Provenance and Tectonic Setting of Lower Cretaceous Huanhe Formation Sandstones, Northwest Ordos Basin, North-Central China

In adjustment to fulfill the requirements of the investigation regarding the lower Cretaceous sandstone uranium deposit in the Naogaodai area within the northwest of Ordos Basin, twenty-seven sandstone samples obtained from the Lower Cretaceous Huanhe Formation were analyzed for major, trace and rare earth elements (REE). The source of clastic and tectonic background was additionally analyzed. The results show that Huanhe sandstone is feldspar rich sandstone, and also the mineral composition is principally quartz, albite and plagioclase; the ratio of light to heavy rare earth elements (LREE/HREE) is 9.25–10.83, with an average value of 10.00; (La/Yb)CN is 10.20–12.53, with an average value of 11.24, demonstrating that LREE is enriched and fractionated compared with HREE. REE distribution patterns are similar, which additionally reveals that Huanhe sandstone has a homogenous source; the Index of Compositional Variability (ICV) value is 1.17–1.73, with an average value of 1.35, both greater than 1, showing an immature property, which may be first-order cycle deposition. The average value of the Chemical Index of Alteration (CIA) is 50.29, suggesting that the source rock has encountered weak chemical weathering; sandstones are near-source first-cycle provenance, which is not significantly influenced by sedimentary sorting and recycling. The geochemical qualities further indicate that Huanhe sandstone was deposited in a passive margin and experienced moderate weathering.


Introduction
In recent years, various exceptional remarkable achievements have been made in recognising the geotectonic background throughout the formation of rocks, the provenance of the basin and the tectonic background of the provenance area, providing a new perspective for understanding the evolution of the basin and its source area [1][2][3][4][5][6][7][8]. Detrital mineralogy and geochemical characteristics of siliciclastic sediments will effectively reveal the provenance area, tectonic settings, transportation, paleoweathering conditions of the source area, and also establish a basis for understanding paleoclimate and diagenesis [9][10][11][12]. Mineralogy and petrography of sandstones have been widely used to determine their provenance [13,14]. Bulk rock geochemistry of sandstones has also been used in the determination of their provenance, tectonic setting and paleoweathering of the source area [8,[15][16][17][18]. In this study, an integrated approach employing mineralogy, petrography together with bulk-rock geochemistry in order to reveal the provenance, tectonic setting, paleoweathering and paleoclimate has been widely documented by many researchers [13,19,20].
Lower Cretaceous strata are extensively distributed throughout the northwestern part of the Ordos Basin. Recently, we have a tendency to find uranium mineralization The Lower Cretaceous in the study area is called the Zhidan group, sandstone bodies are developed and have favorable uranium metallogenic conditions, which can be divided into the Luohe Formation (K1zh 2 ), the Huanhe Formation (K1zh 3+4 ), the Luohandong Formation (K1zh 5 ) and Jingchuan Formation (K1zh 6 ) [21] (Figure 2). From the Luohe Formation to the Huanhe Formation, they are continuous deposits, the maximum thickness of the stratum can be greater than 1200 m, which constitute two major sedimentary cycles: K1zh 2-K1zh 3 + 4 and K1zh 5-K1zh 6 . The strata are generally thin in the east and thick in the west, old in the east and new in the west, reflecting that the missing strata at the top are more in the separation and a high degree of cementation, mainly calcium cementation. The fossils in the Huanhe formation not only have many kinds, but also have a large number, which reflects that they are the products of a relatively humid climate. In particular, the emergence of spruce, cone fern and other plants can accurately explain the characteristics of warm and humid paleoclimate at that time. It can be seen that the sedimentary period of the Zhidan group was dominated by drought and alternating dry and wet paleoclimate.

