Late Triassic Felsic and Mafic Magmatism in the South Qinling Orogen, Central China: Insights from the Petrology, Zircon U-Pb Geochronology, and Geochemistry of the Huoshaodian Pluton

Abstract

The petrology, geochemistry, and zircon U-Pb chronology of the Huoshaodian pluton in the Liuba area of the western part of the South Qinling tectonic belt are investigated in this study. The Huoshaodian pluton consists of gabbro, quartz diorite, and granodiorite, and the dominated rock type is quartz diorite. The results indicate that the Huoshaodian pluton belongs to the calc-alkaline series. In the chondrite-normalized REE, all of the samples showed similar patterns, with an enrichment of light REEs and depletion of heavy REEs, but they showed slight differences in the degrees of Eu anomalies. The primitive mantle-normalized trace element diagram reveals an enrichment of large-ion lithophile elements (LILEs) and light rare earth elements (LREEs), as well as depleted high field strength elements (HFSEs). The zircon U-Pb dating results reveal that the gabbro, quartz diorite, and granodiorite have crystallization ages of 214.9 ± 0.58 Ma, 215.0 ± 1.2 Ma, and 215.4 ± 1.9 Ma, respectively, indicating that the Huoshaodian pluton was emplaced during the late Triassic period (214.9–215.4 Ma). In terms of petrogenesis, the gabbro of the Huoshaodian pluton originates from a transitional lithospheric mantle that has undergone fluid metasomatism and partial melting. Specifically, it originated through 1%–2% garnet spinel peridotite undergoing partial melting. In addition, the gabbro underwent a slight degree of contamination by crustal materials during its ascent and intrusion, with some continental crust material being incorporated. The quartz diorite and granodiorite of the Huoshaodian pluton are formed through partial melting processes occurring within the normal lower crust. Combined with the previous studies on the early Mesozoic tectonic evolution of the South Qinling, this study proposes that the formation mechanism of the Huoshaodian pluton may be as follows: in the early Triassic, the Mianlue Ocean subducted northward beneath the Qinling microblock, resulting in a large-scale continental-continental collision between the North China Block and the Yangtze Block; when the oceanic crust subducted to a certain depth, the detachment of the subducting slab triggered the upwelling of mantle material. The heat from mantle-derived magma caused the partial melting of the mafic lower crust, while the mafic magma entered into the upper granitic magma chamber and began to mix. Due to the high viscosity contrast and temperature difference between the two end-member magmas, incomplete mixing led to the formation of a melt with distinct adakitic characteristics and a mafic melt representing mantle-derived material.

1. Introduction

The Qinling orogenic belt has experienced three important evolutionary processes: the formation and development of the Neoarchean–Mesoproterozoic crystalline basement, the plate tectonic evolution stage characterized by the modern plate tectonic system in the late Neoproterozoic–early Mesozoic, and the intracontinental orogeny since the late Mesozoic. The tectonic framework of the North China Block, the Yangtze Block, and the Qinling microblock between them and the Shangdan and Mianlue tectonic belt separating these blocks is constructed [1]. During the early and middle Paleozoic, when the Shangdan ocean subducted and collided northward [2], in the late Paleozoic, the Mianlue paleo-ocean began to subduct northward, and the collision between the Qinling microblock and the Yangtze Block occurred in the early Mesozoic, forming the southern Mianlue tectonic belt. The North China Block and the Yangtze Block have undergone comprehensive collision, and the main tectonic framework of the Qinling orogenic belt was finally formed [3,4,5,6].

The intense orogeny during the early Mesozoic led to the extensive intrusion of granitic magma in the Qinling area [5]. The Mianlue tectonic belt exhibits a late Triassic granite belt stretching approximately 400 km along its northern side, displaying an east–west distribution. The Dongjiangkou pluton group (Dongjiangkou, Zhashui, Caoping, and Shahewan plutons), the Wulong pluton group (Huayang, Wulong, Laocheng, and Yanzhiba plutons), and the Guangtoushan pluton group (Guangtoushan, Zhangjiaba, Jiangjiaping, Xinyuan, Miba, Liuba, and Huoshaodian plutons) are the three major pluton groups. They form irregular oval-shaped large intrusions, are undeformed, and are situated within the Proterozoic middle-to-high-grade metamorphic sedimentary strata and Paleozoic lower-grade metamorphic sedimentary strata [7]. They provide direct material evidence to unveil the tectonic setting and dynamic background of its formation, and they unravel the tectonic evolution of the Qinling orogenic belt. These plutons are not only spatially related to the Mianlue tectonic belt in the south, but also closely related to the tectonic evolution of the Mianlue belt in terms of magmatic genetic type and intrusive age. In this context, previous studies have been conducted extensively, resulting in substantial progress. The formation age of granodiorite in the Huayang pluton in the east is 228–214 Ma, while the Guangtoushan granite pluton in the western region has shown zircon U-Pb ages ranging from 221 to 199 Ma. Previous studies have shown that the formation age of the Huoshaodian pluton is Carboniferous [8,9]. Its formation age is quite different from neighboring plutons. Therefore, this study selects the Huoshaodian pluton in the Liuba area of the western South Qinling, and through petrology, geochemistry, and LA-ICP-MS zircon U-Pb dating, combined with previous research, explores the formation age, genesis mechanism, and geological significance of the Huoshaodian pluton in the Liuba area of the South Qinling. The aim is to provide new data for interpreting the genesis of the late Triassic granite in the South Qinling and for reconstructing the tectonic evolution of the Qinling orogenic belt.

2. Geological Background and Geological Characteristics of Pluton

2.1. Regional Geological Setting

The Qinling orogenic belt is a major component of the Central China Orogenic Belt (CCOB). It has experienced a multi-stage multi-continental block long-term breakup and assembly of the composite continental collision orogenic belt [1]. The Qinling orogenic belt has undergone a transition into the stage of intracontinental orogenic evolution since the late Mesozoic. The closure of the Mianlue Ocean led to a comprehensive collision between the North China Block and the Yangtze Block, resulting in the formation of two tectonic belts: the Shangdan tectonic belt in the North Qinling and the Mianlue tectonic belt in the South Qinling. Additionally, three distinct belts were formed from north to south: the southern margin of the North China Block, the Qinling microblock, and the Yangtze Block (Figure 1a,b) [10].

