Identification and Geological Significance of Late Cambrian OIB-Type Volcanic Rocks in the Nailenggeledaban Area, Northern Yili Block

Abstract

Paleozoic igneous rocks exposed in the northern Yili Block are thought to have resulted from the subduction of the North Tianshan oceanic crust. However, the exact timing of the transition of the northern margin of the Yili Block from a passive to an active continental margin remains unknown. In this paper, the petrological and geochemical features, zircon U-Pb chronology, Lu-Hf isotopes, and Sr-Nd isotopes of volcanic rocks in the Nailenggeledaban area on the northern margin of the Yili Block were studied. Zircon U-Pb dating results show that the crystallization ages of the volcanic rocks in the Nailenggeledaban area on the northern margin of the Yili Block are 491 ± 2 Ma and 500 ± 2 Ma, suggesting they were formed during the Late Cambrian. Geochemical features show that the volcanic rocks are alkaline basalts with rare earth and trace element distribution patterns similar to OIB, although they exhibit some degree of Zr and Hf depletion. The ε_Hf(t) values of alkaline basalts in the Nailenggeledaban area at the northern Yili Block range from −3.48 to −1.00, with a T_DM1 age of 1152 to 1263 Ma. The ε_Nd(t) values range from −3.53 to −0.96, with a T_DM1 age of 1471 to 2162 Ma. Combined with geochemical data, the alkaline basalt magma in the Nailenggeledaban area on the northern margin of the Yili Block may be derived from the Mesoproterozoic enriched lithospheric mantle. The composition of the mantle source area is potentially garnet lherzolite, and the magma appears to have been either unaffected or only minimally contaminated by crustal materials during the ascending process. On the basis of the research results of the Early Paleozoic tectonic evolution in the northern margin of the Yili Block, this paper proposes that the volcanic rocks in the Nailenggeledaban area, located on the northern margin of the Yili Block, were formed in a back-arc extensional environment resulting from the subduction of the North Tianshan Ocean (or Junggar Ocean) beneath the northern margin of the Yili Block during the Late Cambrian.

1. Introduction

The Central Asian Orogenic Belt (CAOB), located between Eastern Europe, Siberia, the Tarim Basin, and the North China Craton, is the largest Phanerozoic accretionary belt on the Earth [1,2] (Figure 1a). The formation of the CAOB involved collisions of a large number of microcontinents, island arcs, oceanic islands, and accretionary mafic rocks [3,4,5]. The subduction processes of the Paleo-Asian Ocean gave rise to extensive island arc magmatism, contributing to a gradual expansion of the Asian continent during the Paleozoic era. Approximately half of this continental growth was attributed to the formation of juvenile crust [3,6]. The Western Tianshan Orogen Belt is located at the southwestern edge of the CAOB, and investigations of its formation and evolution process are of great significance to the understanding of the accretionary orogenic process. According to previous studies, three major magmatic events developed within the Western Tianshan Orogen Belt, including Neoproterozoic (950~780 Ma) magmatic [7,8,9,10,11], Early Paleozoic (460~395 Ma) magmatic, and Late Paleozoic (375~310 Ma) magmatic events [12,13].

The Paleozoic magmatism of the northern YB has been extensively studied by previous researchers. Hu et al. (2008) first reported the Early Paleozoic plagioclase amphibolite of the Wenquan Group in the northern YB, which was formed in the Late Ordovician (455.1 ± 2.7 Ma and 451.4 ± 5.4 Ma) and had geochemical features similar to island arc rocks. More Ordovician magmatic rocks were found there within the Wenquan area, and it is thought that these magmatic rocks are the result of the southward subduction of the North Tianshan Ocean [9,12,14,15,16,17,18]. Contemporaneous volcanic activity also records this tectonic event [10,11,13,19,20]. However, the exact timing of the transition of the northern YB from a passive to an active continental margin remains unknown.

In this paper, we reported the newly discovered Late Cambrian volcanic rocks in the Nailenggeledaban area, northern YB. Detailed petrological features, zircon U-Pb chronological features, whole-rock geochemical features, and whole-rock Sr-Nd isotope studies were conducted to assess the formation age, petrogenesis, and tectonic setting.

