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6 January 2024

Petrogenesis and Tectonic Evolution of Huashigou Granitoids in the South Qilian Orogen, NW China: Constraints from Geochronology, Geochemistry, and Sr–Nd–Hf–O Isotopes

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1
Gold Mine Resource Exploration and Utilization Technology Innovation Center of Gansu Province, Lanzhou 730050, China
2
The Third Institute of Geology and Minerals Exploration, Gansu Provincial Bureau of Geology and Minerals Exploration and Development, Lanzhou 730050, China
3
College of Geography and Tourism, Hunan University of Arts and Science, Changde 415000, China
4
Key Laboratory of Mineral Resources in Western China, School of Earth Sciences, Lanzhou University, Lanzhou 730000, China
Minerals2024, 14(1), 71;https://doi.org/10.3390/min14010071
This article belongs to the Special Issue Tectonic–Magmatic Evolution and Mineralization Effect in the Southern Central Asian Orogenic Belt
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Abstract

The origin of granitic rocks from the South Qilian orogenic belt is of great significance for understanding the continental tectonic framework of Western China. Currently, scholars have different opinions on the tectonic evolution of the South Qilian. Huashigou granite, which exhibits multiple intrusive episodes, is a suitable example for studying the tectonic evolution of the South Qilian. New zircon U–Pb ages and the whole-rock elemental and Sr–Nd–Hf–O isotopic compositions of Huashigou granitic rocks are presented here to investigate their petrogenesis and discuss the tectonic implications for the evolution of the South Qilian orogenic belt. LA-ICP-MS zircon U–Pb dating yielded crystallization ages of 368.7 ± 3.5 Ma, 261.5 ± 0.63 Ma, and 262.2 ± 1.4 Ma for granodiorites from the Hua1 pluton, quartz diorites from the Hua2 pluton, and porphyritic tonalites from the Hua4 pluton, respectively. Late Devonian granodiorites from the Hua1 pluton belonged to the metaluminous calc-alkaline series and were characterized by an enrichment in LREEs, a depletion in HREEs, negative Eu anomalies, and Sr/Y ratios of 9.17 to 11.67. They showed (87Sr/86Sr)i ratios of 0.712356 to 0.71195, εNd(t) values of −6.56 to −6.14, and an εHf(t) value of −2.06. Middle Permian granitic rocks mainly consisted of quartz diorites and porphyritic tonalites, which are part of the metaluminous tholeiitic series and weakly peraluminous tholeiitic series, respectively. Quartz diorites from the Hua2 pluton were characterized by an enrichment in LREEs, depletions in HREEs and HESEs, weak negative Eu anomalies, and Sr/Y ratios of 13.25 to 14.79. They showed (87Sr/86Sr)i ratios of 0.705905 to 0.705971, εNd(t) values of +0.78 to +0.82, and a δ18OV-SMOW value of 12.4‰. Porphyritic tonalites of the Hua4 pluton were characterized by an enrichment in LREEs, depletions in HREEs and HESEs, weak negative Eu anomalies, and Sr/Y ratios of 9.22 to 12.74. They showed (87Sr/86Sr)i ratios of 0.719528, εNd(t) values of −8.57, and a δ18OV-SMOW value of 11.8‰. We can conclude that Late Devonian granodiorites were derived from the partial melting of enriched and shallow-depth crustal materials, whereas Middle Permian granitic rocks were formed by the delamination of a thickened lower crust after the closure of the Paleo-Tethys Ocean, which caused the underplating of mantle-derived basaltic magma, inducing the partial melting of the lower crust at different depths. Our results show that there were at least two important stages of compressional and extensional tectonic switches in the South Qilian orogenic belt during the Late Paleozoic Era, and the evolution of Altyn Tagh–Qilian–North Qaidam had evident stages.

1. Introduction

The Qilian orogenic belt, located at the northeastern margin of the Tibetan Plateau, is located in the western part of the central orogenic belt that runs across China from east to west [1,2,3,4,5]. The Qaidam Block is located to the south of the belt. The Tarim Craton is located northwest of the belt, delineated by the Altyn Tagh Fault (Figure 1a). A majority of studies have been conducted to investigate the evolution of the Altyn Tagh–Qilian–North Qaidam tectonic system [6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27]. These studies focus on the discovery of an ultrahigh-pressure metamorphic belt (UHPB) in the North Qaidam and Altyn Tagh regions and its comparison to the Altyn Tagh–Qilian–North Qaidam tectonic system [3,28,29,30,31,32,33]. As a ubiquitous intrusive mass in the orogenic belt, granitoids have recorded much information about plate tectonics and crustal evolution [12,34,35,36,37,38,39,40]. Additionally, there are similarities between the tectonic units on both sides of Altyn Tagh.
Figure 1. Tectonic location (a) and geological sketch map (b) of the South Qilian Orogen. (a): 1. Archean–Paleoproterozoic basement; 2. Mesoproterozoic rock; 3. Neoproterozoic rock; 4. Early Paleozoic subduction collision complex belt; 5. Early Paleozoic subduction hyperplasia complex belt; 6. thrust fault; 7. strike-slip fault; and 8. study area. (b): 1. Quaternary; 2. Carboniferous Yanghugou Group; 3. Devonian–Carboniferous Amunike Group; 4. Paleoproterozoic Dakendaban Group (the Third Rock Group); 5. Paleoproterozoic Dakendaban Group (the Second Rock Group); 6. Paleoproterozoic Dakendaban Group (the First Rock Group); 7. Late Devonian granodiorites (Hua1); 8. Middle Permian granitoids (Hua3); 9. quartz mica diorites; 10. Middle Permian quartz mica porphyritic diorites (Hua4); 11. Middle Permian quartz diorites (Hua2); 12. Late Permian monzogranites (Hua5); 13. granodiorites; 14. geological boundary; 15. faults; 16. attitude; 17. sample locations; and 18. copper deposits.
Previous studies have mainly focused on Early Paleozoic granitic rocks in the North Qilian orogenic belt, while studies on Late Paleozoic granitoids are relatively scarce. Since the closure of the Qilian orogenic belt in the Mid–Late Paleozoic, there has been a paucity of pertinent research findings. Consequently, Late Paleozoic granitic rocks serve as a pivotal means to unveil not only their geodynamic implications but also the geological evolution of the Qilian orogenic belt.
In this study, we present results from petrographic, geochronologic, and whole-rock elemental and Sr–Nd–Hf–O isotopic composition analyses in an attempt to investigate the petrogenesis of Paleozoic Huashigou granitoids in the South Qilian along the northeastern margin of the Tibetan Plateau. These data could be used to infer and determine the age, genesis, and tectonic environment of the rocks formed during that period in the South Qilian. The data were used to evaluate the crustal evolution of the Qilian Orogen in the Late Paleozoic.

