Major, trace and rare earth element geochemistry of the Permian Lucaogou oil shales, eastern Junggar Basin, NW China: implications for weathering, provenance and tectonic setting

Abstract The Permian Lucaogou oil shales in the eastern Junggar Basin have long been regarded as important source rocks and reservoirs for tight oil exploration. The oil shales can also be used to assess the provenance and tectonic setting of the East Junggar region using major, trace and rare earth element geochemistry. The low chemical index of alteration ratios (37.36–64.18, 45.54 on average), low plagioclase index of alteration ratios (26.99–62.95, 42.99 on average), and high index of compositional variability ratios (0.64–1.66, 1.10 on average) suggest that the Permian Lucaogou oil shales mostly consist of immature sediments that have experienced a weak weathering intensity. The samples have low TiO2/Al2O3 and TiO2/Zr ratios that indicate a felsic origin with minor intermediate igneous rocks. The high Th/Sc and Zr/Sc ratios suggest a felsic origin without sediment recycling. The samples show fractionated light rare earth elements and relatively flat heavy rare earth elements patterns with weak negative Eu anomalies, which implies that the main provenance is lower Carboniferous intermediate–acid volcanic rocks. In addition, multiple major- and trace-element‐based discrimination diagrams show that the parent rock of clastic sediments formed mainly in a continental island arc and active continental margin environment, consistent with previous studies on the tectonic background of the East Junggar region during the late early Carboniferous. Key Points Immature sediments are present in the Permian Lucaogou oil shales and have a weak weathering intensity. The provenance of the clastic sediments of the Permian Lucaogou oil shales was predominantly early Carboniferous intermediate–acid volcanic rocks. The clastic sediments of the Permian Lucaogou oil shales were developed in a continental island arc and active continental margin environments. The Permian Lucaogou oil shales provide crucial information for tectonic setting and evolution of the northern Xinjiang during the Carboniferous–Permian.


Introduction
The Junggar Basin is located at the structural junction of the Siberian and Tarim plates, one of the crucial areas for studying the tectonic evolution of the Central Asian Orogenic Belt and the closing time limit of the Paleo-Asian ocean Shen et al., 2011;Simonov et al., 2015;Tang et al., 2017;Wu et al., 2022;Yang et al., 2011). In recent years, many scholars have focused on the timing of closure of the Paleo-Asian ocean and subduction and collision processes in northern Xinjiang. Suggestions include late Permian-Triassic Xiao et al., 2008;Zhang et al., 2005), Late Devonian-early Carboniferous (Xia et al., 2006) and late early Carboniferous (Chen et al., 2007;Li, 2004;2020;Nan, 2018;Xu et al., 2014;Zhou et al., 2006). Owing to differences in timing, there are two interpretations on the tectonic setting in northern Xinjiang and its adjacent areas during the Carboniferous-Permian: volcanic island arc environment (He et al., 2001;Lin et al., 1997;Long et al., 2006;Xiao et al., 2008Xiao et al., , 2015 and an intraplate extension environment (Chen et al., 2009;Li, 2020;Nan, 2018;Su et al., 2012;Wu et al., 2009;Xu et al., 2014;Yang et al., 2011;Zhang et al., 2015;Zhao et al., 2022;Zhou et al., 2006;Zhu et al., 2005). The tectonic nature and evolution of northern Xinjiang are still hotly debated.
Numerous studies have implied that the chemical compositions of clastic sediments are generally influenced by the different tectonic settings of the source areas, and many major-and trace-element discrimination diagrams are widely applied to determine the provenance and tectonic setting in sedimentary basins (Armstrong-Altrin, 2015;Armstrong-Altrin, et al., 2013;Armstrong-Altrin & Verma, 2005;Bhatia, 1985;Bhatia & Crook, 1986;Hayashi et al., 1997;Kasanzu, et al., 2008;Li et al., 2022;Liu et al., 2020;Madhavaraju, 2015;Qiao et al., 2023;Ramos-V azquez & Armstrong-Altrin, 2021;Roser & Korsch, 1988;Sugitani et al., 1996;Tao et al., 2017;Wang et al., 2018). The Permian Lucaogou oil shales are widely developed in the north Xinjiang Provence and are generally regarded as source rocks and reservoirs, with significant success in exploration and development of tight oil and shale oil (Lin et al., 2021;Liu et al., 2012Liu et al., , 2019Meng et al., 2022). However, fewer studies have considered provenance and tectonic setting using the oil shales. Tao et al. (2016) proposed that the Shichanggou oil shales of the Southern Junggar Basin are mainly derived from felsic rocks from the western orogenic belt that formed in a continental island arc setting. Liu et al. (2020) suggested that the provenance of salinised lacustrine organic-rich shales in the middle Permian Santanghu Basin was mainly intermediate volcanic rocks derived from a continental island arc setting and that the intermediate-basic volcanic rocks from late Carboniferous in Santanghu area formed in a subduction setting.
In this study, we selected the Permian Lucaogou oil shales in the eastern Junggar Basin and used major, trace and rare earth element geochemistry to identify the chemical weathering conditions, provenance and tectonic setting in the source area, as well as to provide the theoretical basis for the tectonic evolution of the whole northern Xinjiang during the Carboniferous-Permian.