Figure 1

(a) Structural architecture of the Central Asian Orogenic Belt and (b) concise geological map outlining the Western Tianshan Orogen Belt in China (according to Gao et al., 2009a [21]).

[Figure omitted. See PDF]

2. Geological Setting

The Tianshan Orogenic Belt, located between the Tarim and Junggar Blocks, is a Paleozoic accretionary orogenic belt extending about 2400 km. After continuous denudation and pediplanation in the Mesozoic [22], it was uplifted during the Cenozoic due to the far-field effects of the Indo-Eurasian collision, becoming one of the most magnificent mountain ranges in northwest China [23]. The Tianshan Orogenic Belt can be approximately divided into the East Tianshan and the Western Tianshan along the longitude of approximately 88° E [24]. The Western Tianshan Orogen is delineated into four tectonic units by four major regional faults, including the North Tianshan Fault, the North Nalati Fault, the South Central Tianshan Fault, and the North Tarim Fault, from north to south. The four tectonic units are the North Tianshan Accretionary Complex, the Kazakhstan-Yili Block, the Central Tianshan Block, and the South Tianshan Accretionary Complex [23,25,26,27] (Figure 1b).

The North Tianshan Accretionary Complex is an accretionary prism that developed as a result of the subduction of the North Tianshan Ocean beneath the YB during the Paleozoic. The main exposed geological bodies include ophiolite formed during the Late Devonian-Early Carboniferous and Late Devonian-Carboniferous flysch, which form the primary component of the accretive wedge [28]. Additionally, Late Carboniferous volcanic rocks and continental deposits from the Late Permian onwards are also present [29,30].

The YB is situated in a triangular shape between the North Tianshan Accretionary Complex Belt and the Central Tianshan Block, featuring a broader width in the west and a narrower one in the east. The metamorphic Precambrian basement rocks are mainly exposed in the southern and northern margins of the YB. Early Paleozoic strata are exposed sporadically, whereas Late Paleozoic clastic and volcanic-sedimentary rocks, as well as intrusive rocks, developed extensively in the YB [16,23].

The Central Tianshan Block extends in an east-west direction with varying widths. The Precambrian basement metamorphic rock series is exposed, and the Paleozoic strata dominated by the volcano-sedimentary rock series are well developed [31,32,33,34]. These blocks likely collided along the Early Paleozoic suture zone, represented by the Xiate ophiolite in the Middle Ordovician, leading to the formation of a unified Yili-Central Tianshan Block [20,35].

The research region is located in the Nailenggeledaban area, northern YB (Figure 2), which belongs to the northern foot of the Borohoroshan Mountain. The strata exposed in the study area include the Middle Ordovician Nailenggeledaban Group, the Upper Ordovician Hudukedaban Formation, the Upper Silurian Borohoroshan Formation, the Middle Devonian Hanjiga Formation, the Lower Carboniferous Dahalajunshan Formation, the Lower-middle Carboniferous Akeshake Formation, the Middle Carboniferous Dongtujinhe Formation, and the Lower Permian Wulang Formation. The studied volcanic rocks in this paper were formerly assigned to the Nailenggeledaban Group.

3. Petrological Features

The basalt samples were collected from the Middle Ordovician Nailenggeledaban Group (O₂nl) in the Nailenggeledaban area of Jinghe County, northern YB. The basalt is gray-black in color, and the porosity and amygdaloidal structure are well developed. The basalt exhibits a porphyritic texture with phenocrysts of plagioclase (15~25 wt.%) and clinopyroxene (5%~10 wt.%). In addition, the matrix displays a pilotaxitic texture (Figure 3). The plagioclase phenocrysts exhibit a semi-idiomorphic, platy habit with grain sizes ranging from 0.1 mm × 0.3 mm to 1 mm × 0.5 mm and display both zonal structures and polysynthetic twinning. The pyroxene phenocrysts predominantly exhibit a columnar and granular habit, with grain sizes ranging from 0.3 mm × 0.5 mm to 1.2 mm × 0.8 mm, with pyroxene cleavage. Some pyroxene phenocrysts experienced chloritization. The matrix has an intergranular-intercryptic structure, which is composed of plagioclase, pyroxene, and opaque metal minerals (potentially magnetite). Granular minerals and glassy components fill the space between the crystals of plagioclase. The plagioclases in the matrix exhibit a semi-idiomorphic fine plate morphology with grain sizes ranging from 0.01 mm to 0.02 mm, and the polysynthetic twin is developed. Some plagioclase crystallites are oriented and semi-oriented, forming a pilotaxitic texture. The pyroxenes in the matrix are mostly granular and 0.02 mm~0.03 mm in size.

