Geochemistry, U-Pb Zircon Ages and Hf Isotopes of Basement Rocks Beneath the Northeastern Margin of the Ordos Basin: Constraints on the Paleoproterozoic Evolution of the Western North China Craton

Abstract

SHRIMP zircon ages, Hf-in-zircon isotopic compositions and whole rock geochemistry were analyzed on basement metamorphic rocks from drill cores collected from the northeastern margin of the Ordos Basin. Geochemical data from four metasedimentary rocks show large variations in major element compositions, but have similar REE patterns and trace element compositions, with ΣREE = 161.80 × 10⁻⁶~341.82 × 10⁻⁶, δEu = 0.26~0.63 and L_aN/Y_bN = 3.44~25.38. SHRIMP zircon U-Pb dating of granitic gneiss yielded the magmatic zircon ages between 1856 ± 14 Ma and 2188 ± 11 Ma, with the upper intercept age of 2229 ± 88 Ma. The cores have ε_Hf(t) values of −5.46 to +6.57, and Hf model ages vary from 2869 Ma to 2300 Ma. The analyses of metamorphic overgrowths on zircon grains yield an average metamorphic age of 1865 ± 17 Ma (MSWD = 3.5). The zircon cores have ε_Hf(t) values of −3.98 to 1.95, and Hf model ages vary from 2782 to 2416 Ma. Combined with data from earlier studies, we draw the conclusion that the metamorphic rocks in the borehole were formed during middle Paleoproterozoic time and were involved in a major late Paleoproterozoic tectono-thermal event which might be attributable to the collision between the Western and Eastern Block along the TNCO (the Trans-North China Orogen). The basement rocks are petrologically, geochronologically and geochemically comparable with rocks from the Khondalite series. The fact that the material properties of basement rocks beneath the northeastern Ordos Basin are different from those of the Western Block indicates that the extent and exact boundaries of the Ordos Block need to be refined.

1. Introduction

The North China Craton (NCC) is the largest and oldest Precambrian craton in China [1,2]. It is generally accepted that the basement of the NCC is composed of several different blocks separated by major tectonic boundaries, and that they were originally independent crustal entities that eventually collided to form the NCC. There has been wide acceptance of the model proposed by Zhao et al. [3,4,5,6] who initially suggested a 3-fold subdivision of the NCC, based on lithological, structural, metamorphic and geochronological differences [3,4,7]. These authors pointed out that the central zone of the Trans-North China Orogen (TNCO), separates the NCC into Eastern and Western Blocks (Figure 1). In the Eastern Block, the Longgang and Liaonan-Rangrim collided along the paleoproterozoic Jiao-Liao-Ji orogenic belt [8,9,10,11,12,13]. The rocks of the Jiao-Liao-Ji Belt were considered to have been deposited in a rift setting [14], and the rift was thought to have closed at ~1.85 Ga. In the Western Block, the east-west trending of the Khondalite Belt separates the block into two component parts [6]: the Yinshan Block in the north and the Ordos Block in the south. The Yinshan and the Ordos collided along the Khondalite Belt at 1.95–1.92 Ga [6,15,16,17,18,19,20,21,22,23]. The Eastern and Western blocks collided along the TNCO at ~1.85 Ga and formed the basement of the NCC [18,24,25,26].

Since the younger sedimentary rock covered the entire Ordos Basin, the Precambrian crystalline basement of the Ordos Block is the least understood tectonic unit in the NCC. The Ordos Block was considered to be an Archean block that collided with other Archean blocks of the NCC for a long time. In recent years, SHRIMP zircon U-Pb dating on samples from the drill core of the Precambrian basement of the Ordos Basin (Figure 2) indicates that their protoliths were mainly formed in the Paleoproterozoic (~2.0 Ga) and experienced metamorphism of middle pressure granulite to amphibolite facies in ~1.95 Ga [27,28,29]. These latest studies concluded that the Ordos Block experienced a strong tectono-thermal reset during the late Paleoproterozoic. This discovery is not in agreement with the previous Ordos Block Archean basement model.

In this paper, we report SHRIMP U-Pb and LA-ICP-MS Hf isotope analyses of zircons combined with geochemical data from rocks obtained from drill cores that penetrate the basement beneath the Ordos Basin. These data show that the Ordos Block was involved in a late Paleoproterozoic tectono-thermal event and must be smaller than previously thought. This new geochronology and geochemistry information from the basement rocks of the Ordos Basin is important for understanding the tectonic evolution of the NCC.

2. Geological Background

The Ordos Block, in the southern part of the Western Block, is completely covered by a thick sequence of Mesozoic to Cenozoic sediments and surrounded by the Paleoproterozoic basement (Figure 2). Based on the drill core data, the Ordos basement is composed of granulite, para- and ortho-gneiss, (fine grained) leucocratic gneiss, amphibolites, quartzite, schist and marble [20,27,28,29].

The Yinshan Block forms the northern part of the Western Block of the NCC and is mainly composed of TTG gneiss and greenschist to granulite facies ultramafic to felsic volcanic rocks. Metamorphism is characterized by counterclockwise P-T paths and occurred at ~2.5 Ga [7].

The Khondalite Belt to the south of the Yinshan Block is composed of khondalites (high-grade metamorphic Al-rich metapelites), interlayered with charnockites, orthogneiss, paragneiss and various magmatic suites. The khondalites contain detrital and metamorphic zircons with ages of ~2.0 and 1.95 to 1.83 Ga, respectively, indicating a Paleoproterozoic orogeny [19,24,30,31,32,33,34].

In the Lüliangshan area, east of the Ordos Basin, the basement is composed of the Jiehekou Group, Lüliang Group, Yejishan Group (Heichashan Group) and various granitoids. The Jiehekou Group is composed of meta-argillite-arenaceous rocks, marble and metamorphosed volcanic rocks, and its protolith is similar to the khondalites. Rocks of the Jiehekou Group have a lower grade of metamorphism, commonly amphibolite facies, and was formed during the late Paleoproterozoic [19,35]. The Lüliang and Yejishan Groups are composed of meta-volcano-sedimentary rocks which record magmatic and metamorphic zircon ages of ~2.21 Ga and 1.83 Ga, respectively [36].

