Petrogenesis and Geochronology of Late Devonian Intrusive Rocks in Eastern Tianshan, Xinjiang, China: Subduction Constraints of the North Tianshan Ocean

Abstract

We conducted a study on the petrology, geochemistry, and zircon U–Pb dating of Late Devonian intrusive rocks in the Tulargen area of the Eastern Tianshan Orogenic Belt, Xinjiang, China. These intrusive rocks primarily consist of gabbro (382 ± 5 Ma), tonalite (370.9 ± 2.7 Ma), and biotite monzogranite (362.8 ± 4.4 Ma). Gabbro belongs to the low-K calc-alkaline series of quasi-aluminous rocks, with a high Al₂O₃ content (16.46–20.34 wt.%) and Mg^# value (64.55–67.73). Tonalite and biotite monzogranite, which belong to the high-K calc-alkaline series, are metaluminous or weakly peraluminous and also exhibit high Al₂O₃ contents (14.6–15.87 wt.%) and Mg^# values (40.12–62.47). These rocks are enriched in light rare-earth and large-ion lithophile elements (Rb, Ba, and K) and depleted in heavy rare-earth and high-field-strength elements (e.g., Ta, Nb, and Ti), characteristics typical of island-arc magmatic rocks. Gabbro melts are primarily derived from the mantle and result from the partial melting of a depleted mantle that has undergone fluid metasomatism due to subducted plates. Tonalite exhibits high ¹⁷⁶Hf/¹⁷⁷Hf and _εHf(t) values, with a younger two-stage model age (t_DM2) derived from partial juvenile crust melting. The source magma of the biotite monzogranite originated from partial metabasalt melting at a medium crustal depth combined with a new lower crustal material. We concluded that the Late Devonian intrusive rocks in this area formed within the island-arc tectonic setting are associated with the subduction of the North Tianshan Ocean.

1. Introduction

The Central Asian Orogenic Belt (CAOB) is one of the largest Phanerozoic accretionary orogenic belts, located between the Siberian Craton, Tarim Craton, and the North China Craton, extending approximately 7000 km in an east–west direction [1,2,3,4,5,6,7,8,9,10] (Figure 1a). As the southernmost CAOB segment, the Tianshan Orogenic Belt experienced significant tectono-magmatic activity and crustal accretion during the Paleozoic, accompanied by notable economic ore-forming processes [6,11,12,13,14,15,16,17,18,19,20]. Therefore, it is an ideal region for advancing the understanding of the accretionary and collisional processes that formed the southern CAOB, as well as the influence these processes may have on economic mineral system formation [7,21].

A central point of debate regarding region evolution is whether the orogenic processes forming the Tianshan Orogenic Belt extended from the early to late Paleozoic Era [22,23,24,25,26] or ended in the middle Paleozoic and were followed by intracontinental extension [27,28]. Its tectonic evolution has been the focus of many studies, especially regarding the timing of the North Tianshan Ocean closure. Xia et al. [29] and Xu et al. [19] suggested that the North Tianshan Ocean was not part of the Paleozoic Tianshan Ocean basin; instead, they interpreted that a new “Red Sea-type” ocean basin formed after the Paleozoic Tianshan Ocean basin closure in the Late Devonian to early Carboniferous period, which was followed by large-scale rifting in the Tianshan and adjacent areas. Wang et al. [30] and Tang et al. [31] suggested that the Carboniferous adakite, high-Mg andesite, and Nb-rich basalt assemblage in the Tianshan area, thereby suggesting that the North Tianshan Ocean closed in the late Carboniferous to Early Permian. Han et al. [32] suggested that the North Tianshan Ocean closed in the early part of the late Carboniferous based on a zircon U–Pb age of 316 Ma. Zhou et al. [33] suggested that the mafic–ultramafic intrusive rocks and mafic dike swarms in the Tianshan Mountains and surrounding areas formed during 289–241 Ma, possibly in response to mantle plume activity, and that the North Tianshan Ocean closed in the Late Permian. Xiao et al. [25] suggested that compression–extension and strike–slip tectonism coincided with magmatism in the Tianshan area during the late Carboniferous to Early Permian; the ocean basin closure may have occurred as late as the Late Permian or Early Triassic. Previous studies concentrated on the role that early Paleozoic or Carboniferous–Permian magmatism played [19,20,22,29,34,35,36] in the Tianshan Orogenic Belt evolution. However, the tectonic setting and geological evolution during the Devonian remains controversial due to lacking constraints on Devonian magmatism.