4. Analytical Methods

4.1. Whole-Rock Geochemical Features

Whole-rock geochemical composition studies were conducted at the State Key Laboratory of Continental Dynamics, Northwest University, Xi’an, China. An ARL AdvantXP+ wavelength-dispersive X-ray fluorescence spectrometer (XRF) was used for the analysis of major elements. The weight method was used to determine the loss on ignition, with an analytical precision generally exceeding 97%–98%. Trace element analysis of whole rock was conducted by use of the ThermoFisher X Series II ICP-MS system. For elements with a mass fraction exceeding 10 × 10⁻⁶, the analytical precision was better than 5%. For those with a mass fraction below 10 × 10⁻⁶, the precision was better than 10%. The detailed experimental process and data processing method are the same as that described by Chen Fukun et al., 2000 [37].

4.2. Zircon U-Pb Dating Analysis

The zircon selection and sample target preparation were performed at Xi’an Ruishi Geological Technology Co., Ltd., Xi’an, China. U-Pb dating and trace element analyses of zircon were simultaneously conducted using LA-ICP-MS at the Wuhan Sample Solution Analytical Technology Co., Ltd., Wuhan, China. The detailed operating conditions for both the laser ablation system and the ICP-MS instrument, as well as the data reduction procedures, are identical to those described by Zong et al., 2010 [38]. The laser system was a GeolasPro, comprising a COMPexPro 102 ArF excimer laser with a wavelength of 193 nm and a maximum energy of 200 mJ, paired with a MicroLas optical system. To acquire ion signal intensities, an Agilent 7900 ICP-MS instrument from California, USA was utilized. The Excel-based software ICPMSDataCal 10.9 was utilized for offline processing, including the selection and integration of background and analyzed signals, time-drift correction, and quantitative calibration, specifically for trace element analyses and U-Pb dating [39,40]. Concordia diagrams and weighted mean calculations were obtained by use of Isoplot/Ex_ver3 [41].

4.3. Zircon Lu-Hf Isotope Analyses

In situ Lu-Hf isotope ratio experiments were conducted by use of a Neptune Plus MC-ICP-MS (ThermoFisher Scientific, Dreieich, Germany) in combination with a Geolas HD excimer ArF laser ablation system (Coherent, Göttingen, Germany) that was hosted at the Wuhan Sample Solution Analytical Technology Co., Ltd., Wuhan, China. The detailed operating conditions for both the laser ablation system and the MC-ICP-MS instrument, as well as the analytical method, are identical to those described by Hu et al., 2012 [42]. All data were collected from zircon using a single-spot ablation mode with a spot size of 44 μm. The analysis location is near the point where U-Pb isotope data were collected. Helium served as the carrier gas within the ablation cell and was subsequently combined with argon (acting as makeup gas) post-ablation. To enhance the sensitivity of Hf isotopes, a small quantity of nitrogen was introduced into the argon makeup gas flow [42]. To guarantee the reliability of the analysis data, three international zircon standards, including Plešovice, 91500, and GJ-1, were analyzed concurrently with the actual samples. Plešovice is employed for external standard calibration to further refine the analysis and test results. Here, 91500 and GJ-1 serve as secondary standards for quality control in data correction. ICPMSDataCal was used to perform offline selection and integration of analyte signals, as well as mass bias calibrations [40].