The South Qinling tectonic belt is situated between the Shangdan tectonic belt to the north and the Mianlue tectonic belt to the south, while being bordered by the north Qinling tectonic belt on its northern side and the northern margin of the Yangtze Block on its southern side [11]. The Shangdan tectonic belt is the main suture of ocean–continent interaction in the Qinling region during the early Paleozoic [12]. The Mianlue paleo ocean in the southern margin of the south Qinling subducted northward during the late Paleozoic and closed during the Triassic period, leading to its ultimate closure in the late Triassic due to collision between the Qinling microblock and the Yangtze Block, resulting in the formation of the Mianlue tectonic belt [13,14,15]. This signifies the comprehensive collision between the North China Block and Yangtze Block, ultimately leading to the establishment of the predominant tectonic framework of the Qinling orogenic belt [16,17,18]. The South Qinling tectonic belt primarily experienced early Mesozoic magmatic intrusions, while the occurrence of early Paleozoic magmatic intrusions was absent [19,20,21].

Figure 1. (a) Sketch geological map of China [6]; (b) simplified tectonic maps of the Qinling orogenic belt showing the tectonic divisions [1]; (c) sketch map of the distribution of granitic rocks in the South Qinling tectonic belt [22]; (d) simplified geological map of the Huoshaodian pluton in the Liuba area in the South Qinling (after the Regional Institute of Mineral Resources and Geology, the Shaanxi Provincial Bureau of Geology and Mineral Resources, 2000, 1:50,000 Liuba County map). NCB—North China Block; S-NCB—southern margin of the North China Block; LLWF—Lingbao-Lushan-Wu yang fault; KPSZ—KuanPing Suture Zone; NQB—North Qinling belt; SDTB—Shangdan tectonic belt; MBBF—Mianlue-Bashan-Baoguang fault; SQB—South Qinling belt; MLTB—Mianlue tectonic belt; LOB—Longmenshan orogenic belt; YZB—Yangtze Block.

Figure 1. (a) Sketch geological map of China [6]; (b) simplified tectonic maps of the Qinling orogenic belt showing the tectonic divisions [1]; (c) sketch map of the distribution of granitic rocks in the South Qinling tectonic belt [22]; (d) simplified geological map of the Huoshaodian pluton in the Liuba area in the South Qinling (after the Regional Institute of Mineral Resources and Geology, the Shaanxi Provincial Bureau of Geology and Mineral Resources, 2000, 1:50,000 Liuba County map). NCB—North China Block; S-NCB—southern margin of the North China Block; LLWF—Lingbao-Lushan-Wu yang fault; KPSZ—KuanPing Suture Zone; NQB—North Qinling belt; SDTB—Shangdan tectonic belt; MBBF—Mianlue-Bashan-Baoguang fault; SQB—South Qinling belt; MLTB—Mianlue tectonic belt; LOB—Longmenshan orogenic belt; YZB—Yangtze Block.

The South Qinling tectonic belt exhibits the extensive formation of early Mesozoic granitic plutons (252–185 Ma). The majority of these plutons were exposed along the northern margin of the South Qinling and Yangtze Block (Figure 1d), with their formation ages primarily concentrated around 225–220 Ma. From east to west, the formation of three pluton groups (Dongjiangkou, Wulong, and Guangtoushan plutons) can be broadly classified into two stages: (1) the early Triassic granitic pluton (250–240 Ma) is less exposed and it has lower SiO₂ content, higher MgO and CaO content, K₂O/Na₂O < 1, and an A/CNK = 0.70 to 1.31; (2) widely exposed late Triassic granitic pluton (225–185 Ma), consisting of quartz diorite, quartz monzonite, granodiorite, and monzonitic granites with a abundance of mafic microgranular enclaves (MMEs). They predominantly represent I-type or A-type granite transitioning from I-type to A-type, and their geochemistry is primarily characterized by high Na₂O and K₂O, A/CNK ratio ranges between 0.70 and 1.31, as well as an enrichment of LILEs and a depletion of Nb, Ta, P, and Ti [23,24,25].

2.2. Geological Characteristics of Pluton

The Huoshaodian pluton is located in Huoshaodian Town, southwest of Liuba County, Shaanxi Province, covering an area of ~40 km². The pluton is nearly elliptical in plane, and its long axis is in the NNE direction. It is exposed on the north side of the Mianlue tectonic belt and is in intrusive contact with the surrounding lithology (Figure 1d). The surrounding lithology primarily comprises sandstone slate, carbonaceous slate, and phyllite of the Zhouqu Formation, Bailongjiang Group, Silurian period. In addition, there is a partial intrusion of granitic dikes, with locally developed ductile folding, planar fabric, and B-type lineation. The Huoshaodian pluton consists of three distinct rock types, namely the gabbro, quartz diorite, and granodiorite, and the quartz diorite is the dominated rock type.

The exposed strata in the study area are primarily from the Silurian Bailongjiang group, specifically composed of the Middle Silurian Zhouqu Formation (S₂z). The Bailongjiang group is distributed in the area from Xujiaping Town to Baishigou and Baishuijiang Town. To the south, it is in fault contact with the Ordovician Dabao Formation (Od), and to the west, it is unconformably covered by the Cretaceous continental sedimentary strata. The strata of Bailongjiang group are primarily turbidites composed of gray to dark gray mudstone, siltstone, and sandstone, which have undergone significant structure deformation and low-grade metamorphism. The Zhouqu Formation (S₂z) can be classified into two rock units. The lower rock unit predominantly comprises feldspar quartz fine sandstone. The upper rock unit consists of a suite of gray to dark gray medium-thin bedded fine sandstone interbedded with thin layers of silty mudstone and phyllite.

3. Sample Descriptions

The rock assemblage of the Huoshaodian pluton comprises gabbro, quartz diorite, and granodiorite, and the quartz diorite is the dominated rock type.

Samples of gabbroLB012-1 (32°24′42″ N, 106°53′43″ E), LB012-2, and LB2305-2 (33°31′44″ N, 106°54′23″ E) were collected from the central sector of the Huoshaodian pluton. The outcrop of gabbro appears greyish-black to dark grey in the field, while the fresh surface appears dark grey. It has a medium-fine-grained gabbroic texture with massive structure (Figure 2a). The predominant minerals are hornblende (35%–40%), clinopyroxene (10%–20%), plagioclase (30%–45%), and minor biotite. Accessory minerals include zircon and apatite. The clinopyroxene exhibits a pale brown-yellow color and displays weak polychromatic properties. It has a subhedral columnar to allotriomorphic granular shape, with sizes ranging from 5 mm × 8 mm to 1 mm × 2 mm. Additionally, it shows partial replacement via chlorite. The hornblende is subhedral columnar in shape, with a size range from 1.5 mm to 2.0 mm. It exhibits pleochroism and partial chloritization. The plagioclase is subhedral, ranges in size from 4 mm × 8 mm to 1 mm × 2 mm, and exhibits distinct zoning and polyhedral twinning (Figure 2b,c), and the biotite is foliated.

