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Article

Report of CA. 760 Ma Mafic Rocks in the Eastern Himalayan Orogen: Petrogenesis and Geodynamic Implications

by
Yi Yang
1,
Zhi Zhang
2,*,
Guotao Ma
2 and
Suiliang Dong
2
1
School of Engineering, Xizang University, Lhasa 850000, China
2
Chengdu Center, China Geological Survey, Chengdu 610081, China
*
Author to whom correspondence should be addressed.
Minerals 2025, 15(10), 1090; https://doi.org/10.3390/min15101090
Submission received: 13 September 2025 / Revised: 17 October 2025 / Accepted: 17 October 2025 / Published: 20 October 2025
(This article belongs to the Special Issue Tracing Precambrian Pathways: Neoproterozoic Rocks and Their Global Context)
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Abstract

Constraints on the Neoproterozoic evolution of the Himalayan terrane remain poorly understood due to the scarcity of Neoproterozoic magmatic rocks. In this study, we report for the first time Middle Neoproterozoic mafic rocks from the eastern Himalayan orogen. Zircon U–Pb dating indicates that these rocks crystallized at approximately 760 Ma and can be divided into two distinct groups. Group 1 mafic rocks have E-MORB-like compositions and are enriched in incompatible elements and exhibit relatively higher initial (87Sr/86Sr)i ratios (0.7053–0.7063), lower positive whole-rock εNd(t) values (3.0 to 3.4), and zircon εHf(t) values ranging from 4.9 to 10.4. They also show low Nb/Th ratios and high Th/Yb, Nb/Yb, and (La/Sm)N ratios, suggesting a lithospheric mantle source. In contrast, Group 2 mafic rocks have N-MORB-like compositions and are characterized by light rare earth element (LREE)-depleted patterns, lower initial (87Sr/86Sr)i ratios (0.7033–0.7040), and higher positive whole-rock εNd(t) (4.8 to 6.0) and zircon εHf(t) values (4.6 to 10.9). Their high Nb/Th ratios and low Th/Yb, Nb/Yb, and (La/Sm)N ratios indicate an origin involving interaction between the lithospheric mantle and depleted asthenospheric mantle. The absence of coeval volcanic and sedimentary records, combined with high La/Y and Ti/V ratios, suggests that these mafic rocks differ from typical arc or back-arc basin suites but are consistent with an intraplate setting. Integrating previous studies on multistage Neoproterozoic magmatism in India and the Himalayas, we propose that the ca. 760 Ma mafic rocks in the eastern Himalaya were likely formed within an intraplate continental rift system.

1. Introduction

The Himalayan orogeny resulted from the India-Asia continental collision that was initiated during the early Paleogene [1]. Due to the constraints imposed by orogenic activity, research on the tectonic evolution of the Himalayan terrane has primarily focused on the Phanerozoic eon [2]. In contrast, the Precambrian tectonic evolution of the Himalayan terrane remains poorly understood, largely owing to the scarcity of corresponding magmatic records [3].
Thus far, only a few Neoproterozoic magmatic rocks (>800 Ma) have been recognized within the Precambrian magmatic record of the Himalayan terrane [3,4,5,6,7] (Figure 1). However, the origin and geodynamic setting of these Neoproterozoic magmatic rocks remain controversial [3,6,7]. The adjacent Indian terrane, which also formed part of Greater India, exhibits well-documented Neoproterozoic magmatism between 1021 and 750 Ma [8,9,10,11,12] (Figure 1). This magmatic record has been interpreted to reflect an evolution from arc to back-arc basin environments in response to oceanic subduction [11,12]. In contrast, magmatic activity at ca. 760 Ma has not been previously identified in the Himalayan terrane—a significant gap that has hindered a comprehensive understanding of its Precambrian tectonic evolution. Here, we present the first discovery of ca. 760 Ma mafic rocks in the eastern Himalayan terrane. This finding provides critical constraints for reconstructing the Precambrian tectonic evolution of the Himalayan terrane and its paleogeographic relationship with the Indian continent.

