Open Access Published by De Gruyter Open Access February 28, 2020

Provenance and tectonic significance of the Zhongwunongshan Group from the Zhongwunongshan Structural Belt in China: insights from zircon geochronology

Yuan Peng, Yongsheng Zhang, Eenyuan Xing and Linlin Wang
From the journal Open Geosciences

Abstract

The Zhongwunongshan Structural Belt (ZWSB) locates between the Olongbruk Microblock of North Qaidam and the South Qilian Block in China, and it has important implication for understanding the tectonic significance of North Qaidam. Nowadays, there are few discussion on the Caledonian tectonothermal events of the Zhongwunongshan Structural Belt, and there exist different opinions on provenance and tectonic environment of the Zhongwunongshan Group in the ZWSB and its adjacent North Qaidam. In this study, a comprehensive analysis of the detrital zircon geochronological research was carried out on the Zhongwunongshan Group. The detrital zircon U-Pb dating results showed two major populations. The first was Neoproterozoic (966-725 Ma) with a Hf(t) = −15.9 to 9.5, and the other was late Early Paleozoic (460-434Ma) with a Hf(t) = −9.6 to −3.1. In combination with previous research, the dominated provenances were found to be the Neoproterozoic granitic gneiss of the Yuqia-Shaliuhe HP-UHP metamorphic belt and the late Early Paleozoic granite of the Tanjianshan ophiolite-volcanic arc belt in North Qaidam. The Zhongwunongshan Group was deposited in the back-arc sedimentary basin related to the Caledonian collisional orogeny during Middle Silurian-Early Devonian (434-407.9 Ma).

1 Introduction

The Zhongwunongshan Structural Belt (ZWSB) is a northwest-extending structural belt located between the Olongbruk Microblock of North Qaidam and the South Qilian Block [1, 2] (Figure 1). The ZWSB is separated from the South Qilian Block to the north and the Olongbruk Microblock to the south by the Qinghai Nanshan Fault and the Southern Zhongwunongshan Fault, respectively. It extends westward to the Altun Fault, and divides between the West Qingling and South Qilian in the east. Therefore, the geological position of the Zhongwunongshan Structural Belt is significant. It is generally believed that the Zhongwunongshan Structural Belt is a Hercynian-Indosinian tectonic zone developed on the Caledonian block jointly constructed by the Olongbruk Microblock of the North Qaidam and the South Qilian Block [5, 6, 7]. At present, there are few data reported the Caledonian tectonic thermal events and evolution of the ZWSB.

Figure 1 (a): Map showing the location of the North Qaidam. (b): Tectonic units of the North Qaidam. (c): Simplified geological map of the study area (modified after [3, 4])

Figure 1

(a): Map showing the location of the North Qaidam. (b): Tectonic units of the North Qaidam. (c): Simplified geological map of the study area (modified after [3, 4])

The Zhongwunongshan Group is the most important and widely exposed formation in the ZWSB and adjacent North Qaidam. Therefore, the Zhongwunongshan Group is determined to be the ideal object for the tectonic research of the ZWSB. The previous authors carried out a certain research on Zhongwunongshan Group, and most of them believe that it deposited in the Late Carboniferous rift [8, 9, 10]. However, other geologists have had different viewpoints about the tectonic environments and basin types of the Zhongwunongshan Group deposited in. There are two primary views about tectonic environment and basin type: (1) The Zhongwunongshan Group deposited in a stable passive continental margin basin, which irrelevant to plate subduction and collision [7, 10, 11]; and (2) Due to plate subduction and collision, the sediments deposited in the back-arc sedimentary basin along the North Qaidam active continental margin [9, 12, 13]. A consensus in the detritus provenance has also not been reached. Some scholars suggest the provenance was located in the uplift terrane along the southern margin of the South Qilian Block to the north [11, 14, 15], but the others hold the opinion that, the southern part of North Qaidam provided sediments northward [9, 13]. Meanwhile, Geologists also held different opinions about the stratigraphic age of Zhongwunongshan Group. Some believed the strata deposited in Late Carboniferous as indicated by the fusulinids and corals [16]. However, the fossils identification were challenged because of their limited occurrence and metamorphic recrystallization.

Clastic rocks contain lots of useful information, including the material composition and tectonic environment of the source area, and the crustal evolution in the early geological history [17, 18]. The zircon geochronology has been widely used to reveal the petrogenesis of clastic rocks [19, 20], and to reconstruct the characteristics and tectonic environment of source areas [20, 22, 23, 24]. Since sandstone and mudstone are relatively homogeneous and have high level of trace element, they become the first choice for geochemical and chronological studies [21, 25, 26]. Epimetamorphic clastic rocks have also been widely used to study the petrogenesis and tectonic environments of source areas [20, 27]. Based on the previous research with the new geochronology analysis of the Zhongwunongshan Group clastic rocks, this paper aims at providing new data to reconstruct the provenance and tectonic setting of Zhongwunongshan Group, and attempting to find the geological records of the Caledonian tectonic thermal events in order to provide a reasonable explanation for the aforementioned problems.

2 Geological background

Recent studies have well demonstrated that the North Qaidam Structural Belt (NQSB), bounded by the South Qilian Block to the north and Qaidam Block to the south (Figure 1), is a composite tectonic orogenic belt with multi-stage orogenic processes [2, 28]. The NQSB consists of the Shaliuhe-Yuqia high pressure-ultrahigh pressure (HP-UHP) metamorphic belt, the Tanjianshan ophiolitevolcanic arc belt, the Olongbruk Microblock (Figure 1b), from south to north, in view of the significantly diverse geological composition and tectonic evolutionary history [28, 29]. The complex tectonic composition indicates that the sub-tectonic units that make up the NQSB underwent different tectonic evolutionary phase predating the pre-Devonian, including the Paleoproterozoic (2400–1800 Ma) continental crust formation [2, 28]; Neoproterozoic (1030–750 Ma) magmatic activities related to Rodinia supercontinent convergence and breakup [30, 31]; early Paleozoic (542–434Ma) magmatism and HP-UHP metamorphism by oceanic crust subduction followed by collision associated continental deep-subduction [3, 32]. The continental deep subduction marked the assembly of Olongbruk Microblock and the Qaidam Block. Subsequently, the NQSB evolved into the post-orogenic stage indicated by the rifting molasse of the Upper Devonian Maoniushan Group.

The Shaliuhe-Yuqia HP-UHP metamorphic belt is the northern border separating the Qaidam Block from the NQSB (Figure 1b). The belt is predominantly composed of Neoproterozoic-Early Paleozoic granitic gneiss (1011–434Ma) and metamorphic supracrustal rock, with minor sporadically exposed eclogite (1000–800 Ma) [29, 33]. The Tanjianshan ophiolite-volcanic arc belt, to the north of HP-UHP metamorphic belt, is dominated by widespread Early Paleozoic intermediate-basic volcanic (486–450 Ma) and pyroclastic rocks with ophiolite complex, which are intruded by granitic intrusive rocks (465–434 Ma) [4, 34] (Figure 1b). The Olongbruk Microblock has an early Paleoproterozoic crystalline basement including the ~2400 Ma Delingha granitic gneiss and the ~1800 Ma Khondalite– like supracrustal rocks (Figure 1b). Above the basement are a series of unmetamorphosed sedimentary successions that were discontinuously deposited from late Paleoproterozoic to Cenozoic [4, 8, 11, 34, 35, 36].

This study focuses on the ZWSB and the adjacent North Qaidam (Figure 1b). The ZWSB widely outcrops the Upper Carboniferous Zhongwunongshan Group comprising carbonate dominated sedimentary successions with some volcanic interlayers, which are overlain by Permian-Triassic shallow marine clastic rocks and carbonates [6, 8, 14] (Figure 1c). The Zhongwunongshan Group have been further subdivided into two lithostratigraphic units. The lower section is predominately composed of gray green phyllite and sandy slate, with minor medium-basic volcanic layers and limestone lenses,whichwere deposited in the neritic shelf environment. The upper unit mainly consists of gray white banded limestone and dolomitic limestone with gray green intermediate-basic volcanic interlayers, typically formed in sublittoral platform environment (Figure 2).

