Geochemical characteristics of the late Eocene mudstones in the eastern Nima Basin, Xizang: implications for the paleoenvironment, provenance, and tectonic setting

Abstract

Paleoenvironmental reconstruction plays a pivotal role in providing insights into the uplift history of the Xizang Plateau during the Cenozoic. The Nima Basin, situated in the central Xizang Plateau, is crucial for studying the tectonic and geomorphic evolution of this region. The clastic composition and geochemical characteristics of the Niubao Formation hold considerable potential for unravelling the geological history and reconstructing depositional environments of central Xizang in the early Cenozoic. In this study, we present detailed geochemical characteristics to determine their provenance, paleoenvironmental conditions, and tectonic origins. The index of compositional variability (ICV > 1) of mudstones indicates that low compositional maturity sediments underwent weak sedimentary recycling. The chemical index of alteration (CIA: 59.8−72.9) reveals that parental rocks experienced a moderate chemical weathering degree. The paleoclimate indicators of the mudstones suggest an oxidizing and arid depositional environment, with a mean annual temperature (MAT) of 11.64°C ± 4.19°C. The geochemical evidence also demonstrates that the mudstones were derived from mixed felsic and intermediate igneous rocks that formed in a dominantly continental island arc tectonic setting. Similarities in the geochemical characteristics among the Niubao Formation and surrounding igneous rocks indicate that a continental-scale drainage system once drained westward in central Xizang. It is concluded that the central plateau experienced a cooler and drier climate coinciding with the presence of a large-scale drainage system during the late Eocene.

Graphical abstract

Keywords

late Eocene / geochemistry / Nima Basin / mudstone / paleoenvironment

1 Introduction

The early Cenozoic continental collision between the India and Asia accelerated the outward growth of the Xizang Plateau from central Xizang and significantly influenced the Cenozoic global climate and the development of the Asian monsoon system (Raymo and Ruddiman, 1992; Molnar et al., 1993, 2010; Cerling et al., 1997; Dupont-Nivet et al., 2007; Ding et al., 2014; Deng and Ding, 2015; Kapp and DeCelles, 2019; Ingalls et al., 2020; Yang et al., 2022; Feng et al., 2023). Nonetheless, the comprehension of the Cenozoic geomorphological and paleoenvironmental evolution in central Xizang remains a subject of intense debate, primarily due to methodological disparities among studies (Wang et al., 2014a; Su et al., 2019; Spicer et al., 2021a, 2021b; Ding et al., 2022). Several Cenozoic intermountain basins, developed in the Bangong-Nujiang Suture Zone (BNSZ) in central Xizang, accumulated up to 4 km of fluvio-lacustrine sediments that contain crucial paleogeographic evolution records. The isotopic compositions of carbonates obtained from those basins suggest arid and cold conditions during the Paleogene (Rowley and Currie, 2006; DeCelles et al., 2007b), whereas the same successions have yielded fossils of mammals (Deng et al., 2012), fish (Wu et al., 2017), plants (Jia et al., 2019; Jiang et al., 2019; Liu et al., 2019; Su et al., 2019; Tang et al., 2019), and biomarker remains (Jia et al., 2015) that reflect a warm and humid environment. The stable isotope analyses of paleosol carbonate in the Nima Basin point to an arid climate and high paleoelevation (4.5−5 km) by 26 Ma (Rowley and Currie, 2006; DeCelles et al., 2007a; Sun et al., 2014; Ingalls et al., 2020), whereas the paleontological evidence provided by the ecological signal of plant megafossils and fish suggests that tropical-subtropical warm and humid lowland habitats were present in the Xizang interior ca. 26 Ma, probably at an elevation of ~1 km (Wu et al., 2017). Additionally, reliable chronological frameworks (Han et al., 2019; Fang et al., 2020; Xiong et al., 2022) and numerical simulation models (Botsyun et al., 2019; Su et al., 2020) have resolved the disparities between paleontological and isotopic geochemistry investigations.

During the process of weathering, erosion, transport, deposition, and diagenesis, geochemical elements in source rocks exhibit stable chemical characteristics, resulting in minimal differentiation in fine-grained sediments (Nesbitt, 1979; Nesbitt and Markovics, 1997; Duddy, 1980; Bhatia, 1983; McLennan et al., 1983; Taylor and McLennan, 1985; Wronkiewicz and Condie, 1987; Crichton and Condie, 1993; Vital and Stattegger, 2000; Weltje and von Eynatten, 2004; Singh, 2009; Moradi et al., 2016; Caracciolo, 2020; Wang et al., 2020; Amorosi et al., 2022). The geochemistry of clastic sediments is an essential source of information for determining their provenance history (Girty et al., 1993; Cullers and Podkovyrov, 2000; Anderson et al., 2002; Wang et al., 2018a; Ramos-Vázquez and Armstrong-Altrin, 2019; Chen et al., 2020; Banerji et al., 2022), as well as their tectonic background (Roy et al., 2008; Gallala et al., 2009; Armitage et al., 2011; Bai et al., 2015; Wang et al., 2017a; Wang et al., 2018a; Lin et al., 2021; Sun et al., 2022).

