Detrital Zircon Geochronology of the Permian Sedimentary Rocks from the Western Ordos Basin: Implications for Provenance Variations and Tectonic Evolution

Abstract

The western Ordos Basin (OB) is situated at the junction of multiple tectonic units with distinct properties. The prolonged and complex tectonic interactions from adjacent tectonic units have resulted in diverse structural phenomena and intricate evolutionary history in this region. The late Paleozoic represents a critical period for the transition of the tectonic regime in this area. However, due to the effects of intense later-stage modification, the late Paleozoic provenance system and paleogeomorphology of this region remain poorly constrained. Against this background, systematic fieldwork and detrital zircon geochronological analyses of the Youjingshan and Quwushan Permian sections were conducted to determine sediment provenance, and spatial variations in detrital zircon geochronological characteristics across different parts of the OB are further discussed. The results indicate that the detrital zircon age spectra of the Permian Dahuangou and Yaogou formations in the Youjingshan and Quwushan sections are dominated by late Paleozoic (250–360 Ma), early Paleozoic (360–500 Ma), and Paleoproterozoic (1600–2500 Ma) age populations. However, significant differences in age composition are also observed among different samples. This study proposes that the detritus of the Dahuangou Formation in the Youjingshan area was mainly derived from the Alxa Block (AB), while that from the Yaogou Formation was sourced from the Yinshan-Daqingshan-Wulashan Orogenic Belt (YDWOB). In contrast, the West Qinling Orogenic Belt (WQOB) and North Qilian Orogenic Belt (NQOB) were identified as the source areas for the Dahuangou and Yaogou Formations in the Quwushan area. Based on a comprehensive comparison of detrital zircon geochronological data of the Permian strata in the OB, three major provenance systems can be identified: the southwestern source area (WQOB and NQOB); the northwestern source area (YDWOB and AB); and the interior source area (YDWOB). During the Permian, the tectonic-sedimentary evolution of the OB was primarily controlled by the combined effects of the Paleo-Asian Ocean (PAO) to the north and the Paleo-Tethys Ocean (PTO) to the south. Differences in the timing and intensity of subduction/collision between the PAO and the PTO resulted in a general paleogeographic pattern of “higher in the north and lower in the south” in the OB.

1. Introduction

Orogenic belts and sedimentary basins are the two most fundamental geomorphic units on the Earth’s surface [1,2]. They often coexist and develop in a coupled manner [3,4], exhibiting spatial interdependence, morphological adjustment, and material exchange [2]. Provenance analysis of sedimentary rocks serves as a critical link connecting orogenic belts and sedimentary basins. It not only helps determine the composition and sources of detrital materials [4], but also provides key insights into the erosion processes of source areas [5], the tectonic settings of provenance regions [6], and paleogeographic patterns [7,8,9]. As such, it represents an important approach to understanding the coupled evolution of basins and mountains. Among the various methods for provenance analysis, detrital zircon geochronology has proven to be one of the most widely applied and rapidly advancing techniques in provenance analysis, having profoundly transformed our understanding of sedimentary basin provenance [6,10].