Figure 2 Stratigraphic column of the Zhongwunongshan Group

Figure 2

Stratigraphic column of the Zhongwunongshan Group

In this study, two samples (KDLG1505 and BLG1505) were collected from the sandstones of Zhongwunongshan Group exposed in the Baluogenguole (BLG) and Kendelonggou (KDLG) areas (Figure 1c), respectively.

3 Analytical Methods

3.1 Zircon geochronology analysis

The detrital zircons were separated using a conventional heavy liquid and magnetic separation techniques, and then further purified by handpicking under a binocular stereoscope. Subsequently, the grains were mounted in epoxy, and polished to expose the cores of the grains. Cathodoluminescence (CL) images were obtained for all zircons prior to analysis, in order to characterize the internal structures and to choose potential sites for in the in situ U-Pb dating and Lu-Hf analyses.

This zircon U-Pb dating was conducted in the Key Laboratory of Metallogeny and Mineral Assessment of the Ministry of Nature Resources, which is located at the Institute of Mineral Resources of the Chinese Academy of Geological Sciences. The zircon geochronology was performed by Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS), on a Finnigan Neptune equipped with Newwave UP213 laser ablation system, with an ablation spot diameter of 30 μm, frequency of 10 Hz, and energy density of approximately 2.5 J/cm2. Helium was used as the carrier gas. Zircons SRM610, Plesovice, and 91500 were used as the external standards for correcting the mass discrimination and isotope fractionation. The detailed analytical conditions are described in [37]. An ICPMSData-Cal program was used for the data processing [38], and the Concordia diagrams were obtained using an Isoplot4.0 routine [39]. Therefore, in this study a maximum of 80 individual zircon grains were selected randomly from each of the samples. Among the 80 grains, we excluded zircon age analyses with >10% discordance for interpretation.We report the ages derived from 206Pb/238U for < 1000 Ma and 207Pb/206Pb for = 1000 Ma. Visualization of U-Pb zircon ages is achieved using Isoplot 4.0 [39] and Density Plotter program [40].

3.2 Zircon Hf isotope analysis

The Zircon in situ Lu-Hf analysis was completed in the State Key Laboratory of Continental Tectonics and Dynamics of the Institute of Geology, Chinese Academy of Geological Sciences, utilizing a Neptune Plus MC-ICP-MS instrument, and a Complex pro.193nmlaser-ablation system. The ablation spot diameter was 44 μm. The global zircon standard GJ-1 was taken as the reference material, and the analytical points were the same as those of the U-Pb dating. The detailed operating conditions for the LA-MC-ICP-MS and calibration strategy followed those used by [41]. The LA-MC-ICP-MS analyses of the zircon standard GJ-1 during the analytical process achieved the 176Hf/177Hf ratio of 0.282015 ± 8 (2σ, n = 10), which was identical to the literature values within error [42].

4 Results

4.1 Zircon U-Pb dating

The zircon grains from sample BLG1505 range in size from 50 to 100 μm with length/width ratios of 2:1–1:1. These zircons generally display sub-angular to rounded morphology. In CL images, most zircon grains have clear oscillatory zonings, with few zircons showing week zoning or being structureless (Figure 3). All dated zircons show high Th/U ratios of 0.26–2.05, low LREE/HREE (<1) ratios and REE+Y content (<1%), positive Ce and negative Eu anomalies (Table 1), which jointly indicate a magmatic origin [43, 44]. The sample BLG1505 yields 67 concordant U-Pb ages varying from 2456 Ma to 634 Ma with one major age cluster (Table 1, Figure 4a). The most prevailing age cluster of 925–725 Ma contains 53 data and defines two age peaks at 831 Ma and 804 Ma. Additionally, few zircons of Paleo- and Mesoproterozoic age are also present, with subordinate peaks at 2398 Ma and 1746 Ma (Figure 4b).

Figure 3 Cathodoluminescence images (CL) of the representative analyzed detrital zircons and their corresponding U-Pb ages from the Zhongwunongshan Group sandstone samples (BLG1505 and KDLG1505). The open circles represent the positions of ablation pits

Figure 3

Cathodoluminescence images (CL) of the representative analyzed detrital zircons and their corresponding U-Pb ages from the Zhongwunongshan Group sandstone samples (BLG1505 and KDLG1505). The open circles represent the positions of ablation pits