The Paleocene to Eocene Niubao Formation is extensively distributed within the Bangong-Nujiang Suture Zone (BNSZ) and is one of the predominant Cenozoic strata in central Xizang. Although numerous paleontological identifications and hydrocarbon explorations have been conducted on the Niubao Formation, further geochemical studies are required to gain a comprehensive understanding of the paleoenvironment of the Niubao Formation. In this study, the geochemical characteristics of mudstones were analyzed in the Niubao Formation in the Nima Basin, with a comparative examination against neighboring igneous rocks. Additionally, the study applies an empirical formula to calculate the mean annual temperature (MAT) of the source area. The primary objective is to elucidate the origin and tectonic setting of the Niubao Formation and to provide a new perspective on the early Cenozoic paleogeomorphic pattern of central Xizang.

2 Geological background

The Qiangtang and Lhasa terranes are divided by the Bangong-Nujiang Suture Zone, which extends approximately 2000 km from east to west in central Xizang. The Nima Basin, one of the largest Cenozoic sedimentary basins within the BNSZ, spans an east–west-trending region with an area of 3000 km² (Fig.1; DeCelles et al., 2007a, 2007b; Kapp et al., 2007; Chen et al., 2022). The Cenozoic terrestrial strata in the Nima Basin unconformably overlie deformed Mesozoic marine strata. The margins of the basin are strongly deformed by east–west basin-controlling faults along the northern and southern margins. Previous research has revealed that the Niubao Formation was deposited predominantly in a fluvial to deltaic environment during the Paleocene to Eocene. The Niubao Formation is distinguished by red upward-fining or upward-coarsening sequences of conglomerate, sandstone, siltstone, and mudstone, with occasional thin interlayers of marl and limestone that likely represent lacustrine facies. Recent investigations constructed a chronostratigraphic framework for the Niubao Formation using the ages of volcaniclastic interlayers as chronological anchors. Tuffaceous and paleomagnetic stratigraphic results in the Lunpola basin yielded an age range of 41.8−21.5 Ma for the middle to the upper part of the Niubao Formation (Fang et al., 2020). Su et al. (2020) obtained a zircon U-Pb age of 47.5 ± 0.7 Ma from a tephra-rich layer in the Niubao Formation in the Bangoin Basin. Xiong et al. (2022) established a depositional age range of 50−29 Ma for the Niubao Formation, extending from the base to the top of the formation.

In this study, a 410-m-thick stratigraphic section of the Niubao Formation was measured in the field on the south-eastern margin of the Nima Basin (31°57′04.30″N, 88°24′42.87″E; Fig.2). The sedimentary profile is dominated by red and grayish-green mudstones with a total thickness of approximately 200 m. The interbedded conglomerates, sandstones, and siltstones are characterized by kinds of sedimentary structure, including parallel or horizontal bedding, cross-bedding, and bioturbation. This section is near the Kanggale section (31°58′21.38″N, 88°26′10.19″E), where notable late Eocene fossils of Spittlebugs and Lestidae were discovered (Xia et al., 2022; Xu et al., 2022). The fossiliferous strata in Kanggale consist of similar grayish-green mudstones and calcareous shales, with interbedded mudstones, sandstones, and limestones within these layers (Xia et al., 2022). The striking similarity in lithologic composition and sedimentary features between the studied section and the Kanggale section indicates that the depositional age of the measured section in this study is assigned to the late Eocene.

3 Sampling and analytical methods

3.1 Sampling

Eighteen mudstone samples were systemically collected from different stratigraphic intervals for geochemical analysis. The sampling locations are shown in Fig.2. To minimize weathering effects, we selected unweathered samples and carefully removed weathering surfaces. All the samples were crushed in the laboratory before analysis.