Figure 1

Simplified tectonic map showing the Central Asia Orogenic Belt [6] (a); schematic map of the Eastern Tianshan [37] (b); simplified geological map of Tulargen regions [37] (c).

[Figure omitted. See PDF]

We conducted a detailed investigation in the Tulargen Eastern Tianshan Orogenic Belt and identified a series of Mid-to-Late Devonian magmatic rocks to address ongoing debates in the literature. Additionally, we carried out geochronological, whole-rock geochemical, and Hf isotopic analyses of gabbros, tonalites, and biotite monzogranite to establish a new framework for understanding the Devonian evolution of the Eastern Tianshan.

2. Geological Setting

The Eastern Tianshan Orogen, located between the Karamaili and South Tianshan Faults (Figure 1b), is divided into the Bogeda–Harlik belt, Juelotage belt, Tu–Ha Basin, and the Central Tianshan land [37]. Bound by the Kangguer Fault, the Jueluotage belt can be further divided into the Dananhu–Tousuquan and Yamansu sub-belts [37]. Bounding faults tend to mainly congregate in an east–west direction, consistent with the regional structural trends, and provide a primary control group on the location of magmatism and mineralization in the area.

The strata exposed in the study area primarily belong to the Middle Silurian, Lower Devonian, and Lower Carboniferous (Figure 1c). Middle Silurian, Lower Devonian, and Lower Carboniferous rocks include a set of fine-grained clastic and volcanic rock, shallow-marine volcanic–sedimentary, and semi-deep-sea or deep-sea sedimentary formations [38]. Intrusive rocks are mainly Devonian, Late Carboniferous, Late Permian, and Triassic. Late Devonian intrusive rocks were found in the eastern section of the Kangguer shear belt and discovered during a recent 1:250,000 regional geological mapping by the Xi’an Center of Geological Survey. These intrusive rocks include gabbro, tonalite, and biotite monzogranite and are intruded by late Carboniferous mafic rock and acidic granite, in faulted contact with lower Carboniferous rocks to the north and spread out in an east–west direction. Biotite monzogranite intrudes into gabbro and tonalite, but there is no direct contact between gabbro and tonalite.

3. Petrography

Gabbro is medium-grained and unfoliated with a metagabbroic texture. The original mineralogy included 55% augite, 40% plagioclase, and 5% opaque minerals (ilmenite). Augite is subhedral in shape, 2–5 mm in size, and variably replaced by hornblende. Plagioclase is subhedral and tabular and variably replaced by fine-grained epidote. The accessory metallic minerals are anhedral, granular, or platy and small, ranging in size between 0.1 and 0.8 mm (Figure 2a,b).

Tonalite is gray or light gray, fine-grained, with a massive to equigranular and locally weakly foliated texture. It mainly consists of plagioclase (60%–65%), quartz (25%–30%), and biotite (10%), with minor additions of rutile, zircon, and apatite (<0.1% each). The biotite is subhedral, platy, and pleochroic (mostly reddish-brown to light brown) and has a preferred alignment (Figure 2c,d).

Biotite monzogranite is massive and heterogranular texture. It comprises 45% plagioclase, 25% alkali feldspar, 22% quartz, and 8% biotite, with minor additions of apatite and opaque minerals. The plagioclase crystals are tabular or granular, with grain sizes ranging up to 5 mm. The crystals are commonly sericitized and epidotized to varying degrees. The alkaline feldspars are mainly microcline and up to 4 mm in size. Quartz crystals are mostly granular and up to 3.2 mm in size and commonly exhibit wavy or zonal extinction. Biotite is mostly lamellar and commonly replaced by chlorite and epidote. The accessory mineral apatite is columnar or finely granular (Figure 2e,f).

4. Analytical Methods

Gabbro, tonalite, and biotite monzogranite samples were taken during regional geological field investigation and based on a detailed petrographic study. In total, 16 representative texturally homogeneous and unaltered samples were selected for major- and trace-element analyses. Three of the sixteen samples, one of each lithology, were also chosen for zircon U–Pb analysis.

4.1. Zircon U–Pb Data

Samples were crushed to 80–100 mesh and separated using heavy-liquid and electromagnetic methods. Zircons with good crystal shape and high transparency were selected manually, embedded in epoxy resin, and polished. Internal structures were examined using transmitted- and reflected-light microscopy and cathodoluminescence (CL) to identify optimal sites for laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS).

