Geochemical indicators to constrain weathering, provenance and tectonic setting of the Pisha Sandstone (Early–Middle Triassic) in Northeast Ordos Basin, China

Abstract

The Pisha Sandstone is widely exposed in the northeastern margin of the Ordos basin. Since the Mesozoic the basin was subjected to uneven uplift several times, strong weathering and erosion have been occurring and a large amount of sediments derived from these erosional strata are input into the lower Yellow River, posing a fragile ecological environment along the river. However, the geochemical characteristics of the Pisha Sandstone have remained poorly understood. In this study, we focus on the Pisha Sandstone from Early–Middle Triassic Liujiagou, Heshanggou and Ermaying Formation, present a very first petrographic and geochemical data together with detailed field geological characteristics, aiming to place geochemical indicators on weathering, provenance and tectonic setting of the Pisha Sandstone. The results show that sandstones in Pisha Sandstone are classified as arkose, litharenite and wacke. The geochemical proxies including Chemical Index of Alteration (CIA = 67.2), Chemical Index of Weathering (CIW = 80.1), Plagioclase Index of Alteration (PIA = 75.6) and Index of Compositional Variability (ICV = 1.6) indicate Pisha Sandstone experienced first–cycle deposit and moderate to strong chemical weathering. Trace element and rare earth element concentrations together with their ratios (La/Sc, La/Co, Th/Sc, Th/Co, Cr/Th) reveal a felsic provenance, and source rock compositions are predominantly granodiorite and granite from the north margin of the Inner Mongolia Paleo–Uplift (IMPU), with a small amount of mafic or intermediate components. The geochemical signatures and tectonic discrimination diagrams display a collision setting for the Pisha Sandstone and further reveal the sediments had been deposited in a continental island arc setting. The results of this work may provide new theoretical basis for environmental protection in the Pisha Sandstone area.

Highlights

• •Pisha Sandstone is the crux of ecological environmental problems of the Yellow River.

• •Weathering drives erosion and disintegration of Pisha Sandstone into loose sand.
• •Geochemical indicators characterize weathering history and degree of Pisha Sandstone.
• •Provenance and tectonic setting dominate geochemical features of Pisha sandstone.

1. Introduction

Clastic rocks may provide the principal clues to the composition and the timing of exposure of the source rocks [1]. The chemical composition of clastic rocks is a complex function of several variables, including source materials, weathering, transportation physical sorting and diagenesis [[2], [3], [4]]. Consequently, the geochemical signatures of these siliciclastic sedimentary rocks have been widely used to determine the degree of weathering of the source area [[5], [6], [7]], the provenance [8,9], source rock composition [10,11] and depositional tectonic setting [[12], [13], [14]]. The major elements have been used to deduce the weathering history of sediments [15] and to reflect the tectonic setting of the basin [12,13,16]. Additionally, due to their relative immobility, several trace elements such as large ion lithophile elements, high field strength elements and rare earth elements are most suited for discrimination of provenance and tectonic setting [3,17,18].

The Pisha Sandstone is located in northeast Ordos Basin and refers to a set of Late Paleozoic–Mesozoic siliciclastic rocks composed mainly of white and red sandstones, siltstones, mudstones and a small amount of conglomerates. Since the Mesozoic, the Ordos Basin was subjected to uneven uplift several times and strong weathering and erosion occurring in the Early–Middle Triassic strata, especially in the northeastern margin of the basin [19]. Massive coarse sediments originated from these erosional strata are input into the Yellow River and deposited in the downstream region, causing an extremely fragile ecological environment like a hanging river, and thus the loose Pisha Sandstone itself is the crux of the ecological environment problems along the middle and lower reaches of the Yellow River. Due to the harm of these loose rocks to ecological environment like “Pishuang (As₂O₃)” to human beings, these sedimentary successions are called “Pisha Sandstone” by local residents and environmental researchers [[20], [21], [22]]. As the severe soil erosion, the Pisha Sandstone has been a hot research topic in the Yellow River Ji-shaped bend in recent years.

To date, however, the current studies on the Pisha Sandstone mainly focus on soil erosion, runoff and sediment yield, mechanical properties and control measures of soil and water loss [[23], [24], [25], [26]], while less works based on its lithology and geochemistry have related weathering and provenance which may further reveal the weak erosion resistance mechanism of the Pisha Sandstone. In this study, we present a very first petrographic and geochemical data together with detailed field geological characteristics of the Pisha Sandstone from the Lower–Middle Triassic Liujiagou, Heshanggou and Ermaying Formation in Northeast Ordos Basin, in order to examine their provenance and tectonic setting. The results of this study help to unravel weathering history of the Pisha Sandstone, and furthermore may provide new theoretical basis for environmental protection in the Pisha Sandstone area.

2. Materials and methods

2.1. Sample collection and preparation

100 Pisha Sandstone collected from the outcrops of the Liujiagou, Heshanggou and Ermaying Formation sedimentary rocks in Jungar area, Inner Mongolia. However, 63 relatively unweathered sandstone, siltstone and mudstone samples were prepared for petrographical and geochemical analyses. In order to quantificationally evaluate the relationship between the modal mineralogy and chemical composition, 9 standard sandstone thin sections were prepared from the Pisha Sandstone for modal analysis using Leica DM2700P polarizing microscope followed Gazzi-Dickinson point counting method where 300 points were considered for each thin section [27].

2.2. Sample testing methods

Major and trace elements of Pisha Sandstone present in the whole–rock samples consisting of 10 sandstones, 3 siltstones and 2 mudstones were performed at Guizhou Tongwei Analytical Technology Co., Ltd. Major elements were determined using an ARL Perform'X4200 (Thermo Fisher) X–ray fluorescence spectrometer (XRF), with analytical uncertainties better than 0.5 % for all major elements. Loss on ignition (LOI) was measured by weighing before and after being in a 1000 °C furnace for 1 h. Trace elements and rare earth elements were determined using a Thermo Fisher type ICP–MS X2. The ICP–MS analytical procedure follows the protocol of Eggins et al. [28] with modifications as described in Kamber et al. [29] and Li et al. [30]. Internal standards of 10 ppb ⁶¹Ni, 6 ppb Rh, In and Re combined with USGS rock standards W–2a crossed with BIR–1, BHVO–2 were employed to calibrate element concentrations of measured samples in ICP–MS analysis. The results and relative standard deviations were in ppb and 1 sigma.

2.3. Geochemical indicators calculation

Weathering of the Pisha Sandstone has been quantified by several chemical indicators including the Chemical Index of Alteration (CIA = Al₂O₃/(Al₂O₃ + CaO* + Na₂O + K₂O) × 100) [5], the Chemical Index of Weathering (CIW Al₂O₃/(Al₂O₃ + CaO* + Na₂O) × 100) [31], the Plagioclase Index of Alteration (PIA = (Al₂O₃–K₂O)/(Al₂O₃ + CaO* + Na₂O – K₂O) × 100) [32] and the Index of Compositional Variability (ICV (Fe₂O₃ + K₂O + Na₂O + CaO + MgO + MnO + TiO₂)/Al₂O₃) [4]. Within the above formulas, all the major oxides are in the molecular ratios for chemical indices and CaO* calculation. CaO* is CaO content in the aluminum silicate fraction and simplified as CaO* = CaO −10/3 × P₂O₅ [33]. If CaO* is less than Na₂O, then CaO* is considered as representative of CaO in the aluminum silicate fraction; if CaO* is greater than Na₂O, then Na₂O is taken as CaO*.