Materials and Methods
The samples of this study were collected from ZKW2019-1, ZKW2019-2, ZKW2019-3 and ZKW2020-1 boreholes in the Naogaodai area, northwest Ordos Basin (Figure 3). Owing to the effect of sediment granularity on the chemical composition of clastic rocks, the samples were designated as medium and fine sandstone with uniform particles. When sampling, we flushed the surface drilling fluid, cut off the weathered surface, then crushed it to two hundred mesh in the conventional way in Tianjin trilobite Mine Technical Service Co., Ltd. Samples were observed microscopically prior to chemical analysis to monitor potential alteration, mineralization, or secondary weathering. All samples were ground with a noncontaminating crusher, screened in a 200-mesh sieve, and heated in an oven at 80 • C for 3 h to remove moisture. The sample analysis test was completed by the Analytical Laboratory of Beijing Research Institute of Uranium Geology (Beijing, China).  The major element test instrument was an Axiosm AX-X-ray fluorescence spectrometer (Beijing Research Institute of Uranium Geology, Beijing, China, analysis method is based on the Silicate Rock Chemical Analysis Method Part 14: Determination of ferrous oxide (GB/T 14506.  and Silicate Rock Chemical Analysis Method Part 28: Determination of 16 primary and secondary components (GB/T 14506.  and the analysis error was less than 1%. The sample was melted with anhydrous tetraboric acid file, ammonium nitrate as oxidant, lithium fluoride and a small amount of lithium bromide as flux and release agent. The weight ratio of sample to flux is 1:8. The prototype was melted at 1150~1250 degrees to make glass samples. The test voltage was 50 kV and the current was 50 mA.
The analysis instrument of trace and rare earth elements was NexION 300D laser coupled Plasma Mass Spectrometer (ICP-MS) (PerkinElmer, Waltham, MA, USA), the analysis method is based on the Silicate Rock Chemical Analysis Method Part 30: Determination of 44 Elements (GB/T 14506. , and the analysis accuracy is better than 5%. We accurately weighed 25 mg of the sample and placed it in the inner tank of the closed sample dissolver, added hydrofluoric acid and nitric acid and sealed it, put the sample dissolver into the oven, heated it for 2 h, and controled the temperature at 185 • C. After cooling, we placed it on the electric heating plate to heat and steam dry, then added nitric acid to steam dry, and repeated. We added nitric acid again, sealed it, put it into the oven, heated it at 130 • C for 3 h, diluteed it with water after cooling, and tested it by ICP-MS. The error range was 5~10%, the experimental temperature was 21 • C, and the relative humidity was 44%. Paleo-weathering of the source region has been quantitatively assessed by chemical indices, together with the Chemical Index of Alteration (CIA) = Al 2 O 3 /(Al 2 O 3 + CaO* + Na 2 O + K 2 O) × 100 [9], the Plagioclase Index of Alteration (PIA) = ( [22], the Chemical Index of Weathering (CIW) = Al 2 O 3 /(Al 2 O 3 + CaO* + Na 2 O) × 100 [23], and the Index of Compositional Variability (ICV) = (Fe 2 O 3 + K 2 O + Na 2 O + CaO* + MgO + MnO + TiO 2 )/Al 2 O 3 [24]. Within the formula above, the content of all oxides is calculated in line with the molecular ratio, and CaO* solely means the Ca combined with the aluminum silicate component [9,22]. Due to the modification of carbonate minerals, the CaO content within the samples of this analysis varied greatly. If the value of CIA is calculated by the total CaO, this could cause unauthentic conclusions. Therefore, the formula for CaO* is: CaO** = mol CaO − (10/3 × mol P 2 O 5 ); if CaO** < Na 2 O, then CaO* = CaO**, else CaO* = Na 2 O [11]. The discriminant diagram-based major elements were accustomed separate to the provenance of the Huanhe sandstones [25], since major and trace element-based discriminant diagrams have been accustomed to assess the tectonic setting of the Huanhe sandstones [26][27][28][29][30].

Petrography of Sandstones
A petrological identification of allegorical sandstone samples within the western allotment of the Ordos Basin was carried out. Detrital particles incorporate quartz, feldspars, bedrock fragments, micas, and heavy minerals ( Figure 4A-F). Quartz is the predominant detrital grain, a mixture of strained, unstrained and crystalline ( Figure 4). The majority of monocrystalline quartz grains are bright with equant shapes and reasonably undulate eradication. Feldspar crystals incorporate plagioclase (Figure 4), orthoclase and microcline. Potassium feldspars are composed of filamentous twin crystals and Karls twin crystals, which are composed of subhedral particles. Mica includes biotite and muscovite, which are striped or angled (Figure 4), mainly biotite. The grain sizes of quartz, plagioclase, potash feldspar, biotite and muscovite are appropriately 200-450 µm, 150-300 µm, 150-300 µm, 200-500 µm, 100-250 µm, which are fine-medium grained sandstones. majority of monocrystalline quartz grains are bright with equant shapes and reasonably undulate eradication. Feldspar crystals incorporate plagioclase (Figure 4), orthoclase and microcline. Potassium feldspars are composed of filamentous twin crystals and Karls twin crystals, which are composed of subhedral particles. Mica includes biotite and muscovite, which are striped or angled (Figure 4), mainly biotite. The grain sizes of quartz, plagioclase, potash feldspar, biotite and muscovite are appropriately 200-450 µm, 150-300 µm, 150-300 µm, 200-500 µm, 100-250 µm, which are fine-medium grained sandstones.  There are a large number of igneous, sedimentary and metamorphic bedrock fragments present in the lower Cretaceous sandstone. The authigenic minerals, for instance, calcite in sandstone is additionally shown in Figure 4. As observed by thin-sections, due to relatively low matrix concentration, the sandstones are mainly clast-supported.

