Alteration Mechanism of Variscan Granite in a Project Area at the Northern Foot of the Tianshan Mountains, China

Abstract

Long-term erosion of granite—a type of hard rock—by hydrothermal fluids and tectonic movement can lead to a fragmentation of the internal structure of the original rock, transfer/replacement of mineral elements, and alteration of the rock’s basic properties. Such changes can be problematic for the construction of water conservancy, hydropower, and road projects. This study adopted the altered Variscan granite in a water diversion project area at the northern foot of the Tianshan Mountains (China) as the research object, and explored the alteration mechanism using thin section identification, X-ray diffraction, major element analysis, and electron probe and oxygen isotope tests. Results showed that the lithology of the granite in the study area is mainly biotite granodiorite and biotite monzonitic granite. Reductions in both the K⁺ content in plagioclase and the K⁺ and Ti⁴⁺ contents in chlorite indicate that the alteration types within the study area are mainly clayization of feldspar minerals and chloritization of biotite. Biotite granodiorite and biotite monzonitic granite both have low δ¹⁸O values. The δ¹⁸O value of biotite granodiorite decreases with increase in the Loss on Ignition. The low-δ¹⁸O-value granodiorite due to an alteration by hydrothermal fluids transformed from glacier meltwater, groundwater, atmospheric precipitation, and magmatic water; whereas the monzonitic granite might be formed by the reinvasion of low-δ¹⁸O-value granodiorite formed in the early stage, which is remelted, assimilated, and rebalanced. The research findings provide a reference for similar research on altered granite at the northern foot of the Tianshan Mountains, and also lay a foundation for subsequent research on its physical/mechanical properties and engineering characteristics.

1. Introduction

The Tianshan Mountains in the Eurasian hinterland are part of one of the largest inland orogenic belts in the world. The activity of the ancient Tianshan plate can be traced to the middle and late Proterozoic, and the historical development of the tectonic evolution of the modern Tianshan continental crust can be traced to the Paleozoic [1,2,3]. Following the formation of the continental block, it subsequently underwent a stage of intracontinental deformation and basin–mountain evolution throughout the Mesozoic and Cenozoic [4]. Many studies have investigated the long-term tectonic evolution and strong neotectonic movements from chronological and geochemical perspectives that have shaped its unique geomorphological pattern, which includes many types of strata and lithologies and a complex mechanism of rock genesis. Most granites in the Eastern Tianshan region are Early Carboniferous A-type granites, and their genesis mechanism may have originated from a partial melting of the younger crust of the Cenozoic [5]. The granites in the Southern Tianshan region are dominated by dioritic granites, which are characterized by high SiO₂ and alkali content, potassium-rich, and quasi-aluminous rocks [6]. In contrast, the West Tianshan is dominated by quasi-aluminous-weakly peraluminous high potassium-calcium-alkaline granodiorites such as granodiorites and granitic porphyries [7]. However, in the Northern Tianshan, there are not only late Early Carboniferous Bayingou ophiolite and diorite, but also Late Carboniferous-Early Permian weakly peraluminous, high-potassium-calcium-alkaline diorite, diorite, granodiorite, and potassic granite in other areas [8,9,10,11,12,13]. It can be seen that there are more types of granites within the Northern Tien Shan region, and the genesis mechanism is more complicated.

At the same time, multiple phases of volcanic activity in the Late Paleozoic formed widely distributed granite bodies in the Tianshan area. According to the formation age, the Tianshan area granites can be divided into those of the Yangtze period, Caledonian period, Variscan period, and Indosinian–Yanshan period, and the spatiotemporal zoning characteristics of the granites in the Paleozoic are most obvious [14,15,16]. Multistage hydrothermal and tectonic movements have led to the transfer or replacement of minerals and elements within the rocks, changing their original geophysical, chemical, and mechanical properties [17,18]. In many types of alteration, certain damage is caused to the internal structure of the rock that can degrade its physical and mechanical properties to varying degrees. For example, through the development of microcracks, the water content, water absorption, porosity, and expansion rate of rocks will increase with increase in the alteration degree [19,20,21]; whereas the density, longitudinal wave velocity, uniaxial compressive strength, shear strength, and tensile strength of rocks will decrease with increase in the alteration degree [22,23,24,25,26]. Hydrothermal activity provides the temperatures and elements needed for alteration, while tectonic movement crushes the rock and provides channels for hydrothermal flow. Owing to differences in both the original rocks and the multistage hydrothermal activity, the alteration types in each area are different, and there is often a phenomenon of coexistence of various altered minerals within the same area [27,28]. Therefore, it is very important to identify the types of altered minerals and to clarify the changes in the mineral compositions/contents before and after alteration for a comprehensive study of altered rocks.