3. Geological setting

The Ordos Basin, a large multicycle craton depression basin situated in north central China with an area of ca 250, 000 km², is bound to the east by the Lvliang–Taihangshan Orogen, to the west by the Helan–Liupan Mountains, to the north by the Yinshan Terrane and to the south by the Qilian–Qinling Orogen Belt (Fig. 1a). The basin is part of the Western Block of the North China Craton (NCC), which was formed by the assemblage of the Eastern and Western Blocks along the Trans–North China Orogen [34,35]. According to its structural features and evolutionary history, now the basin can be classified into six structural units: 1) Yimeng uplift; 2) Jinxi flexural fold belt; 3) Shanbei slope; 4) Weibei uplift; 5) Tianhuan depression; and 6) Western overthrust belt (Fig. 1b). The overall feature of the basin is characterized by marginal deformation and the interior slightly dipping west [36]. The study area is located at Jungar area in Inner Mongolia situated at the Yimeng Uplift of the northeastern Ordos Basin.

Fig. 1.

(a) Geological sketch of the North China Craton showing the location of Ordos Basin [35]; (b) Structural subdivision of the Ordos Basin and the Pisha Sandstone sedimentary facies distribution in Early–Middle Triassic [40,41].

In the study area, the Pisha Sandstone strata are divided into Liujiagou Formation, Heshanggou Formation and Ermaying Formation in succession upward (Fig. 2). The Early Triassic Liujiagou Formation, a set of fluvial succession around 140 m thick, is mainly composed of feldspathic sandstones. The late Early Triassic Heshanggou Formation is in conformable contact with the Liujiagou Formation. It is a fine-grained lacustrine facies deposit and mainly dominated by mudstones and siltstones [37,38]. The overlying Middle Triassic Ermaying Formation is predominated by sandstones and mudstones, implying deposited in the meandering river [21,39]. To sum up, the depositional environment changed from fluvial (Liujiagou Formation) to fluvial–lacustrine (Heshanggou Formation) then to meandering river (Ermaying Formation) during the Early–Middle Triassic (Fig. 3).

Fig. 2.

Simplified geological map of the Pisha Sandstone area from Fig. 1b [42] and sample locations.

Fig. 3.

The Pisha Sandstone stratigraphic sections in Early–Middle Triassic of northeast Ordos Basin.

4. Results

4.1. Field observation

The underlying Liujiagou Formation consists mainly of variable cross–bedded medium–coarse–grained pinkish and gray–white sandstones, interstratified with small amounts of dark red mudstones and silty mudstones, and sporadically pebbly sandstones occur in the upper section (Fig. 4a and b). The Heshanggou Formation is dominated by mainly rhythmic alternations of brown–red argillaceous siltstones and silty mudstones, interbedded with a small amount of thin medium–grained gray–white subarkoses (Fig. 4c and d). The overlying Ermaying Formation, commonly overlain by Quaternary loess, is mainly composed of cross–bedded gray–green and gray–white medium–coarse–grained subarkoses and pebbly sandstones, with intercalations of dark–red silty mudstones and argillaceous siltstones layers (Fig. 4c and d). Additionally, a great deal of yellowish green argillaceous fragments developed in the gray–white subarkoses (Fig. 4f). Due to strong weathering, the three formations of sedimentary successions are friable to loose and highly susceptible to erosion, consistent with the dense vertical joints (Fig. 4e).

Fig. 4.

Outcrop photographs of the Pisha Sandstone strata. (a) Pinkish and gray–white sandstones developing alternately in Liujiagou Formation; (b) Development of cross–bedding and pebbly sandstone occurring in the upper part of Liujiagou Formation; (c) Boundary of Heshanggou Formation and Ermaying Formation, which overlain by Quaternary loess; (d) Brown–red siltstones and mudstones in turn in Heshanggou Formation and gray–white subarkoses in Ermaying Formation; (e) Development of dense vertical joints in Ermaying Formation; (f) Development of yellowish green argillaceous fragments in gray–white subarkoses of Ermaying Formation.

4.2. Petrography

Based on thin sections observation, Pisha Sandstone samples show poorly sorted and are dominated by subangular to angular detrital minerals (Fig. 5a). The compositions of sandstones are main of quartz (33.3–74.1 %, average 50.3 %), subordinate amounts of feldspars (11.1–48.2 %, average 29.3 %), lithic fragments (0–27.8 %, average 10.4 %) as well as cement and/or matrix 23.7 % (Table 1). Detrital minerals include monocrystalline quartz, polycrystalline quartz, K–feldspar, plagioclase, lithic fragments which are set in matrix and/or cement of clay minerals, calcareous and ferruginous materials (Fig. 5a, c–e, g, i). Biotite, chlorite and heavy minerals such as zircon and epidote are locally recognized (Fig. 5d and e). Ilmenite is common among the opaque grains (Fig. 5f).

Fig. 5.

Photomicrographs of sandstones in the Pisha Sandstone under cross–polarized light. (a) Sandstones, showing poorly sorted and subangular to angular detrital minerals, polycrystalline quartz grains (Qp) and clay cements (Clay cem); (b) Clasts in sandstones containing monocrystalline quartz grains (Qm), sedimentary lithic fragments of chert and claystone (Ls), microcline (Mc) and perthite (Pth); (c) Sparry calcite cements (Cal cem) distributing widely in sandstones; (d) Qm showing undulose extinction (U) and locally metasomatized by Mc, meanwhile feldspar altered into sericite (Ser) and biotite (Bt) exhibiting bright interference color; (e) Plagioclase (Pl), chlorite (Chl) and epidote (Ep) occurring in the thin sections; (f) Zircon (Zrn) and ilmenite (Ilm) developing in sandstones; (g) Engulfed quartz grains embedded in calcite cements; (h) Qm or orthoclase (Or) developing grain–coating clayey rims (yellow arrows); (i) Inherited quartz overgrowths (yellow arrows) occurring in Qm. Abbreviations are according to Kretz [45] and Shen [46].

Table 1.

Modal analysis compositions of sandstones in the Pisha Sandstone.

