Magma Mixing Origin for the Menyuan Granodioritic Pluton in the North Qilian Orogenic Belt, China

Abstract

Magma mixing or mingling is not just a geological phenomenon that widely occurs in granitoid magmatism, but a complex dynamic process that influences the formation of mafic microgranular enclaves (MMEs) and the diversity of granitic rocks. Herein, we carried out a comprehensive study that encompassed the petrology, mineral chemistry, zircon U-Pb ages, Lu-Hf isotopes, whole-rock elements, and Sr-Nd isotope compositions of the Menyuan Granodioritic Pluton in the northern margin of the Qilian Block, to elucidate the petrogenesis and physical and chemical processes occurring during magma mixing. The Menyuan Granodioritic Pluton is mainly composed of granodiorites accompanied by numerous mafic microgranular enclaves (MMEs) and is intruded by minor gabbro dikes. LA-ICP-MS zircon U-Pb dating reveals that these rocks possess a similar crystallization age of ca. 456 Ma. The Menyuan host granodiorites, characterized as metaluminous to weakly peraluminous, belong to subduction-related I-type calc-alkaline granites. The MMEs and gabbroic dikes have relatively low SiO₂ contents and high Mg^# values, probably reflecting a mantle-derived origin. They are enriched in large ion lithophile elements (LILEs) and light, rare earth elements (LREEs) but are depleted in high field strength elements (HFSEs), indicating continental arc-like geochemical affinities. The host granodiorites yield relatively enriched whole-rock Sr-Nd and zircon Hf isotopic compositions (⁸⁷Sr/⁸⁶Sr_i = 0.7072–0.7158; ε_Nd(t) = −9.21 to −4.23; ε_Hf(t) = −8.8 to −1.2), implying a derivation from the anatexis of the ancient mafic lower continental crust beneath the Qilian Block. The MMEs have similar initial Sr isotopes but distinct whole-rock Nd and zircon Hf isotopic compositions compared with the host granodiorites (⁸⁷Sr/⁸⁶Sr_i = 0.7078–0.7089; ε_Nd(t) = −3.88 to −1.68; ε_Hf(t) = −0.1 to +4.1). Field observation, microtextural and mineral chemical evidence, geochemical characteristics, and whole-rock Nd and zircon Hf isotopic differences between the host granodiorites and MMEs suggest insufficient magma mixing of lithospheric mantle mafic magma and lower continental crust felsic melt. In combination with evidence from regional geology, we propose that the anatexis of the ancient mafic lower continental crust and subsequent magma mixing formed in an active continental arc setting, which was triggered by the subducted slab rollback and mantle upwelling during the southward subduction of the Qilian Proto-Tethys Ocean during the Middle-Late Ordovician.

1. Introduction

Magma mixing, or the mingling of distinct compositions, is a common phenomenon in the process of magmatic evolution and has a significant impact on the genesis and compositional diversity of granitoids [1,2,3]. Compared with magma mingling, which refers to physical dispersion without chemical exchanges between compositionally distinct magmas, magma mixing emphasizes magma interactions accompanied by mechanical and chemical exchanges [4,5,6]. The temperature, viscosity, crystallinity, and rheology of both interacting magmas affect the degree of magma homogenization of the magmatic system [7]. Calc-alkaline granitic plutons commonly contain abundant mafic microgranular enclaves (MMEs), and the MMEs and their host granitoids are considered to be the crucial objects to study the genesis and diversity of granites and the interaction between crust and mantle [7,8,9,10,11,12,13]. Several potential models have been put forward to account for the genesis of MMEs, including (1) wall-rock xenoliths [14,15]; (2) refractory remnants from the magma source [16,17,18]; (3) autoliths associated with early-formed cumulate minerals within the cognate magmatic system [19,20,21,22,23,24]; and (4) hybrids generated by the mixing/mingling between mafic and felsic magmas [25,26,27]. Magmatic enclaves occur as a representation of hybrid magma blobs with obvious chemical–physical disequilibrium features, which are considered as one of the most clear pieces of evidence for insufficient magma mixing [5,28,29,30]. To date, there exist two major mechanisms for the formation of magmatic enclaves: heterogeneous magma mixing and homologous magma mixing [19,21,25,31,32]. Verifying the relationship between the MMEs and host granitoids could provide additional insights into the petrogenesis and source of granitoids, the interaction between both magmatic systems, and deep geodynamic processes.

The Qilian Orogen lies in the northeastern margin of the Qinghai-Tibet Platea, stretching in a NW-SE direction. It is bounded by the Alxa Block on the north, the Tarim Block on the west, the Qaidam Block on the south, and the West Qinling Orogenic Belt on the southeast (Figure 1a,b). The North Qilian Belt is considered a typical oceanic suture zone located between the Qilian Block and Alxa Block (Figure 1b [33,34]). The extensive Early Paleozoic granitic magmatism is widespread in the North Qilian Belt, and the majority of these instances of granitic magmatism are suggested to be associated with oceanic–continental subduction and subsequent continental collision [33,35,36,37,38,39,40,41]. In the past decades, an ever-growing quantity of geochemical and isotopic chronological investigations has been conducted in the North Qilian Belt to ascertain the origin of early Paleozoic granitoid plutrons and their MMEs, such as the Chaidanuo, Jinzichuan, Beilinggou, Shidonggou, Niuxinshan, Tuole, Xiagucheng, Yeniutan, Leigongshan, and other plutons [33,37,42,43,44,45,46,47,48,49]. Therefore, the North Qilian Belt is an ideal study area for studying the petrogenesis of the MMEs and their host rocks and the geochemical behavior of elements and isotopes in magmatic systems during mixing. This study is concentrated on the Menyuan Granodioritic Pluton, which is situated on the northern margin of the Qilian Block, which contains abundant enclaves and synplutonic dikes. The petrology, whole-rock geochemistry, Sr-Nd-Hf isotopes, zircon U-Pb ages, and mineral chemistry of the MMEs and their host granodiorites are discussed in this paper. By integrating these new results and prior geological data, our objective is to shed light on the relationship between the MMEs and host granitoids, as well as the possibility of efficient magma mixing during the crust–mantle interaction process during the subduction stage of the Qilian Proto-Tethys Ocean.

