Detrital Zircon Geochronology and Tectonic Evolution Implication of the Middle Jurassic Zhiluo Formation, Southern Ordos Basin, China

Abstract

The Ordos Basin’s southern part is a composite zone made up of numerous continental blocks and has long been influenced by surrounding tectonism. However, only a few studies have investigated the existence of southern provenance supply and the basin’s southern boundary in the Middle Jurassic Zhiluo Formation. Based on sandstone detrital zircon U-Pb dating and zircon rare earth element analyses, net-sand ratio maps, paleocurrent direction and the field outcrop survey, this study establishes the source area and boundary of the Zhiluo Formation in the southern basin, and discusses the tectonic events. The study shows that the four main age peaks in the detrital spectra occurs at 2283 Ma, 1788 Ma, 432.5 Ma and 218.7 Ma, with a few of the zircons dated at 794.5–1235.2 Ma. The North Qilian orogenic belt (N-QLOB), the western part of the North Qinling orogenic belt (NQOB), and the southern margin of the North China Block (SNCB) contributed to the provenance. According to an integrated analysis of the provenance and tectonic background of continental blocks in the basin’s southern margin, the boundary of the basin in the depositional period of the Zhiluo Formation should reach the N-QLOB in the southwest, the NQOB in the south, and the Sanmenxia–Lushan fault belt in the southeast. On the basis of the aforementioned findings, the tectonic evolution of the continental blocks at the southern periphery of the Ordos Basin was restored.

1. Introduction

In the geological history of the Ordos Basin, the Jurassic is a critical period of resource formation and a transitional period of tectonic stress fields and climate change [1,2]. Therefore, in view of the importance of paleogeography, climate, tectonics and energy exploration in this period, geologists have long been interested in the basin boundary, the source of sediment material, and the structural evolution of the basin [3,4,5,6,7].

Geological research on the Zhiluo Formation in the Ordos Basin began in the 1970s and concentrated on the oil and gas discovery areas in its western margin [8]. Recently, the Zhiluo Formation has attracted considerable attention from researchers, with the discovery of large uranium deposits in the basin’s peripheral area (Figure 1) [9,10,11,12].

In the basin’s northern part, previous studies have conducted extensive research on the provenance of the Zhiluo Formation by using paleocurrent direction measurement [5,13,14,15], detrital zircon U–Pb dating [14,16,17], light mineral characteristics [13,18], heavy mineral characteristics [19,20,21], and geochemical indicators [13,17,22], and consensus has been reached that the provenance in the basin’s northern part mainly comes from the old metamorphic and magmatic rocks of the Yinshan Block in the north, and the intermediate–acid magmatic and metamorphic rocks of the Alxa Block in the northwest.

In the basin’s southern part, the question of whether the Zhiluo Formation had a southern source during its deposition is still debated. Based on the basin’s sandstone thickness and net-sand ratio maps, Junfeng et al. [5] proposed that the Zhiluo Formation’s provenance is from the south and southeast direction, but did not indicate the specific provenance. Kaiyu et al. [16] used the detrital zircon dating experiment to conclude that the north QLOB (N-QLOB) and the middle QLOB (M-QLOB) were the primary sources for the Zhiluo Formation in the Binxian area. The paleocurrent direction was measured by Anqing et al. [23] to determine that the QLOB and QOB were the source in the basin’s southern part. Moreover, Xiaopeng et al. [12] employed the paleocurrent direction to suggest that the Zhiluo Formation in the Diantou area had a southwest source, but the provenance was not pointed out. The ambiguity of the southern boundary is due to the disagreement regarding provenance in the south and a lack of related research. Identifying the provenance of the Zhiluo Formation in the south of the basin is crucial for reconstructing the basin paleogeomorphology, reservoir research, and sedimentary-filling evolution during this period, as well as providing a baseline guarantee for uranium exploration.

Detrital zircons in the sedimentary rocks are transported from source rocks and are less impacted by weathering, transportation, diagenesis, and later modification; consequently, they can effectively record the information of the parent rock. Therefore, detrital zircon U–Pb dating plays an important role in inferring sedimentary provenances and geological tectonic background [24,25]. Additionally, the geochemical compositions of detrital zircon are also sensitive indicators to the tectonic setting of source area, so the combination of the zircon U–Pb age and zircon rare earth element characteristics provides a powerful method for provenance analysis [26,27,28,29,30]. During the uplift and denudation of numerous geological terranes in the Ordos Basin’s southern margin, geological information was retained in the Zhiluo Formation. The chronological analysis of zircon can be used to establish the sediment’s provenance, the relative position and boundary of the basin and geological terranes, and the basin’s tectonic evolution [31,32]. Therefore, the samples from the basin’s southern part were selected for U–Pb dating and zircon isotope determination using zircon laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS). By integrating our studies with previous results, we attempt to achieve the following goals: (1) determine the sediment’s provenance by establishing the genetic relationship between the Zhiluo Formation and surrounding geological terranes in the south of the basin; (2) determine the basin’s southern boundary in the Zhiluo Formation; and (3) reconstruct the interaction relationship and tectonic evolution process of the southern Ordos Basin and its surrounding orogenic belts.

2. Geological Setting

The Ordos Basin, located in the western section of the North China Craton (NCC), is China’s second-largest sedimentary basin with an area of approximately 2.5 × 10⁵ km² with substantial energy resources such as oil, gas, coal, and U deposits [33,34]. It presents a rectangular distribution from north to south. Contrasted with the topography of the northwest higher and southeast lower, the Basin’s strata show a gentle west-dipping monocline [35,36]. The Basin is surrounded by the Yinshan and Qinling orogenic belts to the north and south, and the Helan–Liupan mountains and Luliang mountain to the west and east, respectively [37,38] (Figure 1).

Figure 1. (a) Tectonic framework of China, showing the location of the NCC; (b) Schematic diagram of the regional tectonic location in the study area and periphery of the Ordos basin; (c) Net-sand ratio map and Paleocurrent directions of the Zhiluo Formation in the study area. ((a,b) are modified after [39,40]).

Figure 1. (a) Tectonic framework of China, showing the location of the NCC; (b) Schematic diagram of the regional tectonic location in the study area and periphery of the Ordos basin; (c) Net-sand ratio map and Paleocurrent directions of the Zhiluo Formation in the study area. ((a,b) are modified after [39,40]).

