whole-rock ε_Nd(t) value (2.12~2.39) and a negative zircon ε_Hf(t) value (−8.4~−3.8), reflecting the characteristics of Nd-Hf isotopic decoupling. The ε_Nd(t)-(⁸⁷Sr/⁸⁶Sr)_i diagram [80] of Sanchakou gabbros indicates that the magma source may be a depleted mantle (Figure 8a). In the δ¹⁸O-εHf (Paleoproterozoic) diagram [100], most concordant zircons fall in the mixing line between the 3.5 Ga supracrustal component and the depleted mantle with Hf concentration ratios (Hf_pm/Hf_c) of 0.7 and 1.5 (Figure 9), indicating that the magma source may be enriched mantle or suffered from the crustal contamination [74]. In the process of mantle evolution, Nd and Hf are more likely to enter the melts than their parent isotopes, but the mobility of Nd is higher than that of Hf [101]. Lu-Hf isotope system is mainly controlled by Hf-rich minerals such as zircon, apatite, and garnet, and the Nd-Hf isotopic decoupling can occur during partial melting [102,103]. Due to the difference in partial melting conditions and the duration of magmatic events, it is difficult to achieve the isotopic equilibrium, resulting in the Nd-Hf isotopic decoupling between the melt and the source region [104]. However, the Sm-Nd T_DM age (2535~2834 Ma) and the Lu-Hf T_DM age (2444~2609 Ma) are in a similar range, which is older than the formation age of Sanchakou gabbros. Therefore, it is very likely that the Nd-Hf isotopic decoupling occurred during mantle-derived magma mixing with crustal wallrocks in the magma chamber.

The melting depths of the layered complex can be modeled with related trace elements [106,107]. REE ratios and abundances (e.g., Sm/Yb, and La/Yb and Sm) are widely used to determine the origin of magma and the melting degree of mantle [107,108,109,110]. The Sanchakou gabbros plot near the spinel + garnet lherzolite melting curves with primitive mantle (PM) starting compositions. The Sm/Yb ratios are lower than the garnet lherzolite melting curves and higher than the spinel lherzolite melting curves (Figure 10a,b). The parental magma is likely to be derived from a mantle source consisting of spinel + garnet lherzolite. Additionally, approximately 10%~20% partial melting of the lherzolites is required.

The Sanchakou gabbros have a relatively lower Th/Yb ratio and higher TiO₂/Yb ratio, and all of the points fall outside the MORB-OIB array (Figure 10c,d), similar to subduction-related enrichment [111]. All of the samples exhibit a relatively high La/Nb ratio (0.48~33.67) and low La/Ba ratio (0.01~0.16), derived from a similar modified continental lithospheric mantle (CLM) source [112]. The basic magma formed by the partial melting of mantle peridotite that has interacted with fluids usually has relatively high Na₂O and P₂O₅ contents, positive to weakly negative Nb anomalies, and non-negative Ti anomalies relative to the PM [113,114]. It is similar to the characteristics of Sanchakou gabbros, thus supporting the interaction between the CLM and the fluids.

The trace element ratios (e.g., Th/Yb, Ba/La, Ba/Th, and Th/Nb) are widely used to distinguish whether metasomatic agents belong to fluids or sediments [115,116]. All the samples have variable Ba/La (6.21~76.85) and Ba/Th (39.50~18,537.50) ratios but relatively constant Th/Yb (0.01~6.59) and Th/Nb (0.00~2.51) ratios (Figure 10e,f), which can be considered as the addition of fluids into the mantle source [116,117,118]. The highly variable Sr/Nd (2.40~127.39) ratio and relatively low Th/Yb (0.01~6.59) ratio further suggest that the fluids could be derived from the large-scale melting crust [119].

In summary, we believe that crustal contamination plays an important role in the formation of Sanchakou gabbros. The magma originates from the 10%~20% partial melting of the spinel + garnet lherzolite mantle. The Sanchakou gabbros are magmatic crystallization products mixed with crustal wallrocks in the magma chamber. Figure 10

Geochemical diagrams of the Sanchakou gabbros. (a) The Sm/Yb versus La/Yb diagram (after [120]). (b) The Sm/Yb versus Sm diagram (after [107]). Solid and dashed curves are the melting trends from DM (depleted MORB) and PM (primitive mantle). (c) The Th/Yb versus Nb/Yb diagram (after [121]). (d) The TiO₂/Yb versus Nb/Yb diagram (after [121]). (e) The Th/Yb versus Ba/La diagram (after [116]). (f) The Ba/Th versus Th/Nb diagram (after [115]). The major and trace elemental data of Xuwujia gabbronorites and Xigou gabbros are from [35].