Samples of quartz diorite LB2305-1 (33°31′44″ N, 106°54′23″ E), LB2309 (033°31′54″ N, 106°55′34″ E), LB2310 (33°31′38″ N, 106°56′17″ E), LB011 (33°31′52″ N, 106°56′27″ E), LB014 (33°32′46″ N, 106°53′42″ E), LB015 (33°34′55″ N, and 106°55′32″ E) were collected from Qinlinggou, Yuelianghe bridge, and Longwanggou of Huoshaodian. The rock is light grey-green, with a medium-fine porphyritic texture and massive structure (Figure 2d). The predominant minerals are plagioclase (45%–50%), hornblende (10%–15%), K-feldspar (10%–15%), biotite (5%–10%), and quartz (5%–10%). Accessory minerals include zircon and apatite. The majority of the hornblendes are subhedral columnar in shape, yellow-brown to brownish-green, and display polychromatic properties. The grain size spans from 0.4 to 1.4 mm, with swith partial chloritization. The plagioclase is euhedral, arranged in a stacked configuration, with polysynthetic twinning. The K-feldspar displays a subhedral–anhedral form, with Carlsbad twinning, which is indicative of its igneous origin and high-temperature crystallization history. Quartz is anhedral and granular, with a size of 0.2–0.3 mm, filling the spaces between other minerals. Apatite is granular, constituting less than 1% (Figure 2e,f).

Samples of granodiorite LB2304 (33°32′16″ N, 106°52′44″ E), LB2306 (33°33′01″ N, 106°54′46″ E), LB013-1 (33°32′16″ N, 106°52′34″ E), LB013-2, LB016-1 (33°34′56″ N, 106°54′47″ E), LB016-2, and LB016-3 were collected from the Shaofangba and the Tianxingliang of Huoshaodian. The fresh surface of the sample is grayish to light gray, with a medium-to-fine-grained granite texture and a massive structure (Figure 2g). The mineral assemblage in the granodiorites is similar to that in the quartz diorites but with slightly lower proportions of dark minerals. The predominant minerals are plagioclase (45%–50%), K-feldspar (25%–30%), quartz (20%–25%), biotite (5%–10%), and a minor quantity of hornblende accessory minerals are zircon and apatite. The plagioclase is euhedral and tabular, with sizes ranging from 2 mm to 4 mm, and is partially altered to sericite. The hornblende exhibits a subhedral–anhedral granular, with particle sizes range from 0.5 mm to 1.5 mm, and cracks are developed. The biotite is brownish-black, occurring in elongated and platy forms, with particle sizes ranging from 1 mm to 2 mm. Some boundaries may show slight irregularities, while others are irregular or uneven. A small portion of the biotite has undergone chloritization (Figure 2h,i).

4. Analytical Methods

4.1. Whole-Rock Geochemical Analyses

A meticulous collection process was employed to obtain a total of 20 rock geochemical samples and 4 isotopic age samples from fresh gabbro, quartz diorite, and granodiorite rocks. The major, trace, and REEs were analyzed in 3 samples of gabbro, 10 samples of quartz diorite, and 7 samples of granodiorite. The rock samples underwent comprehensive geochemical testing and analysis at Wuhan Sample Solution Technology Co., Ltd. (Wuhan, China), ensuring a rigorous scientific investigation of their chemical composition and properties. After grinding the test sample to a fineness of 200 mesh, the predominant constituent is determined utilizing a wavelength-dispersive X-ray fluorescence spectrometer. The precision and accuracy of this analysis surpass that of the X-ray fluorescence spectrum analysis melt plate method by less than 2%, in accordance with the national standard GB/T14506.28-2010 [26]. The determination of trace elements was performed using the Agilent 7700e inductively coupled plasma mass spectrometer, which demonstrated analytical precision and accuracy exceeding 10%. The chemical analysis procedure followed the methodology described by Chen et al. [27].

4.2. LA-ICP-MS Zircon U-Pb Dating

The isotope dating samples were subjected to crushing, followed by zircon separation, selection and target preparation, the analysis of zircon dating samples, and the cathodoluminescence photography of dating samples, all conducted by Wuhan UP spectrum Analysis Technology Co., Ltd. (Wuhan, China). The zircon dating was performed using the Agilent 7900 inductively coupled plasma mass spectrometer (ICP-MS) and the GeoLas HD coherent 193 nm excimer laser ablation system, employing the LA-ICP-MS technique. To ensure the precise dating of the sample zircon, an initial step was taken to mitigate surface cracks and select the appropriate crystal domain for dating by employing a combination of cathode luminescence images and reflected light transmission images. The 193 nm laser was employed for zircon ablation, with a spot beam diameter of 32 μm. NIST 610 was utilized as the standard sample for trace element correction, while 91,500 served as the standard sample for isotope ratio correction. Moreover, to monitor the isotope ratio accurately, an external standard in the form of a silicate glass NISTRM610 synthesized by the US National Bureau of Standard Materials was incorporated to calculate the content of GJ-1 elements, with inner table element correction performed using 29Si. The experimental data were processed using the ICPMS Data Cal 10.9 program [28,29] and Isopiot4.15 [30], incorporating the ²⁰⁸Pb correction method for common lead correction.

5. Results

5.1. Geochemistry

The whole-rock major and trace element compositions of the Huoshaodian pluton are presented in Table S1. The Huoshaodian pluton was classified as gabbro (3 samples), quartz diorite (10 samples), and granodiorite (7 samples).

The gabbro of the Huoshaodian pluton contains SiO₂ ranging from 49.60 wt.% to 51.62 wt.%, while Al₂O₃ ranged from 16.05 wt.% to 16.18 wt.%. The K₂O levels ranged between 1.34 wt.% and 1.91 wt.%, and Na₂O ranged from 3.05 wt.% to 3.96 wt.%. The combined content of K₂O and Na₂O (K₂O + Na₂O) fell between 4.39 wt.% and 4.54 wt.%. Additionally, TiO₂ was found to range from 0.87 wt.% to 1.00 wt.%, and P₂O₅ varied between 0.23 wt.% and 0.25 wt.%. In the SiO₂-(K₂O + Na₂O) diagram, two samples fell in the subalkaline gabbro region and one sample fell in the monzodiorite region (Figure 3a). The three samples in the QAP diagram all fell in the gabbro region (Figure 3b). The Na₂O/K₂O ratio ranged between 2.15 and 2.28; in the SiO₂ vs. K₂O diagram, all samples fell in the calc-alkaline series region (Figure 3c).