Table 1

Microelement eigenvalues of zircons from the Zhongwunongshan Group sandstones

Sample Spot Σ REE+Y δEu δCe LREE/HREE Sample Spot Σ REE+Y δEu δCe LREE/HREE
Sample BLG1505 Sample KDLG1505
BLG1505-1 984.65 0.30 154.39 0.10 KDLG1505-01 872.27 0.84 1.13 0.43
BLG1505-2 2242.89 0.48 56.63 0.03 KDLG1505-02 870.60 0.23 56.28 0.04
BLG1505-3 1227.22 0.36 18.87 0.03 KDLG1505-03 1237.64 0.40 78.44 0.08
BLG1505-4 3122.88 0.31 5.58 0.02 KDLG1505-04 1364.93 0.36 3.68 0.13
BLG1505-6 935.13 0.50 1.37 0.11 KDLG1505-06 1165.79 0.32 3.50 0.15
BLG1505-7 1891.89 0.67 7.39 0.02 KDLG1505-07 1454.27 0.24 4.65 0.04
BLG1505-8 1956.24 0.36 85.73 0.05 KDLG1505-08 957.66 0.23 56.28 0.04
BLG1505-9 2541.56 0.60 12.42 0.04 KDLG1505-09 2154.00 0.28 4.97 0.05
BLG1505-10 1732.07 0.70 57.82 0.02 KDLG1505-10 1296.68 0.36 3.68 0.13
BLG1505-11 1137.40 0.50 33.45 0.02 KDLG1505-11 2154.00 0.28 4.97 0.05
BLG1505-12 3328.52 0.25 208.57 0.04 KDLG1505-12 3307.58 0.11 1.05 0.12
BLG1505-14 1479.85 0.49 46.33 0.03 KDLG1505-14 914.13 0.23 56.28 0.04
BLG1505-15 3899.46 0.16 1.07 0.09 KDLG1505-15 655.49 0.27 26.48 0.04
BLG1505-16 836.95 0.30 26.55 0.06 KDLG1505-16 1574.60 0.12 69.11 0.03
BLG1505-17 852.27 0.52 76.20 0.14 KDLG1505-17 1508.14 0.44 11.42 0.06
BLG1505-18 3793.98 0.10 14.83 0.04 KDLG1505-18 1099.85 0.25 58.76 0.04
BLG1505-19 1901.71 0.41 98.29 0.03 KDLG1505-19 1724.24 0.14 1.33 0.10
BLG1505-20 1471.52 0.11 132.46 0.04 KDLG1505-20 956.39 0.25 58.76 0.04
BLG1505-22 2451.73 0.88 10.12 0.03 KDLG1505-21 2239.38 0.11 1.00 0.10
BLG1505-23 4735.18 0.49 6.95 0.01 KDLG1505-22 1237.64 0.40 78.44 0.08
BLG1505-25 7874.87 0.38 75.37 0.03 KDLG1505-23 984.18 0.19 2.61 0.04
BLG1505-28 1536.70 0.23 19.74 0.09 KDLG1505-24 1358.25 0.39 4.30 0.16
BLG1505-29 2272.13 0.08 5.10 0.01 KDLG1505-25 2239.38 0.11 1.00 0.10
BLG1505-32 3145.49 0.58 6.29 0.06 KDLG1505-26 1436.78 0.46 22.59 0.07
BLG1505-33 3498.16 0.19 14.17 0.04 KDLG1505-27 927.26 0.32 1.64 0.20
BLG1505-34 2030.05 0.20 17.03 0.02 KDLG1505-28 1547.05 0.40 78.44 0.08
BLG1505-35 3027.49 0.11 1.72 0.04 KDLG1505-29 12019.09 8.48 0.76 0.10
BLG1505-36 839.41 0.60 11.94 0.03 KDLG1505-30 14048.07 4.49 0.60 0.01
BLG1505-39 580.31 0.23 40.26 0.07 KDLG1505-31 1225.84 0.17 43.46 0.01
BLG1505-40 1876.54 0.10 1.51 0.07 KDLG1505-32 1182.91 0.17 1.36 0.14
BLG1505-41 2089.35 0.15 1.48 0.06 KDLG1505-33 914.13 0.23 56.28 0.04
BLG1505-42 1063.46 0.41 109.51 0.02 KDLG1505-34 1086.01 0.20 2.90 0.05
BLG1505-43 1908.81 0.13 33.69 0.03 KDLG1505-35 2110.92 0.28 4.97 0.05
BLG1505-44 2143.49 0.47 3.70 0.03 KDLG1505-36 2632.39 0.11 2.53 0.09
BLG1505-45 1678.47 0.15 4.02 0.03 KDLG1505-40 1243.55 0.37 14.64 0.06
BLG1505-47 2685.93 0.31 242.71 0.03 KDLG1505-41 957.66 0.23 56.28 0.04
BLG1505-48 672.32 0.20 84.41 0.06 KDLG1505-42 1086.07 0.13 7.95 0.03
BLG1505-50 1268.37 0.80 50.15 0.10 KDLG1505-43 1200.39 0.13 7.95 0.03
BLG1505-51 5687.00 0.29 4.16 0.03 KDLG1505-44 1301.33 0.17 43.46 0.02
BLG1505-52 1647.42 0.51 1.83 0.08 KDLG1505-45 2261.70 0.28 4.97 0.05
BLG1505-53 2787.27 0.67 8.64 0.08 KDLG1505-46 2046.30 0.28 4.97 0.05
BLG1505-54 4354.37 0.58 1.04 0.80 KDLG1505-47 966.36 0.23 56.28 0.04
BLG1505-55 3269.21 0.62 1.14 0.43 KDLG1505-49 1332.37 0.39 4.30 0.16
BLG1505-56 3138.72 0.23 7.90 0.03 KDLG1505-50 1245.17 0.17 1.36 0.14
BLG1505-57 870.63 0.81 38.01 0.04 KDLG1505-51 1034.29 0.20 2.90 0.05
BLG1505-58 1916.87 0.57 22.75 0.03 KDLG1505-52 1346.12 0.17 20.68 0.04
BLG1505-60 1606.86 0.28 3.85 0.05 KDLG1505-53 1637.92 0.36 3.68 0.13
BLG1505-61 1332.75 0.37 36.83 0.03 KDLG1505-55 1221.72 0.32 2.86 0.24
BLG1505-62 1985.51 0.63 2.80 0.03 KDLG1505-56 784.81 0.20 53.91 0.03
BLG1505-63 1788.09 0.47 50.54 0.02 KDLG1505-58 1508.14 0.44 11.42 0.06
BLG1505-64 2791.14 0.26 1.21 0.06 KDLG1505-59 1440.29 0.13 5.60 0.04
BLG1505-65 1131.07 0.07 1.11 0.22 KDLG1505-60 1456.91 0.22 98.12 0.03
BLG1505-66 1240.06 0.15 33.60 0.03 KDLG1505-61 1648.08 0.03 23.06 0.01
BLG1505-67 2157.51 0.38 11.21 0.02 KDLG1505-62 668.87 0.27 26.48 0.04
BLG1505-68 1624.76 0.20 21.89 0.04 KDLG1505-64 1130.44 0.12 128.36 0.06
BLG1505-69 1825.65 0.26 5.89 0.08 KDLG1505-65 2027.76 0.32 3.71 0.09
BLG1505-70 1275.80 0.02 36.87 0.03 KDLG1505-67 842.27 0.32 1.28 0.42
BLG1505-71 991.14 0.44 9.89 0.03 KDLG1505-68 2606.20 0.03 20.30 0.01
BLG1505-72 1867.45 0.12 1.52 0.09 KDLG1505-69 1109.86 0.33 36.85 0.06
BLG1505-73 1629.06 0.19 26.61 0.04 KDLG1505-70 1365.42 0.48 183.22 0.09
BLG1505-74 832.37 0.62 104.84 0.04 KDLG1505-71 628.30 0.29 1.75 0.09
BLG1505-75 1125.07 0.45 101.16 0.05 KDLG1505-72 2280.25 0.21 26.12 0.03
BLG1505-76 2615.70 0.46 53.32 0.02 KDLG1505-74 1657.44 0.15 9.50 0.04
BLG1505-77 2792.51 0.39 87.72 0.05 KDLG1505-75 846.18 0.13 55.96 0.03
BLG1505-78 1583.42 0.07 141.86 0.03 KDLG1505-76 3222.01 0.11 1.01 0.12
BLG1505-79 1942.86 0.36 77.16 0.14 KDLG1505-77 1282.74 0.10 23.30 0.02
BLG1505-80 1452.64 0.60 1.89 0.34 KDLG1505-78 1729.99 0.42 9.52 0.02
KDLG1505-79 2018.77 0.11 9.96 0.01
KDLG1505-80 956.39 0.25 58.76 0.04
Figure 4 U-Pb Concordia diagrams and age histograms of the detrital zircons from the Zhongwunongshan Group sandstone samples (BLG1505 and KDLG1505)

Figure 4

U-Pb Concordia diagrams and age histograms of the detrital zircons from the Zhongwunongshan Group sandstone samples (BLG1505 and KDLG1505)

The detrital zircons, in sample KDLG1505, are sub-angular to sub-rounded shapes with length/width ratios of 2:1–1:1. Except for the No. 29 and No. 30 analytical points, the majority of zircons shows well-developed oscillatory zoning or plate internal structure (Figure 3), with Th/U ratios greater than 0.1 (0.26–2.42). Besides, these zircons have similar REE concentration features with sample BLG1505, indicating the magmatic origin [44] (Table 1). Sixty-nine concordant zircon U-Pb ages of 2688–420 Ma were obtained from 69 zircon grains (Table 2, Figure 4c). These concordant ages are mainly grouped into two distinct age populations of 460–434 Ma and 966–726 Ma, defining dominant peaks at 456 Ma with 21 data, and secondary peaks of 828 Ma, 730 Ma and 956 Ma with 24 data (Figure 4d). About one-third of the grains are older than 1500 Ma with peaks at 2540 Ma and 1799 Ma.