Major Elements Geochemistry
The concentration of major elements in sedimentary rocks is regulated by several factors, the foremost vital of that is the mineral composition [31]. For instance, the K 2 O/Na 2 O ratio of sedimentary rocks is controlled by the relative ratio of potassium feldspar and plagioclase, whereas the SiO 2 content is controlled by the abundance of quartz. Weathering can likewise influence the fixation of major elements in sedimentary rocks. Provenance and sediment transport processes also have an impact. For instance, Ti, Si, and P are usually associated with coarse-grained sand in high-energy environments, whereas Al is enriched in clay minerals [32,33].
Major element contents of investigated sandstones are shown in Table 1 [34] shows that Huanhe sandstones are principally distributed within the arkose area ( Figure 5), whereas few samples plotted within the wacke field.

Major Elements Geochemistry
The concentration of major elements in sedimentary rocks is regulated by several factors, the foremost vital of that is the mineral composition [31]. For instance, the K2O/Na2O ratio of sedimentary rocks is controlled by the relative ratio of potassium feldspar and plagioclase, whereas the SiO2 content is controlled by the abundance of quartz. Weathering can likewise influence the fixation of major elements in sedimentary rocks. Provenance and sediment transport processes also have an impact. For instance, Ti, Si, and P are usually associated with coarse-grained sand in high-energy environments, whereas Al is enriched in clay minerals [32,33].

Trace Elements Geochemistry
The trace element contents of the studied sandstones are shown in Table 2. Generally, trace elements contents of the studied sandstones vary widely. Compared to the average contents of the upper continental crust (UCC), the majority of trace element contents are lower. However, the content of large ion lithophile elements, such as Ba and Sr varies greatly, with the average values being 643.89 ppm and 356.74 ppm, respectively. These values are higher than the average UCC values ( Table 2).
The content of Th (3.69-8.24 ppm), U (1.11-7.36 ppm) and Th/U ratio (0.67-3.91) within the studied sandstones vary greatly, but they are consistent with the UCC. Trace elements (such as Pb, V and Cr) are positively correlated with Al 2 O 3 (r = 0.535, 0.50 and 0.494, respectively), as a result of they are typically associated with potash feldspar and clay minerals [35].

Rare Earth Elements (REE) Geochemistry
The contents and proportions of REE of the Huanhe Formation sandstones are reported in Table 3. The REE value show LREE enriched and flat HREE type. The REEs of the Huanhe Formation sandstones are normalized to chondrite (Figure 6), which is comparable to the distribution pattern of REEs of UCC [10,36]. Eu/Eu* anomaly is quantified as: Eu/Eu* = 2 × Eu CN /(Sm CN + Gd CN ). The Ce/Ce* anomaly is determined by the subsequent formula: Ce/Ce* = 2 × Ce CN /(La CN + Pr CN ) [10]. The subscript CN represents the normalized value of chondrite [10]. Table 3    The Huanhe sandstone is rich in quartz and feldspar, containing felsic classics rather than basic rock classics, and is a felsic source rock. Crystalline quartz grains have intergranular boundaries down to slightly curved, which are conducive to the source of deep igneous rocks, whereas the serrated to suture intercrystalline interface reflects the metamorphic source [37,38]. Additionally, only a few crystalline quartz particles enclose one single crystal in an exceedingly bimodal state ( Figure 4). Such quartz particles are likely to come from schist source rocks. Some quartz grains show a part of the inherited quartz overgrowth (Figure 4), that is the result of recycling sedimentary quartz throughout the diagenesis process. The identified non-opaque heavy minerals include ZTR (zircon, tourmaline and rutile), additionally to rare epidote. The ZTR group might be derived from acidic igneous rocks (granite) and/or recycled older sediments, whereas epidote is principally derived from metamorphic rocks [39]. The composition and textural maturity of the Huanhe sandstone indicated that it is a felsic source area, supplemented by recycled quartz sand.

