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17 August 2024

Geochronology and Geochemistry of the Uhelchulu Quartz Diorite-Granodiorite in Inner Mongolia of China: Implications for Evolution of the Hegenshan Ocean in the Early-Middle Devonian

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1
Cores and Samples Center of Natural Resources, China Geological Survey, Sanhe 065201, China
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Tianjin Institute of Geological Survey, Tianjin 300191, China
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Author to whom correspondence should be addressed.
Minerals2024, 14(8), 835;https://doi.org/10.3390/min14080835
This article belongs to the Section Mineral Geochemistry and Geochronology
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Abstract

The Uhelchulu quartz diorite-granodiorite intrusions in Xiwuqi, Inner Mongolia, are exposed along the northwestern margin of the Xilinhot microcontinental block, located within the central and eastern parts of the southeastern Hegenshan suture zone. LA-ICP-MS zircon U-Pb dating yielded crystallization ages of (396 ± 8) Ma for the quartz diorite and (385 ± 5) Ma for the granodiorite, indicating an Early-Middle Devonian magmatic event. The quartz diorite exhibits I-type granite features, characterized by elevated Al2O3 (14.33–15.43 wt%), MgO (3.73–5.62 wt%), and Na (Na2O/K2O = 1.04–1.44), coupled with low P2O5 (0.15–0.20 wt%) and TiO2 (0.73–0.99 wt%). Trace element patterns show relative enrichments in Rb, Th, U, and Pb, while Nb, Ta, Sr and Ti are relatively depleted. Total REE contents are relatively low (123–178 ppm), with significant LREE enrichment (ΣLREE/ΣHREE = 4.75–5.20), and a non-obvious Eu anomaly (δEu = 0.75–0.84). In contrast, the granodiorite displays S-type granite characteristics, with high SiO2 (70.48–73.01 wt%), K (K2O/Na2O = 1.35–1.83), Al2O3 (A/CNK = 1.16–1.31), and a high differentiation index (DI = 76–82). Notably, MgO (1.44–2.24 wt%) contents are low, and significant depletions of Ba, Sr, Ti, and Eu are observed, while Rb, Pb, Th, U, Zr, and Hf are significantly enriched. Total REE contents are relatively low (178–314 ppm), exhibiting significant LREE enrichment (LREE/HREE = 6.17–8.36) and a pronounced negative Eu anomaly (δEu = 0.34–0.49). The overall characteristics point towards an active continental margin arc background for the Uhelchulu intrusions. Previous studies have suggested that the Hegenshan ocean continuously subducted northward from the Early Carboniferous to the Late Permian, but there is a lack of evidence for its geological evolution during the pre-Early Carboniferous. Therefore, this paper provides a certain basis for studying the geological evolution during the pre-Early Carboniferous in the Hegenshan ocean. We preliminarily believed that the Hegenshan ocean underwent a southward subduction towards the Xilinhot microcontinental block in the Xiwuqi area, at least from the Early Devonian to the Middle Devonian and the Hegenshan ocean may might have undergone a shift in subduction mechanism during the Late Devonian or Early Carboniferous.

