Unraveling the mesozoic tectonic transition of the bikou area: a case study of Late Triassic diabase veins

Abstract

Introduction:

The Bikou area, positioned at the junction of the Qinling orogenic belt and the Yangtze Plate, is critical for understanding both continental orogenesis and regional mineralization in China.

Methods:

To better understand the magmatic and metallogenic processes in this highly active region, we investigated the petrogenesis and tectonic setting of exposed diabase veins using petrological, geochronological, and geochemical methods.

Results:

Zircon U-Pb dating constrains the emplacement of these sub-alkaline tholeiitic diabases to the Late Triassic (201–206 Ma). The rocks display moderate SiO₂ (44.57–49.58 wt%) and MgO (3.98–7.46 wt%, Mg^# = 56–63), with flat REE patterns ((La/Yb)_N = 1.93–2.39) and weak positive Eu anomalies (δ_Eu = 0.85–1.21). Trace element geochemistry reveals Nb-Ta-Pb depletion and Ba-K enrichment, while Sr-Nd isotopes show limited variation (⁸⁷Sr/⁸⁶Srᵢ = 0.706627–0.707311; ε_Nd(t) = −1.16 to 0.16). Although the samples retain some arc-related geochemical fingerprints, regional tectonic analysis indicates a post-collisional origin.

Discussion:

We conclude that the Late Triassic diabase magmas were derived from a lithospheric mantle source contaminated by earlier subduction components. Their emplacement was ultimately triggered by lithospheric extension and asthenospheric upwelling in the South Qinling belt. This study refines the current model for the Mesozoic tectonic evolution of the Bikou area.

1 Introduction

The Qinling orogenic belt preserves a long and complex record of geological evolution between the North China Plate and the Yangtze Plate. Having experienced multiple stages of tectonic, magmatic, and thermal events and orogenic processes, it is considered a typical composite orogenic belt (Zhang et al., 2001; Wang Z. Q. et al., 2009a). As an important part of this belt, the South Qinling structural belt is crucial for understanding the regional tectonic evolution. Current models of the tectonic evolution of the South Qinling still differ significantly. For example, some studies suggest that this area underwent rifting in the Devonian, forming a limited ocean basin that subsequently subducted northward and closed in the Triassic (Zhang et al., 2003; Meng and Zhang, 1999; Li S. Z. et al., 2002; Liu et al., 2005; Dong et al., 2011b). Other scholars propose that an ancient ocean existed in this area during the Precambrian, with southward subduction occurring in the Late Paleozoic and ending in the Triassic (Wang et al., 1999; Wang T. et al., 2009). Nevertheless, various viewpoints generally agree that the orogenic belt entered the collisional stage from the Late Triassic onward, accompanied by a series of tectonic activities and magmatic events (Wang et al., 2021). Meanwhile, mineralization also occurred through different stages following continental collision, forming various regional mineralization types during each orogenic evolution period, thus resulting in a distinctive assemblage of mineral deposits. Recent work on the Wuguan Complex has provided new insights into the Paleozoic evolution of the Qinling orogenic belt (Wang et al., 2025), and the Huichizi granite study has further documented the early Paleozoic evolution of North Qinling (Cui et al., 2026).

Mafic dikes are often regarded as effective indicators for determining the timing of major tectonic transformations and thus hold special continental tectonic significance (Lu et al., 2006). Therefore, investigating the age, type, and petrogenesis of mafic magmatic rocks formed during orogenic processes is of great importance for understanding the tectonic evolution of orogenic belts and their metallogenic background (Halls, 1982). The Bikou area, located in the southern part of the South Qinling orogenic belt, represents a key junction where the Qinling orogenic belt, the Songpan-Ganzi orogenic belt, and the Yangtze Plate converge. It is also an important component of the central orogenic system of the Chinese mainland (Zhang G. W. et al., 2001). Meanwhile, this area is also a famous “Golden Triangle” region in Shaanxi Province, hosting a series of small-to medium-sized gold and polymetallic deposits, such as Jianchaling, Dongbagou, Huafanggou, Lijiagou, and Tongchangwan (Zhang et al., 2004; Zhou et al., 2011a; Zhou et al., 2011b; Ji, 2016; Liu et al., 2016), showing considerable exploration potential. Thus, clarifying the regional tectonic evolution process is not only of theoretical significance but also provides a basis for subsequent exploration planning. Although mafic dikes are well developed in these ore-concentrated areas, current research on their petrogeochemical characteristics and petrogenesis remains relatively insufficient. Conducting studies on these mafic dikes can help to better understand the formation ages and tectonic settings of regional rock units, and provide important clues for reconstructing the ancient tectonic framework of the South Qinling orogenic belt. In addition, the petrogenesis of mafic complexes in the Qinling Orogenic Belt has recently been investigated (Zhu et al., 2025).