4.4. Whole-Rock Sr-Nd Isotope Analyses

Sr-Nd isotope analyses were performed by use of a Neptune Plus MC-ICP-MS instrument (ThermoFisher Scientific, Dreieich, Germany) at the Wuhan Sample Solution Analytical Technology Co., Ltd., Wuhan, China. In this study, the exponential law, originally developed for TIMS measurement [43] and still the most widely accepted and employed for MC-ICP-MS, was applied to evaluate instrumental mass discrimination. The detailed experimental process and data processing method are the same as described by Tong et al., 2016 [44].

5. Results

5.1. Whole-Rock Geochemical Composition

Twelve basalt samples were analyzed for whole-rock geochemical features, and the analytical results are presented in Table S1. The LOI values of the basalt samples tested in this study are all above 2.80%, indicating a certain degree of alteration. When utilizing main element oxide analysis data for rock type identification, environmental discrimination, and discussions on the genesis of volcanic rocks, the LOI is initially subtracted to obtain the remaining main element oxide data, which are then recalculated to 100% [45].

5.1.1. Major Elements

The SiO₂ contents in the Late Cambrian basalts of the Nailenggeledaban area in the northern YB range from 47.35 wt.% to 54.44 wt.%, with an average of 50.14 wt.%. The MgO content is 5.14 wt.%~8.61 wt.%, with an average of 6.86 wt.%. The TFe₂O₃ content is 6.30 wt.%~14.17 wt.%, with an average of 11.65 wt.%. The Mg^# value of basalt ranges from 46.8 to 68.3, with an average of 57.9. This value is lower than that of primary magma (Mg^# = 68~75; [46]) but higher than that of the magma resulting from partial melting of the basic lower crust and MORB (Mg^# < 45; [47]).

5.1.2. Rare-Earth Elements and Trace Elements

The basalt samples in this paper belong to alkaline basalt. The REE content of basalts is 90.01–226.49 ppm. The samples are enriched in light rare-earth elements (LREEs) compared to heavy rare-earth elements (HREEs), exhibiting (La/Yb)_N values ranging from 3.63 to 12.81 and slight negative Eu anomalies (δEu = 0.88–1.21; Figure 4a). On the primitive mantle-normalized trace element diagrams, the basalt samples show similar features to OIB as a whole, which are enriched in large ion lithophile elements (LILEs), such as Rb, Ba, Pb, and Sr, and high field strength elements (HFSEs), such as Nb and Ta (Figure 4b). However, unlike OIB, most of the samples display depletion in Zr and Hf.

5.2. Zircon U-Pb Age

A total of 54 zircon grains, sourced from two basalt samples, were subjected to U-Pb dating analysis. Figure 5 displays typical cathodoluminescence (CL) images of the zircon grains that were analyzed. The analytical results are presented in Table S2 and are further depicted in the Concordia diagrams shown in Figure 6. All the analyzed zircon grains exhibited euhedral morphologies, featuring length-to-width ratios ranging from 2:1 to 1:1. The CL images reveal that the majority of the zircon crystals possess magmatic oscillatory zoning (Figure 5).

Thirty zircon crystals were analyzed from the sample TS21-02. Their Th and U contents range from 152 to 859 ppm and 262 to 1860 ppm, respectively, with Th/U ratios varying between 0.15 and 1.38, which suggests an igneous origin. Thirty concordant analyses were obtained from thirty zircon grains. The thirty zircon crystals have a U-Pb age of 490–514 Ma, with a weighted average of 500 Ma (Figure 6a).

Twenty concordant age values were obtained from the 24 analyzed zircon grains of the sample TS2207. The twenty zircon crystals yielded an age of 488–493 Ma, with a weighted average age of 491 Ma (Figure 6b). The twenty zircon crystals exhibit Th contents ranging from 49 to 405 ppm and U contents from 156 to 610 ppm, with Th/U ratios between 0.18 and 1.05, indicating their magmatic origin.