Sample No.	Monocrystalline quartz (Qm)	Polycrystalline quartz (Qp)	Total Quartz (Q)	Feldspar (F)	Lithic fragments (L)	Total lithic fragments (Lt)	Cement and matrix
Ermaying Formation
Emy1	54.9	2.9	57.8	33.3	8.8	11.8	23.1
Emy2	35.7	8.9	44.6	48.2	7.1	16.1	8.7
Emy4	33.3	28.6	61.9	38.1	0.0	28.6	21.6
Emy5	74.1	11.1	85.2	11.1	3.7	14.8	39.5
Emy7	48.8	7.0	55.8	26.7	17.4	24.4	17.1
Emy9	44.2	20.9	65.1	23.3	11.6	32.6	33.3
Emy10	33.3	1.9	35.2	37.0	27.8	29.6	29.6
Liujiagou Formation
Ljg1	60.5	5.3	65.8	31.6	2.6	7.9	19.8
Ljg2	67.6	2.9	70.6	14.7	14.7	17.6	20.6
Average	50.3	9.9	60.2	29.3	10.4	20.4	23.7

Among quartz grains, Qm (50.3 %) are dominant than Qp (9.9 %). Qp are of more than three individual subcrystals, which display different size, extinction and straight sub–grain boundaries (Fig. 5a, d–f). The K–feldspar grains include orthoclase, microcline and perthite, which commonly altered into sericites or clay minerals (Fig. 5b–d, e, i). Feldspars have locally undergone partial dissolution and newformed smectites develop under scanning electron microscope. The lithic fragments are consist of dominantly sedimentary lithic fragments (claystone + chert) and minor volcanic and metamorphic lithic fragments (Fig. 5b). Calcite cements have produced corrosion on detrital minerals, particularly in quartz (Fig. 5g). Locally, Qm or feldspars have grain–coating clayey rims (Fig. 5h). Occasionally, inherited quartz overgrowths occur in Qm (Fig. 5i).

Modal analysis of sandstones in Pisha Sandstone have an average QFL ratio of Q₅₈F_31.1L_10.9 and Q_68.2F_23.1L_8.7 at Ermaying and Liujiagou Formation, respectively. According to the sandstone classification of McBride [43], the studied sandstones are predominantly arkose and lithic arkose, and subordinate amounts of lithic subarkose and subarkose compositions (Fig. 6a). Petrographic characteristics can also be used for inferring provenance information. Based on the QFL and QmFLt provenance–discrimination diagrams of Dickinson et al. [44], most of samples are plotted in the transitional continental and magmatic arc fields (Fig. 6b and c).

Fig. 6.

(a) QFL ternary diagrams for compositional classification of the Pisha Sandstone; (b) QFL, and (c) QmFLt ternary provenance–discrimination diagrams, where Q = total quartz, F = feldspar, L = lithic fragments, Qm = monocrystalline quartz, Lt = total lithic fragments (lithic fragments and polycrystalline quartz).

4.3. Geochemistry

4.3.1. Major elements

Major element contents of Pisha Sandstone in Jungar area are listed in Table 2. The most dominant oxide in analyzed samples is SiO₂ (51.39–82.36 wt%, average = 68.32 wt%), which is markedly enriched in sandstones from the upper part of Ermaying Formation (>80 wt%). The Al₂O₃ content has a wide range (6.83–18.27 wt%, average = 12.95 wt%), and is enriched in siltstones (average = 16.95 wt%) and in mudstones (average = 17.72 wt%), whereas the contents in sandstones from Liujiagou and Ermaying Formation are 8.09 wt% and 11.47 wt%, respectively. The Fe₂O₃ ranges from 0.72 wt% to 9.98 wt%, which is mainly concentrated in siltstones (7.70 wt%), supported by the field observations of their brown–red color and the presence of hematite in XRD analysis. The average K₂O/Na₂O ratio of the studied samples is 7.94, higher than the North American shale composite (NASC) value of 3.5. The high K₂O/Na₂O ratio is due to the predominance of K-feldspar over plagioclase, which is consistent with the petrographic results. The SiO₂/Al₂O₃ ratio shows an increase value from 3.80 in the siltstones and mudstones to 7.39 in the sandstones, suggesting that quartz rules over the coarse fraction while clay minerals dominate the fine fraction. Using the log (SiO₂/Al₂O₃) versus log (Fe₂O₃/K₂O) geochemical classification diagram of Herron [47], the sandstones are plotted in arkose, litharenite and wacke fields (Fig. 7). The result is generally consistent with the QFL petrographic classification diagram.

Table 2.

Major element concentrations of the Pisha Sandstone (wt%) from Ermaying, Heshanggou and Liujiagou Formation.

Sample No.	Ermaying										Heshanggou			Liujiagou
	sand stone	sand stone	sand stone	sand stone	sand stone	mud stone	sand stone	mud stone	sand stone	sand stone	silt stone	silt stone	silt stone	sand stone	sand stone
	Emy1	Emy2	Emy3	Emy4	Emy5	Emy6	Emy7	Emy8	Emy9	Emy10	Hsg1	Hsg2	Hsg3	Ljg1	Ljg2
SiO2	62.68	72.69	45.42	73.02	77.48	62.01	61.56	60.33	40.52	52.71	59.53	57.38	56.49	68.59	73.28
TiO2	0.44	0.36	0.51	0.25	0.42	0.87	0.59	0.63	0.09	0.18	0.61	0.88	0.63	0.21	0.29
Al2O3	12.96	11.15	11.71	6.21	9.55	16.65	13.60	15.57	6.67	9.76	15.08	15.91	14.84	6.69	8.10
TFe2O3	2.15	1.32	4.90	0.66	1.70	5.29	2.68	3.62	0.82	0.92	5.88	9.00	5.96	2.15	0.98
MnO	0.08	0.07	0.25	0.31	0.04	0.02	0.09	0.06	0.12	0.12	0.08	0.04	0.07	0.05	0.05
MgO	1.91	1.13	2.14	0.67	1.46	2.68	1.96	3.61	0.58	0.92	3.84	2.97	3.73	0.83	0.97
CaO	5.92	3.15	14.52	7.98	1.61	1.23	5.79	2.12	26.32	17.02	1.49	1.33	3.73	9.42	5.86
Na2O	2.27	1.99	0.13	0.13	0.14	0.10	2.30	1.61	1.52	2.05	1.35	0.12	1.40	0.16	0.19
K2O	2.95	3.16	1.92	1.61	1.60	2.19	3.19	2.93	2.17	3.06	2.60	2.44	2.65	2.04	2.58
P2O5	0.05	0.09	0.11	0.03	0.08	0.10	0.18	0.18	0.04	0.04	0.03	0.11	0.19	0.06	0.04
LOI	8.56	4.76	18.29	9.02	5.87	8.86	7.98	9.27	21.16	14.26	9.54	9.78	10.28	9.83	7.63
Total	99.97	99.86	99.92	99.89	99.95	100.02	99.93	99.93	100.01	101.04	100.04	99.95	99.98	100.03	99.97
CIA	54.87	52.81	82.35	74.14	81.31	85.96	55.23	64.75	47.54	49.26	67.47	83.94	66.46	71.11	70.38
CIW	63.44	63.04	96.46	93.58	95.36	97.98	64.24	74.58	57.08	59.13	77.21	97.51	76.27	92.89	92.99
PIA	56.67	54.16	95.72	91.30	94.40	97.65	57.27	70.03	46.30	48.88	73.37	97.03	72.16	89.75	89.67
ICV	1.21	1.00	2.06	1.82	0.72	0.74	1.21	0.93	4.72	2.47	1.05	1.05	1.22	2.21	1.34
Al2O3/SiO2	0.21	0.15	0.26	0.09	0.12	0.27	0.22	0.26	0.16	0.19	0.25	0.28	0.26	0.10	0.11
K2O/Na2O	1.30	1.59	14.70	12.42	11.33	21.01	1.39	1.81	1.42	1.49	1.93	19.76	1.89	13.09	13.92
Al2O3/TiO2	29.26	31.24	22.77	24.76	22.59	19.06	23.00	24.77	71.78	54.57	24.70	18.00	23.49	31.55	27.49
SiO2/Al2O3	4.84	6.52	3.88	11.76	8.11	3.72	4.53	3.87	6.07	5.40	3.95	3.61	3.81	10.25	9.05

All the calculated values were based on major-element data which were recalculated to LOI-free basis and adjusted to 100 %.