This study takes the diabase veins developed in the Bikou area as the research object and systematically conducts petrological observations, geochemical analysis, Sr-Nd isotope analysis, and LA-ICP-MS zircon U-Pb dating. Combined with existing regional research results, this paper discusses the petrogenesis and geodynamic background of these mafic dikes, providing new evidence for studying the tectonic evolution of the South Qinling orogenic belt.

2 Methods

Samples were crushed, washed, and enriched by electromagnetic separation at Langfang City Chengxin Geological Service Co., Ltd. Zircons were then manually selected under a microscope. Subsequently, target mounts were prepared, and cathodoluminescence (CL) imaging was performed by Nanjing Hongchuang Company. Only zircon grains with intact crystal shapes and free of cracks or inclusions were selected for analysis.

U–Pb dating was completed at the Institute of Mineral Resources, Chinese Academy of Geological Sciences, using laser ablation MC-ICP-MS (Finnigan Neptune MC-ICP-MS coupled with a Newwave UP 213 laser system). During analysis, the laser spot size was 35 μm and the repetition rate was 10 Hz. GJ-1 was used as the external standard, and Si as the internal standard. One standard sample was analyzed for every ten sample points. Data processing was performed using ICPMSDataCal, and concordia diagrams were generated using Isoplot 3.0 (Ludwig, 2003). Detailed experimental procedures can be found in Hou et al. (2009).

Whole-rock major and trace element analyses were completed at Beijing Yandu Zhongshi Testing Technology Co., Ltd. Samples were crushed, washed, dried, and ground to 200 mesh. For major element analysis, the powdered samples were mixed with Li₂B₄O₇ as a flux in a 1:8 ratio and then heated in a fusion machine to 1150 °C to form homogeneous glass beads. The glass beads were analyzed using XRF (Zetium, PANalytical) with an analytical error of less than 1%. Trace elements were determined by ICP-MS (M90, Analytik Jena) following high-pressure acid digestion, with the standard reference material GSR-2 used for quality control. Typical analytical errors were <5%, and errors for volatile and very low-abundance elements were <10%.

Sr–Nd isotope analyses were performed on a MAT 262 thermal ionization mass spectrometer at the Radioisotope Geochemistry Laboratory of the University of Science and Technology of China. Sr isotope ratios were mass-bias corrected using ⁸⁶Sr/⁸⁸Sr = 0.1194, and the measured value for the NIST 987 standard was 0.710250 ± 7. Nd isotope ratios were corrected using ¹⁴³Nd/¹⁴⁴Nd = 0.7219, and the measured value for the JMC standard was 0.51109 ± 3 (¹⁴³Nd/¹⁴⁴Nd). Based on the zircon U–Pb ages obtained for each gabbroic body, the initial Sr–Nd isotope ratios were calculated for each sample group.

