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Article

Petrogenesis of Intermediate Rocks in Tethyan Himalaya Igneous Province (SE Tibet): The Role of Source Composition and Fractional Crystallization

by
Shengsheng Chen
* and
Haonan Jie
State Key Laboratory of Geological Processes and Mineral Resources, Frontiers Science Center for Deep-Time Digital Earth, School of Earth Science and Resources, China University of Geosciences, Beijing 100083, China
*
Author to whom correspondence should be addressed.
Minerals 2025, 15(12), 1251; https://doi.org/10.3390/min15121251
Submission received: 29 October 2025 / Revised: 21 November 2025 / Accepted: 26 November 2025 / Published: 26 November 2025
(This article belongs to the Special Issue Large Igneous Provinces: Petrogenesis, Mineralization, and Environmental Impact)
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Abstract

The origin of intermediate rocks within large igneous provinces (LIPs), which often exhibit a bimodal compositional distribution, remains poorly understood. To investigate the petrogenesis of such intermediate magmas within the Early Cretaceous Tethyan Himalaya igneous province in Tibet, we present a comprehensive study of zircon U–Pb geochronology, whole-rock geochemistry, and Nd isotopes for the tonalites from the Zhegucuo area. Zircon U–Pb dating yields a crystallization age of 130.81 ± 0.55 Ma. The rocks exhibit low Mg# and compatible element contents, indicating significant fractional crystallization of ferromagnesian minerals, plagioclase, and accessory minerals. Their homogeneous, near-chondritic εNd(t) values (−0.34 to +0.01) preclude significant crustal contamination. Based on field relationships, geochemistry, and isotopic evidence, we conclude that the Zhegucuo tonalites were generated by extensive fractional crystallization of basaltic magmas. FC3MS and FCKANTMS systematics reveal a peridotitic component in the mantle source of the Zhegucuo mafic rocks. The exceptionally high values of these proxies of the Zhegucuo tonalites are attributed to extensive fractional crystallization of evolved magmas.

1. Introduction

Since 120 Ma, the Kerguelen mantle plume has been characterized by prolonged, large-scale magmatism, producing a wide spectrum of igneous rocks (Figure 1a). This activity resulted in the formation of the Kerguelen large igneous province (LIP), which displays both continental and oceanic affinities and contributed significantly to the separation of the Indian, Australian, and Antarctic plates [1,2]. Due to the subsequent divergence of these plates, magmatic rocks originally associated with the Kerguelen plume have been dispersed across multiple tectonic domains [3,4,5,6]. Continental expressions of plume-related magmatism include the Rajmahal–Bengal–Sylhet volcanic systems in northeastern India, which record plume–lithosphere interactions during the breakup of the Indian plate [7]. Oceanic counterparts comprise the northern, central, and southern Kerguelen Plateau, together with the Broken Ridge (Figure 1a).
In addition, several older (147–130 Ma) and relatively localized magmatic provinces are distributed across the eastern Gondwana region, linked to the early tectonic migration of the Indian, Australian, and Antarctic plates. These include the Comei LIP in the Tethyan Himalaya (southern Tibet) and the Bunbury Basalt in southwestern Australia [4,6,8,9]. It is generally accepted that the petrogenesis of the Comei LIP is related to a mantle plume on the basis of geochemical and Sr–Nd–Hf–Pb–Os isotope characters similar to the Kerguelen mantle plume head [10,11,12,13,14,15,16].
The Tethyan Himalaya, located along the northern margin of the Indian continent, serves as a key area for understanding the role of the Kerguelen plume in the breakup of eastern Gondwana, as recorded by widespread Early Cretaceous magmatism (Figure 1b). Zhu et al. (2009) [4] initially identified an extensive Early Cretaceous Comei LIP, composed of gabbroic intrusions, basaltic lavas, mafic sills and dikes, minor layered ultramafic intrusions, and silicic volcanic rocks (Figure 1c). Chen et al. (2018) [5] proposed a new name “Tethyan Himalaya igneous province” (THIP) to recognize early Cretaceous magmatism that appeared across the overall Tethyan Himalaya [4]. While previous investigations have predominantly focused on the mafic and granitic endmembers [10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26], intermediate rocks—such as tonalites—have received considerably less attention, leaving a notable gap in the understanding of the region’s magmatic evolution.
However, the petrogenesis of these intermediate rocks remains contentious. Although fractional crystallization has been proposed as a key mechanism for their formation [27], critical aspects regarding the nature of their source and the composition of the primary melts remain poorly constrained. Resolving the source characteristics and material contributions is therefore essential for constructing a coherent petrogenetic model and for understanding the role of mantle dynamics in the generation of the Comei LIP.
In this paper, zircon U–Pb geochronology, along with whole-rock compositions and Nd isotopic results, are reported for the tonalites from the Zhegucuo area in the eastern Tethyan Himalaya. These results provide critical constraints on the petrogenesis of intermediate, low-volume magmatic rocks within LIPs.