Fig. 7.

Geochemical classification of log (SiO₂/Al₂O₃)–log (Fe₂O₃/K₂O) diagram for the Pisha Sandstone.

4.3.2. Trace elements

The trace element contents of Pisha Sandstone in Jungar area are presented in Table 3. The concentrations of trace elements normalized by the average upper continental crust (UCC) values of [48] are shown in Fig. 8a. Concentrations of the large ion lithophile elements show wide variability within formations, apparently due to greater mobility of these elements during weathering, diagenesis and low–grade metamorphism [49]. Cs shows noticeable depletion in most samples. Rb contents in Liujiagou Formation (Rb = 66.9–73.6 ppm, average = 70.25 ppm) and Ermaying Formation (Rb = 50.8–128 ppm, average = 76.08 ppm) show a slightly depleted pattern with reference to UCC (84 ppm), but conversely relatively enriched in the siltstones from Heshanggou samples (average = 110.27 ppm). Ba values are in the range of 559–1190 ppm (average = 858.13 ppm) and exhibit an enriched pattern in the majority of samples. Sr contents scatter compared to the UCC values and no clear depletion is observed.

Table 3.

Trace element concentrations (ppm) of the Pisha Sandstone from Ermaying, Heshanggou and Liujiagou Formation.

Sample No.	Ermaying										Heshanggou			Liujiagou
	sand stone	sand stone	sand stone	sand stone	sand stone	mud stone	sand stone	mud stone	sand stone	sand stone	silt stone	silt stone	silt stone	sand stone	sand stone
	Emy1	Emy2	Emy3	Emy4	Emy5	Emy6	Emy7	Emy8	Emy9	Emy10	Hsg1	Hsg2	Hsg3	Ljg1	Ljg2
Sc	7.16	4.22	11.00	2.74	7.43	14.20	7.75	10.70	1.85	3.14	11.80	15.00	11.80	3.31	3.89
V	156.00	37.00	72.60	34.40	36.30	140.00	50.90	154.00	17.50	31.90	57.50	88.90	69.10	21.30	27.30
Cr	37.80	23.40	52.30	8.59	23.40	96.00	48.20	90.50	12.20	16.50	72.60	85.80	69.30	17.30	13.40
Co	7.54	3.69	14.70	4.38	10.80	24.40	6.45	14.10	2.30	3.37	14.40	19.10	14.50	2.59	2.24
Ni	12.60	6.49	42.30	2.61	10.20	24.60	12.60	24.40	3.87	5.94	30.10	40.80	28.30	4.59	3.17
Cu	9.33	7.68	19.70	3.02	3.40	69.10	11.60	107.00	2.85	27.00	28.50	25.80	22.00	5.13	4.02
Zn	38.10	19.90	52.10	14.80	35.50	169.00	33.00	75.80	8.87	18.80	68.40	95.20	71.10	18.30	18.70
Ga	14.30	11.20	14.20	6.50	11.30	24.70	13.60	20.80	6.56	9.67	19.70	20.70	19.60	7.58	8.75
Rb	80.00	83.10	80.30	50.80	55.20	128.00	76.20	87.20	51.50	68.50	101.00	140.00	89.80	66.90	73.60
Sr	487.00	371.00	248.00	160.00	165.00	159.00	456.00	418.00	312.00	378.0 0	496.00	229.00	420.00	150.00	163.00
Y	14.50	11.20	31.40	19.10	11.10	33.90	16.10	15.80	10.80	12.90	18.80	13.10	19.60	14.10	14.00
Cs	0.90	0.59	1.84	0.33	0.55	6.17	0.80	1.88	0.32	0.51	2.59	5.34	2.31	0.61	0.53
Ba	1040.00	1190.00	658.00	974.00	679.00	559.00	1070.00	961.00	750.00	988.00	795.00	643.00	937.00	721.00	907.00
Pb	12.10	13.10	11.30	28.30	11.20	27.10	12.80	11.80	9.48	10.70	19.70	16.40	19.50	13.00	11.90
Th	7.60	7.00	4.80	20.20	5.70	10.30	11.10	8.68	2.15	4.74	7.90	10.20	7.79	5.04	5.35
U	2.63	1.02	0.58	5.97	1.13	2.60	1.18	7.27	0.43	1.14	1.35	1.31	1.07	0.95	1.74
Nb	7.62	6.49	7.42	3.06	4.52	14.70	10.00	10.90	1.78	3.75	9.91	14.50	10.90	4.77	7.04
Zr	220.00	169.00	112.00	144.00	251.00	158.00	305.00	236.00	62.90	72.80	194.00	169.00	212.00	132.00	148.00
Hf	5.15	4.02	2.75	3.66	6.16	4.04	6.80	5.59	1.47	1.84	4.77	4.18	5.05	3.16	3.59
La/Sc	6.02	7.68	3.42	12.23	2.77	3.09	6.06	3.16	10.70	7.55	4.30	1.87	3.63	6.53	9.43
Cr/Th	4.97	3.34	10.90	0.43	4.11	9.32	4.34	10.43	5.67	3.48	9.19	8.41	8.90	3.43	2.50
Th/U	2.89	6.86	8.35	3.38	5.04	3.96	9.41	1.19	5.01	4.16	5.85	7.79	7.28	5.31	3.07
Zr/Sc	30.73	40.05	10.18	52.55	33.78	11.13	39.35	22.06	34.00	23.18	16.44	11.27	17.97	39.88	38.05
Th/Sc	1.06	1.66	0.44	7.37	0.77	0.73	1.43	0.81	1.16	1.51	0.67	0.68	0.66	1.52	1.38

Fig. 8.

(a) Multi–element normalized diagram to UCC for the Pisha Sandstone in Jungar area; (b) Chondrite–normalized REEs diagram for the studied samples, the post–Archean average Australian shales (PAAS) normalization values are from McLennan and Taylor [17].

High field strength elements, show similar distribution pattern, being more or less comparable to the UCC values. The normal to low concentrations of Zr and Hf relative to the values of UCC imply that few detrital zircons accumulate and sediment recycling occurs in Pisha Sandstone. However, a general enrichment of these elements in mudstones may be due to grain–size controlled fractionation of heavy minerals into finer sediments [9]. Due to the analogous depleted patterns in all samples, Nb and Ta show pronounced Nb–Ta troughs, which considered as an inherited character of arc magma [50].

