Provenance and Implication of Carboniferous–Permian Detrital Zircons from the Upper Paleozoic, Southern Ordos Basin, China: Evidence from U-Pb Geochronology and Hf Isotopes

: Carboniferous–Permian detrital zircons are recognized in the Upper Paleozoic of the whole Ordos Basin. Previous studies revealed that these Carboniferous–Permian zircons occurred in the Northern Ordos Basin mainly originated from the Yinshan Block. What has not been well documented until now is where this period’s zircons in the Southern Ordos Basin came from, and very little discussion about their provenance. To identify the provenance of the detrital zircons dating from ~350 to 260 Ma, ﬁve sandstone samples from the Shan 1 Member of Shanxi Formation and eight sandstone samples from the He 8 Member of Shihezi Formation were analyzed for detrital zircon U-Pb age dating and in situ Lu-Hf isotopic compositions. The results indicate that the two age clusters of 520–378 Ma and ~350–260 Ma in the Southern Ordos Basin most likely derived from the North Qinling Orogenic Belt–North Qilian Orogenic Belt and the North Qinling Orogenic Belt, respectively. Furthermore, we propose that the zircons aging ~320–260 Ma are representative of the important tectonothermal events occurred in the North Qinling Orogenic Belt during the Late Paleozoic.


Introduction
The Ordos Basin, located in the southwest part of the North China Craton (Figure 1), is the second largest sedimentary basin in China [1] and is over 250,000 km 2 in size [2]. Abundant oil and gas resources occur in the Upper Paleozoic clastic rocks of the basin.

Sampling Information
Five sandstone samples from the Shan 1 Member and eight sandstone samples from the He 8 Member were collected from five boreholes and three field outcrops (Kouzhen, Xuefengchuan and Pingliang outcrop) in the SOB for the detrital zircon U-Pb chronologic and Lu-Hf isotopic analyses (Table 1 and Figure 1a). We also collected zircon U-Pb age data of the 18 samples in the Northern Ordos Basin, and the samples of N14-N18 and N1-N13 are from the Shan 1 Member and He 8 Member, respectively. The sampling locations are shown in Figure 1a. Of these study samples, the research of petrographic characteristics shows that most of the quartz in 13 sandstone samples are sub-angular or sub-rounded ( Figure 3). The quartz overgrowth is common (Figure 3a,b,f). Feldspars (~3%-7%) are found in the samples of ChunT1-01 and 16HC-03. Most of the feldspars are sub-angular and corroded (Figure 3c,e). In addition, lithic fragments are very common in samples, especially in the samples of Luo2-04 and 16HC-03 (Figure 3), and most of them are metamorphic and volcanic fragments with some clay mineral and carbonate cement. Detailed information on the samples is presented in Table 1.

Zircon Separation and CL Imaging
Zircon minerals were separated by traditional heavy liquid and magnetic techniques [37], and handpicked using a binocular microscope. More than 200 zircon grains were randomly selected and fixed in epoxy resin in a 1 cm diameter mount, and then polished until the interior surfaces of all zircons were exposed. All zircons were documented in the optical photomicrographs under the transmitted, reflected light and cathodoluminescence (CL) imaging to uncover their internal structures [37]. The CL images were taken at the State Key Laboratory of Continental Dynamics, Northwest University, Xi'an, China. A detailed description of using instruments was described by Wang et al. [38].

LA-ICP-MS Zircon Dating
The U-Pb isotopic ratios of zircons were measured in situ using a GeoLasPro 193 UV laser ablation system (Lambda Physik AG, Göttingen, Germany) coupled with a 7500a ICP-MS (Agilent Technologies Inc., Santa Clara, CA, USA) at the State Key Laboratory of Continental Dynamics, Northwest University, Xi'an, China. The laser beam size and frequency were 32 µm and 6 Hz, respectively. Three international standard samples of zircon NIST SRM 610, 91500, GJ-1 were tested for every twelve sample analyses, and one standard sample 91500 was tested after the sixth sample of those 12 sample analyses. In order to calculate zircon ages, the zircon 91500 was adopted as an external standard. The detailed instrumental settings and analytical procedures were described by Yuan et al. [39], Diwu et al. [40] and Thomas [41]. The U, Th and Pb concentrations as well as 207 Pb/ 206 Pb, 206 Pb/ 238 U, 207 Pb/ 235 U and 208 Pb/ 232 Th ratios were computed by GLITTER 4.0 software (Macquarie University, Sydney, Australia). Their ages were calculated by Isoplot/Ex v. 3.0 [42]. For the element content analysis, the artificial synthetic silicate glass NIST SRM 610 of the American National Standard Substance Bureau, as an external standard, was used to calibrate U, Th and Pb concentration, while 29 Si was adopted as the internal standard element. All analytical results are shown in Supplementary Table S1.