Table 2

LA-ICP-MS U-Pb analyses of zircons from the Zhongwunongshan Group sandstones

Sample Spot ωB/ppm Th/U Isotropic ration Apparent age (Ma)
Th U 207Pb/206Pb 207Pb/235U 206Pb/238U 207Pb/206Pb 207Pb/235U 206Pb/238U
Sample BLG1505
BLG1505-1 268 223 1.2 0.1522 0.0033 8.0457 0.1883 0.3829 0.0047 2372 31 2236 21 2090 22
BLG1505-2 78 69 1.13 0.0714 0.0057 1.3228 0.0939 0.1374 0.0031 970 163 856 41 830 18
BLG1505-3 253 388 0.65 0.0676 0.002 1.2823 0.0392 0.1378 0.0017 857 63 838 17 832 9
BLG1505-4 245 414 0.59 0.066 0.002 1.2356 0.037 0.1359 0.0017 807 68 817 17 822 9
BLG1505-6 28 41 0.68 0.0713 0.0049 1.3309 0.0913 0.1376 0.0029 969 141 859 40 831 16
BLG1505-7 159 348 0.46 0.0678 0.002 1.0042 0.0299 0.1076 0.0017 861 140 706 15 659 10
BLG1505-8 111 102 1.09 0.0751 0.0049 1.3863 0.0814 0.1358 0.0029 1072 99 883 35 821 16
BLG1505-9 261 418 0.62 0.0658 0.0053 0.9304 0.0708 0.1034 0.0026 1200 164 668 37 634 15
BLG1505-10 83 110 0.75 0.0745 0.0036 1.4032 0.0647 0.1378 0.0018 1054 98 890 27 832 10
BLG1505-11 65 111 0.58 0.0649 0.0024 1.2199 0.0493 0.1366 0.0026 769 78 810 23 826 15
BLG1505-12 423 354 1.2 0.0667 0.0017 1.2643 0.0353 0.1374 0.0021 829 52 830 16 830 12
BLG1505-14 45 56 0.8 0.0692 0.0039 1.1009 0.0592 0.119 0.003 906 117 754 29 725 17
BLG1505-15 342 548 0.62 0.0659 0.0019 1.253 0.0372 0.1377 0.0018 806 61 825 17 832 10
BLG1505-16 336 255 1.32 0.0673 0.0025 1.2903 0.0524 0.1381 0.0022 856 77 841 23 834 12
BLG1505-17 390 373 1.05 0.1576 0.0026 9.4864 0.1874 0.4342 0.0055 2431 28 2386 18 2325 24
BLG1505-18 965 742 1.3 0.0654 0.0014 1.2533 0.0296 0.1381 0.0018 787 44 825 13 834 10
BLG1505-19 110 111 0.99 0.0663 0.0027 1.2711 0.0557 0.138 0.0021 815 85 833 25 833 12
BLG1505-20 150 172 0.87 0.1586 0.0032 9.6778 0.222 0.4395 0.0059 2440 35 2405 21 2348 26
BLG1505-22 340 500 0.68 0.0641 0.002 1.0949 0.0374 0.1232 0.0022 743 67 751 18 749 12
BLG1505-23 381 1440 0.26 0.0978 0.0018 3.7797 0.0783 0.2782 0.0035 1583 33 1588 17 1582 18
BLG1505-25 854 933 0.92 0.0679 0.0013 1.2997 0.0292 0.1377 0.0016 878 38 846 13 831 9
BLG1505-28 1938 1367 1.42 0.0675 0.0014 1.2793 0.0311 0.1366 0.0018 854 43 837 14 825 10
BLG1505-29 113 334 0.34 0.0681 0.0021 1.2887 0.0413 0.1368 0.002 872 95 841 18 827 11
BLG1505-32 753 718 1.05 0.0701 0.0017 1.3323 0.0346 0.1375 0.0016 931 53 860 15 831 9
BLG1505-33 419 372 1.13 0.0694 0.0018 1.3259 0.0372 0.1383 0.0016 909 54 857 16 835 9
BLG1505-34 297 505 0.59 0.0683 0.0015 1.3012 0.033 0.1376 0.0017 880 46 846 15 831 10
BLG1505-35 92 222 0.42 0.0709 0.0023 1.3504 0.0455 0.1378 0.0019 967 67 868 20 832 10
BLG1505-36 26 65 0.41 0.0716 0.0032 1.3391 0.0603 0.1373 0.0025 976 91 863 26 829 14
BLG1505-39 301 293 1.03 0.0681 0.0018 1.2997 0.0398 0.1377 0.002 872 54 846 18 832 11
BLG1505-40 135 138 0.98 0.0706 0.0025 1.3408 0.0495 0.1378 0.0018 946 72 864 21 832 10
BLG1505-41 142 126 1.13 0.069 0.0025 1.311 0.0491 0.1379 0.0022 898 68 851 22 833 12
BLG1505-42 75 141 0.53 0.065 0.0025 1.2316 0.0498 0.1372 0.0021 772 75 815 23 829 12
BLG1505-43 248 342 0.72 0.0652 0.0017 1.1359 0.0301 0.126 0.0014 783 54 771 14 765 8
BLG1505-44 397 761 0.52 0.0663 0.0013 1.267 0.0296 0.1378 0.0016 817 41 831 13 832 9
BLG1505-45 186 270 0.69 0.0641 0.002 1.2264 0.0421 0.1382 0.0019 746 67 813 19 835 11
BLG1505-47 228 245 0.93 0.0667 0.0021 1.2674 0.0437 0.137 0.0019 828 67 831 20 828 11
BLG1505-48 266 220 1.21 0.0651 0.0021 1.243 0.0454 0.1373 0.0017 776 69 820 21 830 10
BLG1505-50 39 59 0.67 0.1302 0.0039 6.6389 0.2058 0.3685 0.0049 2102 52 2065 27 2022 23
BLG1505-51 256 335 0.76 0.074 0.0034 1.5303 0.0685 0.1522 0.0037 1040 94 943 27 913 21
BLG1505-52 467 658 0.71 0.0702 0.002 1.1767 0.0341 0.1216 0.0022 1000 58 790 16 740 13
BLG1505-53 739 1310 0.56 0.1535 0.0033 6.3792 0.1957 0.2972 0.0057 2385 37 2029 27 1677 28
BLG1505-54 1456 2260 0.64 0.0693 0.0016 1.0682 0.0346 0.1104 0.0023 906 48 738 17 675 14
BLG1505-55 441 511 0.86 0.0668 0.002 1.2313 0.0367 0.1328 0.0015 831 63 815 17 804 8
BLG1505-56 195 298 0.65 0.0642 0.0024 1.1864 0.044 0.1333 0.0018 746 50 794 20 807 a
BLG1505-57 205 654 0.31 0.1517 0.0032 7.701 0.1715 0.3657 0.0044 2365 236 2197 20 2009 21
BLG1505-58 63 115 0.55 0.0716 0.0044 1.2472 0.0715 0.1278 0.0021 976 121 822 32 775 12
BLG1505-60 191 273 0.7 0.066 0.0026 1.2226 0.0469 0.1335 0.0018 809 81 811 21 808 10
BLG1505-61 41 51 0.8 0.0844 0.0075 1.4496 0.1081 0.1337 0.0034 1302 172 910 45 809 20
BLG1505-62 70 91 0.76 0.0709 0.0044 1.5008 0.0886 0.1544 0.003 955 122 931 36 925 17
BLG1505-63 46 64 0.72 0.0735 0.0056 1.3289 0.0997 0.133 0.0033 1028 154 858 44 805 19
BLG1505-64 32 41 0.78 0.1035 0.0056 3.4637 0.1882 0.2449 0.0048 1687 99 1519 43 1412 25
BLG1505-65 50 73 0.7 0.1008 0.0035 3.9226 0.1352 0.2822 0.004 1639 64 1618 28 1603 20
BLG1505-66 60 82 0.74 0.0681 0.0045 1.2229 0.0755 0.1334 0.0027 872 139 811 34 807 15
BLG1505-67 141 209 0.68 0.0679 0.0038 1.2329 0.0602 0.1335 0.0022 865 117 816 27 808 13
BLG1505-68 64 118 0.54 0.0797 0.0045 1.4206 0.084 0.1304 0.0025 1191 113 898 35 790 14
BLG1505-69 328 406 0.81 0.0649 0.0018 1.1866 0.0345 0.1322 0.0019 770 53 794 16 801 11
BLG1505-70 297 627 0.47 0.1142 0.0015 4.7994 0.0715 0.3037 0.0028 1866 29 1785 13 1710 14
BLG1505-71 76 143 0.53 0.065 0.0039 1.1151 0.0621 0.1261 0.002 776 121 761 30 766 12
BLG1505-72 162 172 0.94 0.0645 0.0034 1.1186 0.0602 0.1248 0.0019 767 105 762 29 758 11
BLG1505-73 428 487 0.88 0.0673 0.002 1.242 0.0402 0.1337 0.0017 856 64 820 18 809 10
BLG1505-74 81 124 0.65 0.0647 0.0026 1.1824 0.0468 0.1331 0.0022 765 87 792 22 805 13
BLG1505-75 92 225 0.41 0.159 0.0033 9.9756 0.2137 0.4545 0.0057 2456 35 2433 20 2415 25
BLG1505-76 111 130 0.86 0.0671 0.0037 1.2004 0.0635 0.1313 0.0024 843 117 801 29 795 14
BLG1505-77 740 582 1.27 0.0642 0.0019 1.1833 0.0352 0.1334 0.0016 748 62 793 16 807 9
BLG1505-78 119 192 0.62 0.0679 0.0052 1.1445 0.0872 0.1231 0.0023 865 160 775 41 748 13
BLG1505-79 42 70 0.61 0.0816 0.005 1.7359 0.1124 0.1536 0.0027 1236 122 1022 42 921 15
BLG1505-80 758 652 1.16 0.0689 0.0021 1.2704 0.0385 0.1333 0.0016 895 68 833 17 807 9
Sample KDLG1505