In the Daqingshan area, north of the Ordos Basin (Figure 2), the basement is composed of granulites, gneisses, khondalites and plutonic rocks. SHRIMP U-Pb dating of magmatic, detrital and metamorphic zircons revealed multiple tectono-thermal events during the late Neoarchean to late Paleoproterozoic at 2.6 to 2.5 Ga, 2.45 to 2.37 Ga, 2.30 to 2.00 Ga and 1.95 to 1.85 Ga [17,19,30,31,32,36,37,38,39].

In the Qianlishan-Helanshan area, west of the Ordos Basin (Figure 2), the basement is composed of Qianlishan Group and Helanshan Groups, which mostly include granitoids. The field appearance and formation ages of Qianlishan and Helanshan Groups are virtually identical to khondalites in the Daqingshan area [33,34,40].

3. Drill Hole and Samples

Samples were collected from drill hole ZJ-1 in the northeastern Ordos Basin, to the southeast of Yulin City, about 120 km away from Lüliangshan area (Figure 2). All samples were collected from the deepest segment of the drill hole and below the sedimentary cover of the basin.

Drill hole ZJ-1 had a finished depth of 3443.60 m. The strata from top to bottom included: the upper Triassic Yanchang Fm, the middle Triassic Zhifang Fm, the lower Triassic Heshanggou and Liujiagou Fm, the Upper Permian Shiqianfeng Fm, the middle Permian Shihezi Fm, the lower Permian Shanxi and Taiyuan Fm, the upper Carboniferous Benxi Fm, the middle Ordovician Majiagou Fm, the Middle Cambrian Zhangxia and Xuzhuang Fm, and the Paleoproterozoic Group (previously considered Wutai Group of Archean). There were hiatuses of the early Carboniferous to middle Ordovician, early Ordovician to the middle Cambrian and the middle Cambrian to middle Proterozoic. The Cambrian Xuzhuang Fm lies directly on the Proterozoic metamorphic rocks (Figure 3).

The sample ZJ1-1 was collected at the depth of 3426.20 m (Figure 3). It is two-mica plagioclase-gneiss, composed of quartz (25 volume-%), plagioclase (40 volume-%), biotite (20 volume-%), muscovite (10 volume-%) and potassium feldspar (5-10 volume-%). Biotite and muscovite are oriented together and delineate the foliation in the rock. Some plagioclase is altered to sericite and forms irregular or subhedral grains. Strong deformation is also supported by undulate extinction and dynamic recrystallization of most quartz grains (Figure 4a,b).

The sample ZJ1-2 was taken from a depth of 3437.45 m (Figure 3). It is sillimanite-bearing muscovite-biotite-gneiss, composed of quartz (25 volume-%), plagioclase (30 volume-%), muscovite (15 volume-%), biotite (20 volume-%), sillimanite (5 volume-%) and a trace amount of K-feldspar and garnet. The foliation is defined by oriented sillimanite and biotite, whereas quartz shows undulate extinction (Figure 4c,d).

The sample ZJ1-3 was taken from a depth of 3437.54 m (Figure 3). It is granitic gneiss, composed of K-feldspar (15 volume-%), quartz (35 volume-%), plagioclase (35 volume-%), biotite (10 volume-%) and garnet (5 volume-%). Quartz has two different shapes, the irregular grain and the elongated embayed or banding-like grain. K-Feldspars have a mostly irregular grain shape, with a grain size significantly smaller than that of sample ZJ1-1 and ZJ1-2. Plagioclases also show irregular or elongated grains generally with weak sericitization. Garnets are mostly irregular grains, containing rounded quartz, plagioclase and biotite inclusions (Figure 4e).

The sample ZJ1-4 was collected at the depth of 3440.16 m (Figure 3). It is sillimanite gneiss, composed of muscovite + biotite + sillimanite (35~40 volume-%), quartz (35 volume-%), plagioclase (25 volume-%) and a trace amount of garnet. The foliation is defined by oriented biotite, muscovite and sillimanite, whereas quartz shows irregular shape. Plagioclases have an irregular grain or elongated tabular shape with generally weak sericitization (Figure 4f,g).

The sample ZJ1-5 was collected at the depth of 3441.25 m (Figure 3). It is garnet-sillimanite-bearing two-mica gneiss, composed of plagioclase (35 volume-%), quartz (25 volume-%), biotite (30 volume-%), muscovite (5 volume-%), garnet (3 volume-%) and sillimanite (3 volume-%) (Figure 4h,i).

4. Analytical Techniques

The samples were processed involving crushing and initial heavy liquid and subsequent magnetic separation. Zircons were hand-picked and mounted on adhesive tape, enclosed in epoxy resin and polished to about half their thickness and photographed in reflected and transmitted light.

4.1. CL Images

In order to investigate the texture and origin of zircons and choose potential target sites for later U-Pb and Hf analyses, cathodoluminescence (CL) imaging of zircon grains was taken using a CAMECA microprobe at the Beijing SHRIMP Center, Chinese Academy of Geological Sciences (CAGS). CL imaging was carried out using a 305 Hitachi SEM S-3000N (Hitachi Limited., Tokyo, Japan) equipped with a Gatan Chroma CL detector and a DigiSan II data306 recorder (GATAN Inc., Pleasanton, CA, USA). Operating conditions were 9 kv and 99 μA.

4.2. Zircon U-Pb Dating

SHRIMP zircon U-Pb dating of sample ZJ1-3 was carried out using the SHRIMP II (Australian Scientific Instruments, Gladstone Queensland, Australia) at the Beijing SHRIMP Center, CAGS. The analytical procedures and conditions were similar to those described by Williams (1998) [41]. The intensity of the primary O⁻² ion beam was 2–5 nA and spot sizes were 25–30 μm, with each site rastered for 2.5–2 min prior to analysis. Five scans through the mass stations were made for each age determination. Reference materials used were SL13 (U = 238 ppm [41]), M257 (U = 840 ppm [42]) and TEMORA 1 (²⁰⁶Pb/²³⁸U age = 417 Ma [43]). Data processing was carried out using the SQUID and ISOPLOT programs [44]. The common lead correction was based on measured ²⁰⁴Pb and Cumming and Richards (1975) [45] lead composition for the likely age of the rocks. The uncertainties for individual analyses are quoted at the 1 sigma level, whereas the uncertainties on weighted mean ages are quoted at the 95% confidence level.