1. Introduction

The Central Asian orogenic belt is a typical accretion-type orogenic belt located between the Eastern European Craton, Siberian Craton, Tarim Craton, and North China Craton (Figure 1a) [1,2,3,4,5]. Previous studies have shown that multi-ocean basins, multi-subduction zones, and multi-directional convergence are important mechanisms for continental accretion in this belt [6,7]. It is characterized by extensive accretion and collage of continental and oceanic crustal fragments, accretionary complexes, magmatic arcs, ophiolites and other geological bodies [8,9]. From the Paleozoic to the Mesozoic, this belt experienced complex tectonic history marked by the closure of the several Paleo-Asian oceanic branches (including the Hegenshan, Okhotsk, and Solonker oceanic basin) and collage of a number of continental fragments (such as Xilinhot microcontinental block, Xing’an block) and intra-oceanic arcs to the North China Craton [10,11,12,13,14,15,16,17,18,19], with the formation of suture zones such as Hegenshan, Diyanmiao, and Solonker (Figure 1b). The Hegenshan suture zone, specifically, emerged in the eastern-northern portion of the Central Asian orogenic belt through subduction and collision processes involving the Xing’an block and Xilinhot microcontinental block [20,21,22,23,24,25,26,27].
Figure 1. Tectonic division of the Central Asian orogenic belt and location of study area (a) revised after Sengör et al., 1993 [1]; Kai-Jun Zhang, 2014 [8]. (b,c) after Wang et al., 2020 [28]). 1. Cretaceous Meiletu Formation; 2. Cretaceous Baiyin Gaolao Formation; 3. Jurassic Manketu Ebo Formation; 4. Triassic Hongqi Formation; 5. Permian Dashizhai Formation; 6. Permian Shoushangou Formation; 7. Carboniferous Amushan Formation; 8. Carboniferous Ben Batu Formation; 9. Devonian Tarbagate Formation; 10. Cretaceous granite porphyry; 11. Cretaceous monzogranite; 12. Triassic monzogranite; 13. Permian syenite granite; 14. Carboniferous monzogranite; 15. Carboniferous diorite granite; 16. Carboniferous quartz diorite; 17. Carboniferous monzogranite; 18. Devonian gabbro; 19. Devonian granodiorite; 20. Devonian quartz diorite; and 21. Fracture.
Recent regional geological surveys in the Xiwuqi area of Inner Mongolia have identified numerous Carboniferous-Permian magmatic rocks, providing a valuable dataset for understanding the tectonic evolution of the Hegenshan ocean during this period. These magmatic rocks offer insights into distinct stages of tectonic activity within this region. The pre-arc basalt in Dahat, located on the west side of the Diyanmiao ophiolite belt in Xiwuqi, as identified by Li et al., suggests the Hegenshan ocean underwent initial intra-oceanic subduction and oceanic-continental magmatism during Early Carboniferous [29]. Wang et al. discovered adakite, high magnesium andesite, and tonalite in Meilaotuola and believed that the Hegenshan ocean underwent oceanic subduction and island arc magmatism during the Late Carboniferous in Xiwuqi [30,31,32]. Yang et al. suggested that the gabbro-granite in Houtoumiao, Xiwuqi was formed in the subduction background of the Hegenshan ocean in Late Carboniferous [33]. The volcanic rocks in the Dashizhai Formation in the Hanwula area of Xiwuqi, as proposed by Zhang et al. originated from a post-arc spreading environment within the Hegenshan ocean in Early Permian [34,35]. The granites in Xiwuqi, such as Serbeng and Nuheting Sala, according to Fan et al. are related to the northward subduction of the Hegenshan ocean during Late Permian [36]. The above studies all suggested that the Hegenshan ocean had underwent northward subduction during the Carboniferous to Permian, which is consistent with the research results of other regions in this area, including the Early Carboniferous volcanic rocks in the Engeryinpeng area of northern Suzuoqi [37], the Late Carboniferous amphibolite gabbro in Dongwuqi [38], and the Early-Late Carboniferous diorite-quartz diorite-tonalite-granodiorite in the Suzuoqi-Xiwuqi area [39].
However, the understanding of the tectonic background of the Hegenshan ocean before the Carboniferous is still unclear. Currently, only a few magmatic rocks have been identified from the Devonian in this area. Niu et al. proposed that the Hegenshan ocean was already in a post-collision extensional background during the Early Devonian based on their analysis of Huangheshao syenite in Damaoqi [40]. Huang’s research on the Hegenshan ophiolite suggested that the Hegenshan ocean opened during the Middle Devonian and subsequently subducted southward during the Late Devonian, leading to collision [41,42,43,44]. Therefore, in regional geological surveys, we always hope to identify more pre-Carboniferous magmatic rocks and continuously supplement our understanding of the pre-Carboniferous geological evolution in the Hegenshan ocean.
Our study reports the first discovery of intrusive rocks from the Devonian in the Uhelchulu to Guiqinkundui areas of Xiwuqi, offering a valuable opportunity to investigate the tectonic background of the Hegenshan ocean in the Devonian. By conducting zircon U-Pb precise dating and whole-rock geochemical analyses of Uhelchulu quartz diorite and Guiqinkundui granodiorite, this paper aims to shed light on the Devonian tectonic background of the Hegenshan ocean and provide a window for a systematic study of its tectonic evolution.