KDLG1505-01 790 878 0.9 0.0586 0.001 0.727 0.0138 0.09 0.0009 552 36 555 11 555 6
KDLG1505-02 41 68 0.6 0.177 0.0035 12.2597 0.2563 0.5023 0.0052 2625 32 2624 55 2624 27
KDLG1505-03 541 526 1.03 0.0561 0.0014 0.5541 0.015 0.0716 0.0008 458 57 448 12 446 5
KDLG1505-04 386 345 1.12 0.0558 0.0022 0.5469 0.0221 0.071 0.0008 446 87 443 18 442 5
KDLG1505-06 324 299 1.08 0.0557 0.0026 0.5447 0.0258 0.0709 0.0008 440 105 442 21 442 5
KDLG1505-07 253 388 0.65 0.0676 0.002 1.2823 0.0392 0.1378 0.0017 857 63 838 17 832 9
KDLG1505-08 216 178 1.21 0.1838 0.0031 12.3995 0.2285 0.4892 0.0053 2688 28 2635 49 2567 28
KDLG1505-09 140 346 0.41 0.0651 0.0015 1.123 0.0266 0.1252 0.0013 776 47 764 18 760 8
KDLG1505-10 374 269 1.39 0.0559 0.0025 0.5444 0.0245 0.0706 0.0008 448 99 441 20 440 5
KDLG1505-11 111 277 0.4 0.064 0.0017 1.0624 0.029 0.1205 0.0013 741 57 735 20 733 8
KDLG1505-12 343 749 0.46 0.0709 0.0012 1.5623 0.028 0.1598 0.0017 955 34 955 17 956 10
KDLG1505-14 193 386 0.5 0.1583 0.0023 9.9527 0.159 0.4559 0.0049 2438 24 2430 39 2421 26
KDLG1505-15 85 123 0.69 0.1179 0.0019 5.6393 0.0982 0.347 0.0036 1924 29 1922 33 1920 20
KDLG1505-16 169 262 0.64 0.0978 0.0016 3.0352 0.054 0.2251 0.0023 1582 30 1417 25 1309 14
KDLG1505-17 89 277 0.32 0.0564 0.0021 0.5575 0.0216 0.0717 0.0008 469 83 450 17 446 5
KDLG1505-18 87 278 0.31 0.1695 0.0025 11.3624 0.1847 0.4861 0.0051 2553 24 2553 42 2554 27
KDLG1505-19 400 453 0.88 0.0563 0.0015 0.5457 0.0159 0.0703 0.0007 463 61 442 13 438 5
KDLG1505-20 146 215 0.68 0.1366 0.002 7.5941 0.1211 0.4032 0.0041 2184 26 2184 35 2184 22
KDLG1505-21 353 666 0.53 0.071 0.0011 1.5708 0.0257 0.1606 0.0016 956 31 959 16 960 10
KDLG1505-22 420 453 0.93 0.0558 0.0014 0.5462 0.0142 0.0711 0.0007 443 55 443 12 443 5
KDLG1505-23 272 409 0.66 0.0665 0.0012 1.2489 0.0235 0.1363 0.0014 821 36 823 15 824 9
KDLG1505-24 477 773 0.62 0.0561 0.0012 0.555 0.0125 0.0717 0.0008 458 48 448 10 446 5
KDLG1505-25 822 1014 0.81 0.0714 0.001 1.5729 0.024 0.1599 0.0017 968 27 960 15 956 10
KDLG1505-26 111 225 0.49 0.0566 0.0027 0.5569 0.0271 0.0714 0.0008 474 105 450 22 445 5
KDLG1505-27 136 220 0.62 0.0564 0.0027 0.552 0.0263 0.071 0.0008 467 104 446 21 442 5
KDLG1505-28 790 878 0.9 0.0586 0.001 0.727 0.0138 0.09 0.0009 552 36 555 11 555 6
KDLG1505-29 92 1054 0.09 0.0699 0.001 1.4749 0.0229 0.1531 0.0016 924 28 920 14 919 10
KDLG1505-30 136 898 0.15 0.1126 0.0015 4.2313 0.0659 0.2725 0.0029 1842 25 1680 26 1553 17
KDLG1505-31 302 481 0.63 0.0659 0.0012 1.175 0.0232 0.1294 0.0013 802 38 789 16 784 8
KDLG1505-32 107 304 0.35 0.056 0.0162 0.5377 0.1544 0.0696 0.0009 454 640 437 125 434 5
KDLG1505-33 102 195 0.52 0.1615 0.0024 8.4156 0.1353 0.3779 0.0038 2472 25 2277 37 2066 21
KDLG1505-34 965 742 1.3 0.0654 0.0014 1.2533 0.0296 0.1381 0.0018 787 44 825 13 834 10
KDLG1505-35 229 150 1.53 0.064 0.0024 1.0639 0.0403 0.1205 0.0013 743 80 736 28 733 8
KDLG1505-36 319 881 0.36 0.0797 0.0011 1.7772 0.0279 0.1616 0.0016 1191 28 1037 16 966 10
KDLG1505-40 656 648 1.01 0.0563 0.0013 0.5506 0.0129 0.0709 0.0007 465 50 445 10 442 5
KDLG1505-41 269 695 0.39 0.1742 0.0027 10.4835 0.1796 0.4364 0.0046 2599 26 2478 42 2335 25
KDLG1505-42 218 125 1.75 0.0679 0.0021 1.3418 0.0485 0.1432 0.0021 867 65 864 31 863 13
KDLG1505-43 41 51 0.8 0.0783 0.0068 1.57 0.1347 0.1457 0.0039 1155 173 958 53 877 22
KDLG1505-44 237 337 0.7 0.0657 0.0012 1.1946 0.0236 0.1319 0.0013 796 39 798 16 799 8
KDLG1505-45 1411 637 2.22 0.0645 0.0011 1.0997 0.0199 0.1236 0.0013 758 35 753 14 751 8
KDLG1505-46 332 154 2.15 0.0643 0.0025 1.0568 0.0422 0.1193 0.0013 750 82 732 29 726 8
KDLG1505-47 266 541 0.49 0.1707 0.0023 9.906 0.1598 0.4208 0.0047 2565 23 2426 39 2264 25
KDLG1505-49 267 391 0.68 0.0565 0.0016 0.5761 0.0171 0.0739 0.0008 474 64 462 14 460 5
KDLG1505-50 178 261 0.68 0.0561 0.0026 0.5451 0.0267 0.0705 0.0008 456 102 442 22 439 5
KDLG1505-51 429 339 1.27 0.0674 0.0014 1.2778 0.0288 0.1376 0.0014 849 42 836 19 831 9
KDLG1505-52 82 78 1.05 0.1376 0.0024 6.5832 0.1296 0.347 0.004 2197 30 2057 40 1920 22
KDLG1505-53 110 219 0.5 0.0588 0.0013 0.6957 0.0167 0.0857 0.001 561 48 536 13 530 6
KDLG1505-55 372 456 0.82 0.0556 0.0026 0.5483 0.026 0.0717 0.0012 435 136 444 17 447 7
KDLG1505-56 37 153 0.24 0.1521 0.0044 9.2775 0.3286 0.4395 0.0101 2369 49 2366 32 2348 45
KDLG1505-58 324 554 0.58 0.0554 0.0023 0.5455 0.0231 0.0714 0.0011 428 94 442 15 444 6
KDLG1505-59 99 143 0.7 0.0706 0.0038 1.353 0.0684 0.1398 0.0027 946 111 869 30 844 15
KDLG1505-60 65 106 0.62 0.0707 0.0043 1.2933 0.0811 0.1342 0.0031 950 125 843 36 812 18
KDLG1505-61 89 159 0.56 0.0618 0.0041 0.5957 0.0415 0.0701 0.0016 733 144 475 26 436 9
KDLG1505-62 32 76 0.43 0.1185 0.0045 5.7298 0.2059 0.3509 0.0061 1944 101 1936 31 1939 29
KDLG1505-64 65 124 0.52 0.1112 0.0038 5.1238 0.1714 0.3327 0.0061 1820 62 1840 28 1851 30
KDLG1505-65 141 145 0.98 0.065 0.0036 1.0661 0.0553 0.1196 0.0025 772 117 737 27 728 14
KDLG1505-67 172 219 0.79 0.0574 0.004 0.5676 0.04 0.0712 0.0014 506 152 456 26 443 9
KDLG1505-68 104 187 0.56 0.0524 0.004 0.4916 0.0371 0.0674 0.0014 306 179 406 25 420 9
KDLG1505-69 345 383 0.9 0.0555 0.0023 0.5542 0.0239 0.0713 0.0012 435 89 448 16 444 7
KDLG1505-70 402 407 0.99 0.0572 0.0028 0.5649 0.0259 0.0714 0.001 498 112 455 17 445 6
KDLG1505-71 142 264 0.54 0.0686 0.0026 1.2796 0.0475 0.1343 0.0019 887 79 837 21 812 11
KDLG1505-72 373 470 0.79 0.0653 0.0022 1.1107 0.0383 0.122 0.0016 783 72 758 18 742 9
KDLG1505-74 78 123 0.64 0.1232 0.0038 6.2775 0.196 0.3664 0.0058 2003 55 2015 27 2013 27
KDLG1505-75 71 195 0.36 0.0729 0.003 1.5292 0.0647 0.1507 0.0023 1013 83 942 26 905 13
KDLG1505-76 437 301 1.45 0.0753 0.0028 1.6259 0.0638 0.1554 0.0025 1076 69 980 25 931 14
KDLG1505-77 102 239 0.43 0.0707 0.0033 1.3061 0.0616 0.1332 0.0024 950 94 848 27 806 14
KDLG1505-78 30 41 0.73 0.1151 0.0054 5.2916 0.2536 0.3334 0.0064 1883 85 1868 41 1855 31
KDLG1505-79 75 141 0.53 0.0807 0.0027 2.0223 0.0668 0.1801 0.0022 1215 67 1123 22 1068 12
KDLG1505-80 88 185 0.48 0.1448 0.0036 7.6864 0.179 0.3822 0.0042 2287 43 2195 21 2087 20