2. Geological Setting and Samples

The Himalayan orogen, located between the Lhasa and Indian terranes and south of the Indus-Yarlung Zangbo suture (IYZS), is a product of the Indo–Asian continental collision [2]. Stretching over 2500 km, the orogen displays a distinctive arcuate geometry at both its eastern and western termini (Figure 1). It is conventionally divided into four principal geological units from north to south: the Tethys Himalayan Sequence (THS), the Greater Himalayan Crystalline Complex (GHC), the Lesser Himalayan Sequence (LHS), and the Sub-Himalayan Sequence. These units are bounded by the South Tibetan Detachment System (STDS), the Main Central Thrust (MCT), the Main Boundary Thrust (MBT), and the Main Frontal Thrust (MFT), respectively [2,20] (Figure 1).
The Sub-Himalayan Sequence comprises coarse clastic sediments deposited in a foreland basin following Himalayan uplift. The LHS consists predominantly of low-grade metamorphic rocks, including characteristic mica schists [21]. The GHC is composed mainly of high-grade metamorphic rocks, widely interpreted as derived from the Indian cratonic basement [22,23]. These include metapelites and meta-igneous rocks such as granitic gneisses and minor amphibolites, typically occurring within metasedimentary host sequences. Previous studies have identified two main intervals of crystallization ages for these meta-igneous rocks in the Himalayan orogen: (a) 878–800 Ma, attributed to Neoproterozoic back-arc magmatism [3], and (b) 530–467 Ma, associated with early Paleozoic subduction of the Proto-Tethys Ocean [24]. Structural discontinuities within the GHC have also been documented [25,26]. The THS, located south of the IYZS and separated from the GHC by the STDS [2] (Figure 1), constitutes an important Au–Sb–Pb–Zn metallogenic belt [20]. It consists largely of weakly metamorphosed to unmetamorphosed Late Paleozoic to Late Mesozoic clastic and carbonate rocks, interpreted as deposits of a passive continental margin [2,21,27]. From the Paleocene to the Miocene, the Himalayan terrane experienced a tectonic transition from south-directed thrusting to north-directed extension and detachment [2,26,28].
Long-term tectonothermal evolution of the Himalayan terrane has produced widely distributed igneous rocks, including the Neoproterozoic mafic and felsic rocks (>800 Ma) [3,4,5,6,7,29], Paleozoic granitoids (530–467 Ma) [30,31,32,33], Triassic volcanic rocks and bimodal intrusive rocks [34,35], Jurassic felsic volcanic rocks [36], Early Cretaceous mafic and felsic magmatic rocks [37,38,39,40,41], and Eocene–Miocene leucogranites [42,43,44,45,46,47,48,49].
In this study, Middle Neoproterozoic amphibolites (ca. 760 Ma) have been identified within the Cuonadong gneiss dome in the eastern Himalayan orogen (Figure 2). This Cenozoic dome comprises three lithotectonic units: a core (base), a middle unit, and a rim (top). The core consists of Cambrian granitic gneiss intruded by Cenozoic leucogranites [50,51,52], with the former exhibiting amphibolite-facies metamorphism. The rim is composed of Triassic–Jurassic carbonaceous slate [33,53], while the middle unit consists of Paleozoic quartz schist interlayered with mylonitic marble and skarn. Pegmatite dikes are widespread throughout the dome. The amphibolites occur as xenoliths or relict bodies within both the Cambrian granitic gneiss and the Cenozoic leucogranites in the core (Figure 2 and Figure 3A–C).
Samples analyzed in this study were collected from the core unit (sample sites are shown in Figure 2). Three amphibolite samples (CNDXCP, CNDS16-B1, and CNDS16-B2) were selected for LA-MC-ICP-MS zircon U–Pb dating and Lu–Hf isotope analysis. The samples CNDS16-B1 and CNDS16-B2 were collected from the same outcrop, so their corresponding geochemical samples are same. Eleven fresh amphibolite samples—CNDXCPB1–CNDXCPB6 (Group 1) and CNDS16B1–CNDS16B5 (Group 2)—were analyzed for major and trace elements and Sr–Nd isotopes. The main minerals of these amphibolites are composed of hornblende (45–55 vol%), plagioclase (40–50 vol%), biotite (5–10 vol%), pyroxene (3–5 vol%), together with minor quartz, muscovite, zircon, and apatite (<3 vol%) (Figure 3D–F). Although field observations show weakly developed gneissic fabric and some metamorphic or alteration overprints—particularly in plagioclase, hornblende and pyroxene—primary igneous textures are well preserved (Figure 3D–F), suggesting that their protolith was likely gabbro or diabase.

3. Analytical Methods

3.1. Zircon U–Pb Dating and Zircon Trace Element

Zircons were separated by heavy-liquid and magnetic methods. U-Pb dating and trace element analyses of zircon were conducted synchronously by LA-ICP-MS at the Wuhan Sample Solution Analytical Technology Co., Ltd., Wuhan, China. Laser sampling was performed by a KrF excimer laser ablation system (Photo Machine 193 nm, Agilent Technologies (China), Beijing, China), and ion-signal intensities were acquired by the ICP–MS instrument (Agilent 7900, Agilent Technologies (China), Beijing, China). Spot sizes were ~32 μm and Zircon 91500 was analyzed twice every 5 analyses as external standard for U–Pb dating. PLE zircon were used as a reference standard and trace element abundances were calibrated against SRM 610 using 29Si as an internal standard [54]. U–Pb dating and quantitative calibration for zircon trace element analyses were all performed by ICPMSDataCal [54].

3.2. Zircon Hf Isotopic Compositons

Zircon Lu–Hf isotopic analyses were conducted by means of LA–MC–ICP–MS on a Neptune equipped with a NEW WAVE 193 nm FX ArF Laser Ablation System, at the Wuhan Sample Solution Analytical Technology Co., Ltd., Wuhan, China. Analyses were performed with a beam diameter of 44 μm, 9 Hz repetition rate. During the Lu–Hf isotope analyses, zircon GJ–1 was used as external standard, and every ten analyses were followed by one analysis of zircons GJ–1. Detailed analytical methods are listed in Hu et al. [55,56].
The major limitation to accurate in situ zircon Hf isotope determination by LA-MC-ICP-MS is the very large isobaric interference from 176Yb and, to a much lesser extent 176Lu on 176Hf. The 179Hf/177Hf and 173Yb/171Yb ratios were used to calculate the mass bias of Hf (βHf) and Yb (βYb), which were normalized to 179Hf/177Hf = 0.7325 and 173Yb/171Yb = 1.132685using an exponential correction for mass bias [57]. 176Yb/173Yb = 0.79639 is used to correct the Interference of 176Yb on 176Hf and calculate 176Yb/177Hf [57]. 176Lu/175Lu = 0.02656 is used to correct the interference of 176Lu on 176Hf and calculate 176Lu/177Hf [58,59,60].

3.3. Whole-Rock Major and Trace Element Analyses

In this study, eleven fresh amphibolite samples were collected for whole-rock geochemical analyses, which were performed at the Analytical Laboratory Beijing Research Institute of Uranium Geology.
Major element analyses were performed using a sequential X–ray fluorescence spectrometer (Axios MAX, Malvern Panalytical (China), Shanghai, China). Powder after ignition (~0.6 g) was mixed with 6.0 g compound flux (Li2B4O7 : LiBO2 : LiF = 9:2:1), and then was fused in Pt–Au crucibles by heating at ~1100 °C. The mixture was transferred to a fireproofing tile to cool, and form a glass for further analysis.
Trace element analyses were measured using an ICP–MS (Agilent 7700e, Agilent Technologies (China), Beijing, China), Rock powder after drying (50 mg) was dissolved in a Teflon bomb using a mixture of HF (l mL) and HNO3 (1 mL). Then the sealed Teflon bomb was heated at 190 °C for more than 12 h. The final solution was diluted to 100 g with 2% HNO3 in a polyethylene bottle for further analysis.