In previous studies on the northern Tianshan area, the basic rocks represented by ophiolites, volcanic rocks represented by basalts, and symbolic fault scarps have attracted widespread attention, mainly from the aspects of their geological age and geological characteristics [13,29,30,31,32]. From the perspective of structural geology, the genetic background, geological importance, and relevance of some special plate movements to the rocks in each period have been deduced [33,34,35]. However, research on the widely distributed granites in the northern Tianshan area, especially the Variscan granodiorite and monzonitic granite, is relatively weak. The large-scale alteration of the granite caused by long-term hydrothermal activity and tectonic movement, and the deterioration of the original rock properties and its impact on regional large-scale infrastructure projects are also areas of concern. Therefore, this study adopted the altered rock in the granite tunnel section of the ABH water diversion project in the northern Tianshan area as the research object. On the basis of rock mineralogy and geochemistry, thin section identification, X-ray diffraction, major element analysis, and electron probe and oxygen isotope tests, the rock alteration mechanism and the source of altered hydrothermal fluid in the study area were comprehensively investigated to provide insight and reference for further research on similar altered rocks.

Geological Background

The ABH water diversion project is a cross-basin project connecting the northern and southern sides of the northern Tianshan Mountains in the northwest of Xinjiang Province, China (Figure 1). The total length of the tunnel is 41.8 km, and 38% of the tunnel is buried at a depth of >1000 m. The maximum buried depth is 2260 m, of which the granite section is 9.81 km long. The study area is dominated by NWW, NW-trending dextral compressive-shear faults, and EW-trending compressive faults. Over its long history, the geological structure has experienced many tectonic movements and much volcanic activity; consequently, the geological environment is diverse and the stratigraphic lithology is complex. It comprises mainly Paleozoic Ordovician, Silurian, Devonian, and Carboniferous sandstone, metamorphic sandstone, tuff, tuffaceous sandstone, and Variscan granite. There are many altered fracture zones of varying thickness within the granite exposed in the engineering area. The width of the exploration is 0.1–74.2 m. The degree of alteration is uneven and the regularity is poor. The altered rocks in the fracture zone of the surface, exploration adit, and tunnel are mostly grayish white, light flesh red, and flesh red medium–coarse-grained granite. Compared with the unaltered granite rock, the alteration traits of the internal mineral grains are obvious; that is, the surfaces have lost their original luster, grain edge angles are unknown, grain cementation is weak, and rock that has undergone strong alteration can be broken by hand.

2. Methods and Materials

In this study, 29 representative samples of typical altered rocks within the study area were taken from the surface, borehole cores, exploration caves, and the interior of the water tunnels, some of which are shown in Figure 2. The samples were subjected to geophysical and chemical tests, such as thin section identification, X-ray diffraction, whole-rock major element tests, and electron probe micro-analyzer (EPMA (Houston, TX, USA)) and oxygen isotope testing.

In Rock and Mineral Testing Center, Zhengzhou, China, an A1 polarized light microscope and a D/max-2500PC Japanese Rigaku X-ray diffractometer were used for thin section identification and X-ray diffraction, respectively, to observe and analyze the internal structure of the altered rocks, and to investigate the composition and content changes of the major minerals.

The quantitative analysis of in-situ major elements of minerals was performed by using an electron probe microanalyzer in Wuhan SampleSolution Analytical Technology Co., Ltd., Wuhan, China. The analysis was completed using JXA-8230 of JEOL; the voltage and current analyzed are 15 KV and 20 nA, the peak analysis time of the Ca, Ti, Al, Si, Mn, Fe, Na, Mg, K, P, etc., element is 10 s, and the background analysis time is 5 s. The calibration standard samples for the content of major elements use 53 kinds of mineral standard samples, 44 kinds of elemental standard samples, and 15 kinds of rare earth element standard samples provided by SPI company (Chicago, IL, USA). The data correction method adopts the ZAF correction method of JEOL.