2. Regional Geological Background and Petrological Characteristics

The research area is situated within the northwestern edge of the Xilinhot microcontinental block, characterized by its proximity to the south of the Hegenshan suture zone (Figure 1c) [28]. The geological evolution of the research area is closely related to the subduction process associated with the closure of the Hegenshan ocean. This tectonic activity has resulted in a distinctive structural pattern, with geological units aligning along a NE-SW trend, parallel to the extension direction of the Hegenshan suture zone (Figure 1c) [28]. The Hegenshan suture zone acts as a prominent tectonic boundary, separating distinct stratigraphic sequences on either side. On the northwestern edge of the suture, the Devonian geological bodies are the main units, dominated by the marine Tarbagate Formation and intrusions. In contrast, on the southeastern edge of the suture, within the Xilinhot microcontinental block, the geological bodies are more diverse, the stratas include the Carboniferous marine Benbatu Formation, Amushan Formation, the Permian marine-continental Shoushankou Formation, Dashizhai Formation, Zhesi Formation, the Triassic continental Hongqi Formation, and the Jurassic-Cretaceous continental volcanic rocks. The intrusive rocks include gabbro, quartz diorite, granodiorite, tonalite, and diorite from the Carboniferous-Triassic, as well as diorite and granite porphyry from the Jurassic Cretaceous. Prior to our study, no Devonian intrusions have been found within the research area.
The Uhelchulu quartz diorite and Guiqinkundui granodiorite distribute in the prominent northeast-trending belt (Figure 1c). Quartz diorite, located in the southwestern Uhelchulu area, exhibits unconformable contacts with the overlying Lower Permian Dashizhai Formation to the north and the Lower Cretaceous Baiyingolao Formation volcanic rocks to the east. The southern margin of the quartz diorite is intruded by Early Carboniferous monzogranite. The Guiqinkundui granodiorite is overlain unconformably by the Lower Cretaceous Baiyingolao Formation volcanic rocks, the Triassic Hongqi Formation to the northeast and the Upper Carboniferous Benbatu Formation to the west and intruded by Late Triassic monzogranite to the north.

3. Sampling and Analytical Methods

3.1. Sampling

Eight samples were collected from fresh outcrops of the Uhelchulu quartz diorite and Guiqinkundui granodiorite to investigate their petrographic, geochemical, and geochronological characteristics. Among them, four samples from the Uhelchulu quartz diorite (A01, A02, A03, and A04) were selected for petrographic and whole-rock geochemical analyses, while one sample (U01) was collected for zircon U-Pb geochronology. Similarly, four samples from the Guiqinkundui granodiorite (A05, A06, A07, A08) were selected for petrographic and whole-rock geochemical analyses, with one sample (U02) selected for zircon U-Pb geochronology.

3.2. Analytical Techniques

Whole-rock geochemical analyses were carried out at the laboratory of the Regional Geological and Mineral Survey and Research Institute of Hebei Province. Major element analyses were performed using X-ray fluorescence (XRF) with an error margin of less than 2 wt%. Trace element analyses were conducted using inductively coupled plasma mass spectrometry (ICP-MS) with an accuracy exceeding 5 wt%.
Zircon grains were isolated from the rock samples through a meticulous single mineral separation process, completed with assistance from the Hebei Provincial Institute of Regional Geology and Mineral Investigation. Subsequently, cathodoluminescence (CL) imaging was performed using a scanning electron microscope at the laboratory of the Institute of Geology and Geophysics, Chinese Academy of Sciences.
Zircon U-Pb isotopic dating was conducted using LA-ICP-MS analysis at the Tianjin Geological Survey Center of the China Geological Survey. The laser ablation system employed was a UP193FX 193nm ArF excimer system (NewWave, USA, Washington), equipped with a laser from ATL (Germany, Munich) and an Agilent 7500a mass spectrometer for ICP-MS. The laser system operated at a wavelength of 193 nm with a pulse width of less than 4 ns. The laser spot diameter used for analysis was 35 μm. For external matrix correction, the internationally recognized Plesovice (206Pb/238U weighted average age (337.13 ± 0.37) Ma) and Qinghu standard zircons (206Pb/238U weighted average age (159.45 ± 0.16) Ma) were utilized. NIST SRM 612 with 29Si served as the internal standard element. Isotopic ratios and elemental content were calculated using the GLITER-ver 4.0 program (Macquarie University). Ordinary lead correction was performed using Anderson’s ComPbCorr # 3.17 correction program [45]. U-Pb concordia plots, age distribution frequency plots, and age weighted averages were generated using the IsoPlot/Ex_ver 3 program [46]. Detailed descriptions of the analytical methods and data processing procedures can be found in Liu et al. [47,48,49].