Geological background
The Junggar Basin is located in the northern part of Xinjiang Uygur Autonomous Region, NW China ( Figure 1a), with an area of approximately 136 000 km 2 (Liang et al., 2016). It is adjacent to Altay tectonic zone in the northeast, northern Tianshan tectonic zone in the south, and western Junggar tectonic zone in the northwest (Figure 1b). The Junggar Basin is a large superimposed sedimentary basin with a complex tectonic evolution and constitutes the current intracontinental basin-mountain tectonic framework, together with the Turpan-Hami, Santanghu and Yili basins and Altay, Tianshan and Bogda orogenic belts (Fang et al., 2019;Li, 2020). The study area is in two secondary tectonic units of the eastern Junggar Basin, and the Jimsar and Shishugou sags (Figure 1c).
Previous studies have shown that the late Carboniferous-Permian was the key to the structural transformation in the East Junggar region and is mainly composed of basalts, alkali basalts, andesites, granites, alkali granites and volcaniclastic rocks, interbedded with shales/mudstones, sandstones, dolostones, limestones and tuffs (Carroll, 1998;Han et al., 2006;Li, 2020;Su et al., 2010;Yang et al., 2009). In the Jimsar Sag, the strata from bottom to top are lower Carboniferous Batamayineishan Formation, middle Permian Jiangjunmiao and Lucaogou formations, and upper Permian Wutonggou Formation. In the Shishugou Sag, no drilling cores have reached to the strata below the Lucaogou Formation; the other strata are in accordance with the Jimsar Sag ( Figure 2a). The middle Permian Lucaogou Formation is characterised by thick black and dark grey organic-rich shales/mudstones, sandstones, tuffs, dolostones and limestones (Figure 2b, c), which are considered as crucial source rocks and potential tight oil reservoirs in northern Xinjiang in recent years. For example, the Lucaogou Formation in well S consists of 368 m of fine-grained sedimentary rocks such as sandstones, siltstones interbedded with shales, mudstones and tuffs. In the Jimsar Sag, the thickness of the Lucaogou Formation is only 258 m in J1 and 164 m in J2 with a finer grainsize; the grainsize and lithologies are mainly dark shales, mudstones, interbedded with medium to thin layers of grey siltstones, sandstones, dolostones and tuffs (Figure 2b, c).