The Zsx Primus II wavelength dispersive X-ray fluorescence spectrometer (XRF) produced by RIGAKU (Wilmington, MA, USA), Japan was used for the analysis of major elements in the whole rock. The X-ray tube is a 4.0 Kw end window Rh target, and the test conditions are voltage: 50 kV; current: 60 mA. All major element analysis lines are kα, the standard curve uses the national standard material, and the rock standard sample is reference materials [37]. The data were corrected by a theoretical α coefficient method, and the relative standard deviation (RSD) is less than 2%. The sample pretreatment of the whole rock major element analysis was performed by the melting method. The flux is a mixture of lithium tetraborate, lithium metaborate, and lithium fluoride (45:10:5). Ammonium nitrate and lithium bromide were used as oxidant and release agent, respectively. The melting temperature was 1050 °C and the melting time was 15 min. Loss on Ignition (LOI) was determined by the weight loss method LOI = 100 × (m₀ + m₁ − m₂)/m_1. (m₀: crucible mass; m₁: sample mass; m₂: sample residue after 1000 °C loss on ignition + empty crucible mass).

Oxygen isotope testing was conducted at the Beijing Institute of Geology of the Nuclear Industry, using a Delta V Advantage gas isotope mass spectrometer, adopting the “Determination of Oxygen Isotope Composition of Silicate and Oxide Minerals by the Bromine Pentafluoride Method” [38]. The altered granite samples were ground to 200 mesh (0.075 mm) with an agate mortar and pestle, and the specimens containing about 5 mg of oxygen were weighed and placed in a vacuum drying oven, and then baked and degassed at 105 °C for 2 h before being transferred to a desiccator. The nickel reactor was filled with 0.4 MPa argon, and the specimens in the desiccator were loaded into the reactor for vacuum treatment, while the specimens and reactor were heated and degassed externally at 200 °C for 30 min after transferring to a high vacuum, followed by evacuation, freezing, evacuation and thawing again. Finally, the oxygen generated from the fluorination reaction of the specimen was separated, purified, and collected; then, the oxygen isotope composition was determined using a gas isotope mass spectrometer with a two-way injection system.

3. Results

3.1. Mineral Structure of Altered Rocks

Examples of Sample A have a massive, fine-grained granitic structure (Figure 2e,f and Figure 3a–c) that is mainly composed of plagioclase feldspar (22–35%), potassium feldspar (7–24%), quartz (28–33%), chlorite (6–22%), clay minerals (mainly illite; 5–8%), hornblende (3–5%) and minor carbonate (1–3%). Plagioclase feldspar exhibits a semiautomorphic plate-like form with strong alteration, with a well-developed ring-band structure. Potassium feldspar is in the form of anhedral crystal, accounted for by clay minerals. Quartz is in the form of xenomorphic grains, with wave-like extinction. Biotite is scale-like, and polychromatic absorption is remarkable, with Np = light yellow, Ng = tan and green, mostly accounted for by chlorite, and it forms a replacement metasomatic pseudomorph texture. According to the microscopic mineral and structural characteristics, it is determined to be biotite granodiorite.

Examples of Sample B are massive with a fine–medium-grained granular structure (Figure 2g,h and Figure 3d–f) that consists mainly of plagioclase feldspar (21–38%), potassium feldspar (9–33%), quartz (16–32%), chlorite (5–19%), clay minerals (illite and montmorillonite; 2–22%), hornblende (3–5%) and minor carbonate (1–3%). Plagioclase feldspar is semiautomorphic plate-like, which is accounted for by sericite, carbonate, and clay minerals, and cracks are developed. Potassium feldspar is in the form of it-shaped grains; the larger particles are wrapped around other minerals, accounted for by clay minerals. Quartz is in the form of it-shaped grains, with wave-like extinction. Black mica is scale-like (particle size: 0.2–2.0 mm), with Np = yellowish green, Ng = green, and polychromatic absorption is remarkable, mostly accounted for by chlorite. According to the microscopic mineral and structural characteristics, it is determined to be biotite adamellite.

3.2. Mineral Chemistry of Altered Rocks

The data of the electron probe micro-analyzer (EPMA) is shown in Supplementary Materials Table S1. The results show that the plagioclase in the samples is mainly acidic plagioclase (An = 0.16–10.5), although some neutral plagioclase (An = 19.5–37.8) is evident, mainly in the monzonitic granite. The composition of the plagioclase in the granodiorite is albite to oligoclase in composition (An = 0.43–10.5). Plagioclase with high K⁺ content (Or = 2.25–2.37) appears in both the monzonitic granite and the granodiorite. Compared with the typical natural plagioclase, the K⁺ content (0.00–0.34%) of the plagioclase in the samples is generally low, indicating that the plagioclase has obvious clay alteration.