2. Geological Background

From north to south, the Qilian orogen can be partitioned into three distinct tectonic units: the North Qilian Belt, the Qilian Block, and the South Qilian Belt (Figure 1b [41,53,54,55]). The overall Qilian orogen records the complete tectonic evolution of oceanic spreading (the Proto-Tethys Ocean), oceanic plate subduction and accretion, and continent–continent (arc–continent) collision [56,57,58].

The North Qilian Belt stretches in a NW-SE direction for over 1000 km, lying between the Qilian Block to the southwest and the Alxa Block to the northeast (Figure 1b [34]), and is confirmed to be a classic subduction–accretion orogenic belt featuring a complex trench-arc–basin system [33]. The North Qilian Belt predominantly consists of early Paleozoic ophiolite suites, high-pressure/low-temperature (HP/LT) metamorphic rocks, granitoid plutons related to subduction and collision, and arc-related volcanic rocks, overlain by Silurian flysch, Devonian molasse, and Carboniferous–Triassic sedimentary cover sequences [59]. There are two ophiolite belts in the North Qilian Belt: Neoproterozoic–Cambrian ophiolite belts (550–496 Ma) of oceanic ridge origin in the south (including the Aoyougou, Yushigou, and Dongcaohe ophiolites) and Cambrian–Ordovician Ophiolite Belts (490–448 Ma) of back-arc oceanic basin origin in the north (including Jiugequan, Biandukou, and Laohushan ophiolites) [33,54,60]. The widespread granitoids with zircon U-Pb ages ranging from 520 Ma to 440 Ma are predominantly found as elongated bodies within island-arc volcanic rocks in the North Qilian Belt [33,35,39,40,41,46,61]. These granitoids are distinguished by a peraluminous granitic batholith, as well as numerous medium/high-K calc-alkaline I-type diorite-granodiorite-granite intrusions, along with a small number of minor tonalite-trondhjemite associations [35,40,62,63,64,65].

The Qilian Block consists primarily of a substantial Precambrian basement sequence, a thin Phanerozoic sedimentary overlying layer, and early Paleozoic igneous rocks [33,66,67,68,69,70,71]. A few studies have suggested that the lower Precambrian crystallized basement was partitioned into the western Tuolai Group and the Beidahe Group [72,73], the Huangyuan Group in the center [69], and the eastern Maxianshan Group [71,74], and recorded metamorphism from greenschist facies to granulite facies [75,76,77]. Voluminous Early Paleozoic basic to felsic magmatic rocks related to subduction and collision intrude into the Precambrian basement, and they are distinguished by calc-alkaline to high-K calcalkaline and shoshonitic [37,38,51,66,78,79,80,81,82,83].

An Early Paleozoic continental arc, characterized by arc magmatic rocks and associated high-temperature metamorphic rocks, has been discovered in the northern margin of the Qilian Block. The high-temperature metamorphic rocks, traditionally believed to be the part of the lower continental crust from the Qilian Block, consist mainly of granitic gneiss, metasedimentary rocks, and amphibolite (with mafic granulite occurring locally), which experienced different degrees of anatexis during the Cambrian–Ordovician period [38]. Our study area is located in the northern margin of the Qilian Block, with extensively exposed Early Paleozoic mafic to felsic plutonic intrusions, comprising gabbro, diorite, granodiorite, and granite with zircon U-Pb ages of ca. 500–446 Ma (Figure 1c). Geochemical and zircon Hf isotopic analyses show that the Early Paleozoic intrusive rocks have arc-related affinity [37,51]. The Menyuan Granodioritic Pluton, as a large and complex intrusion, provides a unique opportunity and new perspective for researching the petrogenesis and the crust–mantle interaction during the oceanic subduction process.

3. Petrology

A typical granitoid exposure with abundant mafic enclaves was identified in the Menyuan area approximately 100 km NE of Xi’ning City, adjacent to Menyuan County, Qinghai Province, China (Figure 1c and Figure 2a). The Menyuan Granodioritic Pluton in this study intruded into the Precambrian basement including Meso-Neoproterozoic gneiss, schist, and local amphibolite. The Menyuan Granodioritic Pluton is mainly composed of granodiorite intruded by a 1–3-m wide dike of gabbro (Figure 2b). The granodiorite contains a large number of mafic microgranular enclaves (MMEs) throughout the whole pluton (Figure 2c,d).

Granodiorites of the Menyuan Granodioritic Pluton show classic massive textures, and the dominant minerals are of medium- to coarse-grain size (Figure 3a,b). Petrographically, the thin sections reveal that the main minerals contain plagioclase (40–50 vol.%), alkali-feldspar (20–30 vol.%), quartz (10–20 vol.%), and amphibole (15 vol.%) with minor biotite and accessory minerals (e.g., zircon, apatite, sphene, and titanite) (total 5 vol.%). The secondary minerals mainly include sericite, epidote, and opaque phases. Plagioclase typically presents as euhedral to subhedral in shape, displaying clear polysynthetic twinning partly altered to sericite. Almost all alkali feldspars are turbid with the characteristics of plagioclase alteration. These alkali feldspars are microcline featuring typical tartan twinning, and within the alkali feldspar crystals, fine-grained quartz grains can be observed (Figure 3a,b). The amphibole occurs as subhedral tabular crystals and displays pleochroism. Some amphiboles are replaced by alkali feldspar and biotite. The biotite is mainly associated with amphiboles and occurs as subhedral morphologies. The MMEs are widespread in the Menyuan granodiorites and have irregular and rounded morphologies, ranging in size from ten centimeters to thirty centimeters in length, with a chilled margin; they are distributed irregularly in the host rocks (Figure 2c,d). Thin sections indicate that the MMEs have a clear boundary with the host granodiorites (Figure 3c). The MMEs are fine-to medium-fine-grained and dark-colored and show typical magmatic textures (Figure 3d–g). They mainly consist of subhedral plagioclase (~60 vol.%), amphibole (~30 vol.%), minor biotite (~5 vol.%), quartz (~3 vol.%), along with accessory minerals, such as apatite and zircon. The MME contains more mafic minerals than the host granodiorites. In addition, the characteristic microtextures observed within the MMEs encompass euhedral-subhedral plagioclase phenocryst, hornblende featuring zonal and poikilitic structures, skeletal biotite, acicular apatite, and slender hornblende (Figure 3d–g). The gabbro dike intruding into the host granodiorite displays a dark gray color, featuring a medium- to medium-fine-grained texture. It mainly consists of plagioclase (50–60 vol.%), amphibole (30–35 vol.%), and pyroxene (5–10 vol.%), with trace amounts of accessory minerals such as Fe–Ti oxides (Figure 3h).