2. Geological Setting

The Himalayan orogen, situated south of the India–Yarlung Zangbo Suture Zone (IYSZ)—a deformed relic of the Neotethyan Ocean marking the Indo-Asian collision—comprises upper crustal rocks from northern Greater India that were subducted beneath the Asian Lhasa Terrane. This orogen is tectonically subdivided into the Tethyan, Greater, and Lesser Himalaya, bounded by the South Tibetan Detachment System (STDS) and the Main Central Thrust (MCT) [28].
The Tethyan Himalaya, which developed as a passive continental margin on the northern edge of Greater India from the Triassic to the Cretaceous [28], is underlain by a crystalline core that suggests its origin as a rifted micro-continent [29]. It is characterized by Mesozoic isolation, block-in-matrix structures, and mélange and is composed of Paleozoic–Cenozoic carbonates, clastic rocks, and Paleozoic–Mesozoic magmatic suites [30,31]. This tectonic setting is recorded by a sedimentary sequence dominated by sandstone, sandy shale, argillaceous limestone, and slate, identified across multiple stratigraphic units. These include the Upper Triassic Songre, Jiangxiong, Nieru, and Qulonggongba formations, the Lower Jurassic–Upper Cretaceous Sangxiu Formation, the Lower Jurassic Ridang Formation, and the Lower Cretaceous Lakang and Zhela formation. Magmatism in this region occurred in four principal episodes: the Early–Middle Permian (290–264 Ma), Middle–Late Triassic (230–201 Ma), Early–Middle Cretaceous (147–117 Ma), and Eocene (~45 Ma) [25]. Of these, the Early–Middle Cretaceous magmatic phase is the most voluminous, with a pronounced concentration of eruption ages around 147–117 Ma.
Latest Jurassic to Early Cretaceous igneous rocks are predominantly exposed within the Zhela, Langkang, Weimei, and Sangxiu Formations. The Weimei Formation comprises a lower section of interbedded quartz sandstone, silty slate, and silty metamorphic rocks, and an upper section of sericite silty slate with thin intercalations of silty sandstone and arenaceous limestone. Zircon U–Pb geochronology constrains the timing of basaltic lavas within this formation to 138–130 Ma [32,33]. Similarly, the Sangxiu Formation is divided into a lower part of silty slate interbedded with sandstone, basalt, and conglomerate, and an upper part of sericite silty slate interbedded with sandstone, basalt, and rhyolite. Basalt and rhyolite that display similar age and are associated with this formation yield ages between 141 and 124 Ma [4,12,34,35]. The Jiabula Formation consists mainly of quartz sandstone, gray siltstone, bioclastic siliceous rock, shale, and minor basalt interlayers. For the Lakang Formation, the lower succession is characterized by massive basalts, basaltic andesites, and subordinate andesites interbedded with sedimentary rocks, whereas the upper part is composed of silty slate, siltstone, and limestone. Basaltic andesites from this formation have been dated to approximately 147 Ma [36]. In the Taga area, the Zhela Formation—unconformably overlain by the Weimei Formation and underlain by the Lure Formation—is composed of basalt, dacite, sandstone, slate, and limestone. Recent zircon U–Pb geochronology indicates that basalts formed between 147 and 135 Ma [32,37], a temporal range further supported by the youngest detrital zircon ages of ca. 138–127 Ma [38]. This evidence collectively confirms a Late Jurassic to Early Cretaceous, rather than Middle Jurassic, age for the Zhela Formation volcanism.
Numerous E–W trending sills, dykes, and sill-like layered intrusions are emplaced within the Lower Cretaceous Sangxiu Formation and the Late Jurassic to Early Cretaceous Zhela Formation across the area extending from Zhegucuo to Comei [27]. Tonalitic dykes are observed to intrude and cross-cut coevally emplaced dioritic and dolerite sills, with the former typically exhibiting a coarser-grained texture [27].

3. Analytical Methods

3.1. Zircon U-Pb Analysis

The analyses for zircon U–Pb dating were conducted at the Tianjing Geological Survey Center, China. The analyses were performed using an LA-ICP-MS system comprising a RESOlution LR 193 nm laser ablation unit (ASI, Sydney, Australia) coupled to an Agilent 7900 ICP-MS (Agilent Technologies, Santa Clara, CA, USA). Laser sampling was performed using a New Wave and ATL 193 nm ArF excimer laser ablation system, with a short pulse duration (<4 ns) and a range of beam sizes. The spot size for the analysis was set to 35 μm for detailed analytical procedures). Plesovice zircon were adopted as external standards for the matrix-matched calibration of U–Pb dating. Plesovice, were analyzed followed by five-ten sample analyses. Our analysis of the Plešovice zircon standard yielded ages of 336–337 Ma (Table S1), which is consistent with its recommended value of 337.2 ± 0.37 Ma [39]. Off-line isotope ratios were calculated by GLITTER_Ver4.0 [40]. Common Pb correction and ages of the samples were calibrated and calculated using ComPbCorr # 3.17 [41]; U–Pb concordia diagrams, weighted mean calculations and probability density plots of U–Pb ages were made using IsoplotR (version 6.8) online [42].

3.2. Whole-Rock Geochemical Analysis

We selected relatively fresh samples or the central portions of samples for whole-rock compositional analyses. The selected samples were crushed to ~200 mesh. Bulk-rock major and trace element analyses were performed at the Institute of Geology and Geophysics, Chinese Academy of Sciences (IGGCAS), Beijing, employing X-ray fluorescence (XRF) spectrometry and inductively coupled plasma mass spectrometry (ICP–MS), respectively. Approximately 0.5 g of each sample powder was blended with 5 g Li2B4O7 in a CLAISSEFLUXERVI fusion furnace (Fives Group, Quebec City, QC, Canada) at 1050 °C for 20 min to make glass beads. The beads were then analyzed on an AXIOS mineral spectrometer. The analytical uncertainties for major element compositions were generally better than 2%. A 50 mg aliquot of each whole-rock powder was digested with a HF-HNO3 mixture in a Teflon vessel, and then heated at 170 °C for 7 days to drive the remaining acid out of the vessel and change the residue to solution. The relative uncertainty of bulk-rock trace element compositions was better than 5%.