Ferromagnesian trace elements (FMTEs), exhibit variable ranges relative to the UCC values. Ni shows obvious depletion in all formations (average = 16.84 ppm) relative to the UCC (47 ppm). Cr (44.49 ppm), V (66.31 ppm) and Sc (7.73 ppm) are dominantly depleted, which are somewhat enriched in mudstones and siltstones compared with sandstones. Overall, these elements do not exhibit enrichment feature in the sediments. FMTEs are compatible elements and enriched in ultramafic and mafic original rocks [51], hence it is speculated that ultramafic or mafic protoliths may be insignificant components of the source of Pisha Sandstone.

4.3.3. Rare earth elements

The result of rare earth elements (REEs) analysis is presented in Table 4. The chondrite–normalized REEs of Pisha Sandstone display light REE (LREE)–enriched and flat heavy REE (HREE) pattern (Gd_N/Yb_N = 1.24 to 2.26, average = 1.82), which are similar to the values of UCC (Gd_N/Yb_N = 1.62) and PAAS (Gd_N/Yb_N = 1.36, Fig. 8b). The LREE is characterized by high La_N/Yb_N (average = 15.24), La_N/Sm_N (average = 4.95) and ΣLREE/ΣHREE (average = 12.46) values, displaying fractionated features. The Eu/Eu* anomaly and Ce/Ce* anomaly of the studied samples were determined using the following equations: Eu/Eu* = Eu_N/(Sm_N* Gd_N)^0.5 and Ce/Ce* = Ce_N/(La_N * Pr_N)^0.5, where the subscript N denotes the chondrite–normalized values according to Taylor and McLennan [2]. The samples from three formations show slightly negative Eu anomalies (Eu/Eu* = 0.64–1.07, average = 0.82), which are further comparable to those of UCC and PAAS (Eu/Eu* = 0.72 and 0.66, respectively). The Ce/Ce* anomaly varies from 0.69 to 1.06 (average = 0.90). The average ΣREE content of the studied samples (146.73 ppm) is very comparable to the UCC value (148.14 ppm), whereas the results are relatively high in mudstones (ΣREE = 178.22 ppm) followed by siltstones (ΣREE = 172.26 ppm) and relatively low in sandstones (ΣREE = 128.39 ppm). The REE patterns of all formations are generally parallel to sub–parallel and there are no essential distinctions among the different stratigraphic units and lithology, indicating a homogenous source of Pisha Sandstone from varied formations.

Table 4.

Rare earth element concentrations (ppm) of the Pisha Sandstone from Ermaying, Heshanggou and Liujiagou Formation.

Sample No.	Ermaying										Heshanggou			Liujiagou
	sand stone	sand stone	sand stone	sand stone	sand stone	mud stone	sand stone	mud stone	sand stone	sand stone	silt stone	silt stone	silt stone	sand stone	sand stone
	Emy1	Emy2	Emy3	Emy4	Emy5	Emy6	Emy7	Emy8	Emy9	Emy10	Hsg1	Hsg2	Hsg3	Ljg1	Ljg2
La	43.10	32.40	37.60	33.50	20.60	43.90	47.00	33.80	19.80	23.70	50.70	28.00	42.80	21.60	36.70
Ce	71.60	61.90	70.80	67.30	46.00	80.20	80.10	62.50	25.10	40.30	91.60	60.90	61.60	31.60	66.00
Pr	9.27	6.75	8.58	6.93	5.00	10.60	10.30	8.17	3.71	4.88	10.20	6.80	9.06	4.19	7.34
Nd	33.00	23.70	32.30	24.10	19.10	39.90	37.00	30.40	13.20	17.70	36.10	24.70	33.20	14.90	25.30
Sm	5.17	3.60	6.28	3.63	3.46	7.33	5.82	5.12	2.11	2.98	5.61	4.24	5.50	2.48	3.94
Eu	1.15	0.89	1.86	0.69	0.81	1.70	1.23	1.12	0.68	0.83	1.25	0.91	1.23	0.63	0.90
Gd	3.68	2.59	5.85	2.95	2.72	6.40	4.13	3.81	1.78	2.46	4.20	3.19	4.30	2.18	3.03
Tb	0.50	0.36	0.97	0.45	0.37	0.93	0.56	0.53	0.26	0.37	0.61	0.46	0.62	0.35	0.44
Dy	2.59	1.94	5.57	2.68	1.99	5.36	2.95	2.83	1.51	2.13	3.34	2.51	3.43	2.11	2.41
Ho	0.48	0.39	1.12	0.57	0.39	1.08	0.56	0.55	0.31	0.43	0.66	0.49	0.68	0.44	0.47
Er	1.35	1.10	3.10	1.63	1.10	3.02	1.54	1.56	0.91	1.19	1.87	1.37	1.94	1.34	1.32
Tm	0.21	0.17	0.48	0.24	0.18	0.45	0.24	0.24	0.14	0.18	0.30	0.22	0.31	0.21	0.21
Yb	1.32	1.11	3.03	1.45	1.13	2.69	1.56	1.59	0.86	1.15	1.90	1.42	1.99	1.42	1.30
Lu	0.20	0.17	0.46	0.22	0.18	0.41	0.24	0.25	0.13	0.18	0.30	0.22	0.31	0.22	0.20
∑REE	173.62	137.07	178.00	146.35	103.04	203.97	193.24	152.47	70.51	98.48	208.63	135.43	166.97	83.68	149.56
LREE/HREE	15.80	16.50	7.65	13.35	11.78	9.03	15.39	12.42	10.94	11.18	14.84	12.70	11.30	9.11	14.95
Eu/Eu*	0.77	0.85	0.92	0.63	0.78	0.74	0.73	0.74	1.05	0.92	0.76	0.72	0.75	0.81	0.77
Ce/Ce*	0.81	0.94	0.90	0.99	1.04	0.85	0.82	0.86	0.65	0.84	0.90	1.01	0.70	0.74	0.90
La/Yb	32.65	29.19	12.41	23.10	18.23	16.32	30.13	21.26	23.00	20.61	26.68	19.72	21.51	15.21	28.23
LaN/YbN	22.06	19.72	8.39	15.61	12.32	11.03	20.36	14.36	15.54	13.93	18.03	13.32	14.53	10.28	19.08
LaN/SmN	5.25	5.66	3.77	5.81	3.75	3.77	5.08	4.16	5.91	5.01	5.69	4.16	4.90	5.48	5.86
GdN/YbN	2.26	1.89	1.56	1.65	1.95	1.93	2.15	1.94	1.68	1.73	1.79	1.82	1.75	1.24	1.89

5. Discussion

5.1. Source area weathering and paleoclimate

5.1.1. Source area weathering indicators

In addition to source composition, the chemistry of the sedimentary rocks is also controlled by the processes of weathering, transportation, diagenesis, and metamorphism [52,53]. Consequently, chemical composition of the clastic rocks can indicate source area weathering conditions. The chemical weathering degree of source area is generally quantified by CIA and CIW, sometimes in order to avoid the effect of K–metasomatism, PIA and ICV were employed. Commonly, CIA values for unweathered rocks are around 50 or below, CIA values of shales range between 70 and 75, while CIA values of residual clays with high kaolinite, gibbsite and chlorite contents are close to 100 [2]. Pisha Sandstone samples display medium to high CIA values (47.5–86, average = 67.2), and there is no significant variation among Liujiagou, Heshanggou and Ermaying Formation. In terms of lithology, most of the sandstones have markedly low CIA values (average = 59.4) relative to those of the siltstones and mudstones (average = 75.1 and 74.2, respectively). However, except for one sample (CIA value of Emy9 is 47.5), all other samples show higher CIA values than those of unweathered upper crust (the value is 48) [48], suggesting a moderate to strong weathering of source rocks. The CIW values of Pisha Sandstone (57.1–98, average = 80.1) are higher than CIA values, they also indicate the moderate to strong chemical weathering for most of the studied samples.