4. Analytical Methods

In total, more than 30 representative samples were employed for zircon U-Pb dating, thin-section analyses, as well as zircon Hf and whole-rock Sr-Nd isotopes analyses. The detailed analytical methods are described in Appendix A.

5. Results

5.1. Mineral Composition

For the electron probe microanalysis, representative zonal plagioclase and amphibole grains were chosen from both the host granodiorites and MMEs from the Menyuan Granodioritic Pluton. All analyses were conducted on fresh mineral domains to ascertain their compositions. The results are presented in Table S1 and Table S2, respectively.

5.1.1. Plagioclase

Plagioclase grains in the host granodiorite mainly plot in the bytownite and andesine fields in the Ab-Or-An diagram (Figure 4a). Some of the plagioclases in the host granodiorite recorded reversed compositional zonation with variation in the anorthite (An) content from the core (An₄₂) to the rim (An₇₄) (Figure 4b). In addition, plagioclase phenocryst in the MMEs mainly plot in the oligoclase and andesine fields in the Ab-Or-An diagram (Figure 4a). The anorthite (An) content of plagioclase in the MMEs is relatively lower compared to that in the host granodiorite. The plagioclase in the MMEs exhibits complex oscillatory zoning structure from core to rim, and the range of anorthite (An) content varies from 24 to 44 (Figure 4c).

5.1.2. Amphibole

Based on the classification proposed by Leake et al. (1997) [85] all amphiboles from the tested samples are calcic hornblende (Ca_B ≥ 1.5) and can be divided into two groups: (Na + K)_A ≥ 0.5 and <0.5, respectively. Most amphibole analyses fall within the group of (Na + K)_A < 0.5, located in the magnesiohornblende field (Figure 5a); the remaining amphiboles analysis spots classified under the group of (Na + K)_A ≥ 0.5 mainly lie in the edenite field (Figure 5b). Based on the Ca-(Fe²⁺ + Fe³⁺)-Mg ternary diagram, the amphiboles in both the host granodiorite and the MMEs are plotted into the region of crust-mantle mixed-source, implying that the amphiboles may be the products of magmatic mixing(Figure 5c; [86]).

We applied the Al-in-amphibole barometry from Mutch et al. (2016) [87] and the Ti-in-amphibole thermometer from Liao et al. (2021) [88] to estimate the crystallization pressures and temperatures of the host granodiorites. The calculated pressures and temperatures for the host granodiorites range from 1.99 to 3.09 kbar and 714 to 774 °C, respectively. In addition, the Amp-Pl barometry from Molina et al. (2015) [89] and the Ti-in-amphibole thermometer from Liao et al. (2021) [88] were used to calculate the crystallization conditions of the MMEs, yielding pressures of 2.01–3.69 kbar and temperatures of 723–795 °C. Based on the continental crustal pressure gradient of 3.7 km per 1 kbar [90], the Menyuan Granodioritic Pluton might have been emplaced at an intermediate-crustal or shallower depth (7.4–13.7 km).

Figure 5. (a,b) Amphibole classification diagram [91] for the host granodiorite and MME. (c) The Ca-(Fe²⁺ + Fe³⁺)-Mg diagram of amphibole for discriminating the magma source [92].

Figure 5. (a,b) Amphibole classification diagram [91] for the host granodiorite and MME. (c) The Ca-(Fe²⁺ + Fe³⁺)-Mg diagram of amphibole for discriminating the magma source [92].

5.2. Zircon U-Pb Geochronology

The zircon cathodoluminescence (CL) images and Concordia diagrams of three representative samples are listed and presented in Table S3 and Figure 6. The majority of the studied zircons exhibit subhedral to euhedral prismatic, and most of them measure 100 μm to 200 μm in length and 50 μm to 100 μm in width. All the analyzed zircons exhibit clear oscillatory zoning with high Th/U ratios ranging from 0.22 to 0.9, which suggest a magmatic origin [93].

Twenty zircon spot analyses from the gabbro dike 15MY5-02 were obtained. The ²⁰⁶Pb/²³⁸U ages range from 520 Ma to 443 Ma, yielding a weighted average of 456.8 ± 6.3 Ma (MSWD = 2.9; Figure 6a), which can be interpreted as the emplacement age of the gabbro dike. The Th/U ratio of zircons varied between 0.57 and 0.90.

The ²⁰⁶Pb/²³⁸U ages given by twenty-seven analytical spots on the zircons from the MME 14MY3-06 range from 464 Ma to 448 Ma, with a weighted mean age of 456.4 ± 3.2 Ma (MSWD = 0.23; Figure 6b), which represents the crystallization age of the mafic microgranular enclaves in the Menyuan Pluton. The Th/U ratios of all zircons are greater than 0.4 (except for one value of 0.22), ranging from 0.22 to 0.79.

A total of twenty-five dating spots were performed on twenty-five zircons from the host granodiorite 16MY7-08. The ²⁰⁶Pb/²³⁸U ages given by 25 analytical spots on the zircons range from 470 Ma to 443 Ma, with a weighted mean age of 456.6 ± 9 Ma (MSWD = 0.77; Figure 6c), which indicates the emplacement age of the Menyuan host granodiorite. All the measured zircons showed Th/U ratios that varied between 0.36 and 0.83.

5.3. Whole-Rock Geochemistry

Field investigations and thin-section microstructures reveal that the studied samples are essentially fresh with minimal chemical alterations. The whole-rock major and trace element data are presented in Table S4.

The chemical classification reveals a broad lithological variety for the host rocks, enclaves, and gabbro dikes (Figure 7a), from relatively mafic to felsic composition (gabbro-diorite to granite). The host rocks have relatively high and variable SiO₂ contents (60.03–73.34 wt.%), plotting within the diorite, granodiorite, and granite fields, exhibiting sub-alkaline features in the TAS diagram (Figure 7a). The host rocks have high contents of Al₂O₃ (12.68–17.44 wt.%) and K₂O (1.94–3.58 wt.%) and low contents of TiO₂ (0.08–0.73 wt.%), FeO^T (0.73–5.82 wt.%), MgO (0.26–2.64 wt.%), and CaO (2.07–3.47 wt.%) (with corresponding Mg^# values of 35–59). The total alkali content (Na₂O + K₂O) ranges from 5.94 wt.% to 7.45 wt.%, with Na₂O/K₂O ratios of 1.02–2.36. The magma was calc-alkaline to high-K calc-alkaline series (Figure 7b,c) and metaluminous to weakly peraluminous with A/CNK ratios of 0.80–1.15 (Figure 7d).