3.3. Nd Isotope Analysis

At the China University of Geosciences, whole-rock Nd analyses were performed on a Nu Plasma II multi-collector (Nu Instruments Ltd. (an Oxford Instruments company), Wrexham, UK) inductively coupled plasma mass spectrometer (MC-ICP-MS). Samples for Nd isotopic analyses were dissolved in an acidic mixture of HF + HNO3 + HClO4 in Teflon bombs and separated by extraction chromatography techniques with procedures. Measured 143Nd/144Nd ratios were normalized to 146Nd/144Nd = 0.7219 for mass fractionation corrections. The isotopic analyses were conducted utilizing a Thermal Ionization Mass Spectrometer (TIMS) (Thermo Fisher Scientific, Bremen, Germany). The JNdi-1 international standard samples were used to assess instrument stability during data collection. The 143Nd/144Nd ratios of the JNDi-1 Nd standard solutions were 0.512109 ± 10 (2σ). Our measured values for the JNDi-1 standard agree with the published reference value (143Nd/144Nd = 0.512115) [43].

4. Results

Our study centers on the Zhegucuo area (central THIP; Figure 2), where an intrusive suite is exposed and local tonalitic dykes intrude the Zhela Formation (Figure 3). Field observation indicates that these dykes exhibit an E–W orientation, with typical dimensions of several kilometers in length and widths ranging from approximately 10 to 100 m (Figure 2). The field photograph (Figure 3b) reveals textural variations within the dykes and the presence of key features including chilled margins at the contact with the country rock, which provide crucial constraints on their intrusion mechanisms.

4.1. Petrography

Field observation indicates that these dykes exhibit an E–W orientation, with typical dimensions of several kilometers in length and widths ranging from approximately 10 to 100 m (Figure 2). The field photograph (Figure 3b) reveals textural variations within the dykes and the presence of key features, such as chilled margins in contact with the country rock, providing crucial constraints on their emplacement mechanisms. The mineral composition of tonalite is characterized by a predominance of plagioclase (40–50 vol%) and quartz (25–30 vol%), with subordinate amounts of alkali feldspar (Orthoclase) and amphibole (each 5–10 vol%) (Table 1; Figure 3). Amphiboles are predominantly euhedral and fine-grained (<0.5 mm). The plagioclase occurs as medium- to fine-grained, subhedral tabular crystals. In contrast, the alkali feldspar population is medium- to fine-grained (<2 mm) and subhedral-anhedral prism. Quartz typically forms xenomorphic crystals with grain sizes under 2 mm. Accessory minerals commonly include Fe–Ti oxides, apatite, and titanite. In addition, samples ZG01-2 and ZG02-1 to -4 contain more amphibole and opaque Fe-Ti oxide minerals compared to the others (Table 1).
A total of nine representative samples from this dyke swarm were described in this study. All nine samples were selected for whole-rock geochemical analysis, among which three were used for Nd isotope analysis, and one was utilized for zircon U-Pb dating analysis.

4.2. Zircon U–Pb Ages

To determine the timing of magma crystallization, sample ZG02-2 was analyzed for zircon U–Pb dating via LA–ICP–MS. The analytical results are provided in Table S1 and illustrated in Figure 4. Zircons extracted from the tonalite typically range from 40 to 110 μm in length, with length-to-width ratios averaging 1.6:1 and Th/U values between 8.3 and 19.4. Two distinct cathodoluminescence characteristics are observed: most zircons display an oscillatory zoning pattern defined by broad dark and narrow bright zones, whereas the remainder shows a relatively homogeneous structure (Figure S1). The above characters are suggestive of a texture characteristic of magmatic growth [44]. Analyses with concordance levels < 90% or >110% were excluded from the final dataset. Based on 10 spot analyses, the weighted mean 206Pb/238U age of the zircons is 130.81 ± 0.55 Ma (MSWD = 1.5; Figure 4). Previous studies have suggested that some cores of the crystals were interpreted as inherited xenocrysts [6], this study focused on the magmatic rims to reliably constrain the crystallization age. Given that the analytical spots were positioned on zircon rims, the calculated ages are interpreted to record the timing of magma crystallization. This age corresponds to the emplacement of the igneous rocks. It is comparable to that of a gabbro (132.0 ± 1.0 Ma) and a pyroxenite (130 ± 2.0 Ma) intruded into the Middle Jurassic strata on the southern of Zhegucuo [4], a tonalite sample (132.9 ± 1.3 Ma) and a diorite sample (133.4 ± 1.6 Ma) from northwest of Zhegucuo [27] and is also broadly consistent with the established period of magmatic activity in the eastern THIP at ca. 147–117 Ma [6].