The Pisha Sandstone samples display variable K₂O/Na₂O values (1.3–21.01, average = 7.9), implying a secondary K–metasomatism for most of the samples, and hence the PIA value is calculated. There is no obvious PIA value difference among the three formations, and the average value is 80.1. However, the sandstone samples show low PIA values (average = 75.6), with higher values for the siltstones and mudstones (average = 86.9 and 85, respectively), reflecting moderate to intense source rocks weathering. The ICV is applied to measure the compositional maturity of clastic sediments. Compositionally immature sediments have high ICV values (>1), suggesting tectonically active settings and first–cycle deposits, whereas compositionally mature samples have low ICV values (<1), indicating that they may have experienced recycle sedimentation or intense weathering of first–cycle materials [4]. In this study, three samples (Emy5, Emy6 and Emy8) show low ICV values (0.7, 0.7 and 0.9, respctively) and one sample (Emy9) shows high ICV value (4.7), but the majority of samples have ICV values vary from 1.0 to 2.5 (average = 1.6). Thus, this suggests that Pisha Sandstone samples contain low clay mineral contents and belong to the first–cycle deposit or experience moderate to strong weathering in the source area. Furthermore, there is a clear positive correlation between TiO₂ and Al₂O₃ (r = 0.92), suggesting experience of chemical weathering of the source area materials.

The A1₂O₃–(CaO* + Na₂O)–K₂O (A–CN–K) ternary diagram of Nesbitt and Young [5] can be used to determine the mobility of elements during chemical weathering or K–metasomatism, where unweathered rocks plot along the left–hand side of the plagioclase–K-feldspar line (Fig. 9). The compositions of granites and granodiorites from the source area as well as UCC and PAAS are also shown for comparison. The plots of Pisha Sandstone show two distinct weathering trends. In the early stage of weathering, the data are nearly parallel to the A–CN line (the red arrow), reflecting that the plagioclase was firstly decomposed to illite, kaolinite or smectite, accompanied by leaching of calcium and sodium ions. When the CIA value is more than 70, the plagioclase is almost thoroughly weathered. With the increasing weathering degree, since decomposition of potassium–bearing minerals such as biotite, illite and K-feldspar, potassium is released, the trend line is gradually close to the A–K line (the blue arrow).

Fig. 9.

A–CN–K ternary diagram for Pisha Sandstone. The UCC and PAAS values are from Taylor and McLennan [2].

The Th versus Th/U diagram can also be used to reveal successive cycles of chemical weathering and the degree of sediment recycling. Pisha Sandstone samples show much wide ranges of Th/U ratios (1.19–9.41, average = 5.30), which is greater than 4.0 reflecting moderate to intense weathering in source area or sediment recycling. The plots of the Th/U versus Th diagram display two groups straddling the UCC value (Fig. 10a), in which one group (4 samples) has lower Th/U ratio and approaches those of depleted mantle sources, while the other group has higher Th/U ratio than those of UCC, following the weathering trend of McLennan et al. [33].

Fig. 10.

(a) Plots of Th/U versus Th for the Pisha Sandstone, revealing their subjection to varied degrees of weathering; (b) Plots of Th/Sc versus Zr/Sc, showing the degree of zircon enrichment resulting from sedimentary sorting and recycling.

In addition, the Th/Sc ratio can indicate chemical differentiation, while the Zr/Sc ratio is an indicator of sediment recycling [33]. In the first cycle a simple positive correlation is shown between Th/Sc and Zr/Sc ratios, whereas in recycled sediments increase in Zr/Sc ratio more rapidly than Th/Sc ratio is usually observed. On the Th/Sc versus Zr/Sc diagram (Fig. 10b) the samples basically follow the trend of magmatic compositional variation, whereas the sediment recycling seems to be insignificant.

5.1.2. Paleoclimate

Palaeoclimate research is significant for the origin interpretation of rocks and environmental changes in the future. Since chemical weathering usually occurs under humid or tropical conditions characterized by removing of mobile elements and enrichment of immobile elements, some geochemical proxies are suitable for paleoclimatic interpretation [5,16,54]. Plots on the Al₂O₃ + K₂O + Na₂O versus SiO₂ diagram of Suttner and Dutta [54] show most of the Pisha Sandstone samples fall within semi-humid, semi-arid to arid fields (Fig. 11). The result is generally consistent with geochemical proxies (CIA, CIW, PIA and ICV) calculated using major element reflecting that the studied samples experienced the moderate to strong chemical weathering. The above are well supported by the field surveys of Pisha Sandstone.

Fig. 11.

Plots of Al₂O₃ + K₂O + Na₂O versus SiO₂, revealing paleoclimate conditons of the Pisha Sandstone.

5.2. Provenance and source rock composition

5.2.1. Evidences from petrography

The QFL and QmFLt diagrams based on Dickinson et al. [44] show that most of the sandstones in Pisha Sandstone lie within the magmatic arc and transitional continental fields, while a few of them plot within the recycled orogenic fields (Fig. 6b and c). The most arkose and lithic arkose with the characteristics of common felsic rock clasts and absent mafic rock clasts indicate felsic source rocks for the Pisha Sandstone. Polycrystalline quartz grains with straight to slightly curved intercrystal boundaries (Fig. 5a) favor a source deriving from plutonic igneous rocks [55]. In addition, some quartz grains show partial inherited quartz overgrowths (Fig. 5i), reflecting recycled sedimentary quartz during diagenesis [56]. In sum, the compositional and textural features of the Pisha sandstone suggest a felsic source area supplemented by a small amount of recycled components.

5.2.2. Evidences from major elements

Provenance identification of siliciclastic sedimentary rocks has been carried out by using several geochemical discrimination diagrams [12,16,44,57,58]. Using plotting of major elements data on provenance discrimination diagram of Roser and Korsch [59], the majority of Pisha Sandstone samples lie in the quartzose sedimentary and felsic igneous provenance fields (Fig. 12a), which suggest possibly existing of some incorporation from recycled sediments or influenced by strong weathering.

Fig. 12.

(a) Classification plots of F1 and F2 for the Pisha Sandstone samples, revealing their subjection to varied degrees of weathering; (b) Plots of TiO₂ versus Al₂O₃ showing the provenance of the studied samples.