3.4. Whole-Rock Sr–Nd Isotopes

Whole-rock Sr–Nd isotopic analyses were determined on an ISOPROBE-T thermal ionization mass spectrometer (MicroMass, Manchester, Britain) at the Analytical Laboratory of the Beijing Research Institute of Uranium Geology. The measured ratios of 87Sr/86Sr and 143Nd/144Nd were based on 86Sr/88Sr = 0.1194 and 146Nd/144Nd = 0.7219, respectively. The 87Sr/86Sr ratio of the SRM NBS987 Sr standard was 0.710252 ± 0.00004 (2σ), while the 143Nd/144Nd ratio of the JNDI-1 Nd standard was 0.512105 ± 0.00004 (2σ). All the modern Sm/Nd values in CHUR (chondritic uniform reservoir) and DM (depleted mantle) are as suggested by Goldstein et al. [61] and Peucat et al. [62]. Detailed procedures for sample preparation, and column chemistry were described in Lin et al. [63].

4. Results

4.1. Zircon U–Pb Ages and Trace Element Compositions

Zircon U–Pb dating results for the amphibolite samples are provided in Supplementary Table S1. Zircon grains from samples CNDXCP, CNDS16-B1, and CNDS16-B2 are transparent, colorless, and euhedral. They range in size from 40 to 130 μm, with aspect ratios between 1:1 and 1.5:1 (Figure 4A,C,E). All zircons exhibit Th/U ratios ranging from 0.21 to 2.08, with a mean value of 1.08, consistent with a magmatic origin [64,65].
Twenty-seven concordant analyses from sample CNDXCP yield 206Pb/238U ages between 736.1 and 773.1 Ma, giving a weighted mean age of 761.2 ± 3.5 Ma (MSWD = 0.70, n = 27; Figure 4A). Twenty-five concordant analyses from sample CNDS16-B1 yield 206Pb/238U ages ranging from 747.8 to 773.7 Ma, with a weighted mean age of 762.0 ± 4.1 Ma (MSWD = 0.25, n = 25; Figure 4C). Twenty-three concordant analyses from sample CNDS16-B2 yield 206Pb/238U ages from 736.1 to 773.5 Ma and a weighted mean age of 760.3 ± 4.5 Ma (MSWD = 1.09, n = 23; Figure 4E).
Trace element data for zircon grains from the amphibolite samples (CNDXCP, CNDS16-B1, CNDS16-B2) are presented in Supplementary Table S2. The zircons show variable rare earth element (REE) contents ranging from 338 to 2826 ppm (Figure 4B,D,F), along with elevated Th, U, and Y concentrations. Chondrite-normalized REE patterns for all three samples are characterized by light REE (LREE) depletion, positive Ce and Sm anomalies, and negative Eu anomalies (Figure 4B,D,F). Zircons from these samples display Sm contents between 0.88 and 16.60 ppm (mean = 5.17 ppm) and low La contents ranging from 0.01 to 9.09 ppm (mean = 0.37 ppm). These geochemical features are indicative of a magmatic origin for the zircons [66].

4.2. Zircon Hf Isotopes

A total of 48 zircon grains from samples CNDXCP, CNDS16-B1, and CNDS16-B2 were selected for Hf isotopic analysis. The Hf isotopic calculations were performed using the individual spot ages of the corresponding zircon grains. The Lu–Hf isotopic results are presented in Supplementary Table S3. Seventeen concordant analytical spots from sample CNDXCP yield 176Hf/177Hf ratios ranging from 0.282445 to 0.282613, corresponding to initial εHf(t) values of 4.9 to 10.4 (weighted mean = 7.8) and TDMᶜ model ages between 1007 and 1355 Ma. Sixteen concordant analyses from sample CNDS16-B1 show 176Hf/177Hf ratios from 0.282483 to 0.282618, with initial εHf(t) values of 6.3 to 10.4 (weighted mean = 8.7) and TDMᶜ model ages ranging from 1008 to 1272 Ma. Fifteen concordant analyses from sample CNDS16-B2 exhibit 176Hf/177Hf ratios between 0.282436 and 0.282617, yielding initial εHf(t) values of 4.6 to 10.9 (weighted mean = 7.8) and TDMᶜ model ages of 981 to 1378 Ma.

4.3. Major and Trace Elements

Major and trace element data for eleven amphibolite samples are presented in Supplementary Table S4. These samples are divided into two geochemically distinct groups. Group 1 (samples CNDXCPB1–B6) exhibits relatively low SiO2 (47.99–48.83 wt%) and Fe2O3T (11.47–13.08 wt%) contents, moderate CaO (11.05–11.69 wt%) and TiO2 (1.35–1.75 wt%) contents, and comparatively high K2O (0.52–0.68 wt%), Na2O (2.43–2.67 wt%), and MgO (6.89–7.45 wt%) contents. Their Mg# values range from 51.24 to 56.27. In contrast, Group 2 (samples CNDS16B1–B5) shows relatively high SiO2 (50.12–51.22 wt%) and Fe2O3T (13.08–15.24 wt%) contents, moderate CaO (11.00–11.16 wt%) and TiO2 (1.76–1.99 wt%) contents, but lower K2O (0.14–0.32 wt%), Na2O (0.71–1.48 wt%), and MgO (5.81–6.12 wt%) contents. Their Mg# values vary between 43.03 and 47.86. All samples plot within the sub-alkaline field on the SiO2–Zr/TiO2 × 0.0001 diagram (Figure 5A) and are classified as tholeiitic rocks (Figure 5B).
Normalized trace element patterns of all amphibolite samples are characterized by generally flat high-field-strength element (HFSE) and rare earth element (REE) profiles (Figure 6A), with positive Pb and Th anomalies. Chondrite-normalized REE patterns show light REE (LREE) depletion (Figure 6B), similar to normal mid-ocean ridge basalt (N-MORB) [69]. Notably, Group 1 samples (CNDXCPB1–B6) display higher Rb, Sr, Ba, and LREE contents, as well as higher (La/Yb)N ratios [70], whereas Group 2 samples (CNDS16B1–B5) are characterized by higher HREE contents, lower LREE abundances, and lower (La/Yb)N ratios.