4.3. Major- and Trace-Element Geochemistry

The geochemical results for the Zhegucuo magmatic rocks are listed in Table S2. Compositionally, the Zhegucuo tonalites have SiO2 concentrations between 55.03 and 62.68 wt%. They fall within the subalkaline series in the SiO2 vs. Na2O + K2O diagram (Figure 5a) and are predominantly classified as trachyandesites in the Nb/Y-Zr/TiO2 diagram [45] (Figure 5a). According to the nomenclature of Foley et al. (1987) [46], these rocks are high-K rocks (Figure 5c).
The classification based on the magnesium versus iron content [47] reveals that the samples are not uniform. Samples ZG01-1, ZG02-5, -6, and -7 have Fe-number < 0.8, whereas ZG01-2 and ZG02-1 to -4 have Fe-number ≥ 0.8 (Figure 5d). This geochemical classification is petrographically supported by the higher proportion of amphibole and Fe-Ti oxides in the ZG01-2 and ZG02-1 to -4 samples, indicates that the fractional crystallization of ferromagnesian minerals, particularly amphibole and Fe-Ti oxides, was the dominant process driving the magmatic evolution from low Fe-number to high Fe-number. The Aluminium Saturation Index (ASI) shows that, with the exception of ZG02-1 which is peraluminous, all other samples are metaluminous—a characteristic often associated with I-type granitoids. As illustrated in Supplementary Figure S2, the samples exhibit linear compositional trends that extend continuously from mafic rocks toward the compositional field of tonalites [27], suggesting a potential genetic link via magmatic differentiation.
In the normalized REEs diagram, the Zhegucuo tonalites display significant light to heavy REE fractionation, with (La/Yb)n ratios of 10–12, along with weakly positive Eu anomalies (Eu/Eu* ranging from 1.07 to 1.09) (Figure 6a). Their total REE abundances range from 338.9 to 434.3 ppm, similar to those of other contemporaneous tonalites in Zhegucuo (358–362 ppm) [27]. Figure 6b shows that the samples exhibit coherent, subparallel patterns marked by pronounced enrichments in Th, and Pb, and depletions in Nb, Ta, Sr, and Ti. Additionally, these tonalites contain high field-strength elements (HFSE) at elevated concentrations (Nb = 38–62 ppm), while only moderate enrichment is observed for certain fluid-mobile large-ion lithophile elements (LILE).
Nd isotope results for selected samples are provided in Table S3. Initial Nd isotopic ratios were age-corrected to 130 Ma based on U-Pb zircon dating. The tonalites show a narrow range in (143Nd/144Nd)i values (0.51245–0.51247), corresponding to εNd(t) values of –0.33 to 0.03. This is similar to Zhegucuo dolerites with εNd(t) from −0.2 to 3.0 and other Zhegucuo tonalites with εNd(t) values of 0.03 to 0.41 [27]. Their TDM2 model ages range from 924 to 953 Ma.

5. Discussion

5.1. Fractional Crystallization and Crustal Assimilation

The whole-rock Nd isotopic signatures of the Zhegucuo tonalites are notably homogeneous, with εNd(t) values defining a narrow range of −0.34 to +0.01, consistent with a limited role for crustal contamination. This interpretation is further supported by the lack of systematic covariation between Mg# and εNd(t) values (Figure 7a), which would be expected if significant assimilation of crustal material had occurred. In comparison, the Tethyan Himalayan crust is characterized by substantially more radiogenic Nd compositions, with εNd(t) values between −12 and −13 at 132 Ma [13,20]. The analyzed samples, by contrast, exhibit consistently higher and tightly clustered εNd(t) values, closely resembling those of the Zhegucuo tonalites with εNd(t) values of 0.03 to 0.41 [27]. In general, the absence of isotopic dilution and correlative trends indicative of crustal input suggests that the magmas experienced negligible assimilation during ascent and differentiation. Consequently, the geochemical features of the Zhegucuo tonalites can be reasonably attributed to the nature of their mantle source.
Fractional crystallization is widely recognized as a key process governing elemental variations in igneous rock suites across diverse tectonic settings. The studied tonalites exhibit low magnesium numbers (Mg# = 23–36) along with depleted concentrations of chromium (0.8–16.7 ppm) and nickel (0.2–9.5 ppm), indicating significant magmatic differentiation through fractional crystallization. This continuum from magnesian (e.g., ZG01-1, ZG02-5) to ferroan (e.g., ZG02-1, ZG02-4) compositions closely records the fractional crystallization process. Systematic decreases in MgO, Fe2O3T, and CaO with rising SiO2 contents reflect the extraction of ferromagnesian mineral phases (Figure 7b–d). This interpretation is further reinforced by positive correlations between Cr and Ni values (Figure 7e), confirming effective mafic mineral segregation. Pronounced negative anomalies in Sr and Ba, coupled with an inverse relationship between SiO2 and CaO (Figure 7d), provide additional evidence for plagioclase fractionation. The strong positive correlation between Dy and Er (Figure 7f) exhibits coherent behavior with their established hornblende-melt partition coefficients [49], providing robust evidence for hornblende fractionation. Probably, the linear correlation reflects fractional crystallization in equilibrium (coexistence between mineral phases and magma), contrasting with curvilinear trends, suggesting the removal of minerals from the magmatic system throughout the differentiation process.
Fractionation of apatite is demonstrated by the negative P2O5–SiO2 correlation (Figure 7g), while concomitant decreases in Zr and Zr/Hf ratios attest to concurrent zircon fractionation (Figure 7h). Additionally, the high Zr contents (966–1645 ppm), which correlate positively with SiO2 (Figure 7i), indicate that the parental magma underwent extensive fractional crystallization, in particular a mineral assemblage that does not accommodate zirconium. Zirconium is a high-field-strength element and is highly incompatible in the crystal structures of the common rock-forming minerals (e.g., plagioclase, amphibole, zircon) that crystallize early from the magma. Consequently, when these minerals form, Zr is strongly partitioned into the residual melt.