In the chondrite-normalized REE patterns (Figure 4a), three analyzed samples showed similar patterns, with an enrichment of light REEs and depletion of heavy REEs and a distinct differentiation between LREEs and HREEs. The value of (La/Yb)_N was 7.10, and δEu values ranged from 0.82 to 0.95, with a slightly negative value. Based on the chondrite-normalized REE pattern composition in Table S1, the gabbro of the Huoshaodian pluton displayed higher REE contents (∑REE), ranging from 520 ppm to 521 ppm, with an average value of 520 ppm. Furthermore, the content of LREEs was between 307 ppm and 368 ppm, with an average value of 337 ppm, and the ratio of LREEs to HREEs varied between 2.82 and 2.86. The primitive mantle-normalized trace element patterns showed that all samples contain high levels of LILEs, such as Ba, U, Pb, Sr, and Nd. However, they had lower levels of HFSEs like Nb, Ta, Ti, Ba, and P (Figure 4b). Ba depletion may be related to feldspar minerals in the source region (e.g., plagioclase crystallization). The depletion of P indicates the presence of apatite crystallization in the source region. The depletion of Ti can be related to the crystallization of titanium-rich minerals during magma differentiation.

The quartz dioritecontainedSiO₂ ranging from 58.29 wt.% to 62.59 wt.%, while Al₂O₃ was between 15.80 wt.% and 16.09 wt.%. The CaO content fluctuated between 4.84 wt.% and 6.28 wt.%, and TFe₂O₃ ranged from 4.89 wt.% to 6.05 wt.%. MgO was found to range between 3.91 wt.% and 5.37 wt.%, with TiO₂ varying from 0.55 wt.% to 0.65 wt.%. The Mg# values ranged from 64.1 to 69.1, and the combined K₂O and Na₂O (K₂O + Na₂O) content ranged from 5.25 wt.% to 6.48 wt.%. Additionally, the A/CNK values fell between 0.83 and 0.91. The majority of samples fell in the diorite region in the SiO₂-(K₂O + Na₂O) diagram, and two samples fell in the boundary between diorite and granodiorite (Figure 3a). In the QAP diagram, all samples fell in the region of quartz monzodiorite (Figure 3b). The sample in the SiO₂-K₂O diagram fell in the calc-alkaline series (Figure 3c). All samples fell in the metaluminous region in the A/CNK-A/NK diagram (Figure 3d).

In the chondrite-normalized REE patterns (Figure 4c), the LREE/HREE ratio averaged at 4.63, indicating an enrichment of LREEs relative to HREEs. The value of (La/Yb)_N varied from 11.35 to 21.55. δEu values ranged from 0.23 to 0.33. A slight negative of Eu suggests that the source region may have undergone partial plagioclase fractionation [36]. The primitive mantle-normalized trace element patterns showed an enrichment of LILEs and a depletion of HFSEs, similar to the gabbro samples (Figure 4d). This feature may reflect a post-collision environment [37].

The major element composition of the granodiorite showed some variation. The SiO₂ content ranged from 63.79 wt.% to 66.30 wt.%, while K₂O varied from 2.66 wt.% to 4.97 wt.%. The Na₂O content fell between 3.07 wt.% and 4.63 wt.%, and the combined K₂O + Na₂O content ranged from 6.21 wt.% to 8.04 wt.%. The Al₂O₃ content varied from 15.10 wt.% to 16.09 wt.%, TiO₂ ranged from 0.37 wt.% to 0.65 wt.%, and P₂O₅ was found to range between 0.12 wt.% and 0.19 wt.%. The weight ratio of Na₂O to K₂O varied between 0.62 wt.% and 1.92 wt.%. A/CNK varied between 0.90 and 0.94. The sample primarily fell in the granodiorite region in the SiO₂-(K₂O + Na₂O) diagram (Figure 3a), two of the samples fell in the boundary between granodiorite and quartz monzonite, and one of the samples had a relatively high alkalinity and fell within the quartz monzonite region. In the QAP diagram (Figure 3b), the samples were scattered, with most falling in the granodiorite region. In the SiO₂-K₂O diagram (Figure 3c), the data mainly fell in the regions of the high-K calc-alkaline series and the calc-alkaline series. All samples fell in the metaluminous region in the A/CNK-A/NK diagram (Figure 3d).

The chondrite-normalized REE patterns (Figure 4e) and primitive mantle-normalized trace element patterns (Figure 4f) of granodiorite were similar to those observed in quartz diorite, contained high levels of LREEs and LILEs (such as Rb, U, K, Pb, Sr, Nd and Cs), and had lower levels of HREEs and HFSEs (such as Nb, Ta, P and Ti).

5.2. Zircon U-Pb Ages and REE Contents

The LA-ICP MS zircon U-Pb dating results and the REE content of the Huoshaodian pluton are presented in Tables S2 and S3.

5.2.1. Zircon U-Pb Age of Gabbro

The gabbro age sample (LB012) was collected from the central part of the Huoshaodian Pluton. Most of the zircon grain sizes range from 70 μm to 150 μm. and length-to-width ratios of 1:1 to 3:1. They were euhedral, short, and prismatic, and they exhibited distinct oscillatory zoning patterns when observed under cathodoluminescence (CL) imaging (Figure 5a). The luminescence of the fan-shaped growth rings gradually transitioned, with minimal contrast. The wider fan-shaped rings reflected a higher temperature during the crystallization of the zircon. The Th/U ratio of zircon ranged from 0.33 to 0.63, all of which were greater than 0.1 (Figure 6a). In the chondrite-normalized rare earth element (REE) distribution diagram, the zircon grains exhibited a depletion of LREEs and distinct negative Eu anomalies and positive Ce anomalies (Figure 6b), indicative of a magmatic origin [38]. Twenty-four zircon samples from the Huoshaodian gabbro were chosen for zircon U–Pb dating, with detailed U–Pb isotopic data and trace elements listed in Tables S2 and S3. After excluding data with discordance below 95%, a total of 17 valid analysis points were obtained. The zircon ²⁰⁶Pb/²³⁸U age was highly concentrated, displaying a weighted mean age of 214.9 ± 0.58 Ma (MSWD = 0.077, n = 17). This value represents the crystallization age of the gabbro within the Huoshaodian pluton (Figure 7a,b).

