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Article

Amphibole-Based Constraints on Magmatic Evolution and Fe–Ti Oxide Enrichment in the Xiaohaizi Ultramafic–Mafic Intrusion, Bachu, Xinjiang, China

by
Donghui Liu
1,2,
Shigang Duan
1,*,
Maohong Chen
1,
Weicheng Wang
3,
Jinmao Yin
4 and
Maihemuti Maimaiti
4
1
MNR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, Chinese Academy of Geological Sciences, Beijing 100037, China
2
School of Earth Sciences and Resources, China University of Geosciences, Beijing 100083, China
3
Faculty of Land Resources Engineering, Kunming University of Science and Technology, Kunming 650093, China
4
Bachu Natural Resources Bureau, Bachu 843800, China
*
Author to whom correspondence should be addressed.
Minerals 2025, 15(12), 1275; https://doi.org/10.3390/min15121275
Submission received: 30 October 2025 / Revised: 19 November 2025 / Accepted: 28 November 2025 / Published: 1 December 2025
(This article belongs to the Section Mineral Deposits)
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Abstract

A large, low-grade Fe–Ti–V oxide deposit occurs within the Xiaohaizi Ultramafic–Mafic intrusion. Based on petrographic observations and electron probe microanalysis (EPMA) of amphibole, this study examines the magmatic evolution and ore-forming processes of the intrusion through analyses of amphibole occurrence, mineral chemistry, and crystallization conditions. Five textural types of amphibole were identified: (i) inclusions, (ii) co-crystallization with early silicates, (iii) reaction rims, (iv) co-crystallization with late Fe–Ti oxides, and (v) phenocrysts. The amphiboles are calcic varieties, mainly composed of magnesio-hastingsite, kaersutite, and tschermakite. Crystallization occurred at temperatures of 901–1013 °C and pressures of 254–424 MPa, with ΔNNO values ranging from −1.3 to +2.8 and estimated melt H2O contents of 3.3–7.1 wt.%, corresponding to crystallization depths of 9.6–16.0 km. Importantly, the crystallization interval of the Fe–Ti oxides is defined by these amphibole-assemblage conditions, as evidenced by their direct intergrowth. Integration of mineralogical and geochemical data indicates that the Xiaohaizi intrusion underwent four distinct stages of magmatic evolution. During these stages, the crystallization of Fe–Ti oxides was accompanied by notable fluctuations in oxygen fugacity and melt water content. These results suggest that fractional crystallization played a dominant role in ore formation, with possible late-stage liquid immiscibility observed at the mineral scale. Overall, this study proposes that the Xiaohaizi Fe–Ti–V oxide deposit represents a magmatic conduit-type ore-forming system developed within a crystal mush. The enrichment of Fe–Ti oxides is strongly associated with hydrous melts and elevated oxygen fugacity conditions.

1. Introduction

Amphibole (Amp) is a common hydrous chain silicate in Ultramafic–Mafic (UM–M) rocks, with the general crystal-chemical formula A0–1B2C5[T8O22](OH)2. The mineral displays complex chemistry and extensive isomorphic substitution: the A-site is typically vacant or occupied by large cations (e.g., Na+, K+), the B-site commonly hosts Ca2+ and Na+, the C-site accommodates Mg2+, Fe2+/Fe3+ and Al3+ (and occasionally Ti4+), while the T-site is dominated by Si4+ with minor Al3+ [1,2,3].
Amphibole compositional variations are sensitive to temperature (T), pressure (P), oxygen fugacity (ƒO2), and melt water content (H2Omelt) [4,5,6,7]. For example, Al content commonly correlates with crystallization pressure, Fe3+/Fe2+ ratios reflect redox state, and OH− records melt water activity. Thus, amphibole chemistry provides a useful proxy for reconstructing magmatic P–T–ƒO2–H2Omelt paths [8,9,10]. Recent development of single-mineral amphibole thermobarometers allows quantitative reconstruction of P–T–ƒO2–H2Omelt trajectories from amphibole compositions, enhancing the ability to trace magmatic and ore-forming processes [11,12,13,14].
In the Tarim Large Igneous Province (TLIP), UM–M intrusions host large Fe–Ti–V oxide deposits, but ore-forming mechanisms remain contested [15]. Some studies favor fractional crystallization of mantle-derived magmas [16,17,18], whereas others invoke late-stage Fe–Ti-rich melts produced by liquid immiscibility [19,20,21]. To investigate the ore-forming mechanism, traditional mineral-pair approaches (e.g., titanomagnetite–ilmenite) are commonly regarded as suitable indicators for constraining ore-forming conditions. However, these minerals usually develop solid-solution unmixing textures with lamellar ilmenite exsolution, leading to uncertainty in P–T estimates [22,23,24,25]. A further shortcoming is the limited use of amphibole as a petrogenetic indicator, which has not been systematically applied to constrain magmatic evolution in TLIP intrusions. Amphibole, however, commonly coexists with Fe–Ti oxides and forms across a wide range of magmatic environments. It preserves primary compositions more reliably and allows the simultaneous reconstruction of temperature, pressure, oxygen fugacity, and melt water content through well-established single-mineral calibrations [9,26,27].
The Xiaohaizi UM–M intrusion in Bachu County, Xinjiang, represents one of the most Fe–Ti–V-enriched magmatic systems within the Tarim Large Igneous Province. A notable feature of this intrusion is the presence of amphiboles occurring in multiple textural forms, closely associated with Fe–Ti oxides. These characteristics indicate crystallization across different stages of magmatic evolution and make amphibole a key indicator mineral for Fe–Ti mineralization. Consequently, this intrusion provides an ideal natural laboratory for reconstructing its magmatic and metallogenic evolution using amphibole single-mineral thermobarometry. By integrating petrographic observations with detailed amphibole mineral chemistry, this study seeks to achieve three objectives: (1) to establish quantitative constraints on emplacement depth and ore-forming pressure–temperature conditions; (2) to assess how oxygen fugacity and melt water content influenced the enrichment and saturation of Fe–Ti oxides; (3) to construct a coherent genetic model for magmatic and metallogenic evolution of the Xiaohaizi intrusion. These results provide new insights into Fe–Ti–V oxide mineralization in the TLIP and highlight the broader value of amphibole-based thermobarometry for resolving physicochemical processes in complex Ultramafic–Mafic systems.

2. Geological Setting

2.1. Regional Geological Setting

The Tarim Craton in northwestern China is bordered by the Central Asian Orogenic Belt to the north and the Central Orogenic Belt to the south. Within the craton, largely concealed beneath the Taklamakan Desert, lies the TLIP. This basin is flanked by the Tianshan Mountains to the north and northwest, the Altyn Tagh Mountains to the southeast, and the Kunlun Mountains to the southwest (Figure 1a). Regional magmatism comprises extensive continental flood basalts, UM–M intrusive bodies, syenitic and granitic plutons, and minor trachytic to rhyolitic rocks [28,29,30] (Figure 1b).
The Xiaohaizi UM–M intrusion lies within the Bachu Uplift, a fourth-order tectonic unit on the northwestern margin of the Tarim Craton. Bachu uplift is bounded by the Kepingtag thrust fault to the north, the Acha–Tumuxiuke fault to the east, the Selibuya–Mazatag strike-slip fault to the south, and the Badong fault to the west [31]. Facilitated by the network of NE-trending thrust–strike-slip and NW-trending right-lateral faults, magma ascent found conduits and generated radial fractures, which ultimately shaped the Bachu anticline and the Xiaohaizi magmatic diapiric dome [32,33]. Regional sedimentary cover was established during the Cambrian–Ordovician. Following the last major marine transgression that deposited the Upper Devonian–Carboniferous series during the Late Devonian, the Tarim Basin experienced multiple tectonic uplifts (Caledonian, Indosinian–Yanshanian). These events led to the absence of Ordovician–Cambrian and Mesozoic strata, with Quaternary aeolian sands now extensively covering the area [34].
Magmatic activity occurred in two stages: (1) Basaltic eruptions at 292–287 Ma, and (2) Emplacement of Wajilitag kimberlite-like pipes (~285 Ma), the Xiaohaizi composite pluton, and dolerite dike swarms (284–274 Ma), forming an evolutionary sequence from UM–M intrusions to alkaline rocks and late-stage dikes [35]. Regional Fe–Ti–V oxide deposits, such as the Puchang, Wajilitag, and the Xiaohaizi deposits [36], is interpreted to be genetically related to fractional crystallization of mantle plume-derived mafic magmas [32,35].