5.3. Zircon Lu-Hf Isotope Features

A total of 12 zircon crystals from TS21-02 and 14 zircon crystals from TS2207 were selected for in situ Lu-Hf isotope analysis. The results are presented in Table S3. The ¹⁷⁶Lu/¹⁷⁷Hf ratios of all zircon crystals range from 0.000700 to 0.002598, with an average value of 0.001306, indicating a low radiogenic Hf content in the selected zircon crystals for this study. The measured ¹⁷⁶Hf/¹⁷⁷Hf ratio essentially reflects the composition of Hf isotopes at the moment of formation [49].

The 12 zircon crystals selected from TS21-02 exhibit ¹⁷⁶Hf/¹⁷⁷Hf ratios ranging from 0.282374 to 0.282444; ε_Hf(t) values ranging from −3.48 to −1.00, with an average value of −2.53 (Figure 7); and T_DM1 values ranging from 1152 to 1263 Ma.

The 14 zircon crystals selected from TS2207 exhibit ¹⁷⁶Hf/¹⁷⁷Hf ratios ranging from 0.282380 to 0.282434; ε_Hf(t) values ranging from −3.38 to −1.52, with an average value of −2.51 (Figure 7); and T_DM1 values ranging from 1165 to 1241 Ma.

5.4. Whole-Rock Sr-Nd Isotope Features

Eight basalt samples were selected for whole-rock Sr-Nd isotope analyses. The results are presented in Table S4. The basalts exhibit (⁸⁷Sr/⁸⁶Sr)_i ratios ranging from 0.706 to 0.707 (average = 0.706777), which is significantly below that of the current continental crust (⁸⁷Sr/⁸⁶Sr)_i = 0.719. The ¹⁴⁷Sm/¹⁴⁴Nd ratios range from 0.125 to 0.156. The (¹⁴³Nd/¹⁴⁴Nd)_i ratio ranges from 0.5118 to 0.5120, with an average value of 0.511871. The ε_Nd(t) values range from −3.53 to −0.96 (Figure 8), with an average of −2.6, and the T_DM1 ranges from 1471 to 2162 Ma.

6. Discussion

6.1. Redetermination of the Stratigraphic Age of the Nailenggeledaban Group in the Northern YB

The Nailenggeledaban Group was established by Mr. Baoyu Lin (1977), and the standard section was located in the Nailenggeledaban area in the upper reaches of the Nanjifuke River, Jinghe County. The term refers to a collection of upper basic volcanic rocks containing limestone lenses. The lithological features transition to intermediate-to-acidic volcanic rocks and pyroclastic rocks along the strike. The lower part is a stratum composed of sandstone and schists. On the 1:200,000 geological maps of the Jinghe and Sayram Lake sheets, the area is classified as the Middle Ordovician and belongs to the Borohoroshan mountain minor region. The stratum in the Sayram Lake area is predominantly exposed in the Bilikexihe River region to the north of Yining County, with a thickness of approximately 767 m. It primarily consists of grayish-purple, grayish-brown, and dark gray volcanic rocks, accompanied by fine clastic rocks containing minor occurrences of limestone and volcanic tuff. Cao (2024) conducted U-Pb dating of volcanic rocks in the Bilikexihe River section to the north of Yili County and determined that the volcanic rocks within it were deposited during the Late Devonian and Early Carboniferous (380–347 Ma) [51]. It is comparable to the upper Dahalajunshan Formation in the region. The stratum in Jinghe County, which is mainly exposed in the Nailenggeledaban area, has a thickness of 422 m. The zircon U-Pb dating of basalt samples reveals eruption ages of 491 Ma and 500 Ma, corresponding to the Late Cambrian (500–490 Ma). These two samples were collected from the middle and upper parts of the stratum, indicating that the initial eruption of the volcanic rocks was earlier than 500 Ma. On the basis of the dating results of this paper and previous studies, it is imperative to redefine the age and attribution of the Nailenggeledaban Group (O₂nl) in the northern margin of the YB. However, further detailed investigations are required to accurately determine the distribution and age of the Nailenggeledaban Group within the northern YB.

6.2. Petrogenesis

Regarding petrogenesis, the ratios of incompatible elements remain unaffected during fractional crystallization due to their similar partition coefficients. Furthermore, only a marginal alteration in the partial melting process of mantle material occurs, making it a commonly used indicator of the source region’s characteristics [52].