The Ordos Basin is a large intracontinental basin developed over a basement of Archean and Paleoproterozoic metamorphic and Paleozoic neritic sedimentary rocks as well as cap rocks of Mesozoic–Cenozoic continental sedimentary rocks [41,42]. However, the Lower Carboniferous, Devonian, and Silurian strata are absent [43]. During the geological evolution, the Ordos Basin has long been affected by surrounding tectonism [3,44]. The Paleo-Asian tectonic domain in the north, the Tethys tectonic domain in the southwest, and the circum-Pacific tectonic domain in the southeast have been considerably influencing the Ordos Basin’s southern part, particularly since the Mesozoic [1,2,45,46]. In Middle–Late Triassic, the Ordos Basin finally developed into an independent large sedimentary basin, and the Basin’s main tectonic framework was then formed after the intracontinental subduction and collision in the Yanshanian period (205–65 Ma), eventually evolving into the current basin pattern [4,47,48,49].

The study area is largely in the southern part of the Ordos basin (Figure 1b). It is connected to the southern margin of the SNCB in the southeast, the NQOB in the south, and the QLOB in the southwest; therefore, it is more affected by the activities of peripheral structures in the Basin’s south. The N-QLOB is located on the northeastern edge of the Qinghai–Tibet Plateau with the Qilian Block to the south and the Alxa Block to the north [50]. It is divided in the northwest by the Cenozoic Altyn fault zone and related to the QOB in the east, with a general NW–SE distribution [51]. The N-QLOB was generated in the Early Paleozoic by subduction and accretion of the Tethys Ocean, which afforded a full trench–arc–basin system. The oceanic ophiolite, island-arc volcanic complex, and ophiolite belts in the back-arc basin develop successively from south to north [52,53].

The NQOB extends northwest from the Shangnan–Danfeng and Baoji–Luonan–Luanchuan faults. From north to south, the Middle and Neoproterozoic Kuanping, Early Paleozoic Erlangping, Paleoproterozoic Qinling, and Early Paleozoic Danfeng Groups are exposed. Each group is separated from the others by a broad shear and fault zone that thrusts and overlaps [54,55], with local residual Carboniferous, Permian, Triassic, Jurassic, and Cretaceous strata [56]. The Luonan–Luanchuan fault zone in the south, the Sanmenxia–Lushan fault zone in the north, the Huaxian area in Shanxi province as the starting point in the west, and the Lushan area in Henan province in the east form the southern margin of the SNCB. It is typically distributed in the NW–SE direction [57,58]. The crystalline basement of the Proterozoic and Neoarchean Taihua Groups, as well as the multiage caprock on it, form the southern edge of the SNCB, which has an apparent double-layer structure [55,58]. The caprock includes the Mesoproterozoic Xiong’er Group, Guandaokou and Ruyang Groups [58], the Neoproterozoic Luanchuan and Luoyu Groups and a small number of Sinian, Paleozoic, and Mesozoic–Cenozoic sedimentary and volcanic rocks [59,60].

3. Lithological Characteristics and Sample Collection

From bottom to top, the Jurassic in the Ordos Basin is divided into the Fuxian, Yan’an, Zhiluo, Anding, and Fenfanghe Formation. The Zhiluo Formation is widely deposited and distributed over the Basin, and the strata thickness of the Zhiluo Formation becomes thinner from west to east, with a thickness of 150–450 m and an average thickness of approximately 280 m [11,15]. In terms of the stratigraphic contact relationship, the Zhiluo Formation is an unconformity with the underlying Yan’an Formation, with some areas even directly contacting the Yanchang Formation [5,42]. The top of the Zhiluo formation is mainly in conformable contact with the overlying Anding Formation, but due to the uplifting and denudation in the later stage of the basin, the Zhiluo Formation in the basin’s eastern margin presents an angular unconformity with the overlying Lower Cretaceous conglomerate [42,61].

Based on field outcrop, drilling cores, logging data combined with rock assemblage characteristics, and sedimentary facies markers, the sedimentary facies of the Zhiluo Formation in the study area are identified (Figure 2). It is concluded that from early to late deposition, the Zhiluo Formation deposited braided river, meandering river, meandering river delta and lacustrine facies.

Braided river facies is deposited in the lower part of the Zhiluo Formation’s lower member, and it is characterized by a large width/thickness ratio (Figure 3a). The sand body lithology of braided river facies at the bottom is gray, grayish-green medium-coarse-grained thick sandstone with apparent medium–large massive and trough crossbeddings, as well as visible trunk fossils and carbonized plant debris at the bottom (Figure 3b,c). The grain size of the sand body is finer upward, and between the sand bodies are gray, grayish-black silty mudstone and mudstone interlayers with small trough and tabular crossbeddings, and horizontal bedding (Figure 3d). The gamma-ray (GR) curve is a high-amplitude box shape with an extremely thin finger-shaped high-value layer, and the logging curve of this section is a thick layer box shape or box-bell shape.

Meandering river facies is deposited in the upper part of the Zhiluo Formation’s lower member. Gravel, gravel-bearing coarse sandstone, and coarse sandstone form channel floor lag in the lower part of this meandering river facies. The channel floor lag has a visible scouring phenomenon with a lens-shaped distribution (Figure 3e). With the grain size becoming finer upward, the upper part of the meandering river facies develops a marginal bank. Earth-yellow, grayish-green, and purple fine sandstone and siltstone dominate (Figure 3f), and small-scale inclined bedding, tabular crossbedding, and horizontal bedding were observed. The spontaneous potential (SP) and GR curves on the logging curve demonstrate the properties of a bell-zigzag shape combination of similar thickness.

Meandering river delta facies is developed in the lower part of the Zhiluo Formation’s upper member. Earth-yellow, gray, and grayish-green medium-fine sandstone with an evident scouring surface and a large-scale trough crossbedding constitute the lowest section of the delta facies, representing the geological characteristics of a distributary channel (Figure 3g). Multistage sedimentary cycles characterize the upper part, which is interbedded with purple, purplish red, grayish-green mudstone and siltstone (Figure 3h). The logging curve is zigzag-shaped, with strong GR and resistance properties.