3 Geological background

The Bikou area is one of the major tectonic–magmatic activity zones within the Qinling orogenic belt (Figure 1a). Its spatial distribution is controlled by the NE-trending Mengxian–Yangpingguan Fault and the NW-trending Mengxian–Liuyang Fault, and it generally exhibits a triangular, wedge-shaped geometry that opens to the west and converges to the east (Zhang et al., 1995). In an early study, Ye and Guan (1944) classified the exposed rocks in this area as the Bikou Group in a broad sense and assigned them a Carboniferous–Permian age. Subsequently, various scholars have reclassified and subdivided this group (Qin et al., 1992; Yan et al., 2002; Wang et al., 2019), dividing it, from bottom to top, into the Da’an Formation, the Bikou Group (in a narrow sense), and the Yangtianba Formation. These distinct structural units are generally separated by faults or ductile shear zones (Yang et al., 2017). Except for a relatively lower content of acidic volcanic rocks, the Da’an Formation exhibits bulk rock properties similar to those of the Bikou Group. Both units are mainly distributed between the Tongqianba–Fengxiangyuan Fault and the Mengxian–Yangpingguan Fault (Figure 1b) and consist of lava, volcanic breccia, and normal sedimentary rocks. The lava assemblage includes fine-grained basalt, porphyritic rocks (neither acidic nor basic), andesite, quartz porphyritic rocks (acidic), and rhyolite (Wang Z. Q. et al., 2009b). The Yangtianba Formation is mainly composed of feldspar sandstone and argillaceous slate interbedded with meta-volcanic rocks, characterized by abundant volcanic and feldspar fragments but few quartz fragments (Yan et al., 2004; Wang, 2013). It is generally distributed to the north of the Tongqianba-Fengxiangyuan Fault (Miao et al., 2019). The Carboniferous Nantuo Formation, Doushantuo Formation, and Dengying Formation consist of sandstone, siltstone, and dolomite, and are usually separated from the above rock units by faults or occur as lens-shaped bodies. The Silurian Maoxian Group, mainly composed of argillaceous slate, is primarily located to the southeast of the Mengxian-Yangpingguan Fault.

FIGURE 1

Regional tectonic structures are well developed in the area. In the northern part, NW–NWW-trending faults predominate, whereas the southern part is mainly characterized by NEE-trending faults. In addition, nearly N–S-trending fault clusters are superimposed on these structures (Xu et al., 2002). From south to north, the main regional faults include the Mengxian–Yangpingguan Fault, the Qingmucun-Guanshuyao Fault, the Yangba-Cangsheping Fault, and the Tongqianba-Fengxiangyuan Fault (Zhang et al., 2017). Faults with different orientations cut across each other, dividing the rock units into multiple blocks. Within each block, the strata are typically present as monoclines or overturned folds. Igneous rocks in the area are mainly mafic, including basalt, dioritic basalt, and dioritic tholeiite, which are mostly distributed as veins or plutons along the fault zones (Figure 1c). Additionally, a small amount of dark gray ultramafic rocks are sporadically exposed in the area (Figure 1c); they are limited in scale and most of them have undergone intense alteration.

The study area is located north of Dahan Town, Ningqiang County, Shaanxi Province, adjacent to the Mengxian–Yangpingguan Fault. This fault strikes nearly NEE and dips steeply toward the NNW. The main rock units exposed in the area include the Bikou Group, the Doushantuo Formation, and the Dengying Formation, with the latter two covering large areas. The Bikou Group consists mainly of basic igneous rocks and igneous sandstones. The Doushantuo Formation is primarily represented by its second and third members: the second member is composed of gray sericite siltstone and sericite-bearing siltstone, whereas the third member consists of gray-black carbonaceous siltstone and gray laminated micritic limestone. The Dengying Formation is most widely exposed in its second member, with the main rock types being medium-to thick-bedded to massive dolomite, gray massive gravelly dolomite, and dolomitic limestone. The most common magmatic rocks in the area are gabbros, which mainly intrude into the third member of the Doushantuo Formation and the second member of the Dengying Formation. Although individual gabbroic bodies are small in size, they are numerous and are often emplaced as apophyses or veins along structural fractures.