3.2 Analytical methods

Major element, trace element, and rare earth element (REE) analyses were completed at Wuhan Sample Solution Analytical Technology Co., Ltd., Wuhan, China. Whole-rock major oxide concentrations were measured using an X-ray fluorescence spectrometer (XRF). The whole-rock major element analysis samples were prepared by the fusion bead method. For this procedure, samples were collected and pulverized to a size of less than 200 mesh using an agate mortar. Approximately 1 g of fine powdered sample was then fused with a 6 g flux, consisting of a well-balanced mixture of lithium tetraborate, lithium metaborate, and lithium fluoride at a ratio of 45:10:5, to form a uniform glass bead. Ammonium nitrate and lithium bromide were used as oxidant and release agents, respectively. The melting temperature was 1050°C and was maintained for 15 min. Measurements were made using a ZSX Primus II wavelength dispersive XRF instrument (RIGAKU, Japan) with a 4.0 kW Rh target X-ray tube at 50 kV and 60 mA. Quality control was ensured using physical and analysis replicates and the reference material GBW07101-14. The data were corrected by the theoretical α coefficient method, and the relative standard deviation (RSD) was less than 2%.

Whole-rock trace element analysis and REE analysis were conducted on an Agilent 7700e ICP-MS instrument. In the pretreatment phase for geochemical trace and REE analysis, the sample powder, with a 200 mesh size, was initially dried for 12 h at 105°C. A 50 mg portion of this powder was then accurately weighed and placed in a Teflon bomb, to which 1 mL of HNO₃ and 1 mL of HF were added. This mixture was heated in a stainless steel jacket at 190°C for more than 24 h and then cooled. The contents were evaporated to near dryness, treated with HNO₃, and evaporated again. A mixture of HNO₃, Milli-Q water, and 1 mL of internal standard solution with 1 ppm In was added, and the Teflon bomb was resealed and placed in the oven for more than 12 h. The final solution was transferred to a polyethylene bottle and diluted with 2% HNO₃ to a total of 100 g.

The analysis employed a GeolasPro laser ablation system integrated with a COMPexPro 102 ArF excimer laser, featuring a 193 nm wavelength and 200 mJ maximum energy, alongside a MicroLas optical system. Trace element compositions were determined using an Agilent 7700e ICP-MS instrument, with helium as the carrier gas and argon as the makeup gas, which was fed into the ICP through a T-connector. This system includes a “wire” signal smoothing device (Hu et al., 2015). The laser parameters were set to a 40 μm spot size and a 10 Hz frequency. Calibration was conducted using the reference materials NIST 610 and NIST 612. Each sample underwent a 20−30 s background acquisition followed by a 50 s data acquisition from the sample. The off-line selection, integration of the background and analytical signals, time-drift correction, and quantitative calibration were performed using the ICPMS DataCal 10.1 software (Lin et al., 2016).

4 Results

A total of 18 representative samples of the studied section were analyzed for major, trace, and rare earth elements. The results are given in Tab.1, Tab.2, Tab.3.

4.1 Major elements

Major element analyses were carried out on 18 mudstone samples of the Niubao Formation of the Nima Basin, as detailed in Tab.1. The total content of major components in the mudstone ranges from 80.31 to 94.72 wt.%. The results revealed that SiO₂, Al₂O₃, CaO, and Fe₂O₃ were the predominant components, followed closely by MgO and K₂O. The concentrations of other components, such as TiO₂, P₂O₅, and MnO, were generally less than 1.0%.

The mudstone samples exhibited noticeable variations in their elemental compositions. The SiO₂ content varied widely, from 37.23 to 73.10 wt.%, averaging 54.74 wt.%. Al₂O₃ ranged from 7.83 to 19.70 wt.%, with a mean of 14.66 wt.%. The content of total Fe₂O₃ ranges from 3.14 to 6.90 wt.%, averaging at 5.10 wt.%. The TiO₂ content varies between 0.44 and 0.78 wt.%, with a mean value of 0.65 wt.%. The CaO content spaned from 3.01 to 15.47 wt.%, averaging at 7.17 wt.%. The K₂O content is 1.09−3.99 wt.% with an average of 2.89 wt.%. The Na₂O content is in the range of 0.81−2.19 wt.% with an average of 1.21 wt.%. Additionally, the loss on ignition (LOI) ranged from 5.70 to 19.26 wt.%, potentially attributable to organic matter or volatile components.

The ratios of the concentrations of the major elements in the mudstones to those in the upper continental crust (UCC) are shown in Fig.3(a). In addition to some samples being enriched in CaO and K₂O, all the samples exhibited apparent depletion in Na₂O, with the remaining element concentrations plotting near those in the UCC.