Samples and analytical methods
Cores of the Permian Lucaogou Formation from two wells (J1 and J2) in the Jimsar Sag and one (S) from the Shishugou Sag were carefully observed. The rock series mainly consists of grey to black fine-grained sedimentary rocks, including shales/mudstones, sandstone, dolostones and sedimentary tuffs, and these lithofacies are interlaminated or intercalated with each other, which makes it difficult to determine the lithotype by examining core alone. Therefore, in this study, we combined macroscopic and microscopic observation and mineral composition analyses, to exclude dolostones, limestones and tuffs, as well as to select shales enriched in organic matter. A total of 16 samples were collected from three wells (Figure 1c, seven samples from J1, five samples from J2 and four samples from S) for thin-section observation, X-ray diffraction (XRD) analyses, total organic carbon (TOC) analyses and X-ray fluorescence spectrometry (XRF) and inductively coupled plasma-mass spectrometry (ICP-MS) analyses.
For microscopic observation, the samples were cut from a homogeneous section perpendicular to bedding, polished to a thickness of $30 lm and glued on a glass slide. Petrological and mineral features were analysed using a Nikon BX51-P microscope. For XRD analyses, the samples were milled to a 75 lm size, then mixed with ethanol and smeared on glass slides together with a standard sample.
Finally, the qualitative and semi-quantitative interpretations of mineral contents were processed with the Jade software. The XRD analysis of 16 samples was conducted at the Xi'an Center of Geological Survey using a Japanese Rigaku D/max 2500 X-ray diffractometer. For TOC analyses, the powder samples (<200 mesh) were subjected to a Leco CS-230 Carbon and Sulfur Analyzer and Rock-Eval OGE-II analysis at Xi'an Shiyou University. For XRF analyses, the samples were powdered to a 200-mesh size using an agate mill cleaned with acetone to avoid potential contamination. Major elements were analysed using Axios MAX on platinum crucibles and X-ray fluorescence spectrometry, and the analytical precision and accuracy were generally better than 5%. Trace elements were analysed on an Elan DRC-e ICP-MS using sample powders (50 mg) dissolved using an HF þ HNO 3 þ HClO 4 mixture in high-pressure Teflon bombs at 190 C for 48 h. BHVO-2, AGV-2 and the GBW07103 standards were used for analytical quality control with a determined analytical precision of better than 10%. The analytical precision and accuracy for major and trace elements are described by Liu et al. (2008). These 16 samples were selected for whole-rock major and trace-element analyses at the Nanjing Hongchuang Exploration Technology Service Co., Ltd.

Petrological and mineralogical characteristics
The main lithologies of the Lucaogou Formation in the eastern Junggar Basin consist of grey to black fine-grained shales, mudstones, dolostones and tuffs. In this study, we selected shales, and the other lithologies are not described. In addition, the selected samples have a high TOC (7.08-16.8 wt%, 9.33 wt% on average) and are rich in organic matter. Data on XRD and TOC are shown in the Supplemental data (Table S1), and XRD results are illustrated in Figure 3. Most shales are tuffaceous with poorly rounded silt-sized tuffaceous mineral grains and angular crystal fragments (Figure 3c, d). The matrix mainly consists of clay minerals and dark amorphous organic matter with a dark colour (Figure 3a, b). There are obvious differences between the shale in the two depressions. Clay mineral contents are relatively high in the Jimsar Sag (4.0-30.8 vol%, 12.3 vol% on average) but poor in the Shishugou Sag (S, 1.9 vol%, 2567.1 m; Figure 3e-g). Quartz is the main phase of the Lucaogou oil shales and primarily occurs as detrital and authigenic grains. Detrital feldspars are abundant and as important as quartz in the Lucaogou Formation, and consist of plagioclase (albite) and K-feldspar. XRD data show that the felsic minerals in the Shishugou Sag (quartz, 40.3-53.8 vol%, 46.1 vol% on average; plagioclase, 11.3-38.0 vol%, 28.9 vol% on average; Kfeldspar, 5.3-21.7 vol%, 11.5 vol% on average) are slightly higher than those of the Jimsar Sag (quartz, 17.4-81.3 vol%, 35.4 vol% on average; plagioclase, 3.6-44.9 vol%, 30.7 vol% on average; K-feldspar, 0-5.9 vol%, 2.6 vol% on average). Carbonate minerals are mainly calcite and dolomite. Shishugou samples have calcite and dolomite contents (0-13.7 vol%, 3.4 vol% on average), much lower than those of the Jimsar Sag (0-45 vol%, 14.5 vol% on average). In both sags all samples have a small amount of analcime (J, 0-4.9 vol%, 0.6 vol% on average; S, 0-27.7 vol%, 7.3 vol% on average) and pyrite (J, 0-2.4 vol%; S, 0-9.1 vol%).
The results show that the Al 2 O 3 contents are positively associated with the SiO 2 , Na 2 O and K 2 O contents, suggesting the source minerals are mainly feldspar and clay (Ross & Bustin, 2009;Figure 4a, c, d). The positive correlations between Al 2 O 3 and TiO 2 suggest that Ti existed in clay minerals (Ross & Bustin, 2009; Figure 4b). The SiO 2 contents exhibit a slightly negative relationship with Fe 2 O 3 and TiO 2 , which shows that Fe and Ti are not correlated with the felsic minerals (Figure 4e