Zircon CL, LA–ICP–MS, in situ U–Pb, and Lu–Hf dating of zircon were completed at the Key Laboratory for the study of focused Magmatism and Giant ore Deposits, MLR, in Xi’an Center of Geological Survey, China Geological Survey. The experiment was carried out using Agilent 7500 ICP–MS and a COMPex 102 ArF excimer laser (193 nm, 30 μm spot diameter). Helium served as the carrier gas, and NIST SRM 610 was used for optimization. A single-point erosion sampling method was employed, and Andersen’s 3D coordinate method was used to correct common lead. Element contents were determined using NIST SRM 610 and ²⁹Si as standards. Isotopic ratios and element content data were analyzed with ICPMS Data Cal software [39]. Andersen [40] and Isoplot (3.0 edition) [41] software was used for common lead correction, age calculation, and Concordia diagrams, respectively. The analytical methods and instrument settings were consistent with those reported by Li et al. [42].

In situ zircon Hf isotope analyses were performed using a Geolas Pro laser-ablation system coupled to a Neptune plus Multiple-Collector ICP-MS. A 32 μm laser spot was used, with ablated aerosol carried by helium and mixed with argon and nitrogen before entering the ICP-MS. Hf analyses were conducted at the same spots as U–Pb dating, with data quality monitored through reference zircon analyses. Lu and Hf isotopes were calculated using in-house software Hfllow 3.5 (Patent No. ZL 2018 1 0087759.6). For details, refer to the work by Gao et al. [43].

The two-stage model ages were calculated using the fLu/Hf = −0.72 value for the upper continental crust in the zircon Lu–Hf isotope analyses [44]. For detailed calculation methods, refer to Griffin et al. [45], Söderlund et al. [46], and Bouvier et al. [47].

4.2. Geochemical Analyses

Major- and trace-element analyses were completed at the Xi’an Institute of Geology and Mineral Resources. Major-element analysis was performed on a Panalytical PW440 X-ray fluorescence spectrometer, and errors were better calculated at values <5%. Trace elements, including rare-earth elements (REEs), were determined by inductively coupled plasma–mass spectrometry (ICP–MS) on a Thermo Fisher X–Series^II spectrometer. The detection limit for most elements is better estimated from 5 × 10⁻⁹, and the relative standard deviation was better than 5%.

5. Analysis Results

5.1. Zircon U–Pb Age

The zircon surfaces from gabbro (sample JRQ01) are clean, transparent, and dark. Most zircons are euhedral crystals, measuring 50–260 μm in length, with length-to-width ratios ranging from 1:1 to 1:3. CL images reveal that most zircons exhibit a consistent internal structure and clear oscillatory zoning. We determined the U–Pb isotopic ages for 23 zircons from gabbro (Table S1). The U and Th contents of these zircons are relatively high and vary significantly, with U, Th, and Th/U ranging from 103 to 1103 ppm, 58 to 2098 ppm, and 0.31 to 0.95, respectively, indicating that the studied zircons are typical magmatic zircons. The ²⁰⁶Pb/²³⁸U ages indicate that 23 spots are concentrated within the range of 408–371 Ma, with a weighted average age of 382 ± 5 Ma (MSWD = 3.9) (Figure 3a), suggesting that gabbro formed in the Late Devonian.

Zircons from tonalite (sample JRQ02) measure 30–160 μm in length, with length-to-width ratios ranging from 1:1 to 1:2. They are transparent euhedral crystals exhibiting an oscillatory zoning typical of magmatic zircon (Figure 3b). We determined U–Pb isotopic ages for 24 zircons from tonalite (Table S2). These zircons exhibit high U (5.95–336.69 ppm) and Th (1.8–298.03 ppm) contents, with Th/U ratios ranging from 0.3 to 0.95 (Table S2), which is typical for igneous zircon. The ²⁰⁶Pb/²³⁸U ages of the effective 24 points are primarily distributed within the range of 388.5–356.4 Ma, with a weighted average age of 370.9 ± 2.7 Ma (MSWD = 0.56) (Figure 3b), indicating that the tonalite emplacement age is also Late Devonian.