Zircon Lu-Hf Isotopic Analysis
In situ zircon Lu-Hf isotopic analyses were carried out on a Nu Plasma II MC-ICP-MS (Nu Instruments Ltd., Wrexham, UK) connected to a RESOLution M-50 193 nm laser system at the State Key Laboratory of Continental Dynamics, Northwest University (Xi'an, China). The spot size was 44 µm. Lu-Hf isotopic measurements were taken at the same spots or in the same age domains of zircon grains with concordant U-Pb age (discordance <10%). The international standard zircon 91500 was used as an external correction. The running state of the instrument was described by Bao et al. (2017) [43], and detailed information of the analysis strategy and data deduction is stated in the published literature [44]. A 176 Hf/ 177 Hf ratio of 0.282772 and 176 Lu/ 177 Hf of 0.0332 [45] of the chondritic were used to calculate ε Hf (t), and single-stage Hf model ages (T DM1 ) and two-stage Hf model ages (T DM2 ) were calculated by the depleted mantle with 176 Hf/ 177 Hf value of 0.28325 [46], 176 Lu/ 177 Hf of 0.0384 [46], 176 Hf/ 177 Hf of 0.28325 [46] and λ of 1.867 × 10 −11 year −1 [47]. The f Lu/Hf of the upper crust is −0.72 [48], and the f Lu/Hf of the depleted mantle is 0.16 [46]. Calculation formulas of ε Hf (t), T DM1, T DM2 and f Lu/Hf of samples according to Wu et al. (2007) were used [49]. The results are presented in Supplementary Table S2.

U-Pb Ages
In order to study the potential relationship between zircon age and grain size, the grain size and shape parameters of each dated zircon were measured and used to calculate the equivalent spherical diameter (ESD) [50]. The ESD is the cube root of the length values of the three axes of zircon grains [50,51]. Figure 4 shows that the distribution of ages is independent from ESD i.e., the dated zircon ages can be regarded as representatives of whole rocks [50]. A total of 510 and 812 detrital zircons in the Shan 1 and He 8 Member were analyzed, and 404 and 645 grains' ages were concordant (discordance <10%), respectively. Of these, 85 (Shan 1) and 173 (He 8) grains yielded ages of 520-262 Ma (Table S1) Figure 5). Most of the zircons are subhedral to euhedral, displaying a short to long prismatic, and sub-rounded to rounded shapes, with a length of 80-250 µm, which are similar to the magmatic zircon with the length of between 20 and 250 µm [52,53], and a width of 50-230 µm and length/width ratios of 1:1-3.5:1 ( Figure 6). Some zircons exhibit dark or blurry oscillatory zoning ( Figure 6) with Th/U ratios > 0.4, suggesting a magmatic origin [52,[54][55][56][57]. Two grains display fan zoning structures (Figure 6m), which may indicate a metamorphic origin [52,[54][55][56] and yielded ages of 301.1 ± 2.6 Ma and 284.2 ± 3.3 Ma. Generally, the Th/U ratio of igneous zircon is > 0.4 and metamorphic zircon is < 0.1 [53,58,59], which can be used to distinguish the origin of the zircons. Most of the analyzed zircons have the Th/U ratios > 0.4, and 21 grains (9 in Shan 1 and 12 in He 8) with the ratios of 0.1-0.4, and only three grains less than 0.1 (one in Shan 1 and two in He 8) (Table S1 and
In the He 8 Member zircons, 68 analytical data yielded negative ε Hf (t) values from −16.6 to −0.78, with the T DM2 of 1164-1948 Ma (Table S2 and Figure 9). The remaining six grains have positive ε Hf (t) values from 1.28 to 5.18, with the T DM2 of 859-1125 Ma.