Trace elements
The trace-element data of the Permian Lucaogou oil shales in the eastern Junggar Basin (Table 2) consist of LILEs (large ion lithophile elements; Rb, Ba, Th, and U), HFSEs (high-field-strength elements) and transition trace elements. The LILEs are mostly depleted compared with UCC (upper continental crust; Taylor & McLennan, 1985). However, the Sr contents of the Jimsar samples are enriched in comparison with UCC but significantly depleted in samples of the Shishugou Sag ( Figure 5a). The Sr   (Nesbitt & Young, 1982), PIA (Fedo et al., 1995). ICV (Cox, Lowe, & Cullers, 1995).
The oxides are expressed as mol%. CaO Ã refers to the concentration of CaO in silicate minerals, not including carbonate minerals and apatite. The details of the specific calculation method for obtaining CaO contents of the Jimsar samples are positively correlated with Al 2 O 3 contents but have a negative relationship with K 2 O contents, indicating that Sr is mainly hosted in clay minerals. In the Shishugou Sag, the Sr contents show negative relationships with Al 2 O 3 and K 2 O contents, suggesting Sr is not primarily controlled by phyllosilicate. In addition, the Al 2 O 3 contents of the Shishugou samples are negatively related to Th and U contents, revealing that these elements do not exist in clay minerals, which is consistent with the minor clay contents in the  Shishugou Sag. The HFSEs, such as Nb and Hf, are relatively depleted compared with UCC, whereas Zr and Y are slightly enriched in most samples. The Hf content shows a positive relationship with Nb, Zr and Y, indicating that these elements have similar geochemical features or derive from a common source (Bhatia & Crook, 1986). With respect to transition trace elements, the V, Cr, Ni and Cu contents are slightly enriched in most samples, whereas Sc and Co are depleted in comparison with UCC.

Rare earth elements (REEs)
The RREE (total REE contents) contents of the Permian Lucaogou oil shales in the eastern Junggar Basin (Table 3) range from 78.46 to 297.64 ppm (mean of 133.23 ppm), which is lower than that of UCC (146 ppm, Taylor & McLennan, 1985). The concentrations of LREEs (light rare earth elements) are much higher than those of HREEs (heavy rare earth elements), which is consistent with the general distribution of REEs in shales (Gromet et al., 1984;Ketris & Yudovich, 2009). All samples are highly enriched in LREEs relative to HREEs with RLREE/RHREE ratios ranging from 4.65 to 7.09 (5.83 on average). The chondrite-normalised REE diagram is characterised by fractionated LREE patterns (La N /Sm N ¼ 3.82-8.34, 5.82 on average), relatively flat HREE patterns (Gd N /Yb N ¼ 1.01-1.99, 1.46 on average), and show weak negative Eu anomalies (Eu/Eu Ã ¼ 0.24-0.68, 0.57 on average). The chondrite-normalised samples and UCC have similar distribution patterns, suggesting that both originate from a common terrigenous source with negative Eu anomalies inherited from their source rocks (Eskenazy, 1987;Fu et al., 2011;Figure 5b).