The North China Craton (NCC), one of the major tectonic components of the Asian continent, has a history of ~3.85 billion years [11,12]. It is surrounded by the Central Asian Orogenic Belt (CAOB) and Qinling Orogenic Belt (QOB) [13,14,15,16,17]. The formation and evolution of the above-mentioned orogenic belts are closely linked to the tectono-sedimentary evolution of sedimentary basins within the NCC, resulting in the multistage nature, diversity, and complexity of basin development [2,11]. Among these, the Ordos Basin (OB) is the largest and most significant sedimentary basin developed within the NCC. Located in the western part of the NCC, the OB is roughly rectangular in plan view and is peripherally bounded by the Qinling Mountains, Liupan Mountains, Helan Mountains, Daqing Mountains, and Lüliang Mountains. It covers an area of approximately 3.7 × 10⁵ km², making it the second-largest sedimentary basin in China [18,19,20,21]. The OB was formed through the superimposition and modification of multiple prototype basins with varying ages, tectonic attributes, and depositional extents [2,18]. Its evolutionary history can be divided into 4 stages: the Mesoproterozoic–Ordovician marine cratonic basin, the Carboniferous-Permian paralic depressional basin, the Mesozoic intracontinental lacustrine depressional basin, and the Cenozoic intracratonic basin surrounded by rift systems [21]. The late Paleozoic represents a critical stage in the evolution of the OB. This period not only witnessed the transition of the basin’s depositional systems from marine to continental environments but also marked the critical phase during which the large Paleozoic cratonic basin gradually contracted and evolved into an independent intracontinental basin. From a broader perspective, the late Paleozoic was a critical period in global tectonic evolution, characterized by the collision and amalgamation of Laurasia and Gondwana, which ultimately led to the formation of the main body of the supercontinent Pangea [22,23,24]. The OB (western part of the NCC), as one of the constituents of the late Paleozoic Pangea supercontinent, was situated between the Paleo-Asian Ocean (PAO) and the Paleo-Tethys Ocean (PTO). The closure of the PAO and the formation of the CAOB [25], as well as the closure of the PTO and the subsequent formation of the QOB, controlled the tectonic evolution of the northern and southern margins of the OB, respectively [26,27]. Employing the concept of basin-mountain coupling, numerous scholars have extensively discussed the late Paleozoic source-to-sink systems and tectonic framework of the OB. However, previous studies have predominantly focused on either the northern [28,29] or southern OB [4,30]. The current research attention on the western OB remains relatively limited, primarily due to intense tectonic reworking and extensive erosion of late Paleozoic strata in this area. Nevertheless, this region occupies a unique tectonic position (Figure 1a), influenced by the superimposed and shifting interactions between the southern (QOB) and northern (CAOB) major tectonic systems. Studying its provenance system is crucial for further refining the characterization of the overall tectonic-sedimentary framework of the OB during the late Paleozoic. Therefore, the present study selects the Permian strata in the Youjingshan and Quwushan sections of the western OB as the research target (Figure 1b,c). Employing detrital zircon U-Pb geochronology, this study aims to conduct a detailed provenance analysis, compare provenance variations among different regions and temporal stages within the basin, and reconstruct the Permian provenance system of the OB.

2. Geological Setting

The western Ordos Belt (WOB) is situated at the junction of the OB, Alxa Block (AB), Qinling-Qilian Orogenic Belt (QQOB), and CAOB (Figure 1a). The interactions of the aforementioned tectonic units exerted a significant controlling influence on the tectonic framework of the WOB [31,32]. Regional paleo-sedimentary and tectonic evolution studies indicate that the WOB underwent a complex ocean-continent evolution process during the Paleozoic [32], accumulating thick marine sediments (Figure 1d). During the Silurian to early-Middle Devonian, influenced by the collisional orogeny in the North Qilian Orogenic Belt (NQOB) [33], it gradually transitioned into an intracontinental evolutionary stage (Figure 1d). Since the Mesozoic, this area has been modified by multiple phases of tectonic activity, resulting in the presence of several regional unconformities and stratigraphic absences (Figure 1d). In the late Cenozoic, the tectonic framework of the region was ultimately shaped by the uplift and expansion of the Tibetan Plateau.

During the Phanerozoic Eon, the WOB has undergone multiple phases of tectonic movements and a complex evolutionary history, which collectively have shaped its distinct geological characteristics, namely a weak basement, high tectonic activity, and diverse structural styles [31,32,34,35,36]. Based on its tectonic features, the WOB can be divided into two major tectonic units: the Western Margin Thrust Belt and the Liupanshan Arcuate Structural Belt. The major faults developed within WOB, from west to east, are Haiyuan Fault, Yantongshan-Yaoshan Fault, Xiangshan-Tianjingshan Fault, Qingtongxia-Guyuan Fault, Qinglongshan-Pingliang Fault, Anguo Fault, Hui’anpu-Shajingzi Fault, and Baiyanjing Fault. These faults either define the boundaries of the WOB or form the divides between its internal sub-zones, playing significant controlling and segmenting roles in the regional tectonic evolution.

The WOB features a dual basement system comprising Lower Paleozoic to Middle-Upper Proterozoic strata underlain by crystalline rock series. The overlying strata are composed mainly of Silurian shallow-marine deposits, Devonian molasse sediments, Carboniferous marine-continental transitional clastic rocks and carbonates, as well as Permian to Quaternary terrigenous clastic sediments (Figure 1d). The Permian strata within the WOB are sporadically distributed (Figure 1b,c). The stratigraphic sequence from bottom to top comprises the Dahuanggou and Yaogou Formations. The overall thickness of the Permian strata is relatively stable. The Permian system is characterized by a fluvial-dominated depositional environment. The Dahuanggou Formation is composed of variegated sandstone, tuffaceous sandstone, and tuff (Figure S1) [37]. It lies in para-conformity over the underlying Taiyuan Formation or Tupo Formation, and is in conformable contact with the overlying Yaogou Formation (Figure 1d). Based on characteristics such as lithology, and sedimentary structures, the formation can be further subdivided into braided river and meandering river subfacies. The base of the Yaogou Formation consists of gray-white pebbly sandstone, feldspathic sandstone, and purplish-red muddy siltstone (Figure S1) [37]. It has a conformable contact with the Dahuanggou Formation and a parallel unconformable contact with the overlying Lower Triassic Wufosi Formation. Based on lithology, sedimentary structures, and sequence characteristics, it is subdivided into the braided channel microfacies, longitudinal bar microfacies, and interchannel microfacies.