The zircons in biotite monzogranite (sample JRQ03) measure 80–210 μm in length, with length-to-width ratios ranging from 1:1 to 1:4. They are transparent euhedral crystals exhibiting typical magmatic oscillatory zoning. We determined U–Pb isotopic ages for 20 zircons (Table S3). The U and Th contents of these zircons are relatively high and highly variable, with U, Th, and Th/U ratios ranging from 63.76 to 584.52 ppm, 41.59 to 405.81 ppm, and 0.47 to 0.84, respectively, indicating that the studied zircons are typical magmatic zircons. The ²⁰⁶Pb/²³⁸U ages of 20 spots are concentrated within the range of 382–349 Ma, with a weighted average age of 362.8 ± 4.4 Ma (MSWD = 1.06) (Figure 3c), further indicating that the biotite monzogranite emplacement age is Late Devonian.

Accordingly, the altered gabbro, tonalite, and biotite monzogranite in this study all formed in the Late Devonian, between approximately 382 and 362.8 Ma.

5.2. Lu–Hf Isotopes

We performed in situ Lu–Hf–isotopic analysis of zircon in tonalite and biotite monzogranite (Table S4).

Weighted-average age calculations for tonalite, based on in situ Hf–isotopic analysis from 23 spots, yielded ¹⁷⁶Yb/¹⁷⁷Hf and ¹⁷⁶Lu/¹⁷⁷Hf ratios of 0.009484–0.3084 and 0.000284–0.008191, respectively. Eight analyses show that ¹⁷⁶Lu/¹⁷⁷Hf ratios are <0.002, with ¹⁷⁶Hf/¹⁷⁷Hf ratios in the range of 0.282933–0.282991. The corresponding ε_Hf(t) values and crustal model age t_DM^C are in the range of 5.68–7.75 and 808.11–311.45 Ma, respectively.

Weighted-average age calculations for biotite monzogranite, based on in situ Hf–isotopic analysis of 16 spots, yielded ¹⁷⁶Yb/¹⁷⁷Hf and ¹⁷⁶Lu/¹⁷⁷Hf ratios of 0.039275–0.087567 and 0.001418–0.003003, respectively. Five ¹⁷⁶Lu/¹⁷⁷Hf ratios are <0.002, and their ¹⁷⁶Hf/¹⁷⁷Hf ratios are in the range of 0.283021–0.283193. The corresponding ε_Hf(t) values and crustal model age T_DM^C are in the range of 8.81–14.88 and 524.13–62.58 Ma.

5.3. Major- and Trace-Element Geochemistry

5.3.1. Major Elements

Given the limited number of samples obtained for each group from the available outcrop, the geochemical data spread is small (e.g., silica values typically range only a few wt.%) (Table S5). However, the data are clearly sufficient to characterize the geochemical style of magmatism. Gabbro contains 47.9–49.22 wt.% SiO₂, 16.46–20.34 wt.% Al₂O₃, and 2.16–2.63 wt.% total alkalis (Na₂O + K₂O), with Na₂O > K₂O. They straddle the low-K calc–alkaline/tholeiite boundary on a SiO₂–K₂O diagram (Figure 4a). MgO, FeO^T, and Mg^# (5.77–8.74 wt.%, 5.64–7.59 wt.%, and 64.55–67.73, respectively) are relatively high and correspond with the mantle parental magma Mg^# (60–71) and without crystallization differentiation [48,49].

Tonalite has a SiO₂ content of 65.28–68.12 wt.%, with a total alkali (Na₂O + K₂O) content of 6.24–6.84 wt.% and a Na₂O/K₂O ratio of 1.16–1.45. The concentrations of Al₂O₃, MgO, and FeO^T are 14.6–15.3 wt.%, 1.25–1.46 wt.%, and 3.33%–3.72%, respectively, with Mg^# values of 52.44–62.47. The rocks are metaluminous (Figure 4b) but lie close to the peraluminous boundary with an A/CNK ratio of 0.95–0.98. Despite their sodic compositions, the rocks in the field fall under the high-K calc-alkaline series (Figure 4a). As previously demonstrated, high Mg^# might point to a significant component of mantle-derived material.