4.4. Whole-Rock Sr–Nd Isotopes

A total of six amphibolite samples were selected for Sr–Nd isotopic analysis, and the results are presented in Supplementary Table S5. Initial Sr and Nd isotopic compositions were calculated using t = 761 Ma for samples from CNDXCP and t = 762 Ma for those from CNDS16. The results reveal distinct Sr–Nd isotopic signatures between the two groups. Group 1 (samples CNDXCPB4–B6) exhibits relatively high initial (87Sr/86Sr)ᵢ ratios (0.7053–0.7063) and moderately positive εNd(t) values ranging from 3.1 to 3.9 (mean = 3.4). Their TDM2 Nd model ages range from 1125 to 1189 Ma. In contrast, Group 2 (samples CNDS16B1–B3) shows lower initial (87Sr/86Sr)ᵢ ratios (0.7033–0.7040), more strongly positive εNd(t) values between 4.8 and 6.0 (mean = 5.5), and younger TDM2 Nd model ages ranging from 953 to 1051 Ma.

5. Discussion

5.1. Alteration and Metamorphism Effects

Geochemical data indicate that the amphibolite samples exhibit relatively low loss-on-ignition (LOI) values ranging from 0.44 to 1.43 wt%, suggesting a limited alteration effect. However, both field and petrographic observations reveal that these rocks have experienced varying degrees of metamorphism and alteration. Consequently, the use of alkali elements, large-ion lithophile elements (LILEs) and Rb-Sr isotopic ratios in petrogenetic interpretation requires caution, as these elements may have been mobilized during secondary processes [72]. In contrast, high-field-strength elements (HFSEs; e.g., Zr, Nb, Ti, Hf, Ta), rare earth elements (REEs), and transition elements (e.g., Co, Cr, V, Mg, Ni, Fe) are generally considered immobile during metamorphism and alteration [73]. Selected mobile and immobile elements are plotted against LOI in Figure 7 to evaluate the extent of metasomatic and metamorphic overprinting. The results show that K2O increases with rising LOI, confirming its mobility during secondary processes (Figure 7B,D), and thus rendering it unreliable for petrogenetic modeling. Notably, CaO and Rb show no significant correlation with LOI (Figure 7A), suggesting these elements remained relatively immobile and may be used in subsequent interpretations. Similarly, typical immobile elements such as Ti, Ta, and Zr, along with transition elements including V and Ni, exhibit no systematic relationship with LOI (Figure 7C,E–H), supporting their stability under metamorphic conditions. Therefore, in the following discussions, we focus primarily on whole-rock Nd isotopes, immobile and transition elements, and zircon Hf isotopic compositions to constrain the petrogenesis and tectonic setting of these rocks.

5.2. Crustal Contamination and Crystal Fractionation

Previous studies have suggested that ascending mafic magmas may undergo crustal contamination, which can alter their elemental compositions [74]. Therefore, geochemical signatures must be carefully evaluated before being used to interpret the petrogenesis of the amphibolite samples. All samples show no significant correlation between εNd(t) values and SiO2 content (Figure 8A), indicating negligible crustal contamination during magmatic evolution. Furthermore, the absence of clear correlations between Nb/Th and Nb/La ratios, as well as between La/Sm and Nb/La ratios (Figure 8B,C), further supports the lack of substantial crustal assimilation.
Fractional crystallization also plays an important role in the evolution of mafic rocks. The amphibolites exhibit low Mg# (43.03–56.27), Ni (37.0–63.5 ppm), and Cr (69.7–369.0 ppm) concentrations—values lower than those expected for primary mantle-derived magmas in equilibrium with peridotite [75], suggesting that fractional crystallization has occurred. This inference is consistent with their geochemical characteristics. Harker variation diagrams (Figure 9) reveal positive correlations between MgO and compatible elements such as Cr and Ni (Figure 9A,B), indicating fractionation of olivine and/or clinopyroxene. However, the negative correlation between Fe2O3T and MgO (Figure 9H) suggests clinopyroxene fractionation without significant olivine removal. Although Al2O3, CaO, and MgO show positive correlations (Figure 9C,D), the lack of a clear relationship between CaO/Al2O3 and MgO (Figure 9E), as well as between CaO and SiO2 (Figure 9I), points to clinopyroxene fractionation with negligible plagioclase crystallization. This interpretation is supported by the absence of pronounced Sr or Eu anomalies in primitive mantle-normalized diagrams (Figure 6A). Additionally, the negative correlation between MgO and TiO2 (Figure 9F) argues against significant fractionation of Ti–Fe oxides. Finally, the negative or uncorrelated trend between P2O5 and MgO (Figure 9G) precludes apatite fractionation.

