U – Pb Dating and Hf Isotopes Analysis of Detrital Zircons of the Shanxi Formation in the Otuokeqi Area, Northwestern Ordos Basin

The Paleozoic strata are widely distributed in the northwest of the Ordos Basin, and the provenance attributes of the basin sediments during this period are still controversial. In this paper, the detrital zircon LA-MC-ICPMS U-Pb age test was conducted on the drilling core samples of the Shanxi Formation of the Upper Paleozoic in the Otuokeqi area of the Ordos Basin, and the provenance age and the characteristic of the Shanxi formation in the Otuokeqi area in the northwest were discussed. The cathodoluminescence image shows that the detrital zircon has a clear core-edge structure, and most of the cores have clear oscillatory zonings, which suggests that they are magmatic in origin. Zircons have no oscillatory zoning structure that shows the cause of metamorphism. The age of detrital zircon is dominated by Paleoproterozoic and can be divided into four groups, which are 2500 ~ 2300Ma, 2100 ~ 1600Ma, 470 ~ 400 Ma, and 360 ~ 260 Ma. The ﬁ rst two groups are the speci ﬁ c manifestations of the Precambrian Fuping Movement (2.5 billion years) and the Luliang Movement (1.8 billion years) of the North China Craton. The third and fourth groups of detrital zircons mainly come from Paleozoic magmatic rocks formed by the subduction and collision of the Siberian plate and the North China plate. The ε Hf ð t Þ value of zircon ranges from -18.36 to 4.33, and the age of the second-order Hf model T DM2 ranges from 2491 to 1175Ma. The source rock re ﬂ ecting the provenance of the sediments comes from the material recycling of the Paleoproterozoic and Mesoproterozoic in the crust, combined with the Meso-Neoproterozoic detrital zircons discovered this time, indicating that the provenance area has experienced Greenwellian orogeny.


Regional Geology
The Ordos Basin is a typical craton basin that belongs to the southern part of the North China Craton. It merges with the Yinshan block in the north at~1.95 Ga along the Kongziite belt to form the western block. After that, around 1.85 Ga, the eastern and western continental segments were merged into a unified North China landmass (Zhao et al., 2002; [1]). As can be seen in Figure 1, it is adjacent to the Qinling-Qilian Mountains in the south, the Central North China Orogenic Belt in the east, and the Helan Shan-Zhuozi Shan in the west. According to the current structural characteristics, it is divided into 6 structural units, including the western margin thrust belt, Tianhuan depression, Yimeng uplift, Weibei uplift, northern Shaanxi slope, and flexure belt in western Shanxi (Yang Junjie and Pei Xigu, 1996). The surrounding strata of the Ordos Basin are mainly ancient metamorphic rock series composed of various metavolcanic-sedimentary rocks of the Proterozoic (Wei Honghong, 2002 A lx a B a y a n w u la S h a n B a y a n h a o t e B a s in

Analytical Methods and Results
3.1. Zircon U-Pb Dating. The core samples used in the single well in this study weighed more than 1 kg which was crushed in the Laboratory of Langfang Regional Geological Survey Research Institute, Hebei Province. After that, the crushed samples were separated using heavy mineral as well as heavy liquid techniques and then picked under a binocular stereoscope. Subsequently, the representative samples were placed in epoxy resin, then ground and polished to about onethird of the original particle for the subsequent analyses.