5.2. Petrogenesis of the Zhegucuo Tonalites

The origin of intermediate rock remains debated, and several models have been proposed: subduction-related processes including slab melting, hydrous melting of mantle wedge peridotite, partial melting of mélange diapirs (composed of MORB, sediments, and mantle wedge), and hybridization of slab and mantle materials via melt–rock reactions [50,51,52,53]; other models involve crystallization of basalt or basaltic andesite magmas in shallow magma chambers [54], anatexis of mafic lower crust [51], mixing of mantle-derived and crust-derived magmas [55,56], and melting of enriched lithospheric mantle [57]. The Tethyan Himalaya was situated on a passive continental margin of East Gondwana during the Cretaceous [27]. This tectonic setting precludes the application of subduction-related models for the genesis of the intermediate rocks in this study.
Although mantle-derived mafic and crustal-derived felsic magmas can mix to generate andesitic compositions, such a process is precluded for the Zhegucuo tonalites. This conclusion is based on their restricted initial εNd(t) values (from −0.34 to 0.01) and coherent, sub-parallel REE and trace element patterns (Figure 6).
Experimental data demonstrate that partial melting of mafic rocks invariably yields melts with low Mg# values (<41), independent of the extent of melting [58]. Despite possessing low MgO, Mg#, Ni, and Cr—features that might suggest an origin from high-degree melting of mafic lower crust—the Zhegucuo tonalites cannot be explained by this model. Furthermore, their composition (high Y contents: 46.9~60.2 ppm) is inconsistent with the distinctive adakitic geochemistry predicted for melts derived from the Tethyan Himalayan mafic lower crust (Figure 8) [59].
Section 5.1 demonstrates that the Zhegucuo tonalitic magma experienced extensive fractional crystallization before intrusion. Two competing petrogenetic scenarios exist for tonalitic suites: (1) differentiation of mantle-derived basaltic magmas, and (2) direct generation from hydrous primitive andesitic melts [60]. Field relations strongly favor the first hypothesis: the areally extensive mafic–ultramafic assemblages exposed across the region imply that voluminous basaltic magmatism accompanied tonalite petrogenesis. Such a spatial association is most readily reconciled with protracted fractional crystallization of a basaltic parent, rather than with discrete andesitic magma batches. Moreover, the samples compose a geochemical continuum from mafic rocks to tonalites [27], as illustrated in Supplementary Figure S2. This trend is most plausibly explained by magmatic differentiation processes. The compositional continuum observed from dolerite to tonalite, in conjunction with their consistent Nd isotopic signatures, supports a derivation from a common parental magma.
Field relationships provide critical evidence for extensive fractional crystallization within the Zhegucuo intrusive suite. In the study, tonalitic dykes are observed to intrude and cross-cut coevally emplaced dioritic and doleritic sills, with the former typically exhibiting a coarser-grained texture [27]. This field relationship is interpreted as the extraction and emplacement of silicic, volatile-rich residual liquids, derived from advanced fractional crystallization, into the cooler, semi-consolidated parts of the same crystallizing mafic-to-intermediate crystal mush. Thus, the spatial, temporal, and genetic association of these coeval tonalitic residua with their more mafic counterparts provides direct field-based support for the petrogenetic model that the tholeiitic intermediate rocks were produced by high-degree fractional crystallization of a common parental magma. Previous studies attribute the origin of the Zhegucuo mafic rocks not to the asthenospheric mantle itself, but to the secondary melting of the lithospheric mantle, which was thermally triggered by the underlying mantle plume [27]. Recently, Wang and Chen (2024) have conclusively shown that the nearby Zhegucuo dolerites (130 Ma) were derived from an enriched lithospheric mantle source [25]. Given the temporal link, we consider this the most plausible source for the parental magmas of the Zhegucuo suite. Consequently, the Zhegucuo tonalites most probably represent residual liquids generated by extensive fractionation of basaltic precursors. Thus, the Zhegucuo tonalites are best explained by extensive fractional crystallization of a basaltic magma derived from a lithospheric mantle source.

5.3. Mantle Sources

Determining the composition of mantle sources is fundamental to unraveling magmatic processes and the genesis of mantle-derived magmas. Such insights are critical for assessing crustal material recycling and mantle metasomatism—key mechanisms driving mantle heterogeneity. Peridotite and pyroxenite represent the two principal candidate lithologies for the sources of mantle-derived mafic rocks [61,62]. Previous studies argue that the Zhegucuo mafic rocks were generated not from the asthenospheric mantle directly, but by the melting of a lithospheric mantle source, which was thermally triggered by a mantle plume [27]. However, the composition of their source remains unclear, posing a major challenge to a full understanding of their petrogenesis.
Decades of experimental petrology on the partial melting of ultramafic and mafic rocks under varying P–T–fO2 and volatile conditions have significantly advanced our ability to trace mantle magma sources using major element systematics. Parameters such as CaO/MgO [62], FeO/SiO2 [63], FC3MS, and FCKANTMS [64] provide robust discriminants between different mantle lithologies. FC3MS (FeO/CaO − 3 × MgO/SiO2) and FCKANTMS (ln(FeO/CaO) − 0.08 × ln(K2O/Al2O3) − 0.052 × ln(TiO2/Na2O) − 0.036 × ln(Na2O/K2O) ln(Na2O/TiO2) − 0.062 × (ln(MgO/SiO2))3 − 0.641 × (ln(MgO/SiO2))2 − 1.871 × ln(MgO/SiO2) − 1.473) are key geochemical proxies designed to discriminate the lithology of mantle sources from which magmas are derived. The fundamental significance of these parameters lies in their ability to effectively distinguish between melts originating from a common peridotitic mantle and those derived from pyroxenite or other anomalous mantle components. Because partial melting of pyroxenite typically yields melts that are significantly richer in FeO and poorer in CaO compared to peridotite-derived melts, magmas from a pyroxenite source generally exhibit markedly higher FC3MS values. The FCKANTMS parameter further enhances the resolution in identifying mantle source lithologies by incorporating alkali metals and titanium, which are sensitive to variations in mantle mineralogy. The application of these proxies provides critical constraints for understanding magma genesis and the chemical heterogeneity of mantle sources. Among these, FCKANTMS is particularly valuable due to its sensitivity to source composition and relative insensitivity to melting conditions, making it a reliable diagnostic tool.
Experimental data indicate that melts from peridotitic sources typically exhibit FC3MS < 0.65 and FCKANTMS < 0.37, whereas those from pyroxenitic sources show markedly higher values (FC3MS: −0.9 to 1.7; FCKANTMS: >3.7). Regionally, the Cuona picritic porphyrites exhibit FC3MS (0.31–1.05) and FCKANTMS (0.27–0.56) values that are significantly higher than those of melts derived from a pure peridotitic mantle [65]. This systematic enrichment in these specific ratios points to a mantle source distinct from a typical peridotite. In addition, the Zhegucuo mafic rocks have FC3MS = 0.78–2.04 and FCKANTMS = 0.55–1.01 [27]. Similarly, these suggest that they are not derived from a pure peridotitic mantle. However, the less evolved basaltic rock (MgO ~ 6%) is almost at the boundary of the peridotitic and pyroxenitic sources. Furthermore, both FC3MS and FCKANTMS exhibit broadly negative correlations with MgO (Figure 9a,b). The samples in this study display the highest recorded values of FC3MS (2.06–4.14) and FCKANTMS (0.87–1.41). Consequently, we propose that fractional crystallization in evolved magmas (MgO < 6 wt%) is a key process, leading to elevated FC3MS and FCKANTMS values [66].
Based on elevated FC3MS and FCKANTMS values, the mantle source for the Zhegucuo mafic rocks likely contains a peridotitic component, and extensive fractional crystallization of evolved magmas is the key process responsible for the exceptionally high FC3MS and FCKANTMS values observed in our samples.