The lacustrine facies, which is characterized by a shallow lacustrine deposit, is predominantly found in the upper section of the Zhiluo Formation in the north of the study area. The sedimentary rocks are mostly purplish-red, brownish-red, and grayish-yellow mudstone and siltstone, with medium–thin gray and purplish-red fine sandstone sandwiched in the middle, which is highly sorted and rounded [62] (Figure 3i). Limonite and thin-gypsum veins are seen [36,63]. Low SP and high GR values characterize the logging curve.

A set of gray–green and purple (fine) gravel-medium-coarse sandstone intercalated with green and purple mud gravel or lenticular silty sandstone was collected from the Zaoshugou section outcrop in the Yongshou area, southern Ordos Basin for this study. The outcrop has parallel bedding with poor sorting and medium roundness. Feldspathic quartz sandstone, feldspathic sandstone, and feldspathic lithic sandstone constitute the particle composition. The sample’s GPS coordinates are 34°53′30.6″ N and 108°63′40.9″ E (Zaoshugou: YS-01, Figure 1b). The samples were collected from the Zhiluo Formation’s lower member. The U–Pb age spectrogram of detrital zircons from the BX-18 sample of the Zhiluo Formation sandstone in the Binxian area was used in this study to facilitate the comparative analysis of detrital zircons [16].

4. Samples and Analytical Methods

4.1. Zircon U–Pb Dating

A total of 258 zircons in sandstone samples from the Yongshou area and Binxian area [16], southern Ordos Basin, were analysed for rare earth elements (REE) geochemistry and U–Pb dating. Sandstone samples are washed and dried, and the zircon grains were then separated using conventional magnetic and heavy liquid techniques, handpicked under a binocular microscope, mounted and polished in epoxy resin. Cathodoluminescence (CL) was used to obtain the images of zircons.

U–Pb dating analyses were conducted using LA–ICP–MS at Beijing Createch Testing Technology Co., Ltd. (Beijing, China). Detailed operating conditions for the LA system and the ICP–MS instrument have the same detailed operating conditions and data reduction as those reported in Hou et al. [64]. A resolution 193-nm LA system was used for laser sampling. The ion signal intensities were measured using an AnalytikJena PQMS Elite ICP–MS instrument. Helium was used as the carrier gas. Before entering the ICP, the make-up gas was mixed with the carrier gas via a T-connect. Each study included a 15–20 s background collection (gas blank), followed by a 45-s data acquisition from the sample. ICPMSDataCal was used to perform offline raw data selection, integration of background and analytical signals, time-drift correction, and quantitative calibration for U–Pb dating [65].

The external reference for U–Pb dating was Zircon GJ-1, which was evaluated twice every 5–10 analyses. For every 5–10 analyses, time-dependent drifts in the U–Th–Pb isotopic ratios were corrected using linear interpolation (with time) based on GJ-1 fluctuations (i.e., 2 zircon GJ-1 + 5–10 samples + 2 zircon GJ-1) [65]. The sample results were affected by the uncertainty of preferred values for the external standard GJ-1. Owing to the low signal of common ²⁰⁴Pb and high ²⁰⁶Pb/²⁰⁴Pb in all the tested zircon grains, the common Pb correction was unnecessary. NIST 610 was used to calibrate the concentrations of U, Th, and Pb. Isoplot/Ex_ver3 was used to create concordia diagrams and weighted mean computations. The zircon Plesovice was dated as unknown samples, yielding a weighted mean ²⁰⁶Pb/²³⁸U age of 337.4 ± 3.4 Ma (2SD, n = 7), which matches the recommended ²⁰⁶Pb/²³⁸U age of 337.13 ± 0.37 Ma (2SD) [66].

4.2. Net-Sand Ratio Maps

The net-sand ratio denotes the percentage of sandstone in a formation interval. When the net-sand ratio is close to the provenance, it is higher; however, when it is close to the depositional center, it is lower [67]. GR logging curves are of 34 wells used in this study. To identify mudstone and sandstone, selected American petroleum institute units (80 API and above) are used as a baseline [68] and the contour lines were constructed at 10% intervals.

5. Results

5.1. Textures and CL Images

Based on the CL images of the zircons, the morphology of detrital zircons varies from angular to subrounded (Figure 4), and the angular–subangular detrital zircons are common. The morphology of the zircons in the CL image show euhedral or subeuhedral crystals, some of which appear as xenomorphic crystals. Short columnar elliptic shapes are most common in zircons, and long columnar and circular zircons are rare. Zircon crystals are relatively large, measuring between 100–275 μm and 50–100 μm in long and short axis, respectively. In addition, some zircon crystals show magmatic origin with round or polygonal cores and present different gray characteristics, while others have been modified due to metamorphic origin with lack of internal structures (Figure 4).

5.2. Rare Earth Element Characteristics

The trace and rare earth element (REE) concentrations of the detrital zircons from the Yongshou area are listed in Table S1. The Th/U ratios of detrital zircons are generally used to differentiate igneous and/or metamorphic origins: Th/U ratio for zircons of igneous origin is >0.4 and for zircons of metamorphic origin is <0.1 [69,70]. Th/U > 0.4 is found in 109 zircon grains, and the zircons have magmatic oscillatory zoning (Figure 4-4, -21, -24, -37, -55, -80, -100, -101, -118 and -126). Furthermore, 15 zircon grains had 0.1 < Th/U < 0.4, indicating that the zircon has undergone metamorphic recrystallization (Figure 4-87 and -88) and slight metamorphic enlargement (Figure 4-75, -123 and -144). Th/U < 0.1 is found in only one zircon grain, indicating a metamorphic origin (Figure 4-11). After chondrites normalize zircon REEs, the REE domains vary from 928.14 to 21,116.95 ppm and show a relatively steep pattern with low LREE (35.55 to 1724.80 ppm), high HREE (918.75 to 20,636.48 ppm) contents, negative Eu anomalies, positive Ce (Figure 5, Table S1), and high Th/U (between 0.30 and 1.31) ratios (Figure 6). It demonstrates that the majority of detrital zircons are magmatic in origin [71,72,73].