In contrast, the MMEs are characterized by lower SiO₂ (49.77–54.26 wt.%), TiO₂ (0.59–0.94 wt.%), and Al₂O₃ (13.47–15.69 wt.%), but higher MgO (6.93–8.62 wt.%), FeO^T (6.68–9.12 wt.%), CaO (6.80–10.24 wt.%) (with corresponding Mg^# values of 58–68), and total alkali (Na₂O + K₂O = 4.18–6.82 wt.%) contents with variable Na₂O/K₂O ratios of 0.64–2.95. The magma was calc-alkaline to shoshonitic (Figure 7b,c) and metaluminous with A/CNK ratios of 0.56–0.70 (Figure 7d). The gabbro dikes plot within the monzogabbro and monzodiorite fields, and most of the samples belong to alkaline series in the TAS diagram (Figure 7a). In addition, the gabbro dikes are also characterized by low SiO₂ (48.05–54.15 wt.%) and high Al₂O₃ (16.23–18.16 wt.%), FeO^T (7.45–10.84 wt.%), MgO (3.52–5.20 wt.%), and CaO (7.00–9.03 wt.%) (with corresponding Mg^# values of 44–49), combined with low TiO₂ (0.86–1.38 wt.%) contents. The total alkali content (Na₂O + K₂O) ranges from 5.26 wt.% to 6.39 wt.%, with Na₂O/K₂O ratios of 1.04–2.51. The magma was high-K calc-alkaline to shoshonite series (Figure 7b,c) and metaluminous with A/CNK ratios of 0.71–0.82 (Figure 7d).

These studied samples all exhibit similar chondrite-normalized REE distribution patterns, characterized by an enrichment of light rare earth elements (LREEs) in contrast to heavy rare earth elements (HREEs), with no significant Eu anomalies (Figure 8a–c). The primitive mantle-normalized, multi-element diagrams indicate comparable chemical features with enrichment in Ba, Th, Nd, U, and Pb, and depletion in Nb, Ta, Zr, Hf, P, and Ti (Figure 8d–f).

Figure 7. Geochemical discrimination diagrams for the magmatic rocks from the Menyuan Granodioritic Pluton. (a) Total alkalis versus silica diagram [94]; (b) triangle plot of ^TotalFeO versus (Na₂O + K₂O wt.%) versus MgO (wt.%) (after [95]); (c) SiO₂ (wt.%) versus K₂O (wt.%) diagram (after [96]); (d) molar Al₂O₃/(CaO + Na₂O + K₂O) versus molar Al₂O₃/(Na₂O + K₂O) (after [97]). The data for the subduction-related granitoids along the southern margin of the North Qilian Orogenic Belt (NQOB) are from [38,40,51,64].

Figure 7. Geochemical discrimination diagrams for the magmatic rocks from the Menyuan Granodioritic Pluton. (a) Total alkalis versus silica diagram [94]; (b) triangle plot of ^TotalFeO versus (Na₂O + K₂O wt.%) versus MgO (wt.%) (after [95]); (c) SiO₂ (wt.%) versus K₂O (wt.%) diagram (after [96]); (d) molar Al₂O₃/(CaO + Na₂O + K₂O) versus molar Al₂O₃/(Na₂O + K₂O) (after [97]). The data for the subduction-related granitoids along the southern margin of the North Qilian Orogenic Belt (NQOB) are from [38,40,51,64].

Figure 8. Chondrite-normalized REE distribution pattern diagrams (a–c) and primitive mantle-normalized trace element distribution pattern diagrams (d–f) for the magmatic rocks from the Menyuan Granodioritic Pluton. The chondrite normalization values are after [98], and primitive mantle normalization values are after [99]. The compositions for the subduction-related granitoids along the southern margin of the North Qilian Orogenic Belt (NQOB) are from [38,40,51,64].

Figure 8. Chondrite-normalized REE distribution pattern diagrams (a–c) and primitive mantle-normalized trace element distribution pattern diagrams (d–f) for the magmatic rocks from the Menyuan Granodioritic Pluton. The chondrite normalization values are after [98], and primitive mantle normalization values are after [99]. The compositions for the subduction-related granitoids along the southern margin of the North Qilian Orogenic Belt (NQOB) are from [38,40,51,64].

5.4. Zircon Hf and Whole-Rock Sr-Nd Isotopes

Zircon Hf and whole-rock Sr-Nd isotopic compositions for the studied samples are presented in Tables S5 and S6. The Lu-Hf isotopic analysis points are located in the same zircon grains that were used for the U-Pb dating. The initial ¹⁷⁶Hf/¹⁷⁷Hf ratios and ε_Hf(t) values of the zircons were calculated based on their crystallization ages. The whole-rock initial Sr-Nd isotopic compositions and ε_Nd(t) values were determined using a weighted mean age of 456 Ma.

The host granodiorite shows negative ε_Hf(t) values of −8.8 to −1.2 (average −4, 24 zircons; Figure 9a) with corresponding two-stage Hf model ages of 1764–1356 Ma. Zircon grains from the MME show variable ε_Hf(t) values of −0.1 to +4.1 (average +2.5, 27 zircons; Figure 9a) with corresponding depleted mantle model ages of 1075–905 Ma. The gabbro samples have positive ε_Hf(t) values of +4.0 to +8.2 (average +6.13, 20 zircons; Figure 9a) with corresponding depleted mantle model ages of 906–743 Ma.

The host granodiorites have variable initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7072–0.7158 and ε_Nd(t) values of −9.21 to −4.23 (Figure 9b,c), with two-stage Nd model ages (T_DM2) of 1939–1535 Ma, which are closed to the Nd model ages of the Precambrian basement metamorphic rocks of the Qilian Block (ca. 1.8–2.6 Ga, [100,101]). In comparison, the MMEs have slightly lower initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7078–0.7089 and higher ε_Nd(t) values of −3.88 to −1.68 (Figure 9b,c), with corresponding mantle-depleted Nd model ages (T_DM1) of 1711–1306 Ma. The gabbro samples have homogeneous and depleted initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7049–0.7054 and ε_Nd(t) values of +1.23 to +2.56 with T_DM1 model ages of 1187–903 Ma (Figure 9b,c), and plot in the field of the subcontinental lithospheric mantle in the North Qilian Belt.