4. Results

4.1. Mineralogical Characteristics

4.1.1. Uhelchulu Quartz Diorite

The outcrop of the Uhelchulu quartz diorite is exposed in a steep slope (Figure 2a,b), and under the microscope, it presents a medium-to-fine idiomorphic crystal. Its microstructure is shown in Figure 2e, and the main minerals are plagioclase, quartz, hornblende, and biotite. Plagioclase is in a hypidiomorphic platy-shaped crystal, with faintly visible bands and double crystal development, with an abundance of 50 vol% in the rock; Quartz is xenomorphic, with an abundance of 20 vol% in the rock, occupying gaps between feldspars; Biotite occurs as sheets and has an abundance of about 10 vol% in the rock; Hornblende appears as a hypidiomorphic columnar (Figure 2e), with an abundance of about 10 vol% in the rock; and Opaque minerals are in a hypidiomorphic-to-heteromorphic granular-shaped crystal, with an abundance of <1 vol% in the rock and scattered distribution. The secondary minerals are mainly apatite and zircon.
Figure 2. Field and microscopic photographs of the quartz diorite in Uhelchulu and granodiorite in Guiqinkundui. (a,b) field photos of Uhelchulu quartz diorite; (c,d) field photos of Guiqinkundui granodiorite; (e) Uhelchulu quartz diorite with hypidiomorphic crystal (crossed polarizers); (f) Guiqinkundui granodiorite hypidiomorphic crystal (crossed polarizers).

4.1.2. Guiqinkundui Granodiorite

The outcrop of the Guiqinkundui granodiorite is well exposed (Figure 2c,d). Under the microscope, it presents a fine-grained hypidiomorphic structure, fragmented patchy structure, and block-like structure. Its microstructure is shown in Figure 2f, and the main minerals are plagioclase, k-feldspar, quartz, biotite, and hornblende. Plagioclase is in hypidiomorphic platy crystal, with an abundance of 55 vol% in the rock; k-feldspar is hypidiomorphic with a interstitial distribution, with an abundance of 10 vol% in the rock; Quartz is xenomorphic, distributed between feldspars in an interstitial manner, it exhibits slight fragmentation, wavy extinction, deformation patterns, and other stress phenomena, with an abundance of 20 vol% in the rock; Biotite is flaky, with an abundance of 15 vol% in the rock. The hornblende is hypidiomorphic with a relatively low abundance in the rock.
A fine-grained biotite diorite xenolith is observed within the rock (Figure 2d), with an unclear boundary to the medium-to-fine grained granodiorite. It is mainly composed of plagioclase and biotite. The auxiliary mineral are mainly opaque minerals (<1 vol%), ilmenite, magnetite, apatite, zircon, and monazite.
From the relationship within biotite, alkali feldspar, and quartz, it can be preliminarily determined that biotite formed earlier than alkali feldspar and quartz, and the Guiqinkundui granodiorite might have undergone a strong crystallization-differentiation process. The diorite xenolith suggests that magma of the Guiqinkundui granodiorite may originate from partial melting of dioritic rocks.

4.2. Geochronology

Zircon grains extracted from the Uhelchulu quartz diorite and Guiqinkundui granodiorite exhibit a characteristic pink-yellow color and display well-defined morphologies, predominantly prismatic to short prismatic shaped crystal. The grains range in size from approximately 205 to 330 μm in length and 20 to 96 μm in width, with aspect ratios varying between 2:1 and 3:1. Cathodoluminescence (CL) imaging reveals the presence of oscillatory zoning patterns within the zircon (Figure 3a,b), indicative of a magmatic origin. These patterns are characterized by distinct rhythmic zoning structures with minimal visible residual cores. Secondary fractures are relatively limited, and the Th/U ratios of the zircon consistently exceed 0.1 (Th/U = 0.12–0.96), further corroborating a magmatic origin for the zircon grains. The results of LA-ICP-MS zircon U-Pb dating for the Uhelchulu quartz diorite and Guiqinkundui granodiorite are shown in Table 1 and Table 2, respectively.
Figure 3. Cathodoluminescence (CL) images and analysis points of zircons from the Uhelchulu quartz diorite (a) and the Guiqinkundui granodiorite (b).
Table 1. LA-ICP-MS zircon U-Pb dating of the Uhelchulu quartz diorite (U01).
Table 2. LA-ICP-MS Zricon U-Pb dating of the Guiqinkundui granodiorite (U02).
The Uhelchulu quartz diorite exhibits a consistent 206Pb/238U age distribution across all zircon grains, with the exception of those at measurement points 5, 7, 8, and 12. These latter grains may represent inherited zircon incorporated during magmatic processes. The remaining 206Pb/238U age values range from 381 to 405 Ma, clustering along the concordia line (Figure 4a). The weighted average age of 13 206Pb/238U measurements is (395 ± 8) Ma (MSWD = 1.4), suggesting that the Uhelchulu quartz diorite crystallized during the Early Devonian (Figure 4b). Similarly, all 20 zircon grains analyzed from the Guiqinkundui granodiorite exhibit a relatively homogeneous distribution of 206Pb/238U age values, ranging from 371 to 431 Ma, clustering along the concordia line (Figure 4c). The weighted average age of 20 206Pb/238U measurements is (384 ± 5) Ma (MSWD = 1.5) (Figure 4d), indicating that the Guiqinkundui granodiorite formed during the Middle Devonian.
Figure 4. U-Pb concordia diagram, weighted average histogram of zircons from the Uhelchulu quartz diorite (a,b) and the Guiqinkundui granodiorite (c,d).