5.3. Zircon U–Pb Dating

For zircons younger than 1000 Ma, the ages corresponding to ²⁰⁶Pb/²³⁸U and 1σ employed for assessing the U–Pb ages. For zircon older than 1000 Ma, the ages corresponding to ²⁰⁷Pb/²⁰⁶Pb and 1σ are used [75]. The corresponding concordia diagram is created after removing the data with a degree of concordance < 90% (Figure 7).

In total, the zircon grains were studied for the concordia ages with weak age peaks at ca. 2283 and ca. 1788 Ma and prominent age peaks at ca. 432.5 and ca. 218.7 Ma, according to the relative U–Pb age probability curve of detrital zircon (Table S2, Figure 8). In addition, three zircon ages belong to the Early Neoproterozoic, two to the late Mesoproterozoic.

5.4. Net-Sand Ratio Map of the Zhiluo Formation

Within the scope of the study area, a total of 34 wells geophysical well logging data were selected for comparison in the Zhiluo Formation, and the contour lines were drawn at 10% intervals (Figure 1c). The contour map shows that the net-sand ratio in the study area is generally high, with a maximum value of 87% and a minimum value of 35%, mainly in the range of 40% to 80%. In addition, there are three main high net-sand systems in the basin’s southwest, south and southeast margin, while the net-sand ratio gradually decreases from these directions along the edge of the basin to the inside.

6. Discussion

6.1. Potential Source Areas

The study area is a composite zone made up of numerous continental blocks and its sedimentary–tectonic evolution process is related to the activities of the peripheral blocks. During the sedimentary period of the Zhiluo Formation, the Ordos Basin’s eastern part was the depositional area of a large inland lake basin [5,42], the north was Yinshan Block, the northwest was Alxa Block, the southwest-south was N-QLOB and NQOB, and the southeast was the uplift area on the NCC’s southern margin, respectively. These all had the potential to provide clastic materials for the study area.

6.1.1. Mechanical Features

The roundness and microtextures of zircon grains are considered to be very useful tools to infer sediment origin, transport distance [72,76,77]. Based on the CL images, most detrital zircons are angular–subangular shape, implying that they were transported over short distances. Conversely, a few subrounded zircons suggest long-distance transportation or a multistage deposition cycle.

Figure 8. Comparison of zircon age spectra from the Zhiluo Formation in the southern Ordos basin and its adjacent areas (modified after [16,44,78,79,80,81]).

Figure 8. Comparison of zircon age spectra from the Zhiluo Formation in the southern Ordos basin and its adjacent areas (modified after [16,44,78,79,80,81]).

6.1.2. Zircon REE Analysis

Based on the zircon REE diagram analysis of the samples, although there is a small data distribution in the oceanic island arc, the U/Yb versus Hf, Th/Yb versus Y and Nb/Yb versus U/Yb discriminant diagrams demonstrate that most of the zircon data fell in the continental arc (Figure 9a–c). This indicates that the provenance may be related to the trench–arc–basin system. There are data distributions for I-type, A-type, and S-type granites on the Th versus Pb figure; however, the data mostly fell in the field of A-type and S-type granites in continental crust granitic rocks (Figure 9d). The Th/U versus Nb/Hf and Hf/Th versus Th/Nb discriminant diagrams show that zircons are mostly from the magmatic arc/orogenic belt tectonic environment, but the intraplate/nonorogenic belt tectonic environment also contributes moderately, indicating that the Zhiluo Formation zircons come from the continental arc structure and oceanic island-arc environment related to the orogenic belt and intraplate nonorogenic environment (Figure 9e,f).

Studies on the tectonic evolution of the Ordos Basin and its surrounding orogenic belts show that the Qilian–Qinling orogenic belt is a typical accretionary orogenic belt [52]. The strongest subduction–collision occurred in the Caledonian [85,86], resulting in extensive granite intrusions [87] and forming the trench–arc–basin environment on the active continental margin in the basin’s southern margin [88,89]. In the Permian period, the Qilian–Qinling orogenic belt was in the contact and collision stage, and the basin began to evolve into an inland lake basin, making the Qilian–Qinling orogenic belt jointly provide clastic materials for the basin [90,91,92].

Precambrian metamorphic rocks are the basement of Alxa Block. The Alxa Block’s eastern and southern margins in the early Paleozoic belonged to the passive continental margin [93], and then evolved into a foreland basin under the influence of the N-QLOB. In the late Paleozoic, due to the southward subduction–collision of the northern Paleo-Asian Ocean, large-scale thrust structures and related magmatic rocks were formed, which combined the Alxa Block and the NCC into one and entered the intracontinental evolution stage [94,95].

The Yinshan Block underwent continent–continent collision with the Ordos Block at 1.95–1.85 Ga [96,97,98], and formed a unified NCC basement [99,100], after which the Yinshan area had been in a passive continental margin environment. Since the late Paleozoic, with the southward subduction–collision of the Paleo-Asian Ocean, the tectonic setting of the Yinshan area changed and evolved into an Andean-type active continental margin [101,102], and multistage tectonic–magmatic activities occurred in the Mesozoic–Cenozoic [101].

According to the analysis of REEs in zircon and the tectonic background of the basin’s surrounding orogenic belts, the provenance in the study area is related to the trench–arc–basin system and northward subduction–collision of the N-QLOB and NQOB and the material recycling in the SNCB’s southern margin.

6.1.3. Zircon U–Pb Dating Analysis

The YS-01 sample from the lower member of the Jurassic Zhiluo Formation in the Yongshou area, the BX-18 sample from the Zhiluo Formation in the Binxian area in the southern Ordos Basin [16], and the zircon age spectrum results from different areas around the basin were compared and analyzed (Figure 8). The YS-01 and BX-18 samples have similar zircon U–Pb age spectra, with the strongest peak in the Paleozoic (432.5 and 430 Ma, respectively), the weakest peak in the Early Mesoproterozoic–Late Proterozoic and Early Proterozoic (1788 and 1720 Ma and 2283 and 2254 Ma, respectively), and a small amount of zircon in the Neoproterozoic. Although the two samples have varying numbers of zircons in the Early Mesozoic, they both demonstrate tectonic events that correlate to the Indosinian, showing they have good consistency. Therefore, the two samples can be integrated to compare and analyze the Jurassic Zhiluo Formation’s provenance in the basin’s southern part.