Major Tectonothermal Events Analyses of Adjacent Regions
The~520-260 Ma tectonothermal events occurred in the Yinshan Block (YB), North Qinling Orogenic Belt (NQinOB), and North Qilian Orogenic Belt (NQiOB) around the Ordos Basin (Figure 1b). Many zircon U-Pb geochronology have been conducted over the past decades, and a great number of available ages have been obtained from these adjacent regions, providing us with a relatively well constrained framework for the provenance interpretation and correlation for the SOB.
It can be infered that the provenance of 520-378 Ma detrital zircons might mainly come from both the NQinOB and NQiOB. Firstly, the crystallization age distribution, Lu-Hf isotope, and Hf model ages of the 520-378 Ma detrital zircons in the SOB agree well with that of the NQiOB and NQinOB, but they are obviously different from that of the Yinshan Block (Figures 8 and 9). Besides, it is difficult to distinguish the provenance contribution between the NQiOB and NQinOB using this information. Secondly, there are many magmatic zircons with ages from 400 Ma to 520 Ma, and they show several distinct age peaks at~430-410 Ma,~450 Ma and~500 Ma in both the NQiOB and the NQinOB (Figure 8h,i). Furthermore, there are also some metamorphic and magmatic zircons' ages between 400 Ma and 350 Ma in the NQinOB, but few in the NQiOB (Figure 8h,i). Of all samples, three metamorphic origin zircons have ages of 443 Ma, 404.4 Ma, and 391.6 Ma. The~450 Ma and~430-400 metamorphic events widely occurred both in the NQinOB [78,144] and the NQiOB [111,[145][146][147], and~390 Ma metamorphic ages were reported in the NQinOB [74,79]. Therefore, we suggest that the provenance of 520-378 Ma zircons was from the NQinOB, and it is impossible to judge whether the NQiOB had provided any materials. Thirdly, although the ε Hf (t) and T DM2 values from the SOB and the surrounding orogenic belts have a wide range, the Yinshan Block lacks the distribution of ε Hf (t) < −6.2 and > 10, and T DM2 < 1.0 Ga and > 1.8 Ga, while the NQinOB and NQiOB have such a distribution (Figure 9). Finally, the source studies of the SOB by sedimentological methods show that the NQiOB provided some materials for the Southwestern Ordos Basin [4][5][6][7][8][9][10][11]. Therefore, we infer that the provenance of 520-378 Ma detrital zircons likely derived from both the NQinOB and the NQiOB, while the NQiOB provided material only for the local area (e.g., Southwestern Ordos Basin). The 520-378 Ma ages are considered to be a representative attributed to continental collision and accretion in the NQinOB and NQiOB during the Paleozoic. The ages of 480-400 Ma are consistent with the Ordovician-Early Devonian tectonic events resulted from continental collision and accretion occurred in both the NQinOB and the NQiOB (Figure 8h,i). Whereas, the five zircons' ages between 400 Ma and 378 Ma represent the Middle Devonian tectonothermal events occurred in NQinOB.
In summary, we suggest that the detrital zircons of 520-378 Ma in the SOB likely derived from both the NQinOB and the NQiOB. "a" and "d" are numerical ranges of e Hf (t) and T DM2 , respectively; "b" and "e" are numerical averages of e Hf (t) and T DM2 , respectively; "c" is the number of data.
In terms of the~350-260 Ma detrital zircons' Hf isotopic compositions ( Figure 9) and crystallization age distribution (Figure 8) in the Yinshan Block and SOB, they have obvious differences. Firstly, huge magmatism occurred in the Yinshan Block mainly dated~340-320 Ma and~285-260 Ma (Figure 8j), but the age populations in the SOB are dominantly distributed in~320-285 Ma (Figure 8a,b,f). In addition, the proportion of~320-285 Ma detrital zircons preserved in the Northern Ordos Basin (6.8%) [3] is lower than that of in the SOB (10%), which may have two possibilities: one is that the provenance of the above two regions derived from different sources, and another possibility is that the sources of the two regions came from the same