Geochemistry
Geochemical data provide clues to characterize the provenance and source area of clastic rocks [12,16,27,40]. Oxides and hydroxides of Al, Ti and Zr have low solubility in low-temperature aqueous solutions can be considered as immobile elements. Therefore, their ratio is extremely about to that of the source rocks. On the discriminant diagram [25], the Huanhe Formation sandstones are plotted in the felsic and intermediate igneous rock field ( Figure 8A). On the TiO 2 versus Zr binary diagram [41], the studied sandstones are plotted in intermediate igneous provenance fields ( Figure 8B). Al 2 O 3 /TiO 2 ratio < 8 indicates basic source rock, Al 2 O 3 /TiO 2 ratio between 8 and 21 indicates intermediate igneous source rock, and Al 2 O 3 /TiO 2 ratio > 21 indicates felsic igneous source rock [41]. The TiO 2 versus Al 2 O 3 diagram shows the felsic igneous source rocks for most of the studied samples (Figure 9).
Trace elements such as Th, Cr, Ni, V, La, Co, Sc, Nb, Zr, Hf, Y and REEs, are very important in interpreting of provenance and composition of the source area because they are immobile in the post-depositional processes [11,28]. According to the diagram of Hf versus La/Th [42], La/Sc versus Co/Th [11], Th/Sc versus Cr/Th [43], Y/Ni versus Cr/V [44] (Figure 8C-F), the studied sandstone shows that it is a felsic igneous source rock and basic mixed source rock. The ternary graph of Ni-V-Th*10 [45] showed that the Huanhe sandstones fall close to the V-Th*10 line, reflecting a mixed origin of felsic and mafic rock provenance (Figure 7). At the same time, depletion of the Sc, Co, Ni, Cu and Th contents (Average values 6.51, 7.83,17.87, 8.63 and 5.16 ppm, respectively; Table 2), represents the felsic source area [11].
Comparing the Cr/Th, Th/Co, Th/Sc, La/Co and La/Sc ratios of the Huanhe sandstones (69.63, 0.71, 0.80, 3.31 and 3.72, respectively) with those of upper continental crust ( Table 4), shows that the sandstones are derived from felsic rocks [12,35,37]. The distri-bution pattern of rare earth elements in sediments and the abnormal characteristics of Eu provide clues for finding out the feature of the source area. The felsic source rock has the characteristics of high LREE/HREE ratios and negative Eu anomaly, while the basic source rock has the characteristics of low LREE/HREE and missing Eu anomalies [46]. Due to the quartz dilution, the contents of ∑REE within the Huanhe sandstones is low [10], which is consistent with high SiO 2 contents (64.09-75.73%, average 70.99%). In the Eu/Eu* versus fractionation factor (La/Yb) CN bivariate graph, most samples show that felsic source rocks derived from the upper continual crust ( Figure 10A), while in (Gd/Yb) CN versus Eu/Eu* diagram ( Figure 10B), the sandstones studied almost belong to the post-Archean field. This is consistent with the potassium-rich feldspar granite with (Gd/Yb) CN < 2 [10].    [11]. The average chemical compositions of the volcanic rocks are from Condie [47]. (E) Th/Sc versus Cr/Th bivariate diagram [43]. (F) Y/Ni versus Cr/V bivariate diagram [11]. Symbols are the same as in Figure 5.   [42]. (D) La/Sc versus Co/Th bivariate diagram [11]. The average chemical compositions of the volcanic rocks are from Condie [47]. (E) Th/Sc versus Cr/Th bivariate diagram [43]. (F) Y/Ni versus Cr/V bivariate diagram [11]. Symbols are the same as in Figure 5. ls 2021, 11, x 16 of 24 Figure 8. The plot of TiO2 versus Al2O3 of the Huanhe sandstones. UCC [10], granite [48], granodiorite [49], and tonalite [49]. Symbols are the same as in Figure 5. . Ni-V-Th*10 ternary diagram for provenance discrimination of the Huanhe Formation sandstones [45]. Symbols are the same as in Figure 5.  [10], granite [48], granodiorite [49], and tonalite [49]. Symbols are the same as in Figure 5.  [10], granite [48], granodi orite [49], and tonalite [49]. Symbols are the same as in Figure 5.

Figure 9.
Ni-V-Th*10 ternary diagram for provenance discrimination of the Huanhe Formation sandstones [45]. Symbols are the same as in Figure 5. . Ni-V-Th*10 ternary diagram for provenance discrimination of the Huanhe Formation sandstones [45]. Symbols are the same as in Figure 5.  [10]. Symbols are the same as in Figure 5.