Since oxides and hydroxides of Al, Ti and Zr have low solubility in low temperature aqueous solutions, they are generally described as immobile elements and their ratios are very close to their source rocks. According to Hayashi et al. [10], Al₂O₃/TiO₂ ratio<8 indicates mafic source rock, Al₂O₃/TiO₂ ratio between 8 and 21 indicates intermediate igneous source rock, and Al₂O₃/TiO₂ ratio>21 indicates felsic igneous source rock. On the Al₂O₃ versus TiO₂ diagram, most of Pisha Sandstone samples lie in the felsic igneous rock field (Fig. 12b). Additionally, the weathering trends are parallel to the A–CN line [60,61], hence the A–CN–K ternary diagram can also be used to reflect source composition of source rocks. The array of the Pisha Sandstone samples intersects the feldspar join at positions corresponding to the composition between granodiorite and granite in Fig. 9.

5.2.3. Evidences from trace elements

Owing to the nature of relative immobility during post–depositional processes, several trace elements and REEs are employed to interpret provenance and source rock composition. In the La/Th versus Hf bivariate diagram [62], most studied samples of Pisha Sandstone plot on the felsic upper continental crust rock field, whereas a few samples plot on the mixed felsic–mafic or andesitic arc source (Fig. 13a), with a small amount of intermediate components and no input from recycled old sediments. Furthermore, no sample of hafnium (Hf) concentration is more than 10 ppm in Pisha Sandstone, a passive margin setting thus is less unlikely. The relative contents of Ni, V and Th can also be used to indicate sediment sources. On the Ni–V–Th*10 ternary diagram of Cullers [63], Pisha Sandstone samples are plotted near the felsic rock provenance (Fig. 13b). Based on the Cr/Th versus Th/Sc diagram [58], the majority of Pisha Sandstone samples are approximate to mixed source rocks (Fig. 13c), although a few samples have slightly higher Cr/Th ratios. The plots of Cr/V versus Y/Ni diagram from Hiscott [57] show a granite source (Fig. 13d).

Fig. 13.

(a) Provenance discrimination diagrams of the Pisha Sandstone. La/Th versus Hf diagram; (b) Ternary Ni–V–Th*10 diagram; (c) Cr/Th versus Th/Sc bivariate diagram; (d) Cr/V versus Y/Ni diagram.

Additionally, Co/Th versus La/Sc diagram of McLennan et al. [33] displays the studied samples mainly derived from felsic volcanic rock, granodiorite and granite (Fig. 14a). The plots of La/Yb versus REE [64] reflect that the Pisha Sandstone samples are sourced from granite (Fig. 14b). Meanwhile, the La/Sc, La/Co, Th/Co, Th/Sc and Cr/Th ratios of Pisha Sandstone are compared with sediments derived from the felsic and mafic source rocks (Table 5), which reveals the most samples within the range of felsic source rocks.

Fig. 14.

(a) La/Sc versus Co/Th bivariate diagram for the Pisha Sandstone; (b) Plots of La/Yb versus ∑REE showing the provenance of the studied samples.

Table 5.

Elemental ratios of the Pisha Sandstone compared to the range values of siliciclastic sediments derived from felsic and mafic rocks, and UCC.

Elemental ratios	Average of studied samplesa			Range of sediments from felsic sourcesb	Range of sediments from mafic sourcesb	UCCc
	Ermaying	Heshanggou	Liujiagou
Eu/Eu*	0.84	0.77	0.81	0.40–0.94	0.71–0.95	0.72
La/Sc	6.27	3.26	7.98	2.50–16.3	0.43–0.86	2.21
La/Co	5.37	2.65	12.36	1.80–13.8	0.14–0.38	1.79
Th/Sc	1.69	0.67	1.45	0.84–20.5	0.05–0.22	0.75
Th/Co	1.35	0.54	2.17	0.67–19.4	0.04–1.40	0.61
Cr/Th	5.7	8.83	2.97	4.00–15.0	25–500	8.76

^aThis study.

^bCullers [63,65]; Cullers and Podkovyrov [66].

^cRudnick and Gao [48].

5.2.4. Source rock compositon

The source area of clastic sediments of the Late Permian to Triassic in the central and northern parts of the Ordos Basin has been widely investigated, indicating that the source rocks of these sediments are mostly derived from the Late Paleozoic intrusive rocks in the Inner Mongolia Paleo–Uplift at the northern part of the North China Craton [67,68]. This finding also seems to apply to the study area. The average Eu/Eu* values of Pisha Sandstone samples from Ermaying, Heshanggou and Liujiagou Formation (0.84, 0.77 and 0.81, respectively) are similar to the values of Late Paleozoic granitoids in north margin of the North China Craton, as indicated by a comparison (Fig. 15) of the REE patterns in Pisha Sandstone with the results from several researchers [[69], [70], [71], [72]].

Fig. 15.

The comparison of chondrite–normalized REE diagrams. (a) Average REE values of the Pisha Sandstone from Liujiagou, Heshanggou and Ermaying Formaiton; (b) Possible source areas from the northern Inner Mongolia Paleo–Uplift. The chondrite–normalized values from Taylor and McLennan [2], data of SR1 from Zhang et al. [70], SR2 from Zhang and Jian [71], SR3–5 from Zeng et al. [60] and SR6–8 from Wang et al. [72].

The Inner Mongolia Paleo–Uplift, an Andean–style continental margin arc, was exhumed more than 15 km of rocks during the late Paleozoic to early Mesozoic [35,73] and provided a large volume of sediments to the Ordos Basin from Permian Shanxi Formation to Triassic Yanchang Formaion. Hence, the exposed large areas of the intrusive rocks from the Inner Mongolia Paleo–Uplift composed of granodiorite, granites, tonalite and quartz diorite, which are considered to be late Paleozoic to early Mesozoic in age, is a possible provenance.

5.3. Tectonic setting

Bulk–rock geochemical characteristics of siliciclastic sediments has been applied to distinguish the tectonic settings of depositional basins [3,12,13,74]. Therefore, many researchers have attempted to adopt the major, trace and REE element geochemistry to diagnose the tectonic conditions and construct discrimination diagrams or criteria that help to decode the history of sedimentary basins [33]. Plotting on the K₂O/Na₂O versus SiO₂ discrimination diagram (Fig. 16a) [13] show that most the Pisha Sandstone samples fall within active continental margin and passive margin settings. The plots of discriminant scores along Function I versus Function II (Fig. 16b) [12] also reveal that the studied samples are divided into active continental margin and passive margin two groups. Sometimes, the same geological unit sediments had deposited in different tectonic settings may be caused by the most mobile major elements like Na and K. Hence, these discriminant diagrams should be used cautiously [75].

Fig. 16.

(a) K₂O/Na₂O versus SiO₂ discrimination diagram showing the main tectonic settings of the Pisha Sandstone; (b) Plot of discriminant scores along Function I versus Function II, revealing that the studied samples are divided into active continental margin and passive margin.

In contrast, some trace elements are considered to be more reliable in discriminating tectonic settings because of their immobile features under depositional conditions. In the La–Th–Sc, Th–Co–Zr/10 and Th–Sc–Zr/10 ternary diagrams [3], the majority of the data plot in or slightly around the field of continental island arc (Fig. 17). In these ternary diagrams, most of the Pisha Sandstone samples are discriminated into continental island arc setting, whereas two samples are close to passive margin field. However, it should be noted that the results obtained from these diagrams are sometimes considered to be in disagreement with the geological features of the research areas [76].