5.3. Petrogenesis

The amphibolite samples exhibit low Sm/Yb ratios (0.90–1.07) and low (La/Yb)N ratios (0.66–1.19), indicating the absence of garnet in the source region and consistency with a spinel-bearing mantle source [76] (Figure 8D). However, the two groups display distinct Hf–Sr–Nd isotopic signatures and trace element compositions. Group 1 (samples CNDXCPB4–B6) shows relatively elevated initial 87Sr/86Sr ratios (0.7053–0.7063; Figure 10A), lower εNd(t) values (3.1–3.9; mean = 3.4; Figure 10A), and zircon εHf(t) values ranging from 4.9 to 10.4 (mean = 7.8; Figure 10B). These samples are enriched in incompatible elements (e.g., Rb, Sr, Ba; Figure 6A) and exhibit higher Th/Yb and Nb/Yb ratios (Figure 11B), lower Nb/Th ratios (Figure 8B), elevated light REE (LREE) contents (Figure 6B), and high (La/Sm)N ratios (0.88–1.03; mean = 0.95). These geochemical characteristics are consistent with an E-MORB affinity [77,78], suggesting derivation from an enriched mantle source. In contrast, Group 2 (samples CNDS16B1–B3) displays LREE-depleted patterns with higher heavy REE (HREE) contents (Figure 6C,D), which distinguish them from crustal rocks or magmas derived from enriched mantle sources (e.g., E-MORB, OIB, or arc magmas) and instead resemble N-MORB-like mafic rocks [69]. This group is characterized by lower initial 87Sr/86Sr ratios (0.7033–0.7040; Figure 10A), higher εNd(t) values (4.8–6.0; mean = 5.5; Figure 10A), zircon εHf(t) values (4.6–10.9; mean = 8.2; Figure 10B), higher Nb/Th ratios (Figure 8B), and lower LREE contents, Th/Yb, and Nb/Yb ratios (Figure 6D and Figure 8B). These signatures align with those of typical N-MORB-type mafic rocks [69,79] and are comparable to Neoproterozoic N-MORB-like rocks reported in the Lhasa terrane [80,81], indicating a depleted mantle source.
Several models have been proposed to explain the coexistence of coeval N-MORB and E-MORB type mafic rocks, including: (1) varying degrees of partial melting [86], and (2) a heterogeneous mantle source [87,88]. The formation of these contemporaneous magmas is difficult to attribute solely to different melting degrees. In the (87Sr/86Sr)ᵢ vs. εNd(t) diagram (Figure 10A), a binary mixing model shows that Group 1 compositions cannot be reproduced by mixing Group 2 (depleted mantle-derived) with Himalayan Neoproterozoic felsic rocks (ca. 820 Ma; ancient crustal origin) [3]. Hence, the enrichment of incompatible elements in Group 1 amphibolites is not caused by crustal contamination. In addition, the geochemical features of Group 1 cannot be explained by binary mixing between Group 2 amphibolites and the ca. 820 Ma Himalayan Neoproterozoic mafic rocks which were derived from an enriched, subduction-modified continental lithospheric mantle source [3] (Figure 10A). Thus, a different mantle source is required to account for the coeval presence of both N-MORB-like and E-MORB-like mafic rocks in the Himalayan orogen. Specifically, the Group 1 mafic rocks display flat light rare earth element (LREE) patterns, low Nb/Th ratios, and high Th/Yb, Nb/Yb, and (La/Sm)N ratios, suggesting a lithospheric mantle source. In contrast, the Group 2 mafic rocks exhibit depleted LREE patterns, high Nb/Th ratios, and low Th/Yb, Nb/Yb, and (La/Sm)N ratios, indicating an origin involving interaction between the lithospheric mantle and depleted asthenospheric mantle.

5.4. Geodynamic Implications

5.4.1. Tectonic Setting

Previous studies indicate that N-MORB-like mafic rocks can form in the following tectonic settings: (1) normal mid-ocean ridges [69], (2) back-arc basins [89], and (3) intracontinental rift systems [40]. The geochemical signatures of the ca. 760 Ma Neoproterozoic mafic rocks in the Himalayan orogen are distinctly different from those of arc magmas, precluding an arc-related origin [40,90,91]. Although the Zr vs. Ti diagram (Figure 11A) indicates that both Group 1 and Group 2 amphibolites exhibit MORB-like affinities, the Nb/Yb vs. Th/Yb diagram (Figure 11B) shows that Group 1 amphibolites (samples CNDXCP) align with E-MORB-like compositions, consistent with their enrichments in Rb, Sr, Ba, and LREEs. In addition, the Group 2 amphibolites display lower Na2O and Cr contents compared to typical N-MORB type rocks [69]. These characteristics differ from those of N-MORB derived from normal mid-ocean ridges [69]. Additionally, Ti/V ratios have been used to discriminate the tectonic settings of basaltic rocks [85]. Both groups of amphibolites in this study show high V contents and Ti/V ratios ranging from 19.69 to 27.87, which also differ from those of N-MORB-type rocks [71] (Figure 11C). Therefore, these Middle Neoproterozoic MORB-like mafic rocks were likely formed in either a back-arc or an intracontinental rift setting.
However, intense Cenozoic orogenic reworking has largely obscured the Precambrian geological record in the Himalayan terrane [2]. The Neoproterozoic mafic rocks described in this study occur exclusively as enclaves within gneiss domes. As a result, their geochemical signatures alone are inadequate to confidently determine their tectonic setting. Given that the Himalaya has long been considered part of Greater India, the tectonic background revealed by Neoproterozoic magmatic activity in the Indian terrane provides critical constraints for interpreting the tectonic setting of the magmatic activity identified in this study.
As is well known, Neoproterozoic magmatic rocks are widely distributed across India, including the Malani Igneous Suite in the northwest [9,10,11,12], as well as the southern Madurai Block and the Palghat–Cauvery suture zone [8]. These rocks represent three main magmatic pulses: 1021–950 Ma (e.g., Pathanapuram and Moras granites; Figure 1) [12,92], 860–820 Ma (e.g., Pali, Erinpura, and Tangalamvaripatti granites; Figure 1) [8,12,19], and 780–750 Ma (e.g., the Malani Igneous Suite and Chengannur granites; Figure 1) [12,19]. Previous studies indicate that the 1021–950 Ma granites (e.g., the ca. 976 Ma Moras granite in northwestern India) exhibit geochemical signatures characteristic of arc magmas formed in subduction-related settings [12]. In contrast, the 860–820 Ma magmatic suite comprises A-type granites and back-arc basin basalts (BABB), interpreted to have formed in a back-arc setting associated with an embryonic rifting stage [3]. The 780–750 Ma Neoproterozoic magmas also consist of bimodal magmatic rocks [11]. Interpretations of the tectonic settings for these Middle Neoproterozoic magmatic rocks vary, and several models have been proposed to resolve these disputes, including: (1) a continental rift setting [93,94]; and (2) a back-arc setting [11,95,96]. However, the absence of back-arc basin sediments has prompted a reinterpretation of the tectonic setting from a back-arc basin to a continental rift, as evidenced by the sedimentary record preserved in continental rift basins [93,94]. Thus, we favor an interpretation that the 780–750 Ma Neoproterozoic magmas formed in a continental rift setting rather than a back-arc basin.
Moreover, Zhang et al. [3] proposed that the ca. 820 Ma amphibolites in the eastern Himalaya have subalkaline compositions, characterized by enrichments in Rb and depletions in Nb, Ta, Zr, Hf, and Ti. This geochemical signature suggests an arc affinity, which they attributed to partial melting of an enriched, subduction-modified continental lithospheric mantle in a back-arc setting [3,95,97]. In contrast, the mafic rocks reported in this study have high La/Yb and Ti/V ratios, indicating an intra-plate affinity for tholeiitic rocks (Figure 5B). They do not exhibit the arc-like magmatic affinity similar to the 820 Ma mafic rocks, demonstrating that these 760 Ma rocks did not form in a back-arc basin setting. Instead, they are more likely to have originated in a continental rift environment.
When the Neoproterozoic magmatic activities of the Himalayan and Indian terranes are considered as an integrated system, the trend of increasingly negative zircon εHf(t) values from the Early to Middle Neoproterozoic (Figure 10B) suggests significant incorporation of ancient crustal materials into the magma sources over time. This isotopic shift further indicates an evolution in the tectonic setting of the Middle Neoproterozoic magmatism. Integrating these findings with previous studies, we propose that the ca. 760 Ma N-MORB and E-MORB-like mafic rocks in the Himalayan terrane likely formed in an intracontinental rift system [93,94,98,99].