4.2 Trace and REEs

The trace element data of the mudstone samples are listed in Tab.2 and have been standardized in relation to the average UCC values (Taylor and McLennan, 1985; Fig.3(b)). The trace element analyses reveal that, compared with the UCC, the mudstone samples are significantly enriched in Ni, Zn, Rb, Cs, Th, and U and depleted in Co, Sr, Ba, Zr, and Hf.

The REE analysis results are presented in Tab.3 and data are plotted in the chondrite-normalized diagram (Fig.3(c)). The mudstones have total REE (ΣREE) ranging from 124.81 to 204.83 ppm, with an average of 162.34 ppm. the average value is slightly lower than the post-Archean Australian shale (PAAS) value of 211.76 ppm (Taylor and McLennan, 1985; McLennan et al., 2006). In terms of chondrite-normalized REE patterns, the mudstones closely align with PAAS. Light REEs (LREEs) in mudstones range from 109.76 to 183.43 ppm, while heavy REEs (HREEs) range from 13.86 to 21.40 ppm. Generally, the mudstones exhibit higher concentrations of LREEs than HREEs. The ratios of LREEs to HREEs in these samples vary from 7.29 to 9.24, with an overall average of 8.29, indicating relative enrichment of LREEs in the mudstones. Additionally, the mudstones exhibit consistent negative Eu anomalies, with values ranging from 0.59 to 0.70 and an average of 0.65.

4.3 Mineral composition

The high SiO₂ and low Al₂O₃ contents suggest that clay minerals are the main mineral component of the mudstone. The low Na₂O concentrations suggest the minimal presence of plagioclase in the samples (Ross and Bustin, 2009; Zeng et al., 2020). The high levels of CaO are likely attributable to the presence of carbonate minerals, which are present in the formation of cement (Ghosh and Sarkar, 2010). Notably, the mudstones exhibit significant correlations between TFe₂O₃ and Al₂O₃ (R² = 0.847) and K₂O (R² = 0.865; Tab.4), indicating the close association of Fe with clay minerals.

The K₂O/Al₂O₃ ratio provides valuable insight into the primary mineral constituents within sediments. A ratio between 0.2 and 0.3 generally implies the dominance of illite in the sediment. Conversely, a ratio near 0 is indicative of sediment enriched with minerals such as kaolinite, smectite, and vermiculite (Cox et al., 1995; Zhou et al., 2015; Zeng et al., 2020). The K₂O/Al₂O₃ ratios in mudstone vary from 0.13 to 0.25, averaging 0.19 (Tab.5). The highest value was obtained for the ZP-16 sample (0.25), and the lowest value was obtained for the ZP-09 sample (0.13). These data indicate that the ratio aligns with typical values for clay minerals, suggesting that illite is the predominant clay mineral present. Additionally, the notable associations between Sc, V, Zn, Ga, Rb, and both Al₂O₃ and K₂O indicate that these elements are predominantly related to clay minerals (Tab.4).

5 Discussion

5.1 Sediment maturity and paleoweathering

The index of compositional variability (ICV) can determine the original character and maturity of sediments. Generally, ICV value < 1 indicates that the sample is intensively mature, rich in clay minerals, and may have undergone recycling or strong weathering during the first cycle of deposition, while ICV value > 1 indicates that the sample has a low maturity and a low clay mineral content (Barshad, 1966). For the studied samples, the ICV values of all the mudstones exhibit more than 1, except sample ZP17-01, which has an ICV value of 0.97 (Tab.1). Based on these values, it can be inferred that the rocks have a minimal clay content and the sediment is compositionally immature.

The SiO₂/Al₂O₃ ratios depend on sediment recycling and weathering processes, indicating sediment maturity. The average ratio of SiO₂/Al₂O₃ greater than 5.0 indicates progressive maturity sediment (Potter, 1978; Roser et al., 1996; Madukwe et al., 2016; Shekhar et al., 2018). The SiO₂/Al₂O₃ ratios of our samples vary from 2.66 to 8.53 (Tab.5), indicating an intermediate maturity level, which can be compared with the ICV values.

Alternatively, the Th/Sc and Zr/Sc ratios are further used to infer the degree of sedimentary recycling (McLennan et al., 1995; Bai et al., 2015; Qiu et al., 2015; Chen et al., 2020). As recycling progresses, there is a notable increase in Zr/Sc, while Th/Sc remains stable (McLennan et al., 1993; Li et al., 2022a). The Th/Sc ratios in our samples range from 0.72 to 1.58 while the Zr/Sc ratios range from 7.96 to 34.72 (Tab.5). As shown in the Zr/Sc-Th/Sc diagram (Fig.4), the data spots corresponding to all mudstones are relatively concentrated and exhibit a weak sedimentary recycling trend, which suggests insignificant recycling with an initial depositional environment.