Biotite monzogranite contains 69.12–71.1 wt.% SiO₂, 2.97–3.8 wt.% K₂O, and 4.88–5.4 wt.% Na₂O, with a Na₂O/K₂O of 1.28–1.82. The concentrations of MgO, FeO^T, and Al₂O₃ are 0.48–0.62 wt.%, 1.73–2.19 wt.%, and 14.77–15.87 wt.%, respectively, with Mg^# values of 40.12–58.78. These rock ranges demonstrate very weak peraluminous compositions (Figure 4b) (A/CNK 0.99–1.02) and fall into the high-K calc-alkaline series (Figure 4a).

5.3.2. Trace Elements

The total REE content (ΣREE) of gabbro is relatively low, averaging 38.42 ppm and ranging between 36.61 and 41.24 ppm. (La/Yb)_N, (La/Sm)_N, and (Yb/Gd)_N range from 2.18 to 2.56, 1.16 to 1.29, and 0.6 to 0.65, reflecting the weak fractionation of the REEs, with no clear Eu anomaly (Figure 5a). (Ce/Yb)_N > 1 indicates a high degree of magma fractional crystallization. The La/Nb, La/Ta, Ba/Nb, Nb/Yb, and Ce/Pb ratios of gabbros are 3.72–3.88, 28.85–37.3, 57.68–88.25, 0.78–0.98, and 6.5–9.75, respectively.

The ΣREE of the tonalite clearly increases, ranging from 92.27 to 115.16 ppm. (La/Yb)_N, (La/Sm)_N, and (Yb/Gd)_N range from 6.64 to 7.17, 6.64 to 7.17, and 0.67 to 0.7. Significant light to middle rare-earth element (L–MREE) fractionation occurred, but normalized heavy rare-earth element (HREE) patterns remain flat and unfractionated. Eu anomalies are either absent or slightly negative (Figure 5a).

The ΣREE content of biotite monzogranite is 61.92–93.93 ppm, and (La/Y)_N, (La/Sm)_N, and (Yb/Gd)_N range from 7.29 to 8.29, 7.22 to 8.56, and 0.76 to 0.86, respectively. Normalized REE patterns are similar to those of tonalite, with fractionated L–MREE and unfractionated HREE. Eu anomalies are not clearly developed (Figure 5a).

Although mantle-normalized trace-element patterns indicate weak relative enrichment in large-ion lithophile elements (LILEs) such as Rb, Ba, and K (Figure 5b), these remain in low concentrations in gabbros. LILE enrichment is much clearer in tonalites and granites. All rocks show depleted high-field-strength elements (HFSEs), i.e., Ta, Nb, and Ti, in adjacent LILEs on mantle-normalized plots [52] (Figure 5b). The mantle-normalized Nb concentrations in gabbro are lower than those of the HREEs, reflecting a previously melt-depleted mantle source region [53].

6. Discussion

The Devonian intrusive rocks in the East Tianshan area were formed in the Late Devonian (362.8–382 Ma). Each rock type has a certain degree of continuity in time and space, forming a combination of Late Devonian gabbro–tonalite–monzogranite (Figure 1c).

6.1. Petrogenesis

In the Harker diagrams (Figure 6), the correlation between SiO₂ and other common elements in gabbro, tonalite, and biotite monzogranite is generally weak, indicating non-cogenetic magmatic affinity characteristics [53]. This suite of rocks likely represents distinct parental magma products from the same tectonic period.

Incompatible trace element patterns (Figure 5b) show that the studied magmatic rocks are relatively enriched in light rare-earth elements (LREEs) and LILEs (e.g., Rb, Ba, and K) and depleted in HREEs and HFSEs (e.g., Ta, Nb, and Ti). Such features are characteristic of subduction-related magmas, whereas the very low LILE concentrations in gabbros are perhaps indicative of fore- or island-arc magmas [54,55,56]. However, such geochemical characteristics can also arise via mantle-derived rock crustal contamination [57,58]. In the Nb–Y and Rb–(Y + Nb) diagram (Figure 7), the samples of the studied granitic rocks fall into the VAG region.

The La/Nb and Ba/Nb ratios of the gabbros are 3.72–3.88 and 57.68–88.25, respectively, which are much higher than the N–MORB values of 1.07 and 4.30 [60]. However, Nb/Yb ratios between 0.78 and 0.98 are only marginally higher than those of N–MORB and much lower than primitive mantle values and clearly reflect a depleted mantle source. Gabbros have La/Nb ratios greater than 1.5 (3.72–3.88) and La/Ta ratios exceeding 22 (28.85–37.3), suggesting that Jingerquan gabbros originated from the lithospheric mantle [61].