5.2.2. Zircon U-Pb Ages of Quartz Diorite

The zircon in the quartz diorite samples (LB011 and LB015) was euhedral, short, and prismatic, with sizes ranging from 60 to 100 µm and length-to-width ratios of 1:1 to 1:3, as well as a distinct zircon growth zone. Zircon had obvious growth zoning, narrow rhythmic zoning, and large luminescence contrast between zonings. The fan-shaped ring belt was more obvious (Figure 5b,c). The Th/U ratios ranged from 0.29 to 0.60 (Figure 6a). The chondrite-normalized REE patterns revealed an enrichment of HREEs, a relative depletion of LREEs, and a negative Eu anomaly along with a significant positive Ce anomaly, suggesting the magmatic origin of the zircons (Figure 6c,d). The variation in the ²³⁸U content of sample LB015 was significant (673–1590 ppm), but the U content was not entirely positively correlated with age. Additionally, based on the REE content and CL imaging, the possibility of the zircon being inherited or captured zircon was excluded. Therefore, the age calculation for sample LB015 was still carried out using the conventional method. Twenty-four zircons were selected from the quartz diorite sample (LB011) for U-Pb chronology analysis, with the corresponding test results provided in Tables S2 and S3. The weighted mean ²⁰⁶Pb-²³⁸U ages of zircons were 215.1 ± 1.2 Ma (MSWD = 0.0088, n = 17) (Figure 7c,d). The U-Pb chronology of sample LB015 was determined using a selection of 24 zircons, resulting in a weighted mean age of 211.4 ± 1.60 Ma (MSWD = 3.000, n = 21) (Figure 7e,f).

5.2.3. Zircon U-Pb Ages for Granodiorite

The zircon samples from granodiorite (LB013) were similar to the quartz diorite, euhedral, short, and prismatic samples, with sizes ranging from 50 to 160 µm and length-to-width ratios of 1:1 and 1:3. The granite diorite sample exhibited a zircon CL pattern and chondrite-normalized distribution pattern similar to that of the quartz diorite sample. Zircon CL images showed that zircons have a wide and gentle fan-shaped ring belt (Figure 5d). The ²³⁸U content of samples LB013 and LB015 exhibited similar characteristics; thus, the age calculation for both samples was also carried out using the conventional method. The granodiorite sample (LB013) was examined, and 24 zircons were selected for U-Pb chronology analysis. The Th/U ratios of all measured zircons ranged from 0.22 to 0.32, indicating their magmatic origin (Figure 6a). In chondrite-normalized REE patterns, samples also showed a depletion of LREEs, and the positive anomaly of Ce and the negative anomaly of Eu (Figure 6e). After excluding data with discordance below 95%, a total of 21 valid analysis points were obtained. The concentration of the ²⁰⁶Pb/²³⁸U age was relatively high, resulting in a weighted average age of 215.4 ± 1.9 Ma (MSWD = 1.180, n = 21) (Figure 7g,h), which represents the crystallization age of the granodiorite in the Huoshaodian pluton.

6. Discussion

6.1. Age of Formation of the Huoshaodian Pluton

The LA-ICP-MS zircon U-Pb dating results are presented in Tables S2 and S3. The zircon age of one Huoshaodian gabbro sample (LB012) was well constrained, with a weighted mean age of 214.9 ± 0.58 Ma (MSWD = 0.077, n = 17), representing the crystallization age of the gabbro from the Huoshaodian pluton.

The weighted mean ages of quartz diorite samples (LB011 and LB015) from the Huoshaodian pluton were 215.1 ± 1.2 Ma (MSWD = 0.0088, n = 17) and 211.4 ± 1.60 Ma (MSWD = 3.000, n = 21), respectively. Since the formation ages of the two were relatively close, the crystallization age of 215.1 ± 1.2 Ma (with high concordance) was selected for the quartz diorite of the Huoshaodian pluton.

The granodiorite (LB013) from the Huoshaodian pluton exhibited a weighted mean age of 215.4 ± 1.9 Ma (MSWD = 1.180, n = 21).

In summary, the Huoshaodian gabbro, quartz diorite, and granodiorite had similar ages, indicating that the age of the Huoshaodian pluton was 214–216 Ma, corresponding to the late Triassic of the Mesozoic and not the Carboniferous period. The Guangtoushan granite in the western region zircon U-Pb ages ranged from 221 to 199 Ma [40]. The Huayang granodiorite in the eastern region zircon U-Pb ages ranged from 228 to 214 Ma [41]. Therefore, the Huoshaodian pluton was possibly a product of the post-collision phase in the orogenic tectonic setting after the closure of the Mianlue Ocean, during the collision between the Yangtze Block and the North China Block.

6.2. Petrology

Typically, mafic rocks originate from the lithospheric mantle or the asthenosphere mantle [42,43,44]. The gabbro samples from the Huoshaodian pluton exhibited an enrichment of LREEs and LILEs, as well as a depletion of HREEs and HFSEs. Moreover, they demonstrated a weak negative Eu anomaly or no significant Eu anomaly. The partial melting of mafic granulite in the lower crust will result in a negative Eu anomaly [45,46], which suggests the mantle source characteristics of gabbro. The distribution patterns of REEs and trace elements in the gabbro of the Huoshaodian pluton differed from those in N-MORB and E-MORB, but they exhibited similarities to OIBs, suggesting that the lithospheric mantle, rather than the asthenosphere mantle, is likely to be its origin. Meanwhile, basalts derived from the asthenosphere exhibited La/Nb ratios of less than 1.5 and La/Ta ratios below 22, whereas basalts originating from the lithospheric mantle displayed contrasting characteristics [42]. The La/Nb values of the gabbro in the Huoshaodian pluton ranged from 3.02 to 3.33 (>1.5). The La/Ta values ranged from 72.80 to 83.07 (>22), indicating that the gabbro in the Huoshaodian pluton originates from the lithospheric mantle. In the Nb/Th-Zr/Nb diagram (Figure 8a), the samples were primarily plotted between the primitive mantle (PM) and the enriched mantle (EN), indicating that their source is the transitional mantle.

Trace elements with similar distribution coefficients are typically resistant to changes during partial melting or fractional crystallization processes, and, therefore, they usually reflect the characteristics of the parent magma of the magma source region.

The Zn/Fe ratio did not undergo significant fractionation during the partial melting of the mantle peridotite. However, the melt derived from pyroxenite or garnet pyroxenite, which is enriched in orthopyroxene and garnet, tended to have a higher Zn/Fe ratio (>13). In contrast, the three gabbro samples from the Huoshaodian pluton had Zn/Fe ratios of 9.25, 9.04, and 7.58, all of which were lower than 13, indicating that the source of the gabbro was a melt derived from peridotite. The gabbro exhibited a relative enrichment of LREEs, characterized by a high LREE/HREE ratio and low Yb content (1.60–2.17), indicating the presence of residual garnet in the source region. The sample fell close to the garnet spinel lherzolite melt curve in the La/Yb-Yb diagram (Figure 8b), indicating the presence of residual garnet and spinel within the source region, and the ratio of garnet to spinel was 50:50. During the process of partial melting involving varying proportions of clinopyroxene and garnet, distinct variations were observed in the behavior of La/Sm and Sm/Yb ratios, with Sm/Yb remaining constant despite the decrease in the La/Sm ratio [51]. According to the (La/Sm)_N-(Sm/Yb)_N diagram (Figure 8c), the gabbro of the Huoshaodian plutons was primarily the product of the partial melting of garnet–spinel lherzolite, with a ratio of monoclinic pyroxene to garnet ranging from 6:1 to 5:2. The degree of partial melting was between 1% and 2%. Due to the pronounced positive Sr anomaly, there was an absence of plagioclase residue in the source region.