4 Rock and mineralogical characteristics

The samples collected in this study are diabase that intruded into the limestone of the Dengying Formation. In the field, a distinct embayment-shaped intrusive contact can be observed between the diabase and the surrounding rock. In some areas, the diabase is seen intruding along later faults, where the contact line is relatively straight. This diabase generally exhibits a massive structure (Figures 2a,b). Microscopic observation indicates that the rock is mainly composed of plagioclase and pyroxene, with secondary minerals including amphibole and biotite. The overall texture is typical of diabase. Plagioclase content ranges from approximately 45%–55%, mostly occurring as euhedral to subhedral laths, with polysynthetic twinning visible and sericite and kaolinite commonly developed (Figure 2c). Pyroxene accounts for about 35%–45%, mostly in subhedral to anhedral forms, typically filling the triangular interstices between plagioclase crystals (Figure 2d); most pyroxene grains have undergone chlorite alteration. The contents of amphibole and biotite are relatively low and are sparsely distributed. Under reflected light, irregular pyrite grains can be observed enclosed within plagioclase pores. A total of five diabase samples were collected. They are relatively fresh, come from different locations of the diabase veins, and can represent the primary characteristics of the original intrusive diabase veins.

FIGURE 2

5 Results

5.1 Zircon age

Zircons from the diabase are mostly colorless, transparent, short prismatic crystals with euhedral to subhedral morphologies. The grains are generally small, with most being less than 100 μm in length. Length-to-width ratios typically range from 1:1 to 2:1, and in some cases up to 3:1. Cathodoluminescence (CL) images show that most zircons exhibit oscillatory zoning (Figure 3a). A total of 13 zircons showing oscillatory zoning were selected for U-Pb isotope analysis, and the results are presented in Table 1 and Figure 3 (CL images and analyzed spots: Figure 3a; probability plot; Figure 3b; concordia diagram; Figure 3c).

FIGURE 3

Among the 13 analyzed spots, the Th content ranges from 13 to 661 ppm, the U content ranges from 95 to 1,393 ppm, and the corresponding Th/U ratios are mostly >0.1, ranging from 0.11 to 1.21. Based on their magmatic oscillatory zoning, crystal morphology, and CL features, these zircons can be classified as typical magmatic zircons. The obtained ages are relatively dispersed, ranging from 239 to 946 Ma. In addition, two zircons with Th/U ratios <0.1 also display oscillatory zoning and yield ages of 201 Ma and 206 Ma. On the U-Pb concordia diagram (Figure 3c), the two youngest analyses plot near the concordia curve, indicating negligible Pb loss and supporting the reliability of the 201–206 Ma age as the crystallization age of the diabase.

5.2 Major elements of whole rocks

The major and trace element compositions of the diabase are presented in Table 2. The SiO₂ contents of the samples range from 44.57 wt% to 49.58 wt%, MgO contents from 3.98 wt% to 7.46 wt% (Mg^# = 56–63), TiO₂ contents from 0.66 wt% to 1.09 wt%, and FeO_t contents from 5.67 wt% to 7.89 wt%. The ranges of other major elements are as follows: Na₂O = 1.37–4.41 wt%, K₂O = 0.36–1.04 wt%, CaO = 10.92–13.51 wt%, Al₂O₃ = 15.72–18.58 wt%, and P₂O₅ = 0.08–0.25 wt%. Overall, the total alkali contents are relatively low (Na₂O + K₂O = 2.15–4.77 wt%). In the Zr/TiO₂ × 0.0001 versus SiO₂ diagram (Figure 4a), most samples still plot within the sub-alkaline series field, whereas in the AFM diagram (Figure 4b), most samples fall into the sub-alkaline tholeiitic basalt series field.

FIGURE 4

Irvine and Baragar, 1971) of diabases in Bikou area.

The total rare earth element contents (ΣREE) of the diabase samples are relatively low, ranging from 27.25 ppm to 86.20 ppm. The light-to-heavy REE ratio (LREE/HREE) ranges from 2.49 to 2.95 (Table 2). On the chondrite-normalized REE distribution diagram (Figure 5a), the samples generally exhibit a nearly flat pattern, with (La/Yb)N = 1.93–2.39 and δEu = 0.85–1.21. These values indicate weak fractionation between light and heavy REE and a weak positive Eu anomaly, suggesting that fractional crystallization during magma evolution was not significant. Overall, the REE distribution patterns of the samples are similar to those of island-arc tholeiitic basalts (Figure 5a). On the primitive mantle-normalized trace element spider diagram (Figure 5b), the samples show marked depletion in elements such as Nb, Ta, and Pb, and relative enrichment in Ba and K. This element association is also comparable to that of island-arc tholeiitic basalts.