2.2. Geological Characteristics of the Pluton

The Xiaohaizi composite pluton constitutes a multiphase intrusive complex, featuring a concealed UM–M pipes alongside late-stage alkaline stocks (Figure 2c and Figure 3). Country rocks comprise predominantly Devonian Kiziltag Formation quartz sandstones [37], with subordinate Carboniferous Bachu Formation sandstone and mudstone. Extensive Cenozoic erosion and Quaternary cover have obscured over 70% of the intrusion’s exposure [34,35]. Emplacement was controlled by a magmatic diapiric dome, which generated NW–SE-trending (320–340°) radial fractures. These structural conduits were subsequently intruded by dolerite and diorite dikes, collectively forming a typical UM–M dyke swarm [33].
Magmatic evolution at the Xiaohaizi composite pluton records a clear sequence, initiating with early UM–M intrusion (286–282 Ma), followed by alkaline magmatism (279–275 Ma), and culminating with late mafic dike emplacement (275–274 Ma; Figure 2b). Zircon U–Pb ages indicate that pyroxenitic and gabbroic rocks represent coeval products of a single magmatic event [35]. Petrographic analysis reveals that pyroxenitic rock is olivine-rich and plagioclase-poor with high Fe–Ti oxide content (Figure 2d), whereas gabbroic rock is plagioclase-dominated, containing fine-grained Fe–Ti oxides (Figure 2a). Geochemical data suggest both rock facies originated from common parental magma via continuous fractional crystallization [35,38].
A large, low-grade magmatic Fe–Ti–V oxide deposit is hosted within the pluton. Identified Fe–Ti oxide resources approximate 300 Mt, with potential resources estimated up to 2 Gt [34]. Mineralized zones correspond to surface negative magnetic anomalies [33]. Principal ore minerals consist of Titanomagnetite–Ilmenite intergrowths, accompanied by minor pyrrhotite, chalcopyrite, and pentlandite, which locally form massive Fe–Ti oxide ores [35].

3. Petrology

3.1. Petrographic Characteristics

The Xiaohaizi UM–M intrusion mainly comprises olivine clinopyroxenite, plagioclase-bearing olivine pyroxenite, olivine gabbro, gabbro (Figure 4), and later-stage mafic dikes, recording a clear sequence of magmatic evolution and mineral crystallization. The intrusion displays pervasive Fe–Ti mineralization, with Fe–Ti oxides present in all lithologies except for late-stage dikes. Pyroxenitic rocks host the highest proportions of Fe–Ti oxides (18%–25%, locally up to 30%), whereas gabbroic rocks contain moderately lower amounts (5%–15%).
Olivine clinopyroxenite shows a medium- to coarse-grained cumulate texture. Olivine grains are rounded to embayed and exhibit resorption and variable serpentinization. Clinopyroxene occurs as euhedral to subhedral crystals with distinctive octagonal cross-sections; some grains show schillerization produced by exsolved magnetite-ilmenite lamellae or contain inclusions of Fe–Ti oxides and amphibole. Interstitial domains are occupied by Fe–Ti oxides, plagioclase, and amphibole, with locally well-formed oxide crystal faces. Modal proportions are olivine (30–35 vol.%), clinopyroxene (40–45 vol.%), Fe–Ti oxides (18–25 vol.%), plagioclase (<5 vol.%), and amphibole (1–3 vol.%). Olivine grains average ~4 mm in size, whereas clinopyroxene ranges from 2–8 mm, occasionally exceeding 1 cm.
Plagioclase-bearing olivine pyroxenite contains olivine (20–35 vol.%), clinopyroxene (30–35 vol.%), plagioclase (20–25 vol.%), Fe–Ti oxides (10–15 vol.%), and amphibole (3–5 vol.%). Olivine and clinopyroxene typically measure 2–4 mm, while plagioclase is slightly coarser (3–5 mm). Interstitial amphibole that locally reaches up to 3 mm in width suggests that melt–rock interaction was enhanced during late-stage crystallization.
Olivine gabbro exhibits subhedral equigranular textures that locally transition into sub-ophitic to ophitic arrangements. Minor granophyric intergrowths and preferred mineral alignment are also present. Amphibole and apatite phenocrysts are common. Fe–Ti oxides occur as disseminated grains or as fine-grained interstitial aggregates. Modal compositions include clinopyroxene (20–25 vol.%), olivine (10–15 vol.%), plagioclase (30–40 vol.%), Fe–Ti oxides (10–15 vol.%), amphibole (5–8 vol.%), and minor biotite (<3 vol.%). Fine-grained to medium-grained texture (0.2–1 mm) indicates relatively rapid crystallization.
Gabbro is dominated by clinopyroxene (25–35 vol.%), plagioclase (40–50 vol.%), amphibole (8–10 vol.%), and Fe–Ti oxides (~8 vol.%). Accessory apatite and biotite account for <5 vol.%. Clinopyroxene forms subhedral to anhedral granular crystals that intergrow with lath-shaped plagioclase, whereas amphibole occurs as prismatic grains measuring 0.6–0.7 mm. Localized domains of amphibole gabbro contain 15–25 vol.% amphibole forming poikilitic grains (2–3 mm) that enclose prismatic apatite and plagioclase, reflecting increased melt hydration and fluid activity during late-stage crystallization.

3.2. Petrology of Amphibole

As the principal hydrous silicate mineral in the Xiaohaizi UM–M intrusion, amphibole is widely distributed across different lithofacies. Petrographic observations indicate that amphibole abundance is higher in gabbroic rocks (5–15 vol.%), locally reaching up to 25%, and lower in pyroxenitic rocks (<5 vol.%). Under the microscope, it appears brown to reddish-brown, displays two well-developed cleavage sets, and is typically intergrown with olivine, clinopyroxene, and plagioclase. Some phenocrysts are euhedral, showing rhombic or hexagonal cross-sections. Based on textural occurrence, mineral associations, and replacement relationships, amphiboles are classified into five principal textural types, each representing distinct crystallization environments and genetic mechanisms:
(i) Inclusion texture: Amphibole appears as irregular bleb-like inclusions within clinopyroxene, commonly accompanied by small droplets of Fe–Ti oxides or occurring independently. This texture may reflect an early stage of magmatic crystallization and possibly conditions enriched in magmatic water. An alternative interpretation is that these amphiboles formed through local reaction processes together with Fe–Ti oxides, and this possibility cannot be ruled out.
(ii) Co-crystallization texture with early silicates: Amphibole co-crystallizes with olivine, clinopyroxene, and plagioclase, forming sharp and straight grain boundaries without reaction rims, representing the main magmatic crystallization stage from a hydrous melt.
(iii) Reaction-rim texture: Amphibole forms along clinopyroxene margins as partial or complete replacement rims, suggesting metasomatic alteration by late hydrous fluids or magmatic–hydrothermal interaction during the transitional stage.
(iv) Co-crystallization texture with late-stage Fe–Ti oxides: Amphibole encloses or intergrows closely with Fe−Ti oxides as subhedral grains within interstitial spaces, implying co-crystallization from an evolved, Fe–Ti-rich melt enriched in water and metallic components during late magmatic stages.
(v) Phenocrystic texture: Containing inclusions of plagioclase, apatite, and Fe-Ti oxides, the large and well-formed amphibole phenocrysts thereby indicate prolonged crystallization or entrapment of late-stage melt components, consistent with continuous growth under stable physicochemical conditions.
Distinct differences in amphibole textures between pyroxenitic and gabbroic rocks likely result from variations in melt composition, H2Omelt content, and ƒO2 during crystallization, as illustrated in Figure 5.