4.3. Whole-Rock Geochemistry

The analysis of major trace elements along with calculated normative mineral abundances (CIPW-norm) and selected geochemical ratios for the Uhelchulu quartz diorite and Guiqinkundui granodiorite, are shown in Table 3.
Table 3. Contents of major elements (wt%), trace elements (ppm), CIPW-norm of the quartz diorite in Uhelchulu and the granodiorite in Guiqinkundui.

4.3.1. Major Elements

The Uhelchulu quartz diorite exhibits SiO2 ranging from 55.39 to 60.53 wt% (Table 3). Other major element concentrations include: Al2O3 (14.33–15.43 wt%), K2O (1.77–2.39 wt%), Na2O (2.18–2.63 wt%), TiO2 (0.73–0.99 wt%), MnO (0.13–0.18 wt%), MgO (3.73–5.62 wt%), CaO (5.11–6.76 wt%), and P2O5 (0.15–0.20 wt%). The K2O + Na2O content varies between 3.95 and 4.87 wt%, with a Na2O/K2O ratio of 1.04 to 1.44. The A/CNK and A/NK ratios fall within the ranges of 0.83–0.97 and 2.20–2.61, respectively. Normative mineral calculations based on the CIPW norm reveal a quartz content ranging from 9.56 to 21.03 wt%, and an apatite content of 0.37 to 0.48 wt%, with no presence of corundum. The Uhelchulu quartz diorite is characterized by low potassium, high sodium, and high calcium content, along with relatively low concentrations of TiO2, MnO, and P2O5. This geochemical signature is consistent with a rock that has undergone significant fractional crystallization. In the TAS diagram (Figure 5a) [50], the sample points fall within the diorite-gabbro diorite field. The AFM diagram (Figure 5b) [51] indicates that the Uhelchulu quartz diorite belongs to the calc-alkaline series.
Figure 5. TAS and AFM diagram of quartz diorite-granodiorite from Uhelchulu to Guiqinkundui in Xiwuqi ((a) after E.A.K.Middlemost, 1994 [50]; (b) after Irvine T. N., et al. 1971 [51]).
The Guiqinkundui granodiorite displays a relatively high silica content, with SiO2 ranging from 70.48 to 73.01 wt% (Table 3). The K2O + Na2O content ranges from 5.39 to 6.09 wt%, with K2O concentrations ranging from 3.10 to 3.94 wt% and a K2O/Na2O ratio of 1.35 to 1.83. These values indicate a rock that is alkali-rich and specifically potassium-rich. The high CaO/Na2O ratio (0.58–0.76 > 0.3) further supports this conclusion. The Al2O3 content varies from 11.70 to 13.24 wt%, resulting in an A/CNK ratio of 1.16 to 1.31. The presence of corundum molecules in the CIPW norm classifies the rock as peraluminous. The CaO, MnO, P2O5, and TiO2 content is relatively low, resulting in a differentiation index (DI) of 76–82. This high DI value indicates that the rock mass has undergone a significant degree of differentiation and evolution. In the TAS diagram (Figure 5a) [50], the sample points fall within the granite-granodiorite field. Similarly, the AFM diagram (Figure 5b) [51] confirms that the Guiqinkundui granodiorite belongs to the calc-alkaline series.