4.3. Hf-Isotope Analysis

Zircon Hf isotopic analyses were performed on a NU Plasma HR MC-ICP-MS (NU Instruments Ltd., Wrexham Wales, UK) equipped with a GeoLas 2005 (Coherent Inc., Palo Alto, CA, USA) 193 nm ArF-excimer laser-ablation system at the State Key laboratory of Continental Dynamics, Northwest University, Xi’an, China.

Analyses were conducted using a spot size of 44 mm and He as a carrier gas. The laser repetition rate was 10 Hz, and the energy density applied was 15 × 10²⁰ J/cm². Raw count rates for ¹⁷²Yb, ¹⁷³Yb, ¹⁷⁵Lu,¹⁷⁶(Hf +Yb+Lu), ¹⁷⁷Hf, ¹⁷⁸Hf, ¹⁷⁹Hf and ¹⁸⁰Hf were collected simultaneously. The isobaric interference of ¹⁷⁶Lu on ¹⁷⁶Hf was corrected by measuring the intensity of an interference-free ¹⁷⁵Lu isotope and a recommended ¹⁷⁶Lu/¹⁷⁵Lu ratio of 0.02669 to calculate ¹⁷⁶Lu/¹⁷⁷Hf. Similarly, the interference of ¹⁷⁶Yb on ¹⁷⁶Hf was corrected by measuring an interference-free ¹⁷²Yb isotope and using a ¹⁷⁶Yb/¹⁷²Yb ratio of 0.5886 to calculate ¹⁷⁶Hf/¹⁷⁷Hf [46]. Time-dependent drifts of Lu-Hf isotopic ratios were corrected using a linear interpolation according to variations of 91,500 and GJ-1. To check data quality, 91,500 and GJ-1 were reanalyzed as unknown samples. The obtained ¹⁷⁶Hf/¹⁷⁷Hf ratios were 0.282295 ± 0.000027 (n=14, 2σ) for 91,500 and 0.282734 ± 0.000015 (n = 16, 2σ) for GJ-1. These results are in good agreement with recommended ¹⁷⁶Hf/¹⁷⁷Hf ratios (0.2823075 ± 58, 0.282015 ± 0.000019) within 2σ [47].

Data processing was based on a decay constant for ¹⁷⁶Lu of 1.867 × 10⁻¹¹ yr⁻¹ [48], and present-day chondritic ratios of ¹⁷⁶Hf/¹⁷⁷Hf = 0.282772 and ¹⁷⁶Lu/¹⁷⁷Hf = 0.0332 [49] for the calculation of ε_Hf. The single-stage model ages (t_DM1) were calculated by reference to depleted mantle with a present-day ¹⁷⁶Hf/¹⁷⁷Hf ratio of 0.28325 and an ¹⁷⁶Lu/¹⁷⁷Hf ratio of 0.0384 [50]. The two-stage model age (t_DM2) was calculated by projecting the initial ¹⁷⁶Hf/¹⁷⁷Hf of zircon back to the depleted mantle growth curve using a value of ¹⁷⁶Lu/¹⁷⁷Hf = 0.015 for average continental crust [51].

4.4. Major and Trace Element Analyses of Whole Rock

Altered surfaces were carefully removed from all selected samples and the samples were washed and crushed to about 60 mesh in an alumina jaw crusher. Approximately 60 g of sample was powdered in a WC Mill (T1-100, CMT) to less than 200 mesh (75 mm) for whole-rock analyses.

Major oxides were analyzed by XRF (PW4400) using Lieborate glass disks at the Institute of Geological Analysis, Chinese Academy of Geological Science (CAGS). Analyses of international (USGS) rock standards BHVO-1 and AGV-1 indicate precisions and accuracies both better than 5%. Trace and rare earth elements were measured using an ICP-MS (X-series). Approximately 50 mg samples were dissolved in a sealed high-temperature and high-pressure bomb using equal parts of super-pure HF and HNO₃. Analyses of USGS rock standards BHVO-1 and AGV-1 indicate that the analytical precision is mostly better than 5% (expressed in terms of the relative standard deviation).

5. Analytical Results

5.1. Zircon U-Pb Geochronology

The zircons from sample ZJ1-3 are basically elongated in shape (Figure 5). Most of the grains are transparent, except for some that are slightly brownish. Almost all the zircon grains have cracks and inclusions. Most zircon grains are 80~100 μm in length, except for a few small grains that reach 50 μm. CL images show the presence of oscillatory zoning that suggests the cores are of magmatic origin (Figure 5).

Nine analyses for zircon cores and thirteen for zircon rims were conducted for sample ZJ1-3. The zircon cores have 294~592 ppm U and 59~267 ppm Th, and the Th/U ratio ranges from 0.1 to 0.59 (Table 1). In spite of various Pb loss and large discordance, the data define an upper intercept age of 2229 ± 88Ma (MSWD = 2.9) (Figure 6a).

Thirteen zircon grains were analyzed for the rim domains. The zircons have a U content of 164~1666 ppm, Th of 3~77 ppm and Th/U radios of 0.01~0.1, consistent with the characters of metamorphic zircon. Points 6.1 and 10.1 show obvious Pb loss and deviate from the concordia line. Except for the 2 disconcordant points, the rest of the 11 analyses yielded an upper intersection point age of 1897 ± 38 Ma (MSWD = 3.1), consistent with the weighted mean ²⁰⁷Pb/²⁰⁶Pb age (1865 ± 17 Ma, MSWD = 3.5) in the error range (Figure 6b).

5.2. Zircon Hf Isotopes

Nineteen analyses of Hf isotopes were performed on nineteen zircon grains from sample ZJ1-3 that were previously dated on SHRIMP II for U-Pb. The results are listed in Table 2 and shown in Figure 7. Nine Lu-Hf analyses were performed on the magmatic zircon cores, yielding ¹⁷⁶Hf/¹⁷⁷Hf ratios of 0.281371~0.281698 and corresponding ε_Hf(t) values of −5.46 to +6.57, calculated using their corresponding ²⁰⁷Pb/²⁰⁶Pb ages. Their two-stage Hf model ages vary from 2869 Ma to 2300 Ma (Figure 7).