4. Samples and Methods

To systematically investigate the mineral chemistry of amphiboles in the Xiaohaizi UM–M intrusion and its implications for magmatic and metallogenic processes, a total of 28 drill-core samples were collected from two exploration boreholes, providing a representative coverage of different lithofacies and structural units of the intrusion. 10 samples were collected from borehole ZK4605, which mainly consists of olivine clinopyroxenite, olivine gabbro, and gabbro, whereas 18 samples were obtained from borehole ZK1605, dominated by gabbro, olivine gabbro, and plagioclase-bearing olivine pyroxenite. Late-stage diorite, dolerite, doleritic porphyrite, and plagioclase dikes occur as thin intrusions cutting through the drill cores. Detailed borehole logs are shown in Figure 6.
Based on petrographic observations, fresh and unaltered drill-core samples were selected for EPMA. Amp grains with distinct textural and morphological characteristics were identified under a polarizing microscope. Scanning electron microscopy (SEM) was subsequently employed to examine the morphology and assess the compositional homogeneity of target amphiboles. Carbon coating was applied to ensure adequate electrical conductivity prior to EMPA.
Major-element compositions of representative and compositionally homogeneous amphiboles were determined using a JXA–iHP200 electron probe microanalyzer (JEOL Ltd., Akishima, Tokyo, Japan) equipped with an X-Max energy-dispersive spectrometer at the Institute of Mineral Resources, Chinese Academy of Geological Sciences. Analytical conditions were set at an accelerating voltage of 15 kV, a beam current of 20 nA, and a beam diameter of 3–5 μm. Quantitative analyses of major elements (K, Na, Ca, Mg, Mn, Fe, Al, Si, Ti) were performed using wavelength-dispersive spectrometry (WDS). Three types of analyzing crystals were applied to achieve optimal spectral resolution and intensity: PETL for K, Si, and Ca; TAPH for Na, Mg, and Al; and LIFH for Mn, Ti, and Fe. Peak and background intensities were measured using a peak–background–peak counting method, with counting times of 10 s for peaks and 5 s for each background position.
A series of certified high-purity natural and synthetic materials were used as calibration standards during analysis: quartz (SiO2) for Si, corundum (Al2O3) for Al, periclase (MgO) for Mg, hematite (Fe2O3) for Fe (reported as total Fe as FeO), rutile (TiO2) for Ti, jadeite (NaAlSi2O6) for Na, fluorite (CaF2) for Ca, manganotitanite (MnTiO3) for Mn, and potassium niobate (KNbO3) for K. All standards were recently calibrated and verified for analytical stability prior to use. Quantitative data were corrected using the conventional ZAF matrix correction procedure. The final chemical compositions and structural formulae of amphiboles were calculated on the basis of oxygen atoms, with O treated as the anion basis.

5. Results

Amphiboles from the Xiaohaizi UM–M intrusion contain 40.24–43.94 wt.% SiO2 (avg. 41.68 wt.%), 12.00–14.56 wt.% MgO (avg. 13.08 wt.%), 10.28–12.42 wt.% CaO (avg. 11.76 wt.%), 0.20–1.35 wt.% K2O (avg. 0.83 wt.%), 2.07–2.92 wt.% Na2O (avg. 2.45 wt.%), 2.75–5.61 wt.% TiO2 (avg. 4.49 wt.%), and 10.56–12.68 wt.% Al2O3 (avg. 11.37 wt.%). The total oxide contents range from 96.62 to 98.93 wt.% (avg. 97.66 wt.%). The detailed major element data of amphibole are presented in Table A1.
EPMA data indicate that the amphiboles are compositionally homogeneous, characterized by high Mg, Ca, Na, and Ti but low K contents. No exsolution lamellae or phase-separation textures were observed under the microscope, suggesting that the analyzed compositions represent primary magmatic signatures of amphibole crystallization [39,40,41,42,43,44,45,46].
Cation ratio and site occupancies were calculated on the basis of 23 oxygens following the methods of [14,26]. Amphiboles from the Xiaohaizi UM–M intrusion yield Si values of 5.990–6.380 apfu, IVAl (tetrahedral Al) of 1.620–2.010 apfu, Al/Si ratios of 0.283–0.365, Mg/(Fe3+ + Fe2+ + VIAl) ratios of 1.522–2.208, Mg/(Mg + Fe2+) ratios of 0.631–0.780, AlT (total Al) of 1.807–2.192 apfu, and Si/(Si + Ti + Al) ratios of 0.680–0.752. The detailed Cation ratio data of amphibole are presented in Table A3.
All amphiboles are classified as calcic amphiboles (CaB ≥ 1.5). Although Na+ occurs at the B-site, NaB < 1.00 apfu, indicating that Ca2+ is the dominant cation. Some amphiboles contain Ti ≥ 0.5 apfu and are therefore identified as high-Ti varieties. The detailed site occupancy data of amphibole are presented in Table A2. A classification diagram based on Si content and Mg# [3] further refines the amphibole types as Magnesio-hastingsite, Kaersutite, and Tshermakite (Figure 7).
Most amphiboles in the Xiaohaizi UM–M intrusion exhibit uniform interference colors, suggesting that they are not products of post-magmatic hydrothermal alteration [39]. In Al2O3–TiO2 classification diagram for calcic amphiboles (Figure 8), display features typical of mantle-derived magmatic amphiboles, with Si/(Si + Ti + Al) ≤ 0.765, consistent with petrographic observations [40].
Ref. [41] proposed that TiO2 content in amphibole serves as a reliable indicator of magma alkalinity. As shown in TiO2–K2O correlation diagram (Figure 9), it can be concluded that the magma from which the amphiboles in the Xiaohaizi UM-M intrusion crystallized was alkaline in nature. In TiO2–IVAl diagram (Figure 10), data points along the primary magmatic trend, indicating that the amphiboles are of primary mantle-derived magmatic origin. In Mg/(Fe3+ + Fe2+ + VIAl)–Al/Si diagram (Figure 11), all data points plot within the compositional field of intermediate to Mafic-Intermediate magmatic amphibole [40].

6. Discussion

6.1. Magmatic Crystallization Conditions (T–P) Recorded by Amphibole

Amphibole chemical composition provides a key tracer for physicochemical conditions during magmatic crystallization. This study applied empirical thermobarometry from [13,14,26,42,43,44] to quantitatively constrain the crystallization conditions of magmatic amphiboles in the Xiaohaizi UM-M intrusion. Developed from extensive experimental petrology datasets, these models are applicable to calcic amphiboles crystallized from calc-alkaline to alkaline melts, which enables quantitative estimation of P–T–ƒO2–H2Omelt conditions. To ensure the reliability of thermobarometric interpretations, only amphiboles preserving primary magmatic compositions were included in the dataset. Amphiboles affected by subsolidus reactions or hydrothermal overprinting most notably the reaction-rim varieties (Amp-iii) were excluded, as their compositions are inconsistent with equilibrium magmatic crystallization. The resulting P–T–ƒO2–H2Omelt estimates therefore reflect conditions recorded by the magmatic textural types (Amp-i, ii, iv, v).
Model results indicate crystallization temperatures ranged from 912 to 1013 °C (average 965 °C), with corresponding pressures of 254 to 424 MPa (average 323 MPa). Both pyroxenitic rocks (Figure 12a) and gabbroic rocks (Figure 12b) display a clear inverse correlation between temperature and pressure, documenting magma ascent and cooling within the upper crust at estimated depths of 9.6–16.0 km. The overlapping pressure–temperature fields among different textural types of amphibole further indicate continuous crystallization across multiple magmatic stages.
To validate these results, amphibole Al2O3 (10.6–12.7 wt.%) and TiO2 (2.7–5.6 wt.%) contents were projected onto the P–T lattice of [45] (Figure 13). The interpolated temperature range (925–1040 °C) shows close agreement with estimates from the [14] model. Although pressures derived from this method (<0.52 GPa) are moderately higher, they remain within the same order of magnitude, suggesting that the resulting P–T estimates are broadly consistent. Application of the [7] amphibole thermometer yielded crystallization temperatures of 936–1021 °C, consistent with the results above and further supporting the reliability of the dataset.