FIGURE 5

5.3 Sr-Nd isotopes

The diabase formed at ca. 200 Ma has ⁸⁷Rb/⁸⁶Sr ratios ranging from 0.20986 to 0.37017 and ¹⁴⁷Sm/¹⁴⁴Nd ratios from 0.1671 to 0.1770 (Table 3). The initial Sr isotope ratios are relatively low, with (⁸⁷Sr/⁸⁶Sr)_i ranging from 0.706627 to 0.707311, whereas εNd(t) values vary within a narrow range from −1.16 to 0.16. The two-stage Nd model ages (T_2DM) range from 972 to 1,079 Ma.

6 Discussion

6.1 The formation age of diabase

Mafic to ultramafic rocks generally have low Si and Zr contents, and thus it is often difficult for them to grow abundant zircons. Nevertheless, in many orogenic belts, both mafic and ultramafic rocks may contain variable amounts of zircon grains. These zircons could be inherited grains derived from crustal materials that have been recycled into the mantle source region, zircons modified by mantle-derived melts or fluids, or zircons crystallized directly from basaltic magmas (Cai et al., 2025). Detailed studies of such captured zircons allow us to infer the evolutionary history of geological bodies surrounding the magma conduits of basalts, acting as “probes” that penetrate the upper part of the lithosphere.

Zircons in mafic rocks also include inherited grains, and sometimes yield ages significantly younger than those of the host rocks or related structural units. Several explanations exist for these young zircons: (1) the basalts are younger products, and the youngest zircon population records the intrusion and crystallization age of the basalts (Song et al., 2014); (2) the ages of young zircons represent the timing of late metamorphic, magmatic, tectonic, or hydrothermal events (Belousova et al., 2015).

LA-ICP-MS U–Pb dating of zircons from diabase veins in the Bikou area yielded a minimum age of 201–206 Ma. The exposed volcanic–sedimentary rock series in the Bikou area has a metamorphic grade mainly corresponding to the low chlorite–garnet facies. Most of the ultramafic to mafic rocks within this series have undergone intense hydrothermal alteration but not medium-grade metamorphism. Consequently, typical metamorphic zircons are not developed. However, zircons that have been overprinted and modified by hydrothermal fluids commonly exhibit complex internal structures, appearing porous or spongy, containing abundant hydrothermal mineral inclusions and fluid inclusions, and showing relics of primary textures. The young zircons investigated in this study from the Bikou area occur as short prismatic crystals with well-developed oscillatory zoning. Based on these morphological features, the ages of 201–206 Ma (weighted mean ²⁰⁶Pb/²³⁸U age of 203.5 ± 3.5 Ma, n = 2) are interpreted to represent the formation age of the diabase.

Field geological relationships show that the diabase veins either intrude directly into dolomite or are emplaced along brittle fractures within the dolomite. The contact interface between the diabase and dolomite is mostly embayed or nearly straight, indicating that the intrusion and brittle fracturing were roughly synchronous or that the intrusion occurred shortly after fracturing. The brittle fractures in the Bikou area were mostly controlled by the Mianxian–Yangpingguan Fault, belonging to a unified fault system. As the bounding fault of the Bikou block, the Mianxian–Yangpingguan Fault formed during the interaction between the Bikou block and its surrounding blocks. Previous studies have shown that in the Late Triassic, the Bikou block began to interact with the Qinling orogenic belt, leading to the formation of the Mianxian–Yangpingguan Fault (Wang E. Q. et al., 2001; Li et al., 2002). Wen D.H. (1994) and Wang Y. L. et al. (2001) pointed out that during the Indosinian period, this fault was mainly controlled by NW–SE compression and was accompanied by left-lateral strike-slip motion. Li et al. (2002) suggested that this fault recorded three phases of Indosinian tectonic deformation: ductile reverse thrusting (D1), left-lateral brittle–ductile thrusting (D2), and right-lateral brittle slip (D3). Ren and Liu (2011) further subdivided its activity into six stages, proposing that the fault activity extended from the late Indosinian to the Himalayan period. Collectively, these studies indicate that brittle activity along the Mianxian–Yangpingguan Fault and its branch faults mainly occurred in the late Indosinian. The age and field characteristics of the diabase obtained in this study are consistent with previous understandings, suggesting that there was indeed a Late Triassic tectonic event in this region, namely, the transition from ductile to brittle deformation.