Ten Hf analyses were obtained from ten metamorphic zircon grains, exhibiting ¹⁷⁶Hf/¹⁷⁷Hf ratios of 0.281483~0.281658. Their ε_Hf(t) values, calculated using their corresponding ²⁰⁷Pb/²⁰⁶Pb ages, varied from −3.98 to +1.95, yielding two-stage Hf model ages of 2782 to 2416 Ma (Figure 7).

5.3. Geochemical Characteristics

5.3.1. Major Elements Compositions

Geochemical data and calculated parameters of five analyzed samples are listed in Table 3. Four paragneiss samples (ZJ1-1, ZJ1-2, ZJ1-4 and ZJ1-5) show variations in major element compositions. SiO₂ ranges from about 52.21 to 61.76 weight-% and TiO₂ from 0.74 to 0.9 weight-%. Additionally, a characteristic of these four paragneiss is the high Al₂O₃ content (18.69~21.53 weight-%), FeOt/MgO ratios (2.8~3.14) and narrow ranges of Al₂O₃/SiO₂ and Al₂O₃/TiO₂ ratios (0.30~0.41 and 24~28.5, respectively). Theses relative contents are similar to those average compositions of the upper crust (Rudnick et al., 2004). The relative contents of Al₂O₃, CaO, Na₂O and K₂O are close to those of the Post Archean average Australian shale (PAAS) (Taylor and McLennan, 1985). SiO₂, TiO₂ and Fe₂O₃ contents are slightly lower than those average compositions of the PAAS. MgO relative content is slightly higher than the average compositions of the PAAS.

In contrast, the granitic gneiss sample (ZJ1-3) displays lower Al₂O₃ (13.2 weight-%), Fe₂O₃ (1.2 weight-%) and K₂O (2.7 weight-%) and higher TiO₂ (1.04 weight-%) and MgO (6.5 weight-%) concentrations. A characteristic of sample ZJ1-3 is the low FeO^t/MgO (1.93), Al₂O₃/SiO₂ and Al₂O₃/TiO₂ ratios (0.23 and 12.7, respectively).

5.3.2. Trace Elements Compositions

The granitic gneiss (sample ZJ1-3) has notably lower ΣREE (= 55.32 × 10⁻⁶) content than other four paragneiss samples (sample ZJ1-1, ZJ1-2, ZJ1-4 and ZJ1-5) (ΣREE = 161.80 × 10⁻⁶~341.82 × 10⁻⁶).

All samples have similar chondrite-normalized REE patterns and trace element compositions (Table 3, Figure 7) moderately enriched in light REE (LREE/HREE = 3.23~10.46, La_N/Yb_N = 3.44~12.27) and show flat heavy REE with strong negative Eu anomalies (δEu = 0.26~0.63), except sample ZJ1-2 (LREE/HREE = 14.01, LaN/YbN = 25.38).

On the Primitive mantle (PM) normalized multi-element spider diagram, samples show depletion of Ta, Nb, Sr and Ti with strong negative Ba anomalies (Figure 8b).

6. Discussion

6.1. Protolith Age of the Basement Rocks from the Boreholes

The zircon morphology and their CL images clearly indicate magmatic origin of zircon cores from the sample ZJ1-3. SHRIMP zircon U-Pb analyses revealed that the magmatic zircon core ²⁰⁷Pb/²⁰⁶Pb ages between 1856 ± 14Ma and 2188 ± 11Ma, with an upper intercept age of 2229 ± 88 Ma. Dating results of the granitic gneiss from the basement of Ordos Basin indicate that the magmatic intrusion occurred at ~2230 Ma. However, previous age analysis results of the granitic gneiss in this area provide a good reference for this interpretation. Hu et al. (2013) obtained a U-Pb zircon age of ~2035 Ma for the two-mica granitic gneiss in the boreholes LT1, about 30 km to the west north of the sample location [27]. U-Pb isotopic analyses on detrital zircons obtained from the metasedimentary rocks also indicated that the protolith of the basement rocks in the boreholes LT1 was formed between 2.2 Ga and 2.0 Ga [28,29]. So far, Archean terranes have not been found in basement of the Ordos Basin, though previous study has revealed some inherited zircon grains form metamorphic sedimentary rocks [29]. We thus suggest that the granitic gneiss and metasedimentary rocks in the boreholes were formed during middle Paleoproterozoic time. These age data indicate that widespread magmatism occurred during the middle Paleoproterozoic (2.2~2.0 Ga) in the Ordos Basin. In addition, the middle Paleoproterozoic magmatic activities have previously been reported from adjacent parts of the NCC, including the Khondalite Belt and the middle part of the TNCO [19,32,34,37,40,53,54,55]. U-Pb isotopic dating on metamorphosed volcanic rocks from the Lüliangshan area reveals that they formed at ~2210 Ma [35,56,57], about 80 km to the east of sample location. Dating on the Khondalite series rocks involving the Jining, Helanshan, Daqingshan and Qilianshan complexes reveal that they formed at 2.3~2.0 Ga. These age data suggest that the Paleoproterozoic mobile belts involving the Khondalite Belt and the TNCO could be wider than presently seen at the surface.

6.2. Age of the Metamorphic Zircon

Most of zircons from the granitic gneiss sample ZJ1-3 have a metamorphic rim with a metamorphic age of c. 1865 Ma, supporting the idea that regional metamorphism occurred at ~1860 Ma.