6.2. Evolution of Oxygen Fugacity (fO2) and Melt Water Content (H2Omelt) Recorded by Amphibole

Amphibole composition constrains the redox state (ƒO2) and water content of parental melts. Calculated using P–T-independent formulations from [14], amphibole crystallization in the Xiaohaizi UM-M intrusion yields ƒO2 values of ΔNNO − 1.3 to +2.8 (average ΔNNO + 0.4) and corresponding H2Omelt contents of 3.3–7.1 wt.% (average 4.3 wt.%). This ΔNNO range indicates dynamic fluctuations in both oxidation state and volatile content during magmatic evolution.
In pyroxenitic rocks, amphiboles record consistently high ƒO2 values across a temperature range of 912–1013 °C, suggesting that crystal accumulation occurred under persistently oxidizing conditions (Figure 14a). Amphiboles coexisting with early silicate minerals exhibit relatively lower ƒO2 values, likely reflecting a reduction in oxygen fugacity associated with extensive early magnetite crystallization. In contrast, gabbroic rocks, representing residual melt crystallization, display a clear trend of increasing ƒO2 with decreasing temperature (Figure 14b), attributed to progressive fractionation of Fe2+-bearing silicate minerals during magma ascent [47,48]. Large amphibole phenocrysts record relatively lower ƒO2 values, implying crystallization prior to melt water saturation.
Regarding H2Omelt, amphiboles in pyroxenitic rocks exhibit a broad range of values, possibly reflecting disequilibrium crystal-melt interactions during crystal accumulation or heterogeneity in the intercumulus melt composition (Figure 15a). Conversely, gabbroic rocks show a narrower H2Omelt range, indicating that late-stage residual melts were comparatively homogeneous and water-rich (Figure 15b). Collectively, these observations demonstrate that the magmatic system evolved toward a more oxidized and hydrous state during late-stage crystallization, creating favorable conditions for Fe–Ti–V oxide enrichment [16,49,50]. A summary of calculated physicochemical conditions is provided in Table A3. Complementing the graphical diagrams, Table 1 and Table 2 offers a comparative summary of these parameters to facilitate direct comparison.

6.3. Magmatic Evolution and Metallogenic Process: Constraints from Amphibole

6.3.1. From Static Magma Chamber to Dynamic Crystal-Mush Conduit: A New Petrogenetic Model

Traditional static magma chamber models fail to explain key features of the Xiaohaizi UM-M intrusion. Pronounced textural diversity of amphiboles within individual thin sections, combined with the continuous temperature–pressure trends in Figure 11, is consistent with crystallization in a dynamic crystal-mush conduit system. Amphibole crystallization occurred not at discrete pressure levels but along a continuous pressure gradient, recording progressive re-equilibration during magma ascent rather than stratified crystallization in a stagnant chamber. These findings suggest that Xiaohaizi intrusion formed through a melt-poor conduit system, with melt fractions estimated to be less than 20%, linking deep and shallow crustal levels [51,52]. Within this system, early-formed amphiboles crystallized at varying depths and were subsequently transported upward by later melt pulses to shallower levels, as crystals and melt experienced continuous replenishment, migration, and differentiation.

6.3.2. Delineation of a Four-Stage Magmatic Evolutionary Sequence

Integrating amphibole-derived data on temperature, pressure, ƒO2, and H2Omelt, magmatic evolution of the Xiaohaizi intrusion is divided into four successive stages, each marking a critical transition in Fe–Ti–V oxide mineralization.
  • Stage I: Deep Crystallization and Initial Mineralization;
This stage is characterized by amphiboles (Amp-i) occurring as inclusions within clinopyroxene, recording high-T (~1000 °C), high-fO2 (~ΔNNO + 1.5), and hydrous (~4.8 wt.%) conditions at lower-crustal depths (~15 km). Under these conditions, Fe–Ti oxides reached early saturation and co-crystallized with clinopyroxene, marking the onset of mineralization. The high H2Omelt of magma suppressed plagioclase crystallization and drove the melt toward Fe–Ti enrichment in the residual liquid.
  • Stage II: Mush Ascent and Disequilibrium Reaction Stage;
This key ore-forming stage is represented by reaction-rimmed amphiboles (Amp-iii) and Co-crystallization structure with silicates and Fe–Ti oxides (Amp-ii, Amp-iv). As the crystal mush ascended and cooled (~12 km depth), fractional crystallization of anhydrous minerals such as olivine and clinopyroxene consumed Fe2+, increasing the Fe3+/ΣFe ratio [48], inducing “self-oxidation” of the melt. Concurrent volatile enrichment further elevated ƒO2, triggering renewed saturation and abundant crystallization of Fe–Ti oxides. Widespread Amp–oxide intergrowths record this process.
  • Stage III: Mid-Depth Differentiation and ƒO2 Fluctuations;
Amphibole phenocrysts containing apatite inclusions (Amp-v) record the highest P–T values among the gabbroic rocks, showing substantial overlap with those of amphiboles co-crystallizing with silicates (Amp-ii), indicating a stable differentiation and growth phase at mid-crustal levels. A temporary decrease in ƒO2 (down to ~ΔNNO − 0.6) likely reflects melt enrichment in phosphorus, as PO43− complexes Fe3+, reducing its activity in the melt [53].
  • Stage IV: Shallow Solidification and Late Melt Infiltration;
At this stage, magma ascended to shallower levels (~9 km), forming typical gabbroic textures and showing increased apatite abundance. Two amphibole types are observed: (1) Large phenocrystic amphiboles enclosing fine plagioclase, Fe–Ti oxides, and clinopyroxene along their rims, suggesting re-equilibration after transport from deeper levels; (2) Anhedral interstitial amphiboles co-crystallized with Fe–Ti oxides until the final solidification, locally modifying earlier-formed minerals. A moderate ƒO2 increase during this stage may have resulted from Fe3+ release during apatite crystallization [39,53].
  • Timing of Fe–Ti Oxide Mineralization;
Integrating all stages, Amp data constrain Fe–Ti oxide crystallization to the temperature range of 1013–901 °C (from Stage I to Stage IV), corresponding to pressures of 254–424 MPa and depths of 9.6–16.0 km [54]. The peak mineralization is closely associated with the “self-oxidation” process during Stage III.
Complementing the textual description, Table 2 summarizes the physicochemical conditions characteristic of each defined stage, Figure 16 suggest a model for the evolution of the studied mafic-ultramafic intrusion.

6.3.3. Metallogenic Mechanism: Fe–Ti–V Enrichment Dominated by Fractional Crystallization

The Xiaohaizi Fe–Ti–V oxide deposit is characterized by disseminated mineralization and gradational lithological transitions, with no evidence of rhythmic layering. These features indicate formation within a relatively isolated crystal-mush system dominated by fractional crystallization. The parental Fe–Ti–rich magma, derived from a mantle plume source metasomatized by ancient subduction-related fluids, was inherently oxidized and hydrous, providing the fundamental prerequisites for ore formation [35,55].
Petrographic features, including reverse zoning in plagioclase and localized vermicular symplectites, suggest grain-scale liquid immiscibility during the late stages of fractional crystallization (Figure 4) [56,57,58,59,60]. Although this localized segregation further promoted Fe–Ti enrichment, it was not the principal mechanism on a regional scale [32,61,62,63].
Accordingly, the metallogenic model for the Xiaohaizi Fe–Ti–V oxide deposit can be summarized as follows: a high-ƒO2, hydrous parental magma evolved within a dynamic crystal-mush conduit system, where continuous fractional crystallization—particularly the “self-oxidation” process during Stage III—progressively enriched Fe and Ti and ultimately led to Fe–Ti oxide precipitation from the residual melt. Amphibole, as a key magmatic witness, systematically records this complex and dynamic evolutionary process.