4.3.2. Trace Elements

The trace element compositions of the Uhelchulu quartz diorite and Guiqinkundui granodiorite exhibit notable similarities and differences (Table 3, Figure 6a,b). Both rock types show REE patterns with significant light over heavy REE enrichment and little fractionation within the heavy REE when normalized to chondrite [52], indicative of enrichment in both light and heavy REEs with significant fractionation between the two groups.
Figure 6. (a) Chondrite-normalized REE distribution patterns (after Boynton, 1984 [52]) and (b) Primitive mantle-normalized trace elements distribution patterns (after Sun and McDonough, 1989 [53]) for the Uhelchulu quartz diorite and Guiqinkundui granodiorite, Xiwuqi.
The total REE content (∑REE) is slightly lower than it in the Uhelchulu quartz diorite (123–178 ppm) compared to the Guiqinkundui granodiorite (178–314 ppm). However, the Guiqinkundui granodiorite exhibits a more pronounced negative europium anomaly (δEu = 0.36–0.49) compared to the Uhelchulu quartz diorite (δEu = 0.75–0.84).
Overall, both rock types are characterized by elevated trace element concentrations with respect to the primitive mantle [53]. They display significant depletion of Ba, Nb, Ta, Sr, Eu, and Ti, while Rb, Th, U, Pb, Zr, and Hf are notably enriched. The Guiqinkundui granodiorite displays more pronounced “peak” values for these elements compared to the Uhelchulu quartz diorite.
The geochemical signatures of the Uhelchulu quartz diorite and Guiqinkundui granodiorite suggest that they likely originated from similar source rocks potentially related to island arc-type magmatism. The source may have involved partial melting of lower crustal diorite rocks, followed by assimilation of crustal materials during intrusion. The Guiqinkundui granodiorite appears to have undergone more significant fractional crystallization compared to the Uhelchulu quartz diorite.

5. Discussion

5.1. Intrusion Age

Previous studies on the Uhelchulu quartz diorite and Guiqinkundui granodiorite have lacked precise geochronological constraints. The 1:200,000 Maodeng regional geological survey report by the Bureau of Inner Mongolia Geology (1976) [54] classified the Guiqinkundui granodiorite as a middle acidic intrusive rock of Late Hualixi age, corresponding to the Late Permian. Furthermore, the Institute of Inner Mongolia Autonomous Region Geological Survey (2008) [55], in their 1:250,000 Chaokewula regional geological survey report, speculated the unit to be Late Jurassic.
This study presents precise zircon U-Pb dating results, revealing ages of (396 ± 8) Ma (MSWD = 1.4) for the Uhelchulu quartz diorite and (385 ± 5) Ma (MSWD = 1.5) for the Guiqinkundui granodiorite. The measured zircons exhibit exclusively magmatic characteristics, confirming the Early Devonian and Middle Devonian ages of these intrusions, respectively. These findings contradict previous assumptions of a late Variscan or Late Jurassic origin for these rocks.
These newly determined ages significantly expand the record of tectonic-magmatic activity in the Xiwuqi area from the Carboniferous-Permian to the Devonian. This discovery provides a crucial chronological framework for investigating the tectonic evolution of the Hegenshan ocean during the Devonian period.