The chemical index of alteration (CIA) has been commonly used to estimate the intensity of chemical weathering in the source region (Nesbitt and Young, 1982). The formula can be shown as CIA = [Al₂O₃ / [Al₂O₃ + CaO* + Na₂O + K₂O] × 100, with all the elements expressed in molecular percentages. Here, CaO* represents the amount incorporated in the silicate fraction of the rock, defined as CaO* = CaO−10/3 × P₂O₅ (McLennan et al., 1993). The CIA values between 50 and 60 represent a low degree of chemical weathering in cold, dry climates. A range of 60−80 indicates moderate weathering, which is typical of warm, humid conditions. Finally, values ranging from 80 to 100 represent intense weathering observed in hot, humid tropical-subtropical climates, indicating that the parent rock has undergone substantial weathering (Nesbitt and Young, 1982; Fedo et al., 1995; Bock et al., 1998; Xu and Shao, 2018). The CIA values of the mudstone samples are given in Tab.1. They range between 59.79 and 72.90 with an average of 66.70, indicating that the Niubao Formation mudstones suffered from low to moderate weathering. As depicted on the A-CN-K (Al₂O₃-CaO + Na₂O-K₂O) ternary diagram, the mudstone samples show an evolution trend from the upper crust to kaolinite along the A-CN axis (Fig.5). Even though the samples also deviate from the ideal weathering trend (IWT) and are inclined toward illite, which could be attributed to K-metasomatism during diagenesis (Fedo et al., 1995; Michalopoulos and Aller, 1995), the subtle divergence of samples implies a weak diagenetic impact (Ma et al., 2015).

5.2 Paleoenvironmental conditions

5.2.1 Paleosalinity and paleoclimate

The Sr/Ba ratios are a diagnostic indicator for determining paleosalinity and paleoclimate variations. High Sr/Ba ratios, typically above 1.0, signify water columns with high salinity prevalent in arid climatic settings, whereas values less than 0.5 represent water columns with low salinity associated with humid climatic conditions (Meng et al., 2012; Fu et al., 2016; Wang et al., 2017a). Among the analyzed samples, six samples had values more than 1, while two samples had values less than 0.5 (Tab.5; Fig.2), potentially highlighting variation in paleosalinity over time and the transient dry and humid climate change of the sedimentary environment. In addition, Sr/Cu ratios less than 5.0 are indicative of humid climate conditions, whereas those exceeding 5.0 are characteristic of arid climate conditions (Lerman, 1989; Meng et al., 2012; Jia et al., 2013; Liang et al., 2014; Cao et al., 2015; Fu et al., 2016, 2018). The Sr/Cu ratio was close to 5.0 for four samples, over 5 for all other samples, and greater than 12 for most of the samples, indicating relatively dry climate conditions (Tab.5).

Certain elements, such as V, Co, and Cu, are recognized as redox-sensitive and commonly accumulate in anoxic sediments (Algeo and Maynard, 2004; Tribovillard et al., 2006; Lézin et al., 2013; Ayinla et al., 2017). Key trace element ratios, such as V/Cr and Ni/Co ratios, are reliable indicators of redox conditions. The V/Cr ratios, which typically increase with decreasing oxygen content in a water column, can be used to classify environments: ratios less than 2.0 suggest oxic conditions, those between 2.0 and 4.25 indicate dysoxic conditions, and values above 4.25 signify anoxic conditions (Jones and Manning, 1994). The V/Cr ratios of the mudstones fluctuate from 0.77 to 1.81 (Tab.5; Fig.2), indicating oxic conditions. Similarly, Ni/Co ratio values less than 5.0 indicate an oxidizing environment, values between 5.0 and 7.0 suggest suboxic to dysoxic conditions, and values over 7.0 indicate reducing conditions (Jones and Manning, 1994). Our mudstone samples have Ni/Co ratios between 3.16 and 6.02 (Tab.5; Fig.2), further supporting oxic conditions during deposition.