Tectonic Setting
Bulk-rock geochemistry of siliciclastic sediments has been accustomed to distingu the tectonic settings of sedimentary basins [26][27][28]. The discrimination diagrams based the major elements were drawn [26,28,29], showing that the majority of the sandsto samples fall within the passive margin field; however, four samples exist in the act continental margin field ( Figure 11).
The distribution of REE is additionally accustomed distinguish between active a passive marginal tectonic environments [11,26,35]. The passive margin includes a R distribution pattern similar to the average of UCC and Post-Achaean Australian Sh (PAAS) [10,26] and an obvious Eu anomaly. Within Figure 4, the distribution of REE studied samples is extremely almost like the typical average UCC and PAAS, and Eu i negative anomaly, indicating that the deposition was at the passive margin after the Ea Cretaceous rifting event. Cyclic alternations of fluvial and shallow lake sediments of Lower Cretaceous Huanhe Formation reflect base-level changes. Geochemical data sh that before the late Cretaceous, the early Cretaceous rifting period in the southern edge the Neo-Tethys Ocean ceased, and then passive margin development and the deposit of the Huanhe Formation began after rifting appeared.  [36], (B) (Gd/Yb) CN versus Eu/Eu* bivariate diagram [10]. Symbols are the same as in Figure 5.

Tectonic Setting
Bulk-rock geochemistry of siliciclastic sediments has been accustomed to distinguish the tectonic settings of sedimentary basins [26][27][28]. The discrimination diagrams based on the major elements were drawn [26,28,29], showing that the majority of the sandstone samples fall within the passive margin field; however, four samples exist in the active continental margin field ( Figure 11).
The distribution of REE is additionally accustomed distinguish between active and passive marginal tectonic environments [11,26,35]. The passive margin includes a REE distribution pattern similar to the average of UCC and Post-Achaean Australian Shale (PAAS) [10,26] and an obvious Eu anomaly. Within Figure 4, the distribution of REE in studied samples is extremely almost like the typical average UCC and PAAS, and Eu is a negative anomaly, indicating that the deposition was at the passive margin after the Early Cretaceous rifting event. Cyclic alternations of fluvial and shallow lake sediments of the Lower Cretaceous Huanhe Formation reflect base-level changes. Geochemical data show that before the late Cretaceous, the early Cretaceous rifting period in the southern edge of the Neo-Tethys Ocean ceased, and then passive margin development and the deposition of the Huanhe Formation began after rifting appeared. Recently, two major element diagrams of siliciclastic rocks based on discriminant function using the geochemical composition of Neogene-Quaternary sediments [30]. Consistent with the high silica multidimensional function diagram [30], the sandstones studied are mainly distributed within the continental rift field, except that three samples belong to the collision field ( Figure 12). Continental rifts are caused by extension processes within the continent, including passive margins [30]. Plotting of both major and trace elements of Huanhe sandstones using the multidimensional discriminant function diagrams [50] reflects a passive margin setting of the source area ( Figure 13). The previous studies are basically consistent with the passive margin interpretation of the Huanhe Formation.  Recently, two major element diagrams of siliciclastic rocks based on discriminant function using the geochemical composition of Neogene-Quaternary sediments [30]. Consistent with the high silica multidimensional function diagram [30], the sandstones studied are mainly distributed within the continental rift field, except that three samples belong to the collision field ( Figure 12). Continental rifts are caused by extension processes within the continent, including passive margins [30]. Plotting of both major and trace elements of Huanhe sandstones using the multidimensional discriminant function diagrams [50] reflects a passive margin setting of the source area ( Figure 13). The previous studies are basically consistent with the passive margin interpretation of the Huanhe Formation. Recently, two major element diagrams of siliciclastic rocks based on discriminant function using the geochemical composition of Neogene-Quaternary sediments [30]. Consistent with the high silica multidimensional function diagram [30], the sandstones studied are mainly distributed within the continental rift field, except that three samples belong to the collision field ( Figure 12). Continental rifts are caused by extension processes within the continent, including passive margins [30]. Plotting of both major and trace elements of Huanhe sandstones using the multidimensional discriminant function diagrams [50] reflects a passive margin setting of the source area ( Figure 13). The previous studies are basically consistent with the passive margin interpretation of the Huanhe Formation.   Multidimensional discriminant function diagrams [50] using major elements (A) and combined major and trace elements (B) respectively, to distinguish active from passive margin settings. Symbols are the same as in Figure 5.