provenance, but the supplying source direction was from south to north, which is just opposite to the previous research result that the source supplying directions for the Shan 1-He 8 Member in the SOB were from southwest to northeast, southeast to northwest and south to north [4][5][6][7][8][9][10][11]. Therefore, we speculate that the provenance of~320-285 Ma detrital zircons in the Northern Ordos Basin and SOB was different. Secondly, the ε Hf (t) of 350-320 Ma and 320-285 Ma in the Yinshan Block are from −9.81 to 3.2 (average −2.6, all ε Hf (t) > −10) and −25.49 to −3.97 (average −7.86, most of ε Hf (t) between −8.5 to −4), and that in the SOB are from −15.13 to 2.92 (average −7.05, 33% of ε Hf (t) < −10) and −20.84 to 5.18 (average −8.65, 52% of ε Hf (t) < −8.5), respectively (  Table 2). Instead of the Yinshan Block, there are 33% of ε Hf (t) < −10 and 10% of T DM2 < 1.1 Ga of the 350-320 Ma zircons in the SOB. Besides, there is 79% of ε Hf (t) from −15 to −5, 52% of ε Hf (t) < −8.5, and 42% of T DM2 < 1.52 Ga of the 320-285 Ma zircons in the SOB, but it is significantly different from the Yinshan Block (Table 2). Therefore, it can be infered that the provenance of~350-260 Ma detrital zircons is not coming from the Yinshan Block.
Thus we suggest that the provenance of~350-260 Ma zircons is from the NQinOB. Firstly, an age group of~350-260 Ma of the detrital zircons from the Carboniferous-Triassic sandstones also presented in the Liuyehe Basin [13,14] (Figure 8g). Besides, the~350-260 Ma detrital zircon content (23.2%) in the Liuyehe Basin [13,14] is higher than that of in the SOB (12.7%). Gao et al. and Li et al. indicated that the provenance of Carboniferous-Triassic (~350-260 Ma) zircons in the Liuyehe Basin was from the NQinOB, and the Liuyehe Basin and SOB have the same source [13,14]. Secondly, there are a few reports about metamorphic ages between 350 Ma and 260 Ma in the NQinOB (Figure 8d), such as the ages of 324 Ma by U-Pb isotopic ratios titanite dating [68], 335-345 Ma [85], 341.7 ± 3 Ma [85] and 330-348 Ma [65] by LA-ICP-MS zircon dating, and 347 ± 6 Ma by SHRIMP zircon dating [70]. Moreover, Zhang et al. obtained 312 Ma and 263 ± 2 Ma ages in the NQinOB amphibolite by the method of mineral Rb-Sr isochron [148]. These indicate that tectonothermal events may have occurred in the NQinOB during 350-260 Ma, particularly during~320-260 Ma. Finally, the source supplying directions of the Shan 1 and He 8 Member in the SOB show that the clastic sources were from the southwestern, southeastern and southern orogenic belts surrounding the SOB [4][5][6][7][8][9][10][11].
In summary, the~350-260 Ma zircons of the Upper Paleozoic in the SOB might originate from the NQinOB. Besides, there are both magmatic origin and metamorphic origin zircons of~320-260 Ma in the Shan 1-He 8 Member. Furthermore, the magmatism and metamorphism of~320-260 Ma possibly occurred in the NQinOB, even though, to date, there have been few reports on tectonothermal events of the~320-260 Ma period. Therefore, we tentatively speculate that the~320-260 Ma age records tracing the Carboniferou-Permian tectonothermal events might be preserved in some geologic bodies in the NQinOB, unfortunately, few records have been found due to the intensified denudation during the later orogenic uplift.

Conclusions
The 520-378 Ma detrital zircons preserved in the Upper Paleozoic in the Southern Ordos Basin are representatives of source contribution from both the NQinOB and the NQiOB. While the~350-260 Ma detrital zircons in the Southern Ordos Basin are agree very well to those coeval zircons recognized from the Carboniferous-Triassic sandstones in Liuyehe intermountain basin, which suggested that not only the NQinOB turn into the major source for the Southern Ordos Basin during the Late Paleozoic, but important tectonothermal events also occurred from~320 Ma to 260 Ma in the NQinOB after oceanic subduction and continental collision during Early Paleozoic.