3. Sampling and Methodology

A total of four sandstone samples were collected in the field. For this study, the sandstone samples from Dahuanggou Formation (NN-23-74, N 37°00′43″, E 105°15′19″) and Yaogou Formation (NN-23-75, N 37°00′43″, E 105°15′32″) were collected from the Youjingshan section (Figure 1b). Additionally, to compare Permian provenance differences across distinct regions, supplementary sandstone samples from Dahuanggou (NX-16-53, N 36°34′49″, E105°2′40″) and Yaogou (NX-16-52, N 36°35′31″, E105°1′51″) Formations were collected from Quwushan section (Figure 1c). The Permian sequences in the two studied sections are interpreted as braided river subfacies.

During sample pretreatment, the conventional heavy liquid separation method is typically employed to isolate zircon grains by exploiting density differences, supplemented by magnetic separation as an auxiliary technique. Heavy liquid effectively separates zircon from other impurities based on their contrasting densities, while magnetic separation targets the removal of impurities with magnetic properties. Following the separation process, zircon grains are carefully hand-picked. Subsequently, the selected grains are mounted in epoxy resin, which provides excellent adhesion and securely encapsulates the grains. After mounting, the sample undergoes precise polishing to achieve a flat and smooth surface. Transmitted and reflected light images of the zircon grains are then captured under a microscope. Microscopic examination allows for clear magnification of the grains: transmitted light images reveal internal structures, while reflected light images are used to examine surface features. Prior to instrumental analysis, cathodoluminescence (CL) imaging is performed using a scanning electron microscope to highlight internal characteristics of the zircon grains.

Zircon U-Pb isotopic dating was conducted simultaneously by Wuhan SampleSolution Analytical Technology Co., Ltd. (Wuhan, China) using LA-ICP-MS. Detailed instrument parameters and analytical procedures are described in [38]. The experiment utilized a GeolasPro laser ablation system (COMPexPro 5 ArF 193 nm laser + MicroLas optical system) coupled with an Agilent 7700 ICP-MS (Agilent Technologies, Santa Clara, CA, USA). Helium served as the carrier gas and argon as the make-up gas, which were mixed via a T-connector before introduction into the ICP; the system was equipped with a signal-smoothing device [39]. Experimental parameters: laser spot size 32 μm, frequency 5 Hz. Zircon 91,500 (for U-Pb dating) and NIST610 glass (for trace elements) were used as external standards for fractionation correction. Each analysis point involved collecting 20–30 s of background signal and 50 s of sample signal. Data were processed offline using the software ICP-MS Data Cal V10.9 [40,41] (including selection of sample and blank signals, correction for instrumental sensitivity drift, calculation of elemental contents, U-Pb isotopic ratios, and ages). IsoplotR was employed to generate concordia diagrams and calculate weighted mean ages for the zircon samples.

4. Results

4.1. Petrographic Features

Four sandstone samples were collected from Youjingshan and Quwushan sections. Both samples NN-23-74 and NX-16-53 were collected from the grayish-green medium-grained feldspathic quartz sandstone of the Dahuanggou Formation, displaying consistent petrological characteristics. The clastic grains are dominated by quartz, feldspar, and lithic fragments: quartz grains feature clean surfaces, with a minority exhibiting undulatory extinction; feldspar consists predominantly of plagioclase and K-feldspar; lithic fragments are mainly chert and argillaceous lithics. The reddish-purple medium-grained feldspathic quartz samples (NN-23-75 and NX-16-52) collected from the Yaogou Formation exhibit consistent lithological characteristics. The clastic grains are dominated by quartz, feldspar, lithic fragments, and mica, mostly with angular shapes. Quartz grains have clean surfaces, a minority show undulatory extinction, and some are polycrystalline; feldspar is mainly plagioclase and K-feldspar, with slight surface argillization; lithic fragments are primarily chert and argillaceous lithics; mica occurs mostly as anhedral flakes, some of which are bent and deformed.