Paleoweathering
In humid climate conditions, chemical weathering is dominant and strongly controls the major and trace element composition of siliceous clastic sediments [9,11,22,23,51], within which larger cations (such as Ba, Al, Rb) are preferentially fixed in weathering residue, while smaller cation (such as Na, Ca, Sr) are selectively removed [47]. These geochemical features can eventually be transferred to sediments, providing important information for monitoring the weathering status of the sediment source region [9]. At the same time, physical weathering is common in arid climate, together with the mechanical decomposition of source rock into smaller grain sizes without significant changes in minerals or chemical composition. Therefore, geochemical indicators give an honest methodology for paleoweathering in the source area. This may be due to the leaching of alkaline elements (such as Na, K and Ca) and also the enrichment of immobile elements (such as Al and Si) throughout weathering process, and also the development of plagioclase and K-feldspars in the direction of clay minerals ( Figure 14) [9]. Using Al2O3 − (CaO* + Na2O) − K2O (A-CN-K) ternary diagram, where Al2O3, CaO*, Na2O and K2O are arranged in molecular proportions [52], and also the Chemical Index of Alteration (CIA) [9] determined the mobility of elements during in the diagenesis process. The CIA value is widely plotted in the A-CN-K ternary diagram [53] (Figure 14) to judge the weathering history and source rock composition [52]. They were utilizing the values of major elements, chemical indices such as CIW [23], PIA [22], ICV [24] and other chemical indicators to evaluate the paleoweathering in the source area, avoiding the matter of potassium enrichment.
The Index of Compositional Variability (ICV) is employed to identify the compositional maturity of sediments and has been successfully applied in several studies. Typical diagenetic minerals such as feldspar, amphibole, and pyroxene have ICV values larger than 1, while alteration products such as kaolinite, illite, and muscovite have ICV values less than 1 [12,24]. Therefore, sediments with immature composition and high ICV value belong to the sediments with high recirculation and strong weathering. The ICV values of the studied samples are 1.17-1.72 (average is 1.35), indicating that Huanhe sandstone includes a low content of clay and belongs to the first-order cycle sediments. Most of the Figure 13. Multidimensional discriminant function diagrams [50] using major elements (A) and combined major and trace elements (B) respectively, to distinguish active from passive margin settings. Symbols are the same as in Figure 5.