2. Regional Geology and Petrography

The Huashigou area is located at the southwest margin of the Qilian Orogen, which is at the junction of the Altyn Tagh–Qilian–North Qaidam tectonic belt. Paleoproterozoic (Dakendaban Group), Devonian (Amunike Formation), and Carboniferous (Keluke Formation) strata are widely distributed in the study area. The Paleoproterozoic Dakendaban Group represents a major stratigraphic formation developed in this area. The Devonian and Carboniferous strata are mainly distributed at the center of the study area. Faults extend in the north–south and north–west directions. The Huashigou area consists of five types of granitic rocks, including granodiorite, biotite granodiorite, porphyritic tonalite, quartz diorite, and monzonitic granite (Figure 1b).
Three granodiorite intrusions exist, with sizes ranging from 2 to 0.1 km2, which intruded into Paleoproterozoic metamorphic rocks (Dakendaban Group). The Hua1 pluton is composed of plagioclase (47%), quartz (20%), albite (18%), biotite (10%), hornblende (5%), and accessory minerals, including apatite, zircon, sphene, and magnetite. Plagioclases are wide and tabular and are intensively altered, with sericitization, decarbonation, and clayzation alterations. Some plagioclase grains wrapped in microcline have an obvious albite reaction rim. Quartzes are in a xenomorphic–granular form, and some of them show wavy extinction in rare situations.
The Hua2 pluton is mainly located south of the study area, with extensive exposure intruding in the Paleoproterozoic Dakendaban Group and Carboniferous Keluke Group, with a long strip layout and small areas in general. The pluton exhibits a subhedral–granular texture and a blocky structure and consists of plagioclase (70%), normal hornblende (15%), quartz (10%), and biotite (5%), with minor accessory minerals, including apatite, zircon, sphene, and magnetite. Plagioclases are mainly tabular and dotted with small amounts of secondary minerals, such as clay, microcryptocrystalline zoisite, sericite, and carbonates. Hornblende is intensively altered, with actinolitization along fractures.
The Hua3 pluton is located north of the study area, covering about 2 km2 in a nearly elliptical shape. With a fine-grained granite texture, the pluton mainly has a blocky structure and is composed of plagioclase (49%), orthoclase (15%), quartz (18%), biotite (15%), microcline (3%), and accessory minerals, including apatite, zircon, and magnetite. The tabular biotite is slightly wrapped in orthoclase, and the plagioclase has a visible polysynthetic twin and a mostly rapakivi structure. LA-ICP-MS zircon U–Pb dating indicated that the pluton was emplaced in the Middle Permian (261.1 ± 3.8 Ma [41]).
The irregular zonal Hua4 pluton is mainly distributed along the F1 Fault. The pluton shows typical porphyritic textures, and its phenocrysts include plagioclase and quartz. Its groundmass is composed of plagioclase, quartz, sericite, and biotite, with microgranular, cataclastic, and blocky structures. The plagioclase phenocrysts are in lenticular and elliptical shapes, and some parts have a wide tabular shape, with particle sizes ranging from 0.6 to 1.0 mm. Quartz phenocrysts have an elliptical shape, aggregating along the fractures with a sericite particle size of ±1.0 mm.
The Hua5 pluton intruded into the Devonian Amunike Group, with an exposed surface area of about 0.4 km2. The pluton displays a fine-grained granitic and blocky structure and consists of plagioclase (45%), quartz (23%), biotite (12%), perthite (12%), microcline (7%), and minor accessory minerals, such as apatite, sphene, zircon, and magnetite. LA-ICP-MS zircon U–Pb dating indicated that the pluton was formed in the Late Permian (252.0 ± 2.1 Ma [42]).

3. Analytical Methods

Sixteen samples were collected from the granitic plutons in the Huashigou area, including seven from the Hua1 pluton (YQ57–YQ63), four from the Hua2 pluton (YQ81–84), and four from the Hua4 pluton (YQ89–92). The locations of the analyzed samples are shown in Figure 1b.