[Figure omitted. See PDF]

[Figure omitted. See PDF]

5.3. Tectonic Implications

It is generally believed that the Khondalite Belt is the tectonic amalgamation belt of the Ordos and Yinshan Blocks at ~1.95 Ga, which is a typical Paleoproterozoic continent-continent collisional belt [20,43]. This view is supported by the Precambrian granulites/gneisses, but it is still unclear whether it is consistent with the magmatic rocks. A few studies have focused on 2.45~2.10 Ga, 1.97~1.92 Ga, and 1.85~1.84 Ga basic intrusive rocks in the Jining terrane, indicating that they were involved in the subduction and collision processes during the formation of the Khondalite Belt [34,35,36,37]. However, the detailed tectonic environment is still controversial.

For the tectonic environment of Jining terrane at 1.95~1.82 Ga, researchers have proposed a variety of models, including mantle plume events [21], mid-ocean ridge subduction [22,35,86], and post-collision mantle upwelling [30,122]. The mid-ocean ridge subduction usually forms double metamorphic zones and adakitic rocks [123,124], which have not been found in the Jining terrane. The post-collision mantle upwelling model also seems to be insufficient, because the duration of UHT metamorphism formed under this condition is generally within ~30 Myr [81,82], and the upper-temperature limit is usually less than 1000 °C [125,126], which contradicts the fact that extremely high-temperature metamorphic rocks are exposed in the Jining terrane [32]. In addition, the back-arc basin has twice the heat flow value compared with the normal craton, which is also an ideal environment for UHT metamorphism [127,128]. The geothermal gradient is only 20~25 °C/km [127,129], and high-grade metamorphic rocks usually have anticlockwise P-T paths [82,130]. However, it is contrary to the fact that most UHT granulites/gneisses reported in the Jining terrane have clockwise P-T paths [20,43]. Combined with previous studies, the layered complex often forms in the tectonic environment associated with mantle plume or intracontinental rifting, such as the Stillwater complex, the Duluth complex in America, and the Muskox complex in Canada [10,11,12,13]. Only a few form in the late-orogenic or post-orogenic extensional environment, such as the Bjerkreim-Sokndal complex and the Fongen-Hyllingen complex in Norway [131,132]. For basic intrusive rocks in the Jining terrane, the magma emplacement temperature is as high as ~1400 °C, and the mantle potential temperature is about ~1550 °C [35] which is slightly higher than that of the Paleoproterozoic mantle (~1500 °C) [133,134,135]. It is probably caused by the upwelling asthenosphere. The duration of UHT metamorphism depends on the duration of asthenosphere upwelling.

Therefore, we produce a hypothetical tectonic framework for the Khondalite Belt to explain the tectonic evolution at 1.95~1.82 Ga. With the end of the amalgamation between the Ordos and Yinshan Blocks at ~1.95 Ga [20,43], the asthenosphere upwelling resulted in large-scale crustal melting and long-term magmatism. The magma assimilated crustal wallrocks, and then fractional crystallized to form the layered complex (Figure 11), in which several fluids of granulites/gneisses were mixed. Meanwhile, the surrounding granulites/gneisses were heated to form the UHT metamorphism.

6. Concluding Remarks

Based on the in situ zircon U-Pb isotopic analysis of the Sanchakou gabbros, we have found that they have experienced a slow cooling process from ~1.94 Ga to ~1.82 Ga, with a weighted average age of 1923 ± 28 Ma. Combined with the study of elemental and Hf-O-Sr-Nd isotopic analysis, we believe that they are the crystallization products of assimilating crustal wallrocks after 10%~20% partial melting of spinel + garnet lherzolite mantle, probably formed by the asthenosphere upwelling after the amalgamation of the Ordos and Yinshan Blocks.