5.2.2 Mean average temperature (MAT) calculation

Chemical weathering processes are significantly influenced by climatic factors (Nesbitt and Young, 1982). In the weathering of granitic bedrock soils, the sodium depletion fraction index (τNa) effectively indicates terrestrial chemical weathering intensity (Gaillardet et al., 1999; Anderson et al., 2002; Rasmussen et al., 2011; West, 2012; Garzanti et al., 2013). The intensity of chemical weathering in the topsoil of weathered granite bedrock shows a positive correlation with the mean annual surface temperature (MAT) in modern non-glacial areas (Yang et al., 2016). The empirical equation can express this relationship: MAT = −24.2τNa−0.9 (r² = 0.84, P < 0.0001). Four specific criteria must be considered before employing this equation. First, annual precipitation should range between 400 mm and 4000 mm, avoiding extreme aridity or excessive humidity conditions. Secondly, the source rock should be felsic igneous rocks. Thirdly, the physical erosion rate should be 2−100 m/m.y. Fourthly, sorting and sedimentary recycling processes during transport need to be comparatively weak, and finally, diagenetic processes should be minimal.

Evidence from late Eocene fossils suggests the presence of tropical to subtropical climates (Fang et al., 2020). The diagram of Zr vs TiO₂ for the samples points out that the mudstones predominantly originate from the felsic igneous rocks source area (Fig.6(b)). Furthermore, the erosion rate in the central regions of the plateau during the Eocene (~45 Ma) was below 50 m/m.y (Rohrmann et al., 2012). It is coupled with weak recycling effects (Fig.4) and the absence of diagenetic alteration minerals, which conform to the application conditions of the empirical formula. Using granite and granodiorite as the original rock (Chi, 2007), the τNa of 18 mudstone samples spans from 0.93 to 2.42 with an average of 1.34. These results suggest that the average land surface temperature during the late Eocene is estimated to be 11.64°C ± 4.19°C (Tab.1; Fig.2).

The carbonate formation temperature of paleosol carbonates derived from clumped isotope analysis can represent the surface temperature during formation (Ghosh et al., 2006). The carbonate formation temperatures in the upper member of the Niubao Formation range between 10.1°C and 12.9°C, with an average of 11.9°C and a standard deviation of 2.8°C (Xiong et al., 2022). Ingalls et al. (2020) interpreted the T(Δ47) values of ~30°C to 36°C in soil carbonate in the late Eocene to Oligocene upper Niubao Formation in the Lunpola Basin as the maximum possible surface temperature during summer. It has been shown that terrestrial soil carbonates recorded T(Δ47) values approximately 18°C warmer than MAT at midlatitudes during the Paleocene-Eocene (Snell et al., 2013). Thus, the findings suggest an annual average temperature of 12°C to 18°C during the late Eocene to Oligocene.

The numerical climate modeling and proxy thermal regimes yield consistent results, indicating a shift toward a drier and cooler climate in central Xizang during the mid-late Eocene (Su et al., 2019; Chen et al., 2021; Zhang et al., 2022). The presence of multiple animal fossils alongside a diverse grass community, palms, and a variety of other woody species suggests the existence of a subtropical open woodland ecosystem, with an estimated dry bulb mean annual temperature of ~15.6°C (Deng et al., 2012; Wu et al., 2017; Wang et al., 2018b; Mao et al., 2019; Del Rio et al., 2020; Xu et al., 2022; Zhang et al., 2022).

In summary, the paleotemperature inferred from the geochemical proxies in mudstones in the Nima Basin is in concordance with the results obtained from paleontological and isotopic investigations, providing support for the hypothesis that the central plateau exhibited a cooler climatic regime during the late Eocene.

5.3 Source lithotypes and tectonic setting

5.3.1 Source lithotypes

The REE parameters are relative to the sources of sedimentary rocks due to their high stability during weathering, erosion, and early diagenesis (McLennan et al., 1995; Bock et al., 1998; Asiedu et al., 2000; Cullers and Podkovyrov, 2000; Kasanzu et al., 2008; Armstrong-Altrin et al., 2009). Generally, negative Eu anomalies reflect felsic rocks, whereas minimal or no Eu anomalies mainly indicate mafic rocks (Roddaz et al., 2006; Kasanzu et al., 2008; Armstrong-Altrin et al., 2017). The distribution patterns of mudstone samples chondrite-normalized REEs are relatively consistent, indicating that the mudstones originated from the same source. The REE profiles of the mudstones also exhibit significant negative Eu anomalies (Fig.3(c)), strongly indicating that the mudstones were derived from felsic rocks. Additionally, the REE exhibit a negative correlation with CaO (R² = 0.617; Tab.3), indicating that the REE concentrations in the mudstone samples are predominantly controlled by terrigenous material (Zeng et al., 2020).