4.2. Detrital Zircon Morphology and Origin

Approximately 300 zircon grains were separated from each sample collected from the Youjingshan and Quwushan sections in the WOB, then randomly selected and mounted. The zircon grains of selected samples are about 100–150 μm with length to width aspect ratios in the range of 1:1 to 2:1. The majority of zircons are euhedral to subhedral, while a small number display rounded morphologies with some abrasion. Cathodoluminescence (CL) images reveal that most zircons exhibit well-developed oscillatory zoning or platy morphology (Figure 2), with moderate sorting. Generally, magmatic zircons have Th/U ratios greater than 0.1, whereas metamorphic zircons have Th/U ratios less than 0.1 [42]. The Th/U ratios of detrital zircons from the Youjingshan Permian samples range from 0.04 to 2.11, with most exceeding 0.1 (Figure 3a,b). Similarly, the Th/U ratios of detrital zircons from the Quwushan Permian samples range from 0.09 to 2.11, with the majority also above 0.1 (Figure 3c,d). These characteristics indicate a magmatic origin for the vast majority of the zircons. A very small number of zircons with Th/U ratios below 0.1 are interpreted as metamorphic in origin.

In addition, the overall characteristics of zircon rare earth element (REE) distribution patterns, their slope direction, as well as the presence of Ce and Eu anomalies, can also be used to infer zircon genesis [43]. The REE distribution patterns of detrital zircons from the Dahuanggou (sample NN-23-74) and Yaogou (sample NN-23-75) Formations from the Youjingshan section (Figure 4a,b) indicate a magmatic origin, as evidenced by their characteristic LREE depletion, HREE enrichment, left-sloping profiles, negative Eu anomalies, and positive Ce anomalies. The REE distribution patterns of detrital zircons from the Dahuanggou (sample NX-16-53) and Yaogou (sample NX-16-52) Formations from Quwushan section (Figure 4c,d) display similar features, indicating magmatic origin as well. Furthermore. a limited number of zircons from all samples display subdued LREE depletion, moderate HREE enrichment, along with faint negative Eu and slight positive Ce anomalies, consistent with a metamorphic origin.

4.3. Detrital Zircon U-Pb Geochronology

For this study, 85 analysis points were performed for each sample. Specifically, a total of 83 concordant data points were obtained from the Dahuangou Formation (sample NN-23-74) at the Youjingshan section, and 68 concordant data points from the Yaogou Formation (sample NN-23-75) at the same location. Similarly, 83 concordant data points were acquired from the Dahuangou Formation (sample NX-16-53) at the Quwushan section, and another 83 concordant data points from the Yaogou Formation (sample NX-16-52) at this section. The detailed age results are presented as follows:

The Dahuanggou Formation from Youjingshan section (Sample NN-23-74): the dating results range from ca. 388 to 2550 Ma. These ages can be subdivided into the following groups: (a) ca. 360–500 Ma (52 data points, accounting for ~63%), with a peak age of ca. 438 Ma; (b) ca. 500–900 Ma (9 data points, ~11%); (c) ca. 900–1150 Ma (14 data points, ~17%), with a major peak at ca. 938 Ma; (d) ca. 1150–1600 Ma (3 data points, ~4%); (e) ca. 1600–2500 Ma (4 analyses, ~5%); and (f) >2500 Ma (1 data point, ~1%) (Figure 5a and Figure 6a).

The Yaogou Formation from Youjingshan section (Sample NN-23-75): the dating results range from ca. 272 to 2543 Ma. These ages can be subdivided into the following groups: (a) ca. 250–360 Ma (16 data points, accounting for ~24%), with a peak age of ca. 301 Ma; (b) ca. 360–500 Ma (3 data points, ~4%); (c) ca. 1600-2500 Ma (48 data points, ~71%), with major peak ages at ca. 1836 Ma and 2404 Ma, respectively; and (e) >2500 Ma (1 data point,~1%) (Figure 5b and Figure 6b).

The Dahuangou Formation from Quwushan section (NX-16-53): the dating results range from ca. 256 Ma to 2532 Ma. These ages can be divided into the following groups: (a) ca. 250–360 Ma (57 data points, accounting for about 69%), with a major peak age of ca. 288 Ma; (b) ca. 360–500 Ma (16 data points, accounting for about 19%), with a major peak age of ca. 432 Ma; (c) ca. 900–1150 Ma (3 data points, accounting for about 4%); (d) ca. 1600–2500 Ma (3 data points, accounting for about 4%); (e) >2500 Ma (1 data point,~1%) (Figure 5c and Figure 6c).