The primary mantle-derived magma typically undergoes contamination and metasomatism with crustal materials during the ascent process. Additionally, fractional crystallization can also induce changes in the elemental composition and isotopic ratios of the primary magma. Crustal materials generally exhibit relatively low Nb/La, La/Ba, and Nb/Ba values [53,54]. The high primitive mantle-normalized Th/Nb ratios (>>1; [55]) and the low Nb/La ratios [56] are features of the primary magma undergoing crustal contamination. The studied basalt samples exhibit low La/Ba values ranging from 0.03 to 0.11, with a mean value of 0.08; (Th/Nb)_N values ranging from 0.83 to 1.71, with a mean value of 1.03; and high Nb/La values ranging from 0.93 to 1.40, with a mean value of 1.13. In the Nb/La-(Th/Nb)_N diagrams (Figure 9a) and La/Ba-La/Nb diagrams (Figure 9b), the majority of basalts are classified as uncontaminated volcanic rocks, indicating very limited crustal contamination during the magma ascent process.

The low Mg^#, Ni, and Cr values of the sample, along with the observed negative correlation between SiO₂ and MgO, TiO₂, TFe₂O₃, and CaO (Figure 10), suggest that the samples do not represent primary magma. The process of separation and crystallization of magnesite minerals, such as peridot, clinopyroxene, and iron and titanium oxides, potentially occurred [59]. The δEu values of basalt range from 0.88 to 1.21, with an average value of 1.03, suggesting the absence of significant plagioclase separation and crystallization during magmatic evolution. The enrichment of Y and Yb by garnet has been demonstrated in previous studies [60]. All the studied samples show an enrichment in LREEs relative to HREEs ((La/Yb)_N is 6.94) and deficiencies in Y and Yb compared to Ti and Zr, suggesting that the basalt may have originated from a mantle source region containing garnet. The Sm/Yb-Sm diagram indicates that the samples are concentrated within the range of the garnet + lherzolite curve and the garnet + spinel lherzolite curve, implying a mantle source region composed of garnet and lherzolite with 5%-10% lherzolite (Figure 11). In the Zr/Nb-La/Y diagram, the sample is positioned between N-MORB and OIB, with closer proximity to OIB, suggesting that the source region may consist of mantle material resembling oceanic island basalt (Figure 12a). The basalts exhibit elevated La/Nb and La/Ba ratios, placing them within the OIB region on the La/Ba-La/Nb diagram (Figure 12b).

According to the Zr/Nb ratios of the mantle, it can be categorized into primitive mantle (Zr/Nb ratio = 18), enriched mantle (Zr/Nb ratio < 18), and depleted mantle (Zr/Nb ratio > 18). The Zr/Y-Zr/Nb diagram (Figure 13a) and Zr/Y-Y/Nb-Zr/Nb diagram (Figure 13b) [63] show that the input points of basalt samples in the Nailenggeledaban area on the northern margin of the YB all fall near the enriched mantle region. The ε_Hf(t) values of sample TS21-02 ranged between −3.48 and −1.00, with an average value of −2.53, and the T_DM1 spanned from 1152 to 1263 Ma. The ε_Hf(t) values of TS2207 ranged between −3.38 and −1.52, with an average value of −2.51, and the T_DM1 spanned from 1165 to 1241 Ma. This evidence suggests that the primary magma might originate from the Mesoproterozoic enriched mantle [49]. The ε_Nd(t) values of the basalts range from −3.53 to −0.96, with an average of −2.6, indicating variation in isotopic composition within this basaltic suite. The T_DM1 ranges from 1471 to 2162 Ma, providing further evidence for the potential derivation of these basalts in the Nailenggeledaban area on the northern margin of the YB from the Mesoproterozoic subcontinental lithospheric mantle.

To summarize, the Late Cambrian basalts in the Nailenggeledaban area on the northern margin of the YB are likely derived from an enriched middle Proterozoic lithosphere mantle, with minimal contamination by crustal materials during magma ascent.