Paleoweathering
In humid climate conditions, chemical weathering is dominant and strongly controls the major and trace element composition of siliceous clastic sediments [9,11,22,23,51], within which larger cations (such as Ba, Al, Rb) are preferentially fixed in weathering residue, while smaller cation (such as Na, Ca, Sr) are selectively removed [47]. These geochemical features can eventually be transferred to sediments, providing important information for monitoring the weathering status of the sediment source region [9]. At the same time, physical weathering is common in arid climate, together with the mechanical decomposition of source rock into smaller grain sizes without significant changes in minerals or chemical composition. Therefore, geochemical indicators give an honest methodology for paleoweathering in the source area. This may be due to the leaching of alkaline elements (such as Na, K and Ca) and also the enrichment of immobile elements (such as Al and Si) throughout weathering process, and also the development of plagioclase and K-feldspars in the direction of clay minerals ( Figure 14) [9]. Using Al 2 O 3 − (CaO* + Na 2 O) − K 2 O (A-CN-K) ternary diagram, where Al 2 O 3 , CaO*, Na 2 O and K 2 O are arranged in molecular proportions [52], and also the Chemical Index of Alteration (CIA) [9] determined the mobility of elements during in the diagenesis process. The CIA value is widely plotted in the A-CN-K ternary diagram [53] (Figure 14) to judge the weathering history and source rock composition [52]. They were utilizing the values of major elements, chemical indices such as CIW [23], PIA [22], ICV [24] and other chemical indicators to evaluate the paleoweathering in the source area, avoiding the matter of potassium enrichment.  The calculated average values of CIA within the studied sandstones vary from 43.60 to 53.82 (average is 50.29) ( Table 1). These values indicate that the sediments were derived from the source rocks that have undergone weak chemical decomposition. Several authors use the chemical index of weathering (CIW) to raise weathering conditions in the source area [22]. The CIW values of Huanhe sandstones are 47.67-60.42, with an average of 55.89 (Table 1). These CIW values indicate that the degree of chemical weathering of source rocks/sediments ranges from weak to moderate [23]. The data of the Huanhe sandstones are located in the A-CN-K composition space, nearly parallel to the A-CN edge ( Figure  14), showing that Na2O and CaO are removed from the source rock throughout low-moderate chemical weathering process of plagioclase feldspars. The PIA values (42.27-54.89, average = 50.40, Table 1) also indicate that there is weak chemical weathering in the source area [22].
The chemical weathering of the source area is additionally determined according to the proportion of trace elements [11]. The studied sandstones plot at the starting point of sedimentary recycling trend on Zr/Sc versus Th/Sc bivariate diagram ( Figure 15A). The lower Zr/Sc ratios (5.04-12.53) indicate that the Zr concentration is lower throughout the circulation and sorting processes of the sediment. The chemical weathering in a semi-humid to humid environment is especially an oxidization reaction. During this process, some elements such as U are oxidized (U 4+ to U 6+ ) and removed by a natural process, such as leaching, unlike Th remaining in insoluble residues [54]. Therefore, Th versus Th/U diagram can be used to evaluate the continuous cycles of chemical weathering and sediment recirculation. The Huanhe sandstones have a high Th/U ratio (average is 2.47), indicating that it has reducing conditions (less than 3.8 of UCC) [10]. On the Th versus Th/U bivariate diagram, most samples plot along the depleted mantle source ( Figure 15B). The ratio of Th/U and Th/Sc reflects the influence of chemical weathering or special oxidation-redox conditions, instead of the chemical composition of the source rocks. The Index of Compositional Variability (ICV) is employed to identify the compositional maturity of sediments and has been successfully applied in several studies. Typical diagenetic minerals such as feldspar, amphibole, and pyroxene have ICV values larger than 1, while alteration products such as kaolinite, illite, and muscovite have ICV values less than 1 [12,24]. Therefore, sediments with immature composition and high ICV value belong to the sediments with high recirculation and strong weathering. The ICV values of the studied samples are 1.17-1.72 (average is 1.35), indicating that Huanhe sandstone includes a low content of clay and belongs to the first-order cycle sediments. Most of the sediments in the first cycle do not change the geochemical characteristics of the samples, and their CIA can be used to discuss the weathering conditions within the source area.
The calculated average values of CIA within the studied sandstones vary from 43.60 to 53.82 (average is 50.29) ( Table 1). These values indicate that the sediments were derived from the source rocks that have undergone weak chemical decomposition. Several authors use the chemical index of weathering (CIW) to raise weathering conditions in the source area [22]. The CIW values of Huanhe sandstones are 47.67-60.42, with an average of 55.89 (Table 1). These CIW values indicate that the degree of chemical weathering of source rocks/sediments ranges from weak to moderate [23]. The data of the Huanhe sandstones are located in the A-CN-K composition space, nearly parallel to the A-CN edge (Figure 14), showing that Na 2 O and CaO are removed from the source rock throughout low-moderate chemical weathering process of plagioclase feldspars. The PIA values (42.27-54.89, average = 50.40, Table 1) also indicate that there is weak chemical weathering in the source area [22].
The chemical weathering of the source area is additionally determined according to the proportion of trace elements [11]. The studied sandstones plot at the starting point of sedimentary recycling trend on Zr/Sc versus Th/Sc bivariate diagram ( Figure 15A). The lower Zr/Sc ratios (5.04-12.53) indicate that the Zr concentration is lower throughout the circulation and sorting processes of the sediment. The chemical weathering in a semihumid to humid environment is especially an oxidization reaction. During this process, some elements such as U are oxidized (U 4+ to U 6+ ) and removed by a natural process, such as leaching, unlike Th remaining in insoluble residues [54]. Therefore, Th versus Th/U diagram can be used to evaluate the continuous cycles of chemical weathering and sediment recirculation. The Huanhe sandstones have a high Th/U ratio (average is 2.47), indicating that it has reducing conditions (less than 3.8 of UCC) [10]. On the Th versus Th/U bivariate diagram, most samples plot along the depleted mantle source ( Figure 15B). The ratio of Th/U and Th/Sc reflects the influence of chemical weathering or special oxidation-redox conditions, instead of the chemical composition of the source rocks.  [11]. (B) Th versus Th/U bivariate diagram [11]. Symbols are the same as in Figure 5.