Our results are comparable with the previous reported metamorphic ages of the basement rocks below Ordos Basin. Based on the zircon data, two age groups are recognized from the metamorphic basement of the Ordos Basin. One age group is concentrated in ~1.95 Ga and the other 1.88~1.85 Ga [27,28,29]. These two late Paleoproterozoic metamorphic ages, interpreted to be two metamorphic events, have been widely recognized from the rocks surrounding the Ordos Basin, including the northwestern part of the Khondalite series and middle part of the TNCO [19,33,34,35,37,39,40,54,58]. The ~1.95 Ga metamorphic event was corresponding to the age of collision between the Ordos basement and the Yinshan Block, and the metamorphic event at 1.85 Ga was corresponding to the age of collision between the West and East Block of the NCC. For example, in the Qianlishan-Helanshan area, the supracrustal rocks yield metamorphic ages of 1.95 and 1.92 Ga [33,59]; in the Wulashan area, north of the Ordos Basin, the metamorphic age of khondalites is ~1.94 Ga [20]; in Huai’an metamorphic complex, at the boundary area between the north Khondalite Belt and the TNCO, two groups of zircon ages have been obtained at 1.95 Ga and 1.85 Ga [30]; in the TNCO, metamorphic ages concentrated in 1.85 Ga, such as in the Chengde area [60], Xuanhua-Huai’an area [61,62], Fuping-Hengshan-Wutai area [26,31,63,64,65], Lüliang area [35,36,50,66,67,68,69] and Zanhuang area [70,71]. The metamorphic event at ~1.86 Ga, recorded by the granitic gneiss collected from the drill hole in this study, might be attributable to the collision between the West and East Block along the TNCO [6], but a better understanding of the Ordos Block requires further investigation into the entire basement.

6.3. Nature of the Source Rocks

The nature of the source rocks of the basement metamorphic rocks from Ordos Basin is important in understanding their depositional tectonic setting. Khondalite series are widely distributed across the world. In China, the Khondalite series mainly developed in the Precambrian regions of the NCC; the most typical Khondalite series occurred in the Jining-Datong areas [72]. The basement metamorphic rocks from the Drill hole ZJ-1 consist of garnet-sillimanite pelitic gneisses and felsic gneisses which are aluminum-rich paragneiss similar to that of the typical Khondalite series found elsewhere in the NCC. The diagram of wΣ(REE)/10⁻⁶ − w(La)/w(Yb) shows that three analyzed points (ZJ1-1, 2, 5) drop in the zone of shale and claystone, and only one analyzed point (ZJ1-4) drop in the zone of sandstone and greywacke (Figure 9a). So, the protolith of the basement paragneiss should be argillaceous and arenaceous rock. In the Log((CaO + Na₂O)/K₂O) − Log(SiO₂/Al₂O₃) diagram, the metasedimentary rocks collected from the Drill hole ZJ-1 fall within the field of shales and far away the igneous trend which are well correlated with the Khondalites in the southern India and the NCC (Figure 9b). The Khondalites in the NCC and adjacent areas have generally higher content of K₂O and lower of Na₂O, and their REE patterns are characterized by low LREE/HREE ratios and flat heavy REE with strong negative Eu anomalies [68]. Furthermore, in the rock/chondrite element diagram, they show a significant negative Ba anomalies and relative depletion of Nb, Sr and Ti (Figure 8a). The geochemistry element content values are very close to the corresponding values of meta-sediments inside consist of the Khondalite series in the NCC and its adjacent areas, so that the basement rocks from ZJ-1drill hole is comparable with other typical Khondalites (Figure 9b). In terms of the tectonic position, the drill hole ZJ-1 is located in the eastern of the Ordos Basin which close to the Lüliangshan area. Liu et al. (2013) and Wan et al. (2000) pointed out that the Jiehekou Group in Lüliangshan area, being of Khondalite series properties, share a provenance with it [67,68].

In addition, regarding to the geological time, both the age of protolith and the metamorphic age are comparable with the other complexes from the Khondalite Belt and the TNCO. Previous dating results of detrital zircons form the Khondalite Belt indicates that the source rocks of the khondalite protoliths are predominantly 2.2~2.0 Ga in age [19,20,21,33,34,37,40,58,59], and these 2.2~2.0 Ga detrital zircons have Hf model ages between 2.6~2.1 Ga [21,35,58,59]. Paleoproterozoic magmas are widely distributed in the Lüliangshan area, Fuping area and Wutai area of the TNCO during 2.2~2.1 Ga [73,74,75,76,77]. In our study, SHRIMP zircon U-Pb analyses revealed the magmatic zircon core ²⁰⁷Pb/²⁰⁶Pb ages between 1856 ± 14 Ma and 2188 ± 11 Ma, with the upper intercept age of 2229 ± 88 Ma. These zircons have Hf isotopic compositions with ε_Hf(t) values and Hf crustal model ages of −5.5 ~+6.6 and 2.8~2.3 Ga, respectively.

Controversies existed about the sedimentary environments of the Khondalite series, such as the active continental margins [58,78], cratonic basins [72] and passive continental margins [4,67,68,79]. The diagram of chondrite-normalized REE shows the pattern of the average metamorphic clastic sedimentary rocks form drill hole ZJ-1 located between active continental margins and passive continental margins (Figure 10a). The diagram of La/Sc-Ti/Zr diagram (Figure 10b) shows most of the analyzed points drop in the zone of active continental margins and only sample ZJ1-4 drops in the zone of continental island arc. The diagrams of the La-Th-Sc and Dc-Th-Zr/10 denote the source of the meta-argillo-arenaceous rocks collected form drill hole ZJ-1have tectonic setting of continental island arc (Figure 10c,d). On the other hand, the diagram of wΣ(REE)/10⁻⁶ − w(La)/w(Yb) shows the protolith of the paragneiss from the metamorphic basement should be argillaceous and arenaceous rock (Figure 9a). Magmatic arcs provide most of the materials for the deposit of both fore-arc basins and island arcs in active continental margin, and the maturity of the detrital components in this kind of tectonic setting is usually extremely low. However, the sediments of back-arc basins may come from both stable continents and island arcs which can provide detrital mature and immature materials, respectively. Additionally, only this kind of sedimentary environment can explain the geochemical data of the metamorphic clastic sedimentary rock from the basement beneath the northeastern Ordos Basin. So, the sedimentary environment of the metasedimentary rock should be the active continental margin with the nature of back-arc basin.