3.1. Zircon U–Pb Age Analytical Method

Firstly, zircon grains were separated using conventional magnetic techniques and handpicked using a binocular microscope (BX51, Olympus, Tokyo, Japan). The zircon grains were mounted in an epoxy mount and polished down to a half-section. Prior to the isotope analysis, all grains were photographed under transmitted and reflected light and subsequently examined using cathode luminescence (CL) images (JSM-6510, JEOL, Tokyo, Japan). The zircon LA-ICP-MS U–Pb analyses were performed on a 7500 ICP-MS (Agilent, Santa Clara, CA, USA) with a UP193SS LA (New Wave, Seattle, WA, USA) at the Key Laboratory of Lithospheric Tectonics and Exploration of the Ministry of Education, China University of Geosciences (Beijing), China. The samples were analyzed in runs, with four GJ-1 zircon standards measured every ten to twelve samples. The analytical data were then reduced, calculated, and plotted using the Squid 1.0 and Isoplot programs (version 4.00.08.09.16, by KR Ludwig, Berkeley Geochr. Ctr., Berkeley, CA, USA) [43]. The compositions of common Pbs were corrected following the Andersen method [44]. Errors during the individual analyses are cited at 1σ, and the weighted age uncertainties are quoted at a confidence level of 95%.

3.2. Whole-Rock, Sr–Nd–O, and Hf Isotopic Analytical Method

The whole-rock major elements, trace elements, and Sr–Nd–Hf isotopes were determined at the State Key Laboratory of Continental Dynamics, Northwest University, China. The samples for the whole-rock chemical analysis were crushed to 200 mesh. The major-element contents of the samples were analyzed using the RIX-2100 (RIGAKU, Tokyo, Japan) X-ray fluorescence spectrometer (XRF). The analytic uncertainties were generally less than 2% for most of the major elements and 2% to 5% for the major elements with <1 wt.% contents. Trace elements and rare-earth elements (REEs) were analyzed using the Elan-6100DRC model ICP-MS (Perkin Elmer, Waltham, MA, USA). About 50 mg of the powder samples was dissolved in high-pressure Teflon bombs using a HF + HNO3 mixture. The analytical precision was better than 5%. The Nd, Sr, and Hf isotopic analyses were performed using MC-ICP-MS (Nu Instruments, North Wales, UK), and the isotopic ratios were, respectively, normalized to 143Nd/144Nd = 0.7219, 87Sr/86Sr = 8.3721, and 176Hf/177Hf = 0.7325. The oxygen-stable isotopes of the whole-rock samples were determined at the Institute of Mineral Resources, Chinese Academy of Geological Science (Beijing, China). The oxygen isotopic compositions (δ18O) were analyzed using the BrF5 method and measured using the MAT-253 mass spectrometer (MAT-253, Thermo Fisher Scientific, Waltham, MA, USA), according to international standards (SMOW), with a precision of ±0.2 ‰.

4. Results

Fifteen samples were collected for the major- and trace-element analyses, of which three samples were used for zircon U–Pb dating, six samples were used for Sr–Nd isotopic analyses, one sample was used for Hf isotopic analyses, and two samples were used for O isotopic analyses. The detailed data are presented in Table 1, Table 2 and Table 3.
Table 1. Major- and trace-element compositions of Huashigou granitoids.
Table 2. Sr–Nd–Hf–O isotopic compositions of Huashigou granites.
Table 3. LA-ICP-MS U–Pb zircon data of Huashigou granites.

4.1. Geochronology

In this study, three granitic plutons from the Huashigou area were chosen for zircon U–Pb dating. The zircons used were primarily colorless to transparent and euhedral to subeuhedral, with elongated to stubby grains (50–250 μm long with length-to-width ratios of 2:1–4:1). Typical magmatic oscillatory zonation was common, and several inherited cores were observed (see insets in Figure 2). The contents of Th (16–415 × 10−6) and U (40–440 × 10−6) were different among these zircons, and the Th/U ratios were consistent with a magmatic origin [45,46,47,48]. The detailed analytical results are listed in Table 3. The U–Pb concordia diagrams for Huashigou granitic rocks are shown in Figure 3.
Figure 2. Cathodoluminescence images of zircon grains of granitic rocks from the Huashigou area (ages in Ma).
Figure 3. Zircon concordia diagram of granitic rocks. The weighted mean 206Pb/238U ages for samples YQ-57, YQ-81, and YQ-89 are 368.7 ± 3.5 Ma (MSWD = 2.0; 2σ), 261.5 ± 0.63 Ma (MSWD = 0.102; 2σ), and 262.2 ± 1.4 Ma (MSWD = 1.2; 2σ), respectively.