Chemical weathering and compositional maturity
Generally, the alkali metal elements such as Na, K and Ca are commonly lost during the chemical weathering processes, whereas Al and Ti are enriched in weathered products (Fedo et al., 1995). Therefore, the chemical index of alteration (CIA; Nesbitt & Young, 1982) was proposed to study the degree of chemical weathering. Compared with CIA, the PIA (plagioclase index of alteration) reflects the weathering of plagioclase after removing K and Al in K-feldspar (Fedo  Taylor & McLennan, 1985;chondrite, Sun & McDonough, 1989;primitive mantle, McDonough & Sun, 1995; early Carboniferous basalts, andesites and rhyolites, late Carboniferous rhyolites, Zhang et al., 2014;early Permian rhyolites, Li et al., 2013. Generally, the CIA values (Table 1) of a provenance with intense weathering (80-100) are relatively higher than the source with weak weathering (50-70), and the fresh rocks usually show low CIA and PIA values of $50. High values of CIA and PIA up to 100 reflect the transformation of feldspar to aluminous clay minerals (such as kaolinite, illite and gibbsite) (Fedo et al., 1995., Nesbitt & Young, 1982. The CIA and PIA values of the Permian Lucaogou oil shales range from 37.36 to 64.18 (45.54 on average) and from 26.99 to 62.95 (42.99 on average), respectively, suggesting a low degree of weathering. On an A-CN-K ternary diagram (Figure 6), all samples plot around the plagioclase-potash feldspar join, which indicates that the source area experienced a weak weathering intensity. In addition, the Rb/Sr ratio is widely used to reflect the weathering conditions of sediments. High Rb/Sr ratios (>1) indicate intense weathering, whereas the fresh rocks show low Rb/Sr ratios (UCC ¼ 0.32) (Hossain et al., 2017;McLennan et al., 1993). In our study, the Rb/Sr ratios of the Permian Lucaogou oil shales range from 0.02 to 0.96 (0.28 on average; Table 2), suggesting a low degree of weathering. Combined with the above features, we conclude that the source area had a weak weathering intensity.
The SiO 2 /Al 2 O 3 ratios are generally applied to the discrimination of compositional maturity and sediment sorting, and the high ratios correlate with high maturity . Generally, the average SiO 2 /Al 2 O 3 ratios of basic and acid rocks are 3 and 5, respectively, and the ratios exceeding 5 indicate a moderate to high compositional maturity (Roser et al., 1996). The SiO 2 /Al 2 O 3 ratios of samples in our study range from 3.69 to 5.47 (average of 4.58; Table 1, except one sample with 9.77), suggesting a low compositional maturity. The index of compositional variability (ICV; Cox et al., 1995) evaluates the compositional maturity of sediments. In general, mature samples (ICV <1; Table 1) contain high clay mineral contents, such as kaolinite, illite and muscovite, which means that they are recycled or strongly weathered under the first deposition conditions, whereas the immature samples (ICV >1) contain relatively high typical rock-forming minerals such as feldspar, amphibole and pyroxene, belonging to a first sedimentation related to tectonic activity (Cox et al., 1995;Cullers & Podkovyrov, 2000). In this study, the ICV values of samples vary between 0.64 and 1.66 (1.10 on average; Table 1), which demonstrates that the degree of clastic sediments in the Permian Lucaogou oil shales in the eastern Junggar Basin vary from low to moderate maturity.