5.2. Petrogenesis

The petrogenesis of granitic rocks is widely understood to be governed by the source characteristics, leading to the distinction of granites into I-type, S-type, A-type, and M-type, with M-type granites being relatively uncommon [56]. A-type granites are typically distinguished by the presence of alkaline mafic minerals [57]. Chemically, A-type granites are characterized by elevated concentrations of elements like Zr, Nb, Ce, Ga, and Y, as well as high silicon, potassium, and iron. These granites are also typically associated with high crystallization temperatures [58,59,60].
The FeO*/MgO ratios of the Uhelchulu quartz diorite and Guiqinkundui granodiorite (1.53–2.13) fall well below the significantly iron-rich A-type granite classification (FeO*/MgO > 10) [61]. Detailed petrographic analysis revealed the absence of alkaline mafic minerals, a key characteristic of A-type granites. Furthermore, zircon saturation temperatures, determined through whole-rock major element and Zr content analysis, suggest relatively lower crystallization temperatures compared to A-type granites. The Uhelchulu quartz diorite exhibits a zircon saturation temperature range of 734–762 °C (average 746 °C; Table 3), while the Guiqinkundui granodiorite displays a range of 817–839 °C (average 819 °C; Table 3) [62]. Both of these temperature ranges fall below the average A-type granite temperature of 833 °C [62].
The Ga/Al ratio (10,000 × Ga/Al) ranges from 1.21 to 1.79, significantly lower than the established A-type granite threshold of 2.6 [61]. Similarly, the sum of Zr + Nb + Ce + Y (249–427 ppm, average 303 ppm) falls below the A-type granite lower limit of 350 ppm [61] (Table 3). These geochemical observations further corroborate the notion that the Uhelchulu quartz diorite and Guiqinkundui granodiorite do not exhibit the defining petrological and geochemical characteristics of A-type granites. Instead, their features are more consistent with those of I-type or S-type granites, as supported by relevant diagrams (Figure 7a,b) [61].
Previous studies have established that apatite solubility decreases with increasing SiO2 during magma differentiation in metaluminous to weakly peraluminous magmas [62,63]. However, in strongly peraluminous magmas, the solubility trend is reversed. The Uhelchulu quartz diorite exhibits an extremely low P2O5 content (0.15–0.20 wt%) that consistently decreases with increasing SiO2 content (Figure 7c) [64], aligning with the established evolutionary trend of I-type granites. Furthermore, the Uhelchulu quartz diorite samples plot within the I-type granite field on the Na2O-K2O diagram (Figure 7d). In contrast, the P2O5 content of the Guiqinkundui granodiorite shows a positive correlation with SiO2 (Figure 7c) [64], and its samples fall within the S-type granite field on the Na2O-K2O diagram (Figure 7d) [64]. Additionally, CIPW normative mineral calculations for the Guiqinkundui granodiorite indicate the presence of corundum (1.79–2.85 vol%), confirming its peraluminous nature.
Therefore, we tentatively conclude that the Uhelchulu quartz diorite has I-type granite characteristics, while the Guiqinkundui granodiorite has S-type granite characteristics. Previous studies and experimental petrology have shown that the genesis of I-type and S-type granites is mainly due to partial melting and fractional crystallization of crustal materials [65,66,67].
Minerals 14 00835 g007
Figure 7. Geochemical discrimination diagrams for Uhelchulu quartz diorite and Guiqinkundui granodiorite samples in Xiwuqi. (a) (K2O + Na2O)/CaO versus Zr + Nb + Ce + Y (ppm) (after Whalen et al., 1987 [61]); (b) (K2O + Na2O)/CaO versus 10,000*Ga/Al (after Whalen et al., 1987 [61]); (c) Na2O versus K2O (after Collins et al., 1982 [64]); and (d) P2O5 versus SiO2 (after Collins et al., 1982 [64]).

5.3. Tectonic Implications

The Devonian tectonic environment of the Hegenshan ocean remains poorly understood, primarily due to limited research and a scarcity of geological evidence. Existing interpretations of the Hegenshan ocean Devonian evolution depend largely on studies of Devonian magmatic rocks within the Hegenshan ophiolitic mélange belt. Huang proposed that the Hegenshan ocean began forming during the Middle-Late Devonian, underwent northward subduction during the Early Carboniferous, and ultimately closed during the Late Carboniferous [41]. Shao et al. based on the age of the Gegenaobao volcanic rocks and the emplacement of ultrabasic rocks, suggested that the Hegenshan ocean closed during the Late Devonian-Early Carboniferous, with an unconformity observed between the serpentinite and the overlying strata in the Hegenshan area [68].
The first discovery of Devonian intrusive rocks in the Xiwuqi area during the regional geological survey provides valuable insights into the Devonian tectonic environment of the Hegenshan ocean. Despite the limited availability of other geological evidence from the same period, this study offers a preliminary exploration of the tectonic setting associated with the formation of the Uhelchulu quartz diorite and Guiqinkundui granodiorite, drawing primarily on their geochemical characteristics.
Using Nb-Y and Rb-(Y + Nb) discrimination diagrams (Figure 8a,b) [69], all samples of Uhelchulu quartz diorite and Guiqinkundui granodiorite plot within the volcanic arc environment. This geochemical evidence, combined with the detailed petrogenesis analysis, supports the formation of these rocks in an active continental margin arc environment, thus strongly contracting earlier interpretations, providing compelling evidence for a distinct tectonic setting during their formation.
Figure 8. Discrimination diagrams for Uhelchulu quartz diorite and Guiqinkundui granodiorite in Xiwuqi, (a) Y-Nb and (b) (Y + Nb)-Rb (after Pearce et al., 1984 [69]). Syn-COLG (syn-collision granite); WPG (within plate granite); VAG (volcanic arc granite); and ORG (mid ocean ridge granite).
Petrological and geochemical analyses of the Uhelchulu quartz diorite and Guiqinkundui granodiorite indicate that the Xiwuqi area occupied an active continental margin arc setting within the Xilinhot microcontinental block during the Early to Middle Devonian. This interpretation suggests that the Hegenshan oceanic crust underwent southward subduction during this period (Figure 9a). Previous research has suggested that the Hegenshan ocean transitioned to northward subduction at least during the Early Carboniferous (Figure 9b). Therefore, we preliminarily believed that the Hegenshan ocean may might have undergone a shift in subduction mechanism during the Late Devonian or Early Carboniferous.
Figure 9. Tectonic evolution model of the middle-east section of the Hegenshan ocean in the Early-Middle Devonian (a) and Early Carboniferous (b).
Profeta et al. (2016) proposed a fitting relationship between the whole-rock chemistry (La/Yb)N of intrusions and crustal thickness: H = 21.277ln(1.0204w(La)N/w(Yb)N), Where H is the thickness of the crust and w(La)N/w(Yb)N is the ratio after homogenization of chondrite meteorite [70]. This calculation needs to remove the samples that do not meet the requirements, such as La > 1 × 10−6, Th/U < 0.1, and SiO2 > 75% in the whole-rock chemistry [71,72]. We calculated the variation in crustal thickness from the Early Devonian to Early Cretaceous in the research area (Figure 10) based on the data from this paper and other researchers (Table 4) [30,36,39,73,74,75,76,77]. The crustal thickness in the Early Devonian is relatively thin (~28.52 km), but it increased to 43.13 km during the Middle Devonian. The crustal thickening might continue until the Middle Carboniferous (~42.60 km), which might be related to the continuous southward subduction of the Hegenshan ocean. In the late Early Carboniferous, there was a sudden thinning of the crust, which might be related to the shift in the subduction mechanism of the Hegenshan ocean in the Early Carboniferous. This calculation result might further support the understanding of our study. Subsequently, it underwent crustal thickening and thinning processes several times until the Early Cretaceous in the research region, which is consistent with the geological evolution processes studied by previous researchers.
Figure 10. Crustal thickness evolution in the research area.
Table 4. The calculation results of crustal thickness in the research area.