3.2.
Results. The detrital zircons samples from Well S100, Well E59, Well E73, and Well E77 in the Shanxi Formation in the study area have similar morphology which are presented in Figure 2. The cathodoluminescence (CL) photos show that the detrital zircons are mostly gray and light brown with the semiautomatic short column-columnar or round shape. The zircon particles are well preserved and have a clear core-edge structure. Most of the cores have clear oscillatory zoning, and the Th/U ratio is over 0.4 suggesting a typical magmatic origin. Some zircon has a nonoscillatory zoning structure which is indicating the metamorphism origin ( [4]; Wu Yuanbao and Zheng Yongfei, 2004). In this study, the LA-ACP-MS U-Pb age test was performed on the detrital zircon particles of sandstone samples from Well S100, Well E59, Well E73, and Well E77 in the Shanxi Formation. Use Isoplot software to process the concordant data and draw the zircon U-Pb concordia diagrams and the age distribution histogram. When the age value is greater than1000 Ma, the U-Pb isotope age is taken as the age value corresponding to 207 Pb/ 206 Pb; when the age value is less than 1000 Ma, the U-Pb age is taken as the age value corresponding to 206 Pb/ 238 U, and U-Pb concordia diagrams and histogram can be seen at Figure 3.
A total of 130 detrital zircons were obtained from the samples from Well S100. The age of 42 zircons is discordant, and the rest of zircons grains are selected for the final interpretation. Among them, three grains yielded age of (2530:3 ± 26) Ma,   9 Ma. Also, one Paleoarchean metamorphic zircon ( Figure 2 E77-018) with the age of (3398:8 ± 31) Ma and one Neoproterozoic metamorphic zircon ( Figure 2 E77-015) with the age of (949:1 ± 9) Ma was found in this sample which is recording the important geological events.
To sum up, the zircon age spectrum of the sandstone samples in the study area is consistent. Four principal age peak ranges are recognized, 470~400 Ma, 360~260 Ma, 2100~1600 Ma, and 2500~2300 Ma. The age is dominated by Paleoproterozoic and Carboniferous-Early Middle Perm-ian and followed by Neoarchean, Mesoproterozoic and Early Paleozoic detrital zircons.
3.3. Zircon Hf Isotopic Characteristics. In this paper, in situ Lu-Hf isotope analyses were carried out on selected 38 young zircon grains with concordance ages of Well E 59 and Well S100 to calculate εHf ðtÞ values and two-stage depleted mantle model ages (T DM2 ). As illustrated in Table 2, except for the sample E 59-126 zircon whose 176 Lu/ 177 Hf ratio is greater than 0.002, the 176 Lu/ 177 Hf ratio of other zircons is all less than 0.002 with the average value of 0.0009, indicating a low level of radioactive Hf accumulation after the zircons were formed. Given that the Lu/Hf ratio (f Lu/Hf = −0:97) of zircon grains is significantly smaller than that of the silicoaluminous continental crust (f Lu/Hf = −0:72), the εHf ðt Þ values of all zircons fall below the depleted mantle evolution curve which can be seen in Figure 4; so, the two-stage model age (T DM2 ) more truly reflects the time when the source material was extracted from the depleted mantle ( [5] [12][13][14]). The basement of the Helanshan area is the Helanshan metamorphic complex, which is composed of a large number of high-grade metamorphic aluminum-rich gneiss (Helanshan rock group), marble, and metamorphic deformed granite. It contains a certain amount of S-type granite. The age is dominated by Paleoproterozoic (Hu Neng Gao Gao, 1994; Zhou Xiwen and Geng Yuansheng, 2009; [12][13][14], Li Liming et al., 2014)) and also contains Archean rocks (Zhou Xiwen and Geng Yuansheng, 2009; [15]).
Hu and Yang [16] used the single-grain zircon evaporation method to obtain the biotite-monzonite bearing granulite in the Helan Mountain area. The ages are 2102~1902 Ma, and the ages of the fibrolite-garnetcordierite bearing gneisses are 1898~1853 Ma. The age of medium granite bearing garnet is 1975~1893 Ma. Dong et al. [15] used SHRIMP to obtain the average age of the residual zircons of the garnet-mica bearing monzonite gneiss of the Helanshan rock group as (1978 ± 17) Ma, and the age of the magmatic zircon of the gneissic granite in the Bayanwula-Helanshan area is (2323 ± 20) Ma, the ages of metamorphic zircons are (1923 ± 28) and (1856 ± 12) Dan et al. [12,13] dated the porphyritic moyite invaded into the Khondalite series in the Zongbieli area and obtained the magma crystallization age of (1947 ± 6) Ma. The age of the biotite garnet gneiss is (2056 ± 18) Ma, and the crystallization age of the biotite granite in the Gunzhongkou area is (1956 ± 19) Ma. Yin et al. [14] measured the crystallization age of the S-type granite in the Helanshan Complex to be (1840 ± 19) Ma, and the age of the detrital zircon was 2.02~1.95 Ga. Geng et al. [17] used SHRIMP zircon U-Pb dating of the Helanshan metamorphic granite to determine that the biotite plagioclase gneiss was formed at (2053 ± 58) The base of the Qianlishan area is the Qianlishan complex, which is mainly composed of granulite facies highgrade regional metamorphic rock series (Qianlishan rock group) and a small amount of S-type granite. Yin et al. (2009) obtained the (1875 ± 19) Ma crystallization age from the S-type granite in Qianlishan, the sedimentary age of the clastic zircon original rock is slightly later than 2 billion years ago, and the metamorphic age is 195 million years ago. Darby and Gehrels' [20] Detrital zircon dating of Neoproterozoic to Ordovician sedimentary rocks in the Qianlishan area, there is a major age peak of 2.06-2.0 Ga.