4.2 Zircon Hf isotopic analysis

Zircon Lu-Hf isotopic analysis were carried out on samples KDLG1505 and BLG1505 with completed zircon U-Pb dating. The related results are listed in Table 3. For samples KDLG1505 and BLG1505, the detrital zircons of the Zhongwunongshan Group were found to be dominated by late Early Paleozoic and Neoproterozoic (Table 2, Figure 4). The Hf isotopic analysis results indicated that the vast majority of the zircons had negative Hf(t) (Figure 5). Such Hf(t) distribution characteristics confirmed that the detrital zircons of the Zhongwunongshan Group were mainly sourced from crustal recycling. However, some zircons with positive Hf(t) in BLG1505 proved a depleted mantle source existed (Table 3, Figure 5). The zircons of the Zhongwunongshan Group in the eastern section of the study area had two major U-Pb age populations of 460–434 Ma and 966–726 Ma (Figure 4d). Also, the corresponding Hf(t) were calculated at −9.6 to −3.1, and −15.9 to −0.8, respectively, with the exception of the No.21 point which had an Hf(t) of 3.8 (Figure 5). The two-stage model ages (TcDM) were between 1621 Ma and 2029 Ma, and between 1581 Ma and 2815 Ma respectively. The averages were determined to be 1864 Ma and 2362 Ma, respectively (Table 3). In the western section of the study area, the zircons had a major U-Pb age population of 925–725 Ma (Figure 4d); vHf(t) between −11.9 and 9.5 (Figure 5); and average TcDM of 1692 Ma, which was lower than that of the Neoproterozoic zircons in the eastern section.