Paleoclimate
Geochemical proxy indicators are widely utilized in the paleoclimate interpretation of the source areas [9,11,12,55]. In humid climate conditions, chemical weathering is dominant and affects the chemical composition of sediments, just like the concentration of mobile elements (Na, K and Ca) and immobile elements (Al, Si) throughout the process [9].
The major elements bivariate diagram (Al2O3 + K2O + Na2O% versus SiO2) of Suttner and Dutta (1986) was applied to the studied sandstones, reflecting the prevalence of arid paleoclimate conditions ( Figure 16). In keeping with the geochemical indicators (CIA, CIW, PIA and ICV) of the major element composition of siliciclastic sediments (Table 1), sandstones have experienced weak to moderate weathering beneath arid paleoclimate conditions. This is often in sensible agreement with the previous documents within the survey area. Figure 16. Al2O3 + K2O + Na2O% versus SiO2% bivariate diagram for distinguishing climatic conditions of the Huanhe sandstones [55]. Symbols are the same as in Figure 5.

Conclusions
This study takes the Huanhe Sandstone within the west Ordos Basin, China as the research object, the provenance, sedimentary structure background, and paleoweathering  [11]. (B) Th versus Th/U bivariate diagram [11]. Symbols are the same as in Figure 5.

Paleoclimate
Geochemical proxy indicators are widely utilized in the paleoclimate interpretation of the source areas [9,11,12,55]. In humid climate conditions, chemical weathering is dominant and affects the chemical composition of sediments, just like the concentration of mobile elements (Na, K and Ca) and immobile elements (Al, Si) throughout the process [9].
The major elements bivariate diagram (Al 2 O 3 + K 2 O + Na 2 O% versus SiO 2 ) of Suttner and Dutta (1986) was applied to the studied sandstones, reflecting the prevalence of arid paleoclimate conditions ( Figure 16). In keeping with the geochemical indicators (CIA, CIW, PIA and ICV) of the major element composition of siliciclastic sediments (Table 1), sandstones have experienced weak to moderate weathering beneath arid paleoclimate conditions. This is often in sensible agreement with the previous documents within the survey area.  [11]. (B) Th versus Th/U bivariate diagram [11]. Symbols are the same as in Figure 5.

Paleoclimate
Geochemical proxy indicators are widely utilized in the paleoclimate interpretation of the source areas [9,11,12,55]. In humid climate conditions, chemical weathering is dominant and affects the chemical composition of sediments, just like the concentration of mobile elements (Na, K and Ca) and immobile elements (Al, Si) throughout the process [9].
The major elements bivariate diagram (Al2O3 + K2O + Na2O% versus SiO2) of Suttner and Dutta (1986) was applied to the studied sandstones, reflecting the prevalence of arid paleoclimate conditions ( Figure 16). In keeping with the geochemical indicators (CIA, CIW, PIA and ICV) of the major element composition of siliciclastic sediments (Table 1), sandstones have experienced weak to moderate weathering beneath arid paleoclimate conditions. This is often in sensible agreement with the previous documents within the survey area. Figure 16. Al2O3 + K2O + Na2O% versus SiO2% bivariate diagram for distinguishing climatic conditions of the Huanhe sandstones [55]. Symbols are the same as in Figure 5.

Conclusions
This study takes the Huanhe Sandstone within the west Ordos Basin, China as the research object, the provenance, sedimentary structure background, and paleoweathering were studied by petrography and bulk-rock major and trace element geochemistry. Most Figure 16. Al 2 O 3 + K 2 O + Na 2 O% versus SiO 2 % bivariate diagram for distinguishing climatic conditions of the Huanhe sandstones [55]. Symbols are the same as in Figure 5.

Conclusions
This study takes the Huanhe Sandstone within the west Ordos Basin, China as the research object, the provenance, sedimentary structure background, and paleoweathering were studied by petrography and bulk-rock major and trace element geochemistry. Most of the sandstone is feldspar sandstone. Both chemical index of alteration and chemical index of weathering values indicate that the degree of chemical weathering of most studied samples is weak to moderate. In the Th/Sc versus Zr/Sc diagram, all samples are amid at the starting point of the sedimentary recycling trend, indicating that the degree of sediment recycling (zircon addition) is low. Different indicators (such as La/Th versus Hf) indicate that the studied samples are felsic and mixed (felsic-mafic) source rocks in the upper continental crust. Similarly, the Cr/Th versus Th/Sc and Cr/V versus Y/Ni commixture models conjointly indicate that Huanhe Sandstone has felsic to mixed source rocks. Their geochemical characteristics further indicate that the Huanhe Sandstone was partially deposited in a passive marginal environment and has undergone post-deposition potassium metasomatism.

Data Availability Statement:
No new data were created in this study. Data sharing is not applicable to this article.