Fig. 17.

Ternary diagrams of (a) La–Th–Sc; (b) Th–Co–Zr/10; (c) Th–Sc–Zr/10, used for discriminating tectonic settings of the Pisha Sandstone. OA: Oceanic island arc; CA: Continental island arc; ACM: Active continental margin; PM: Passive margin.

Recently, Verma and Armstrong-Altrin [77] have re–evaluated these discrimination diagrams and proposed new multidimensional tectonic discrimination diagrams. They had constructed the tectonic discrimination diagrams of low–silica [(SiO₂)_adj = 35–63 wt%] and high–silica [(SiO₂)_adj = 63–95 wt%] rocks to divide main tectonic settings into island or continental arc, continental rift and collision, where (SiO₂)_adj value is obtained by adjusting the ten major elements to 100 wt% after removing the LOI. All the studied samples are plotted on the collision field on the high–silica and low–silica multidimensional diagrams (Fig. 18). In addition, several trace and REE characteristic parameters from Bhatia [12] and Bhatia and Crook [3] are employed to decipher the tectonic settings of sedimentary basins (Table 6). The average of the Pisha Sandstone samples show a relatively better correlation with discrimination parameters for a mixture setting of continental island arc and active continental margin, in which the REE parameters are prone to continental island arc setting.

Fig. 18.

Tectonic discrimination diagrams for the Pisha Sandstone. (a) High–silica [(SiO₂)_adj = 63–95 wt%]; (b) Low–silica [(SiO₂)_adj = 35–63 wt%].

Table 6.

Trace and REE characteristic parameters comparison of the Pisha Sandstone from various tectonic settings.

	Oceanic island arc	Continental island arc	Active continental margin	Passive margin	Ermaying	Heshanggou	Liujiagou
La/Y	0.48 ± 0.12	1.02 ± 0.07	1.33 ± 0.09	1.31 ± 0.26	2.1	2.34	2.1
La/Th	4.26 ± 1.2	2.36 ± 0.3	1.77 ± 0.1	2.2 ± 0.47	5.0	4.89	5.6
La/Sc	0.55 ± 0.22	1.82 ± 0.3	4.55 ± 0.8	6.25 ± 1.35	6.3	3.26	8.0
Ti/Zr	56.8 ± 21.4	19.7 ± 4.3	15.3 ± 2.4	6.74 ± 0.9	14.7	21.48	9.9
Sc/Cr	0.57 ± 0.16	0.32 ± 0.06	0.3 ± 0.02	0.16 ± 0.02	0.2	0.17	0.2
La	8 ± 1.7	27 ± 4.5	37	39	33.5	40.5	29.2
Ce	19 ± 3.7	59 ± 8.2	78	85	60.6	71.4	48.8
∑REE	58 ± 10	146 ± 20	186	210	145.7	170.3	116.6
La/Yb	4.2 ± 1.3	11 ± 3.6	12.5	15.9	22.7	22.6	21.7
(La/Yb)N	2.8 ± 0.9	7.5 ± 2.5	8.5	10.8	15.3	15.3	14.7
∑LREE/∑HREE	3.8 ± 0.9	7.7 ± 1.7	9.1	8.5	12.4	12.9	12.0
Eu/Eu*	1.04 ± 0.11	0.79 ± 0.13	0.6	0.56	0.84	0.77	0.81

Data from Bhatia and Crook [3] and Bhatia [12].

In the Late Permian, several Paleozoic island arcs and accretion–subduction complexes (named “Mongolian arc terranes”) collided with the northern part of the North China Craton [73], which was subsequently followed by post–collisional extension and magmatic activities, and then provided massive sediments for the Ordos Basin [[69], [70], [71], [72]]. Combined with the results obtained from the geochemical criteria and tectonic discrimination diagrams, a continental island arc or active continental margin setting is proposed for the Pisha Sandstone. Actually, continental island arc and active continental margin are similar tectonic settings, since they are both characterized by development of convergent plates, orogenic and subduction zones, and then are underlain by continental crust. Consequently, a continental island arc–related post–collision tectonic setting is the most probable setting for the source rocks of the Pisha Sandstone. This conclusion is in agreement with the regional geologic evolution by Qiao et al. [78] who reported that from the Early Triassic Liujiagou Formation to Heshanggou Formation the tectonic settings of the source areas have gone through the transition from the active continental margin to the continental island arc.

6. Conclusions

Geochemical characteristics of the Pisha Sandstone are significant for comprehensively understanding formation mechanism and principle of bedrock erosion of these loose clastic rocks. In this study, we present a very first petrographic and geochemical data together with detailed field geological characteristics of the Pisha Sandstone in northeast Ordos Basin, and establish several indicators to unravel weathering history, provenance and tectonic setting of the Pisha Sandstone. The results show that the geochemical indices including CIA, CIW, PIA and ICV indicate the first–cycle deposit and moderate to strong chemical weathering for most of the Pisha Sandstone samples. These sediments experienced two distinct weathering trends, in which one reflects decomposition of plagioclase firstly and the other refers to decomposition of potassium–bearing mineral in the late stage. Different proxies such as Al₂O₃ versus TiO₂, La/Th versus Hf, La/Sc, La/Co, Th/Co, Th/Sc and Cr/Th reveal a felsic source rock (granite and granodiorite) of Upper Continental Crust for the Pisha Sandstone, with a little mafic or intermediate components. Source rock compositions of granodiorite and granite from the north margin of the Inner Mongolia Paleo–Uplift are identified based on the diagrams of A–CN–K, Cr/V versus Y/Ni, Co/Th versus La/Sc and La/Yb versus REE combined with comparison patterns in REEs. Meanwhile, the tectonic discrimination diagrams display a collision setting for the Pisha Sandstone originally deposited in a continental island arc setting. In the future, researches will be concentrated on the assessment of weathering conditions and the relationship between weathering of the Pisha Sandstone and soil formation. This work highlights basic geological materials for the Pisha Sandstone and may provide new theoretical basis for environmental protection in the Pisha Sandstone area.

Funding

This research was supported by National Natural Science Foundation of China (42207392, 41967008), Natural Science Foundation of Inner Mongolia Autonomous Region (2020BS04006, 2023LHMS05025 and 2021ZD07), Program for Improving the Scientific Research Ability of Youth Teachers of Inner Mongolia Agricultural University (BR220134) and Program for High–level Talent Introduction of Inner Mongolia Agricultural University (NDYB2019–36).

Data availability statement

Data will be made available on request.

CRediT authorship contribution statement

Shuhua Fan: Writing – review & editing, Writing – original draft, Methodology, Investigation, Conceptualization, Data curation, Formal analysis, Funding acquisition. Fucang Qin: Writing – review & editing, Funding acquisition, Conceptualization, Supervision. Zhihui Che: Writing – review & editing, Supervision, Investigation, Funding acquisition.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.