Figure 9. Plots of (a) ε_Hf(t) and (b) ε_Nd(t) values versus ages and (c) (⁸⁷Sr/⁸⁶Sr)_i versus ε_Nd(t) diagram for samples from the NQOB. In (c), a binary mixing curve between the depleted mantle-derived Paleozoic basalt (Sample LH57, Sr = 201, (⁸⁷Sr/⁸⁶Sr)_i = 0.7038, Nd = 12.2, ε_Nd(t) = 6.57; [102]) and amphibolite within the Precambrian Qilian basement (Average: Sr = 250, (⁸⁷Sr/⁸⁶Sr)_i = 0.7187, Nd = 46.5, ε_Nd(t) = −10.01; [103]) are also shown. Blue dots on the binary mixing curve represent the mixing ratios. Data sources: Menyuan magmatic rocks [38]; the Precambrian basement rocks in the Qilian Block [101,103,104,105]; enriched mantle-derived mafic rocks in the NQOB [106,107,108]; depleted mantle-derived ophiolites and adakites in the NQOB [102,109,110,111]; coeval arc-type mafic rocks derived from metasomatized asthenospheric mantle in the NQOB [61]; thickened lower crust-derived adakitic rocks and old crust-derived granites in the NQOB [43,46,81,101,112].

Figure 9. Plots of (a) ε_Hf(t) and (b) ε_Nd(t) values versus ages and (c) (⁸⁷Sr/⁸⁶Sr)_i versus ε_Nd(t) diagram for samples from the NQOB. In (c), a binary mixing curve between the depleted mantle-derived Paleozoic basalt (Sample LH57, Sr = 201, (⁸⁷Sr/⁸⁶Sr)_i = 0.7038, Nd = 12.2, ε_Nd(t) = 6.57; [102]) and amphibolite within the Precambrian Qilian basement (Average: Sr = 250, (⁸⁷Sr/⁸⁶Sr)_i = 0.7187, Nd = 46.5, ε_Nd(t) = −10.01; [103]) are also shown. Blue dots on the binary mixing curve represent the mixing ratios. Data sources: Menyuan magmatic rocks [38]; the Precambrian basement rocks in the Qilian Block [101,103,104,105]; enriched mantle-derived mafic rocks in the NQOB [106,107,108]; depleted mantle-derived ophiolites and adakites in the NQOB [102,109,110,111]; coeval arc-type mafic rocks derived from metasomatized asthenospheric mantle in the NQOB [61]; thickened lower crust-derived adakitic rocks and old crust-derived granites in the NQOB [43,46,81,101,112].

6. Discussion

6.1. Petrogenesis and Source Nature of the Menyuan Granodioritic Pluton

6.1.1. Petrogenesis of the Menyuan Granodioritic Pluton

The principal rock-forming minerals of the Menyuan Granodioritic Pluton are plagioclase and hornblende, lacking typical peraluminous minerals like muscovite, garnet, and cordierite. The host granodiorites are metaluminous to weakly peraluminous with A/CNK ratios ranging from 0.80 to 1.16 (with an average of 0.85), low K₂O/Na₂O ratios (0.42–0.98), and low Zr saturation temperatures (with an average of 746 °C, see Table S4), indicating affinity with I-type granites [113]. Furthermore, the studied samples of the Menyuan Granodioritic Pluton have a significant difference from those of sanukitoid magma featuring depleted CaO and enriched MgO elements [114] (Figure S1). These rock samples show arc-type magmatic features with enrichment of large ion lithophile elements (LILEs, e.g., Rb and Pb) and LREEs but depletion of HFSEs (e.g., Nb, Ta, Zr, Hf and Ti). Integrated with the geochemical characteristics of arc magma and low (Na₂O + K₂O)/CaO and TFeO/MgO ratios (Figure 10), the Menyuan granodiorites are classified as unfractionated calc-alkaline I-type granites. The fractional crystallization from mantle-derived basaltic magma, partial melting of a single heterogeneous source, such as pure crust or lithospheric mantle, as well as magma mixing between mantle-derived mafic melts and crust-derived felsic melts are the three primary mechanisms for the petrogenesis of calc-alkaline I-type granites [115,116,117].

Before discussing the petrogenesis and characteristics of the source nature, it is essential to consider the potential of crustal contamination during the ascent and emplacement of magma [119,120]. Crustal contamination is expected to result in positive Zr-Hf anomalies and positive correlation between (⁸⁷Sr/⁸⁶Sr)_i ratios and SiO₂ contents or negative correlation between ε_Nd(t) values and SiO₂ contents [121]. However, our data exhibited significant negative Zr-Hf anomalies (Figure 8), with no obvious correlation between SiO₂ contents and both (⁸⁷Sr/⁸⁶Sr)_i ratios and ε_Nd(t) values for the MMEs and gabbro dikes (Figure 11a,b), and the ε_Nd(t) values of host granodiorites do not decrease significantly with the increase of SiO₂ contents (Figure 11b). Meanwhile, ancient inherited zircon grains are not present in the Menyuan-dated samples, indicating that contamination by ancient crustal materials is insignificant or negligible for the Menyuan Granodioritic Pluton. Thus, the geochemical characteristics of the Menyuan Granodioritic Pluton can effectively reflect their petrogenesis and magma sources.

Several genetic mechanisms have accounted for the formation of MMEs in the granitoids and their connection to the host granites: (1) xenolith: fragments from wall rocks that are incorporated during the ascent of the host magmas (e.g., [120,122]); (2) refractory remnants from the magma source region (e.g., [123,124]); (3) autoliths associated with early-formed cumulate minerals originating from a cogenetic magma (e.g., [20]); and (4) hybrid magma blobs resulting from mafic and felsic magma mixing (e.g., [7,125]). The question widely applied to their formation are whether they are of co-genetic origin or that of heterogeneous magma mixing [21,25,31,126]. The MMEs hosted in the Menyuan Granodioritic Pluton are ellipsoidal or irregular in shape with clear edges and they display apparent magmatic textures, and the MMEs are distributed throughout the Menyuan Granodioritic Pluton (Figure 2c,d). The fact that the MMEs and the host granodiorites have indistinguishable crystallization ages and a similar mineral assemblage eliminates the possibility that the MMEs are restites or xenoliths from magma sources or country rocks.