6. Conclusions

This study provides novel insights into the Devonian tectonic evolution of the Hegenshan ocean, based on the petrological and geochemical characterization of the Uhelchulu quartz diorite and Guiqinkundui granodiorite in the Xiwuqi area.
LA-ICP-MS zircon U-Pb dating yielded ages of (396 ± 8) Ma (MSWD = 1.4) for the Uhelchulu quartz diorite and (385 ± 5) Ma (MSWD = 1.5) for the Guiqinkundui granodiorite, conclusively demonstrating Early to Middle Devonian magmatic activity. This discovery significantly expands the known extent of Devonian magmatism, previously documented only north of the Hegenshan suture zone, to the southern Xiwuqi area. These ages provide critical chronological constraints for further elucidating the complex tectonic evolution of the Hegenshan ocean during this critical period.
The mineralogical and whole-rock geochemical characteristics suggest that the Uhelchulu quartz diorite has I-type granite characteristics, while the Guiqinkundui granodiorite has S-type granite characteristics.
Geochemical analysis of the tectonic environment indicated that the Uhelchulu quartz diorite and Guiqinkundui granodiorite formed in an active continental margin arc background, which may be related to the southward subduction of the Hegenshan ocean in the Early to Middle Devonian. The Hegenshan ocean might have undergone a shift in subduction mechanism during the Late Devonian or Early Carboniferous.

Author Contributions

Conceptualization, T.C., and W.Y.; methodology, T.C. and W.Y.; software, T.C., W.Y., C.T. and X.Y.; validation, T.C., W.Y. and D.X.; formal analysis, T.C. and W.Y.; investigation, T.C., W.Y., C.T. and X.Y.; resources, T.C. and W.Y.; data curation, T.C., W.Y., C.T. and X.Y.; writing—original draft preparation, T.C. and W.Y.; writing—review and editing, T.C. and W.Y.; visualization, T.C. and W.Y.; supervision, W.Y.; project administration, T.C. and W.Y.; funding acquisition, T.C. and W.Y. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by grants from China Geological Survey (Grants [2010]01-19-01, DD20221814, DD20242250).

Data Availability Statement

The authors confirm that the data generated or analyzed during this study are provided in full within the published article.

Acknowledgments

We thank the editors and anonymous reviewers for their critical reviews and excellent suggestions that helped to improve this manuscript. We thank Xuebin Zhang and Jie Shao for their help and valuable comments. We also thank Xiqing Cheng, Jun Cao for their valuable help. We would like to express our gratitude to the leadership and staff of the Exploration Department at the Tianjin Geological Survey and Research Institute for their assistance and support with the field research.

Conflicts of Interest

All the authors declare that there is no conflict of interest regarding the publication of this article.

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