5.4.2. A Possible Neoproterozoic Evolutionary Model for the Indian–Himalayan Terrane

Integrated with previous studies and new data presented here, we propose a possible geodynamic model for the Neoproterozoic evolution of the Indian–Himalayan terrane (Figure 12).
During the Early Neoproterozoic (ca. 970 Ma), the Mozambique Ocean crust subducted beneath the Indian–Himalayan terrane. This process resulted in a subduction-modified enriched lithospheric mantle and the generation of enriched basaltic melts, which underplated the base of the crust and contributed to the formation of a juvenile mafic lower crust. Subsequent partial melting of this juvenile lower crust gave rise to arc-type magmas, such as the Moras granites [12]. At ca. 820 Ma, back-arc extension was likely triggered by the onset of slab rollback, which led to the formation of abundant A-type granites and minor back-arc basin basalt (BABB)-type mafic rocks, such as those documented in the Cuonadong region [3]. Around 760 Ma, the post-subduction intraplate evolution was initiated by extensional thinning of the Indian–Himalayan continental lithosphere, which facilitated asthenospheric upwelling. The associated thermal flux first caused partial melting of the lithospheric mantle, producing the Group 1 amphibolites. Second, the interaction between upwelling asthenospheric material and the partial melting of continental lithospheric mantle produced the Group 2 amphibolites (Figure 12).
In summary, our findings support an intracontinental rift setting, rather than an Andean-type margin, for the Indian–Himalayan terrane during the Middle Neoproterozoic (ca. 760 Ma). From the Early Neoproterozoic (>950 Ma) to ca. 760 Ma, the region underwent a tectonic transition from a continental arc to a back-arc extensional system and to an intracontinental rift setting (Figure 12). It should be noted, however, that this interpretation is based on limited geochemical data from Neoproterozoic magmatic rocks in the Himalayan orogen. Further research will be necessary to evaluate and refine this hypothesis.

6. Conclusions

  • In situ zircon U–Pb geochronology indicates the presence of ca. 760 Ma Neoproterozoic magmatism within the Himalayan orogen.
  • These mafic rocks can be classified into two geochemically distinct groups. Group 1 is enriched in incompatible elements and exhibits relatively higher initial (87Sr/86Sr)ᵢ ratios, lower positive whole-rock εNd(t) values, and lower positive zircon εHf(t) values, consistent with an E-MORB-like affinity and suggesting a lithospheric mantle source. In contrast, Group 2 shows lower initial (87Sr/86Sr)ᵢ ratios and higher positive whole-rock εNd(t) and zircon εHf(t) values, consistent with an N-MORB-like affinity and indicating an origin involving interaction between the lithospheric mantle and depleted asthenospheric mantle.
  • The ca. 760 Ma mafic rocks in the eastern Himalayan orogen are interpreted as products of an intracontinental rift setting. These findings support a tectonic transition of the Indian–Himalayan terrane from an arc to a back-arc and finally to a continental rift between the Early and Middle Neoproterozoic.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/min15101090/s1: Table S1: In situ LA–ICP–MS U–Pb dating results for zircons from the amphibolites; Table S2: Trace element compositions of Zircons from the amphibolites; Table S3: Zircon Hf isotopic compositions of the amphibolites [59,60,100]; Table S4: Major element (wt.%) and trace element (ppm) analytical results of the amphibolites [70]; Table S5: Whole rock Sr and Nd isotopic compositions of the amphibolites [61,62].

Author Contributions

Y.Y.: Conceptualization, Investigation, Writing—Original Draft, Writing—review & editing, Supervision; Z.Z.: Formal analysis, Investigation, Funding acquisition; G.M.: Investigation, Resources; S.D.: Formal analysis, Visualization. All authors have read and agreed to the published version of the manuscript.