Author Contributions

Conceptualization, W.-P.Z. and W.T.; methodology, W.-P.Z.; software, W.-P.Z.; validation, W.-P.Z., W.T. and B.W.; formal analysis, W.-P.Z.; investigation, W.-P.Z., Y.-H.Z. and C.-J.W.; resources, Y.-H.Z.; data curation, W.-P.Z. and W.T.; writing—original draft preparation, W.-P.Z.; writing—review and editing, W.T. and C.-J.W.; visualization, B.W.; supervision, W.T.; project administration, W.T.; funding acquisition, W.T., Y.-H.Z. and C.-J.W. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

Not applicable.

Acknowledgments

Constructive suggestions and comments from anonymous reviewers led to great improvements in the quality of this article.

Conflicts of Interest

The authors declare no conflict of interest.

Footnotes

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Figures and Tables

Figure 1. The regional geological map of the Jining terrane in the Khondalite Belt of the NCC. (a) The structural sketch of the NCC (modified from [20]); (b) The distribution map of structural units contained in the Khondalite Belt and adjacent structural units of the Khondalite Belt (modified from [20]); (c) The lithology distribution map of the Jining terrane (modified from [34]).

Figure 2. Petrological characteristics of the Sanchakou gabbros from the Jining terrane. (a) The Sanchakou gabbro drilling core (The arrows indicate the slicing and sampling positions); (b) The main rock-forming minerals, including the diopside, pargasite, and plagioclase; (c) The equilibrium assemblage of magnetite and ilmenite. Mineral abbreviations: Amp, amphibole; Cpx, clinopyroxene; Ilm, ilmenite; Mag, magnetite; Pl, plagioclase.

Figure 4. Zircon U-Pb isotopic analytical results of the Sanchakou gabbros from the Jining terrane. (a) Typical zircon CL images, marked with spot positions of in situ zircon U-Pb and Hf-O isotopic analysis; (b) The zircon U-Pb concordia diagram; (c) The 207Pb/206Pb age probability density histogram together with the 207Pb/206Pb weighted average age distribution.

Figure 6. Binary classification diagrams for the Sanchakou gabbros. (a) The (Na2O + K2O) versus SiO2 diagram [77]; (b) The K2O versus SiO2 diagram [78]. The major elemental data of Xuwujia gabbronorites and Xigou gabbros are from [35].

Figure 7. Chondrite-normalized REE (a) and primitive mantle-normalized trace element (b) patterns for the Sanchakou gabbros. The normalization values are from [79]. The trace elemental data of Xuwujia gabbronorites and Xigou gabbros are from [35].

Figure 9. The zircon δ18O versus εHf(t) diagram of the Sanchakou gabbros. The dotted lines indicate the two-component mixing trends between the depleted mantle and the supracrust-derived magma. We assume that the supracrustal zircons have εHf(t) = −12 and δ18O = 10‰, the depleted mantle zircons have εHf(t) = 12 and δ18O = 5.3‰ [105]. Hfpm/Hfc is the Hf concentration ratio between the parental mantle-derived magma and supracrustal components indicated for each curve. The small circles on the curves indicate 10% mixing increments. The zircon Hf-O isotope compositions of the supracrustal component (S-type granites) are from [105], and those of the depleted mantle are from [75].

Figure 11. Cartoons showing the tectonic evolution of the Khondalite Belt between ~1.95 Ga and ~1.82 Ga.

Table 1

The major elemental data (wt.%) of representative minerals in the Sanchakou gabbros.

Mineral	SiO2	TiO2	Al2O3	Cr2O3	TFeO	MnO	NiO	MgO	CaO	Na2O	K2O	Total
Cpx	52.42	0.25	1.99	0.01	8.09	0.36	0.01	14.05	23.87	0.39	0.01	101.45
Amp	43.89	2.01	10.65	0.03	13.03	0.21	0.02	12.98	12.38	1.46	1.21	97.87
Pl	55.91	0.01	28.08	0.01	0.15	0.01	0.03	0	11.17	5.11	0.25	100.73
Mag	0.31	0.10	0.17	0.04	93.65	0.02	0.01	0.14	0.11	0.06	0.01	94.62
Ilm	0.03	50.58	0.03	0.02	46.29	2.63	0.01	0.05	0.01	0.03	0.00	99.68

Notes: Amp, amphibole; Cpx, clinopyroxene; Ilm, ilmenite; Mag, magnetite; Pl, plagioclase.