The Menyuan host granodiorites have variable SiO₂ (60.03–73.34 wt.%), high total FeO (0.73–5.82 wt.%) and MgO (0.26–2.64 wt.%) contents, with relatively high Mg^# (35.28–58.66, mean value = 43.76) values, which belong to calc-alkaline and high-K calc-alkaline rocks, and have metaluminous-to-weakly-peraluminous features (Figure 7d). Compared to the host granodiorites, MMEs have relatively lower SiO₂ (49.77–54.26 wt.%) and higher MgO (6.93–8.62 wt.%) contents and Mg^# (57.73–67.54) concentrations, reflecting a significant mantle contribution [121,127]. The Nb/Ta ratios of our studied samples are well above the average range of the continental crust and are inconsistent with typical crustal origin. Experimental studies have revealed that melts that originated from the partial melting of pure lower crust with no mantle material input generally correspond to low Mg^# melts (<42), but high Mg^# melts (>60) are derived from mantle-derived basic rocks [128,129]. Moreover, the host granodiorites have significantly enriched Sr-Nd isotope compositions with initial ⁸⁷Sr/⁸⁶Sr ratios (0.7072–0.7158) and whole-rock ε_Nd(t) values (−9.21 to −4.23), which are obviously distinct from the lithospheric mantle-derived melts in the North Qilian Belt (Figure 9c). Therefore, they are not controlled by partial melting of the mafic lower crust or lithospheric mantle.

Previous studies have shown that the mafic minerals crystallized at an early stage in the magma chamber are characterized by coarse-grained and cumulate textures [130]. However, the predominantly rock-forming minerals of the MMEs are obvious smaller compared to their host granites, and the MMEs are randomly distributed without mafic minerals’ cumulate texture. Likewise, given that the REEs are strongly incompatible elements, if the MMEs and host granites have a cognate origin and host granodiorites are derived from fractional crystallization of the basaltic magma, the contents of REEs of the host granodiorites were higher than the MMEs (e.g., [96]), but the MMEs exhibit REE contents comparable to those of the host granodiorites (Figure 8a). In addition, the fractional crystallization of the amphibole, biotite, or plagioclase was not detected in the diagrams of fractionation trends for the Menyuan Granodioritic Pluton (Figure 11c–f). The fractionation crystallization from mantle-derived basaltic magma demands contemporaneous and larger amounts of mafic rocks than granites; however, the Menyuan granodiorites cropped out as large-scale plutons and gabbro dikes have an intrusive contact with the granodiorite. These observations are inconsistent with cognate or autolith origin whereby the MMEs were produced during early-formed phases within the same magma chamber as the host granodiorites. Combined with a clear whole-rock Sr-Nd and zircon Hf isotopic contrast between the host granodiorites and MMEs (Figure 9), we suggest that they are unlikely to have been formed by fractional crystallization of a single heterogeneous source. The wide ranges of major element contents and trace elemental ratios of the host granites and MMEs reflect contrasting mantle and crustal sources. Thus, the heterogeneous magma mixing from different magma sources is the most possible petrogenesis for the Menyuan Granodioritic Pluton.

There are several pieces of evidence suggesting that the petrogenesis of the Menyuan Granodioritic Pluton may be related to incomplete magmatic mixing. The MMEs are irregular ellipsoidal in shape, and have sharp and occasionally diffusive contact with the Menyuan granodiorites, which indicate that parental mafic magma was still partially ductile while being injected into the host granitic magma and during their mixing [1,131]. Plagioclase xenocrysts captured from the host granodiorite and surrounded by fine-grained mafic minerals can be identified in MME (Figure 3e). The megacrysts of plagioclase in MMEs can be considered as the consequence of mechanical transfer occurring between felsic and mafic magmas in the course of the magmatic mingling process [115,132]. Felsic back-veins with a mineral assemblage resembling that of the host granodiorites are detected in the MMEs (Figure 2d), and they are caused by the formation of fractures due to magma-quenching crystallization when hotter mafic magma was injected into relatively cooler felsic host magma [7,116]. In addition, disequilibrium textures are common in the MMEs and host granodiorites, which are considered to be evidence of mechanical stirring of minerals between felsic and mafic magmas during magma mingling. Mostly, the plagioclase grains are rounded and embayed with dissolution textures in the MMEs, which indicates that the plagioclases crystallized in the felsic magma chamber and experienced remelting in relatively high-temperature mafic magma after mechanical transfer. The occurrences of skeletal biotite and sieve-textured hornblende in the MMEs and some euhedral-subhedral plagioclase grains rimmed with fine-grained mafic minerals in the host granodiorites probably reflect that partial dissolution and overgrowth of principal rock-forming minerals have occurred in the hybrid magma [117]. The occurrence of acicular apatite and slender hornblende (Figure 3g) can be considered to be a result of undercooling and rapid crystallization under water saturation conditions when the hotter basic magma was injected into relatively cool felsic magma [131,133]. Furthermore, plagioclase in the host granodiorites exhibit reversed compositional zonation with the rapid increase in anorthite (An) content (Figure 4b), indicating the chemical and thermal changes during plagioclase crystallization, which may be related to magmatic mixing [134,135,136]. Plagioclase in MMEs exhibits complex oscillatory zoning structure with oscillating orthite (An) content (Figure 4c). This may be attributed to the injection of high-temperature basaltic magma that caused the dissolution of the previous low-An plagioclase in the shallow magma chamber, leading to the formation of sieve textures [137]. Subsequently, the sieve-textured plagioclase continued to grow in the hybrid magma, and during the magma mixing process, these plagioclase macrocrysts were successively captured by the MMEs. These features suggest that the Menyuan Granodioritic Pluton was most likely derived from a mixed source of mantle and crustal origin, while the MMEs represent mafic-hybrid magma globules scattered throughout the host granodiorites by convection.