The Yaogou Formation from Quwushan section (Sample NX-16-52): the dating results range from ca. 253 to 2558 Ma. These ages can be subdivided into the following groups: (a) ca. 250–360 Ma (39 data points, accounting for ~48%), with a major peak age of ca. 281 Ma; (b) ca. 360–500 Ma (13 data points, ~16%), with a major peak age of ca. 432 Ma; (c) ca. 900–1150Ma (4 data points, ~4%); (d) ca. 1150–1600Ma (3 data points, ~4%), with a major peak age of ca. 1339 Ma; (e) ca. 1600–2500 Ma (23 data points, ~28%), with a major peak age of ca. 1842Ma; and (f) >2500 Ma (3 data points, ~4%) (Figure 5d and Figure 6d).

5. Discussion

5.1. Provenance Analysis

This study systematically compiled zircon geochronological data from potential source areas surrounding the WOB to facilitate subsequent provenance tracing. As an important tectonic unit in the western NCC, the AB is located northwest of the OB, bounded by the Helanshan Tectonic Belt [44]. Its zircon U-Pb age distribution is shown in Figure 7a with six peak ages: ca. 295 Ma, 438 Ma, 966 Ma, 1432 Ma, 1747 Ma, and 2479 Ma. The NQOB, situated west of the WOB [45], exhibits a U-Pb age distribution as shown in Figure 7b, with five peak ages: ca. 274 Ma, 452 Ma, 774 Ma, 1815 Ma, and 2459 Ma. The WQOB, located south of the WOB, exhibits a U-Pb age distribution as shown in Figure 7c, with five peak ages: ca. 240 Ma, 452 Ma, 849 Ma, 1849 Ma, and 2486 Ma. The YDWOB, located along the northern OB, shows a U-Pb age distribution illustrated in Figure 7d, with four peak ages: ca. 295 Ma, 966 Ma, 1890 Ma, and 2943 Ma.

The detrital zircon U-Pb ages from the Dahuanggou Formation at Youjingshan section (Sample NN-23-74) show that the ages are predominantly concentrated in the early Paleozoic and Neoproterozoic, with major peak ages of ca. 438 Ma and 938 Ma, respectively (Figure 7e). In contrast, the detrital zircon U-Pb ages from the Dahuanggou Formation at Quwushan section (Sample NX-16-53) exhibit distinctly different characteristics: the ages are primarily late Paleozoic (peak at ca. 288Ma), with minor Neoproterozoic and Archean detrital zircons (Figure 7f). The detrital zircon U-Pb ages from the Yaogou Formation at Youjingshan section (Sample NN-23-75) indicate that the zircon grains are mainly distributed in the late Paleozoic and Paleoproterozoic (Figure 7g). Compared to other samples, there is a significant proportion of Paleoproterozoic ages, with peak ages at ca. 301 Ma, 1836 Ma, and 2404 Ma (Figure 7g). Meanwhile, the detrital zircon U-Pb ages from the Yaogou Formation at Quwushan section (Sample NX-16-52) reveal that the zircon grains are dominated by early and late Paleozoic ages, followed by Paleoproterozoic, with minor Archean zircon grains, and exhibit major peak ages at ca. 281 Ma, 432 Ma,1842 Ma, and 2521 Ma (Figure 7h).

The dominant early Paleozoic age population and peak ages of the Dahuanggou Formation at Youjingshan section (NN-23-74) show similarities with the WQOB, NQOB, and AB (Figure 7e). However, its Neoproterozoic age distribution (ca. 900–1000 Ma) and peak age (ca. 938 Ma) are most comparable to those of the AB, indicating the AB was the primary provenance. The dominant late Paleozoic age population (ca. 250–360 Ma) and peak age (ca. 301 Ma) of the Yaogou Formation at Youjingshan section (NN-23-75) find contemporaneous counterparts in the YDWOB, NQOB, and AB (Figure 7g). However, the characteristic Paleoproterozoic age distribution (ca. 1600–2500 Ma) in this sample aligns more closely with the YDWOB. Furthermore, the distinctive Neoproterozoic ages (ca. 700–1000 Ma) that typify the AB and NQOB are not observed in this sample. Therefore, it is preliminarily concluded that the provenance of the Yaogou Formation in the Youjingshan area was likely derived from the YDWOB.