Figure 9

wΣ(REE)/10⁻⁶ − w(La)/w(Yb) and Log(CaO + Na₂O)/K₂O) − Log(SiO₂/Al₂O₃) plot for the basement rocks [68,72,80,81]. (a) showing the protolith of the paragneiss from the metamorphic basement should be argillaceous and arenaceous rock (after Bhatia, 1985). (b) showing the distribution of the different Khondalite series. 1- Distribution range of Khondalite series in southern India (after Dash et al., 1987); 2- distribution range of Khondalite series in the NCC and adjacent areas (after Wan et al., 2000). I - shale field; II - sandstone field; III - igneous trend (after Condie et al., 1992).

[Figure omitted. See PDF]

In summary, the metamorphic basement beneath the northeastern Ordos Basin has the characteristics of the Khondalite series because of the similarities in the fabric, geochemical characteristics, geochronology and sedimentary environment with other typical Khondalite series.

6.4. Geological Implication for the Paleoproterozoic Evolution of the Ordos Basin

On the basis of the precious studies and the age data in this paper, there is no evidence that Archean source terranes were exposed in the area during the deposition of the late Paleoproterozoic sediments [28]. It seems that the upper crust of the northeastern Ordos Basin is composed of Paleoproterozoic rocks, and the Ordos basement was involved in a major late Paleoproterozoic tectono-thermal event. The isotopic age data of this study is more consistent with the former age of the Western and Eastern Block collision during ~1.85 Ga.

Previous studies suggested that the Eastern Block and the TNCO experienced crustal growth at 2.8~2.7 Ga and strong magmatic activity occurred during 2.6~2.5 Ga [69,75,77,83,84]. Geng et al. (2012) further added that both the Eastern Block and the TNCO share similar evolutionary history, and they have endured crust-mantel differentiation event during the early Neoarchean (2.8~2.65 Ga) and vast magmatic activity occurred at the end of Neoarchean (2.6~2.5 Ga) [84].

The zircon Hf isotope data of the basement rocks beneath the Ordos Basin show a large variation in ε_Hf(t) from −5.46 to +6.57, suggesting that ancient crustal sources as well as juvenile components may have contributed to their formation during the middle Paleoproterozoic. However, the zircon T_DM range from 2869 Ma to 2300 Ma, with only two of them being less than 2.5 Ga. Most zircon Hf isotopic composition values are locked in the crustal evolution region of 2.7 Ga to 2.5 Ga (Figure 7). This comparison is consistent with the 2.6~2.5 Ga age reported in the previous studies [23,36,85,86]. However, the Western Block experienced strong magmatic activity at 2.3~2.0 Ga [59,78,87]. So, the material properties are different from those of the Western Block but similar to the Eastern Block and the TNCO. As previously mentioned, the Paleoproterozoic basement rocks beneath the northeastern Ordos Basin have the similar characteristics of the Khondalite series. Consequently, the size of the Ordos Block must be smaller than previously thought based on the research in petrology and tectonics, and this result is similar to the viewpoints of Wang et al. (2014) [29]. Since there were little studies focused on the crustal evolution of the Ordos Block, more investigation into the deep lithosphere is necessary to improve our understanding of the entire basement beneath the Ordos Block.

7. Conclusions

(1) SHRIMP U-Pb analyses revealed magmatic zircon core ²⁰⁷Pb/²⁰⁶Pb ages between 1856 ± 14 Ma and 2188 ± 11 Ma, with the upper intercept age of 2229 ± 88Ma, which suggests the five subtypes of gneiss in the borehole ZJ-1 were formed during middle Paleoproterozoic.

(2) SHRIMP U-Pb analyses of zircon metamorphic accreted rims yield a metamorphic age of 1865 ± 17Ma (MSWD = 3.5), which constrains the timing of metamorphism of the borehole basement rock to the late Paleoproterozoic, might be attributable to the collision between the Western and Eastern Block along the TNCO.

(3) The metamorphic basement rocks beneath the northeastern Ordos Basin, being of Khondalite series properties, were likely formed in a tectonic environment of the active continental margin with the nature of back-arc basin.

(4) The material properties of basement rocks beneath the northeastern Ordos Basin are different from those of the Western Block but similar to the Eastern Block and the TNCO, and the Ordos Block is smaller than previously thought.

Author Contributions

E.Y.X. has completed the main experiments of the research and the manuscript writing. M.P.Z. and Y.S.Z. have provided ideas and guidance for this paper, and reviewed the conclusions and understandings of the paper. S.J.W. have completed the analyses of Hf isotopes. B.L.G. and Y.P. assisted in the collection of the samples and the data. All authors have read and agreed to the published version of the manuscript.

Acknowledgments

We thank Ming Hua Ren, Shuan Hong Zhang, and Yong Jie Lin for their useful and helpful suggestions. The editor Guochun Zhao and two anonymous reviewers are thanked for their constructive comments.

Conflicts of Interest

The authors declare no conflict of interest.

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Figures and Tables

Figure 1. Tectonic map of the North China Craton showing Paleoproterozoic orogenic belts [6] (modified after Zhao et al., 2005). see Ordos Basin in Figure 2.

Figure 2. Geological map of the Ordos Basin and surrounding areas [27,28,29] (modified after Wan et al., 2013), showing the sample location in this study and those of Hu et al., 2013, Wan et al., 2013 and Wang et al., 2014. (A): Yinshan Block; (B): Khondalite Belt; (C): Ordos Block; (D): Trans-North China Orogen; (E): Qingling Orogen.

Figure 3. Columnar sections for drill hole ZJ-1 in the northeastern Ordos Basin. Sample locations are marked by open circular.

Figure 4. Microphotographs of typical textures in Paleoproterozoic rocks obtained from drill cores beneath the Ordos Basin. (a,b): two-mica plagioclase-gneiss (ZJ1-1); (c,d): sillimanite-bearing two-mica muscovite-biotite-gneiss (ZJ1-2); (e): granitic gneiss (Zj1-3); (f,g): sillimanite gneiss (ZJ1-4); (h,i): garnet-sillimanite-bearing two-mica gneiss (ZJ1-5).

Figure 5. Cathodoluminescence (CL) images for zircons from the basement sample ZJ1-3.

Figure 6. Concordia diagrams showing SHRIMP zircon data for the basement samples ZJ1-3: (a) for the magmatic zircons; (b) for the metamorphic overgrowths on zircon grains.