7. Conclusions

Amphiboles in the Xiaohaizi Ultramafic–Mafic intrusion can be classified into magmatic and hydrothermal types. Magmatic amphiboles, primarily magnesio-hornblende, tschermakite, and pargasite, occur as inclusions, reaction rims, symplectites, and phenocrysts, providing clear evidence of multiphase crystallization. Their compositional characteristics indicate crystallization at temperatures of 901–1013 °C and pressures of 254–424 MPa (corresponding to depths of 9.6–16.0 km), under oxygen fugacities ranging from ΔNNO −1.3 to +2.8 and melt water contents of 3.3–7.1 wt.%. These parameters define a continuous magmatic evolution from deep to shallow levels.
The continuous variation in amphibole crystallization conditions and the coexistence of multiple amphibole generations contradict the concept of a static magma chamber and instead support a dynamic crystal-mush conduit system. Pyroxenitic and gabbroic rocks, interpreted as products of continuous differentiation from a common parental magma, were emplaced within this conduit network. Variations in emplacement depth record the progressive ascent and accumulation of magma during the evolution of the intrusion.
Fe–Ti oxide mineralization occurred within a temperature range of 1013–901 °C and was primarily governed by fractional crystallization. The crystallization of anhydrous silicate minerals (such as olivine and clinopyroxene) promoted self-oxidation of the melt, triggering large-scale Fe–Ti oxide saturation. At later stages, residual interstitial melts may have undergone localized liquid immiscibility, further enhancing the enrichment of ore-forming elements.
The high ƒO2 and hydrous signatures recorded by amphibole compositions, together with regional geological evidence, indicate that the parental magma was derived from an enriched lithospheric mantle that had been metasomatized by ancient subduction-related fluids. These inherited characteristics—namely high oxidation state and elevated water content—controlled early Fe–Ti oxide crystallization and ultimately facilitated their enrichment and accumulation within the crystal-mush conduit system.

Author Contributions

Conceptualization, D.L., S.D. and M.C.; Methodology, D.L., S.D. and M.C.; Software, D.L., S.D. and M.C.; Validation, D.L., S.D., M.C. and W.W.; Formal analysis, D.L., S.D., M.C. and W.W.; Investigation, D.L., S.D., M.C., W.W., J.Y. and M.M.; Resources, S.D., M.C., J.Y. and M.M.; Data curation, D.L.; Writing – original draft, D.L.; Writing – review & editing, D.L., S.D. and M.C.; Visualization, D.L., S.D. and M.C.; Supervision, S.D. and M.C.; Project administration, S.D. and M.C.; Funding acquisition, S.D., M.C., J.Y. and M.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the following projects: National Science and Technology Major Project “Deep Earth Probe and Mineral Resources Exploration” (Grant No. 2024ZD1003403); China Geological Survey Project (Grant No. DD20240204803); Bachu County Project (Grant No. GYZB-BCKC2025-01).

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Acknowledgments

Field investigation and sample collection in this study were greatly supported and assisted by Ni Kang and Zhang Bing of Xinjiang Hanqing International Mining Co., Ltd. The authors express their sincere gratitude for their valuable help and contributions.

Conflicts of Interest

The authors declare no conflicts of interest.