The Alxa Block is in the Ordos Basin’s northwest. Although magmatic rocks of various ages are distributed (exposed), the Hercynian magmatic rocks of the Alxa Block are the most common due to the southward subduction–collision and closure of the Paleo-Asian Ocean in the late Paleozoic [94,103]. However, the samples obviously lack Hercynian zircons (260–380 Ma) and do not develop Hercynian magmatic rocks. In addition, the Neoproterozoic rocks in the Alxa Block are predominantly found in the western part of the block [104,105], so the block’s western clastic materials cannot be transported to its eastern region due to the uplift terrain in the middle. Therefore, the Alxa Block cannot be the study area’s provenance.

The Yinshan Block is in the Ordos Basin’s north. It was also affected by the subduction–collision of the Paleo-Asian Ocean to the North China Plate (NCP) in the late Paleozoic, so the Yinshan area developed magmatic rocks of various ages and the Hercynian magmatic rocks are also the most common [101,103]. This is inconsistent with the lack of Hercynian zircons in the samples. Studies show that the Meso-Neoproterozoic strata in the Yinshan area were lost due to its later uplift and denudation [44,106]. The Yinshan area belonged to the tectonic background of the continental extension environment during the Meso-Neoproterozoic period, so the Yinshan area and the NCP’s northern margin did not participate in the Rodinia supercontinent convergence [107]. The result is also quite different from these sample results. In addition, even though the khondalite series and Mesozoic–Cenozoic magmatic rocks in Yinshan area have always provided material sources for the basin, the sedimentary facies analysis shows that the depositional center of the Zhiluo Formation was in the basin’s southeastern part (Ansai–Yan’an–Fuxian area), and the clastic materials of Yinshan Block basically transported to the sedimentary area and are difficult to deposit to the study area across the lake basin. Therefore, the Yinshan Block cannot provide the provenance for the study area.

The Helan Mountain in the basin’s northwest had not been uplifted in the Early-Middle Jurassic, and only partially rose in the Late Jurassic. The large-scale uplift occurred in the Eocene [108]. The Liupan Mountain in the basin’s southwest began to rise in the Late Jurassic-Early Cretaceous [109,110], so the Liupan Mountain area belonged to the basin’s part during the depositional period of the Zhiluo Formation. The Luliang Mountain in the basin’s east had always been a sedimentary area; it began to be gradually uplifted in the Late Jurassic and entered the main uplift stage since the late Early Cretaceous [4,40]. This indicates that the Luliang Mountain area and the Ordos Basin belonged to the same large sedimentary basin during the Zhiluo Formation’s sedimentary period. Therefore, the Helan Mountain, Liupan Mountain and Luliang Mountain cannot provide material sources for the Zhiluo Formation.

The Northern Qinling region is characterized by a scarcity of Hercynian magmatic rocks, but the Early Paleozoic magmatic rocks, which was the main body of magmatic rocks in the NQOB, are widely distributed [111,112]. Its formation is related to the northward subduction–collision–closure of the Qinling–Qilian Ocean basin during this period, with a peak age of around 450 Ma [87,113]. The contact zone between the SNCB and the NQOB established an active continental margin trench–arc–basin system, which produced ophiolites, island-arc volcanic complex, and other related volcanic rocks [114,115].

The N-QLOB has a similar geological history to the NQOB, but it lacks Hercynian magmatic rocks and is dominated by Paleozoic magmatic rock mass with a peak age of around 430–450 Ma. It develops rock types such as ophiolites and island-arc volcanic complexes and has a complete trench–arc–basin structural system [52,116]. Furthermore, the N-QLOB and NQOB zircon age spectra range from 750–1000 to 1600–2500 Ma [117,118], which corresponds to the YS-01 and BX-18 samples.

The West Qinling Mountains showed obvious tectonic activity around 1800 Ma [119,120], which contradicted the peak-age characteristics of the YS-01 and BX-18 samples. Geographically, the West Qinling Mountains have always been on the southwest of the N-QLOB and NQOB, and its clastic materials are difficult to transport to the study area; thus, the West Qinling Mountains are not the provenance area.

The basement of the SNCB’s southern margin has geological information of tectonic magmatic-metamorphism events with zircon U–Pb peak ages between ~2.5 and ~1.85 Ga [121,122]. With a U–Pb weak peak age of around 931 Ma in the Neoproterozoic, tectono-magmatic signatures occur and U–Pb peak ages of 273 and 427 Ma are also the most widely distributed. The age spectrum of the SNCB’s southern margin correlates well with the YS-01 and BX-18 samples, implying that the provenance is primarily from the SNCB’s southern margin.

6.1.4. Net-Sand Ratio Analysis

In general, the net-sand ratio is higher than 40%, indicating that the study area is closer to the source area and has the characteristics of near provenance and short-distance transportation. In addition, the net-sand ratio of Zhiluo Formation gradually decreases in the basin’s southwest, south and southeast, reflecting the characteristics of multi-sources. Using a contour map of sandstone thickness and net-sand ratio in the basin, Junfeng et al. [5] determined that the Zhiluo Formation’s provenance is in the south and southeast. Therefore, the SNCB’s southern margin, N-QLOB, and NQOB may all contribute to the Zhiluo Formation’s source in the study area.

6.1.5. Paleocurrent

In the basin’s south, the paleocurrent directions of the Cuijiagou section in Xunyi County, the Shuibeigou section in Binzhou city and the Shayaozi section in Tongchuan city are largely concentrated between ~0°–45° and ~350°–360°, respectively. Thus, the QLOB and QOB have provenance in the southwest and south, respectively [23] (Figure 1c). The paleocurrent direction in the Nanyukou Diantou’s lower member of the Zhiluo Formation is 60°–70°, indicating a southwest provenance in the early sedimentary period in the south of the study area.

Based on the above analysis, it is considered that the source of Zhiluo Formation in the study area should be jointly provided by the SNCB’s southern margin, N-QLOB and NQOB with higher topography.