Figure 7. Diagram of εHf(t) vs. U-Pb age for zircons from Paleoproterozoic rocks obtained from drill cores beneath the northeastern Ordos Basin and the Lüliangshan area [28].

Figure 8. Chondrite-normalized REE patterns (a) and primitive mantle normalized spider diagrams (b) [52] (after Sun and McDonough, 1989).

Figure 10. Discriminate diagrams of tectonic settings for rocks from the Ordos Basin basement [52,82] (a): after Sun and McDonough, 1989; (b–d): after Bhatia and Crook, 1986) (The average data of clastic sedimentary rock in different tectonic environment in (a): are from Bhatia and Crook, 1986. Abbreviations of average: OIA—oceanic island arc greywacke; CIA—continental arc greywacke; ACM—active continental margin greywacke; PM—passive continental margin shale and greywacke; ※: average of REE of metamorphic clastic sedimentary rocks form drill hole ZJ-1; (b–d): A—oceanic island arc; B—continental island arc; C—active continental margin; D—passive continental margin).

Table 1

SHRIMP U-Pb analyses of zircons for granitic gneiss sample ZJ1-3 from drill cores ZJ-1 at northeastern Ordos Basin.

Spot	206Pbc (Weight-%)	U (Weight-ppm)	Th (Weight-ppm)	232Th/238U	206Pb* (Weight-ppm)	206Pb/238U Age(Ma)		207Pb/206Pb Age(Ma)		208Pb/232Th Age(Ma)		207Pb/206Pb*	±%	207Pb*/235U	±%	206Pb*/238U	±%	Errcorr	Discordant (%)
1.1R	0.34	848.12	76.85	0.09	146	1171	±8.3	1839	±11	1.201	±59	0.11245	0.610	3.09	0.99	0.20	0.78	0.84	36.35
2.1C	0.55	471.30	107.09	0.23	79.8	1153	± 8.7	1856	±14	1251	±34	0.11346	0.790	3.07	1.10	0.20	0.82	0.80	37.85
3.1R	0.81	264.22	2.70	0.01	34.4	904	± 8.6	1800	±34	3630	±1200	0.11000	1.900	2.28	2.10	0.15	1.00	0.65	49.79
4.1C	1.98	466.14	266.92	0.59	92.6	1317	±11	1961	±57	1421	±68	0.12030	3.200	3.76	3.30	0.23	0.89	0.35	32.84
5.1C	0.44	301.37	79.82	0.27	79.1	1713	±14	2161	±14	1743	±61	0.13470	0.780	5.65	1.20	0.30	0.94	0.83	20.74
6.1R	0.37	458.37	35.96	0.08	86.5	1276	± 9.5	1776	±13	1606	±82	0.10861	0.720	3.28	1.10	0.22	0.82	0.81	28.19
7.1C	0.38	525.11	117.37	0.23	94.8	1226	± 8.7	1893	±12	1438	±30	0.11587	0.680	3.35	1.00	0.21	0.78	0.81	35.26
8.1R	4.07	865.76	71.57	0.09	137	1049	± 7.7	1817	±36	1170	±370	0.11110	2.000	2.71	2.10	0.18	0.79	0.73	42.28
9.1C	0.14	415.07	125.94	0.31	92.3	1482	±11	2064	±13	1874	±26	0.12750	0.720	4.54	1.10	0.26	0.83	0.74	28.21
10.1R	1.31	1144.79	45.80	0.04	304	1715	±11	2233	±12	1943	±430	0.14046	0.670	5.90	1.00	0.30	0.76	0.89	23.21
11.1C	0.48	294.33	89.43	0.31	88.2	1920	±14	2188	±11	2361	±43	0.13687	0.640	6.55	1.00	0.35	0.83	0.83	12.23
12.1C	1.34	306.12	64.87	0.22	84.2	1770	±15	2045	±58	2609	±290	0.12610	3.300	5.50	3.40	0.32	0.96	0.33	13.40
13.1R	0.27	1666.28	30.11	0.02	294	1201	± 7.9	1882	±8.6	1641	±190	0.11514	0.480	3.25	0.86	0.20	0.72	0.86	36.17
14.1R	0.21	526.20	10.82	0.02	112	1422	±10	1863	±12	1797	±250	0.11393	0.660	3.88	1.00	0.25	0.81	0.82	23.65
15.1C	0.35	296.42	75.45	0.26	66.3	1488	±12	2006	±14	1697	±37	0.12338	0.800	4.42	1.20	0.26	0.87	0.78	25.83
16.1C	0.34	592.32	58.53	0.10	142	1585	±11	2108	±9.6	1980	±67	0.13072	0.540	5.03	0.96	0.28	0.79	0.86	24.78
17.1R	0.62	561.33	55.59	0.10	92.4	1124	± 8.2	1842	±14	449	±75	0.11263	0.780	2.96	1.10	0.19	0.79	0.84	38.98
18.1R	0.67	669.15	11.52	0.02	140	1401	±10.0	1843	±13	1784	± 510	0.11266	0.730	3.77	1.10	0.24	0.79	0.84	23.97
19.1R	0.57	506.15	9.92	0.02	110	1450	±11	1876	±14	1319	±470	0.11474	0.780	3.99	1.10	0.25	0.81	0.83	22.72
20.1R	0.62	484.47	6.25	0.01	105	1446	±10	1870	±13	1609	±680	0.11437	0.730	3.97	1.10	0.25	0.80	0.84	22.67
21.1R	0.89	561.05	36.60	0.07	126	1483	±11	1854	±16	605	±200	0.11330	0.910	4.04	1.20	0.26	0.85	0.84	19.97
22.1R	0.53	542.84	10.23	0.02	114	1401	±10	1913	±13	2071	±440	0.11711	0.740	3.92	1.10	0.24	0.80	0.83	26.75

Table 2

Zircon Lu-Hf isotopic data of zircons for granitic gneiss sample ZJ1-3 from drill cores ZJ-1 at northeastern Ordos Basin.