The K-feldspar in the biotite granodiorite is orthoclase, with K₂O content (15.6~16.9%) slightly lower than that of typical fresh orthoclase (16.7%), and Na₂O content ranging from 0.13% to 0.98%. MgO (7.98~11.6%), FeO (24.2~30.0%), TiO₂ (1.04~4.71%), K₂O (0.89~9.51%), Fe/(Mg + Fe) = 0.72~0.75 in biotite. Chlorite has MgO (10.4~13.1%), FeO (28.6~29.8%), TiO₂ (0.00~1.98%), K₂O (0.00~0.01%), Fe/(Mg + Fe) = 0.69~0.74.

In the biotite adamellite, the K₂O content of potassium feldspar (14.7~16.9%) is slightly lower than that of typical fresh orthoclase (16.7%), and the Na₂O content is 0.11~1.5%. The MgO (7.1~7.2%), FeO (24.6~25.8%), TiO₂ (4.38~4.71%), K₂O (0.00~9.51%), and Fe/(Mg + Fe) = 0.78 were found in biotite. Chlorite has MgO (10.0~12.9%), FeO (26.8~29.7%), TiO₂ (0.03% to 0.52%), K₂O (0.02% to 0.19%), and Fe/(Mg + Fe) = 0.70~0.75.

A comprehensive comparison of the internal oxide contents of the biotite granodiorite and the biotite adamellite reveals that the FeO content and Fe/(Mg + Fe) ratio in chlorite are essentially the same as those of the biotite. The MgO content is slightly higher, and the K₂O and TiO₂ contents are notably reduced.

3.3. Geochemistry of Altered Rocks

The results of the geochemical test are shown in

Table 1. Contents of the major elements, and the LOI and δ¹⁸O values of the sample rocks.

Sample Number	Rock Character	SiO2	TiO2	Al2O3	TFe2O3	MgO	CaO	Na2O	K2O	LOI	δ18O
		Mass%	Mass%	Mass%	Mass%	Mass%	Mass%	Mass%	Mass%	Mass%	‰
1	Fresh-Bio-adamellite	70.68	0.27	15.41	1.92	0.65	2.59	3.7	3.58	0.74	8.6
2	Bio adamellite	67.58	0.47	13.60	2.94	1.58	3.30	3.20	3.13	3.51	11.4
3		80.97	0.25	7.66	1.63	0.67	2.20	2.20	2.06	2.06	2.7
4		72.64	0.46	12.11	2.85	1.32	1.76	3.29	3.05	2.17	2.7
5		75.11	0.47	11.07	2.87	1.21	1.21	2.57	3.32	1.49	2.6
6		69.85	0.52	13.97	3.11	1.13	2.56	3.24	3.70	1.38	11.1
7		66.37	0.50	14.05	3.04	1.26	4.04	3.50	3.26	3.82	11.3
8		70.93	0.47	13.61	3.05	1.32	2.26	3.58	3.08	1.40	6.2
9		64.74	0.52	14.44	3.17	1.32	4.22	3.30	3.44	4.15	11
10		66.85	0.51	13.45	3.12	1.24	3.66	3.98	3.42	3.39	11.5
11		70.47	0.54	13.54	3.43	1.39	2.80	3.47	2.98	0.60	9.3
12		68.98	0.58	13.75	3.60	1.45	2.32	3.94	2.93	2.01	5.1
13		68.97	0.61	13.66	3.79	1.54	2.54	3.57	3.13	2.33	5.2
14		75.36	0.19	11.56	1.54	0.34	1.41	3.37	4.34	1.29	5.7
15		73.03	0.41	12.99	2.73	0.66	1.50	3.23	4.07	0.86	7.9
16		73.17	0.41	13.05	2.66	0.66	1.69	3.27	4.19	0.75	8.6
17		72.83	0.38	13.09	2.56	1.34	1.00	1.78	4.36	2.52	8.7
18		69.81	0.55	13.36	3.46	1.01	2.07	3.43	4.09	1.76	7.7
19	Fresh-Bio granodiorite	71.71	0.44	13.29	2.75	0.98	2.29	3.39	3.93	0.62	10.8
20	Bio granodiorite	54.81	0.52	12.47	3.21	1.46	5.75	3.93	2.94	9.23	1.9
21		57.94	0.63	16.77	3.87	1.75	4.84	5.20	3.90	4.77	1.9
22		67.27	0.59	14.42	3.52	1.38	2.01	3.17	4.06	2.76	3.6
23		68.16	0.49	12.70	3.10	1.24	3.69	2.73	3.30	4.09	3
24		68.45	0.65	10.64	4.02	1.29	4.94	2.70	2.44	4.11	8.4
25		71.29	0.50	12.90	3.13	1.30	2.02	3.31	3.50	1.82	6.5
26		59.03	1.67	13.01	9.98	2.86	3.51	2.70	2.39	3.84	8.3
27		68.74	0.71	13.86	5.12	1.45	2.89	3.00	3.03	0.53	13.3
28		71.19	0.51	13.33	3.36	1.13	2.50	3.79	2.51	1.24	9.6
29		70.52	0.55	13.00	3.55	1.29	2.50	3.33	3.46	1.19	10.5