Th, U, LREE, and LILE enrichments mainly accompany higher ratios against Nb and Ta than expected from depleted mantle melt contamination with an average continental crust. With one exception (9.75), gabbros have Ce/Pb ratios within 2.77–7.21, a range that includes the global average (2.9) for oceanic sediments [62]. The relatively low Nb/U and Ta/U ratios (4.47–7.48 and 0.36–0.91, respectively) of the studied gabbros are similar to those of magma derived from partial mantle wedge metasomatism melting with fluids from a subducted plate [63]. Accordingly, we interpret the magma-crystallized gabbros to have formed by melting a depleted mantle source that had undergone fluid metasomatism during subduction.

In addition, given that biotite monzogranites lack an accompanying arfvedsonite [64,65], the studied samples should not be considered A-type granites. The SiO₂ contents of tonalites and biotite monzogranites are negatively correlated with P₂O₅, which typically decreases in the evolution process of granitic magmas, as is characteristic of I-type granite [66,67]. The granite-type discrimination diagram (Figure 8) shows that the granite in the area includes both I-type and S-type. No Al-rich minerals, such as cordierite and muscovite, are found in the samples, which are obviously different from typically Al-rich S-type granite [66]. The Rb/Sr, Rb/Ba, and K/Ba ratios of the samples are 0.08–0.17, 0.03–0.07, and 0.0021–0.0034, respectively, which are consistent with those of the parent magma of I-type granite [68], thereby further indicating their affinity to I-type granite.

On the covariant diagram for (K₂O + Na₂O)/CaO and Zr + Nb + Ce + Y (Figure 8), all the samples plot within the area of undifferentiated granite, indicating that crystallization differentiation was poor during magmatic evolution. However, the strongly negative P and Ti anomalies suggest that apatite and Fe–Ti oxide fractional crystallization may have occurred in the ascending magma (Figure 5b).

Tonalites’ Nb/Ta values (12.23–12.84) are between the average values for the crust (~11) [70] and the mantle (17.5) [52], while their Zr/Hf values (31.44–34.33) are less than the average value for the crust (44.68) [71] but greater than the average value for the mantle (30.74) [70], suggesting crustal-source contribution. The depleted Nb and Ta values (Figure 5b) also suggest continental crust contribution. Tonalites have high initial ε_Hf(t) (5.68258–7.74965) values and corresponding two-stage model age t_DM2 (724.47–592.17 Ma), which also indicates that tonalite originated from partial newly formed crust melting [13,14].

The low Cr (6.48–11.78) and Ni (1.49–3.88) contents in biotite monzogranite indicate that it did not derive from primitive mantle-source magma [72]. Similar to tonalites, the Nb/Ta radio (8.2–9.72) is lower than the average value for the crust (~11) [70], while the Zr/Hf radio (36.53–37.37) falls between the average mantle (30.74) [70] and crustal values (44.68) [71], indicating significant crustal contribution. The depleted Nb and Ta values (Figure 5b) are typical of continental crusts. The sodic nature of these rocks suggests a mafic source.

In the Th/Yb and Ta/Yb diagram [73] (Figure 9), all samples are near the crustal area. The flat medium to heavy rare-earth element (M–HREE) patterns show that they were not formed by melting in the presence of garnet. The whole-rock geochemical composition diagram (Figure 10) shows that all biotite monzogranite samples are within the metabasalt area, suggesting their magma primarily resulted from partial metabasalt melting. Metabasalt has a higher Mg content than meta-andesite. If biotite monzogranites are derived from partial metabasalt melting, this would have occurred above the garnet-stable region (<10 kb) [74,75].

Biotite monzogranite formed via this process exhibits high SiO₂ (69.12–71.1 wt.%) and low MgO (0.48–0.62 wt.%) contents, indicating formation under middle-crustal pressure [76,77,78]. Its peraluminous characteristics indicate that melting occurred under H₂O-unsaturated conditions [77,78,79,80,81]. Additionally, positive ε_Hf(t) values (8.81–14.88) and a two-stage model age (t_DM2 < 524.13 Ma) indicate that the original magma also derived from juvenile crust. Therefore, the parental magma for biotite monzogranite was derived from partially melted metabasalt at medium-crustal depth, incorporating juvenile lower-crust material. This melt resembles I-type granite, characterized by poorly differentiated crystallization and H₂O-unsaturated conditions.