The Alxa block is located in the northwestern part of the study area. It has long been considered to be the west extension of the Yinshan block of the North China Craton which collided with the Ordos block at~1.95 Ga to form the western block of the North China Craton [1]. In recent years, with the development of more research work, multiple periods of tectonic thermal events have been identified in the Alxa area, with a complex evolutionary history (Li Junjian, 2006; [21,22]; Li Jinyi, etc., 2012; [23]). The Early Precambrian basement of the Alxa block is mainly exposed in the Alxa Left Banner area in the east of the block.  [21,24,25]). The western basement of the block mainly exposes the Beidashan Complex in Alxa Right Banner [26] and the Longshoushan Complex on the north side of the Hexi Corridor [27,28]. In recent years, a large number of early Cambrian metamorphic basement ages have been obtained in the East Alashan block through zircon dating. The zircon ages of the diabetic hornblende plagioclase gneisses of 3570-2750 Ma were obtained from the Dibsge complex. At the same time, two metamorphic ages of Late Neoarchean (2.69~2.5 Ga) and Late Paleoproterozoic (1.9~2.0Ga) were obtained [25]. The Rb-Sr isochron age of the amphibolite is 3219 Ma, and the Sm-Nd whole-rock isochron age is 3018 ± 49 Ma whose age belongs to the Middle Archean (Li Junjian, 2006 [25]. The magma crystallization age of the gneissic granite is 2323 ± 20 Ma and also acquired two metamorphic ages of 1923 ± 28 Ma and 1856 ± 12 Ma [15]. The magma crystallization age of orthogneiss is 2344 ± 122 303 ± 5 Ma, the two metamorphic ages are~1.9Ga and~1.8Ga, respectively, and the magma age of the granite dike is 1895 ± 28 [12,13]. The magma crystallization ages of orthogneisses are 2301 ± 20~2232 ± 18 Ma, and their metamorphic ages are~1.91 Ga and~1.85Ga, respectively [29]. The diagenetic ages of the intrusive rocks in the Borostamiao complex are 1839 ± 18 Ma et al., 2004). The ages of the detrital zircons in the paragneisses are mainly concentrated between 2.16 and 2.01 Ga, and the magma crystallization ages of the granite gneiss are~2.33Ga,~2.17Ga, and~2.04 Ga, respectively. Both the diorite gneisses and granodiorite gneisses with TTG characteristics in the Beidashan complex have the magma crystallization ages of 2. 55 to 2. 51 Ga [26]. The Alxa block has experienced 2.7~3.0Ga continental crust growth,~2.5Ga TTG magmatic-metamorphic event, 2.0~2.3Ga magma event, and the two important metamorphic events of 1. 9~1.95Ga and 1.80~1.85Ga, which have strong consistency with the zircon age distribution of Shanxi Formation in the study area. 8 Geofluids The Langshan structural belt is located at the junction of the northern margin of North China and the southern margin of the Central Asian Orogenic Belt. The longterm multistage strong magmatism had complex and diverse rock types and well-exposed bedrocks. It is an important structure-magma evolution place for the study area. Sun et al. (2013) used dating techniques to conduct isotope chronological research on the metamorphic intrusions and zircons in the Baoyintu group quartzite and obtained the metamorphic intrusive gneissic monzonitic granite in the Baoyintu block. The SHRIMP zircon U-Pb age is 1672 ± 10 Ma, and the detrital zircon LA-ICP-MS zircon U-Pb age range of the Baoyintu group quartzite is 2896~1426 Ma. Liu et al. [30] obtained that the detrital zir- Based on the above analysis, it can be seen that the Guyang, Wuchuan, Langshan, and Alxa areas of the Yinshan block of the North China Craton have Neoarchean zircons that are similar in age to the study area. The main body of the Neoarchean zircon obtained by the research may be derived from the ancient TTG gneiss and basic granulite in the Yinshan block. The ages of a large number of zircons in the Daqingshan-Wulashan-Qianlishan-Helanshan area of the Kongzi rock belt are concentrated in 2.3~1.8Ga, and there are two metamorphic event ages of~1.95Ga and 1.85Ga exist, indicating that the Paleoproterozoic gneiss and granite in the Kongzi rock belt can provide the main source material of Paleoproterozoic for the study area. At the same time, there are also Paleoproterozoic zircons in Langshan and Alxa areas, showing that they may also participate in supplying Paleoproterozoic zircon provenance in the study area which is showed in Figure 5. This is consistent with the previous conclusions drawn by methods of sedimentary debris, heavy mineral, and lithofacies paleogeographic distribution patterns in the study area [2].
As can be seen in Figure 2, a Paleoarchean metamorphic zircon with an age of (3398:8 ± 31) Ma was found in the detrital zircon of Well E 77 in this study (E 77-018). The U-Pb age of (3690 ± 35) Ma was obtained from the detrital zircons in the Wushenqi area on the northern margin of the Ordos Basin [31]. Geng et al. [25] measured the U-Pb age of the zircon core of the Diabushige Group of the Diabushige Group in Alxa area to be (3570 ± 7) Ma. Therefore, there may be an ancient basement for more than 3 billion years old in the western of the North China Craton. The exposed basement has disappeared due to long-term weathering and erosion. However, these ancient materials are well preserved in the sediments of the basin, which can provide more comprehensive information for the tectonic evolution of the North China Craton [5,11].