Appendix A

Table A1. Amphibole major elements Composition in the Xiaohaizi UM–M Intrusion (wt.%).
Table A1. Amphibole major elements Composition in the Xiaohaizi UM–M Intrusion (wt.%).
SampleRock Type *PointAmp TypeSiO2TiO2Al2O3FeOMnOMgOCaONa2OK2OH2O *Total
4605-4OC1i40.475.4911.7911.070.1313.0412.102.331.351.1398.90
4605-15POC1i40.325.0012.3011.220.1112.9812.352.321.171.2098.97
4605-19OC1i40.245.2412.4412.480.1112.0012.162.450.631.1898.92
4605-23OC1i40.255.2912.4511.570.1112.5312.222.550.691.1898.85
OC2i40.315.6112.4811.350.1312.3312.272.500.731.1498.84
OC3i41.663.4012.499.550.1014.1412.372.920.201.5898.41
OC4ii41.164.3312.2710.400.1013.4512.422.760.591.3998.86
4605-13OC1ii41.164.8412.3110.440.1213.5412.152.361.251.2499.40
OC2ii41.705.1411.7511.160.1012.4712.222.320.761.2598.87
4605-22OC1ii40.965.5011.6510.790.1013.3911.892.391.121.1498.93
1605-43POC1ii41.344.6011.3012.190.1412.6711.612.510.701.3298.37
POC2iv41.014.6911.7912.160.1612.5011.702.590.761.3098.65
POC3iv41.384.5711.2812.300.2212.9811.692.580.671.3499.01
4605-1OC1iv41.654.5112.2310.840.1313.5012.242.700.731.3699.88
4605-12OC1iv41.154.0812.6810.530.1013.8312.312.830.371.4299.30
1605-5POC1iv40.735.0711.3611.280.1113.0511.842.251.051.2097.93
1605-7POC1iv42.224.3411.8710.960.1013.9512.072.231.191.32100.25
POC2iv41.004.9910.9811.760.1212.6512.082.151.171.2398.13
1605-16POC1iv41.994.3911.1411.980.1212.7012.012.151.111.3598.94
POC2iv41.524.7810.8511.750.1212.8811.852.091.321.2698.42
1605-2OG1iv41.714.8311.2211.440.1313.1411.802.271.161.2698.96
1605-4OG1iv41.744.2610.9412.060.1212.8611.812.331.211.3798.69
OG2iv42.234.1310.8511.980.1012.8111.952.171.051.3998.65
1605-9OG1iv43.193.4911.009.950.1114.5612.172.191.021.5299.20
1605-20G1iv41.924.6411.0312.270.1413.0111.962.071.221.2899.52
G2iv42.274.7111.1512.380.1112.9011.862.161.231.27100.02
1605-3OG1iv40.775.0411.3411.410.1212.7611.732.391.071.2297.85
1605-4OG1iv42.003.9911.4211.460.1513.3811.862.320.951.4198.95
1605-4OG2iv42.474.1711.0211.470.1113.5911.622.410.931.3999.18
1605-20G1iv43.343.3510.8611.310.1113.9111.852.150.881.5499.29
G2iv43.013.4411.0311.450.1113.8411.892.170.891.5199.33
1605-3OG1iv42.244.1511.2711.400.1113.6610.862.270.971.3498.27
1605-39AG1v41.344.9811.2612.200.1813.0911.392.620.661.2698.97
1605-41AG1v41.194.7411.5012.180.1812.8111.522.530.641.2998.57
AG2v41.304.6011.3712.380.2212.5611.432.610.681.3498.48
AG3v41.864.7611.1311.910.2012.7411.442.650.661.3498.68
AG4v41.604.5711.1512.680.1612.7811.512.700.641.3499.12
AG5v42.205.0111.5412.440.1712.9411.122.600.651.2599.91
1605-42AG1v41.234.5411.5011.890.2112.7011.442.640.711.3598.20
AG2v41.134.4511.3412.090.1512.8511.712.560.861.3498.46
AG3v41.574.5811.4112.220.1613.1111.972.520.731.3399.60
1605-44AG1v41.364.4111.5012.540.1613.1311.722.630.681.3499.47
AG2v41.584.6011.6312.280.1513.0911.822.640.691.3299.81
1605-45G1v41.924.9611.2512.540.2012.7011.632.570.661.2999.73
G2v41.784.9811.3412.510.1912.9411.652.650.671.2899.99
4605-7OG1v41.944.7511.2912.810.1812.5110.282.500.871.2698.39
OG2v41.684.7211.4712.550.1512.4711.282.480.791.2998.89
1605-43OG1v41.584.6611.2812.200.1712.7711.692.570.661.3398.90
OG2v41.764.7311.2312.120.1712.9011.712.680.721.3399.36
1605-44AG1v41.874.5511.4012.380.1712.8511.662.480.821.3399.50
AG2v41.954.9311.4812.700.1712.7611.612.610.721.28100.19
1605-37AG1v41.684.7611.2412.080.2112.9111.722.500.711.3299.13
AG2v41.474.2411.2011.540.2013.2911.642.320.921.3798.18
AG3v41.774.6010.9912.370.1913.0811.492.480.851.3299.14
AG4v41.664.4911.0211.860.1913.2611.662.370.841.3498.70
AG5v40.834.6111.3412.750.2012.9511.682.400.751.2998.78
1605-39AG1v41.584.5510.7112.300.1613.3011.372.410.781.3198.47
1605-39AG2v41.484.0311.5312.170.1412.9911.682.580.761.4198.77
AG3iv41.904.4510.9712.470.1612.5711.502.580.811.3798.76
AG4iv42.043.7311.3511.770.2213.3911.782.281.041.4699.05
1605-40AG1iv41.444.2711.0511.940.1813.2211.512.450.831.3798.25
1605-46AG1iv41.664.2411.3512.010.1912.9812.262.460.801.4199.34
1605-37AG1iv42.054.7110.8312.430.2113.0011.522.570.701.3499.34
1605-45AG1iv42.383.9910.7812.560.1713.3211.622.430.771.4399.43
AG2iv42.453.8510.7512.610.1913.2011.592.420.801.4699.32
1605-13OG1iv42.444.1510.6512.010.2213.4811.602.420.731.4399.11
OG2iv42.063.9710.7712.210.1613.0911.852.270.821.4398.62
4605-20OG1iv43.302.8510.9111.860.1713.6012.132.430.501.6799.41
OG2iv43.942.7510.5611.760.1014.2011.342.290.541.6599.12
* The following abbreviations are used in Table A1, OC: Olivine Clinopyroxenite; POC: Plagioclase-bearing Olivine Clinopyroxenite; AG: Amphibole Gabbro; OG: olivine gabbro; G: Gabbro. The H2O content was obtained by assigning a stoichiometric coefficient of 1 (H2O pfu).
Table A2. Crystal-chemical site occupancy analysis of amphibole based on major element composition (apfu).
Table A2. Crystal-chemical site occupancy analysis of amphibole based on major element composition (apfu).
SamplePointAmphibole Formula *
[4]T[6]CBA
SiIVAlVIAlTiFe3+MgFe2+MnCaNaNaK
4605-416.031.970.090.611.082.900.310.021.930.070.600.26
4605-1516.002.000.150.561.152.880.260.011.970.030.640.22
4605-1916.002.000.180.591.012.660.560.011.940.060.650.12
4605-2315.992.010.170.591.042.780.420.011.950.050.690.13
26.011.990.200.630.892.740.530.021.960.040.680.14
36.151.850.320.380.863.110.320.011.960.040.790.04
46.101.900.240.480.922.970.370.011.970.030.760.11
4605-1316.061.940.190.540.832.970.460.011.920.080.590.23
26.201.800.260.570.452.760.940.011.950.050.620.14
4605-2216.061.940.090.610.882.950.470.011.890.110.570.21
1605-4316.161.840.140.510.752.810.780.021.850.150.580.13
26.101.900.170.530.792.770.740.021.860.140.610.14
36.121.880.080.510.942.860.600.031.850.150.590.13
4605-116.101.900.210.500.832.950.510.021.920.080.690.14
4605-1216.031.970.220.451.063.020.240.011.930.070.740.07
1605-516.091.910.090.570.962.910.470.011.900.100.550.20
1605-716.131.870.160.470.773.020.580.011.880.120.510.22
26.161.840.110.560.902.830.580.021.940.060.570.22
1605-1616.241.760.190.490.602.810.900.021.910.090.530.21
26.201.800.110.540.742.870.750.011.900.100.500.25
1605-216.171.830.130.540.682.900.750.021.870.130.520.22
1605-416.221.780.140.480.762.860.750.021.890.110.560.23
26.291.710.190.460.592.840.910.011.910.090.530.20
1605-916.321.680.210.380.543.170.680.011.910.090.530.19
1605-2016.181.820.100.510.802.860.730.021.890.110.480.23
26.201.800.130.520.632.820.900.011.860.140.480.23
1605-316.121.880.130.570.842.860.610.011.890.110.580.20
1605-416.201.800.180.440.702.940.730.021.870.130.540.18
26.241.760.140.460.592.970.830.011.830.170.510.17
1605-2016.331.670.200.370.483.030.920.011.850.150.470.16
26.291.710.190.380.573.020.850.011.860.140.480.17
1605-316.201.800.140.460.462.990.970.011.710.290.350.18
1605-3916.091.910.050.550.862.880.660.021.800.200.550.12
1605-4116.101.900.110.530.822.830.710.021.830.170.560.12