Provenance
Major-element contents and/or ratios of clastic sediments are commonly used to discriminate the provenance of sedimentary rocks because they maintain stable chemical properties during deposition, transportation and diagenesis (Armstrong-Altrin, 2015; Madhavaraju, 2015). Sugitani et al.  Figure 6. A -CN -K ternary diagram for Permian Lucaogou oil shales in the eastern Junggar Basin, after Nesbitt and Young (1982). UCC, upper continental crust (Taylor & McLennan, 1985). A, Al 2 O 3 ; C N , CaO Ã þ Na 2 O; K, K 2 O.
(1996) proposed that the TiO 2 /Al 2 O 3 ratios decrease from mafic to felsic igneous sources. In the TiO 2 /Al 2 O 3 vs Fe 2 O 3 /Al 2 O 3 and TiO 2 /Al 2 O 3 vs TiO 2 /Al 2 O 3 diagrams ( Figure  7a, b), most samples plot around andesite source with a small number as granite. The Al 2 O 3 /TiO 2 ratios of the samples in our study (J1, 20.07-30.78; J2, 21.13-49.13; S, 21.45-48.76) are close to those of felsic igneous rocks (21-70, Hayashi et al., 1997). In the Al 2 O 3 vs TiO 2 and Zr vs TiO 2 diagrams (Figure 7c, d), most samples plot in the field of felsic igneous rocks with a small number as intermediate igneous rocks. Trace-element (e.g. Sc, Cr, Co, La, Hf and Th) concentrations and/or ratios in clastic sediments are also commonly used to deduce their provenance because these immobile elements are unaffected by diagenesis and metamorphism Bhatia & Crook, 1986;Liu et al., 2020;Wang et al., 2018). Felsic rocks generally show enrichments in La and Th, whereas Sc, Cr and Co are more abundant in the mafic rocks Cullers & Berendsen, 1998). The Th/Sc ratios of our samples (J1, 0.36-0.92; J2, 0.32-0.68; S, 0.65-2.65) are much lower than the Zr/Sc ratios (J1, 13.24-21.30; J2, 14.06-24.92; S, 23.77-46.84), and most samples exhibit a positive linear relationship in the Zr/Sc vs Th/Sc diagram (Figure 7e), indicating a felsic origin without sediment recycling. The Co/Th ratios (J1, mean of 1.85; J2, mean of 1.14; S, mean of 1.84) are generally higher than that of UCC (0.9, Taylor & McLennan, 1985). In the La/Sc vs Co/Th diagram (Figure 7f REEs are also regarded as indicators for determining the origin and provenance of rock owing to their immobile properties during sedimentation and diagenesis. dEu values (Eu/Eu Ã ) are commonly inherited from source rocks (Armstrong-Altrin et al., 2012;Bhatia, 1985;Kasanzu et al., 2008;Ramos-V azquez & Armstrong-Altrin, 2021;Wang et al., 2018). The chondrite-normalised REE distribution patterns of the Permian Lucaogou oil shales in the eastern Junggar Basin show fractionated LREE and relatively flat HREE patterns with weak negative Eu anomalies, which is very similar to early Carboniferous rhyolites developed around Karamaili structural belt in the northeast Junggar region and early Permian intermediate-acid rocks in the Zhaheba area, East Junggar region, but a different trend to the late Carboniferous rhyolites developed in the Batamayi Formation in the East Junggar region (Figure 5b). Most samples have similar early Carboniferous intermediate-acid rock patterns in primitive-mantle normalised trace-element diagrams (Figure 5c), with a few samples from the Jimsar Sag showing enrichment in Sr, which is consistent with early Carboniferous basalts and andesites. In addition, the early Carboniferous basalts and andesites and our studied samples both show enrichment in the LILEs (including Ba, Th, U and Pb) and depletion in the HFSEs (including Nb, Ta and Ti) compared with their trace-element data. However, there are clear differences between the studied samples and the late Carboniferous rhyolites on the primitivemantle-normalised incompatible trace-element diagram ( Figure 5c). Therefore, we infer that the main provenance of the Permian Lucaogou oil shales is primarily early Carboniferous intermediate-acid volcanic rocks with minor mafic rocks.

Tectonic setting
Major-and trace-element contents and/or ratios of clastic sediments are generally influenced by different tectonic settings, such as arc, rift and collision, so discrimination diagrams based on major and trace elements are widely applied to determine the tectonic setting of sedimentary basins (Armstrong-Altrin & Verma, 2005;Bhatia & Crook, 1986;Li et al., 2022;Liu et al., 2020;Roser & Korsch, 1988;Tao et al., 2017). On the MgO þ Fe 2 O 3 vs TiO 2 and MgO þ Fe 2 O 3 vs Al 2 O 3/ SiO 2 diagrams, most samples plot in the field of the continental island arc and active continental margin (Figure 8a, b). In recent years, some specific discrimination diagrams based on trace elements have been proven to be feasible for identifying tectonic settings (Armstrong-Altrin & Verma, 2005;Liu et al., 2020;Ryan & Williams, 2007). In the diagrams of La/Sc vs Ti/Zr and Sc/Cr vs La/Y ( Figure  8c, d), and the ternary diagrams of Th-Co-Zr/10, Th-Sc-Zr/10 and La-Th-Sc (Figure 8e, f, g), most samples from the study area plot in the continental island arc and active continental margin environments.
In addition, Verma and Armstrong-Altrin (2013) proposed a multidimensional major-element discrimination diagram based on the oxides of Cenozoic siliciclastic sediments to identify their tectonic settings (arc, rift and collision). Samples are divided into high-silica [(SiO 2 ) adj ¼ 63-95 wt%] and low-silica types [(SiO 2 ) adj ¼ 35-63 wt%], and the subscript adj of (SiO 2 ) adj means the SiO 2 content acquired by adjusting all major element contents to 100% (excluding the LOI). In this study, the (SiO 2 ) adj contents of the Permian Lucaogou oil shales range from 58.79 to 76.49 wt% (65.63% on average). Twelve samples belong to the high-silica group, and the other four samples are of the low-silica group. On the DF1(Arc-Rift-Col) m1 vs DF2(Arc-Rift-Collision) m1 diagram (Figure 9a), most high-silica samples fall into the arc field except one in the rift and two in the collision. On the DF1(Arc-Rift-Col) m2 vs DF2(Arc-Rift-Collision) m2 diagram (Figure 9b), all low-silica samples are located in the arc area, indicating that the Permian Lucaogou oil shales are mainly derived from an arc environment.