Table 2

The in situ zircon U-Pb isotopic data of the Sanchakou gabbros.

Sample	Th (×10−6)	U (×10−6)	Th/U	207Pb/206Pb	±1σ	207Pb/235U	±1σ	206Pb/238U	±1σ	207Pb/206Pb Age (Ma)	±1σ	Corrected 207Pb/206Pb Age (Ma)	±1σ
SCK-1	98.74	146.87	0.67	0.1201	0.0025	5.4627	0.1757	0.3271	0.0044	1958	60	1958	37
SCK-2.1	31.93	280.56	0.11	0.1184	0.0024	5.5976	0.1810	0.3429	0.0050	1943	45	1932	35
SCK-2.2	99.94	42.62	2.34	0.1187	0.0030	5.5456	0.2253	0.3400	0.0065	1936	81	1936	44
SCK-3.1	286.49	529.80	0.54	0.1196	0.0026	5.8190	0.1880	0.3529	0.0043	1979	43	1950	38
SCK-3.2	84.66	110.80	0.76	0.1212	0.0025	5.4406	0.1839	0.3233	0.0053	1976	57	1974	36
SCK-4.1	79.50	149.93	0.53	0.1399	0.0033	7.2351	0.2538	0.3703	0.0050	2226	56	2225	41
SCK-4.2	112.83	82.50	1.37	0.1187	0.0033	5.3588	0.2084	0.3262	0.0043	1936	71	1936	49
SCK-5.1	175.25	349.17	0.50	0.1526	0.0029	8.5282	0.2523	0.4003	0.0049	2376	46	2375	32
SCK-5.2	70.99	95.85	0.74	0.1136	0.0037	4.9834	0.2247	0.3182	0.0047	1924	58	1924	57
SCK-6	97.22	265.73	0.37	0.1505	0.0032	8.8052	0.2818	0.4242	0.0053	2355	48	2352	36
SCK-8.1	91.48	13.31	6.87	0.1214	0.0052	5.6946	0.3648	0.3514	0.0088	1977	123	1977	75
SCK-8.2	65.32	164.50	0.40	0.1175	0.0027	5.6357	0.1866	0.3434	0.0042	1920	66	1919	40
SCK-9.1	126.56	162.62	0.78	0.1405	0.0028	7.5763	0.2212	0.3870	0.0042	2235	50	2234	34
SCK-9.2	68.40	113.10	0.60	0.1212	0.0027	5.9214	0.2012	0.3507	0.0048	1974	56	1974	40
SCK-10.1	127.14	248.59	0.51	0.1516	0.0027	9.4564	0.2680	0.4488	0.0057	2365	46	2364	30
SCK-10.2	155.94	183.12	0.85	0.1180	0.0025	5.3711	0.1641	0.3291	0.0035	1926	55	1926	38
SCK-11	78.17	140.98	0.55	0.1131	0.0025	5.0615	0.1605	0.3238	0.0036	1850	57	1849	40
SCK-12.1	110.85	271.20	0.41	0.1250	0.0028	6.1952	0.1955	0.3589	0.0038	2029	56	2029	39
SCK-12.2	172.81	136.29	1.27	0.1133	0.0025	5.1886	0.1703	0.3318	0.0043	1854	58	1853	39
SCK-12.3	94.04	21.42	4.39	0.1107	0.0042	4.9726	0.2740	0.3304	0.0066	1810	109	1810	68
SCK-13.1	95.48	133.61	0.71	0.1101	0.0023	4.9251	0.1510	0.3237	0.0037	1811	57	1802	38
SCK-13.2	104.55	13.89	7.53	0.1111	0.0044	4.7167	0.2796	0.3271	0.0077	1817	136	1817	70
SCK-14	62.50	193.00	0.32	0.1130	0.0020	5.0281	0.1381	0.3212	0.0036	1850	50	1848	32

Table 3

The in situ zircon Hf-O isotopic data of the Sanchakou gabbros.