Funding

This research was financially supported by the National Natural Science Foundation of China (grants 42572112 and 41702080), the Deep Earth probe and Mineral Resources Exploration-National Science and Technology Major Project (grant 2025ZD1006306), and the China Geological Survey (grants DD20240069 and DD202402028).

Data Availability Statement

The data are contained within this article.

Acknowledgments

The authors are thankful to the reviewers and scientific editor, whose constructive criticism and recommendations helped us to significantly rework and improve the manuscript.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Tectonic units of the Tibetan Plateau and adjacent areas (modified after [6]). STDS = South Tibetan Detachment System; MCT = Main Central Thrust; MBT = Main Boundary Thrust; IYSZ = Indus–Yarlung Zangbo suture zone. Data sources of ages and locations of the Neoproterozoic magmatic rocks: Cuonadong [3]; Black Mountain [13]; Cuona [3,6,7]; Peshawar [5]; Amdo [14]; Chor [15]; Bhutan [16]; Hapoli [4]; Malani [11,17]; Namco [18]; Chengannur and Pathanapuram [19]; Pali, Mirpur, and Moras [12].
Figure 1. Tectonic units of the Tibetan Plateau and adjacent areas (modified after [6]). STDS = South Tibetan Detachment System; MCT = Main Central Thrust; MBT = Main Boundary Thrust; IYSZ = Indus–Yarlung Zangbo suture zone. Data sources of ages and locations of the Neoproterozoic magmatic rocks: Cuonadong [3]; Black Mountain [13]; Cuona [3,6,7]; Peshawar [5]; Amdo [14]; Chor [15]; Bhutan [16]; Hapoli [4]; Malani [11,17]; Namco [18]; Chengannur and Pathanapuram [19]; Pali, Mirpur, and Moras [12].
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Figure 2. Geological map of the Cuonadong area with the sample locations. Data sources: Cuonadong [3]; Cuona-Xiaozhan [3,6,7].
Figure 2. Geological map of the Cuonadong area with the sample locations. Data sources: Cuonadong [3]; Cuona-Xiaozhan [3,6,7].
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Figure 3. Outcrop and photomicrographs of representative ca. 760 Ma Neoproterozoic mafic rocks. (A,D) Outcrop and microphotograph of sample CNDXCP-B1, respectively. (B,E) Outcrop and microphotograph of sample CNDS16-B1, respectively. (C,F) Outcrop and microphotograph of sample CNDS16-B2, respectively. Hbl—hornblende, Px—pyroxene, Pl—plagioclase, Bt—biotite, and Q—quartz.
Figure 3. Outcrop and photomicrographs of representative ca. 760 Ma Neoproterozoic mafic rocks. (A,D) Outcrop and microphotograph of sample CNDXCP-B1, respectively. (B,E) Outcrop and microphotograph of sample CNDS16-B1, respectively. (C,F) Outcrop and microphotograph of sample CNDS16-B2, respectively. Hbl—hornblende, Px—pyroxene, Pl—plagioclase, Bt—biotite, and Q—quartz.
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Figure 4. Zircon U-Pb diagram and chondrite-normalized REE pattern of zircon of ca. 760 Ma Neoproterozoic mafic rocks. (A) Zircon U-Pb diagram of sample CNDXCP-B1. (B) Chondrite-normalized REE pattern of zircon of sample CNDXCP-B1. (C) Zircon U-Pb diagram of sample CNDS16-B1. (D) Chondrite-normalized REE pattern of zircon of sample CNDS16-B1. (E) Zircon U-Pb diagram of sample CNDS16-B2. (F) Chondrite-normalized REE pattern of zircon of sample CNDS16-B2.
Figure 4. Zircon U-Pb diagram and chondrite-normalized REE pattern of zircon of ca. 760 Ma Neoproterozoic mafic rocks. (A) Zircon U-Pb diagram of sample CNDXCP-B1. (B) Chondrite-normalized REE pattern of zircon of sample CNDXCP-B1. (C) Zircon U-Pb diagram of sample CNDS16-B1. (D) Chondrite-normalized REE pattern of zircon of sample CNDS16-B1. (E) Zircon U-Pb diagram of sample CNDS16-B2. (F) Chondrite-normalized REE pattern of zircon of sample CNDS16-B2.
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Figure 5. (A) SiO2 vs. Zr/TiO2 × 0.0001 diagram for the ca.760Ma mafic rocks (modified after [67]). (B) FeOT/MgO vs. SiO2 diagram for the ca.760Ma mafic rocks (modified after [68]), showing these ca.760Ma amphibolites are tholeiitic. Data for the Malani ca.760 Ma mafic rocks and felsic rocks are from [11,17].
Figure 5. (A) SiO2 vs. Zr/TiO2 × 0.0001 diagram for the ca.760Ma mafic rocks (modified after [67]). (B) FeOT/MgO vs. SiO2 diagram for the ca.760Ma mafic rocks (modified after [68]), showing these ca.760Ma amphibolites are tholeiitic. Data for the Malani ca.760 Ma mafic rocks and felsic rocks are from [11,17].
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Figure 6. Diagrams of Primitive mantle-normalized trace element patterns (A,C) and chondrite-normalized REE patterns (B,D) of ca. 760 Ma Neoproterozoic mafic rocks. Chondrite, primitive mantle, normal mid-ocean ridge basalt (N-MORB), enriched mid-ocean ridge basalt (E-MORB), and oceanic island basalt (OIB) data are from [69]. Normalization values after [69]. Data of Malani ca. 760 Ma mafic rocks are from [11,17]. Data for Okinawa BABB are from [71].
Figure 6. Diagrams of Primitive mantle-normalized trace element patterns (A,C) and chondrite-normalized REE patterns (B,D) of ca. 760 Ma Neoproterozoic mafic rocks. Chondrite, primitive mantle, normal mid-ocean ridge basalt (N-MORB), enriched mid-ocean ridge basalt (E-MORB), and oceanic island basalt (OIB) data are from [69]. Normalization values after [69]. Data of Malani ca. 760 Ma mafic rocks are from [11,17]. Data for Okinawa BABB are from [71].