. During the process of rock alteration in the study area, accompanied by changes in the mineral composition and geochemical characteristics, the contents of the primary major rock elements have also changed. Compared with fresh rocks, the overall trend of the contents of TiO₂ (0.11–0.71%), Na₂O (1.78–5.20%), and K₂O (2.06–4.36%) in the altered granite is that of reduction. Compared with previous studies [39,40,41,42,43,44,45], the contents of MgO (0.18–2.86%) and CaO (0.54–10.75) in the samples obtained in this study are more anomalous with the change of LOI, mostly showing a trend of increase with increasing LOI. This phenomenon reflects the continuous intrusion of carbonates with groundwater into strongly altered granites with pore development. Some granites exhibit evidence of cataclastic rockization. The maximum LOI (4.77–9.23%) of the cataclastic lithified granodiorite indicates a high degree of alteration of this rock type. The LOI of the cataclastic monzonitic granite is broadly consistent with that of other samples (0.59–4.15%). Meanwhile, the mineral particle size of the granodiorite might be related to the degree of alteration. Compared with the fine-grained and cataclastic granodiorite, the medium-grained granodiorite shows a smaller LOI, which is closer to that of fresh rock, indicating a lower degree of alteration. No significant correlation was observed between the LOI and the mineral particle size of the monzonitic granite samples.

3.4. Whole-Rock Oxygen Isotope of Altered Rocks

The results of the oxygen isotope test are shown in Table 1. The δ¹⁸O values of samples of fresh granite from within the study area are 8.6–10.8‰, which are broadly the same as those of the oxygen isotopes of both the granites in the eastern part of the northern Tianshan Mountains (8.02–8.91‰), and the rock masses in the JH area (8.29–10.25‰; mean: 9.25‰) measured in similar studies [8,46]. However, different from previous studies [47], nearly half of the altered rock samples in the study area have low δ¹⁸O values. The δ¹⁸O values of the granodiorite samples range from 1.9‰ to 13.3‰ (average: 7.07‰). Among them, the δ¹⁸O values of five groups of samples are in a state of deficit, whereas the δ¹⁸O values of one group of samples are higher. The δ¹⁸O values of the monzonitic granite samples range from 2.6‰ to 11.3‰ (average: 7.70‰), and the δ¹⁸O values of 10 groups of samples are in a state of deficit.

4. Discussion

4.1. Genetic Mechanism of Alteration

The K⁺ content of some altered rock samples obtained in the study area is markedly lower than that of fresh noneroded rocks (Figure 4a,b); conversely, the MgO content of almost all of the altered rock samples is higher than that of fresh noneroded rocks. Moreover, the MgO content shows a marked trend of increase with increasing LOI (Figure 4c,d), indicating that the chloritization phenomenon is more widely distributed and is prevalent in most rock samples. In the clayification stage, K⁺-rich acidic hydrothermal fluids extracted Na+ and Ca²⁺ ions from the plagioclase feldspars, which were replaced with K⁺ ions present in the fluids to form sericite and clay minerals such as kaolinitite, montmorillonite and illite [27,28,48,49]. Electron microprobe data show that the K content of the plagioclase in the samples (0.00–0.34%) is generally low compared with that of typical fresh plagioclase, which is indicative of the more pronounced clay alteration of the plagioclase in the obtained samples. The low K⁺ content in the potassium feldspar is also indicative of claying alteration. These changes in elemental content indicate that magmatic activity provided large amounts of hydrothermal fluid for alteration, leading to chloritization of smectite and clayification of feldspar minerals, which are products of medium–low-temperature hydrothermal alteration [27,28,48,49].