The gabbro of the Huoshaodian pluton exhibited an enrichment of LREEs and LILEs, as well as a deficiency in HFSEs. In the La/Nb-Nb/Th diagram (Figure 8d), the samples were positioned away from both oceanic island basalts (OIBs) and mid-ocean ridge basalts (MORBs), which fell in the field of island arc magmatic rocks. This phenomenon could potentially be attributed to the existence of subduction fluids or the integration of island arc components [52,53,54]. The gabbro of the Huoshaodian pluton exhibited high TiO₂ content and a relatively prominent negative Ce anomaly, with Cr (275–465 ppm), Co (24.2–34.5 ppm), and Ni (62.5–102 ppm) contents surpassing those observed in normal mid-ocean ridge basalt (N-MORB); these characteristics indicate that the source region is subject to melt metasomatism [55,56], and this trend is similar to that observed in the Th/Zr-Nb/Zr diagram (Figure 8e). However, the negative anomalies of Nb and Ta, higher Ce/Th ratio (average of 29.84), and Ba/Th ratio (average of 341.23) were inconsistent with the characteristics of melt contamination [55,56]. In the Th/Yb-Ba/La diagram (Figure 8f), the samples exhibited distribution patterns along two distinct pathways of fluid and melt replacement, indicating that the gabbro source region might have experienced dual metasomatism involving both fluid and melt.

During the ascent and emplacement of mantle-derived magma, the assimilation and contamination of crustal materials have the potential to occur. The primitive mantle-normalized (Th/Nb) _PM value (>1) [57,58] and Nb/La (<1) [59,60,61] were two reliable trace element indices for distinguishing crustal mixing. The (Th/Nb) _PM value of the gabbro from the Huoshaodian pluton ranged from 1.58 to 2.85, while the Nb/La value ranged from 0.30 to 0.33. The relatively low (Th/Nb) _PM value indicates a distinct magma source, potentially influenced by the assimilation of crustal materials resulting in Nb enrichment [62,63]. This indicates that the gabbro from the Huoshaodian pluton experienced a certain degree of crustal contamination during the ascent of the magma. The magmas mixed with the crust generally exhibited elevated Th/Nb values (>5) and Th/Ta values (>10), while the La/Sm ratio increased to over 5 [64,65,66]. The gabbro from the Huoshaodian pluton exhibited relatively low Th/Nb ratios (0.18–0.34, with an average of 0.26), Th/Ta ratios (4.47–8.25, with an average of 6.36), and La/Sm ratios (3.28–3.57, with an average of 3.425). The primitive mantle-normalized trace element diagram did not exhibit any significant positive anomalies in Zr and Hf, indicating that the gabbro magma from the Huoshaodian pluton experienced minimal contamination from crustal materials during its ascent and evolution.

The gabbro in the Huoshaodian pluton originated from a transitional lithospheric mantle that was altered by fluid–rock interactions and melt processes resulting from the partial melting (1%–2%) of garnet–spinel lherzolite. During its ascent and emplacement, the gabbro experienced minimal crustal contamination, incorporating only a limited amount of continental crustal materials.

The quartz diorite samples from the Huoshaodian pluton P₂O₅ content exhibited a relatively low range, varying from 0.15% to 0.17%, and they exhibited a declining trend with increasing SiO₂ content. The trace element suggests that the quartz diorite from the Huoshaodian pluton displayed certain adakite traits, including high Sr contents of 591 to 690 ppm, a high Sr/Y ratio of 41.72 to 45.97, and a (La/Yb)_N ratio between 11.21 and 21.55; in addition, it had low Y contents of 13.1 to 16.5 ppm along with a subtle negative Eu anomaly [67]. The quartz diorite samples from the Huoshaodian pluton fell in the adakite field in both (La/Yb)_N-Yb_N and Sr/Y-Y diagrams (Figure 9a,b).

The previous studies suggest that the late Triassic adakite in the South Qinling region was formed by the partial melting of the thickened lower crust [17,67,70,71,72]. According to experimental petrology studies, the partial melting of the lower crust to form adakitic magma requires conditions within the garnet stability field [68,73,74]. The stability of garnet in the temperature range of 800–1000 °C varies depending on the composition of the source rock, with pressures ranging from approximately 0.9 to 1.4 GPa [75,76,77,78,79,80,81]. A common feature of adakite rocks with different origins or tectonic settings is that garnet and rutile are present as stable or residual phases during melt formation [77,82,83]. The intermediate-acid melt mentioned above is believed to have originated in the lower crust at a depth of approximately 40–50 km, indicating its distinctiveness from other intermediate-acid rocks lacking an adakitic composition. The granites formed in the thickened mafic lower crust (>50 km) exhibited high Sr/Y, Sr/CaO, and (La/Yb)_N ratios, which demonstrate a distinct positive correlation [84,85,86]. The significant distinguishing features between adakite and other formation processes (such as assimilation–fractional crystallization, crust–mantle magma mixing, and inherited source regions) are primarily reflected in its high Sr/Y and (La/Yb)_N ratios, which show clear differences from typical granites derived from the normal crust [87,88,89]. The quartz diorite in the Huoshaodian pluton, with its Sr and CaO content, as well as Sr/Y and (La/Yb)_N ratios, all fell within the range of typical granites formed by crustal processes. This suggests that the source region is unrelated to a thickened lower crust. Numerous studies have indicated that the significant differentiation between LREEs and HREEs in granites, as well as the steep distribution pattern of HREEs, can more reliably indicate the involvement of garnet and better characterize the features of adakite rocks [74,90,91,92]. Compared to granites from the thickened lower crust, the Sr and CaO contents in the quartz–feldspar rocks from the Huoshaodian pluton showed significant differences. Additionally, the distribution pattern of HREEs also deviated from the typical adakite characteristics. All high-Sr/Y granite samples from the Huoshaodian pluton were close to the amphibole field and far from the rutile field (Figure 9c). Considering that the minimum pressure limit for rutile stability is 1.5 GPa [77,93], it can be inferred that the source depth of the high Sr/Y granites in the Huoshaodian pluton should not exceed 50 km. Therefore, their origin is unlikely to be related to the melting or decompression of granulite facies in the lower crust. Recent experiments have shown that the partial melting (10%–40%) of a mafic lower crust at pressures of 1–1.5 GPa and temperatures of 800–900 °C can produce adakitic melts with high Sr/Y and La/Yb ratios [73,74,75,94]. This suggests that adakitic rocks from the lower crust do not necessarily indicate an excessively thick crust (greater than 50 km). Under these specific melting conditions, the remaining mineral assemblage consists of amphibolite facies and garnet-bearing granulite facies, resulting in acidic melts with higher Yb content compared to typical adakites [73,95]. This feature aligns with the samples collected from the Huoshaodian pluton, indicating an origin depth of approximately 30–40 km.