6.2. Primary Provenance

The study shows that the four main age peaks in the detrital spectra occurs at 2283 Ma, 1788 Ma, 432.5 Ma and 218.7 Ma, corresponding to the tectonic events of Early Luliang (~2800–2050 Ma), Late Luliang–Early Jinning (~2050–1200 Ma), Late Caledonian and Indosinian, respectively. In addition, there are 3 zircon ages corresponding to the early Neoproterozoic and 2 zircon ages corresponding to the late Mesoproterozoic. The zircon ages in this range correspond to the Late Jinning tectonic event (~1200–720 Ma).

(1)

From 2831.79 to 2035.49 Ma, the peak is 2283 Ma, the zircon age in this range corresponds to the Early Luliang tectonic event.

During this period, the Columbia supercontinent convergence occurred [123], the N-QLOB and NQOB were combined, and the SNCB experienced an intracontinental orogenic evolution process from rift formation to closure [46,124].

The Longshan Complex (2350 and 2500 Ma) and metasandstone of the Hulushan group (2.58–2.30 Ga) in the eastern section of the N-QLOB are the consequence of tectonism [117,125]. The gneiss with a zircon U–Pb age of 2000–2300 Ma is widely distributed in the Qinling group at the NQOB and N-QLOB junction [126]. The Kuanping group’s Guchuangou granite has a zircon U–Pb age of 2197 ± 19 Ma [127]. The zircon U–Pb age of tonalite in Guyang is 2440 ± 35 Ma [128]. The paleocontinental crust of the NCC developed between 2900 and 2700 Ma [129,130,131].

The zircon ages from 2831.79 to 2035.49 Ma matched the magmatic and metamorphic rock ages in the SNCB’s southern margin, N-QLOB, and NQOB, which were all the provenance for the Zhiluo Formation, and the NQOB should be the main provenance area in this age range.

(2)

From 1947.84 to 1561.12 Ma, the peak is 1788 Ma, the zircon age in this range corresponds to the Late Luliang–Early Jinning tectonic event.

The SNCB, NQOB and N-QLOB underwent convergence, splicing, and continental margin accretion episodes during this period and various parametamorphic rocks and granites corresponding to the process are also widely exposed [115,132].

In the N-QLOB, the zircon SHRIMP U–Pb ages of garnet muscovite schists in the upper part of the Nieyuan group are 1.6–1.8 Ga, and the dating age of potassic granite porphyry in the Tiemahe section of Longxian County is 1814 ± 12 Ma [133,134]. In the western section of the NQOB, the U–Pb ages of both gongjiangou-deformed intrusive rocks and monzonitic granite in the Taibai batholith are 1741 ± 12 Ma and those of monzonitic granite in the Hudian are 1770 ± 13 Ma [111]. In the southern margin of the NCC, the volcanic rocks of the Luoyu and Ruyang groups are developed between 1750 and 1600 Ma [135]. The age peaks of the metamorphic basement, granitic intrusive rocks, and pegmatite dikes in the NCC range from 1900 to 1700 Ma [136]. These reflect that they are related to the tectonic event from the Late Luliang to the Early Jinning.

The aforementioned results indicate that the zircons of the Zhiluo Formation mostly came from the schist and granite porphyry of the N-QLOB, the parametamorphic rock and granite of the NQOB, and the metamorphic basement and granite bodies in the southern margin of the NCC. The southern margin of the SNCB was the main provenance area during this period.

(3)

From 1235.18 to 794.54 Ma, there are only 5 zircon ages (794.54 Ma, 799.08 Ma, 915.92 Ma, 1012.96 Ma and 1235.18 Ma). The zircon age in this range corresponds to the Late Jinning tectonic event.

The Rodinia supercontinent disintegration in the Late Jinning is closely linked to the magmatism [137,138]. During this time, the NCC evolved into an independent plate and the basin’s southern edge and SNCB became a stable passive continental margin [40].

The rocks corresponding to this age range are still found. In the N-QLOB’s eastern part, the intrusion age of the syn collision S-type granitoids at 917 ± 12 Ma was discovered in the Huangyuan Group [139]. In the NQOB, the zircon U–Pb ages of the Kuanping group’s metabasic volcanic rocks and the amphibolite in the Muqitan Formation are 943 Ma [140] and 762.5 ± 4.6 Ma [141], respectively. The Rb–Sr dating of the Longwang Shitong alkaline granite at the southern margin of the SNCB is 1076 Ma, corresponding to the age of a few zircons in the late Paleoproterozoic [142].

The results show that in the N-QLOB’s eastern part, the NQOB and the NCC’s southern margin also provide a small amount of materials to the Zhiluo formation.

(4)

From 461.04 to 412.61 Ma, the strong peak is 432.5 Ma. The zircon age in this range corresponds to the Late Caledonian tectonic event.

Caledonian is the most critical period of the ocean–continent transformation of the QLOB and QOB [85,86], which is the most intense age of northward subduction of the Qinling–Qilian ocean basin. The QLOB, along with the QOB, provided provenances for the south and southwest of the Ordos Basin [92].

Zircons in this age range are extensively developed in the western part of the North Qinling and North Qilian, and a certain distribution in the southern margin of the SNCB occurs. For example, the zircon U–Pb age of diabase (dike) in Guanshangou is 440.9 ± 1.7 Ma [115]. The crystallization age of Huangyanghe potash feldspar granite in the Lenglong area is 402 ± 4 Ma [143]. In the western section of the NQOB, the formation age of the Honghuapu tonalite diorite is 450.5 ± 1.8 Ma [144], and diorite and quartz diorite in the Baihua area are 439–430 Ma [145]. The zircon U–Pb age of the I-type granite in the Huichizi area of North Qinling is 421 ± 27 Ma, confirming Caledonian continental margin subduction and collision [146].

During the Caledonian period, the SNCB was in a moderately stable craton area; therefore, the zircons of this period in the southern margin of the SNCB should have emerged from the NQOB and N-QLOB. Furthermore, the zircons in the Zhiluo Formation should be provided by the circulation of sedimentary materials in the southern margin of the SNCB. The reasons are as follows: (1) the zircon age spectra of the Zhiluo Formation are closely matched with those of the southern margin of the SNCB; (2) the Middle Jurassic Yan’an Formation in the basin’s southern part lacks many top strata, and the Zhiluo and Yan’an Formations are in a high-angle unconformity contact relationship; and (3) many Paleozoic and Mesozoic strata in the basin’s southern part missed in varying degrees [59,60].