6.3. Tectonic Setting and Implications

The basalts in the Nailenggeledaban area on the northern YB are classified as alkaline basalts, originating from an enriched Mesoproterozoic lithosphere mantle. These volcanic rocks exhibit typical features of intracontinental rift valleys. Ti, Zr, Y, and other elements are relatively stable and are minimally affected by crustal or lithospheric mantle contamination. They can be used to identify the tectonic environment of volcanic rocks influenced by crustal or lithospheric mantle contamination [64,65]. In the tectonic setting discrimination diagrams of volcanic rocks (Figure 14), all the samples fall into the intraplate basalt region.

Some researchers suggest that the northern margin of the YB was a passive continental margin before the Ordovician [66,67]. The identification of Middle and Late Ordovician continental arc-type intrusive rocks in the Wenquan area suggests that the North Tianshan Ocean (or Junggar Ocean) had already subducted beneath the YB [14,15]. Further evidence came from Late Ordovician (460–458 Ma) arc-type tuff interlayers within the siliceous shales in the Guozigou area [19]. Therefore, the northern YB had already been an active continental margin in the Middle Ordovician [15,19]; however, the transformation process remains unclear. The subduction of the North Tianshan Ocean (or Junggar Ocean) in the western Junggar region commenced in the Early Cambrian and persisted until at least the conclusion of the Silurian, as evidenced by previous studies [68,69,70,71]. Previous studies suggest that the western Junggar area and YB potentially exhibit a linear distribution before the formation of the Kazakhstan Orocline [4,72,73,74]. Therefore, it is postulated by certain scholars that the subduction of the North Tianshan Ocean (or Junggar Ocean) might have started in the Cambrian and gradually extended from west (the West Junggar region) to east (the northern YB) [19,20].

The absence of Late Cambrian-Early Ordovician magmatic rocks in the northern YB suggests a period of quiescence in magmatic activity. However, we have identified numerous detrital zircon grains from the Late Cambrian-Ordovician sandstones in the northern YB (~500 Ma, unpublished data of the research group). The Hf isotope features of the detrital zircon are similar to those observed in the mid-late Ordovician intrusive rocks in the northern YB. This finding indicates the existence of Late Cambrian-Early Ordovician magmatism in the northern YB, although the exact bedrock remains undiscovered. It might still lie beneath the surface or may have been completely eroded away. Although the Late Cambrian volcanic rocks in the Nailenggeledaban area studied in this paper exhibit overall similarities to OIB, they also display a certain degree of Zr and Hf depletion, potentially influenced by subduction fluids. Consequently, it is possible that this suite of volcanic rocks was formed within a back-arc extensional setting.

Figure 14

Construction environment discrimination diagram in basalts. (a) [75]: WPB: within-plate basalt; MORB: mid-ocean ridge basalt; VAB: volcanic arc basalt. (b) [76]: I: N-MORB area at the edge of plate divergence; II: basalt area at the edge of the convergent plate (II1: ocean island arc basalt; II2: continental margin island arc and continental margin volcanic arc basalt area); III: ocean island, seamount basalt area, and TMORB, E-MORB area; IV: continental intraplate basalt area (IV1: intracontinental rift valley and continental rift valley basalt area; IV2: intracontinental rift alkaline basalt area; IV3: continental extension zone (or initial rift) basalt area); V: mantle plume basalt area. (c) [77] (Cabanis and Lecolle, 1989). (d) [78]: AI, AII: in-plate alkaline basalt; AII, C: intraplate tholeiitic basalt, B: enriched mid-ocean ridge basalt, D: depleted mid-ocean ridge basalt; C, D: volcanic arc basalt.

[Figure omitted. See PDF]

7. Conclusions

• (1). The LA-ICP-MS U-Pb age of the alkaline basalts from the Nailenggeledaban area is 490.8 ± 2.0 Ma and 499.9 ± 1.8 Ma, indicating that these rocks were formed in the Late Cambrian.

• (2). The basalt samples exhibit significant enrichments in LILEs such as Rb, Ba, Pb, and Sr, as well as HFSEs, including Nb and Ta. These geochemical characteristics resemble those of OIB. The primary magma in these samples was mainly enriched mantle material, and the degree of contamination by the new crustal material was very limited.