The Mg# value is an important criterion for distinguishing whether magma is derived from a crust–mantle hybrid source or the lower crust [96]. The Mg# value of the pluton with a crust–mantle hybrid origin is >45, whereas the Mg# value of the pluton resulting from the partial melting of the lower crust is generally <4 [97]. The Mg# values of quartz diorite in the Huoshaodian pluton ranged from 64.1 to 69.1, indicating that the magma may have undergone some degree of crustal magma interaction during its evolution, with the involvement of mantle-derived material. The weak negative Eu in the samples suggests that plagioclase residue was limited in the source region. Given its adakite-like characteristics, the possible presence of garnet residue in the source and the relatively high HREE content (Y = 13.1–16.5 ppm; Yb = 1.26–1.54 ppm) suggest a low degree of control via the garnet [98]. The samples in the Yb-(La/Yb)_N diagram (Figure 9a) fell close to the melting curve of amphibolite with 25% garnet, indicating that the potential source rock for the quartz diorite of the Huoshaodian pluton may be amphibolite containing approximately 25% garnet.

The granodiorite samples from the Huoshaodian pluton exhibited geochemical characteristics similar to adakite, SiO₂ content greater than or equal to 56%, Al₂O₃ content exceeding 15%, Na₂O > K₂O, an enrichment of LILEs and LREEs, a minimal negative Eu anomaly, an enrichment of Sr, and a high Sr/Y ratio. In the Yb_N-(La/Yb)_N diagram (Figure 9a), all samples fell into the region where the residual mineral phases in the source were primarily hornblende and garnet. In the Sr/Y diagram (Figure 9b), all samples fell in the range of adakite. Compared with adakite in the island arc environment, the K₂O of the rocks was obviously higher, so the granite with adakite may have a special genetic mechanism.

Due to the incompatibility of K during the partial melting process, a partial melt with a low degree (<10%) will result in a high-K and high-Si composition, along with a low Na₂O/K₂O ratio (0.9–0.12). On the other hand, a higher degree of melting (10%–30%) will lead to the formation of sodium-rich adakite rocks (Na₂O/K₂O ratio > 1.5–2.0) [99,100]. The granodiorite in the Huoshaodian pluton exhibited high K content and an average Na₂O/K₂O ratio of 1.14, and most of it fell into the high-K calc-alkaline region (Figure 3c). The Al₂O₃/TiO₂ ratio in granodiorite can be used as an indicator of the partial melting temperature of the source region. A higher Al₂O₃/TiO₂ ratio (>100) indicates a lower partial melting temperature (<875 °C), whereas a lower ratio suggests a higher partial melting temperature [101,102]. The Al₂O₃/TiO₂ ratios of granite within the Huoshaodian pluton exhibited a range between 26.98 and 43.3 (<100), suggesting an elevated partial melting temperature within the source region, with values reaching 875 °C. The granodiorite of the Huoshaodian pluton exhibited a significantly developed perthite under the microscope, suggesting that the granites may have formed in a high-temperature and hydrologically restricted environment [101,103].

The distribution coefficient of Sr in plagioclase is significantly higher compared to that in other minerals. In magmas with similar source rock compositions and no apparent differentiation, the enrichment or depletion of Sr can serve as an indicator for the incorporation of plagioclase into the melt [104,105]. The low Rb/Sr ratio of granodiorite in the Huoshaodian pluton (with an average of 0.19) implies that the pluton has not undergone significant degrees of crystallization differentiation [106]. The simultaneous occurrence implies a substantial influx of Sr into the melt, resulting in a decrease in the ratio. The geochemical characteristics of the samples display similarities to those of adakite, indicating the presence of residual garnet in the source region. The garnet is strongly enriched in HREEs, while the hornblende exhibits a relatively higher enrichment of MREEs. Therefore, when amphibole is the dominant residual phase, the melt shows a relatively flat HREE pattern (Y/Yb ≈ 10; (Ho/Yb)_N ≈ 1), and when the garnet is the dominant residual phase, the melt exhibits an inclined HREE distribution pattern, with (Y/Yb) >10 and (Ho/Yb)ₙ > 1.2 [70,107]. The geochemical parameters of the granodiorite from the Huoshaodian pluton exhibited a Y/Yb ratio ranging from 9.56 to 13.18, with an average value of 10.88. The (Ho/Yb)_N ratio ranged from 1.01 to 1.60, with an average value of 1.15, and showed a relatively flat HREE pattern. This indicates that the source region is probably composed of garnet and hornblende as the main residual phase, and it contains no or little plagioclase. The granodiorite samples from the Huoshaodian pluton fell in the partial melting region of the amphibolite in the Nb/Ta-Zr/Sm diagram (Figure 9c). The granodiorite of the Huoshaodian pluton likely represented an adakitic magma derived from the dehydration melting of amphibolite containing 25% garnet in the mafic lower crust.

To conclude, the gabbro of the Huoshaodian pluton originates from a transitional lithospheric mantle that is modified by both fluids and melts, formed by 1%–2% partial melting of garnet–spinel lherzolite. The magma of the Huoshaodian quartz diorite and granodiorite originates from the mafic lower crust at depths of 30–49 km, formed by 25% partial melting of garnet-bearing amphibolite. The different rock types present in the Huoshaodian pluton were closely associated spatially and had similar formation ages. The Harker diagram (Figure 10) showed a linear relationship between the major oxides (TiO₂, Fe₂O₃^T, MnO, MgO, CaO, K₂O, and P₂O₅) and SiO₂, indicating a genetic relationship consistent with the evolution of a parental magma. Due to variations in certain elements among the different rock types, suggesting that these rocks contained different components and that magma mixing was incomplete, it was hypothesized that the gabbro represents mafic magmas from a mantle source, and their mixing with crust-derived magmas results in significant compositional differences in the pluton. Therefore, the genesis of the different rock types in the Huoshaodian pluton is attributed to the melting of deep crustal materials, followed by the intrusion of mafic magmas from a mantle source into its magma chamber. The incomplete mixing between the two magmas was caused by differences in their viscosity and temperature.