Table 3

Hf isotopic compositions of zircons from the Zhongwunongshan Group sandstones

Sample U-Pb Age (Ma) 176 Yb 177 Hf 176 Lu 177 Hf 176 Hf 177 Hf ϵHf(0) ϵHf(t) T D M C
Sample BLG1505
BLG1505-01 2372 0.014893 0.000503 0.000008 0.281265 0.000023 -53.3 -0.95 2960
BLG1505-02 830 0.034457 0.001056 0.000039 0.282392 0.000027 -13.43 4.34 1446
BLG1505-03 832 0.020833 0.000634 0.000002 0.28227 0.000022 -17.75 0.27 1704
BLG1505-04 822 0.054682 0.001565 0.000026 0.2824 0.000022 -13.16 4.16 1452
BLG1505-06 831 0.018978 0.000557 0.000003 0.28253 0.000025 -8.57 9.5 1120
BLG1505-07 659 0.033059 0.001168 0.000058 0.282396 0.000023 -13.29 0.74 1542
BLG1505-08 821 0.037872 0.00114 0.000028 0.282345 0.000024 -15.11 2.4 1561
BLG1505-10 832 0.040049 0.001355 0.000015 0.282495 0.000022 -9.79 7.86 1225
BLG1505-11 826 0.02147 0.000714 0.000007 0.282353 0.000023 -14.8 3.05 1523
BLG1505-12 830 0.097608 0.003171 0.000059 0.282416 0.00002 -12.58 4.02 1467
BLG1505-14 725 0.049159 0.001682 0.000156 0.282393 0.000028 -13.41 1.79 1527
BLG1505-15 832 0.051351 0.001449 0.000066 0.28213 0.000027 -22.72 -5.15 2045
BLG1505-17 2431 0.02254 0.00073 0.000003 0.281209 0.000024 -55.29 -1.97 3068
BLG1505-18 834 0.077842 0.002077 0.000018 0.281954 0.000027 -28.92 -11.66 2455
BLG1505-19 833 0.022269 0.00071 0.000003 0.282398 0.000025 -13.24 4.8 1421
BLG1505-20 2440 0.017897 0.000506 0.000002 0.280988 0.000019 -63.1 -9.27 3519
BLG1505-22 749 0.02431 0.000792 0.000016 0.282313 0.000025 -16.22 -0.09 1663
BLG1505-23 1583 0.062695 0.001971 0.000062 0.281681 0.000022 -38.57 -5.47 2634
BLG1505-25 831 0.100069 0.003164 0.00004 0.2825 0.000026 -9.64 6.98 1280
BLG1505-28 825 0.018694 0.000616 0.000008 0.281991 0.000026 -27.61 -9.73 2327
BLG1505-29 827 0.019528 0.00055 0.000023 0.282137 0.000019 -22.44 -4.48 2000
BLG1505-32 831 0.036456 0.001106 0.000002 0.282304 0.000026 -16.54 1.21 1644
BLG1505-33 835 0.040226 0.001201 0.000007 0.282356 0.000026 -14.72 3.07 1530
BLG1505-34 831 0.026978 0.000853 0.00001 0.282167 0.000025 -21.39 -3.5 1941
BLG1505-35 832 0.049137 0.001592 0.00002 0.282503 0.000025 -9.52 8 1217
BLG1505-36 829 0.014769 0.000522 0.000049 0.282239 0.000021 -18.87 -0.86 1773
BLG1505-39 832 0.008091 0.000247 0.000001 0.282022 0.000027 -26.53 -8.3 2242
BLG1505-40 832 0.038534 0.001343 0.000065 0.282252 0.000025 -18.37 -0.75 1768
BLG1505-41 833 0.039793 0.001258 0.000039 0.282269 0.000028 -17.77 -0.06 1727
BLG1505-42 829 0.030067 0.001048 0.000023 0.282508 0.000025 -9.33 8.43 1187
BLG1505-43 765 0.032088 0.001025 0.000015 0.282177 0.000026 -21.04 -4.69 1965
BLG1505-44 832 0.060327 0.00168 0.000023 0.282253 0.000024 -18.34 -0.9 1777
BLG1505-45 835 0.025155 0.000839 0.000011 0.282158 0.000025 -21.7 -3.73 1959
BLG1505-47 828 0.046628 0.001702 0.000024 0.28241 0.000027 -12.8 4.54 1430
BLG1505-48 830 0.009163 0.000271 0.000001 0.281923 0.000026 -30.01 -11.85 2463
BLG1505-50 2102 0.007978 0.000196 0.000003 0.281127 0.000023 -58.16 -11.48 3398
Sample KDLG1505
KDLG1505-02 2625 0.010742 0.000327 0.00001 0.281012 0.000023 -62.23 -3.86 3331
KDLG1505-03 446 0.019216 0.000612 0.000009 0.282379 0.000021 -13.88 -4.26 1696
KDLG1505-04 442 0.019617 0.000607 0.000031 0.282333 0.000021 -15.53 -5.98 1802
KDLG1505-06 442 0.015493 0.000484 0.000015 0.282233 0.000026 -19.08 -9.48 2023
KDLG1505-07 832 0.013424 0.000359 0.000012 0.281942 0.000021 -29.36 -11.2 2424
KDLG1505-08 2688 0.047057 0.001472 0.000005 0.28101 0.000028 -62.32 -4.6 3425
KDLG1505-09 760 0.049388 0.001607 0.000047 0.281925 0.000027 -29.95 -14.01 2545
KDLG1505-10 440 0.017338 0.000533 0.000003 0.282231 0.000026 -19.13 -9.6 2029
KDLG1505-11 733 0.017129 0.000497 0.000012 0.282114 0.000022 -23.28 -7.35 2108
KDLG1505-12 956 0.018144 0.000532 0.000007 0.282162 0.000024 -21.56 -0.75 1864
KDLG1505-13 439 0.021354 0.000672 0.000009 0.282355 0.000021 -14.74 -5.27 1755
KDLG1505-14 2438 0.0212 0.000651 0.000022 0.281421 0.000024 -47.76 5.87 2593
KDLG1505-15 1924 0.017898 0.00052 0.000014 0.281571 0.000025 -42.48 -0.2 2570
KDLG1505-16 1582 0.046345 0.001719 0.000076 0.281804 0.000028 -34.23 -0.84 2349
KDLG1505-17 446 0.040986 0.001226 0.000023 0.282275 0.000025 -17.59 -8.16 1941
KDLG1505-18 2553 0.017914 0.000595 0.000008 0.281258 0.000025 -53.54 2.77 2871
KDLG1505-19 438 0.027397 0.000829 0.000025 0.282303 0.000022 -16.58 -7.16 1875
KDLG1505-20 2184 0.021474 0.000679 0.000034 0.281287 0.000025 -52.53 -4.7 3045
KDLG1505-21 960 0.057063 0.001823 0.000071 0.282312 0.000027 -16.27 3.81 1581
KDLG1505-22 443 0.015489 0.000483 0.000006 0.282413 0.000024 -12.71 -3.09 1621
KDLG1505-23 824 0.05065 0.001531 0.000043 0.281915 0.00002 -30.32 -12.98 2529
KDLG1505-24 446 0.03988 0.001225 0.000039 0.282374 0.000026 -14.08 -4.62 1720
KDLG1505-25 956 0.053563 0.001465 0.000019 0.281839 0.000024 -33 -12.83 2617
KDLG1505-26 445 0.036234 0.001077 0.000012 0.282293 0.000022 -16.95 -7.47 1898
KDLG1505-27 442 0.01839 0.000546 0.000005 0.28231 0.000024 -16.33 -6.76 1853
KDLG1505-28 555 0.027264 0.000786 0.000013 0.282192 0.000027 -20.52 -8.58 2052
KDLG1505-30 1842 0.029848 0.00085 0.000005 0.28161 0.000023 -41.08 -1.04 2559
KDLG1505-31 784 0.058477 0.001719 0.000023 0.281977 0.00003 -28.1 -11.71 2419
KDLG1505-32 434 0.02181 0.000654 0.000013 0.282242 0.000021 -18.73 -9.38 2010
KDLG1505-33 2472 0.027235 0.000883 0.000008 0.280971 0.000025 -63.68 -9.74 3572
KDLG1505-34 834 0.043409 0.001189 0.000005 0.281869 0.000026 -31.94 -14.21 2612
KDLG1505-35 733 0.055346 0.001726 0.000011 0.282044 0.000028 -25.75 -10.43 2300
KDLG1505-36 966 0.011427 0.00034 0.000006 0.281726 0.000023 -37 -15.87 2815
KDLG1505-40 442 0.019077 0.000597 0.000011 0.282325 0.000021 -15.82 -6.26 1820
KDLG1505-42 877 0.049528 0.001589 0.000013 0.282037 0.00003 -25.98 -7.54 2228
KDLG1505-43 1152 0.01379 0.000446 0.000007 0.28186 0.000021 -32.26 -7.1 2409
KDLG1505-44 799 0.045299 0.001299 0.000012 0.281855 0.000025 -32.44 -15.52 2667
KDLG1505-45 751 0.080713 0.002377 0.000026 0.281896 0.00003 -30.97 -15.59 2637
KDLG1505-46 726 0.087725 0.00254 0.000021 0.281989 0.00003 -27.69 -12.93 2450
KDLG1505-47 2565 0.047276 0.001282 0.000049 0.28114 0.000023 -57.73 -2.36 3195
KDLG1505-49 460 0.018305 0.000567 0.000021 0.282323 0.000021 -15.88 -5.93 1813
KDLG1505-50 439 0.044257 0.001376 0.000035 0.282259 0.000029 -18.15 -8.91 1983
KDLG1505-51 831 0.029402 0.000888 0.00001 0.282014 0.00003 -26.81 -8.96 2282
KDLG1505-52 2197 0.015031 0.000448 0.000003 0.281306 0.000025 -51.84 -3.34 2972
KDLG1505-53 530 0.030277 0.000878 0.000002 0.282029 0.00003 -26.28 -14.94 2432
Figure 5 Plot of zircon Hf isotopes vs. crystallization age from the Zhongwunongshan Group sandstones

Figure 5

Plot of zircon Hf isotopes vs. crystallization age from the Zhongwunongshan Group sandstones