4.2. Whole-Rock Geochemistry

4.2.1. Hua1 Pluton

Seven samples of granitic rocks (YQ57–YQ63) from the Hua1 pluton were analyzed for major- and trace-element compositions. Loss on ignition (LOI) for all samples was less than 1 wt.%. The granitic samples showed high contents of SiO2 (67.78–68.48 wt.%) and Al2O3 (15.26–15.77 wt.%) but low contents of MgO (1.19–1.38 wt.%) with low Mg# (about 38). Their contents of CaO and TiO2 were 3.39–3.80 wt.% and 0.40–0.48 wt.%, respectively. The Hua1 pluton samples showed total alkali contents ranging from 6.36 to 6.67 wt.% and a slight sodium enrichment (Na2O/K2O = 1.16–1.39). They belong to the calc-alkaline series in the SiO2-K2O diagram (Figure 4a). Their A/CNK ratios ranged from 0.97 to 0.99, indicating that the Hua1 granites are metaluminous (Figure 4b).
Figure 4. (a) Diagram of SiO2-K2O contents; (b) ANK-ACNK diagram of granitic rocks from the Huashigou area. A/NK = Al2O3/Na2O + K2O; A/CNK = Al2O3/CaO + Na2O + K2O.
The Late Devonian granodiorites showed low Sr contents (205.34–226.54 × 10−6) and high Y contents (19.41–23.82 × 10−6), with Sr/Y ratios of 9.17–11.67. The Nb/U and Nb/Ta ratios of the Late Devonian granodiorites were 7.77–10.21 and 12.42–14.39, respectively, which are comparable to those of the Earth’s crust, i.e., 9–12 [49,50,51,52] and 12–13 [53]. The Late Devonian granodiorite samples contained relatively high REE contents (ΣREE = 570–766 × 10−6) and were relatively enriched in light rare-earth elements (LREEs) with moderate LREE/HREE fractionation ((La/Yb)N = 12.99–16.05 and (La/Sm)N = 6.1–6.91). Additionally, they showed a flat distribution pattern of heavy rare-earth elements (HREEs) and negative Eu anomalies (δEu = 0.64–0.89). In a primitive-mantle-normalized trace element spider diagram (Figure 5a), the samples showed a notable enrichment in Rb, K, La, Th, and Ce and a relative depletion in Ba, Sr, and Cs. In terms of high-field-strength elements (HFSEs), a notable depletion in Nb, Ta, and Ti and a relative enrichment in Zr and Hf can be observed for these samples.
Figure 5. Primitive-mantle-normalized trace element spider diagrams for granitoids (a); chondrite-normalized REE patterns of granitic rocks from the Huashigou area (b) (PM values and chondrite values are derived from [54]).

4.2.2. Hua2 Pluton

The quartz diorites (YQ81–YQ84) from the Hua2 pluton showed relatively low contents of SiO2 (52.41–53.32 wt.%) and high contents of Al2O3 (19.67–19.69 wt.%), TiO2 (1.4–1.58 wt.%), TFe2O3 (8.71–9.23 wt.%), MgO (2.99–3.25 wt.%), and CaO (7.91–8.11 wt.%). Their total alkali contents ranged from 5.38 wt.% to 5.56 wt.%, and the quartz diorites were evidently sodium-enriched and potassium-depleted (Na2O/K2O = 7.01–7.55); thus, they were plotted in the calc-alkaline field in a SiO2-K2O diagram (Figure 4a). Based on the plot of the A/NK vs. A/CNK diagram, all of the samples are peraluminous (Figure 4b).
As can be seen from the primitive-mantle-normalized diagram (Figure 5a), the quartz diorites were enriched in LILEs, Ba, Rb, Th, and U and depleted in HFSEs, Nb, Ta, and Ti, with high Sr (573.85–593.13 × 10−6) and Y (40.1–43.32 × 10−6) contents and a low Sr/Y ratio (13.25–14.79). As shown in the chondrite-normalized distribution pattern (Figure 5b), the quartz diorites were slightly enriched in LREEs ((La/Yb)N = 3.89–4.76, (La/Sm)N = 1.54–1.88, and (Gd/Yb)N = 1.81–1.89), with slightly negative Eu anomalies (δEu = 0.81–0.89).

4.2.3. Hua4 Pluton

The Middle Permian porphyritic tonalites (YQ89–YQ92) from the Hua4 pluton showed high contents of SiO2 (63.64–68.51 wt.%), Al2O3 (14.89–16.62 wt.%), Mg (MgO = 0.71%–2.5 wt.%, Mg# = 45–51), and total alkali (Na2O + K2O = 7.3–8.02 wt.%) and low contents of CaO (1.47–2.93 wt.%). In the K2O vs. SiO2 diagram (Figure 4a), all of the samples are plotted as part of the calc-alkaline series. Based on the plot of A/NK vs. A/CNK, all of the samples are peraluminous (Figure 4b).
As can be seen in the primitive-mantle-normalized diagram (Figure 5a), the porphyritic tonalites were enriched in Rb, Ba, and Th and relatively depleted in Nb, Ta, and Ti, with low contents of Sr (172.43–217.28 × 10−6) and Y (17.05–20.98 × 10−6) and a low Sr/Y ratio (9.22–12.74). As shown in the chondrite-normalized distribution pattern (Figure 5b), the porphyritic tonalites were enriched in LREEs ((La/Yb)N = 10.34–14.36, (La/Sm)N = 5.38–6.98, and (Gd/Yb)N = 1.35–1.47), with negative Eu anomalies (δEu = 0.68–0.76).

4.3. Whole-Rock Sr–Nd–O and Hf Isotopes

Whole-rock Sr–Nd–O isotope data are presented in Table 2. The Late Devonian granodiorites (YQ57 and YQ63) from the Hua1 pluton showed high values for the initial 87Sr/86Sr ratio (0.712356 and 0.71195) and low values for the initial 143Nd/144Nd ratio (0.511827 and 0.511815), with negative εNd(t) values of −6.56 and −6.14 and a model age (TDM) of 1.6 Ga. In addition, the sample YQ63 showed an initial 176Hf/177Hf ratio value of 0.282484, with an εHf(t) value of −2.06 and a crustal model age of 1.4 Ga.
The Middle Permian granites from the Hua2 pluton showed high ISr values of 0.705906 and 0.705970 for quartz diorites (YQ83 and YQ84). Porphyritic tonalite (YQ91) showed the highest (87Sr/86Sr)ir value of 0.719527. The quartz diorites (YQ83 and YQ84) showed εNd(t) values of +0.78 and +0.82. The porphyritic tonalite (YQ91) from the Hua4 pluton showed the lowest εNd(t) value of −8.57. In addition, the values of δ18OV-SMOW for quartz diorite (YQ83) from the Hua2 pluton and porphyritic tonalite (YQ91) from the Hua4 pluton were all relatively higher (12.4‰ and 11.8‰, respectively).