The Dahuanggou Formation (NX-16-53) and Yaogou Formation (NX-16-52) samples from Quwushan section exhibit generally similar characteristics, both dominated by late Paleozoic ages (ca. 260–360 Ma) (Figure 7f). However, the Yaogou Formation sample (NX-16-52) shows a greater proportion of early Paleozoic (peak age at ~432Ma) and Paleoproterozoic ages (peak age at ~1842 Ma) (Figure 7h). Although late Paleozoic ages (ca. 260–360 Ma) in these samples have contemporaneous records in the adjacent WQOB, NQOB, AB, and YDWOB, the age spectra notably lack significant contributions from the characteristic early Paleozoic (peak age at ~432Ma) and Meso-Neoproterozoic ages (ca. 700–1600 Ma) of the AB, as well as the typical Paleoproterozoic ages (ca. 1600–2500 Ma) of the YDWOB. Therefore, it is inferred that the provenance of the Dahuanggou and Yaogou Formations at Quwushan area were primarily derived from the WQOB. Given the presence of a paleocurrent direction towards the northeast in this region, the NQOB may have also contributed a minor amount of sediment.

Previous studies indicate that the Permian paleocurrents on the WOB exhibited multi-sourced characteristics, with a dominant NE-directed sediment transport direction in the south, and predominantly SE-, S-,and SW-directed flows in the north [4,63,64,65,66,67,68]. These patterns are consistent with the zircon age comparison results presented above. Consequently, this study concludes that the provenance of the Permian strata in the Quwushan area was primarily derived from the WQOB and/or NQOB. In contrast, the provenance of the Permian strata in the Youjingshan area was likely mainly sourced from the AB and YDWOB. Notably, the two samples from Youjingshan show significant differences in their age distributions and peak ages, indicating a marked shift in provenance during this period. This shift is potentially related to intense uplift and erosion along the northern OB during the deposition of the Yaogou Formation. Tao [5] employed an empirical equation correlating crustal thickness with zircon Eu/Eu* ratios to calculate the range of crustal thickening in the orogenic belts along the northern segment of the OB. Their results revealed large-scale uplift events in this region during the late Permian, providing further robust support for the aforementioned conclusion.

5.2. Provenance Variations in the Permian of the Ordos Basin

To comprehensively constrain the relationship between the Permian provenance system of the OB and the tectonic evolution of its surrounding areas, this study carried out a systematic zircon geochronological investigation, comparing Permian strata from the OB with the potential source areas, e.g., WQOB, YDWOB, NQOB, and AB. This work has reconstructed a basin-scale Permian provenance system and provides a reliable basis for further restoring the original basin configuration of the OB during the Permian.

The locations of the Permian detrital zircon U-Pb geochronology samples collected in this study are shown in Figure 8a. From the perspective of geochronological characteristics, the Permian strata of the OB primarily exhibit four major age distribution zones: late Paleozoic (ca. 260–360 Ma), early Paleozoic (ca. 420–500 Ma), early Paleoproterozoic (ca. 1600–1900 Ma), and late Paleoproterozoic (ca. 2300–2500 Ma) (Figure 8b). A small number of samples also contain Neoproterozoic ages (ca. 700–1000 Ma) (Figure 8b). Additionally, some samples show an absence of either late Paleozoic (ca. 260–360 Ma) or early Paleozoic ages (ca. 420–500 Ma), along with variations in the proportion of ages from other periods (Figure 8b).

This study categorized collected Permian detrital zircon U-Pb geochronological samples from the OB into distinct groups for comparative analysis, aiming to elucidate variations in their provenance characteristics. The detrital zircon age composition of the early Permian Shanxi Formation in the OB is generally uniform, characterized by dominant late Paleozoic (ca. 260–360 Ma), early Paleozoic (ca. 420–500 Ma), and Paleoproterozoic age (ca. 1600–2500 Ma) populations (Figure 9a). However, certain individual samples exhibit variations in the proportional representation of these age groups (Figure 9a). For example, sample Zh12 shows a lower proportion of Paleozoic detrital zircon ages (ca. 260–500 Ma), while sample Zh17 contains a reduced number of early Paleoproterozoic grains (ca. 1600–1900 Ma). The Multidimensional Scaling (MDS) statistical analysis of detrital zircon ages shows relative strong correlations among all Shanxi Formation samples (Figure 9b). This indicates that the provenance system recorded by the detrital zircon ages had not yet undergone significant differentiation at this time. The primary provenance for the early Permian OB was derived from the YDWOB [50,69], with additional minor or localized contributions from adjacent areas such as the AB and the QQOB.