Age Spot	Hf Spot	Age(Ma)	176Yb177Hf	176Lu177Hf	2σ	176Hf177Hf	2σ	εHf(0)	εHf(t)	TDM	TDMc	f Lu/Hf
1.1	3-1.1	1839	0.013333	0.000494	0.0000196	0.281583	0.0000406	−42.06	−1.68	2306	2596	−0.99
2.1	3-2.1	1856	0.032978	0.001220	0.0000068	0.281595	0.0000334	−41.61	−1.75	2332	2612	−0.96
4.1	3-4.1	1961	0.006096	0.000320	0.0000020	0.281581	0.0000311	−42.12	1.24	2298	2509	−0.99
5.1	3-5.1	2161	0.025171	0.000988	0.0000073	0.281630	0.0000299	−40.37	6.57	2270	2335	−0.97
7.1	3-7.1	1893	0.018373	0.000703	0.0000058	0.281450	0.0000212	−46.76	−5.46	2499	2869	−0.98
8.1	3-8.1	1817	0.025503	0.000957	0.0000120	0.281555	0.0000301	−43.04	−3.71	2372	2705	−0.97
9.1	3-9.1	2064	0.035497	0.001294	0.0000108	0.281593	0.0000287	−41.71	2.64	2340	2502	−0.96
10.1	3-10.1	2233	0.022739	0.000970	0.0000053	0.281445	0.0000290	−46.94	1.61	2523	2697	−0.97
11.1	3-11.1	2188	0.020053	0.000797	0.0000082	0.281371	0.0000273	−49.54	−1.77	2612	2869	−0.98
13.1	3-13.1	1882.1	0.014843	0.000539	0.0000073	0.281483	0.0000255	−45.6	−4.32	2444	2791	−0.98
14.1	3-14.1	1863	0.020868	0.000783	0.0000248	0.281658	0.0000201	−39.4	1.17	2220	2437	−0.98
15.1	3-15.1	2006	0.033023	0.001217	0.0000127	0.281698	0.0000190	−38	5.19	2191	2300	−0.96
16.1	3-16.1	2107.60	0.012961	0.000463	0.0000058	0.281422	0.0000225	−47.74	−1.26	2520	2776	−0.99
17.1	3-17.1	1842	0.015034	0.000629	0.0000188	0.281594	0.0000165	−41.66	−1.36	2298	2578	−0.98
18.1	3-18.1	1843	0.028107	0.001092	0.0000116	0.281651	0.0000231	−39.64	0.12	2248	2489	−0.97
19.1	3-19.1	1876	0.025430	0.000982	0.0000146	0.281559	0.0000171	−42.9	−2.29	2368	2663	−0.97
20.1	3-20.1	1870	0.016556	0.000630	0.0000052	0.281545	0.0000141	−43.4	−2.5	2365	2669	−0.98
21.1	3-21.1	1854	0.035072	0.001356	0.0000253	0.281618	0.0000186	−40.81	−1.16	2309	2574	−0.96
22.1	3-22.1	1913	0.040506	0.001503	0.0000530	0.281632	0.0000150	−40.33	0.43	2299	2522	−0.95

Table 3

Major and trace element contents of whole-rock sample from the northeastern Ordos Basin basement.

Sample No.	ZJ1-1	ZJ1-2	ZJ1-3	ZJ1-4	ZJ1-5	Sample No.	ZJ1-1	ZJ1-2	ZJ1-3	ZJ1-4	ZJ1-5
SiO2	52.2	55.0	58.3	61.8	55.6	Th	17.8	21	4.75	13.1	24.7
TiO2	0.9	0.9	1.0	0.8	0.7	U	2.38	3.2	1.63	2.55	4.82
Al2O3	21.5	21.3	13.2	18.7	21.1	La	64.3	54.5	8.77	34.1	71.1
Fe2O3	1.7	1.5	1.2	1.4	1.3	Ce	113	105	17.8	64.3	134
FeO	7.4	6.0	10.2	5.6	6.5	Pr	15.7	14.3	2.51	8.46	18.1
MnO	0.1	0.1	0.1	0.1	0.1	Nd	57.8	54.5	10.3	32.3	68.4
MgO	3.6	2.9	6.5	2.5	2.7	Sm	9.04	9.46	2.44	5.27	12
CaO	2.2	1.0	1.1	0.5	1.1	Eu	1.59	0.75	0.41	1.05	1.59
Na2O	3.4	0.4	1.0	1.4	2.9	Gd	7.3	7.43	3.15	4.71	9.91
K2O	3.1	6.1	2.7	4.5	4.5	Tb	1.08	0.88	0.6	0.69	1.52
P2O5	0.1	0.0	0.1	0.1	0.1	Dy	6.2	4.05	3.89	4.15	9.32
H2O+	3.3	3.7	3.8	2.6	2.4	Ho	1.23	0.66	0.74	0.79	1.9
CO2	0.3	0.3	0.2	0.3	0.3	Er	4.2	1.96	2.27	2.8	6.7
Total	99.7	99.1	99.4	100.1	99.4	Tm	0.62	0.24	0.33	0.41	0.93
Cr	143	137	423	110	118	Yb	3.76	1.54	1.83	2.38	5.49
Ni	52.5	46.2	99.3	40.9	39.4	Lu	0.6	0.27	0.28	0.39	0.86
Ga	33.1	59.7	21.6	26.7	29.1	Pb	19.3	8.4	3.08	8	15.1
Sc	25.3	23	27.5	20.8	26.7	LaN/LuN	11.1	21.0		9.1	8.6
Rb	169	686	301	236	229	La/Sc	2.5	2.4		1.6	2.7
Ba	538	609	149	559	317	Th/Sc	0.7	0.9		0.6	0.9
Sr	128	23.8	31.9	67.3	117	δEu	0.58	0.26	0.45	0.58	0.63
Nb	14.4	37.8	6.68	13.5	14.1	δCe	0.85	0.90	0.92	0.90	0.89
Ta	0.76	29.9	0.58	0.91	1.06	ΣREE	286.4	255.5	55.3	161.8	341.8
Zr	221	244	125	223	184	LaN/YbN	12.3	25.4	3.4	10.3	9.3
Hf	6.03	7.3	3.34	6.4	5.6	Th/U	7.5	6.6	2.9	5.1	5.1
Y	35.4	19.1	20.3	23	52.5	Rb/Sr	1.3	28.8	9.4	3.5	2.0

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