6.2 The origin of diabase

The whole-rock composition of the basalt in the Bikou area exhibits sub-alkaline tholeiitic characteristics. Since porphyritic basalt can form in various tectonic settings, including oceanic islands, mid-ocean ridges, island arcs, marginal basins, active continental margins, and stable continental margins, further discrimination using geochemical diagrams is necessary. The SiO₂ (44.57–49.58 wt%), TiO₂ (0.66–1.09 wt%), and FeO_t/MgO (0.96–1.31) contents of the basalt in this area are similar to those of island-arc porphyritic basalts (Miyashiro, 1975). On the 2Nb–Zr/4–Y and Hf/3–Th–Ta diagrams (Figures 6a,b), the sample points plot near the volcanic-arc basalt and island-arc basalt fields. On the Ta/Hf–Th/Hf and Ta/Yb–Th/Yb diagrams (Figures 6c,d), the samples fall within the oceanic island-arc field. The REE and trace element patterns (Figure 5) of the basalt are distinctly different from those of most intra-oceanic basalts (e.g., MORB and OIB) but are similar to those of island-arc porphyritic basalts related to oceanic plate subduction. In addition, pronounced negative Nb and Ta anomalies in the trace element patterns indicate significant involvement of continental materials in the magma genesis (Crawford et al., 1987; Davidson, 1987; Jahn et al., 1999; Martin, 1999).

FIGURE 6

High-field-strength elements (e.g., Ti, Zr, Y, Nb, Ta, Hf, Th) and rare earth elements are less affected by later alteration, and their geochemical characteristics can effectively reflect the nature of the magma source (Taylor and Mclennan, 1985; Rudnick and Gao, 2003). Crustal material can enter the lithospheric mantle during subduction and undergo partial melting and modification, or it can be added to mantle-derived magma via crustal contamination during magma ascent. The Ba/Nb and La/Nb ratios of the studied samples range from 23 to 151 and from 2.1 to 2.7, respectively, which are significantly higher than the corresponding ranges for MORB, OIB, alkaline basalt, and kimberlite (1–20 and 0.5–2.5), indicating that continental material played an important role in the evolution of the basaltic magma (Dungan et al., 1986). However, because the average La/Nb ratio of the continental crust is approximately 2.2, and some samples have values not lower than this level, it is more likely that the geochemical characteristics of the basalt in the Bikou area are related to source region modification rather than simple shallow crustal contamination. Continental basalts that have not been contaminated by the crust typically have Nb/La >1, whereas after crustal contamination, the Nb/La ratio drops below 1 (Zeng et al., 2015). Furthermore, the average Nb/U ratio of oceanic basalts is 52 ± 15 (Hofmann et al., 1986), whereas that of the crust is much lower (∼6.2; Rudnick and Gao, 2003). A low Nb/U ratio can be used as an indicator of crustal assimilation. The diabase studied in this paper has Nb/La ratios of 0.37–0.47 (<1), low Nb/U ratios (9.94–17.63), pronounced negative Nb and Ta anomalies, and relatively low Nb/Ta ratios (12.8–16.6). These lines of evidence initially suggest either crustal contamination or modification of the source region by subduction components.

However, it is worth noting that if crustal contamination occurred during magma ascent, significant variations in La/Nb and Th/Nb ratios would be expected (Hawkesworth et al., 1995), along with depletions in Zr and Hf (Sun and McDonough, 1989). In contrast, the diabase samples in this study exhibit relatively constant La/Nb (2.14–2.68) and Th/Nb (0.29–0.32) ratios, and show positive Zr and Hf anomalies (Hui et al., 2020). Moreover, crustal contamination alone cannot explain the extremely low Nb/U ratios (9.94–17.63). Additionally, if the rocks had been mixed with crustal materials (⁸⁷Sr/⁸⁶Sr)ᵢ would be expected to increase and εNd(t) to decrease with increasing SiO₂ content during magma emplacement. However, in the Bikou diabase samples, the Sr–Nd isotope ratios remain stable with increasing SiO₂. Therefore, it is concluded that crustal contamination during magma ascent was not significant. Instead, this diabase most likely originated from a partial melt of depleted asthenospheric mantle that had been modified by subduction components.