The Dahuanggou and Shihezi Formations in the OB are primarily characterized by early Paleozoic (ca. 260–360 Ma), late Paleozoic (ca. 420–500 Ma), and Paleoproterozoic ages (ca. 1600–2500 Ma) (Figure 10a). However, the proportional representation of these age groups varies significantly among samples from different locations (Figure 10a). For instance, the Dahuanggou Formation from the WOB is dominated by Paleozoic ages (ca. 260–500 Ma), with very few Paleoproterozoic ages (ca. 1600–2500 Ma). In contrast, the Shihezi Formation within the interior of the basin exhibit a dominance of Paleoproterozoic ages, but the proportion of Paleozoic ages differs markedly between locations. Multidimensional Scaling (MDS) analysis of detrital zircon age spectra identifies four strongly correlated age assemblages within the Dahuanggou and Shihezi Formations of the OB (Figure 10b). Samples from the Shihezi Formation in the interior of the OB (Sh31, Zh41, Zh14, T32, Zh32, YG, SGZ, Y1008S-2, Y1149FH1, CJG) possess relatively similar age compositions, with provenance primarily derived from the YDWOB [69]. Samples from the southern OB (Zr1–Zr6), and the western OB (NN-23-74, HL, HL-04, SWOP-02, Su230, Y1257, DK-13, E27) also show relatively similar age compositions, dominated by late Paleozoic (ca. 420–500 Ma) and Paleoproterozoic ages (ca. 1600–2500 Ma). Their provenance were interpreted as a mixture derived from both the YDWOB and QQOB [4,69]. Samples from the central segment of the western OB (NX-16-53, 11HL-57, L3) share a similar age signature, characterized by dominant proportion of Paleozoic ages (ca. 260–500 Ma) and a significantly lower proportion of Paleoproterozoic ages (ca. 1600–2500 Ma). This characteristic indicates their provenance was primarily from the AB. The absence of Paleoproterozoic ages rules out the YDWOB as a major sediment source for this area.

The existing samples from the late Permian Yaogou and Shiqianfeng Formations are primarily distributed in the western OB, exhibiting concentrated age distributions in the early Paleozoic, late Paleozoic, and Paleoproterozoic (Figure 11a). As shown in the MDS statistical analysis diagram, the Dahuangou and Shiqianfeng Formations in the OB display two strongly correlated age assemblages (Figure 11b). The samples from the central segment of the western OB (NX-16-52,11HL-51,11HL-55) are dominated by late Paleozoic ages, with fewer Paleoproterozoic ages, indicating that their provenance was mainly derived from the AB or the NQOB, while the YDWOB in the north did not supply significant sediment to this area. The age compositions of samples from the northwestern, southwestern, and interior OB (SWOP-03, WS-06, Y1147F-2, NN-23-75, SQF) are relatively similar. Compared to the samples from the central segment of the western OB, these show a significantly higher proportion of Paleoproterozoic ages. Furthermore, the western samples are dominated by early Paleozoic ages, whereas the interior basin samples are dominated by late Paleozoic ages. This suggests that the YDWOB in the northern part of the basin was the main provenance for these regions, with the QOB in the south potentially providing minor contributions [50].

In summary, the surrounding tectonic units, including the AB, YDWOB, NQOB, and QOB, all served as source areas of the Permian strata of the OB. Provenance patterns, however, varied both temporally and spatially across the basin. Moreover, from the Shanxi stage to the Yaogou/Shiqianfeng stage, the overall provenance heterogeneity of the basin progressively increased.