5.4. Implications for Intermediate Magmatism in the LIPs

Bimodal magmatism is a frequent occurrence in LIPs, which is often accompanied by a notable scarcity of intermediate rocks—a phenomenon known as the “Daly gap” [67,68]. Current genetic models for intermediate to acidic magmatic rocks in such settings are primarily divided into two viewpoints: (1) intermediate-alkaline rocks form mainly through fractional crystallization of mantle plume-derived basaltic magmas, involving phases such as olivine and clinopyroxene [69,70]; or (2) they originate from high-temperature partial melting of crustal materials under mantle plume-related thermal conditions [71]. Additionally, other mechanisms such as the melting of basaltic underplates, assimilation coupled with fractional crystallization (AFC), and liquid immiscibility have also been proposed as potential genetic processes [72,73,74,75,76].
Since the dominant magmatic products in LIPs are basalts, a key question is which factors control the evolution from basaltic to intermediate compositions. In this study, we propose that the formation of these intermediate magmas is critically governed by extensive fractional crystallization from parental basaltic magmas (see Section 5.2). This process is initiated when a rising mantle plume thermally erodes and melts the lithospheric mantle. During subsequent ascent and stagnation in crustal magma chambers, the basaltic magmas undergo extensive fractional crystallization. The removal of mafic phases such as olivine, clinopyroxene, plagioclase, and amphibole drove the residual melt toward intermediate compositions such as tonalite. Thus, the melts are already compositionally evolved and consistent with the Zhegucuo intrusive suite, which exhibits a continuous compositional range from basaltic to intermediate stages (SiO2 = 51.2–62.6 wt%) [27].
In light of the regional context and our geochemical data from the eastern Tethyan Himalayan belt, we propose that the generation of the Zhegucuo intermediate magmas can be effectively explained by fractional crystallization of mafic magmas. This process, involving significant mineral fractionation, provides a viable mechanism to bridge the compositional gap in this specific bimodal magmatic suite.

6. Conclusions

An integrated investigation of zircon U-Pb geochronology, whole-rock geochemistry, and Nd isotopes yields the following key conclusions for the Zhegucuo tonalites:
  • The zircon U-Pb weighted mean age of the Zhegucuo tonalites is 130.81 ± 0.55 Ma, indicating that they were products of Early Cretaceous magmatism.
  • The rocks exhibit low Mg# values and low concentrations of Cr and Ni, indicating that the parental magma underwent significant fractional crystallization. The homogeneous εNd(t) values (−0.34 to +0.01), which are distinctly different from those of the regional crust (εNd(t) = −12 to −13), demonstrate that the magmas experienced negligible crustal contamination during their ascent and differentiation.
  • Based on field relationships, geochemistry, and isotopic evidence, the Zhegucuo tonalites are most plausibly explained as residual liquids derived from extensive fractional crystallization of a basaltic parental magma originating from an enriched lithospheric mantle source.
  • Based on FC3MS and FCKANTMS systematics, the mantle source of the Zhegucuo mafic rocks likely contains a peridotitic component, and extensive fractional crystallization of evolved magmas accounts for the exceptionally high FC3MS and FCKANTMS values in our samples.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/min15121251/s1, Table S1: Zircon U-Pb data for tonalite of the eastern Tethyan Himalaya igneous province; Table S2: Bulk-rock major (wt.%) and trace (ppm) element data for Zhegucuo tonalites of the eastern Tethyan Himalaya igneous province; Table S3: Whole-rock Nd isotopic compositions for Zhegucuo tonalites of the eastern Tethyan Himalaya igneous province. Figure S1: Zircon cathode luminescence (CL) images of ZG02-2; Figure S2: Silica-variation diagrams showing a nonlinear compositional continuum of the Zhegucuo intrusive suite.

Author Contributions

Conceptualization, S.C.; methodology S.C. and H.J.; software, H.J.; writing—original draft preparation, S.C.; writing—review and editing, S.C. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the National Natural Science Foundation of China (Grant 42172055).