The elemental geochemical and isotopic characteristics of the whole rocks further suggest that the magma mixing was insufficient during the formation of the Menyuan Granodioritic Pluton. The MgO versus TFeO distribution shown on the whole-rock discrimination diagram conforms to the magma mixing trend (Figure 12a). In the Al₂O₃/CaO versus Na₂O/CaO and CaO/Al₂O₃ versus K₂O/MgO diagrams, there exist clear linear and curved correlations which are also in accordance with the magma mixing trend (Figure 12b,c). The behavioral differences between compatible and incompatible elements are conducive to the identification of magmatic mixing [138,139]. The Rb/V versus 1/V diagram plots along the mixing arrays of the melts, which can indicate magma mixing (Figure 12d). Additionally, similar normalized elemental patterns (Figure 8) for the studied samples of the Menyuan Granodioritic Pluton reflect that elemental diffusion and re-equilibration of trace elements that occurred during the magma mixing process. Zircon Hf isotopes characterized by high closure temperatures, high stability, and early crystallization in magmatic systems can effectively resist later isotopic exchange and re-equilibration between the interacting magmas [140]. As illustrated in Figure 9a, diverse Hf isotopic compositions recorded the magma source and mixing process. For the Sr-Nd isotopic system, Sr exhibits much higher mobility than Nd in both fluids and melts [141,142], which results in comparable initial ⁸⁷Sr/⁸⁶Sr ratios (ranging from to 0.7072 to 0.7101) but somewhat distinct ε_Nd(t) values for the Menyuan MMEs and host granodiorites during the magma mixing process (Figure 9c). Nevertheless, the wide range of whole-rock Sr-Nd and zircon Hf isotopic compositions observed in the MMEs and host granodiorites (Figure 9a–c) indicates restricted chemical exchange between the two magma end-members from distinct sources during magma mixing, and isotopic homogenization was not achieved prior to the complete solidification of the hybrid magma [140]. Consequently, we deduce that the most likely explanation for the formation of the Menyuan Granodioritic Pluton involves the incomplete mixing of magma derived from mantle and crustal sources, and the MMEs are the relics of local heterogeneity within the mixed melt serving as direct evidence of magma mixing. Effective magma mixing requires the onset of local superheating of the felsic magma [143]. The occurrence of contemporaneous Menyuan gabbroic dikes indicates a regional mafic magma injection event, and the mafic mantle-derived magma underplating provides adequate energy for the partial melting of the lower crust to generate crustal melts.

6.1.2. Source Nature of the Menyuan Granodioritic Pluton

The key to clarifying the crust–mantle magma mixing process lies in revealing the components of crust–mantle magmatic end-members. The host granodiorites, MMEs, and gabbro dikes in this study share similar diagenetic ages, normalized elements distribution patterns, as well as spatiotemporal correlation, suggesting that they may have a genetic connection. Notably, similar geochemical and isotopic compositions between the MMEs and coeval gabbro dikes indicate that they probably originated from a similar mantle source. The finding of magma mixing origin illustrates that the geochemical characteristics of the MMEs and coeval gabbro dikes cannot accurately represent the original compositions of the magma derived from the mantle. However, we can deduce some crucial information about the mantle source from the geochemical and isotopic features of these studied rocks. The gabbro dikes are characterized by much more positive and homogeneous ε_Nd(t) (+1.23 to +2.56) and ε_Hf(t) (+4.0 to +8.2) values compared to other Menyuan samples in this study with depleted mantle Hf model ages of 906–743 Ma. Additionally, the MMEs have moderately enriched ε_Nd(t) (−3.88 to −1.68) and ε_Hf(t) (−0.1~+4.1) values with depleted mantle Hf model ages of 1075–905 Ma, which indicate an early Neoproterozoic-enriched mantle source. Notably, the Nd isotope data of the MMEs and the gabbro dikes are lower than those of the coeval depleted mantle-derived basalts-basaltic andesites (ε_Nd(t) = +3.0 to +5.9) in the North Qilian Belt (Figure 9c [61]), but they overlap with those of enriched mantle-derived mafic rocks in the North Qilian Belt. The MMEs and the gabbro dikes in the Menyuan Granodioritic Pluton are distinctly enriched in LILEs (Ba, K, and Sr) and LREEs, and are depleted in HFSEs (Nb, Ta, Zr, Hf, and Ti; Figure 8e,f), which is consistent with arc-like trace elements distribution patterns. In the La/Yb versus Nb/La diagram, most samples of the MMEs and the gabbro dikes fall in the source region of the lithosphere mantle (Figure 13a). Combined with the relatively low La/Ba and high Th/Yb ratios (Figure 13b,c), these geochemical characteristics indicate that the mafic magma was produced by partial melting of a subduction-modified continental lithospheric mantle source [145,146,147]. Combined with the Nb/Y versus Ba, Nb/Y versus Rb/Y, and Sr/Nd versus Th/Yb diagrams (Figure 13d–f), these mafic samples display a fluid-related enrichment trend, indicating that the lithospheric mantle source was probably metasomatized by subducted slab-derived fluids prior to melting [148]. Moreover, the presence of hornblende in the studied samples from the Menyuan Granodioritic Pluton also reflects the participation of fluids. Thus, we suggest that the mafic magma end-member in this study was derived from partial melting of the enriched lithospheric mantle metasomatized by slab-derived fluids in the North Qilian Belt.

The Menyuan host granodiorites are K-rich, calc-alkaline, and high-K calc-alkaline rocks in geochemistry and they have negative ε_Hf(t) values of −8.8 to −1.2, corresponding with two-stage Hf model ages of 1764–1356 Ma, which indicates that they might be predominantly associated with the recycling of Paleoproterozoic-to-early-Mesoproterozoic crustal materials within the North Qilian Orogenic Belt. Their relatively low ε_Nd(t) values range from −9.21 to −4.23, with two-stage Nd model ages of 1939–1535 Ma, which are close to the isotopic composition range of the meta-basaltic rocks from the Qilian basement, but deviate from data related to the paragneiss and Neoproterozoic granitoids from the Precambrian basement (Figure 9c). It can be inferred that the crustal-derived source of the Menyuan Granodioritic Pluton was produced by the partial melting of meta-basaltic rocks from the Qilian basement. Abundant meta-basaltic rocks have been identified in the Qilian Precambrian basement, and their mantle-depleted Nd model ages range from 1.8 to 2.6 Ga [100,101]. Considering all the mentioned facts, it is reasonable to confirm that the ancient mafic lower continental crustal rocks from the Qilian Block were the crustal source of the Menyuan Granodioritic Pluton, which is reinforced by the presence of amphibolite enclaves within the Menyuan Granodioritic Pluton [38]. In this scenario, we applied a two-component mixing model of a whole rock Sr-Nd isotope to roughly simulate the interaction between the ancient mafic lower continental crust and depleted mantle-derived melts. The simple binary mixing simulation indicates that the proportions of mantle components involved in the formation of the Menyuan Granodioritic Pluton varied from 10% to 30%, while the proportions of ancient crustal materials varied from 70% to 90% (Figure 9c).