6.3 Implications of tectonic

Dong et al. (2011a) systematically divided the evolution of the southern and northern Qinling structural belts into several stages, including the Precambrian to early Paleozoic evolution of the North Qinling, the late Paleozoic to Mesozoic subduction–collision stage of the South Qinling, and the Jurassic–Cretaceous intracontinental orogeny. Liu et al. (2011), based on regional magmatic rock data, subdivided the Mesozoic evolution of the Qinling region into the Mianlue Ocean basin closure stage (248–216 Ma), the North Qinling island-arc complex collision to orogenic belt collapse stage (215–201 Ma), and the post-collision subsidence stage (200–195 Ma). Chen et al. (2020), integrating regional sedimentary, metamorphic, and magmatic data, also proposed that the West Qinling experienced three stages: subduction (248–235 Ma), co-contraction (235–225 Ma), and post-collisional (225–195 Ma). Collectively, these studies indicate that by approximately 200 Ma, the Qinling orogenic belt had already entered the post-collisional stage (Liu et al., 2011). This interpretation is further supported by evidence from magmatic rocks (Zhang et al., 2008; Wu et al., 2009; Chen et al., 2009; Jiang et al., 2010; Qin et al., 2010; Wang X. X. et al., 2011; Dong et al., 2012; Lü et al., 2014; Zheng et al., 2015; Zhang et al., 2019; Wang et al., 2024), metamorphic rocks (Zhang et al., 2002; Wan et al., 2005; Liu et al., 2006), and tectonic deformation (Li et al., 1999). The crystallization age of the diabase in the Bikou area obtained in this study is Late Triassic, which coincides with the time frame of the post-collisional stage, thus constraining its formation setting. Within the same structural belt, Sun et al. (2004) also obtained a Late Triassic ⁴⁰Ar/³⁹Ar plateau age from the basic rock wall group at the Qinling–Kunlun junction. These results indicate that a tectono-magmatic event occurred in the South Qinling during the Late Triassic, after the closure of the Mianlue Ocean basin and during the post-collisional stage.

A complete collisional orogeny typically includes a compression stage, a transition stage from compression to extension, and an extensional stage. During the compression-to-extension transition, the orogenic belt generally experiences a special tectonic regime of reduced pressure and increased temperature, which facilitates melting and enhances crust–mantle interaction and magmatic activity (Zhu et al., 2009). During the Late Triassic, the stress state of the Qinling orogenic belt changed, and it entered the compression-to-extension transition stage (Feng et al., 2003). In this context, the orogenic belt experienced pressure release, and mantle-derived materials rose and intruded into the lower crust, causing partial melting and forming the diabase veins in the Bikou area. Therefore, these diabase veins can be regarded as products of mafic magma intrusion in an extensional environment.

Although the geochemical characteristics indicate that the diabase in the Bikou area has a composition similar to continental margin arc basalt, previous studies have pointed out that magmatic rocks with volcanic arc signatures do not necessarily form in a typical subduction environment (Peccerillo, 1998; Xia, 2014). Mafic to intermediate rocks formed during the post-collisional stage often exhibit geochemical compositions resembling island-arc basalts because they inherit the geochemical fingerprint of a mantle source that was previously metasomatized by subducted slab-derived components (El Hassan et al., 2010; Wang et al., 2013). Based on the analytical results of this study and previous work in the South Qinling region, we conclude that the diabase in the Bikou area formed during the post-collisional stage rather than as a direct product of an island-arc tectonic setting. This understanding is of great importance for a comprehensive view of the tectonic and metallogenic processes in the Bikou area.

7 Conclusion

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