Data Availability Statement

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

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. (a) Tectonic sketch map depicting the spatiotemporal distribution of magmatic rocks associated with the Kerguelen mantle plume within circum-eastern Gondwana [6]. (b) Plate reconstruction of eastern Gondwana at ~132 Ma, illustrating the relative positions of the Bunbury Basalt in Australia, the Tethyan Himalaya Igneous Province in India, and the Naturaliste Plateau in Antarctica [4]. (c) Geological map of the THIP, displaying the location of its magmatic rocks and sample locations [4,6]. Abbreviations: RT = Rajmahal Traps; SP = Shilong Plateau; WP = Wallaby Plateau; BR = Broken Ridge; BB = Bunbury Basalt; SB = Skiff Bank; EB = Elan Bank; KA = Kerguelen Archipelago; NP = Naturaliste Plateau; NKP = North Kerguelen Plateau; CKP = Central Kerguelen Plateau; SKP = South Kerguelen Plateau; IYSZ = India–Yarlung Zangbo Suture Zone; THIP = Tethyan Himalaya Igneous Province.
Figure 1. (a) Tectonic sketch map depicting the spatiotemporal distribution of magmatic rocks associated with the Kerguelen mantle plume within circum-eastern Gondwana [6]. (b) Plate reconstruction of eastern Gondwana at ~132 Ma, illustrating the relative positions of the Bunbury Basalt in Australia, the Tethyan Himalaya Igneous Province in India, and the Naturaliste Plateau in Antarctica [4]. (c) Geological map of the THIP, displaying the location of its magmatic rocks and sample locations [4,6]. Abbreviations: RT = Rajmahal Traps; SP = Shilong Plateau; WP = Wallaby Plateau; BR = Broken Ridge; BB = Bunbury Basalt; SB = Skiff Bank; EB = Elan Bank; KA = Kerguelen Archipelago; NP = Naturaliste Plateau; NKP = North Kerguelen Plateau; CKP = Central Kerguelen Plateau; SKP = South Kerguelen Plateau; IYSZ = India–Yarlung Zangbo Suture Zone; THIP = Tethyan Himalaya Igneous Province.
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Figure 2. A geological map of Early Cretaceous magmatic rocks in Zhegucuo [25,27].
Figure 2. A geological map of Early Cretaceous magmatic rocks in Zhegucuo [25,27].
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Figure 3. Representative field photographs (a,b) and photomicrographs (c,d) of the Zhegucuo magmatic rocks. Image (a) shows the Zhegucuo tonalitic dykes intruding the Zhela Formation, and image (b) displays the constituent minerals. Abbreviations: Amp = Amphibole; Pl = Plagioclase; Afs = Alkaline feldspar; Qz = Quartz.
Figure 3. Representative field photographs (a,b) and photomicrographs (c,d) of the Zhegucuo magmatic rocks. Image (a) shows the Zhegucuo tonalitic dykes intruding the Zhela Formation, and image (b) displays the constituent minerals. Abbreviations: Amp = Amphibole; Pl = Plagioclase; Afs = Alkaline feldspar; Qz = Quartz.
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Figure 4. U–Pb concordia diagrams depicting zircon analyses from the Zhegucuo tonalites in the eastern THIP. The figure presents a zircon U-Pb concordia diagram generated using the IsoplotR online, displaying isotopic data with 1σ absolute error ellipses. All data points are shown with 95% confidence ellipses and are overlain on the concordia curve. A concordia age is calculated from a cluster of concordant or nearly concordant analyses, with the result displayed on the plot including the age value, its uncertainty (reported at 1σ or 2σ as specified), and the Mean Square of Weighted Deviates (MSWD) and probability as measures of data consistency. The axes are clearly labeled as 206Pb/238U and 207Pb/235U ratios, with the plotting range typically set between 0.10–0.17 for X and 0.015–0.025 for Y to optimally frame the data. The numbers in the zircon images represent the analysis spot numbers.
Figure 4. U–Pb concordia diagrams depicting zircon analyses from the Zhegucuo tonalites in the eastern THIP. The figure presents a zircon U-Pb concordia diagram generated using the IsoplotR online, displaying isotopic data with 1σ absolute error ellipses. All data points are shown with 95% confidence ellipses and are overlain on the concordia curve. A concordia age is calculated from a cluster of concordant or nearly concordant analyses, with the result displayed on the plot including the age value, its uncertainty (reported at 1σ or 2σ as specified), and the Mean Square of Weighted Deviates (MSWD) and probability as measures of data consistency. The axes are clearly labeled as 206Pb/238U and 207Pb/235U ratios, with the plotting range typically set between 0.10–0.17 for X and 0.015–0.025 for Y to optimally frame the data. The numbers in the zircon images represent the analysis spot numbers.
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Figure 5. Geochemical discrimination diagrams for the Zhegucuo tonalites (eastern THIP): (a) SiO2 vs. Na2O + K2O [27], (b) Nb/Y vs. Zr/TiO2 [42], (c) Na2O vs. K2O [43] and (d) SiO2 vs. Fe* (FeOT/(MgO + FeOT)) [47].