6.3. Tectonic Environment Discrimination

Currently, there are diverse perspectives regarding the dynamic background of late Triassic magmatic activity in the South Qinling region, including the subduction of the Mianlue ancient oceanic crust to the north, a collision and compression convergence environment, the transition from post-collision compression to an extensional tectonic regime, and the post-collision extensional environment [7]. The ages of the late Triassic magmatic rocks in the South Qinling area, as determined by the Nd-Hf two-stage model, are predominantly concentrated in the Mesoproterozoic (1.1–1.5 Ga) [108,109]. The source rock is unlikely to be derived from the Paleozoic Mianlue oceanic crust or to have formed in a subduction environment associated with oceanic crust. The experimental petrology results also show that the late Triassic magmatic rocks in the Qinling orogenic belt, with their high-K characteristics, are distinctly different from the Na-rich melts (Na₂O/K₂O > 2) generated under high-pressure and high-temperature conditions from young oceanic crust [110]. The above characteristics suggest that the late Triassic magmatic activity in the South Qinling area could not have formed in a northward subduction environment of the Mianlue ancient oceanic crust. The peak age of late Triassic magmatic activity in the South Qinling is between 210 Ma and 220 Ma [110], and the timing of this event clearly falls outside the time frame for comprehensive collision between the Yangtze Block and the North China Block (221–242 Ma). This is further supported by geological characteristics in the region, such as the lack of significant deformation or only weak deformation in the late Triassic plutons, and given the fact that some plutons are placed within regional tectonic sutures in a passive manner [7,71,110,111,112], it is evident that these formations cannot have been formed within a co-collision and compression–convergence tectonic environment. The Huoshaodian pluton showcases an ellipsoidal morphology without evident deformation and demonstrates a distinct intrusive contact relationship with the adjacent rock, thus signifying its post-tectonic emplacement in an actively evolving environment. Therefore, the formation of the Huoshaodian pluton can be attributed to a post-collision tectonic setting, characterized by the transition from compression to extension following the collision between the Yangtze Block and Qinling microblock.

In terms of geochemistry, post-collision granites exhibit geochemical characteristics similar to those of island arc volcanic rocks (enriched in LILEs and LREEs, depleted in HFSEs). However, they demonstrate significantly elevated alkali and strontium content, as well as higher ratios of (Fe₂O₃ + FeO) to MgO and K₂O to Na₂O compared to island arc volcanic rocks [113,114]. The Huoshaodian pluton primarily consists of calc-alkaline granites, and the mineral assemblage corresponds to K-rich granites (KCGs) that were formed during the uplift and denudation stage subsequent to the main collision orogeny. In the final stage of continental block collision, materials from the accretionary wedge formed during the pre-collision subduction phase are typically incorporated into the collision belt, and they may become a component of the source material for subsequent magmatic activity. Hence, the source rocks of post-collision magmatism are primarily composed of crustal material, but they also include mantle components or newly formed crustal material that exhibits characteristics of both igneous and sedimentary rocks [115,116]. The geochemical characteristics of the Huoshaodian pluton imply a mechanism involving the mixing of crustal and mantle-derived magmas, likely influenced by island arc materials originating from subduction zone processes. Their formation process was influenced by the incorporation of subducted material during both the pre-collision and collision stages. Additionally, the Huoshaodian pluton exhibits geochemical characteristics of post-collision high-K calc-alkaline granite, indicating that it may be a product of the stress relaxation phase during the transition from compression to extension, following the post-collision between the south Qinling microblock and the Yangtze Block along the Mianlue tectonic belt.

The Liuba pluton, located north of the Huoshaodian pluton, and the Guangtoushan pluton, located in the west, exhibit MMEs that suggest a mixing of crustal and mantle magmas. The Huoshaodian pluton contains basic end-members, while the above discussion excludes the possibility of large-scale crustal contamination affecting the basic end-members. Therefore, the presence of this basic end-member may indicate that the formation of the rock body involved a bimodal mixing of mantle-derived basic and crust-derived acidic magmas, suggesting the addition of mantle-derived magma during the magma formation process. The predominant collisional orogeny of a continental orogenic belt is typically characterized by crustal thickening and surface uplift, and isostatic adjustments lead to an extension in the upper lithospheric crust, an uplift of mantle material, and the generation of substantial amounts of basic magma through decompression and melting [117]. The heat from the basic magma causes the partial melting of the upper crust material, and due to the mixing of mantle-derived basic magma with the upper granitic magma chamber, bimodal magma mixed with granitic melts is formed [118]. Previous researchers believe that this process is primarily caused by the detachment of oceanic crust and that the depth at which this detachment occurs is relatively shallow [117,119]. This also supports the conclusion that the granitic source region of the Huoshaodian pluton, which exhibits adakitic rock characteristics, is the result of the partial melting of the lower crust.

The formation mechanism of the Huoshaodian pluton can be inferred based on the aforementioned factors. During the early Triassic period, the Mianlue oceanic block underwent northward subduction beneath the Qinling microblock along the Mianlue suture zone, resulting in a significant continental–continental collision between the North China Block and the Yangtze Block. In the late Triassic, as the oceanic lithosphere subducted to a certain depth, slab breakoff induced mantle upwelling. The thermal input from underplating magma triggered the partial melting of the mafic lower crustal material, which then mingled with the upper granitic magma chamber. Due to significant differences in viscosity and temperature between the two types of magma, incomplete mixing occurred, leading to the generation of melts exhibiting distinct adakitic characteristics and mafic melts representing mantle-derived materials (Figure 11).

7. Conclusions

(1)

The LA-ICP-MS zircon U-Pb isotopic dating results indicate that the gabbro, quartz diorite, and granodiorite in the Huoshaodian pluton have crystallization ages of 214.9 ± 0.58 Ma, 215.1 ± 1.2 Ma, and 215.4 ± 1.9 Ma, respectively, suggesting that the formation age of the Huoshaodian pluton ranges from 214.9 to 215.4 Ma during the late Triassic period.

(2)

The gabbro samples from the Huoshaodian pluton exhibit an enrichment of LILEs and a depletion of HFSEs, which is indicative of typical geochemical features observed in arc-related magmatic rocks. During the ascent and evolution of the magma, processes of fractional crystallization and crustal contamination occurred. The quartz diorite and granodiorite derived from the Huoshaodian pluton display geochemical characteristics resembling adakitic rocks, suggesting that they were formed through the melting of mafic lower crust triggered by mantle-derived magma underplating.

(3)

The formation mechanism of the Huoshaodian pluton involves the subduction of oceanic crust to a specific depth, where the detachment of the subducting slab induces mantle material upwelling. Magma underplating brings heat, triggering the partial melting of the mafic lower crust. The resulting mafic magma ascends into the granitic magma chamber. Due to temperature differences and high viscosity between mafic and granitic magmas, incomplete mixing occurs, leading to the generation of mafic melts and melts with distinct adakitic characteristics.