As discussed above, the magma originating from the lithospheric mantle and the melts of the ancient mafic lower continental crust beneath the Qilian Block made an substantial contribution to the magmatic sources of the Middle-Late Ordovician Menyuan Granodioritic Pluton.

6.2. Geodynamic Implication for the Menyuan Granodioritic Pluton

The LA-ICP-MS zircon U-Pb dating data obtained from the granodiorites, MMEs, and gabbroic dikes suggest that the magma emplacement time for the Menyuan Granodioritic Pluton can be constrained to the Middle-Late Ordovician period (470–443 Ma). These ages are consistent with the previous magmatic ages (ranging from 520 to 440 Ma) associated with the subduction of the North Qilian Ocean in the northern margin of the Qilian Block and adjacent regions [35,36,37,38,40,51,55,151]. For example, the ca. 466 Ma felsic volcanic rocks in the Qingshuigou were attributed to an arc setting [152].The quartz diorites (ca. 476 Ma) from the Niuxinshan and granodiorites (ca. 463 Ma) from the Minyueyaogou were formed in an active continental margin setting [153]. The 503–450 Ma I-type granite and mafic rocks, 461–443 Ma arc volcanic rocks, and 508–450 Ma metamorphic rocks at Menyuan Lenglongling-Dabanshan in the northern margin of the Qilian Block were formed during the south-dipping subduction of the North Qilian Ocean in the Early Paleozoic [37,38,51,154,155]. In this regard, the Menyuan Granodioritic Pluton in this study was likely produced in an active continental arc setting related to the southward subduction of the North Qilian Oceanic slab. All studied samples from the Menyuan Granodioritic Pluton are calc-alkaline in composition and have similar normalized elemental patterns with enrichment in LILEs (e.g., Ba, Rb, and Pb) and depletion in HFSEs (e.g., Nb, Ta, and Ti; Figure 8e,f), typically characterizing subduction-related magmatism [145,156,157,158]. In addition, the MMEs and gabbro dike samples are mainly distributed in the island-arc basalt field in the Y/15-La/10-Nb/8 and Ti/100-Zr-Y*3 ternary diagrams (Figure 14a,b), and show affinities close to the continental arcs in the Nb/Yb versus Th/Yb diagram (Figure 13c), which support an active continental arc setting. Therefore, the U-Pb zircon ages from the Menyuan Granodioritic Pluton (ca. 456 Ma), in combination with the chemical characteristics described above, suggest subduction-related magmatism. The Menyuan Granodioritic Pluton can be regarded as continental arc rocks formed in an active continental arc setting, which was associated with the southward subduction of the North Qilian Ocean beneath the northern margin of the Qilian Block in the Middle-Late Ordovician period.

The triggering mechanism of Middle-Late Ordovician arc magmatism along the northern margin of the Qilian Block during the subduction-dominated stage constitutes a pivotal scientific question that needs to be discussed. The subducted slab rollback and breakoff can induce asthenospheric upwelling coupled with partial melting of the overriding lithospheric mantle. Then, the thermal perturbation migrates upward, initiating crustal anatexis within the continental crust, which establishes favorable conditions for extensive crust–mantle interaction [161]. The slab breakoff model is commonly invoked to explain syn- to post-collisional magmatism in orogenic systems, wherein detachment initiates after the complete consumption of the oceanic lithosphere, which seems inadequate to interpret the arc magmatism of the Menyuan Granodioritic Pluton [162,163]. Therefore, we consider that the formation of Menyuan Granodioritic Pluton in this study was most likely initiated by the slab rollback under the force of gravity during the Middle-Late Ordovician period.

Clarifying the petrogenetic affiliation of the Menyuan Granodioritic Pluton can provide important insights to the emplacement process of orogenic magmas and extensive crust–mantle interactions. The Menyuan Granodioritic Plutonic rocks were related to mixing between the lithospheric mantle-derived magma and ancient continental crustal melts in the active continental arc setting. During subduction, the subducted slab rollback caused partial melting of the overlying lithospheric mantle metasomatized by slab-derived fluids and the thermal upwelling of high-temperature, mantle-derived basaltic magmas. Meanwhile, the high-temperature basaltic magmatic underplating could provide sufficient energy for the anatexis of the ancient mafic continental lower crust beneath the Qilian Block to form primitive felsic melts. These crust-derived felsic melts subsequently experienced variable mixing with mantle-derived basaltic magma during ascent, ultimately emplacing as the Menyuan Granodioritic Pluton in the upper crust along the northern margin of the Qilian Block (Figure 15a,b). Consequently, our results advocate for a magmatic underplating mechanism in which the upwelling of high-temperature mantle-derived magma induced extensive crust–mantle interaction.

7. Conclusions

(1) The Menyuan Granodioritic Pluton primarily consists of enclave-bearing granodiorites with minor gabbroic dike. These studied rocks samples have a consistent LA-ICP-MS zircon U-Pb age of ca. 456 Ma, suggesting a Middle-Late Ordovician mafic to felsic magmatism within the North Qilian orogen;

(2) The host granodiorites and the MMEs exhibit similar crystallization ages, trace elemental geochemistry, and Sr isotopic compositions, yet display heterogeneous whole-rock Nd and zircon Hf isotopic characteristics. This can be interpreted as a result of insufficient chemical and isotopic equilibration during the magma mixing process;

(3) The Menyuan host granodiorites and the MMEs were likely derived from a hybrid magma, which was composed of components derived from the lithospheric mantle and ancient mafic lower continental crust. The felsic magma was generated from the anatexis of the meta-basalt from the Precambrian Qilian basement, and the mafic magma originated from the melting of the enriched lithospheric mantle that intruded into the lower continental crust through underplating. Then, the mafic magmas mixed with felsic melts to form arc magmatic rocks ranging from gabbroic to granodioritic in the shallow crust;

(4) The geochemical and isotopic characteristics of the gabbro dikes indicate that they originated from the lithospheric mantle metasomatized by slab-derived fluids. Their presence reflects the existence of evolved mafic magmas in the region that did not directly participate in mixing with the granodiorites and provides evidence for mantle-derived magmatism;

(5) Detailed field observations, petrographic, and geochemical data suggest that the Menyuan Granodioritic Pluton formed in an active continental-arc setting induced by the northward rollback of the Qilian Proto-Tethys Ocean in the Middle-Late Ordovician period.