5.3. Tectonic Implications

By comparing detrital zircon geochronology from surrounding potential source areas and the OB, and integrating previous data on paleocurrents and detrital compositions, the Permian provenance of the OB can be categorized into three major systems. (1) The southwestern provenance system: the source areas are primarily the WQOB and NQOB. Zircon ages from these sources are dominated by late Paleozoic ages (ca. 255–360 Ma), containing some early Paleozoic (ca. 420–500 Ma) and Paleoproterozoic ages (ca. 1600–2500 Ma). The lithic assemblage is characterized by high contents of quartzite and phyllite lithic fragments [72]. This system primarily supplied sediment to areas such as Pingliang and the Quwushan areas on the southwestern OB. (2) The northwestern provenance system: the source areas are primarily the AB and YDWOB. Zircon ages from these sources are mainly late Paleozoic (ca. 280–360 Ma), early Paleozoic (ca. 420–500 Ma), and Paleoproterozoic (ca. 1600–2500 Ma). This system primarily supplied sediment to areas such as the Helan Mountains and the Youjingshan areas on the northwestern OB. (3) The interior provenance system: the source area is primarily the YDWOB, with minor contributions from the QOB. Zircon ages are predominantly Paleoproterozoic (ca. 1600–2500 Ma), late Paleozoic (ca. 280–360 Ma), and early Paleozoic (ca. 420–500 Ma). The rock assemblage includes metamorphic rocks, various magmatic rocks, with minor sedimentary rock characteristics [73]. For the western OB, the provenance exhibits characteristics of dual influence from both northern sources (YDWOB and AB) and southern source (QQOB). Meanwhile, due to its north–south-trending belt-shaped distribution, the dominant provenance transitions northward to the YDWOB and AB, and southward to the QQOB. (Figure 12). The aforementioned research provides substantial support and key evidence for further comprehensive understanding of the original basin configuration of the OB during the Permian period.

Resting on an Archean–Paleoproterozoic crystalline basement, the OB is a multi-cycle superimposed basin featuring a sedimentary record extending from the Mesoproterozoic to the Cenozoic [19,74]. The western OB underwent a complex evolution, progressing through Meso–Neoproterozoic rifting (including the Helan Aulacogen), an Early Paleozoic passive margin stage, and a Late Paleozoic active margin and collisional orogeny, before stabilizing as an intracontinental basin in the Mesozoic [21,75,76]. In the Late Paleozoic, the block resided in an intraplate depression bounded by active margins to the south and north, with tectonically distinct sectors [21]. Furthermore, this study indicates that the Permian tectono-sedimentary framework of the OB was primarily controlled by the influence of two major tectonic domains (i.e., the Paleo Asian Ocean and Paleo Tethys Ocean tectonic domains) (Figure 12) [63,77,78]. This tectonic evolution led to continuous uplift along both the southern and northern margins of the OB, supplying detrital materials to the interior of the basin. Specifically, from the Carboniferous to the late Permian, the subduction of the PAO caused significant crustal thickening [5] and large-scale uplift and exhumation along the northern margin of the OB [78]. This large-scale uplift provided substantial provenance to the interior of the basin [4,5,72], and also led to notable changes in sediment sources, sedimentary hiatus, and climatic shifts in certain regions [5]. Following the final closure of the PAO, the tectonic nature of the northern OB fundamentally transformed into an intracontinental depression basin, dominated by continental clastic deposition and forming large-scale fluvial-deltaic systems [79,80]. Similarly, due to the subduction of the PTO [4,8,81,82], the QOB also experienced uplift during the Permian, supplying sediments to the Permian strata in the southern part of the basin [4,9]. However, the southern OB remained in a marine environment during this period, characterized by relatively low and gentle topography [5], indicating that the magnitude and scale of uplift in the south were relatively limited. Therefore, due to differences in the timing and intensity of subduction/collision between the PAO and PTO, the OB exhibited a characteristic “high in the north and low in the south” during this period (Figure 12) [4,69,77,83].

6. Conclusions

(1)

By comparing the detrital zircon U-Pb ages from the study area with those of potential source regions, it is concluded that the provenance of the Dahuangou Formation in the Youjingshan area was the Alxa Block, while the provenance of the Yaogou Formation was mainly derived from the Yinshan-Daqingshan-Wulashan Orogenic Belt. In contrast, the source areas for both the Dahuangou and Yaogou formations in the Quwushan area were the West Qinling Orogenic Belt and North Qilian Orogenic Belt. It is noteworthy that the two samples from Youjingshan exhibit significant differences in age distributions and peak ages, indicating a pronounced shift in provenance during this period.

(2)

The Permian provenance of the Ordos Basin can be classified into three systems: the southwestern system sourced from the West Qinling Orogenic Belt and North Qilian Orogenic Belt; the northwestern system sourced from the Alxa Block and Yinshan-Daqingshan-Wulashan Orogenic Belt; and the interior system primarily sourced from the Yinshan-Daqingshan-Wulashan Orogenic Belt.

(3)

During the Permian, the northern margin of the Ordos Basin was influenced by the Paleo Asian Ocean tectonic domain, while its southern margin was affected by the Paleo Tethys Ocean tectonic domain. Due to differences in the timing and intensity of subduction/collision between the Paleo Asian Ocean and the Paleo Tethys Ocean, the paleogeographic pattern of the Ordos Basin exhibited a characteristic “higher in the north and lower in the south” during this period.