Figure 10

Compositional fields of experimental melts derived from dehydration melting of various bulk compositions [82,83,84,85,86,87,88,89,90].

[Figure omitted. See PDF]

6.2. Tectonic Setting

Within the Tianshan Orogenic Belt, limited studies have reported Late Devonian magmatic rocks. Some researchers suggest that the Late Devonian magmatic activity was associated with an oceanic slab subduction setting [91,92,93,94], while others propose that the Late Devonian intrusions formed during a collisional orogenic stage, as suggested by Gao Jinggang et al. [95].

Basaltic magmas with distinctive geochemical signatures and specific tectonic settings can provide relatively reliable insights into tectonic environments. The Jingerquan gabbros in this study generally show enriched light rare-earth elements (LREEs) relative to heavy rare-earth elements (HREEs), with significant negative anomalies in Ta, Nb, and Ti, indicating that gabbros may be closely associated with magmatic activity linked to an arc setting. Both the Hf/3-Th-Ta (Figure 11a) and Th/Yb-Nb/Yb (Figure 11b) diagrams identify the Jingerquan gabbros as island-arc basalts. In summary, Jingerquan gabbros were formed in a subduction-related environment.

Between the Tianshan and Beishan Orogenic Belts, there is a general unconformity separating Late Devonian or early Carboniferous strata from underlying sedimentary strata or igneous rock units [98]. This indicates that there was a strong folding and uplifting orogenic event in the Tianshan area before the early Carboniferous and may indicate that the partial oceanic convergence in Tianshan was Late Devonian to early Carboniferous. After the early Carboniferous, the Tianshan area entered the post-collisional rifting stage. The magma that formed the volcanic rocks exposed in the Tianshan Orogenic Belt was of alkaline and tholeiitic basalt composition. In addition, evidence of island-arc volcanism (such as low Nb, Ta, and Ti) suggests that continental crust or continental lithosphere contamination occurred, and some areas show bimodal volcanic characteristics [30,99,100,101].

As previously mentioned, the crystallization ages of gabbro, tonalite, and biotite monzogranite in this study are 382 ± 5 Ma, 370.9 ± 2.7 Ma, and 362.8 ± 4.4 Ma, respectively. Gabbro is the product of the partial melting of depleted mantle metasomatized during active oceanic lithosphere subduction, most likely associated with the North Tianshan Ocean closure. Both tonalite and biotite monzogranite are calc-alkaline magmas with geochemical characteristics such as enrichments in Rb, Ba, and K and depleted in Ta, Nb, and Ti, consistent with magmas produced via active continental margin crust melting, with or without the contribution of direct mantle contribution magmas. Zhou [102] studied the origin of granites in the Jueluotage area and suggested that the Late Devonian granites (386.5–369.5 Ma) initially formed in the pre-collision stage and entered the main collision stage in the early Carboniferous. This conclusion is consistent with the subduction tectonic setting for the studied gabbro. The corresponding tonalite and biotite monzogranite fall under the end of the subduction stage.

Subduction in the early part of the Late Devonian resulted in the melting of fluid-metasomatism-depleted mantle parental magmas to gabbro. At the same time, the ascent and emplacement of mantle-derived magma resulted in crustal accretion, which not only provided new crustal material but also generated considerable heat. Under emplacement magma heating, the partial metabasalt melting at a medium-crustal depth and with a young lower-crustal material form monzonitic magma, while the partial melting of newly formed crust forms tonalitic magma. Tonalite and monzogranite intrusion led to significant vertical crustal accretion (Figure 12).

7. Conclusions

(1) Zircon U–Pb dating determined the crystallization ages for gabbro, tonalite, and biotite monzogranite of 382 ± 5 Ma, 370.9 ± 2.7 Ma, and 362.8 ± 4.4 Ma, respectively, which indicate that there was Late Devonian basic and acidic magmatism in the North Tianshan Orogenic Belt.

(2) The Late Devonian assemblage of gabbro, tonalite, and biotite monzogranite developed in the Tulargen area in Eastern Tianshan. Gabbro belongs to the low-K calc-alkaline series. Tonalite and biotite monzogranite are of the calc-alkaline or high-potassium calc-alkaline series. These all have the characteristics of arc-related magmatic rocks.