Sample	207Pb/206PbAge (Ma)	176Lu/177Hf	±2σ	176Hf/177Hf	±2σ	εHf (t)	εHf (0)	±2σ	TDM(Ma)	δ18O(‰)	±2σ
SCK-1	1958	0.000770	0.000020	0.281411	0.000012	−5.5	−48.1	0.4	2558	8.57	0.25
SCK-2.1	1943	0.000738	0.000006	0.281396	0.000012	−5.3	−47.1	0.4	2526	8.68	0.21
SCK-2.2	1936	0.000383	0.000004	0.281424	0.000012	−7.6	−47.8	0.4	2546	8.50	0.18
SCK-3.1	1979	0.000995	0.000026	0.281385	0.000013	−3.9	−48.7	0.4	2553	8.29	0.17
SCK-3.2	1976	0.000563	0.000013	0.281403	0.000012	−7.1	−48.2	0.4	2520	9.06	0.28
SCK-4.2	1936	0.000609	0.000047	0.281388	0.000013	−8.4	−48.5	0.5	2531	8.87	0.20
SCK-5.2	1924	0.000355	0.000016	0.281407	0.000012	−7.4	−47.7	0.4	2493	9.04	0.26
SCK-8.1	1977	0.000839	0.000001	0.281446	0.000012	−7.8	−48.2	0.5	2512	9.00	0.21
SCK-8.2	1920	0.000042	0.000001	0.281418	0.000012	−5.2	−46.1	0.5	2444	8.72	0.18
SCK-9.2	1974	0.000744	0.000007	0.281383	0.000013	−6.3	−48.6	0.4	2576	8.38	0.27
SCK-10.2	1926	0.000858	0.000005	0.281439	0.000012	−5.0	−47.7	0.4	2516	8.84	0.24
SCK-11	1850	0.000761	0.000004	0.281420	0.000012	−6.3	−49.1	0.5	2609	9.12	0.30
SCK-12.1	2029	0.000329	0.000023	0.281394	0.000012	−5.1	−48.4	0.4	2555	8.08	0.19
SCK-12.2	1854	0.000139	0.000002	0.281409	0.000012	−6.6	−48.9	0.5	2578	8.78	0.25
SCK-12.3	1810	0.000152	0.000002	0.281402	0.000015	−5.9	−48.3	0.4	2536	9.22	0.16
SCK-13.1	1811	0.000031	0.000001	0.281424	0.000011	−5.2	−46.9	0.4	2515	9.11	0.24
SCK-13.2	1817	0.000011	0.000000	0.281408	0.000013	−3.8	−47.9	0.4	2501	8.41	0.16
SCK-14	1850	0.000187	0.000012	0.281468	0.000013	−6.1	−49.1	0.5	2595	9.26	0.21

Table 4

The whole-rock major elemental data (wt.%) of the Sanchakou gabbros.

Sample	SCK-1	SCK-2	SCK-3	SCK-4	SCK-5	SCK-6
SiO2	45.01	44.87	43.89	43.45	43.75	43.50
TiO2	2.06	2.29	2.46	2.65	2.52	2.59
Al2O3	13.64	13.50	13.11	12.73	13.04	13.15
TFeO	17.17	17.87	19.18	20.27	19.20	19.96
MnO	0.26	0.27	0.27	0.28	0.26	0.28
MgO	7.77	7.61	7.68	7.61	7.44	7.56
CaO	11.41	11.09	11.17	10.79	11.38	10.46
Na2O	2.25	2.24	2.12	2.05	2.13	2.17
K2O	0.23	0.17	0.21	0.16	0.19	0.14
P2O5	0.21	0.23	0.25	0.27	0.26	0.27
LOI	−0.14	−0.24	−0.24	−0.33	−0.14	−0.01
Total	99.86	99.90	100.08	99.94	100.02	100.08
Na2O + K2O	2.47	2.41	2.33	2.21	2.32	2.31
K2O/Na2O	0.10	0.08	0.10	0.08	0.09	0.06
Mg#	44.64	43.16	41.66	40.08	40.84	40.32
CaO/Al2O3	0.84	0.82	0.85	0.85	0.87	0.80

Table 5

The whole-rock trace elemental data (×10⁻⁶) of the Sanchakou gabbros.