Different from previous studies, the content of CaO was found to increase with increase in the LOI after a small initial decrease (Figure 4e,f). The main reason for this phenomenon is that calcite is common within the samples, as shown by X-ray diffraction [42]. Furthermore, with increase in the LOI, the degree of rock alteration deepens, and rock porosity increases. Carbonate is a soluble mineral, and water-encapsulated carbonate continuously invades into the altered rock via cracks in surrounding rocks. As the porosity increases, more Ca²⁺ is invaded. Clayification is the process of Ca²⁺ precipitation [50], and the CaO content first decreases and then increases with increase in the LOI. The content of Na₂O decreases slightly with increase in the LOI, and this phenomenon is more obvious in the biotite granodiorite.

Through field investigation, dark-mineral-enriched inclusions appear locally in the surface rock mass near the water conveyance tunnel (Figure 5), very similar to the Mafic Microgranular Enclaves [51] found in the granite of the Kekesala Fe–Cu deposit near the Tianshan area. Rock and mineral identification revealed that the granite here is porphyritic biotite monzonitic granite, and that the black inclusions are adamellite or plagioclase rich in tourmaline and dark minerals, indicating that the quartz content in the dark inclusions is low and that the rock is generally basic. The formation of tourmaline is mainly related to the activity of B-rich fluid in magmatic activity. B-rich fluid reacts with the surrounding rock or early crystallized basic minerals, such as biotite and hornblende to form tourmaline in granite [52,53]. The formation of tourmaline and the changes in the mineral element contents in the altered granite samples are related to hydrothermal action. Therefore, our findings reconfirm that the alteration of granite within the study area was mainly due to the rise of medium–low-temperature hydrothermal fluid along the fault structure and its reaction with the surrounding rocks on both sides of the fault.

In summary, the study area is in the northern Tianshan unit of the Tianshan tectonic belt, between the Junggar and Tarim basins, and it lies in the orogenic belt between the Indian Ocean plate and the Eurasian plate. The North Tianshan unit underwent a long and complex tectonic evolution process, and experienced extensive and strong magmatic activity in the Paleozoic, especially the Late Paleozoic Variscan magmatic activity. As the magma rose and was emplaced, it underwent fractional crystallization and continued to evolve. In the latter stage of evolution, most of the silicate components formed granite via fractional crystallization, and the residual components mainly comprised low-temperature hydrothermal fluids composed of water-rich and volatile active metal elements. The low-temperature hydrothermal fluid rose from the deep along fractures and joints and reacted with the surrounding rocks on both sides of the fracture, where material exchange occurred and the surrounding rocks were altered. Multistage magmatic activities provided large numbers of injections of low-temperature hydrothermal fluid, and the low-temperature hydrothermal activity occurred over a long duration in a large range, resulting in large-scale wall rock alteration.

4.2. Constraints of Oxygen Isotope on Causes of Alteration

Many studies have identified that the whole-rock δ¹⁸O values of the granites in the study area are generally stable at 8.02–10.8‰ after testing the oxygen isotopes of granites of different periods in the northern Tianshan Mountains. In the process of hydrothermal alteration, owing to inhomogeneities of the fluid channel, duration, and water–rock ratio in different parts of the rock, the degree of hydrothermal alteration differed, resulting in a difference in the oxygen isotope composition of the altered rocks at different scales [54]. The oxygen isotope test on the altered granites with different buried depths in the study area revealed that some altered granites have a low δ¹⁸O value. Previous studies found that clay minerals from hydrothermal alteration have the smallest δ¹⁸O values, generally 2–14‰, whereas clay minerals produced by surface weathering have moderate δ¹⁸O values, usually 15–19‰ [55,56]. This finding also verifies that most of the samples with low δ¹⁸O values were from deeper parts of the study area (i.e., 600–700 m below the surface), while the samples obtained at the surface have normal δ¹⁸O values. Glacial meltwater, groundwater, atmospheric precipitation, and other fluids with low δ¹⁸O values can infiltrate along tectonic channels. When the conditions required for water–rock reaction are satisfied, fluids with low δ¹⁸O values react with the surrounding rock mass, thereby reducing the δ¹⁸O value of the rock within a certain depth range [57,58]. However, some rock belts with high δ¹⁸O values appear above the granite belt with low δ¹⁸O values. This reflects continued exchange between hydrothermal fluids and the upper granite in the subsequent rising process that results in the growth of the δ¹⁸O value, resulting in the appearance of altered rocks with high δ¹⁸O values in the upper part. It also indicates that the deep altered rocks are related to atmospheric precipitation, and that they might also be caused by a deep hydrothermal metamorphism [59,60].