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Figure 7. LOI versus representative major and trace elements diagrams for the ca. 760 Ma mafic rocks. (A) LOI versus CaO diagram. (B) LOI versus K2O diagram. (C) LOI versus TiO2 diagram. (D) LOI versus Rb diagram. (E) LOI versus V diagram. (F) LOI versus Ni diagram. (G) LOI versus Ta diagram. (H) LOI versus Zr diagram.
Figure 7. LOI versus representative major and trace elements diagrams for the ca. 760 Ma mafic rocks. (A) LOI versus CaO diagram. (B) LOI versus K2O diagram. (C) LOI versus TiO2 diagram. (D) LOI versus Rb diagram. (E) LOI versus V diagram. (F) LOI versus Ni diagram. (G) LOI versus Ta diagram. (H) LOI versus Zr diagram.
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Figure 8. (A) εNd(t) vs. SiO2, (B) Nb/La vs. Nb/Th, (C) La/Sm vs. Nb/La and (D) Sm vs. Sm/Yb diagrams for the ca. 760 Ma mafic rocks from the eastern Himalayan orogen.
Figure 8. (A) εNd(t) vs. SiO2, (B) Nb/La vs. Nb/Th, (C) La/Sm vs. Nb/La and (D) Sm vs. Sm/Yb diagrams for the ca. 760 Ma mafic rocks from the eastern Himalayan orogen.
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Figure 9. (A–H) MgO versus representative major and trace elements and (I) CaO vs. SiO2 diagrams for the ca.760 Ma mafic rocks.
Figure 9. (A–H) MgO versus representative major and trace elements and (I) CaO vs. SiO2 diagrams for the ca.760 Ma mafic rocks.
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Figure 10. (A) Sr-Nd isotope systematics of ca. 760 Ma Neoproterozoic mafic rocks. Data for Mozambique N-MORB mafic rocks are from [81]. Data for the Malani ca.760 Ma mafic rocks and felsic rocks are from [11,17]; Data for the Neoproterozoic juvenile lower crust are from [12]; Data for the Himalayan basement are from [16,82]. (B) zircon U-Pb age vs. εHf(t) diagram for the ca. 760 Ma mafic rocks. Data for Chengannur and Pathanapuram granites are from [19]; Data for Pali, Mirpur, and Moras granites are from [11]); Data for Cuonadong amphibolite and gneisses are from [3]; Data for Malani ca.760 Ma felsic rocks are from [11,17]; Data for end-member A are CNDS16B3; Data for end-member B are CNDXCPB4; Data for end-member C are from [3]; Data for end-member D are from [16].
Figure 10. (A) Sr-Nd isotope systematics of ca. 760 Ma Neoproterozoic mafic rocks. Data for Mozambique N-MORB mafic rocks are from [81]. Data for the Malani ca.760 Ma mafic rocks and felsic rocks are from [11,17]; Data for the Neoproterozoic juvenile lower crust are from [12]; Data for the Himalayan basement are from [16,82]. (B) zircon U-Pb age vs. εHf(t) diagram for the ca. 760 Ma mafic rocks. Data for Chengannur and Pathanapuram granites are from [19]; Data for Pali, Mirpur, and Moras granites are from [11]); Data for Cuonadong amphibolite and gneisses are from [3]; Data for Malani ca.760 Ma felsic rocks are from [11,17]; Data for end-member A are CNDS16B3; Data for end-member B are CNDXCPB4; Data for end-member C are from [3]; Data for end-member D are from [16].
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Figure 11. (A) Zr vs. Ti (after [83]), (B) Nb/Yb vs. Th/Yb (after [84]) and (C) V vs. Ti/1000 (after [85]) diagrams for ca. 760 Ma Neoproterozoic mafic rocks from the India-Himalayan terrane. Data sources: Malani mafic rocks (760 Ma) [11,17]; and Okinawa BABB [71].
Figure 11. (A) Zr vs. Ti (after [83]), (B) Nb/Yb vs. Th/Yb (after [84]) and (C) V vs. Ti/1000 (after [85]) diagrams for ca. 760 Ma Neoproterozoic mafic rocks from the India-Himalayan terrane. Data sources: Malani mafic rocks (760 Ma) [11,17]; and Okinawa BABB [71].
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Figure 12. Cartoon illustrating tectonic evolution of the India-Himalayan terrane from the Early Neoproterozoic to Middle Neoproterozoic.
Figure 12. Cartoon illustrating tectonic evolution of the India-Himalayan terrane from the Early Neoproterozoic to Middle Neoproterozoic.
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Yang, Y.; Zhang, Z.; Ma, G.; Dong, S. Report of CA. 760 Ma Mafic Rocks in the Eastern Himalayan Orogen: Petrogenesis and Geodynamic Implications. Minerals 2025, 15, 1090. https://doi.org/10.3390/min15101090

AMA Style

Yang Y, Zhang Z, Ma G, Dong S. Report of CA. 760 Ma Mafic Rocks in the Eastern Himalayan Orogen: Petrogenesis and Geodynamic Implications. Minerals. 2025; 15(10):1090. https://doi.org/10.3390/min15101090

Chicago/Turabian Style

Yang, Yi, Zhi Zhang, Guotao Ma, and Suiliang Dong. 2025. "Report of CA. 760 Ma Mafic Rocks in the Eastern Himalayan Orogen: Petrogenesis and Geodynamic Implications" Minerals 15, no. 10: 1090. https://doi.org/10.3390/min15101090

APA Style

Yang, Y., Zhang, Z., Ma, G., & Dong, S. (2025). Report of CA. 760 Ma Mafic Rocks in the Eastern Himalayan Orogen: Petrogenesis and Geodynamic Implications. Minerals, 15(10), 1090. https://doi.org/10.3390/min15101090

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