Sample	SCK-1	SCK-2	SCK-3	SCK-4	SCK-5	SCK-6
Rb	1.28	1.17	0.77	0.72	1.02	0.66
Sr	271.97	282.96	258.95	257.62	289.98	287.52
Ba	45.74	67.76	37.23	35.06	84.33	31.27
Th	0.06	0.05	0.04	0.03	0.19	0.04
U	0.02	0.05	0.01	0.06	0.16	0.05
Zr	90.73	101.63	95.27	109.80	101.95	92.60
Hf	2.59	2.67	2.71	2.99	2.79	2.56
Nb	9.02	9.90	10.74	11.30	10.74	10.41
Ta	0.50	0.56	0.58	0.61	0.57	0.58
Sc	47.28	46.05	51.06	50.66	54.06	49.49
La	6.17	6.59	5.95	5.52	6.85	5.03
Ce	19.61	18.62	20.04	17.80	20.48	15.44
Pr	3.06	2.72	3.11	2.80	3.15	2.35
Nd	15.94	14.00	16.83	15.17	17.13	12.80
Sm	4.44	3.85	4.86	4.49	5.09	3.87
Eu	1.68	1.44	1.75	1.56	1.74	1.37
Gd	5.11	4.42	5.78	5.60	6.22	4.79
Tb	0.83	0.73	0.93	0.88	1.01	0.77
Dy	5.24	4.60	5.94	5.65	6.52	4.99
Ho	1.06	0.95	1.21	1.14	1.32	1.02
Er	3.01	2.70	3.42	3.24	3.65	2.82
Tm	0.44	0.39	0.49	0.47	0.52	0.41
Yb	2.75	2.51	3.06	2.90	3.25	2.57
Lu	0.41	0.37	0.45	0.41	0.46	0.37
Y	28.98	24.92	31.89	29.86	34.37	26.19
ΣREE	69.73	63.88	73.81	67.63	77.40	58.60
LREE/HREE	2.70	2.84	2.47	2.33	2.37	2.30
LaN/YbN	1.61	1.88	1.39	1.37	1.51	1.41
δEu	1.08	1.07	1.01	0.95	0.95	0.97
Sc/Y	1.63	1.85	1.60	1.70	1.57	1.89
Eu*/Eu	2.85	2.87	3.04	3.24	3.25	3.16
Sm/Yb	1.62	1.53	1.59	1.55	1.57	1.51
La/Yb	2.24	2.63	1.94	1.90	2.11	1.96
Th/Yb	0.02	0.02	0.01	0.01	0.06	0.02
TiO2/Yb	0.75	0.91	0.80	0.92	0.77	1.01
La/Nb	0.68	0.67	0.55	0.49	0.64	0.48
La/Ba	0.13	0.10	0.16	0.16	0.08	0.16
Th/Yb	0.02	0.02	0.01	0.01	0.06	0.02
Ba/La	7.41	10.28	6.26	6.35	12.31	6.21
Ba/Th	782.96	1243.80	1008.25	1185.70	447.10	739.64
Th/Nb	0.01	0.01	0.00	0.00	0.02	0.00

Table 6

The whole-rock Sr-Nd isotopic data of the Sanchakou gabbros.

Sample	87Rb/86Sr	87Sr/86Sr	±1σ	(87Sr/86Sr)i	147Sm/144Nd	143Nd/144Nd	±1σ	εNd (t)	TDM (Ma)	TDM2 (Ma)
SCK-1	0.0136	0.704313	0.000005	0.703942	0.1684	0.512389	0.000002	2.12	2563	2226
SCK-2	0.0120	0.704346	0.000004	0.704019	0.1664	0.512364	0.000002	2.13	2535	2225
SCK-3	0.0086	0.704425	0.000005	0.704191	0.1743	0.512474	0.000002	2.33	2619	2209
SCK-4	0.0080	0.704634	0.000004	0.704415	0.1787	0.512528	0.000003	2.28	2716	2213
SCK-5	0.0102	0.704731	0.000005	0.704454	0.1797	0.512545	0.000002	2.39	2713	2204
SCK-6	0.0066	0.704882	0.000004	0.704702	0.1826	0.512573	0.000003	2.21	2834	2219

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