• (3). Combined with previous studies, our results suggest that the volcanic rocks in the Nailenggeledaban area, located on the northern margin of the Yili Block, were formed in a back-arc extensional environment resulting from the subduction of the North Tianshan Ocean (or Junggar Ocean) beneath the northern margin of the Yili Block during the Late Cambrian.

Author Contributions

Conceptualization, M.W., Y.C. and S.Z. (Shaowei Zhao); methodology, M.W.; software, D.X.; formal analysis, M.W. and Y.C.; investigation, D.X., M.C., M.W., Y.C., S.Z. (Shaowei Zhao), S.Z. (Shengqiang Zhu), T.W. and Z.B.; resources, M.W.; data curation, D.X., M.W. and M.C.; writing—original draft preparation, D.X., M.C. and M.W.; writing—review and editing, M.W., M.C., Y.C. and S.Z. (Shaowei Zhao); supervision, M.W.; funding acquisition, M.W. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement

The original contributions presented in the study are included in the article/Supplementary Material.

Conflicts of Interest

The authors declare no conflicts of interest.

Footnotes

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Figures

Figure 2. Location map of Late Cambrian basalt samples in the Nailenggeledaban area on the northern YB (modified after Dong et al., 2009 [36]).

Figure 3. (a) TS21-02, (b) TS2207 field photos, and (c) TS21-02, (d) TS2207 microscope photos of basalts in the Nailenggeledaban area. Pl: plagioclase, Px: pyroxene.

Figure 4. (a) Chondrite-normalized REE patterns and (b) space primitive mantle-normalized trace element (normalization values are from Sun et al., 1989 [48]).

Figure 5. Zircon CL images of basalts (in the figure, the red solid circle represents the laser ablation position of zircon age, and the yellow dotted circle represents the analysis position of zircon Lu-Hf isotope).

Figure 6. Zircon U-Pb age concordia diagrams of basalts in the Nailenggeledaban area.

Figure 7. εHf(t)-t-plot for the zircon crystals of basalts in the Nailenggeledaban area.

Figure 8. (87Sr/86Sr)i-εNd(t) plot for the basalts in the Nailenggeledaban area (modified after Zimmer et al., 1995 [50]). DM: depleted mantle, MORB: Mid-Ocean Ridge Basalt, OIB: Ocean Island Basalt.

Figure 9. (a) Nb/La-(Th/Nb)N diagram and (b) La/Ba-La/Nb diagram of basalt in the Nailenggeledaban area (base map according to Fitton et al., 1991, 1995 [57,58]). OIB: Ocean Island Basalt.

Figure 10. Harker variation diagram of the basalts in Nailenggeledaban area.

Figure 11. Sm/Yb-Sm diagram of basalt in the Nailenggeledaban area (the trends of the partial melting models are derived from Aldanmaz et al., 2000 [61]). DM: depleted mantle, PM: enriched mantle, N-MORB: Normal Mid-Ocean Ridge Basalt, E-MORB: Enriched Mid-Ocean Ridge Basalt.

Figure 12. (a) Zr/Nb-La/Y diagram and (b) La/Ba-La/Nb diagram of basalts in the Nailenggeledaban area (modified after Xia et al., 2019 [62]). OIB: Ocean Island Basalt, MORB: Mid-Ocean Ridge Basalt, N-MORB: Normal Mid-Ocean Ridge Basalt.

Figure 13. (a) Zr/Y-Zr/Nb diagram and (b) Zr/Y-Y/Nb-Zr/Nb diagram of basalts in the Naile-nggeledaban area (modified after Fodor et al., 1984 [63]).

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/min15010007/s1, Table S1: Major element (%) and trace and rare earth element (×10⁻⁶) analysis data of the basalts from the Nailenggeledaban area; Table S2: LA-ICP-MS zircon U-Pb age test results of the basalts in the Nailenggeledaban area; Table S3: Zircon Lu-Hf isotopic compositions of the basalts from the Nailenggeledaban area; Table S4: Sr-Nd isotopic compositions of the basalts from the Nailenggeledaban area.

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