26.141.860.130.510.722.780.840.031.820.180.570.13
36.211.790.150.530.632.820.860.021.820.180.580.13
46.151.850.090.510.812.820.770.021.820.180.600.12
56.141.860.110.550.472.801.070.021.730.270.470.12
1605-4216.141.860.160.510.772.820.730.031.830.170.590.14
26.131.870.120.500.912.850.620.021.870.130.610.16
36.121.880.090.510.982.880.540.021.890.110.610.14
1605-4416.081.920.070.491.052.880.510.021.850.150.600.13
26.101.900.110.510.922.860.600.021.860.140.610.13
1605-4516.161.840.110.550.682.780.880.031.830.170.560.12
1605-4526.121.880.070.550.832.820.730.021.830.170.580.12
4605-716.171.830.130.530.432.741.180.021.620.380.330.16
26.161.840.160.520.542.751.040.021.790.210.490.15
1605-4316.161.840.130.520.752.820.780.021.860.140.590.12
26.161.840.120.530.742.840.770.021.850.150.620.14
1605-4416.161.840.130.500.702.820.840.021.840.160.550.15
26.131.870.110.540.712.780.870.021.820.180.560.13
1605-3716.151.850.110.530.762.840.750.031.850.150.570.13
26.161.840.120.470.832.940.620.021.850.150.520.17
36.161.840.070.510.802.880.750.021.820.180.520.16
46.161.840.080.500.842.920.650.021.850.150.530.16
56.051.950.020.511.192.860.420.021.850.150.540.14
1605-3916.151.850.020.510.912.930.640.021.800.200.490.15
26.141.860.160.450.842.870.680.021.850.150.590.14
36.231.770.150.500.712.780.850.021.830.170.570.15
46.201.800.170.410.752.940.710.031.860.140.510.20
1605-4016.161.840.090.480.872.930.630.021.830.170.540.16
1605-4616.171.830.150.470.952.870.550.021.950.050.650.15
1605-3716.191.810.070.520.722.850.840.031.820.180.550.13
1605-4516.221.780.080.440.792.910.780.021.830.170.520.14
26.241.760.110.430.722.890.850.021.830.170.520.15
1605-1316.241.760.080.460.712.950.790.031.830.170.520.14
26.241.760.130.440.782.890.750.021.880.120.540.15
4605-2016.351.650.240.310.682.970.780.021.910.090.600.09
26.381.620.190.300.483.070.970.011.760.240.410.10
* The cation population and Fe3+ were estimated using the “Amp-TB2” calculation scheme [14].
Table A3. Cation ratios and physicochemical parameters of amphibole calculated from major element compositions.
Table A3. Cation ratios and physicochemical parameters of amphibole calculated from major element compositions.
SamplePointCation RatioParameters
Al/SiMg/(Fe3+ + Fe2+ + VIAl)Mg/(Mg + Fe2+)AlTSi/(Si + Ti + Al)T *PΔNNOH2OmeltPutirka–T *Putirka–T (P) *
4605-410.341.950.732.070.691000.83398.492.714.271025.311020.90
4605-1510.361.840.722.160.69989.14414.112.314.871021.351016.03
4605-1910.361.520.682.180.68995.09396.52−0.584.701017.621011.40
4605-2310.361.710.702.180.681005.47406.760.574.901029.721023.52
20.361.680.702.190.681013.11416.201.775.671032.061027.15
30.352.070.792.170.71972.89377.510.624.481012.121003.25
40.351.930.752.140.70983.35377.502.283.561018.371010.87
4605-1310.351.990.742.140.69988.30423.482.194.051018.381013.97
20.331.670.712.060.70969.12330.072.844.08996.64992.56
4605-2210.342.050.732.030.701003.66382.280.833.721025.201020.69
1605-4310.321.680.701.980.71966.97308.58−0.554.82992.05984.85
20.341.630.692.070.70983.75368.35−0.487.111003.03996.23
30.321.760.701.970.71979.41310.29−0.415.10997.91989.87
4605-110.351.900.732.110.70986.53383.110.916.111014.811007.98
4605-1210.361.980.752.190.70991.36401.920.246.321024.801016.21
1605-510.331.920.732.000.70977.82328.680.363.671007.331001.84
1605-710.332.000.742.030.71962.58375.571.744.38993.80989.01
20.321.780.711.950.71955.37291.282.234.82989.69984.55
1605-1610.311.670.701.950.72934.15291.381.475.01969.67964.58
20.311.800.711.910.72944.80287.571.544.11979.84975.12
1605-210.321.860.721.960.71961.54311.970.663.79992.12986.93
1605-410.311.720.711.920.72935.06281.781.294.32974.64968.06
20.301.680.711.900.73920.84269.811.285.07960.35954.93
1605-910.302.210.771.900.73923.31285.502.195.05965.76960.55
1605-2010.311.760.701.920.72943.16289.610.924.01975.63970.42
20.311.700.691.930.72941.43292.370.724.06975.36970.20
1605-310.331.820.722.010.70975.54324.761.014.091006.061000.15
1605-410.321.830.731.990.72946.72316.851.044.57979.42973.05
20.311.900.731.910.72946.04290.430.804.22978.89972.41
1605-2010.301.890.741.870.74911.58272.201.445.21949.22943.54
1605-2020.301.880.741.900.73916.71280.871.495.19955.41949.34
1605-310.311.900.731.950.72963.36360.610.733.90980.64975.76
1605-3910.321.830.191.960.71993.50318.35−0.813.321008.451000.79
1605-4110.331.720.202.010.71983.57330.78−0.773.761001.54994.27
20.321.640.231.990.71979.52323.34−0.793.74995.34987.80
30.311.710.241.940.71977.24304.59−1.063.63993.66986.85
40.321.680.221.940.72969.95292.67−0.613.85993.90985.27
50.321.690.281.980.71994.35358.11−1.323.40999.86993.84
1605-4210.331.700.212.020.71987.17362.79−0.513.83999.49992.72
20.321.740.181.990.71965.04310.230.294.09995.89987.93
30.321.780.161.980.71970.22305.79−0.074.02997.03989.16
1605-4410.331.760.151.990.71973.05313.730.064.02999.93990.94
20.331.750.172.010.71975.59320.15−0.194.021002.44994.18
1605-4510.321.670.241.950.71980.02304.06−1.193.55994.98988.02
20.321.740.211.960.71986.25308.54−0.933.481003.18995.46
4605-710.321.580.301.960.71985.04371.71−0.983.25986.01980.65
20.321.590.272.000.71970.13328.52−0.793.83988.96982.57
1605-4310.321.700.221.970.71972.24304.93−0.723.90993.94986.57
20.321.740.211.950.71974.69299.38−0.543.76998.15990.40
1605-4410.321.680.231.980.71964.25311.94−0.193.99988.33981.34
20.321.650.241.980.71979.30314.84−0.913.65997.83990.51
1605-3710.321.750.211.960.71977.13307.89−0.673.68994.88988.03
20.321.870.171.960.72962.12317.160.704.00986.68980.18
30.311.780.211.910.72968.64295.41−0.043.59989.18981.88
40.311.860.181.920.72966.17299.380.193.76988.51981.76
50.331.750.131.980.71978.28317.16−0.053.72998.59990.48
1605-3910.301.870.181.870.72965.85281.580.033.57987.99980.51
20.331.710.192.010.71955.67316.990.484.55988.29979.90
30.311.630.231.920.72953.83283.83−0.364.02980.25972.89
40.321.800.201.970.72942.92317.971.494.51972.28965.62
1605-4010.311.840.181.940.72963.00302.760.433.92988.21980.68
1605-4610.321.740.161.980.72953.96296.150.534.56985.35977.78
1605-3710.301.760.231.880.72972.42281.90−0.733.48988.32980.95
1605-4510.301.770.211.860.73942.26269.600.434.20969.90962.07
1605-4520.301.730.231.860.73937.86269.090.504.27964.63956.88
1605-1310.301.870.211.840.73953.91272.210.113.85973.94966.88
20.301.750.211.880.73934.09268.810.514.49965.18958.35
4605-2010.301.750.211.890.74905.90256.070.875.99943.80935.80
20.281.880.241.810.75901.32254.101.075.51935.52928.43
* T: The amphibole crystallization temperature was calculated using the thermometer formula proposed by [14]; Putirka–T: The amphibole crystallization temperature was calculated using the thermometer formula (8) proposed by [7]; Putirka–T (P): The calculation of amphibole crystallization temperature, based on [7] thermometer Formula (9), requires the input of a known pressure.