Overall, the zircon source in the Zhiluo Formation is complex, including diabase, gabbro, basalt, and granite from the N-QLOB and diorite and granite from the NQOB. Furthermore, the recycled sedimentary sources in the southern margin of the SNCB also provide a small amount of materials to the study area.

(5)

From 257.65 to 208.44 Ma, the strong peak is 218.7 Ma. The zircon age in this range corresponds to the Late Caledonian tectonic event.

In the Late Indosinian (~237–205 Ma), geochronological data show large-scale magmatism in the SNCB and North Qinling region [147,148].

The presence of Indosinian granite bodies in the NQOB correlates to this period’s tectonic collision. For instance, Baoji granite has a zircon age of 212 ± 1 Ma [149], whereas magmatic rocks in the Heigouxia area of QOB have a 221–242 Ma age [150]. The SNCB has reported Laoniushan magmatic rocks with a zircon age of 223–214 Ma [151]. Basalts in the Ruyang Group have a sensitive high-resolution ion microprobe (SHRIMP) zircon U–Pb age of 213.5 ± 2.4 Ma [152].

In the Late Indosinian, the zircon relative probability density diagrams show no zircon age record in the QLOB, indicating that magmatic activity was moderately weak and not exposed. Furthermore, during the sedimentary period of the Zhiluo Formation, the magmatic rocks in the QLOB may not have been exposed to the surface; thus, the QLOB may not provide clastic matter to the basin at this period. The NQOB has just a few zircon age records from the Indosinian, indicating that the Indosinian magmatic rocks in the QOB were primarily underground during the Zhiluo Formation’s sedimentary phase. Only a few regions were exposed to the surface at this time, causing a scarcity of clastic matter in the basin’s southern part. The SNCB’s southern margin has the highest proportion of zircon age record, which corresponds to the samples’ zircon U–Pb dating. Therefore, the SNCB’s southern margin mostly provides Indosinian magmatic rocks to the Zhiluo Formation, whereas the QOB solely provides a small amount of clastic materials.

6.3. Basin’s Southern Boundary during the Zhiluo Formation

For a long time, the Zhiluo Formation’s boundary in the Ordos Basin was thought to be far beyond the current basin’s scope [42,153,154]. However, research on the exact location of the boundary in the basin’s southern margin during the Zhiluo Formation is limited and unclear and existing perspectives differ. Extensional faults controlled the basin’s southern margin during the Zhiluo Formation, according to studies on tectonic deformation in the middle Jurassic, and the boundary of the basin’s southern margin was the QOB [154,155]. Junfeng et al. [5,156] proposed that the basin’s southwestern boundary is the Liupanshan basin’s western margin, and the basin’s southern boundary is the Weihe basin’s southern margin, based on the geomorphology and sedimentary facies combination characteristics of the Zhiluo Formation in the basin’s southern area.

The subduction of East Asia’s Izanagi plate affected the basin during the upper Yan’an Formation’s sedimentary period (174.1–168.3 Ma), according to studies [157,158,159]). The top of Yan’an Formation in the southern Ordos Basin was severely denuded or perhaps destroyed, enabling the Zhiluo Formation to come into unconformable contact with the underlying Yan’an Formation, with some sections even directly contacting the Yanchang Formation [5]. On the southern margin of the SNCB, there are Early Yanshanian intermediate-acid rocks. The Rb–Sr age of the Yinjiagou monzonite porphyry in western Henan province is 152 Ma [160], the U–Pb age of the trachyandensite in the Luanchuan area is 165 ± 1 Ma, and the basaltic andesite age is 165 ± 2 Ma [161]. The magmatic activity generated by the intracontinental subduction collision in the Early Yanshan episode afforded these products.

Based on the provenance analysis, the southern margin of the SNCB is considered one of the provenance areas of the Zhiluo Formation in the southern Ordos Basin, with the remaining lower members of the Zhiluo Formation producing fine gravel-bearing medium-coarse sandstone and uncommon conglomerate. The study region should be close to the basin’s margin, as it belongs to the sedimentary environment of braided rivers. In the study area’s southeastern part, the upper part of the Zhiluo Formation in the Shayaozi section of Tongchuan, purplish-red medium-coarse-grained conglomerates intercalated with sandstone layers of unequal thickness are commonly found, representing arid alluvial fan and showing typical marginal facies characteristics [162]. As a result, the Sanmenxia–Lushan fault zone at the northern boundary of the SNCB’s southern margin can be regarded as the Zhiluo Formation’s southeastern boundary, providing sufficient material sources to the basin.

The Liupan Mountain began to rise in the Late Jurassic–Early Cretaceous [109,110]; therefore, the Liupan Mountain was part of the Ordos Basin during the Zhiluo Formation’s sedimentary period. In the Tanshan area of Liupan Mountain, alluvial fan and braided river facies consistent with the Zhiluo Formation have been found, which are considered to be close to the boundary of the Ordos Basin [5]. The North Qilian–North Qinling Mountain finally merged with the NCC at the end of the Late Paleozoic and has been in a state of uplift for a long time, providing a provenance for the southern Ordos Basin [46,90,114,163]. During the Zhiluo Formation, the southwest boundary of the Ordos Basin should be on the northeast side of the N-QLOB, and the south boundary should be on the north side of the N-QOB, based on tectonic setting and provenance (Figure 10).

6.4. Tectonic Evolution of the Basin’s Southern Margin

The tectonic events of the surrounding geological terranes correlate well with the zircon age spectra of the Zhiluo Formation in the southern Ordos Basin, which has a substantial impact on the basin’s paleogeography.

Precambrian: The NCC is an ancient craton with a history of about 3.8 billion years [31], which successively experienced the processes of the initial continental core formation, growth and convergence [129,130,131], and formed a unified NCC basement during the Luliang movement [129,131].