As shown in Figure 6, only the δ¹⁸O value of the granodiorite in the samples shows a notable trend of change with the LOI, which decreases with increase in the LOI, and some granodiorites have the characteristic of a low δ¹⁸O value (1.9–3.6‰). Analysis of the major elements revealed that the altered granodiorite with deeper burial depth has a larger LOI (2.76–9.23%) (Figure 7), indicating that the altered granodiorite within the study area is mainly caused by meteoric water and magmatic hydrothermal alteration, although some granodiorite caused by meteoric water hydrothermal alteration has low δ¹⁸O characteristics (1.9–3.6‰). Through consideration of the geological history and the δ¹⁸O value characteristics of the study area, it can be inferred that in the main part of the late Paleozoic early Variscan subcycle, under the influence of plate movement, geosynclinal folding uplift occurred, accompanied by large numbers of magmatic intrusions, which formed large quantities of granodiorite. After the tectonic movement of the middle Variscan subcycle, the ancient Tianshan Mountains were formed, accompanied by frequent intermediate–acidic magmatic activities. The previously formed low-δ¹⁸O-value granodiorite was remelted, assimilated, and rebalanced by the magmatic hydrothermal fluid to form new granite. The samples of altered monzonitic granite with large buried depth have LOI values that are not high (0.6–2.33%), but they also have low δ¹⁸O characteristics (2.6–5.2‰), and the δ¹⁸O values of the monzonitic granite, which are higher than the δ¹⁸O values of the granodiorite, have no obvious trend of change with the LOI values. This phenomenon again verifies that the altered monzonitic granite in the study area might have been formed mainly by the remelting, assimilation, and rebalancing of the early low-δ¹⁸O-value granite via magmatic hydrothermal action.

5. Conclusions

In this study, the altered granite in an engineering area at the northern foot of the Tianshan Mountains (China) was taken as the research object. Following field investigation and sampling, the geochemical characteristics and alteration mechanism of the granite in this area were systematically studied via a series of laboratory tests, which included thin section analysis, X-ray diffraction, major element analysis, and electron probe and oxygen isotope tests. The main conclusions derived are as follows.

The lithology of the granite in the study area is biotite granodiorite and biotite monzonitic granite, and the K⁺ content in the plagioclase and K-feldspar are generally low. The K⁺ and Ti⁴⁺ contents in the chlorite are markedly lower than those in the biotite, and the MgO content is higher than that in fresh unaltered rocks. The MgO content increases notably with increase in the LOI. These phenomena indicate that the granite in this area has been subjected to low-temperature hydrothermal alteration over a long period. The alteration types are mainly chlorite of biotite and clay of feldspar minerals. The abnormal change in CaO content with the LOI (initial slight reduction and then gradual increase) is mainly due to the widespread existence of calcite and the continuous intrusion of carbonate with groundwater into the altered rock along cracks in the surrounding rocks.

Most of the altered granodiorites in the study area have low δ¹⁸O values. The δ¹⁸O value decreases with increase in the LOI, and the low-δ¹⁸O-value samples mostly appear in the water conveyance tunnel with large buried depth. With consideration of the geological history of the northern foot of the Tianshan Mountains, it can be inferred that the genetic mechanism was mainly water–rock reaction involving glacial meltwater, groundwater, atmospheric precipitation, and magmatic hydrothermal fluid. The irregular low-δ¹⁸O-value phenomenon in the monzonitic granite indicates that the monzonitic granite might have formed from the early low-δ¹⁸O-value granodiorite, which was remelted, assimilated, and rebalanced following reintroduction by magma.