The geochemical compositions of detrital deposits provide vital insights into their provenance (Wesolowski, 1992; Ayers and Watson, 1993; Cullers and Podkovyrov, 2000; Verma and Armstrong-Altrin, 2013, 2016; Armstrong-Altrin et al., 2013, 2015a, 2015b, 2017; Madhavaraju, 2015). The Al₂O₃/TiO₂ ratio in clastic sediments is widely considered an effective indicator of the lithology of sedimentary source rocks. Mafic rocks have Al₂O₃/TiO₂ ratios ranging from 3 to 8, intermediate rocks have ratios ranging from 8 to 21, and felsic rocks have ratios ranging from 21 to 70 (Hayashi et al., 1997; Armstrong-Altrin et al., 2015a, 2017; Zhou et al., 2015; Moradi et al., 2016; Verma and Armstrong-Altrin, 2016; Wang et al., 2017b). The ratios of our samples range from 13.55 to 32.89, averaging 22.53 (Tab.5; Fig.6(a)). Most samples exhibit values characteristic of felsic rocks, while only six samples display values associated with intermediate rocks. Similarly, the TiO₂/Zr ratio can be used to investigate the origin of sediments. A TiO₂/Zr ratio up to 200 suggests that the sediments originated from the erosion of mafic rocks. Alternatively, ratios between 55 and 200 are indicative of intermediate igneous rock sources, and ratios less than 55 are indicative of felsic igneous rock sources (Hayashi et al., 1997; Zeng et al., 2020; Kirubakaran et al., 2023). The Zr-TiO₂ graph (Fig.6(b)) and the Al₂O₃ vs TiO₂ (Fig.6(a)) diagram for the mudstone samples indicate a predominance of felsic igneous source rocks and subordinate intermediate igneous source rocks.

The Co/Th ratio is a distinctive indicator of felsic versus mafic sources, as felsic rocks typically exhibit lower Co/Th values than mafic rocks (Amorosi et al., 2002; Cullers, 2002). Our collected samples feature Co/Th values between 0.55 and 1.37, which are notably lower than the benchmarks of the UCC (1.65) and PAAS (1.37), suggesting a predominantly felsic source (Tab.5; Ma et al., 2015). Similarly, the La/Sc ratio has been recognized as a reliable source composition indicator (Floyd and Leveridge, 1987; Condie, 1993; McLennan et al., 1995; Gu et al., 2002; Ma et al., 2015). T he La/Sc-Co/Th plot (Fig.7(a)) reveals that the samples are predominantly concentrated between granodiorite and adamellite. Additionally, on the Zr/Sc-Th/Sc plot (Fig.7(b)), the majority of the data plot in the felsic source area.

In conclusion, geochemical results suggest that the mudstones of the Niubao Formation predominantly originated from the source area of felsic rocks with abundant felsic igneous rocks and minor intermediate igneous rocks.

5.3.2 Tectonic setting

The geochemical characteristics of sedimentary rocks could reveal the composition and tectonic setting of source rocks (Bhatia and Crook, 1986; Roser and Korsch, 1988). Clastic sediments from various tectonic environments can exhibit unique terrane-specific imprints, leading to diverse geochemical compositions (Roser et al., 1996; Bai et al., 2015).

The discriminant function diagram illustrates the mudstones plotted in the collision area (Fig.8; Verma and Armstrong-Altrin, 2013). The La-Th-Sc plot (Fig.9(a)) shows that most of mudstone samples cluster within and near the edges of the continental island arc field. In the Th-Sc-Zr/10 plot (Fig.9(b)), there is an observable spread of sample points, with most of them positioned in the continental island arc and a few scattered within the passive margin. The Th-Co-Zr/10 diagram (Fig.9(c)) displays some mudstone samples within the continental island arc field, while others gravitate toward the boundaries of the active continental margin. In addition, the mudstones have high ΣREE values (124.81−204.83 ppm), (La/Yb)_N ratios (8.47−10.99), and negative Eu anomalies (Eu/Eu* values ranging from 0.59 to 0.70) reinforce the conclusion of the continental island arc (Bhatia and Crook, 1986; Tab.4).