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Figure 1. (a) Regional Geology Map of the Tarim Craton and Surrounding Areas; (b) Tectonic subdivision map of the Tarim Block and the distribution of major Ultramafic–Mafic intrusions (Modified from [31]).
Figure 1. (a) Regional Geology Map of the Tarim Craton and Surrounding Areas; (b) Tectonic subdivision map of the Tarim Block and the distribution of major Ultramafic–Mafic intrusions (Modified from [31]).
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Figure 2. Field photographs of the Xiaohaizi pluton. (a) Dark gray, fine- to medium-grained gabbro; (b) Contact between a dolerite dyke and gabbro; (c) Syenite hill in the northern part of the pluton; (d) Strongly mineralized olivine pyroxenite outcrop.
Figure 2. Field photographs of the Xiaohaizi pluton. (a) Dark gray, fine- to medium-grained gabbro; (b) Contact between a dolerite dyke and gabbro; (c) Syenite hill in the northern part of the pluton; (d) Strongly mineralized olivine pyroxenite outcrop.
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Figure 3. Geological map of the Xiaohaizi intrusion (Modified from [34]). 1. Diorite; 2. Lower Carboniferous Bachu Formation quartz sandstone; 3. Olivine gabbro; 4. Pyroxenite; 5. Olivine pyroxenite; 6. Syenite; 7. K-feldspar granite; 8. Positive magnetic anomaly contour; 9. Magnetic zero contour; 10. Negative magnetic anomaly contour; 11. Inferred magnetic anomaly; 12. Drill hole locations and numbers; 13. Quaternary sediments; 14. Reservoir.
Figure 3. Geological map of the Xiaohaizi intrusion (Modified from [34]). 1. Diorite; 2. Lower Carboniferous Bachu Formation quartz sandstone; 3. Olivine gabbro; 4. Pyroxenite; 5. Olivine pyroxenite; 6. Syenite; 7. K-feldspar granite; 8. Positive magnetic anomaly contour; 9. Magnetic zero contour; 10. Negative magnetic anomaly contour; 11. Inferred magnetic anomaly; 12. Drill hole locations and numbers; 13. Quaternary sediments; 14. Reservoir.
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Figure 4. Photographs of drill core samples from the Xiaohaizi UM–M intrusion. (a) Core of olivine pyroxenite; (b) Cross-section of gabbro core; (c) Split core sample of olivine gabbro.
Figure 4. Photographs of drill core samples from the Xiaohaizi UM–M intrusion. (a) Core of olivine pyroxenite; (b) Cross-section of gabbro core; (c) Split core sample of olivine gabbro.
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Figure 5. Microscopic textures of amphibole in the Xiaohaizi UM–M intrusion. (a) Inclusion-type amphibole within clinopyroxene in pyroxenitic rock (XPL); (b) Amphibole coexisting with silicate minerals in pyroxenitic rock (XPL); (c–e) Amphibole intergrown with Fe–Ti oxides in pyroxenitic rock (reflected light, PPL, XPL); (f) Reaction-rim amphibole around clinopyroxene in olivine gabbro (PPL); (g,h) Zoned plagioclase in olivine gabbro (XPL); (i,j) Phenocrystic amphibole in gabbroic rock (XPL); (k,l) Symplectic intergrowths in gabbroic rock (SEM). Abbreviations: Pl: plagioclase; Ox: Fe–Ti oxides; Amp: amphibole; Cpx: clinopyroxene; Ol: olivine; Ap: apatite; Sul: sulfide; Mt: magnetite; Ilm: ilmenite.
Figure 5. Microscopic textures of amphibole in the Xiaohaizi UM–M intrusion. (a) Inclusion-type amphibole within clinopyroxene in pyroxenitic rock (XPL); (b) Amphibole coexisting with silicate minerals in pyroxenitic rock (XPL); (c–e) Amphibole intergrown with Fe–Ti oxides in pyroxenitic rock (reflected light, PPL, XPL); (f) Reaction-rim amphibole around clinopyroxene in olivine gabbro (PPL); (g,h) Zoned plagioclase in olivine gabbro (XPL); (i,j) Phenocrystic amphibole in gabbroic rock (XPL); (k,l) Symplectic intergrowths in gabbroic rock (SEM). Abbreviations: Pl: plagioclase; Ox: Fe–Ti oxides; Amp: amphibole; Cpx: clinopyroxene; Ol: olivine; Ap: apatite; Sul: sulfide; Mt: magnetite; Ilm: ilmenite.
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Figure 6. Lithological borehole logs of the Xiaohaizi UM–M intrusion. 1. Quaternary deposits; 2. Olivine clinopyroxenite; 3. Olivine gabbro; 4. Gabbro; 5. Plagiocrite; 6. Plagioclase-bearing olivine pyroxenite. Late-stage dikes are not shown due to their small thickness.
Figure 6. Lithological borehole logs of the Xiaohaizi UM–M intrusion. 1. Quaternary deposits; 2. Olivine clinopyroxenite; 3. Olivine gabbro; 4. Gabbro; 5. Plagiocrite; 6. Plagioclase-bearing olivine pyroxenite. Late-stage dikes are not shown due to their small thickness.
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Figure 7. Classification diagram of amphibole (modified after [3]).
Figure 7. Classification diagram of amphibole (modified after [3]).
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Figure 8. Al2O3–TiO2 correlation diagram of amphibole (modified after [40]).
Figure 8. Al2O3–TiO2 correlation diagram of amphibole (modified after [40]).
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Figure 9. TiO2–K2O correlation diagram of amphibole (Modified after [41]).
Figure 9. TiO2–K2O correlation diagram of amphibole (Modified after [41]).
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Figure 10. TiO2–IVAl correlation diagram of amphibole (Modified after [41]).
Figure 10. TiO2–IVAl correlation diagram of amphibole (Modified after [41]).
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Figure 11. Mg/(Fe3+ + Fe2+ + VIAl)–Al/Si diagram for amphibole, showing the compositional field of amphiboles from intermediate to mafic magmas (Al/Si = 0.10–0.67; Mg/(Fe3+ + Fe2+ + VIAl) = 1.50–2.00; modified after [42]).
Figure 11. Mg/(Fe3+ + Fe2+ + VIAl)–Al/Si diagram for amphibole, showing the compositional field of amphiboles from intermediate to mafic magmas (Al/Si = 0.10–0.67; Mg/(Fe3+ + Fe2+ + VIAl) = 1.50–2.00; modified after [42]).
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Figure 12. Temperature–Pressure (T–P) correlation diagram of amphibole. (a) Amphiboles from pyroxenitic rocks showing inclusion textures (i), co-crystallization textures with silicates (ii), and co-crystallization textures with Fe–Ti oxides (iv). (b) Amphiboles from gabbroic rock displaying phenocrystic textures (v) and co-crystallization textures with Fe–Ti oxides (iv). (Modified after [14]).
Figure 12. Temperature–Pressure (T–P) correlation diagram of amphibole. (a) Amphiboles from pyroxenitic rocks showing inclusion textures (i), co-crystallization textures with silicates (ii), and co-crystallization textures with Fe–Ti oxides (iv). (b) Amphiboles from gabbroic rock displaying phenocrystic textures (v) and co-crystallization textures with Fe–Ti oxides (iv). (Modified after [14]).
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Figure 13. Temperature–Pressure (T–P) grid diagram for amphibole crystallization (Modified after [45]). The dashed lines represent isopleths of Al2O3 (wt.%) and TiO2 (wt.%) contents in amphibole, with the numbers indicating their respective concentrations. The shaded field marks the estimated P–T conditions for amphiboles from the Xiaohaizi UM–M intrusion.
Figure 13. Temperature–Pressure (T–P) grid diagram for amphibole crystallization (Modified after [45]). The dashed lines represent isopleths of Al2O3 (wt.%) and TiO2 (wt.%) contents in amphibole, with the numbers indicating their respective concentrations. The shaded field marks the estimated P–T conditions for amphiboles from the Xiaohaizi UM–M intrusion.
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Figure 14. Temperature–log(ƒO2) correlation diagram of amphibole (Modified after [14]).
Figure 14. Temperature–log(ƒO2) correlation diagram of amphibole (Modified after [14]).
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Figure 15. H2Omelt–Temperature correlation diagram of amphibole (Modified after [14]).
Figure 15. H2Omelt–Temperature correlation diagram of amphibole (Modified after [14]).
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Figure 16. Four-Stage Evolutionary Framework of the Ultramafic–Mafic Intrusion within the Mantle Plume–Lithosphere System.
Figure 16. Four-Stage Evolutionary Framework of the Ultramafic–Mafic Intrusion within the Mantle Plume–Lithosphere System.
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Table 1. Physicochemical conditions of amphibole crystallization in the Xiaohaizi UM–M intrusion.
Table 1. Physicochemical conditions of amphibole crystallization in the Xiaohaizi UM–M intrusion.
Rock TypeAmp TypeT (°C)P (MPa)ƒO2 (ΔNNO)H2Omelt (wt.%)
Pyroxenitic rocksi1013–973416–378+2.7–−0.65.7–4.3
ii1004–967424–330+2.8–−0.64.8–3.6
iv991–934402–288+2.2–−0.57.1–3.7
Gabbroic rocksv994–956372–282+0.7–−1.34.6–3.3
iv976–901360–254+2.2–−0.76.0–3.5
Table 2. Summary of magmatic evolution stages and corresponding physicochemical parameters for the Xiaohaizi UM–M intrusion.
Table 2. Summary of magmatic evolution stages and corresponding physicochemical parameters for the Xiaohaizi UM–M intrusion.
Parameters
Stage		IIIIIIIV
T (°C)1013–9881004–967994–955978–901
P (MPa)424–397423–309371–281329–254
ƒO2 (ΔNNO)+2.7–−0.6+2.8–−0.6+0.7–−1.3+2.2–−0.7
H2Omelt (wt.%)5.7–4.37.1–3.64.6–3.36.0–3.5
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Liu, D.; Duan, S.; Chen, M.; Wang, W.; Yin, J.; Maimaiti, M. Amphibole-Based Constraints on Magmatic Evolution and Fe–Ti Oxide Enrichment in the Xiaohaizi Ultramafic–Mafic Intrusion, Bachu, Xinjiang, China. Minerals 2025, 15, 1275. https://doi.org/10.3390/min15121275

AMA Style

Liu D, Duan S, Chen M, Wang W, Yin J, Maimaiti M. Amphibole-Based Constraints on Magmatic Evolution and Fe–Ti Oxide Enrichment in the Xiaohaizi Ultramafic–Mafic Intrusion, Bachu, Xinjiang, China. Minerals. 2025; 15(12):1275. https://doi.org/10.3390/min15121275

Chicago/Turabian Style

Liu, Donghui, Shigang Duan, Maohong Chen, Weicheng Wang, Jinmao Yin, and Maihemuti Maimaiti. 2025. "Amphibole-Based Constraints on Magmatic Evolution and Fe–Ti Oxide Enrichment in the Xiaohaizi Ultramafic–Mafic Intrusion, Bachu, Xinjiang, China" Minerals 15, no. 12: 1275. https://doi.org/10.3390/min15121275

APA Style

Liu, D., Duan, S., Chen, M., Wang, W., Yin, J., & Maimaiti, M. (2025). Amphibole-Based Constraints on Magmatic Evolution and Fe–Ti Oxide Enrichment in the Xiaohaizi Ultramafic–Mafic Intrusion, Bachu, Xinjiang, China. Minerals, 15(12), 1275. https://doi.org/10.3390/min15121275

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