Regional tectonic setting of northern Xinjiang during the Carboniferous-Permian
Many previous studies have shown that the volcanic rocks of the study area are formed in an arc environment closely related to an active continental margin during the early Carboniferous and the ancient oceanic basin finally closed in the late Carboniferous (Huang et al., 2018;Liu et al., 2017;Long et al., 2012;Nan, 2018;Xiao et al., 1992). During the early Carboniferous, the ancient ocean basin represented by the Karamaili ophiolite belt was subducted northward in the late early Carboniferous, resulting in the development of island arc volcanic rocks (328.9 ± 1.9 Ma, zircon U-Pb,  and marine terrestrial volcaniclastic rocks in the East Junggar region (Nan, 2018;Xiao et al., 1992), which belonged to the continental margin of the Siberian plate. During the late Carboniferous, the Karamaili ancient ocean basin was finally closed, marking the transformation of the eastern Junggar Basin into a post-orogenic intraplate basin. Late Carboniferous volcanic rocks are widely developed in the eastern Junggar and its adjacent areas, and zircon U-Pb dating results show that the age is mainly between 320 and 305 Ma Long et al., 2006;Zhang et al., 2014). Zhang et al. (2014) suggested that the volcanic rocks of the Batamayishan Formation in the eastern Junggar region were calc-alkaline volcanic rocks with high potassium contents (319.7 ± 5.9 Ma, zircon U-Pb) and were produced in a back-arc basin environment. According to the regional geological environment, subduction and collision of the northern Xinjiang ended and entered a period of extensional transformation in the late  Allegre & Minster, 1978); (c) La/Sc vs Ti/Zr and (d) Sc/Cr vs La/Y (after Roser & Korsch, 1988); Ternary diagrams of (d) Th, Co and Zr/10; (e) Th, Sc and Zr/10; and (f) La, Th and Sc (after Bhatia & Crook, 1986). A, oceanic island; B, continental island arc; C, active continental margin; D, passive continental margin.
Carboniferous, forming a series of grabens, with fluviallacustrine sedimentation in the early Permian (Han et al., 2006;Long et al., 2006;Nan, 2018;Zhou et al., 2006). Some studies support this interpretation of the sedimentary rocks (Liu et al., 2020;Tao et al., 2016). In our study, the clastic sediments of the Permian Lucaogou oil shales in the eastern Junggar Basin are mainly derived from early Carboniferous intermediate-acid volcanic rocks with minor mafic rocks, which is consistent with published interpretations of the tectonic background of the East Junggar region during the late early Carboniferous. Therefore, we propose that the clastic sediments of the Permian Lucaogou oil shales mainly originated from early Carboniferous intermediate-acid volcanic rocks developed in a continental island arc environment, which provides crucial evidence for understanding the regional tectonic setting in northwest China.

Conclusions
In this study, major, trace and rare earth element geochemistry analyses have been conducted to explore the chemical weathering intensity of the source areas and the provenance and tectonic setting of the Permian Lucaogou oil shales in the eastern Junggar Basin. The main conclusions are as follows: 1. The studied samples indicate that immature sediments in the Permian Lucaogou oil shales have experienced a weak weathering intensity. 2. The main provenance of the clastic sediments of the Permian Lucaogou oil shales was early Carboniferous intermediate-acid volcanic rocks with minor mafic rocks. 3. Multiple discrimination diagrams showed that the clastic sediments of the Permian Lucaogou oil shales were developed in a continental island arc and active continental margin environments, which support the nature and tectonic background of the East Junggar region during the late early Carboniferous and provide theoretical information for the regional tectonic regime of northern Xinjiang.

Disclosure statement
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Data availability statement
The authors confirm that the data supporting the findings of this study are available within the article or its supplementary materials.