The N-QLOB and NQOB were distinct microcontinents on ocean islands beyond the SNCB at the time, but the North Qinling microcontinent began to move toward the SNCB around 1600 Ma and for the first time overlapped on the SNCB [114]. The SNCB, NQOB and N-QLOB underwent convergence, splicing, and continental margin accretion episodes during the Greenville orogeny [132]. The oceanic basin and rift valley on the continental margin were closed successively in the early Late Proterozoic, and the blocks accumulated to form the Rodinia supercontinent [164]. The Rodinia supercontinent disintegrated again in the middle–late Proterozoic, and the NCC progressively became an independent plate, whereas the Ordos microcontinent developed into a stable passive continental margin on the NCC’s south side [40,164]. The NQOB and N-QLOB experienced similar tectonic evolution events and ocean–continent transition processes and subduction from the south to the north throughout the Late Neoproterozoic–Early Paleozoic [115].

Caledonian: The southern margin of the Ordos microcontinent belonged to a passive continental margin sediment environment during the Early and Middle Cambrian, and it was near to the Qinling–Qilian Ocean [165,166]. The tectonic stress in the Ordos Basin progressively shifted from north–south extension to north–south compression from the Late Cambrian to the Early Ordovician, under the influence of the Early Caledonian movement, and the entire basin began to uplift [4]. The north–south compression was prevalent in the middle–late Ordovician due to the influence of the Caledonian migration [164]. On the southern margin, this was the time of the Qinling–Qilian Ocean’s most extreme northward subduction, accompanied by granite intrusion induced by the subducting Shangnan–Danfeng Fault [87]. The active continental margin’s trench–arc–basin system was distributed in the basin’s southern margin, creating island- and back-arc basins in the N-QLOB spreading NW–SE and island- and back-arc basins in the NQOB spreading WE [50,88,89].

Hercynian: The collision and uplifting effect of the Late Caledonian were inherited by the tectonic action of the Early Hercynian movement, and the Ordos Basin continued to uplift and withstand denudation until the Late Carboniferous [167]. The N-QLOB and NQOB entered the stage of full-scale contact and collision in the Permian, while the Qinling–Qilian Ocean continued to subduct northward, resulting in the overall uplift of the NCC [90,91], and the basin evolved into an inland lake basin. The NQOB exhibited a tectonic pattern that connected the southern margin of the SNCB to the north and the South Qinling Block to the south during this period [46].

Indosinian: The Ordos Basin inherited Late Permian paleogeographic pattern and sedimentary characteristics in the Early–middle Triassic, with higher topographic features in the north and lower in the south, steeper in the south and slower in the north [10]. The Alxa Block in the northwest pushed and thrust to the southeast near the end of the Middle Triassic, the Ordos Basin was squeezed to the northeast by the Paleotethys oceanic crust in the southwest, and the Yangtze plate was pushed to the north. Furthermore, the joint position of the South Qinling block and the Yangtze plate shifted to the west of the Qinling Mountains in the later stage, and the West Qinling block was wedged under the Qilian block, gradually splicing and closing of the Qinling–Qilian Trough and uplifting of the North Qinling–Qilian orogenic belt [33,150]. The Yangtze block and the NCC were connected during this period. The Ordos Basin was a large sedimentary basin that formed independently and completely, with paleogeomorphic features, such as high in the west and low in the east and high in the north and low in the south [49,168]. The NQOB and N-QLOB became sediment source areas in the basin’s southern part during the Late Triassic [163]. The Triassic Ordos basin began to rise unevenly during this period because of the tectonic consequences of the Late Indosinian movement. The basin’s sedimentary range shifted westward with time, and the top layer underwent differential denudation, resulting in a stratigraphic discontinuity near the end of the Triassic [148].

Yanshanian: The paleotectonic stress field in China underwent a dramatic alteration during the Early–Middle Jurassic period (late sedimentary period of the Yan’an Formation) [6,157,158]. The circum-Pacific plate began to subduct vigorously into the NCC in the southeast, the Indian plate subducted northward, the Siberian plate moved southward in the north, and the SNCB’s northern margin was compressed and thrust [169]. Therefore, the Ordos Basin’s paleotectonic pattern shifted from north–south to east–west, resulting in paleogeographic characteristics of depression in the west and inclined uplift in the east [156]. The Ordos Basin was raised in the Late Jurassic because of multidirectional tectonic compression of the Paleotethys basin, the Siberian plate, the Alxa Block, and the circum-Pacific Plates, producing exposure and denudation once again. The basin’s western margin produced strong thrust deformation and nappe structures, whereas the eastern part exhibited denudation. The Luliang Mountain in the east, and the Liupan and Helan Mountains in the west, gradually uplifted throughout this period [4]. The western margin of the Ordos Basin transitioned from an early extensional environment to a tectonic setting of thrust nappe compression inside the basin during the Early Cretaceous, under the influence of Episode III of the Yanshan movement [49]. The Ordos Basin and its periphery continued to uplift and suffer from weathering and erosion during the Late Cretaceous, resulting in the absence of Upper Cretaceous Paleocene strata. After that, the Ordos Basin’s main tectonic framework stopped developing, resulting in a paleogeomorphic structure with peaks in the east and low points in the west, a north–south distribution, and a west-dipping monocline, similar to the current basin pattern [42,47,48].

7. Conclusions

• The source area’s tectonic background of the Zhiluo Formation in the study area is related to the trench–arc–basin system, subduction–collision orogeny and material recycling, respectively. Combining multiple methods, it was determined that the provenance was from the N-QLOB in the southwest, the western part of the NQOB in the south, and the SNCB’s southern margin in the southeast, respectively, surrounded the Ordos Basin.
• The detrital zircon ages of the Zhiluo Formation in the study area cluster into five groups: 2831.79–2035.49, 1947.84–1561.12, 1235.18–794.54, 461.04–412.61 and 257.65–208.44 Ma, which are coeval with the tectonic–thermal events of the paleocontinental crust development and the splicing of microcontinents in the Early Luliang movement, the formation of a unified NCC basement in the Late Luliang movement–Early Jinning movement, the Rodinia supercontinent disintegration in the Late Jinning, the ocean–continent transformation of the QLOB and QOB in the Late Caledonian, and the subduction of Yangtze plate to the SNCB in the Indosinian, respectively.
• During the sedimentary period of the Zhiluo Formation, the Ordos Basin’s southwest boundary should reach the northeast side of the N-QLOB, the south boundary should reach the north side of the N-QOB, and the southeast boundary should be along the Sanmenxia–Lushan fault zone.