5 Discussion

5.1 Sources of detritus

The zircon U-Pb age histogram of the clastic samples showed two major populations (460–434 Ma and 966–725 Ma), which illustrated that the zircons of the Zhongwunongshan Group were mainly derived from Neoproterozoic and late Early Palaeozoic detritus source rocks (Figure 4). The Neoproterozoic granitic gneiss with similar formation ages was found in the Yuqia-Shaliuhe HP-UHP metamorphic belt, located on the southern margin of the NQSB [3, 27]. Due to the continental collision between the Olongbruk Microblock and the Qaidam Block, the most important Neoproterozoic tonalite-monzonitic granites were formed, and intruded into the Yuqia-Shaliuhe HP-UHP metamorphic belt during the Jinningian period (1000–800 Ma) [28, 29, 36, 45]. In addition, the Hf isotopic results showed that the Neoproterozoic zircons from the sample KDLG1505 also had similar model age (2815–1581 Ma, with a peak of 2362 Ma), and a similar origin (dominated by ancient crust melting) as the Neoproterozoic Shaliuhe granitic gneiss in the eastern section of the Yuqia-Shaliuhe HP-UHP metamorphic belt [2, 46]. However, since the two-stage model age (TcDM) of sample BLG1505 in the western section was lower than that in the eastern section, the Hf(t) was found to be significantly higher (Table 2, Figure 5). These results indicated that the Neoproterozoic detritus source rock in the western section may have been mixed with depleted mantle. A previous survey revealed that eclogite which developed in the Xitieshan granitic gneiss of the western section was originally the oceanic ridge basalt, with crystallization ages of 1000–800 Ma and positive Nd (t) [47, 48]. It confirmed that the Neoproterozoic granite gneiss in the west may have been formed during a crust-remelting process, and mixed with depleted mantle, which caused the model age TCDM to be below the normal value in the east [49, 50]. Therefore, in the Yuqia-Shaliuhe HP-UHP metamorphic belt of North Qaidam, the Neoproterozoic granitic gneiss can likely be one of the major detritus source for the Zhongwunongshan Group.

The detrital zircon U-Pb age histogram also showed that the late Early Paleozoic detritus source rocks within the age population of 460–434 Ma were another major source of the Zhongwunongshan Group (Figure 4). However, the zircons in this age range only appeared in the eastern section of the study area. It was indicated that the detritus source rocks were slightly different between the eastern and western sections of the study area. The Hf isotopic analysis showed that the late Paleozoic detritus source rocks were mainly derived from the remelted magma of the lower crust reservoir (Figure 5). A previous survey indicated the sustained oceanic crust subduction occurred when the Qaidam block northward dived beneath the Olongbruk Microblock during the Late Ordovician [36, 63]. Due to the rapid subduction, the thickened lower continental crust melting took place which formed the active epicontinental arc tonalite-granodiorite (450–440 Ma) in the Tanjianshan ophiolite-volcanic arc belt [3, 12, 28, 51]. As the only Early Paleozoic continental arc granites within the age population in the study area, the Tomorit quartz diorite–monzogranite series approaching to the south of Ulan was another source for the Zhongwunongshan Group.

5.2 Age of the Zhongwunongshan Group

Because the deposition age of sedimentary formation must be younger than the youngest detrital zircon, which can be regarded as the maximum deposition age for this formation [52, 53, 54]. This approach has been successfully applied to the time determination for sedimentary strata, especially to successions where biostratigraphy cannot be used [55, 56].

In this study, the 206Pb/238U age of the youngest detrital zircon (KDLG1505-32) of the Zhongwunongshan Group is 434±5 Ma (Table 2), and can be regarded as the maximum deposition age of the Zhongwunongshan Group. As the major provenances for the Zhongwunongshan Group, the significant input of detritus materials derived from the Yuqia-Shaliuhe HP-UHP metamorphic belt and the Tanjianshan ophiolite-volcanic arc belt to the south (Figure 1b). The widespread Maonianshan Group in Devonian period of the above source regions was characterized of the youngest detrital zircon age of 407.9 Ma [57], which limited the deposition of the Maoniushan Group to be later than the Early Devonian. However, the detrital zircon U-Pb ages of the Zhongwunongshan Group were all older than 407.9 Ma, which indicated the absence of detritus materials of 407.9 Ma in the source regions during the Zhongwunongshan group deposited, and limited the depositional age of that strata to be older than early Devonian (407.9 Ma).

5.3 Tectonic significance

In the period from late Late Neoproterozoic to Middle Ordovician (580–465 Ma), the North Qaidam oceanic crust subducted northward beneath the Olongbruk Microblock, resulted in the formation of the Tanjianshan volcanic island arc [32, 58, 59] and Yuqia-Shaliuhe ultrahigh pressure metamorphic rocks [60, 61]. This subduction also induced compensatory convection of the mantle wedge behind the island arc, and forced deep-seatedmagmatoupwellwhich resulted in back-arc extension [63].

During the Late Ordovician to Middle Silurian (465–435 Ma), the North Qaidam Ocean closed and the Qaidam Block collided with the Olongbruk Microblock. Due to the rapid subdution, the thickened lower continental crust melting took place which formed the epicontinental arc tonalites-granodiorites (450–440 Ma) in the Tanjianshan ophiolite-volcanic arc belt [3, 12, 28]. At the same time, the back-arc basin to the north of the North Qaidam deposited thick flysch sediments in the ZWSB and the adjacent South Qilian area during the Silurian [63].

The Late Silurian-Early Devonian (435–400 Ma) is an important transition period for the Early Paleozoic tectonic evolution in the North Qaidam. The continuous movement of the asthenosphere with respect to the lithosphere at this stage promoted detachment within the lithosphere and caused partial melting of the sub-ducted continental crust. Meanwhile the alkaline basaltic magma, derived from remelting of deep subduction of the oceanic crust, uplifted and mixed with the continental crust magma. The mixed magma was finally emplaced in both sides of the HP-UHP zone, which formed the Early Devonian gneissic granite with the characteristics of intraplate cracking. The zircons from the gneissic granite yielded a U-Pb isotopic age about 400 Ma [3, 12], marking the completion of the Caledonian collisional orogeny in the NQSB.

In conclusion, the Zhongwunongshan Group was deposited during the period from the late Early Silurian to the early Early Devonian (434–407.9 Ma), when the North Qaidam was in the orogenic stage of the end of the early Paleozoic. Its provenance mainly came from the uplifting and erosion of the Yuqia-Shaliuhe HP-UHP metamorphic belt and the Tanjianshan ophiolite-volcanic island arc belt of the NQSB. Furthermore, the Zhongwunongshan Group was located in the back-arc area to the Early Paleozoic Tanjianshan volcanic island arc belt of North Qaidam. With this information, it suggests that the Zhongwunongshan Group in the ZWSB and adjacent area is obviously dominated by the intense collisional orogeny of the North Qaidam in the end of the Early Paleozoic, which formed in the late Early Paleozoic back-arc basin.

6 Conclusion

Our new data allow the following major conclusions:

  1. The detrital zircon U-Pb dating results of the Zhongwunongshan Group showed two major populations. The first was Neoproterozoic (966-725 Ma) with a Hf(t) = −15.9 to 9.5, and the other was late Early Paleozoic (460-434 Ma) with a Hf(t) = −9.6 to −3.1. Based on the regional studies, the dominated provenanceswere found to be the Neoproterozoic granitic gneiss of the Yuqia-Shaliuhe HP-UHP metamorphic belt and the late Early Paleozoic granites of the Tanjianshan ophiolite-volcanic arc belt in the NQSB.

  2. In combination with previous research, the youngest detritus zircon ages of 434 Ma suggest that the Zhongwunongshan Group was deposited in the back-arc sedimentary basin associated with the Caledonian collisional orogeny of the NQSB during the Middle Silurian-Early Devonian (434–407.9 Ma)

Acknowledgement

We are grateful to Mr Dongdong Yu, Suyang Jiang and Kai Li for their help during the zircon U-Pb and Lu-Hf analyses. This study was financially supported by the subject of National Key R&D Program of China (subject No.2017YFC0602806) and the National Natural Science Foundation of China (grant number 4157021100).

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Received: 2018-08-06
Accepted: 2019-12-15
Published Online: 2020-02-28

© 2020 Y. Peng et al., published by De Gruyter

This work is licensed under the Creative Commons Attribution 4.0 International License.