Figure 5. Geochemical discrimination diagrams for the Zhegucuo tonalites (eastern THIP): (a) SiO2 vs. Na2O + K2O [27], (b) Nb/Y vs. Zr/TiO2 [42], (c) Na2O vs. K2O [43] and (d) SiO2 vs. Fe* (FeOT/(MgO + FeOT)) [47].
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Figure 6. (a) Chondrite-normalized REE patterns and (b) primitive mantle-normalized trace element spider diagrams for the Zhegucuo tonalites (eastern THIP). Normalization values are from Sun and McDonough (1989) [48]. The data for the Zhegucuo tonalites [27] are also plotted for comparison.
Figure 6. (a) Chondrite-normalized REE patterns and (b) primitive mantle-normalized trace element spider diagrams for the Zhegucuo tonalites (eastern THIP). Normalization values are from Sun and McDonough (1989) [48]. The data for the Zhegucuo tonalites [27] are also plotted for comparison.
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Figure 7. (a) εNd(t) versus MgO, (b) SiO2 versus MgO, (c) SiO2 versus Fe2O3T, (d) SiO2 versus CaO, (e) Cr versus Ni, (f) Er versus Dy, (g) SiO2 versus P2O5, (h) Zr versus Zr/Hf, and (i) SiO2 versus Zr for the Zhegucuo tonalites (eastern THIP). The data for the Zhegucuo tonalites [27] are also plotted for comparison.
Figure 7. (a) εNd(t) versus MgO, (b) SiO2 versus MgO, (c) SiO2 versus Fe2O3T, (d) SiO2 versus CaO, (e) Cr versus Ni, (f) Er versus Dy, (g) SiO2 versus P2O5, (h) Zr versus Zr/Hf, and (i) SiO2 versus Zr for the Zhegucuo tonalites (eastern THIP). The data for the Zhegucuo tonalites [27] are also plotted for comparison.
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Figure 8. Y versus Sr/Y diagrams for the Zhegucuo tonalites (eastern THIP). The data for the Zhegucuo tonalites [27] are also plotted for comparison.
Figure 8. Y versus Sr/Y diagrams for the Zhegucuo tonalites (eastern THIP). The data for the Zhegucuo tonalites [27] are also plotted for comparison.
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Figure 9. (a) MgO vs. FC3MS and (b) MgO vs. FCKANTMS for diagrams for the Zhegucuo tonalites (eastern THIP). The data are from [27,65].
Figure 9. (a) MgO vs. FC3MS and (b) MgO vs. FCKANTMS for diagrams for the Zhegucuo tonalites (eastern THIP). The data are from [27,65].
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Table 1. Petrographic Summary of the Zhegucuo Intrusive Suite.
Table 1. Petrographic Summary of the Zhegucuo Intrusive Suite.
SampleLithologyTextureEssential Minerals (vol%)Accessory Minerals
ZG01-1TonaliteMedium-grained hypidimorphicPlagioclase (40–50) + quartz (25–30) + alkaline feldspar (5–10) + amphibole (5–10)Apatite, titanite, Fe–Ti Oxides
ZG01-2TonaliteMedium-grained hypidimorphicPlagioclase (40–50) + quartz (25–30) + alkaline feldspar (5–10) + amphibole (5–10)Apatite, titanite, Fe–Ti Oxides
ZG02-1TonaliteMedium-grained hypidimorphicPlagioclase (35–45) + quartz (30–35) + alkaline feldspar (10–15) + amphibole (8–12)Apatite, titanite, Fe–Ti Oxides
ZG02-2TonaliteMedium-grained hypidimorphicPlagioclase (35–45) + quartz (30–35) + alkaline feldspar (10–15) + amphibole (8–12)Apatite, titanite, Fe–Ti Oxides
ZG02-3TonaliteMedium-grained hypidimorphicPlagioclase (35–45) + quartz (30–35) + alkaline feldspar (10–15) + amphibole (8–12)Apatite, titanite, Fe–Ti Oxides
ZG02-4TonaliteMedium-grained hypidimorphicPlagioclase (35–45) + quartz (30–35) + alkaline feldspar (10–15) + amphibole (8–12)Apatite, titanite, Fe–Ti Oxides
ZG02-5TonaliteMedium-grained hypidimorphicPlagioclase (40–50) + quartz (25–30) + alkaline feldspar (5–10) + amphibole (5–10)Apatite, titanite, Fe–Ti Oxides
ZG02-6TonaliteMedium-grained hypidimorphicPlagioclase (40–50) + quartz (25–30) + alkaline feldspar (5–10) + amphibole (5–10)Apatite, titanite, Fe–Ti Oxides
ZG02-7TonaliteMedium-grained hypidimorphicPlagioclase (40–50) + quartz (25–30) + alkaline feldspar (5–10) + amphibole (5–10)Apatite, titanite, Fe–Ti Oxides
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Chen, S.; Jie, H. Petrogenesis of Intermediate Rocks in Tethyan Himalaya Igneous Province (SE Tibet): The Role of Source Composition and Fractional Crystallization. Minerals 2025, 15, 1251. https://doi.org/10.3390/min15121251

AMA Style

Chen S, Jie H. Petrogenesis of Intermediate Rocks in Tethyan Himalaya Igneous Province (SE Tibet): The Role of Source Composition and Fractional Crystallization. Minerals. 2025; 15(12):1251. https://doi.org/10.3390/min15121251

Chicago/Turabian Style

Chen, Shengsheng, and Haonan Jie. 2025. "Petrogenesis of Intermediate Rocks in Tethyan Himalaya Igneous Province (SE Tibet): The Role of Source Composition and Fractional Crystallization" Minerals 15, no. 12: 1251. https://doi.org/10.3390/min15121251

APA Style

Chen, S., & Jie, H. (2025). Petrogenesis of Intermediate Rocks in Tethyan Himalaya Igneous Province (SE Tibet): The Role of Source Composition and Fractional Crystallization. Minerals, 15(12), 1251. https://doi.org/10.3390/min15121251

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