5. Discussion

5.1. Petrogenesis

5.1.1. Petrogenesis of the Hua1 Pluton

The Late Devonian granodiorites (YQ57–YQ63) from the Hua1 pluton were rich in Al but poor in Mg, and they were slightly sodium-enriched and potassium-depleted. Additionally, they belong to the metaluminous calc-alkaline series (Figure 6). They showed high contents of Cs, Ba, Rb, Th, and Pb, low contents of Nb, Ta, Sr, and Ti (Figure 5a), high total REEs and LREEs, and a flat HREE pattern (Figure 5b) [34,55,56]. The flat pattern of HREEs implies that there is no garnet residual in the source region. The low Sr and negative Eu anomalies suggest fractional crystallization of plagioclase during magma evolution, indicating that the depth of partial melting is shallow [57]. The negative anomalies of P and Ti, high ISr values, and negative εNd(t) and εHf(t) values are potentially the result of the fractional crystallization of apatite and Fe-Ti oxide in the plagioclase-stable region. The εNd(t) vs. εHf(t) diagram is similar to that of the lower crust (Figure 7). The Sr–Nd–Hf isotopic compositions of granites suggest that they may have been derived from the partial melting of the underplated lower crust.
Figure 6. Plot of 10,000 × Ga/Al vs. (Na2O + K2O) in Huashigou granites.
Figure 7. Plots of (a) ISr vs. εHf(t) and (b) εNd(t) vs. εHf(t) of granites.

5.1.2. Petrogenesis of the Hua4 Pluton and Hua2 Pluton

Middle Permian granodiorites resemble the features of the continental crust because of their higher contents of SiO2, Al2O3, and K2O, as well as because they are rich in Ba, Rb, and Th but poor in Nb, Ta, Ti, Sr, etc., and have a higher positive Pb anomaly relative to REEs (Figure 5a), with high ISr, negative εNd(t), and high δ18OV-SMOW values. The fractional crystallization of plagioclase occurred during the process of rock formation, as evidenced by a slightly negative Eu anomaly (Figure 5b). The negative P and Ti anomalies imply the fractional distillation of apatite and Fe-Ti oxide. In addition, the low Nb/U ratio and Nb content are related to the contamination of materials from the upper crust [53,58]. The high content of Sr and the low contents of Yb and Y in the granodiorites were similar to the contents of those elements [59,60]. However, the isotopic characteristics of low Mg# content (≈40), high ISr, low INd, and positive εNd(t) (+5.51, Sample YQ3) values are clearly different from the isotopic characteristics of typical adakites formed by the partial melting of juvenile subduction slabs [59,60,61,62,63,64]; however, they are close to the isotopic characteristics of adakites generated by the partial melting of the underplated basaltic lower crust (Figure 5a) [65,66,67]. Thus, these adakites were formed in crustal environments with thickened slabs at high temperatures after magmas were contaminated by upper crustal materials during magma ascent and certain fractional crystallization of plagioclases [41].
As for Middle Permian granodiorites, the Nb/U and Nb/Ta ratios were slightly higher than those of the continental crust, and the positive εNd(t) values indicate that granodiorites originated from the partial melting of the underplated basaltic lower crust with the involvement of mantle materials. The low Nb/U ratio of quartz mica porphyritic diorite also implies magma contamination with upper crust materials during its ascent. The δ18OV-SMOW values of quartz diorite and quartz mica porphyritic diorite were significantly higher than the typical mantle values (5.4 ± 0.3 ‰ [68]), indicating that these granitic rocks are derived from the partial melting of the lower crust. With regard to this, we speculate that Middle Permian granitic rocks are the products of post-collisional slab break-off [69].
The geochemistry characteristics of Middle Permian granitoids are similar to those of Late Permian monzogranites from the Hua5 pluton [42] because of their high contents of Si and Al and low contents of Ti and Mg, as well as Cs, Ba, Rb, Th, and Pb enrichment and Nb, Ta, Sr, and Ti depletion. The Sr–Nd–Hf isotopic compositions showed that monzogranites derived from the partial melting of the underplated Mid-Neoproterozoic basaltic lower crust from the underplating heating of Late Paleozoic basaltic magma [42].
The εNd(t) values of Middle Permian quartz diorite and Late Permian monzogranite were greater than 0 [42], which is similar to some granites formed in environments with extensional tectonics. Granitic rocks with these characteristics have been found only in several areas in the world, such as the New England Batholith [70], northeastern US, southern Quebec of Canada [71,72], the Central Asia–Mongolia fold belt [73,74,75], etc. These two types of granitic rocks in this area have even lower εNd(t) values. Generally, the characteristic of εNd(t) < 0 indicates that a depleted mantle component played an important role in magma formation. In addition, the geochemical characteristics of Middle Permian quartz diorite and Late Permian monzogranite did not show any evidence of crust–mantle mixing in the source region [42]. Comparing this to other granites with εNd(t) > 0, granites in this area had lower εNd(t) (closer to 0) values. Therefore, the characteristic of the εNd(t) value, which is slightly greater than 0, may indicate that Middle Permian quartz diorite and Late Permian monzogranite were derived from the partial melting of basic materials in the lower crust formed by underplating.