(3) Gabbro melt was produced via the partial melting of a depleted mantle, which previously underwent fluid metasomatization during subduction. Tonalite was derived from the partial melting of the new crust. The original source magma for biotite monzogranite was derived from the contributions of partially melted metabasalt at a medium-crustal depth and with a young lower-crust material.

(4) The research results indicate that during the Late Devonian period (lasting 380–362 Ma), the Tianshan Ocean was still under oceanic continental subduction, and the closure time of the Southern Tianshan Ocean was after the Late Devonian period.

Author Contributions

Conceptualization, Y.M. and X.Z.; methodology, Y.M. and X.Z.; software, Z.L., H.Z. and Y.H.; validation, H.Z. and X.Z.; formal analysis, Y.M., X.Z., K.W. and Z.L.; investigation, Y.M., X.Z., Z.L., K.W., H.Z, Y.H. and Y.Q.; resources, Y.M. and X.Z.; data curation, Y.M. and X.Z.; writing—original draft preparation, Y.M. and X.Z.; writing—review and editing, Y.M., X.Z. and K.W.; visualization, Y.M.; supervision, Y.M. and X.Z.; project administration, Y.M. and X.Z.; funding acquisition, Y.M., K.W. and Z.L. 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 Materials.

Acknowledgments

Our thanks are extended to the chief editor and the anonymous reviewers for their constructive reviews, which have greatly improved our manuscript.

Conflicts of Interest

The authors declare no conflicts of interest.

Footnotes

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Figures

Figure 2. Field photographs and photomicrographs of the Tulargen intrusions in the Eastern Tianshan: (a) hand specimen of gabbro; (b) field photograph showing tonalite; (c) field photograph showing biotite monzogranite; (d) gabbro, consisting of Pl and Pxz; (e) tonalite, consisting of Pl, Q, Bt, Ms and Chl; (f) biotite monzogranite, consisting of Pl, Kfs, Q, Bt, and Mc. Bt = Biotite; Chl = Chlorite; Ep = Epidote; Kfs = Potash feldspar; Mc = Microcline; Ms = Muscovite; Pl = Plagioclase; Px = Pyroxene; Q = Quartz.

Figure 3. Zircon U–Pb concordia and 206Pb/238U weighted-average diagrams of the studied intrusive rocks: (a) gabbro; (b) tonalite; (c) biotite monzogranite.

Figure 4. Major-element classification of Devonian magmatic rocks: (a) SiO2– K2O diagram [50]; (b) A/CNK–A/NK diagram [51].

Figure 5. Chondrite-normalized rare-earth element (REE) patterns (a) and primitive mantle-normalized trace-element patterns (b) for gabbro, tonalite, and biotite monzogranite [52].

Figure 6. Harker diagrams for the major selected elements from the studied intrusive rocks.

Figure 7. Tectonic setting discrimination diagrams for the studied intrusive rocks: (a) Nb–Y and (b) Rb–(Y + Nb) [59].

Figure 8. Rock-type discrimination for Late Devonian granite: (a) (K2O + Na2O)/CaO vs. (Zr + Nb + Ce + Y) diagram; (b) FeOT/MgO vs. (Zr + Nb + Ce + Y) diagram [69].

Figure 9. Th/Yb vs. Ta/Yb diagrams [73].

Figure 11. Tectonic setting discrimination diagrams from Jingerquan gabbro: (a) Hf/3-Th-Ta [96] and (b) Th/Yb-Nb/Yb [97]. IAT, island arc tholeiitic basalt; CAB, continental-arc basalt; WPAB, within-plate alkaline basalt; WPT, within-plate basalt; N-MORB, normal mid-ridge basalt; E-MORB, enriched mid-ridge basalt; OIB, oceanic island basalt.

Figure 12. Tectonic model for the intrusive rocks in Eastern Tianshan.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/min14111144/s1, Table S1: Zircon LA-ICP-MS U-Pb data of gabbro in Tulaergen area, Xinjiang, China; Table S2: Zircon LA-ICP-MS U-Pb data of tonalite in Tulaergen area, Xinjiang, China; Table S3: Zircon LA-ICP-MS U-Pb data of biotite monzogranite in Tulaergen area, Xinjiang, China; Table S4: Zircon in situ Lu-Hf isotope analyses data of the tonalite and biotite monzogranite in Tulaergen area, Xinjiang, China; Table S5: Major (wt.%) and trace (ppm) element concentrations of gabbro, tonalite and biotite monzogranite in Tulaergen area, Xinjiang, China.

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