5.4 Provenance interpretation

As an intermontane basin proposed by previous work, it is reasonable to speculate that the neighboring geological units fed the Niubao Formation in the Nima Basin, namely the southern Qiangtang, the northern Lhasa, and the BNSZ (Rowley et al., 2015; Mi et al., 2018; Han et al., 2022), which are characterized by widely distributed pre-Cenozoic felsic and intermediate igneous rocks, particularly Cretaceous and Jurassic in age (Chapman and Kapp, 2017). According to the detrital zircon U-Pb age distributions of the Niubao Formation, zircons originating from these igneous rocks are the predominant components of the distinct peaks (Kapp et al., 2007; Donaldson et al., 2013; Zhu et al., 2017; Li et al., 2022b; Liu et al., 2023). Several representative intrusive and volcanic rocks are compared to the trace elements and REE of the mudstone samples (Lee et al., 2012; Li et al., 2013; Wang et al., 2014b; Li et al., 2016; Yan et al., 2016; Chen et al., 2017; Hu et al., 2017; Liu et al., 2017). Fig.10(a) displays that the Cretaceous and Jurassic granodiorites from Amdo and the southern Qiangtang, along with the mudstone samples, show enrichment in Th and U elements relative to the upper crust. In contrast, the volcanic rocks on the west side of the Nima Basin do not show similar enrichment. In addition, the REE concentrations in the volcanic rocks from the west of the Nima Basin are lower than in the mudstone samples (Fig.10(b)), suggesting that the volcanic rocks on the west side of the Nima Basin are unlikely geochemically related to the mudstones. Beyond that, the abundance of limestone clasts in the conglomerates of the Niubao Formation is indicative that the extensively distributed Mesozoic marine strata of the southern Qiangtang and northern Lhasa terranes also contributed an enormous amount of debris to the Niubao Formation (Han et al., 2019; Kong et al., 2019; Liu et al., 2023).

5.5 Palaeogeography of the drainage basin and implications for the landscape evolution

The increasing amount of quantitative paleoaltitude data sets based on various methods suggests a lowland valley with an elevation of 2−3 km during the Eocene, which ran between the highlands of the Gangdese arc (Currie et al., 2005; Ding et al., 2014; Currie et al., 2016) to the south and the central Qiangtang terrane to the north (Zhu et al., 2013, 2015; Wu et al., 2017; Su et al., 2019; Fang et al., 2020; Ding et al., 2022; Li et al., 2022c; Xiong et al., 2022; Bi et al., 2023). However, the west of central Xizang was near sea level during the late Eocene (Wei et al., 2016). Apatite fission track studies suggest that the Amdo area, a few hundred kilometres east of the Nima Basin, underwent rapid cooling in the early Cretaceous times (ca. 110 Ma; Bi et al., 2023; Lu et al., 2024). Low temperature thermochronological data demonstrated that the west of the Nima Basin did not experience rapid cooling until the late Cretaceous (ca. 70 Ma; Xue et al., 2022). On the basis of the differential cooling history, it is possible that the land surface uplift in the eastern portion of central Xizang commenced prior to that in the western portion, thereby establishing an elevation gradient from eastern highlands to western lowlands. The early Cenozoic sedimentary basins along the BNSZ accumulate several kilometres thick of fluvial-lacustrine sediments and show great similarities in sedimentary structural characteristics and sedimentary facies. It could be inferred that these basins may have been interconnected (e.g., Ma et al., 2015; Han et al., 2019). Furthermore, the westward direction of paleocurrents in the Lunpola and Bangoin basins suggests the possibility of a drainage system extending from east to west across the central part of the Xizang Plateau (He et al., 2012; Ma et al., 2015; Han et al., 2019). Hence, a westward drained source-to-sink system likely developed since the Cenozoic. In this geomorphologic pattern, it makes more sense that the mudstones of the Nima Basin have a stronger affinity for the igneous rocks to their east.

6 Conclusions

The geochemical analysis of the late Eocene of the Niubao Formation was undertaken to study the paleoclimate conditions, provenance, and tectonic settings. The main conclusions are summarized as follow.

1) The Niubao Formation mudstones are characterized by high contents of SiO₂, Al₂O₃, and CaO and are primarily composed of clay minerals.

2) The mudstone samples have a total rare earth element (ΣREE) content of 124.81−204.83 ppm. They exhibit a higher proportion of light rare earth elements (LREEs) compared to heavy rare earth elements (HREEs) and display notable negative Eu anomalies.

3) The sediment is first recycled and compositionally immature. The CIA values (59.8−72.9) indicate that the source area of the Niubao Formation experienced moderate chemical weathering.

4) The mudstones were deposited in an oxidizing and arid depositional environment with a mean annual temperature (MAT) of 11.64°C ± 4.19°C, reflecting a cool environmental conditions in central Xizang during the late Eocene.

5) The mudstones were predominantly derived from felsic and intermediate igneous rocks that originated in a continental island arc setting. Igneous rocks from the north, south, and east of the Nima Basin contributed significantly to the composition of these mudstones.

6) Combining data of paleoaltimetry and low temperature thermochronology, the geochemical characteristics of the Niubao Formation suggest the possible development of a westward-draining source-to-sink system since the Cenozoic.

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