5.2. Tectonic Setting of Huashigou Granitoids

Wu et al. [76] considered that granite in Phase 4 is related to isostatic adjustments between and among different rock bodies after orogenesis. Leake et al. [77] identified that, during the continental crash with the orogenic belt, the granitic emplacements caused by local temperatures and stresses while intrusions and tensility slumps happened in many ancient orogenic belts in the world were synchronal, e.g., the Sevier orogenic belt in southeast California, USA [78]; Guitiriz granite in the Variscan belt in Northern Spain [79]; and the Proterozoic orogenic belt in South Greenland. Foster et al. [80] showed that rapid exhumation could cause the decompression and partial melting of the lower or upper mantle, leading to the formation of Eocene intrusive rocks. For Late Devonian Huashigou granitoids, the geochemical characteristics indicate that they were formed from the partial melting of the lower crust, and their formation age is the same as that of Phase 4 granitic magma in the North Qaidam, as classified by [76]. In addition, the granites from Tula in the South Altyn Tagh Fault and Yusupuleke Tagh belong to the same period [39,76]. All of them were formed in the background of extensional tectonics after orogenesis. In Haker’s diagram (Figure 8), granitic rocks from different eras show favorable correlations, and their contents of Al2O3, MgO, TiO2, and TFeO decrease gradually as the SiO2 content increases, indicating a distinct negative correlation. K2O and Na2O show a positive correlation, and Rb, Sr, and Zr increase gradually as SiO2 contents increase. Additionally, Ba shows a negative correlation (Figure 9). These characteristics showed that the sources of different types of granitic rocks in the Huashigou area are similar. As shown in Figure 10, all samples are plotted in a post-collisional tectonic setting. Combined with the regional tectonic setting, we believe that the rocks were formed during a post-collisional tectonic setting where the previously subducted slab underwent detachment, leading to slab break-off magmatism and the formation of a significant number of granitic rocks.
Figure 8. Harker diagrams of Huashiou granitic rocks.
Figure 9. Trace element variation diagrams of granitic rocks from the Huashigou area.
Figure 10. Rb − (Y + Nb) diagram for discriminating the tectonic setting of granitic rocks from the Huashigou area. VAG: volcanic arc granite; ORG: ocean ridge granite; WPG: within-plate granite; syn-COLG: syn-collisional granite; post-COLG: post-collisional granite (after [81]).
Mid–Late Permian granitoids are comparable to Phase 5 Paleozoic granites in the North Qaidam, as determined by Wu et al. [76], due to the closing of the Paleo-Tethys Ocean, causing the regional tectonic setting to change from tensility to shrinkage. The continental subduction happened before the crust thickened rapidly. In addition, the delamination of the lower crust induced the underplating of basaltic magma, which further elicited the partial melting of the lower crust. Eventually, Mid–Late Permian granitoids were formed. Possessing characteristics of adakites, Mid–Late Permian granodiorites were formed under pressures greater than 1.5 GPa [64,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87], indicating that the crust thickness could be over 50 km [87,88]. Quartz diorites with high Sr and Yb contents were formed under medium or relatively high pressures, while quartz mica porphyritic diorites with high Sr and Yb contents were formed under medium pressures. The formation depth is estimated to be around 30 km. The depth of the source regions of partial melting decreases gradually from Middle Permian granodiorite to quartz diorite to quartz mica porphyritic diorite. Furthermore, Late Permian monzogranite is similar to granite, with low Sr but high Yb contents. Granite formed under low pressure and potentially at normal crustal thicknesses (≈30 km). Therefore, the thickness of the continental crust in the study area might have changed from thick to normal during the Late Permian.
It can be concluded that there were at least two important compressional–extensional tectonic transformation phases (c. 369 Ma and c. 262–252 Ma, respectively), with intrusions of granitic magma, which happened in the study area during the period of the Late Paleozoic. Additionally, this demonstrated that the evolution of the Altyn Tagh–Qilian–North Qaidam tectonic mechanism was evidently divided into phases. The magmatic activities and geochemical characteristics of granites indicate that the crust thickness before c. 369 Ma may have been less than 35 km, with low contents of Sr and high contents of Y. This corresponds to Late Devonian continental lava and volcanic pyroclastic deposits (products of tectonic evolution in the post-orogenic period) developed in the North Qaidam and East Kunlun regions, which are related to extensions of the continental crust [89]. After a short period of extension in the Devonian, the North Margin of the Qinghai–Tibet Plateau was influenced by tectonic movements in the Paleo-Tethys tectonic domain, especially subductions [89]. The ancient Tethys Ocean’s collision transpired between the Carboniferous–Permian continents of Asia and the Gondwana continent as the crust was renewed and intensified the compression. The crust underwent rapid thickening, reaching a thickness of up to 50 km in the Mid–Late Permian (262~252 Ma). Simultaneously, there was a transition phase in the tectonic system from compression to extension, and the basaltic lower crust was partially melted due to the heating of underplated basaltic magma, causing the formation of Mid–Late Permian granites. In conclusion, magmatic activities in the study area in the Early Paleozoic–Devonian were mainly affected by the Altyn Tagh–Qilian–North Qaidam tectonic mechanism, and those of the Carboniferous–Mesozoic–Cenozoic were restricted by the tectonic mechanism of the Paleo-Tethys Plate.