Probable
Provenance Area in the Proterozoic (1492.8~949. 1 Ma). In this study, nine Mesoproterozoic detrital zircons were found in sandstone samples from Well E 59, Well E 73, and Well E 77, which are well concordant, with ages ranging from 1492.8 to 1165.7 Ma. Also, the samples from Well E 77 A detrital zircon with a U-Pb age of (949:1 ± 9) Ma were also found. The zircon ages discovered in the Mid-New Proterozoic period have a good correspondence with the orogeny of the Greenville period. In the eastern part of the Alxa block, Liu et al. [30] conducted a chronological study on the Kyanite-bearing garnet mica quartz schist of the Deerhetongte Formation in the Altanobao area and found that the age of detrital zircon ranges from 3306 to 1146         A lot of Lu-Hf isotope analysis on Paleozoic magmatic zircons had been done before. The comprehensive analysis result is demonstrated in Figure 6; it can be found that the Hf isotope characteristics of the Paleozoic magmatic zircons in Daqingshan-Wulashan in the North China Craton and the north area showed that the source rock is the recycle of ancient materials from the Neoarchean-Paleoproterozoic crust, while the magmatic source rock in the Paleozoic in the Alxa and Langshan areas contains the Mesoproterozoic crust and mantle material, which is similar to the Hf isotopic characteristics obtained from the zircons from Well E 59. Therefore, the Meso-Palaeozoic detrital zircons from the sandstones of the Shanxi Formation in the study area are likely to come from the igneous rock bodies formed in the Paleozoic in Langshan and Alxa areas.

Indicative
Significance to the Greenville Orogen. At present, most researchers believe that the Rodinia supercontinent in the late Mesoproterozoic-Early Neoproterozoic was the combination of the extensive Greenwell and several continental blocks which separated at the early orogeny period of the same period from 1.3 to 1.0 Ga after the collapse of the Colombian supercontinent [38]. However, there is still a lack of systematic research on whether there is a corresponding Greenville orogen in the northern margin of the North China block. The Meso-New Proterozoic detrital zircon and Paleozoic detrital zircon Hf isotopic two-stage model ages the Ordos sediments in this study, it is reasonable to believe that the Alxa block has corresponding Greenwellian orogeny and participated in Rodinia (Rodini Convergence and fragmentation of subcontinents).

Data Availability
Most of the data generated or analyzed during this study are included in this manuscript, and all of the data are available from the author on reasonable request.

Conflicts of Interest
The authors declare that they have no conflicts of interest.