5.3. Tectonic Implications

5.3.1. Tectonic System of Altyn Tagh

Altyn Tagh is an important tectonic belt in the Northern Qinghai–Tibet Plateau, which includes a calc-alkaline granitic body dated at c. 575 Ma [90]. These granodiorites were formed at the active continental margin, meaning that plate subduction happened between the end of the Neoproterozoic and the beginning of the Paleozoic. In the Early Paleozoic, the Altyn Tagh region had started the ocean–continent evolution, as evidenced by two major ophiolite belts of such age. The c. 508–449 Ma Hongliugou–Lapeiquan ophiolite [91,92] is comparable to the Mangnai ophiolite, dated at c. 481 Ma [91]. In recent years, the discovery of high-pressure and ultrahigh-pressure metamorphic rocks in southern Altyn Tagh revealed the existence of a UHP metamorphic belt [7,11,12,28,29,93], formed ca. 504–486 Ma [94], which is 40–60 Ma older than that of eclogite in the UHP metamorphic belt in the North Qaidam Margin. Post-collisional extension was the major tectonic evolution process in the Altyn Tagh region, with weak magmatic activity and small rock masses [89].

5.3.2. North Qaidam–Qilian Tectonic System

North Qaidam–Qilian is a Cambrian–Ordovician orogenic belt. The Southern Qilian region is a small, active ocean basin. The oceanic crust subducted toward the Middle Qilian continental crust, and the ocean basin soon closed and formed the tectonic pattern of today. In recent years, the discovery of eclogites in the Xitie Mountain, Dachaidan, Yuka River, Dulan, and Shaliu River [9,12,95] proved that, during ca. 495–466 Ma, the subducted oceanic crust towed the Qaidam continental crust, causing it to subduct continuously. As a result, different types of rocks in Northern Qaidam experienced Early Paleozoic UHP metamorphism [12,17,18]. Furthermore, coesite and pseudomorph in eclogite and paragneiss [9,12], garnet peridotite, and diamond inclusions, as well as the phenomenon that clinopyroxene and rutile were exsolved in garnet [7], are consistent with these results. Several rocks were subducted to a mantle depth of 100–200 km. In the Early Silurian–Devonian, the continental crust began an exhumation process because of the subducted oceanic crust’s earlier delamination and violent crustal uplift during orogenesis, with magmatic activities in the crust. Molasse formations were broadly developed in the Early Devonian [19,21,96]. Furthermore, quartz diorite of the Aolao River in North Qaidam, small rock bodies southeast of the Bagaqaidam Lake, Datouyangou rock bodies in the Dachaidan region, and Late Devonian granodiorites were formed by the partial melting that occurred during extensions between different blocks in the orogenic zone after the UHP metamorphic belt was subducted and exhumed, indicating the uplifting stage of the North Qaidam–Qilian region after its orogenesis. This orogenesis stopped in the Carboniferous when the marine-paralic continental deposition belt covered the whole Qilian Caledonides tectonic belt [21]. In the Late Carboniferous, intracontinental subduction happened in North Qaidam as the Paleo-Tethys Ocean closed. Large amounts of granodiorite–monzogranite magmas were created in the middle–lower crust [97], such as the Sanchagou rock body in the west section of North Qaidam [21] and Late Permian monzogranite. These granites were formed under local tectonic stress changes from extension to compression, leading to intracontinental subduction and the upward intrusion of deep-mantle-derived materials. The middle–lower crust was eventually remelted.

5.3.3. Tectonic Setting of Altyn Tagh and North Qaidam–Qilian

Ge et al. [98] considered the tectonic units on both sides of the Altyn Tagh Fault before the Cenozoic Era to be comparable to the Alashan–Beishan–Tarim Block, the North Qilian–Hexi Corridor–Manjiaer Rift Trough, the Middle Qilian Uplift–Middle Uplift of Tarim, the South Qilian–North Qaidam–South Tarim, and the East Kunlun–West Kunlun. Based on a study of eclogites in the North Qaidam and South Altyn Tagh UHP metamorphic belts, Yang et al. [9] concluded that the South Altyn Tagh UHP metamorphic belt was a westward extension of the Northern Qaidam UHP metamorphic belt, with a stagger of 400 km by the Altyn Tagh Fault. Chen et al. [99] considered the east section of Altyn Tagh and the west section of the Qilian Mountain to be similar and comparable, estimating that the total displacement of the sinistral strike-slip of the Altyn Tagh Fault is about 400 km. Research on the metamorphic ages of eclogites has been conducted in Altyn Tagh and North Qaidam. Liu et al. [93] found that the peak metamorphic age of rocks in the Altyn Tagh high-pressure–ultrahigh-pressure metamorphic belt (c. 504–480 Ma) was c. 40–60 Ma earlier than that of Northern Qaidam (c. 461–420 Ma), which is c. 30–55 Ma earlier than the retrograde metamorphic age. Thus, it is considered that eclogites in Altyn Tagh and Northern Qaidam do not belong to the same high-pressure–ultrahigh-pressure metamorphic belts.
Late Paleozoic granites in Northern Qaidam–Qilian can be distinguished in the Late Devonian (c. 385–367 Ma) and Permian (c. 275–252 Ma). However, Late Paleozoic granites are rare in Altyn Tagh. Chen et al. [99] considered that the Early Paleozoic Era (c. 490–385) was the active stage due to magmatic activities in the north margin of Altyn Tagh. Hence, the granite in Altyn Tagh was not found in the study area, which may be caused by the different active ages of the Altyn Tagh and North Qaidam UHP metamorphic belts. Accordingly, the South Altyn Tagh and North Qaidam UHP metamorphic belts may not belong to one UHP metamorphic belt.

5.3.4. Nd Model Age and Attribution of Precambrian Basement in the Qilian Block

The Proterozoic crystalline basements within the Qilian orogenic belt predominantly emerge in the Yemanan Mountains in the western segment, Huangyuan in the central portion, and Maxianshan in the eastern section. These formations comprise high-grade metamorphic and granitic rocks. Generally, there are two models of the tectonic affinity of the Precambrian basement in the Qilian orogenic belt. Researchers consider that the Qilian Block belongs to the North China Craton [100,101,102]. However, geochemical and isotopic geochronological results led some researchers to consider that the Qilian Block was genetically related to the Yangtze Block. By analyzing the age spectrum of detrital and magmatic zircons in the Qilian Block, we concluded that the Qilian Block and Yangtze Block may have been in a uniform block in the Neoproterozoic Era and both belonged to Gondwana [33,103,104]. Through the study of Precambrian basement rocks in the Qilian orogenic belt and Pb–Nd isotopic compositions of granites, Zhang et al. [88] concluded that the Qilian orogenic belt is similar to the Yangtze Block and expanded the northwest boundary of the Yangtze Block to the north of the Qilian orogenic belt.
Based on the Nd isotope model ages of granites, we concluded that the basement of the North China Plate was formed in the Archean. The most important crustal growth period was between 2.8 and 2.6 Ga, and the less important crustal growth period was from 1.8 to 2.2 Ga [105]. Simultaneously, through comprehensive data processing and analysis, Wu et al. [106] determined that the Nd isotope model ages of the North China Craton are mainly between 2.4 Ga and 3.6 Ga, and the Nd isotope model ages of the whole South China Craton are mainly between 0.9 Ga and 2.3 Ga, while the intermediate acid igneous rocks in the Yangtze Block are between 0.7 and 2.2 Ga [72]. Furthermore, the Nd model ages of Huashigou granitoids are evidently divided into two phases: 0.9–1.1 Ga, represented by Middle Permian granodiorite, Middle Permian quartz diorite, and Late Permian monzogranite, and 1.6~1.7 Ga, represented by Late Devonian granodiorite and Middle Permian porphyritic tonalite [41,42]. This confirmed that the Mesoproterozoic Era was a crustal growth period in the Qilian region. The results indicate that the crustal growth history of the Qilian orogenic belt is similar to that of the South China Craton but different from that of the North China Craton. Furthermore, the Nd isotope model ages indicated that the Qilian Block and Yangtze Block might have been in a uniform block in the Mesoproterozoic Era.
The results highlighted two stages of granitic production in the Huashigou area, which included Late Devonian granodiorite (c. 369 Ma), Middle Permian biotite granodiorite, quartz diorite, porphyritic tonalite (c. 262–261 Ma), and Late Permian monzonitic granite (252 Ma).

6. Conclusions

(1)
Zircon U–Pb dating yielded crystallization ages of 368.7 ± 3.5 Ma, 261.5 ± 0.63 Ma, and 262.2 ± 1.4 Ma for the Hua1, Hua2, and Hua4 plutons, respectively.
(2)
Trace elements and Sr–Nd–Hf–O isotopes indicated that Huashigou granitoids were derived from the partial melting of the lower crust.
(3)
Huashigou granitoids were formed in the orogenic–post-orogenic extensional tectonic setting, and they were derived from different tectonic levels or crustal depths. Late Devonian granitoids were formed from the decompression melting of enriched and shallow-depth crustal materials. Middle–Late Permian granitoids resulted from the partial melting of the basaltic lower crust through heating by underplating basaltic magma. Additionally, they may have been formed by slab break-off magmatism.

Author Contributions

Y.W.: conceptualization, investigation, visualization, data analysis, writing—original draft, writing—review and editing. W.C.: writing—original draft, writing—review and editing. J.W.: investigation, writing—review and editing. Z.J.: investigation, writing—original draft. Q.T.: writing—original draft, writing—review and editing. P.D.: investigation, writing—review and editing. All authors have read and agreed to the published version of the manuscript.

Funding

This study was funded by the projects of the Science and Technology Program of Gansu Province (22YF7GA050), the Gansu Province Youth Talent Project (2020RCXM152), the Central Government Guides Local Funds for Scientific and Technological Development (YDZX20216200001297), the Second Comprehensive Scientific Expedition to the Qinghai–Tibet Plateau (2019QZKK0704), PhD Research Startup Foundation of Hunan University of Arts and Science (No.23BSQD05, No.23BSQD07, and No.21BSQD32), the National Key Research and Development Program of China (2023YFC2908700), the National Natural Science Foundation of China (U2244204), Key Talent Projects in the Gansu Province (2023), and Gansu Province Key Research and Development Program–Industrial Projects (23YFGA0001).

Data Availability Statement

The original contributions presented in the study are included in the article.

Acknowledgments

We appreciate three anonymous reviewers for their constructive suggestions, which greatly improved the manuscript.

Conflicts of Interest

This manuscript has not been published elsewhere and is not under consideration by another journal. The authors have approved the manuscript and agree with its submission to Minerals. There are no conflicts of interest to declare.

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Study on the Differential Distribution Patterns of Fine-Grained Sedimentary Rocks in the Lower Third Member of the Shahejie Formation in Zhanhua Sag, Bohai Bay Basin
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